1984 peavey VTX Heritage2 Tube Change to EL34

[Cygnus X1]

Owner wants me to change it back to 6L6'
He tells me that he took it to a music store to get a new set of 6L6's put in, and they put in EL34's instead.
Now his tone isn't there.

Two questions.
Interweebbie claims the amp does not have to be rebiased when using matched tubes.
True or not?

And,
Any chance they threw in the EL34's and did not rebias?

Actually 3rd question.
Schematic Heaven used to have the schematic, now it can't be found except to pay.
Does anyone have one?

Thanks in advance!

[nashvillebill]

peavey-heritage-vtx.pdf Ask and ye shall receive...

The Heritage VTX and Classic VTX amps were unique. Their bias comes from a transistor setup, not a traditional adjustable or fixed bias. I wil quote from Mr. Fahey : "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"

. The amp therefore does not have to be biased when changing tubes and I believe I have a Peavey ad which claims these amps could use either a EL34 or 6L6 merely by changing tubes without any other changes made.

[Cygnus X1]

Thank you Bill.
We'll change them out and double check.

[Chuck H]

JM2C... From a repair standpoint I would say plug them in and take some measuremnents and be done. If "I" owned one of these amps I would mod it for a more typical bias circuit for the added control and tone manipulation.

[Cygnus X1]

Thanks CH.
Owner wants to try some winged C 6L6GC's.

Anyone know what tube brand was original to this amp?
Svetlana?

[Gtr_tech]

Sylvania STRs were in that amp when it was on the showroom floor.

[Enzo]

Chuck, what are you proposing? Grid bias? meaning you'd creat5e a new PI and bias supplies?

[Cygnus X1]

Good for theoretical discussion but the owner would FLIP if I even asked to change it.
Turns out our original premise was wrong.
The music store installed cheap Sovtek 6L6WGB whatever.
I showed him that I have boxes of them, all I use them for is troubleshooting and hard break-ins on new equipment.

Hopefully the Winged C's will bring back the "tone in his head".
If not, I'll open it up and see if there is anything funny with the IC/diode/robotically controlled biasing.
I suggested NOS GE's, but he said the tubes were twice what he could get for a replacement amp.
True.

Thing that causes me some pause is he has the originals "somewhere", and never played the amp
much more than a few times a year.
If he can find them and they are low hours, he might be wasting his money.
We shall see!

[nashvillebill]

I bought my first Classic VTX (the Heritage's little brother) back in '82 or '83, kept it many years then sold it because I wasn't playing it. I really missed it though, and eventually I came across another Classic VTX very very cheap; the owner said it had a "hum" so I got it for $20. Turned out the power supply caps were shot, B+ was half what it was supposed to be. (Thanks Enzo for quickly diagnosing THAT issue). Anyhow, I went on to recap it completely, and I found another electrolytic--a coupling cap-- that was completely dried out too. (I was measuring them, both capacitance and ESR, as I swapped them). I found a bad op-amp too.

Upon finishing the job, the amp sounded EXACTLY like I remembered. Very smooth clean channel, the preamp is solid-state so the distortion is the classic '80's "hair metal" if you crank the saturation up, but keep the saturation down andthe distortion sounds surprisingly smooth too. What I'm leading up to: the Heritage VTX, being from the same time era, may also be suffering from dried-out caps in the preamp or power supply. It's also possible the old Scorpions (or did the Heritage have Black Widows?) may be getting long in the tooth. I would keep the amp as-is but refurbish it, before embarking on changes to the bias circuit or other drastic mods. Like its competitor, the Musicman amps with the same bias setup, these really are outstandingly good-sounding amps IMHO when they are functioning like new.

Oh, and I've tried the Sovtek 5881's as well as JJ 6L6's...but I came across some Amperex-labelled NOS GE's as part of a big lot of stuff I bought cheap, and that's what's in mine now. I'm not a "golden ears" audiophile though so I can't swear they sound that much better than the other tubes. But I feel nice having them in there.

[Chuck H]

Chuck, what are you proposing? Grid bias? meaning you'd creat5e a new PI and bias supplies?

Holy crap. No. I just looked at the schem. Not what I was expecting.

[Enzo]

Just an opinion, but in my mind, as on most Peavey power amps, these are designed to be clean and loud, not pushed to the edge of oblivion. And as such, I tend to think the choice of 6L6 is not a major component of the tone. Others may disagree.

[Cygnus X1]

Seems to me that power tubes can sound bad all across their range.
Not just "pushed".

Bill, I will look at the caps and do the measurements.

Anyone have a good schematic?
Or do I have to actually buy one?

[Jazz P Bass]

Anyone have a good schematic?
Or do I have to actually buy one?

Uhm, Nashville Bill posted it in the very first reply.

[Enzo]

Not only was it posted as the first line of post#2, Schematic Heaven STILL has it up for free:

http://www.webphix.com/schematic%20h...ritage_vtx.pdf


And beyond even that, you can always get any Peavey schematic for the asking from customer service at Peavey.

[Cygnus X1]

:Facepalm:

I didn't see it!

Thanks Bill, and all.
Carry on...I will keep you updated.
I ought to pop in some GE's while she's here and see what's up.