Amp repair?!

[phatt]

@ 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
 

[galaxiex]

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...

[phatt]

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.
Phil.

[Dimi Pana]

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? 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!
 

[galaxiex]

Well, you could make one of these bucking transformer things...

scroll to Page 2 of the pdf.

[Dimi Pana]

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!

[galaxiex]

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

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

http://sluckeyamps.com/

[phatt]

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.

[solderer25]

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!

[Dimi Pana]

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!

[phatt]

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.
Don't over think it,, If it was on my bench I'd just drop the heater voltage with a resistor problem solved.
Phil.

[Dimi Pana]

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?

[phatt]

You don't have to check all that or pull the PCB;
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 cause I've forgotten how anyway
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.
Rebundle the 2 sets of wires and re-tie then you know it's safe.

Phil.

[Dimi Pana]

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!   

[phatt]

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.