Solid State Guitar Amp Forum | DIY Guitar Amplifiers

Solid State Amplifiers => The Newcomer's Forum => Topic started by: Dimi Pana on January 23, 2019, 12:35:01 AM

Title: Amp repair?!
Post by: Dimi Pana on January 23, 2019, 12:35:01 AM
I have an Epiphone Valve Junior Combo amplifier that needs repair. It used to play fine until I decided to swap in/out a few different tubes both at pre and power section. I suspect the repeated removal/insertion of tubes and retention spring must have done something to the socket. I simply cannot turn the amp on and start playing and in less that a few minutes there is a weird crackling noise, then some hiss/static noise and then silence. I then check the tube with a tester it's dead, both the heaters (pins 4 & 5) show no continuity. I have ..."lost" two brand new tubes so far, so I am not risking putting another in there unless I identify the problem. This is a very simple design the only thing that could be the culprit is a resistor and cap and of course the voltage that is provided to the power tube. BUT, there is absolutely no problem at all with the preamp tube (which I also swapped in/out many times) so I am kinda stuck. I can follow instructions, I am good at soldering, I have a decent DMM and tube tester (albeit borrowed from a friend), I understand electronic schematics although definitely not an expert. I believe I can do the troubleshooting with someone's guidance. It is simply not worth paying a tech to look at it, it will probably cost me more than the amp's value. I am also posting the schematic, however at this early point I am primarily concerned about asking questions in a Solid State forum about a tube amp. Let me know, I hope I did not break any rules. Thank you!

EDIT (1/23): I uploaded another schematic I found online, the tube section is identical but there are some "interesting"(?!) differences in the power supply. Indeed the transformer outputs 6.3 VDC for the heaters obviously after the bridge it should be a bit higher, so indeed the first schematic is wrong (we already know that).  ;)
Title: Re: Amp repair?!
Post by: phatt on January 23, 2019, 09:00:23 AM
First you have to determine which schematic is your amp as I think there are a few variations.
The schematic you have uploaded makes little sense as you have heater voltage at transformer reading 6.3VAC then a full wave bridge and output reading 6.3VDC.
As that can't happen then that means one of those readings is wrong.

Here is a way to check;
If your circuit has that 8Amp Bridge driving into C12 (4,700uF/16V) then you have a DC heater circuit.
If so then set your meter to read DC.
Now with valves removed and amp powered on, read the DC voltage between pins 4&5. Then post your findings. Does not have to be exact,, +/- 20% is ok but 9 VDC will fry the heating filament in the valve.

If you do not have the DC heater circuit then it's obviously going to be AC so set meter to read AC volts and post those numbers.
Phil.
Title: Re: Amp repair?!
Post by: Dimi Pana on January 23, 2019, 09:38:04 AM
Phil -

Thanks for your quick and informative answer. I have version 3 of the Epiphone valve Junior which luckily has the DC heater circuit. Thanks for confirming that there's something strange with the schematic as you can see it's lifted from a venerable source but you never know right? I'll try to find a better schematic I'm almost certain there's one floating online for all different versions of the EVJ. I am at work now I will do the measurements you asked me when I get back home. Just to clarify you are asking me to do that with both tubes, not only the power tube that's giving me the problem. Also you want me to measure between 4 and ground and then 5 and ground or between 4 and 5 only? Finally I might be able to obtain an identical working amp from a friend-of-a-friend etc LOL which could be useful for comparison purposes. Much obliged for the help!
Title: Re: Amp repair?!
Post by: phatt on January 24, 2019, 02:40:27 AM
Yes pins 4&5 ONLY. no ground needed,, you just want to see if heater sockets are delivering the correct voltage,,Yes check 4&5 on both valve sockets to be certain.

It may be fine but as the schematic is wrong then it's a potential issue.

A couple of other more likely things to check while the power is on and no valves inserted is the Screen grid voltage at pin 9 of the power tube socket and the bias resistor.

 Screen grid check;
Set meter to highest DC voltage,,
With Black lead on chassis (or other convenient ground point)
Probe pin 9 of power tube socket with the red lead.
As long as you have around 300VDC then screen is safe if no voltage then I think that would cause a failure.
While you are there check pin7 it should be a little higher in voltage than the screen voltage.
That covers the 2 main voltages of the power tube. B1 and B2 on schematic.

** Safety Note, when probing high voltage connect the black lead to ground point *FIRST* **

Another test;
With power OFF and meter set to *Ohms*
measure the value of R14 is should be close to 220 Ohms.
(This is a Cathode biased amp and R14 sets the idle for the power valve)
If it reads dead short then unsolder C5 at one end then read the resistor again. If it then reads normal C5 is shorted out and that would burn out a power tube real fast. In which replace C5.

If you are new to this?  Things to be aware of around live circuits.
Remove; Rings, watches, necklaces. Work with your left hand away from chassis. it is tempting to hold chassis with left hand while probing but if you do come into contact with high voltage that voltage will travel through your heart,, hence right hand working is safer.
Don't work in bare feet. And don't work on it after a long drinking session. :duh

Hope it helps,,Phil.
Title: Re: Amp repair?!
Post by: Dimi Pana on January 24, 2019, 01:53:31 PM
OK, sorry for delaying to do what you asked me, I am digressing, but we expect snow and subzero temps in Chicago as of later today so I decided to stay home and prepare the "fort" (salt-check, snow blower-check, electric generator-check, etc, you get my point).

But I also managed to find the time and took the measurements, so here it is.

I first checked R14 (did that BEFORE even powering up) and it read 217 Ω, which I think is good (so I did not mess with removing C5) .

Then, with 125 VAC coming from the grid here's what I get:


Preamp tube

Probing DC pin# to ground:

Pin 1 ---> 345 V

Pin 2, 3, 7, 8, 9 ---> 0 V

Pin 4, 5 ---> 9.30 V (!?)

Pin 6 ---> 345

  *Probing between pin 4 and pin 5 ---> 9.45 V (?!? but read comment below about the heater pins)


Power tube

Probing DC pin# to ground:

Pin 1, 2, 3 , 4, 6 and 8 all show 0 V

Pin 5 ---> 9.43 V

Pin 7 ---> 385 V

Pin 9 ---> 384 V

 *Probing between pin 4 and pin 5 ---> 9.61 V (hmmm ?!?)

BUT wait !!!

With time (as circuit warmed up more?) the heater pins voltage also kept going up in both tubes!?! I watched it go all the up past 9.85 V and eventually powered-off at 10.05 V fearing something is (likely) wrong and it was still rising!? Think we found the culprit?

Thanks again for all the advice, waiting for your diagnosis and possible remedies. I hope this is an easy (and not so expensive) repair!  ;)

- Dimi.






Title: Re: Amp repair?!
Post by: phatt on January 25, 2019, 02:10:28 AM
Hi Dimi,
OK the EL84 Plate (pin7) and Screen (pin9) are ok.

Cathode resistor is fine.

But Oh dear,, Ouch!! yes my fear was correct.
The heater is way over voltage.  :o

The heater filament is designed to run on 6.3 Volts (AC or DC)
The problem is that there is only a +/-10% tolerance for filament voltages. Outside of that you are shortening the life span of the Valves.

So 6.3v + 10% = 6.93v max,,, so 7 Volts is the outer limit for safe operation. At 9Volts you are striping the cathode rapidly until the filament gets too hot and burns out. (your comment that you had no continuity between pins 4&5 on the power valve confirms the heater has burnt out)
Bear in mind that those voltages you post are *No load* readings and will be lower with valves inserted,, but likely heater is still too high.

If you pull the 2 filament wires from the pcb and read the AC voltage at the transformer output it should read somewhere around 7Volts AC with no load.
That will tell you that the filament voltage is correct for AC heaters but NO go for DC. :trouble :trouble :trouble :trouble
Adding a Full wave bridge rectifier adds 1.4 times to the AC Volts.

So 6.3VAC x 1.4 = 8.82VDC,, So yes some clueless designer screwed up badly. :duh

You could add 5 watt resistors to drop the voltage or Remove the rectifier and run the heaters on ACvolts.
But I'd guess that would mean cutting some tracks on the PCB.
Others here who know this amp well might have a better option for you.

Re the rising voltage,, with no load the voltage will float a bit but no big deal,, with no valves not much can go wrong. :tu:
Phil.
Title: Re: Amp repair?!
Post by: Dimi Pana on January 25, 2019, 01:21:27 PM
Phil, hi and thank you again for the insight.

The weird thing is this amp was working OK for about a month after I purchased it, early last Nov 2018. Well, I did feel that the front face-plate was (probably) running a little bit on the hot side, but had no other to compare, the amp was working, so I ignored that. Then in mid-December while I was playing it started the crackling and hissing noises, and the amp stopped making sound. I took it to a friend who has the same amp, he removed the tubes, and found the power tube had burned out.

I felt that for a used amp off of eBay that was expected so I embarked on replacing both tubes with new so I can have a fresh start and a baseline. After some research and because I felt the amp distorted too soon, I decided to go with a pair of JJ tubes only instead of a 12ax7, I put a 5751 in the preamp which has 30% less gain and it did help a bit with what I was looking to accomplish.

It is at this point that I tried about 4-5 different brands just to see if my ears could tell the difference. I decided I could not tell any (significant) difference so I settled with the JJs. However, at this point the problem started getting serious. During my tube swapping I did not blow any tubes probably because I never played the amp too much or too hard.

So now that I settled on two JJ preamp + power tubes I cannot play more than 2-3 minutes and the amp sound crackles, hisses and fizzles and the power tube (ONLY the power tube is gone!?)

So what could cause the higher voltage to the heaters? Btw, this version 3 of the EVJ was specifically made with DC heater voltage to eliminate noise which was a problem with earlier versions. So there is no purpose going back to AC.

But what's causing the excess voltage going to the heaters? Could it be the transformer? Or the next logical think would be the rectifier. I look at the schematic and do not see any resistors after the bridge rectifier so I am assuming the path is:

120 VAC ---> transformer ---> 6.3 VAC ---> bridge rect. ---> ~8 VDC ---> tube heaters.

Am I correct?

Also, if for some reason the DC to the filaments is so high how come the preamp tube does not burn out. Plus when I did the first measurement I found over 9 volts, so am I right thinking that the bridge rectifier may have gone bad?

Now that I am getting more comfortable with all this, I will go back and do some more careful and invasive measurements. In the mean time, how can I "advertise" in this forum what my problem is in the hopes that someone particularly expert with this amp can help me out?

Thanks again!     
Title: Re: Amp repair?!
Post by: galaxiex on January 25, 2019, 02:50:45 PM
Is it possible that a damaged heater bridge rectifier could cause the high heater voltage?
Eg; one of the diodes shorted or open?

Or... maybe the transformer heater winding has a problem?
Title: Re: Amp repair?!
Post by: g1 on January 25, 2019, 03:13:48 PM
Reading the heater voltage without the tubes installed is in this case not much help.
You need to check with the tubes installed.
Post your results.
Title: Re: Amp repair?!
Post by: Dimi Pana on January 25, 2019, 03:45:44 PM
OK, I can certainly do that (test heater voltages with tubes installed) BUT I am afraid I will burn the power tube again. If I just turn the EVJ ON but with no guitar plugged in and volume all the way down, am I still stressing the tubes, especially the power (e.g. if there's nothing to output) or I am going to be OK for at least a minute or so I need to take the measurement?

On a different note, to test the bridge rectifier I need to remove it from the PCB correct?

--

So just to clarify because the schematic does not help, there are two components that can be failing between 120 VAC input and the tubes heater filaments: The power transformer and the bridge rectifier. I see a capacitor but that is not in series it is going directly to ground. Shouldn't there be some sort of resistor in there too? I am no expert but my limited knowledge tells me there should be at least one controlling the voltage to the heaters. YES/NO?!

Thank you all for the input!
Title: Re: Amp repair?!
Post by: Dimi Pana on January 25, 2019, 10:10:54 PM
I was about to test the heater voltages with the tubes on when I had the idea to test the transformer output BEFORE it goes into the bridge rectifier. Well, it is 7.8 VAC guys, so I think we found the problem. For some reason the transformer voltage is too high. Not sure what cause it, but that's what I think is going on. Am I right/wrong? Chime in please! So after it exits the bridge and gets rectified, it will be even more hence the 9+ voltages I was measuring yesterday. Still have not put in the tubes, but does it make any difference at this point? Is it normal that all of a sudden the PT will output more of its nominal voltage. If you think I should still measure the heater voltages with tubes installed let me know - Thank you!

EDIT: While I was thinking what could have caused the PT to do that, I remembered I have a mains step-down transformer at work. I am a PC technician and sometimes we need to bring down the mains AC if it is too high so it won't fry PC equipment. Anyway, I plugged it all in and guess what. With the AC stepped down the heater voltage is a healthy 6.5 Volts. At this point I plugged the tubes in and played my guitar through the amp, no problem at all. I am posting some pictures, so what do you think? I am so lost about WHAT could have caused the PT to go ...crazy!?
Title: Re: Amp repair?!
Post by: phatt on January 25, 2019, 10:37:46 PM
Ed;
Hi Dimi,
Wow now you are running hot mate,, good work.
You will be the expert soon.

You posted just as I finished this but I'll post it anyway as it's relevant and may help.
-----
Number 1;
I think it would be wise to establish what AC voltage is coming from the transformer that powers the heater filaments,, FIRST!
Then move on to other possible problems.

Remove Valves
Set meter to read AC volts;
REMOVE the 2 filament wires that go to Bridge rectifier.
(in the pics I see on the net they might be Orange but you better check)
Insert probes in those 2 wires and read the AC voltage,, then post results.

For AC filaments the no load voltage should be around 7VAC, give or take a few points.
If the transformer was wound for full wave rectification then it should read less,, maybe 5VAC.

Yes you can power up with a power tube in place and read the heater pins on the back of socket, it will only take a minute
If it worries you then you can likely remove the high voltage wires from the pcb but leave the filament wires in place to power the heater,, (looks like all the transformer wires are spade cons so should be easy to remove B+) should only take a minute to read the loaded heater voltage, then turn off.

Regards AC hum,, Again misguided design Xpurts.
The PCB tracks are in the wrong place hence heater hum. :duh

my guess;
To lazy to reroute the pcb with better layout so they just slapped a bridge on the heater supply.
which did help hum but now the filaments are running too hot.
Do they care?  I doubt it. :-X
They sell trashy crap with a brand name on it and people buy it cause it has a big name brand on the front. I fixed a couple of Fender pro Juniors (some models had a really bad hum)
The most sensitive signal path in the whole circuit ran a trace right next to the B+ track.  :loco :loco :loco
I simply Replaced that track with a shielded cable directly to valve socket and hum was gone. ;)

And that is just FENDER,, other big names make similar F*** ups.
And you pay big dollars for the privilege of their name on the front.

From the RCA Valve manuals; "Triodes should last for 10,000 Hours at least,, power valves 5,000 hours."
If they don't it's usually because of a crap design in the circuit.
I read a report that a uni had a valve computational circuit which run 24/7 for 30 years.
Yet The best names in guitar amp world still don't get the joke,,, but the shops sell a lot of new valves every 2 years. go figure :-X
Marshall are notorious for burning out EL34's because they run the screens way over voltage,, and still doing it.

Regards Amp distorts too soon;
Easy to understand once you study a bit of history. 8|
Back in the 40/50's these type of amps were popular because a jazz player could lift has volume.
Those old guitars had low output PU's and there was enough power to lift the sound to balance against the Brass section.
Now even the cheapest guitar with crappy PU's has many times the output of those old semi acoustic jazz guitars,,, Result = your Amp cannot play clean with a high gain PU's,, Unless you turn right down.
If you want clean then you need a bigger amp to get clean headroom.
Rant over. :grr :grr :grr
Phil.
Title: Re: Amp repair?!
Post by: phatt on January 25, 2019, 10:53:49 PM
Still have not put in the tubes, but does it make any difference at this point? Is it normal that all of a sudden the PT will output more of its nominal voltage. If you think I should still measure the heater voltages with tubes installed let me know - Thank you!
YES!
EDIT: While I was thinking what could have caused the PT to do that, I remembered I have a mains step-down transformer at work. I am a PC technician and sometimes we need to bring down the mains AC if it is too high so it won't fry PC equipment. Anyway, I plugged it all in and guess what. With the AC stepped down the heater voltage is a healthy 6.5 Volts. At this point I plugged the tubes in and played my guitar through the amp, no problem at all. I am posting some pictures, so what do you think? I am so lost about WHAT could have caused the PT to go ...crazy!?

A stupid crazy clueless Valve amp designer :lmao: :lmao: :lmao: :lmao:
Phil.
Title: Re: Amp repair?!
Post by: galaxiex on January 25, 2019, 11:37:11 PM
Is it possible for the transformer heater winding to partially short a few winds and cause the higher AC heater voltage?
Title: Re: Amp repair?!
Post by: phatt on January 25, 2019, 11:58:28 PM
Is it possible for the transformer heater winding to partially short a few winds and cause the higher AC heater voltage?

If so surely the voltage would be lower not higher??
Phil.
Title: Re: Amp repair?!
Post by: phatt on January 26, 2019, 12:07:29 AM
@ Dimi,,
Oppsy :-[,, A Correction on heater pins;

The preamp is for an AX7 which can be wired for 6.3Volts as well as 12.6Volts.
So for 6.3 volt operation the heater pins 4&5 are tied together and pin 9 is the other end. so probe pin9 & 4 or 5.

For the EL84 power valve, it only runs on 6.3Volts so heater voltage is between pin 4&5.

This is what happens when you have not had a coffee and don't work on valve gear for a long time. :-[
Phil
 
Title: Re: Amp repair?!
Post by: galaxiex on January 26, 2019, 12:11:08 AM
Do I have this backwards?
I thought fewer turns on the secondary (like a shorted winding would cause)
= higher voltage.

Ooops. Yes sorry I had it backwards.
Carry on...
Title: Re: Amp repair?!
Post by: phatt on January 26, 2019, 12:30:41 AM
Do I have this backwards?
I thought fewer turns on the secondary (like a shorted winding would cause)
= higher voltage.

Ooops. Yes sorry I had it backwards.
Carry on...

I know how you feel,, hey you only got it backwards,, while I had it inside out, back to front and upside down. :lmao:
Phil.
Title: Re: Amp repair?!
Post by: Dimi Pana on January 28, 2019, 06:12:30 PM
ΟΚ, so here in the NW 'burbs of Chicago, I've been spending most of my free time removing snow, maintaining the snow blower and preparing for the next snow, so amp repair was put in the back burner. But here's something interesting, I was able to use the amp with no problem and no tubes burning up by powering it through the step down transformer. It sounds good too! So what's the remedy for my ...blues?! A new power transformer? Or calculating the values and adding some resistors in-series to bring down the heater voltage? Or perhaps a ...new amp? xP Chime in please with your experts' advice, my boss is hounding me to bring the step down transformer back to the shop but unless I find a permanent solution this ain't gonna happen soon!
 :dbtu:
Title: Re: Amp repair?!
Post by: galaxiex on January 28, 2019, 06:32:37 PM
Well, you could make one of these bucking transformer things...

scroll to Page 2 of the pdf.

Title: Re: Amp repair?!
Post by: Dimi Pana on January 28, 2019, 06:54:32 PM
WOW galaxiex !!! Where on earth did you find this?! I love building little projects like that. Essentially I am making a step down transformer that is more portable than what I already have now and although I was looking for a more direct approach this workaround is certainly an option plus a bucking transformer is always a useful thingy to have on my bench. Thank you so much, where did this come from, do you have more projects you can share like that? I'd be very interested. Thank you!
Title: Re: Amp repair?!
Post by: galaxiex on January 28, 2019, 07:19:11 PM
WOW galaxiex !!! Where on earth did you find this?! I love building little projects like that. Essentially I am making a step down transformer that is more portable than what I already have now and although I was looking for a more direct approach this workaround is certainly an option plus a bucking transformer is always a useful thingy to have on my bench. Thank you so much, where did this come from, do you have more projects you can share like that? I'd be very interested. Thank you!

You are very welcome.  :)

I got it from the Hoffman amp forum.
A member/moderator there by the name of "sluckey" is a pretty smart guy with amps.
That is his scrapbook of amp stuff.

Forum here.

https://el34world.com/Forum/index.php (https://el34world.com/Forum/index.php)

Edit; Here's sluckey's web site...

http://sluckeyamps.com/ (http://sluckeyamps.com/)
Title: Re: Amp repair?!
Post by: phatt on January 28, 2019, 09:19:17 PM
You could just insert a low value 5 Watt resistor in series with the filament supply.
You may have to fiddle with a few values but maybe around 5 Ohms would work.
A 7806 reg may also work but a sigle resistor would be simple and Cheap. :-\
Phil.
Title: Re: Amp repair?!
Post by: solderer25 on January 29, 2019, 06:51:56 AM
Hi. I would return and check the bridge rect. and smoothing cap for the heater supply before suspecting the transformer. These can both be easily tested with a DMM. Check also the solder connections for both as problems there could also give the symptoms you are having with heater supply overvoltage. The design of this part of the circuit really is primitive - cost cutting gone mad!
Title: Re: Amp repair?!
Post by: Dimi Pana on January 29, 2019, 01:08:58 PM
Thank you all for the suggestions. I am providing you with some extra feedback I was able to collect from a friend of a friend who has an identical EVJ and also need confirmation about some ideas of my own.

1. By this week's end I should be able to have that identical EVJ borrowed and I will do some measurements and compare with mine. At that point we will be able to determine which part or the "equation" is ...faulty. The PT, the bridge, something else, who knows?

2. I am trying to confirm that the bridge that rectifies the AC to DC for the heaters is that black square component with the four legs. You are probably not able to see it well in the pics I uploaded, it sits between the two spade connectors where the two orange wires are going in from the PT and the (big) 4700μF capacitor. However there is also an array of four black diodes forming -clearly- a rectifying network, what I do not know for sure is which goes to the heaters, and which to the rest of the amp. Just by their location I am guessing the diodes are for the main amp circuit and the bridge for the heaters.

3. Can you provide either a quick (if possible) guide how to test all possible components or a link where I can read up? I mean I understand an ideal scenario is to remove them from the PCB and test independently however this is not possible at this time, I will wait until I had the chance to compare with the other working EVJ and draw some -hopefully final- conclusions. Still, how do I test a bridge, or a diode array, or that big capacitor? I am also a bit concerned about high voltages. Can you please advice.

4. Someone pointed me to the possibility that all this tube swapping I did (remove, insert, remove, etc.) may have loosened the pins in the socket. So the theory is that a pin that makes no contact at all or worse one that makes intermittent contact may be the culprit. Who knows, maybe I was hasty and damaged it, but again bear in mind that it is ONLY the power tube and not the preamp tube that's acting up. Still the heater voltage is the same at both the power and preamp socket. Do you see where I am driving at?

Anyway, thank you again, looking forward to your replies.

PS: Not sure where you guys are but here in the NW suburbs of Chicago we are bracing for one of the coldest 48 hours in this area's history, we expect temps of about -13 and probably double that with the windchill. All I can say is: Good luck!
Title: Re: Amp repair?!
Post by: phatt on January 30, 2019, 08:14:36 AM
Pretty easy to identify heater circuit from HT circuit,, just read the voltage rating of the caps on each rectifier output.
You would not use a 400Volt rated cap on a 6volt heater circuit. ;)

If you want to see if the rectifier is wonky set meter for ACVolts and report your findings.
There will likely be a tiny AC ripple on the heater DC rail but not much if working ok. 
It's unlikely any fault,, just plain old cheap crap made to a price not a standard.
Close enough is good enough and they know it will last just till the warranty runs out. :-X
Don't over think it,, If it was on my bench I'd just drop the heater voltage with a resistor problem solved. 8|
Phil.
Title: Re: Amp repair?!
Post by: Dimi Pana on January 31, 2019, 06:42:22 PM
Phil hi -

Yes, it makes sense, basically follow the heater circuit starting from the two orange cables coming from the PT, through the rectification then to the cap. The 4700μF cap is obviously the one from the heater wires to ground. The only problem with doing all this AND taking measurements is removing the PCB from the case, turning it upside down, re-attaching all wiring, etc etc. The way the PCB is, its solder pads side is hidden under the visible side. Anyway, I will definitely get the identical EVJ this Saturday and will be able to assess what's going on after I take measurements and compare. I can certainly put in there a better (higher quality) bridge with the desired DC output OR like you said the easiest MOST effective remedy is to calculate the appropriate resistor value. Thanks and I will be back this weekend with more ...news!

Edit: Btw, how do I calculate the value of the resistor? I know I have to use Ohm's Law (R=U/i) I know the target voltage and I need to find the Resistance. But how do I know what the Amperage is? Are you saying I should measure the amperage without any modifications and go from there? What about the tubes' filaments, that's resistance too, right? Shouldn't that be taken into account also? ???
Title: Re: Amp repair?!
Post by: phatt on February 01, 2019, 01:58:09 AM
You don't have to check all that or pull the PCB; 8|
ALL you need do is measure the Heater AC, at those 2 orange leads.
Using the pic from previous page your meter reads 7.8 VAC so to find out what the DC voltage should be After the rectifier you just multiply that number by 1.4
Answer = 10.92VDC (NO LOAD)
IIRC you mentioned it read over 9 volts and climbing over 10VDC with no load. So it's working right,, (just too high a voltage cause some clown fluffed up in the design dept)

It won't be perfect match due to a some loss in the diodes but within a volt so will tell you the rectifier is working.
Under load it will drop a volt or so but obviously not enough to get back to 6.3 VDC

So Insert a 5Watt resistor in series with one of the orange leads, The AC side of Rectifier
Start with 4.7 Ohm and read the DC voltage,, work up or down as needed. saves a whole lot of maths :-X cause I've forgotten how anyway :-[ :lmao: :lmao:
When it's close to 7Volts DC,, insert some valves and read the filament voltage again to check you are within the +/- 10% tolerance,
wait for the Valves to heat up to get working voltage.

AX7 filament is around 300mA and EL84 is about 700mA so Total heater current will be around 1Amp.
You can use block connectors to add the 5W resistor and mount the resistor on a chassis then run wires back to PCB.

Oh while you are inside this thing,, cut those cable ties.(we call them zip ties)
Separate the **Mains wires** from the *Secondary wires*.
A common practice but potentially fatal if mains wires melts onto secondary wires.
May never happen but that practice would be an unacceptable safety risk in hospital and military equipment. :trouble :trouble :trouble :trouble :trouble :trouble
Rebundle the 2 sets of wires and re-tie then you know it's safe.

Phil.


Title: Re: Amp repair?!
Post by: Dimi Pana on February 01, 2019, 12:57:08 PM
Τhanks for the info, it prompts me to study more, and that's a good thing. Tube amps seem simple from a first look but it can get complicated once you understand how each component feeds into the other from one stage to another. I am really excited about getting my hands to a working EVJ and taking some measurements comparing to mine. I should be able to report back by tomorrow afternoon. In the mean time can anyone elaborate (but not much and you can always provide a ink) on what is the effect voltage variance has on the heater filaments. And by "effect" I mean both electronically as well as from an audio (tone) perspective. Thanks!   
Title: Re: Amp repair?!
Post by: phatt on February 02, 2019, 04:55:45 AM
If filements run too hot they conduct a little more,, too cold they conduct less but I doubt you would notice any mind blowing tone mojo diff between too hot or cold.
Might distort a little earlier but as you have noticed you have to replace valves every few months due to burnt out heaters. :'(

I believe the Old RCA Valve hand books are now downloadable but a novice might find that hard reading.
Valve Wizard site is good,, you can buy books from him.
I recall Chapter one is a free down load now.
Phil.
Title: Re: Amp repair?!
Post by: Dimi Pana on February 10, 2019, 01:11:41 AM
OK, sorry for the late reply, got busy at work this past week. However, I was able to do some more "digging" and this is what I found. Also, keep in mind that all following measurements were taken with standard 125VAC grid voltage (e.g. without using the step down transformer) and with the tubes inserted, speaker connected, no guitar connected and volume all the way down. I did not let the amp stay on more than 1-2 minutes, I took the measurements and powered-OFF. Nothing "bad" happened, tubes were tested and found OK, no ...smoke or funny smells, you know what I mean eh?!  ;)

So, here's what I found:

#1. The PT outputs 7.8 VAC which feeds the tube heaters circuit. As correctly pointed out by Phil, the heaters of a 12ax7 and a EL84, combined, should draw about 1A. However, an actual measurement showed that at power-ON the current "rushes" to a little below 3A but it quickly starts falling and settles around 1.8A after about 30 seconds or so (I am assuming this is when the heaters have reached full operating temperature). Now, if I measure the heaters' voltage at that point, I get a consistent 8.5VDC at both of them.

#2. Through trial and error I found that the amount of resistance needed to bring the heaters voltage (measured at the pins with tubes inserted) down to its optimal range (e.g. +/- 5% of 6.3 VDC) should be between 0.5 and 1Ω. How so you may ask? Well, I had a few 5W wirewound resistors (5, 2.2 and 1 Ω) and tried them all. I started with a 5Ω but it brought the voltage too low (about 1.85VDC) the tubes did not even glow, then I tried the 2.2Ω, voltage went up but still low so tubes were barely glowing, then I tried a 1Ω and the voltage went up to about 5.38VDC, the tubes did glow but I ...chickened and did not try to play through the amp, I was afraid I might do some damage.

So, now I have two questions:

A. How do I (accurately and confidently) calculate the proper resistance value in order to achieve smack 6.3VDC at the pins of the heaters?

B. Assuming this is accomplished, can I consider this a solid/final solution or is it a necessary workaround/hack that works but might cause problems down the road. If it is not a, then what do I have to do to bring the voltage down without using the brute force solution (in series resistor). Someone I asked mentioned something about building a simple zener diode voltage regulator. I googled that and from the little I read it sounds like a viable solution. Can anyone please weigh in on this?

And as a ..."bonus question" can anyone explain why instead of about 1A, I measured 1.8A drawn by the heaters circuit? Also, Phil, by "block connectors" you mean that in the pic right?
Title: Re: Amp repair?!
Post by: phatt on February 10, 2019, 06:44:29 AM
Yes that is what i meant :tu:

You can bolt them onto the chassis and run wires,, here's a pic for example of how to do it. The big 10Watt R's are mounted on Terminal/Connector blocks,, well that is what I call them.

OK you found 1 Ohm is very close then you need to halve that. Well try .5 Ohm
 If you can't get OR47 (.47) resistors then Two 1 Ohms in parallel will give you 1/2 an Ohm. :dbtu:

It does not have to be Exact,, anywhere in that 10% window is fine (5.5Volts up to 7Volts).
A Zener would need to go on the DC side and yes it works but another fail point down the track as Zeners fail more often than resistors. up to you.
The wall outlet will go up and down a bit and that alters the result,, depends on time of day you measure the voltage.
So EXACT will never happen. :lmao:
Phil.
Title: Re: Amp repair?!
Post by: Dimi Pana on February 11, 2019, 01:04:15 AM
Success!  :dbtu:


Well, it is is not finalized yet but I managed to bring the heaters voltage down to 5.89 VDC, so then I was able to play with the EVJ for about an hour, clean, overdriven, pedals, straight into it, no funny sounds, high or low volume, no crackling, amp is not overheating, and most importantly it did not blow any tubes.

So the method Phil suggested has been tried and it works, I had to use a combination of four (!) resistors, 1, 2.2, 5 and 15 Ω (that's what I had available at home), in parallel, to achieve the lowest possible resistance value, which in theory is .58Ω but in practice my cheap DMM showed 0.8 Ω (well these were wirewounds at 10%).

Still 5.89 VDC is on the low side (not sure if that's good or bad) but sound/tone wise it had no perceivable change. I think an actual value between 1/2 and 3/4 of an Ohm should land me in the sweet spot of around 6.3 VDC, and actually I'd rather run a little on the "colder side" for tube longevity, am I right about that?

So a few questions:

1. As expected, the resistors got a little warm, still was able to comfortably touch them I mean after an hour of operation, they did not burn hot at all, so that's good. I could be wrong but the lower the resistance the warmer they appeared to be, the 15Ω was almost not warm at all. They were all 5W at 10% so what do you think for the final fix, should I get one at a higher wattage (thus bigger) or it is better to split the load (heat) across many of equal value. And I do not really need a heat sink, just bolt them against the amp chassis, right?  Also can you suggest where I can buy such resistors?

2. This is more of a philosophical question but it is funny how the PT the designers decided to use is putting out more voltage than what it is healthy for the tubes. Or you think the PT is developing a problem? From previous posts I understand that if the PT is on its way out, most likely it would have provided less voltage not more, am I right?

Thank you all and especially Phil for sticking with me along the way, so far this has exceeded my expectations, I really appreciate your help!

 :tu:
Title: Re: Amp repair?!
Post by: Enzo on February 11, 2019, 01:37:42 AM
You are WAY overthinking this, in my personal view.  The difference in tube life between 6.3v and 5.89 is negligible.   Phenomena like that don't necessarily "track" high numbers to low.  In other words having 9v there instead of 6 will have a large effect.  Having 6.9v versus 6.6 won't be detectable.

If trying to measure low resistance, always first touch the meter probes together to see how far above zero they result.  Then subtract that from any resistance readings you take.  If your probes have 0.2 ohm resistance, and you measure a 0.5 ohm resistor, the reading will show 0.7 ohms.

Transformers don't have a failure mode where their voltages drift.  They work on turns rations, and are simply wire wrapped around a form inside.  They don't wear out and lose voltage.

I doubt any designer sat there thinking he would run excess heater voltage through anything.  I suspect mains voltages change, yours, his, everyone's.  The load can change.  Perhaps the tubes he was using at the factory drew more heater current than the type you use.  In any case, I would assume the designer specified a transformer that put out about 6.3v at the designed load.


As to parallel resistors, Ohm's Law tells you the lower resistance will pass the most current, they won't share equally. 

FInd one resistor of the value you need.  yes, electrically ten resistors that add up the same will also work, but really...   Is 5w enough?  I don't know what is really flowing through your circuit.  Want to use a 10W?  Go ahead, teh cost is a few cents more, so why not.
Title: Re: Amp repair?!
Post by: Dimi Pana on February 11, 2019, 03:17:37 AM
You are correct, overthinking is a novice's natural reaction while treading unknown territory for the first time. IMO, I'd rather overthink than underthink, with high voltage amps it is also safer, I think.

Now, you have given me a lot of good advice, i.e. indeed the probes amount to about 0.2 Ω so the actual value I measured is about 0.6 Ω which is indeed within the theoretical 0.58 Ω. Ι looked up online, cost will not be an issue, any such resistor (.5 to .75 Ω @ 5 or 10 W or ...more) is going to cost way less than actually shipping it first class mail. Not worth ...overthinking that.

Can you also please help me with the following: Without inserting the voltage dropping resistor, there are two (orange color) wires coming from the PT providing 7.8 VAC for the heaters circuit and the tubes according to their data sheet should draw about 1A combined (in practice I measured 1.8 A but what do I know?). Anyway, according to Ohm's law the power should then be (that's easy P=VxI) a theoretical 7.8 W to an actual 14.04 W. Provided that my thinking is correct, a 5W resistor is insufficient in both cases. It is my understanding that the more power a resistor is rated at, the better it dissipates the heat. So what is the recommended value in my case?

Also, what is best: One resistor for the entire heaters circuit in series to one of these orange wires, or should I split the total recommended Ω value in half and add one resistor per each wire?

Regarding the PT voltage (and with my limited knowledge about amps) I doubt the designer screwed up so bad I mean the schematic I got says the PT provides 6.3 VDC but I am reading close to 8 VDC in mine, which after rectification you well know is going to be over 10, so as Phil said, someone screwed up big time or something is iffy with this particular PT. I still have not been able to borrow a friends identical EVJ to compare with mine, it will be interesting to see what's going on.

Anyway, thanks also for the info about running cold or hot, since I still have to get the proper resistor, I will try to tweak it just a tad higher, just to be OK according to the tube's data sheet. Btw, a few weeks ago when I started having problems with this amp I had no clue about heaters voltage etc. Now I understand that tube manufacturers say 6.3 V is ideal, a +/- 5% is acceptable and the maximum tolerance is up to +/- 10%. So, I hope around 6 V is OK and I heard somewhere that less heater voltage might help the amp distort a little later and the EVJ being as a simplistic design as it is, it does suffer from that also.

Again all this is just an excuse to play and learn with an amp that I can afford to ...destroy (not intentionally of course) so thank you all, and thank you Εnzο for the input.
Title: Re: Amp repair?!
Post by: phatt on February 11, 2019, 07:30:13 AM

Enzo has spent his life working on this stuff so I'd take his advice.  8|

Obviously .5 Ohms is all you need and gets you into the -5% mark which as Enzo has tried to point out is perfect.
IF heat worries you then use a 10Watt but two 1 Ohm 5 Watt resistors (in parallel) will give you .5 Ohms 10Watts
As long as the resistor/s are not giving you first degree burns then close the lid and it's done. HINT.

Regards mounting;
As all those resistors are only warm then mount them in a convenient place as long as it's away from the mains.

Think about the mounting point of your resistors before you cut the orange wire as you may have enough wire length in the orange wire to not need extra wire.
And No you don't need resistors on both sides of the AC.

Re transformers;
No the person who wound the transformer made no mistake.
The clown back at the factory who was under pressure to reduce hum due to bad track layout thought hey we can run the heaters on DC but forgot that doing so puts the heater voltage way over the limit.
I have a home built 10 Watt Valve Amp that has run on 6.9VAC heaters for over 15 years and it always runs flat out <3)
Phil.


Title: Re: Amp repair?!
Post by: Enzo on February 11, 2019, 03:45:13 PM
There us no reason to use two resistors.  The current is AC and flows both directions.

Power is determined by voltage and current.  A resistor will dissipate whatever those data indicate.  A 5 watt or 10 watt or 20 watt will dissipate the same watts.  Larger ones will just be a less concentrated hot spot in the chassis, that is all.

Ohm's LAw pertains to the resistor.  Remember, the entire 7.8v is not dropping across the resistor.  The resistor is there to drop 1.5v.  So at 1A, that would mean 1.5 watt by my math.  1.8A?  1.5 x 1.8 = 2.7 watt.  So 5w should be fine or 10W if you have the space.

At some point you mentioned something to the effect you had the "standard" 125v mains.  Well 125v is not a standard even if common, look at old schematics.  120v.  I grew up with 117v.  115v, and older still 110v.  A transformer could have been wound for any of those standards.  If you run a 110v transformer on your 125v, then the secondaries will all be 13% high.  Now what results from an increase of 13% at 6.3v?  7.2v
Title: Re: Amp repair?!
Post by: Dimi Pana on February 12, 2019, 12:36:53 PM
Thank you both for the feedback, I do appreciate the free tutorials, and as a formerly clueless end-user, I can now confidently assert that hey, when it comes to expert opinion there is no substitute, you have definitely opened up my eyes to matters that I was completely unaware of.  :duh

So,

#1. The amp is fixed and packaged back to its original state and it works REALLY well. I was able to find and ..."borrow" a few low Ω resistors from work, and after some mucking around, guess what? A .27Ω resistor (5W @10%) brings the heater voltage to its ideal range. Actually, at start up it shoots up to about 6.38V then goes down to around 6.28V and finally stabilizes exactly at 6.3V. Sorry for being pedantic, I was just playing with different values and just got lucky! I used some spade terminals soldered to the resistors leads dressed the connections nicely with heat shrink tubing, found a nice spot away from the mains and used some existing holes in the chassis (the ones where the heater wires enter from the PT) to thread a zip tie that fastened the resistor against the chassis with enough space for air to circulate around it. Like already said, even after of plenty of time of playing the amp, the resistor is comfortable to the touch, warm but not hot, actually while still in the open (e.g. chassis not back in the cabinet) a laser thermometer showed about 78F and after I put it in the enclosure that spot (touched from the outside) never felt hot, actually it was barely warm, perhaps because the chassis now dissipates the heat better(?!?). So it is done and THANK YOU SO VERY MUCH both/all!   :dbtu:

#2. I installed a new JJ EL84 and instead of a 12AX7 I opted for a new 5751 also JJ. The sound is sweet, it distorts much later and although it is probably the psychological aspect of it, I think it sounds better than before. So, looks like my $75 investment at a recent garage sale paid off. Even better, now I know a lot more about how this thing works (my first tube amp, btw) I did not spent a dime for the repair (other than time and readily available parts) and guess what, all this reading made me realize the EJV is probably one of the most modded low-power amps out there, to the point that I now feel ..."dangerous" enough to attempt some of this mods. I know what you're thinking, we'll never get rid of this guy, he "will be back"!!!   :grr

#3. I was also able to borrow that other EVJ from my friend and guess WHAT?!? As far as voltages, it is EXACTLY THE SAME !!! The PT provides 7.6VAC for the heaters and with my house's 125 VAC mains (btw, is that normal, or a bit high?), and tubes installed, when measured at the pins, I read a ..."nice" 6.93VDC. My friend said he had this amp for about 5 years now, he does not use it for gigging, just home practice/jamming, and he has never changed tubes at all!?! Again it is probably my ears, but my amp is quieter at any volume level and actually if cranked mine sounds more defined, the other is getting "muddier" earlier. Anyway, ... So you were of course right all along (not that I doubted any of you) still I am not sure why mine had the problem I experienced which led to the discovery of the "elevated" heater voltage, but honestly, I am not going to ...overthink this any more/longer.   :-[

#4. Regarding the wattage measurements, I used the 7.8VAC coming from the PT, and the 1A nominal current the two tubes draw per their data sheets.  But then I measured with my DMM the actual current and found it to be 1.8A, thus the widely varying figures I stated in an earlier post. I now understand how it really works, thanks Enzo and Phil. I am not sure why though, currently with the fix that draw has gone down to about 1.35A so there is still something on top of the filaments that draws the .35A current. Not worrying about it, after all the schematic does not show the heater circuit so I do not know if there is something else in there other than the heater's filaments.   8|

#5. This is not a question but I would be ungrateful if I did not mention again how appreciative I am for your help. If you guys ever happen to visit the "Windy City" PLEASE drop me a line, I'd like to return the favor.

 <3)

Cheers to all and take care!       
Title: Re: Amp repair?!
Post by: phatt on February 12, 2019, 11:51:32 PM
No worries, glad it worked out. :tu:

If your mates amp reads 7.6VAC and the heaters read 6.93VDC then it maybe a different model/revision.
There are several ways to rectify AC into DC and they all have different outcomes.
It may only have a halfwave rectifier which would yeild; .9 times the AC.
So 7.6VAC x .9 = 6.84VDC.
A much better outcome for the heater voltage and half wave Only requires one Diode,,, but Full wave gives better noise results.
It's debateable if you would notice the difference in buzz or hum.

http://www.hammondmfg.com/pdf/5c007.pdf
Note the maths below each type of rectifier setup.

Re heater current draw.
EL84 draws 760mA
AX7 about300mA
There will always be deviations due to manufacturing processes. So don't expext perfect results.

And don't forget the meter you use may not give the exact same readings as another DMM.
Oh and be careful reading mains voltage with cheap DMM's as some are not really up to the task.

Re mains voltage fluctuations;
I think **Enzo** has already made comment on that?
It fluctuates all the time and the amount depends on where you live,, how far from the power station,, how close is the booster transformer,, are you at the end of a long line,, and so forth.

Re zip ties;
They are plastic and will melt or at least stretch if that 5 Watt resistor gets hot.
Which is why I mentioned block connectors as you can screw them down.
You use a double on each end and that gives you one mounting hole on each end of the resistor. This gives the resistor a solid mount at each end.

The noise problem explained,,
And why DC heater still won't fully fix bad hum:
I quote from this page
(http://web.archive.org/web/20071024154401/http://users.telenet.be/svokke/valve%20junior%20mods.htm)
--------
"The layout problem is mainly in two places. At number one there is Crosstalk between the heater and the trace from the volume pot wiper to V1B. At number two there crosstalk between heater line and the trace between anode of first gain stage to anode load resistor (R3). I can't believe that they didn't discover this at epiphone. After all, they should be the pro's. There is also some crosstalk between heater and the trace between anode of second gain stage to anode load resistor. On this location it is less important though because the gain following this stage is low."
--------
Same old problem as my mates Fender Pro Junior.
Cross talk between tracks.
Obviously people in front of a cad program who have no idea what happens when you put high voltage tracks right next to noise sensitive hiZ tracks.
they hit the auto route and go to the pub,,meantime the consumer has to put up with
The HUUUUUUUUUUUUUUUUUUUUUM. :duh :loco :grr

This particular model of Pro Junior was a shocker and I fixed that by cutting tracks and running shielded cable direct to valve pins.
hum gone with no DC heater needed <3) <3) <3)

Phil.
Title: Re: Amp repair?!
Post by: Dimi Pana on February 13, 2019, 12:11:30 AM
Excellent points, now I have lots of reading to do. Hmmm, I did not think about the zip tie getting loose because of the resistor heat, it's not much but it does warm up. I wanted a quick solution so I can put the amp back together and play. I will revisit this hopefully in a few months, I intend to do a few mods to this amp, the double block connectors is a solid solution I will do that. My friends EVJ is indeed a newer production (judging by the serial number) but still the exact same version 3 (identical PCB, components, etc.). So I am not sure what's going on but for sure this amp was designed by mistake/accident who knows, with heaters voltage higher than what it should be. Anyway, I am so glad mine now works and (to my ears) it works well. Thanks again, cheers!