Solid State Guitar Amp Forum | DIY Guitar Amplifiers

Solid State Amplifiers => Tubes and Hybrids => Topic started by: ohmy!! on July 08, 2010, 04:28:32 PM

Title: Peavey Heritage VTX ..uh oh..
Post by: ohmy!! on July 08, 2010, 04:28:32 PM
Hey guys, I'm having an issue with my VTX.  It was working fine until the other day.  It started sounding really bad and I had looked at the tubes and one of them was out completely.  So, naturally, I got a new set of the same tubes that were in there and have been in there for a few years (Sovtek 6L6WXT+).  I had plugged them in and started it up, after about 2-3 minutes of it being on it makes a loud pop and then nothing but loud humming.  I check the tubes, 2 of the tubes are glowing insanely bright red.  I flip it off, unplug it and check the fuses later.  One of the fuses had gone, I went ahead and replaced all of them, filter caps look fine.  So I figured maybe those two tubes were bad, I went through and swapped tubes between each plug and the same two plugs cause the tubes to get real hot and lots of loud humming going on.  So I unplug those 2 tubes, turn it on and it's working, no humming.  Any tubes I plug into those 2 spots get red hot and cause it to hum very loud.  I'm not sure if there's a bad resistor anywhere or what's going on. Any help would be appreciated.  I snapped off a picture for you guys to see it.

(http://img16.imageshack.us/img16/6396/img0638r.th.jpg) (http://img16.imageshack.us/i/img0638r.jpg/)

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Title: Re: Peavey Heritage VTX ..uh oh..
Post by: skynyrd on July 08, 2010, 05:13:41 PM
Sounds like screen grid resistors are shot, the big cement looking resistors on top of the tubes. If you have a multimeter I would check the ohmage on the 2 spots compared to the other 2.
Title: Re: Peavey Heritage VTX ..uh oh..
Post by: ohmy!! on July 08, 2010, 06:01:25 PM
Here's a picture of the circuit board where the two plugs are and the resistors.

(http://img29.imageshack.us/img29/3717/img0642r.th.jpg) (http://img29.imageshack.us/i/img0642r.jpg/)

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Title: Re: Peavey Heritage VTX ..uh oh..
Post by: ohmy!! on July 08, 2010, 06:24:28 PM
So I need to replace both of those power resistors there, the 100ohm 5W 10% above the sockets?
Title: Re: Peavey Heritage VTX ..uh oh..
Post by: bry melvin on July 08, 2010, 06:54:17 PM
I've had this happen when a driver transistor goes Thats the large transistor on the heat sink. It's happened about every seven years actually.

If the resistors check out...look at the transistors.  I've always changed the pair and the transistor just before it in the schematic.
Careful of the latter Subs with the same pin out are rare.

(q6 q7 q8 q9)
Title: Re: Peavey Heritage VTX ..uh oh..
Post by: ohmy!! on July 08, 2010, 07:10:34 PM
(http://img138.imageshack.us/img138/1725/img0646dc.th.jpg) (http://img138.imageshack.us/i/img0646dc.jpg/)

Uploaded with ImageShack.us (http://imageshack.us)

this pair here?
Title: Re: Peavey Heritage VTX ..uh oh..
Post by: bry melvin on July 08, 2010, 07:16:22 PM
yes if the resistors check the same (good) one of those transistors q7 q9  I always change q6 and q8 along with them...

You'll need the cross reference I uploaded...you'll never find the original peavey #s.

You will need to remove the transistors from the circuit to test them properly.

Make sure you drain the caps before sticking your hands in there.
Title: Re: Peavey Heritage VTX ..uh oh..
Post by: ohmy!! on July 08, 2010, 07:28:29 PM
Now I just need to find my multimeter...off to the garage to look through some boxes (gotta love moving, right?).  I'll have to check it out tomorrow and report back what I figure out. Thanks guys!
Title: Re: Peavey Heritage VTX ..uh oh..
Post by: bry melvin on July 08, 2010, 07:34:26 PM
https://docs.google.com/fileview?id=0B5Lr8KpxtYCuYWQwZDY5M2YtOThjYy00NzE2LWEyMzYtYWI2ZTdkMDAyZDc1&hl=en&authkey=CN3fuK0M

heritage vtx schematics   
Title: Re: Peavey Heritage VTX ..uh oh..
Post by: Jack1962 on July 09, 2010, 08:54:08 AM
first thing you have to do is replace the screen resistors , then buy new tubes those did whayt is known as red plating , this means they are now damaged . and then you ave to rebias your amp , anytime you replace power tubes in ANY amp you have to rebias.

                                                    Rock On
Title: Re: Peavey Heritage VTX ..uh oh..
Post by: bry melvin on July 09, 2010, 11:25:10 AM
VTX is cathode bias  there is no adjustment to change.

If the tubes were red for more than a very brief period of time...particularly if they are recent manufacture tubes they ARE damaged, ( Current tubes have more metal impurities AND are not as deep a vacuum as old US and British made) how badly depends on whose tubes and how long the red.  Red plating causes metal impurities to build as gas in the tube. I would check the tubes before throwing them away but you would need a tube tester for that.

Use a matched pair or better a quad for best results.

This amp has very little tone difference with different ones Ruby Tube set from musicians friend is proabably the cheapest that's not junk.


BTW while you are doing all this ...testing....make sure you have the power on LOW setting it reduces the voltage to the tubes.

Amp usually sounds better there anyway ....I've only used that amp on High in auditoriums.
Title: Re: Peavey Heritage VTX ..uh oh..
Post by: Jack1962 on July 10, 2010, 10:50:01 AM
Sorry bro (brv melvin) ANYTIME powertubes a changed any GOOD technician the bias is adjusted because the current flow is never the same in any 2 tubes , weather or it be fixed or cathode bias has nothing to do with it.


                                                     Rock On
Title: Re: Peavey Heritage VTX ..uh oh..
Post by: J M Fahey on July 10, 2010, 11:49:31 AM
In this particular circuit (which is not like any other tube amp, except the Music Man) , the bias is *always* being adjusted automatically, no matter what the tube, new or old, because it has a SS servo bias circuit.
The tubes will always pass whatever current the cathode-driver transistors feed them, until they die suddenly at the end of their life.
Their grid, in fact, always has +15V (+24v in MM's) and their cathodes will float to whatever voltage is necessary to pass the current fed by the drivers.
Those redplating ones either had some internal short or whatever *or* were fed too much current by a malfunctioning drive circuit.
Title: Re: Peavey Heritage VTX ..uh oh..
Post by: bry melvin on July 10, 2010, 02:18:56 PM
Quote
ANYTIME powertubes a changed any GOOD technician the bias is adjusted

well it's not truly an normal cathode bias ...but the point was there is no way to "adjust"

Most cathode bias involve substituting different values of resistors... not "adjusting" a pot

Personally if it's "within limits" as such I never change resistors. If I have to suddenly throw in new tubes in a performance I prefer the resistors be the factory values. I've actually seldom had tubes go performing....transportation is usually the cause of unplanned tube changes for me.

As JM says this one is even different. There simply is no way to adjust it.

The symptoms described are somewhat common and usually caused by a shorted driver transistor q7 or q9 depending on which tube pair is acting up.



I've had to do this twice on one of my VTX once on the other.  that's around  5000 hours of use.

Title: Re: Peavey Heritage VTX ..uh oh..
Post by: ohmy!! on July 10, 2010, 09:59:12 PM
In this particular circuit (which is not like any other tube amp, except the Music Man) , the bias is *always* being adjusted automatically, no matter what the tube, new or old, because it has a SS servo bias circuit.
The tubes will always pass whatever current the cathode-driver transistors feed them, until they die suddenly at the end of their life.
Their grid, in fact, always has +15V (+24v in MM's) and their cathodes will float to whatever voltage is necessary to pass the current fed by the drivers.
Those redplating ones either had some internal short or whatever *or* were fed too much current by a malfunctioning drive circuit.

You're right, with this amp you pretty much just stick whatever 6L6 types of tube in there.  I've never had an issue with any tubes in it.  The only thing besides tubes that's been changed over the years is the filter caps which I changed last year.  The previous owner had it since the early-mid '80s.  He even gave me the original tubes that came with it which were still working.  I haven't been able to test anything because I couldn't find my multimeter and I've got too many other things to pay for to really spend any more money until I get paid at the end of the week.  I'll have an update on what went wrong or if I run into any problems.
Title: Re: Peavey Heritage VTX ..uh oh..
Post by: Jack1962 on July 11, 2010, 04:43:49 PM
are you sure about that +15 volts on a tube grid JM
Title: Re: Peavey Heritage VTX ..uh oh..
Post by: ohmy!! on July 11, 2010, 05:07:42 PM
The resistors check out to the proper ohms as marked.  It's one of the transistors.  Q7 is bad.  I'll just go ahead and replace them all like you recommended, they're 25+ years old after all.
Title: Re: Peavey Heritage VTX ..uh oh..
Post by: ohmy!! on July 11, 2010, 05:34:02 PM
Also, in case anyone else needs these, there's a site  www.tubesandmore.com (http://www.tubesandmore.com) that has them with the original parts numbers.

just search SJ6465 and MPS 6531
Title: Re: Peavey Heritage VTX ..uh oh..
Post by: Enzo on July 12, 2010, 02:24:31 AM
+15v on the control grid is perfectly reasonable when the cathode sits at +90v.

Here, explore the power amp circuit.
http://www.schematicheaven.com/newamps/peavey_heritage_vtx.pdf
Title: Re: Peavey Heritage VTX ..uh oh..
Post by: teemuk on July 12, 2010, 09:52:50 AM
The Peavey VTX -series power amp's biasing circuit -related patent is U.S. #4,439,742.

http://www.google.com/patents/about?id=Tsc6AAAAEBAJ&dq=4,439,742
Title: Re: Peavey Heritage VTX ..uh oh..
Post by: Jack1962 on July 13, 2010, 11:21:42 AM
no doubting your word guy's , however , evertube amp I have ever worked on cathode or fixed bias , had a negitive voltage on the pin 5 the grid.

                                                   Rock On
Title: Re: Peavey Heritage VTX ..uh oh..
Post by: Jack1962 on July 13, 2010, 11:25:36 AM
just looked at the diagram , that's very interesting.

                                                 Rock On
Title: Re: Peavey Heritage VTX ..uh oh..
Post by: Enzo on July 13, 2010, 09:06:08 PM
Look at the Music Man amps, they are of similar circuitry.


When the cathode is grounded like most fixed bias amps, then the grid is indeed generally at negative voltage.   But cathode biased amps generally will not have a negatiove vokltage on the grid, because the cathode is made positive instead.

The whole point of bias is the relationship between the cathode and the grid.  What the voltages actually are is less important.   Having a tube grid at -25v and the cathode grounded is exactly the same as having the cathode at +25 and the grid grounded, as far as the tube is concerned.

SO having a cathode at +90 and a grid at +15, is the same to the tube as having the cathode grounded and the grid at -75v.   Either way the grid is 785v more negative than the cathode.   And in the case of the Heritage here, if you think about it, that is pretty cool bias.  These amps are working close to class B.


Obviously since the voltage across the tube plate to cathode is important, B+ would be different in each example.
Title: Re: Peavey Heritage VTX ..uh oh..
Post by: phatt on July 14, 2010, 01:48:20 AM
Hi Enzo Thanks for the explaination of the vastly complex.

But ya know One can't help but think Geezz that's about the most complex way around a simple problem? :o
Phil.
Title: Re: Peavey Heritage VTX ..uh oh..
Post by: Enzo on July 14, 2010, 06:48:10 AM
Oh poop.   It might have been even clearer if I had typed that the grid is 75v more negative than the cathode instead of 785v.  Oh well.

Well, when you are designing something it is only complex once.  Once it is done it is done.   Peavey had a number of amps using that basic power amp design, just as Music Man did.   I couldn't tell you who thought of it first.

I am no expert on this, but for the user, the design complexity is not an issue.  But out of it he gets a pretty efficient amp that is strong and won't really have any bias issues.   These darn things ar LOUD.

Hartley Peavey has written a number of white papers on various aspects of his product designs.  This is just the sort of thing he might write about, though I haven't looked.
Title: Re: Peavey Heritage VTX ..uh oh..
Post by: Jack1962 on July 14, 2010, 07:36:17 AM
All I can say about the design of the bias circuit is it is rock solid and allows the end user to change tubes without rebiasing the amp , which most don't know how to or aren't capable of doing , which is a great marketing tool lol . 2 thumps up on this design . AS for the operation of the vacuum tube , cathode has to be more negative charged than plate , grid introduce's a small signal to produce a large signal at the plate, if you need more info read Jack Darr's book lmao

                                              Rock On
Title: Re: Peavey Heritage VTX ..uh oh..
Post by: ohmy!! on July 14, 2010, 02:31:45 PM
The new transistors fixed it, other than the tubes which are all bad and sound god-awful.  Looks like I'll be ordering some tubes soon.
Title: Re: Peavey Heritage VTX ..uh oh..
Post by: bry melvin on July 14, 2010, 03:28:10 PM
The last tube sets I put in mine were chinese st shape "Chinese 6L6GC Small ST Shape"  from Danny's amp service in NJ  with his label

I THINK the ones antique radio supply (tubesandmore.com) are the same thing...39USD for the quad.

FWIW those sets were bought in 94  still working.


Title: Re: Peavey Heritage VTX ..uh oh..
Post by: ohmy!! on July 14, 2010, 04:17:16 PM
I was running some Sovtek 6L6 WXT+ tubes in there and they held up great until those transistors went out.  I may get another set of them.
Title: Re: Peavey Heritage VTX ..uh oh..
Post by: Jack1962 on July 16, 2010, 03:42:50 PM
Ohmy you can order a set of Ruby 6L6's from musicians friend probably cheaper than anywhere else ($34 a pair I think) I have them in one of my amps and have had for a few years they seem to be pretty good tubes for rebranded tubes lol lol lol
Title: Re: Peavey Heritage VTX ..uh oh..
Post by: ohmy!! on July 20, 2010, 02:49:19 PM
Well I got the tubes in.  What I thought was noise from bad tubes apparently isn't  :-\.  The transistors fixed the tube red lighting problem, but I'm wondering if maybe the filter caps were killed off too when everything happened  ???.  I had swapped them out a year and a half ago so it's kind of a bummer.

(http://img33.imageshack.us/img33/1677/img0681nt.th.jpg) (http://img33.imageshack.us/i/img0681nt.jpg/)

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Title: Re: Peavey Heritage VTX ..uh oh..
Post by: ohmy!! on July 20, 2010, 07:58:22 PM
I got lucky, only one of the smaller caps actually needed replaced and I was able to get it at radioshack for $1.60  :tu:.
Title: Re: Peavey Heritage VTX ..uh oh..
Post by: ohmy!! on August 30, 2010, 09:41:53 PM
Hate to bring this thread back, but.. Q6 and Q8 transistors keep dying.  I'll replace them and after a good 3-4 hours of playing the amp will just start to sound like some kind of awful fuzz monster with volume jumping up and down.  I've replaced them like 3 times now.  Any suggestions as to why this is happening?

Title: Re: Peavey Heritage VTX ..uh oh..
Post by: bry melvin on August 30, 2010, 10:12:35 PM
check your data sheets: make sure you are putting them in correctly

The last ones I bought had a different pin out from the original peavey transistors
Title: Re: Peavey Heritage VTX ..uh oh..
Post by: ohmy!! on August 30, 2010, 10:51:12 PM
Do you know the pin out of the originals? I can't seem to find the info.
Title: Re: Peavey Heritage VTX ..uh oh..
Post by: ohmy!! on August 30, 2010, 11:24:57 PM
Here's what I have.  Still can't find the original's pin out.

http://www.fairchildsemi.com/ds/MP%2FMPS6531.pdf
Title: Re: Peavey Heritage VTX ..uh oh..
Post by: bry melvin on August 31, 2010, 12:18:12 AM
http://www.datasheetcatalog.com/datasheets_pdf/M/P/S/6/MPS6530.shtml

looks the same...flat should be as in the layout linked earlier on this thread

looking at notes: I used 2n3642 per peavey cross reference list.
Title: Re: Peavey Heritage VTX ..uh oh..
Post by: ohmy!! on August 31, 2010, 01:27:29 AM
Maybe that's not the problem then?  Seemed like when I'd replace it, the amp would go back to sounding right.

I just switched the amp back on a bit ago and it all of a sudden sounded normal, then after about 15 minutes went back to sounding all fuzzy and the volume dropping and raising.  ???
Title: Re: Peavey Heritage VTX ..uh oh..
Post by: Enzo on August 31, 2010, 01:50:54 AM
Stick with the stock MPS6530/6531.  It is what the amp is designed around, all MPS6531 will have the same EBC pinout. Mounted flat side away from the drivers. I can't speak for the 2N3642, I don't know what package they are in.

Those are limiters, the amp should work fine if you simply remove them.  Of course that would also remove some of the amp's own protection.

Like any limiter, they turn on when their base is energized.  Look at the circuit.  When the voltage across resistors R155, R159 gets large enough, the transistors turn on and ground off the signal at the base of the drive transistors.   And what causes that voltage in R155, R159?  Current through them, the same current flowing through the tubes.

SO the question arises, have you put different tubes in it?

And for that matter we could have a failing drive transistor, or two.

And look at those resistors, they are the two 5 watt resistors next to the drive transistors.  DO both measure 3 ohms?  And is the solder to each in good shape?

Measure the voltage across each resistor, does it meet the spec on the schematic?  20-40mv each?  And when it fuzzes out, do those voltages change?
Title: Re: Peavey Heritage VTX ..uh oh..
Post by: ohmy!! on August 31, 2010, 02:50:22 AM
Well I had Sovtek 6L6 WXT+ tubes in there before and now I have the Ruby Tubes 6L6 since replacing the transistors Q6,Q7,Q8,Q9.  The tubes appear to be fine visually, but that doesn't really mean anything I guess. I'll have to take it apart and check those voltages later.  For the last hour or so I've been playing it and the amp has been fine. 

Thanks
Title: Re: Peavey Heritage VTX ..uh oh..
Post by: ohmy!! on August 31, 2010, 07:56:31 PM
When I measure the ohms of those resistors, the meter's needle just drops down below zero.
Title: Re: Peavey Heritage VTX ..uh oh..
Post by: Enzo on August 31, 2010, 08:33:54 PM
You are not measuring them with the amp on are you?   If your ohm meter goes backwards, that means there is voltage in the circuit.  Flip it to volts and see.   At worst, unsolder one end of each resistor and lift it up to measure it.
Title: Re: Peavey Heritage VTX ..uh oh..
Post by: ohmy!! on August 31, 2010, 08:56:09 PM
I have the amp unplugged.  It's measuring other resistors in there, but it's off by a bit (the 100ohm is measuring at 80 and the 150ohm measuring at 130-140). The meter could be off by a bit.  If I flip it to volts the needle doesn't move at all.  Then again I couldn't measure the voltage on these if they're supposed to be 20-40mv, my meter's selection for volts starts at 10.
Title: Re: Peavey Heritage VTX ..uh oh..
Post by: ohmy!! on August 31, 2010, 09:09:31 PM
Pulled the resistor out completely, same issue. Reads below the zero, just as if I put the two probes together.  I need to find my digital meter.
Title: Re: Peavey Heritage VTX ..uh oh..
Post by: ohmy!! on August 31, 2010, 11:26:29 PM
Can't find my digital meter. I did notice though, that one of the tubes has bit of a blue glow to it and when it starts getting fuzzy and lowering volume that the blue glow dies down.  I tapped on a couple of the tubes a little bit and the sound returned to normal, the blue glow on the one tube returning as well.  Maybe bad tube and/or bad driver transistor?   :-\
Title: Re: Peavey Heritage VTX ..uh oh..
Post by: bry melvin on September 01, 2010, 01:12:21 AM
check the tube pin contacts: they often spread and don't make good contact. Some blue is often normal with mordern tubes (often test gassy too...not as deep a vaccumm) however loose tubes can cause lots of problems. As you saw a change when tapping the tubes  would take a good look at contacts AND clean any carbon trails off of the sockets especially since you started with a redplating problem on this amp.
Title: Re: Peavey Heritage VTX ..uh oh..
Post by: ohmy!! on September 01, 2010, 02:15:43 AM
The tubes were a little loose.  I also took the board out to check the soldering.  One of the driver transistors needed to be soldered better, it also had a huge dead bug stuck to the pins  :o.   Transistor  Q6 was also a bit loose.  So far it's playing normal now, actually sounds a little different than before.  Hopefully it will stay that way.  I'll clean off the contacts on the tube sockets for good measure.
Title: Re: Peavey Heritage VTX ..uh oh..
Post by: J M Fahey on September 01, 2010, 06:59:36 AM
The poor little 6 leg bug wanted to make love  <3) to the huge 8 leg tube and got zapped.
Love is blind :( .
Title: Re: Peavey Heritage VTX ..uh oh..
Post by: ohmy!! on September 01, 2010, 09:50:28 PM
 ;D It would seem so.  Anyhow I think it's all good now, I played it for hours on end today and cranked it up really loud to test it (about half-way on both channels combined with a distortion pedal).