Taming the Peavey "Nasties"

soulsonic · Post by **soulsonic** » 26 Oct 2008, 07:24

This past weekend I found myself repairing two Peavey amps for friends. They're both from the same general era and have the same general design. One was a VTM 120 and the other was a Butcher. They're both basically copies of the JCM800; the Butcher is has almost the exact same preamp, and the VTM has a bunch of mods done to it... I'm sure many of you are aware of this already. Anyway, these amps all have a serious flaw in my opinion: the power amp sections have some bad mistakes in them; the main one being the size of the Screen Grid resistor. All these amps have 100 ohm 5 watt screen resistors. I believe this allows far too much Screen current to flow and causes any number of problems in the amp and power supply. I am certainly distressed by what happens in the power supply; there is this large 400 ohm 10 watt resistor that precedes the screen voltage which I believe is a regular source of failure in these power supplies. I've seen a Butcher with this resistor failed open, and I've seen serious burn marks on adjacent connectors and wires in the VTM. I am convinced that the reason why this resistor is getting burned is because too much screen current is flowing; there is nothing else other than the screen grids that could potentially be pulling enough current to do this other than the possibility of a component has failing in the preamp (which I haven't seen yet).

So, I decided to replace these poorly-chosen Screen Grid resistors in the Butcher with a different value. I chose 470 ohm 5 watt resistors because this is a value commonly used with 6L6 types and it seemed like a fair value to experiment with. Along with this, I also modified the bias supply to be adjustable. I'm sure there's already discussions out there about adding adjustable bias to these amps, but I must note that the bias voltage had to be changed from the original range to accommodate the change in Screen voltage, so if you decide to change the screen resistors, expect to require experimenting with the bias supply a bit to find a good range. After playing around with it, I got the tubes to idle at around 19-20 watts each, which they seem to be happy with. This is a little bit cold for a 6L6, but not too much, and it gives a little bit of safety room... it certainly doesn't sound as cold as the factory bias does! The crossover distortion with the factory bias on these amps is so bad you can really hear how brittle it is; so much nicer to warm it up.

It hasn't been road-tested yet, but I believe that changing these Screen Grid resistors to lower the Screen current is a real improvement that can do alot to help the reliability of these amps and improve the sound as well. This applies to the Butcher, VTM, and 5150 that I know of, and there are probably many others of this era that share this same flaw. If you have a Peavey amp that has 100 ohm Screen Grid resistors and that big 400 ohm 10 watt power supply resistor, I highly recommend this experiment.

chromaticdeth87 · Post by **chromaticdeth87** » 27 Oct 2008, 21:05

Yes and double yes, that is what killed my triple xxx for about two weeks. That resistor and fucking cheap ass rectifier diodes, they all fried like catfish when I was playing in a poorly wired garage for a show.

iwantmypie · Post by **iwantmypie** » 28 Oct 2008, 02:56

soulsonic wrote:This past weekend I found myself repairing two Peavey amps for friends. They're both from the same general era and have the same general design. One was a VTM 120 and the other was a Butcher. They're both basically copies of the JCM800; the Butcher is has almost the exact same preamp, and the VTM has a bunch of mods done to it... I'm sure many of you are aware of this already. Anyway, these amps all have a serious flaw in my opinion: the power amp sections have some bad mistakes in them; the main one being the size of the Screen Grid resistor. All these amps have 100 ohm 5 watt screen resistors. I believe this allows far too much Screen current to flow and causes any number of problems in the amp and power supply. I am certainly distressed by what happens in the power supply; there is this large 400 ohm 10 watt resistor that precedes the screen voltage which I believe is a regular source of failure in these power supplies. I've seen a Butcher with this resistor failed open, and I've seen serious burn marks on adjacent connectors and wires in the VTM. I am convinced that the reason why this resistor is getting burned is because too much screen current is flowing; there is nothing else other than the screen grids that could potentially be pulling enough current to do this other than the possibility of a component has failing in the preamp (which I haven't seen yet).

So, I decided to replace these poorly-chosen Screen Grid resistors in the Butcher with a different value. I chose 470 ohm 5 watt resistors because this is a value commonly used with 6L6 types and it seemed like a fair value to experiment with. Along with this, I also modified the bias supply to be adjustable. I'm sure there's already discussions out there about adding adjustable bias to these amps, but I must note that the bias voltage had to be changed from the original range to accommodate the change in Screen voltage, so if you decide to change the screen resistors, expect to require experimenting with the bias supply a bit to find a good range. After playing around with it, I got the tubes to idle at around 19-20 watts each, which they seem to be happy with. This is a little bit cold for a 6L6, but not too much, and it gives a little bit of safety room... it certainly doesn't sound as cold as the factory bias does! The crossover distortion with the factory bias on these amps is so bad you can really hear how brittle it is; so much nicer to warm it up.

It hasn't been road-tested yet, but I believe that changing these Screen Grid resistors to lower the Screen current is a real improvement that can do alot to help the reliability of these amps and improve the sound as well. This applies to the Butcher, VTM, and 5150 that I know of, and there are probably many others of this era that share this same flaw. If you have a Peavey amp that has 100 ohm Screen Grid resistors and that big 400 ohm 10 watt power supply resistor, I highly recommend this experiment.

Would you be able to explain a little on how you modified the bias supply to be adjustable? I have an old Peavy Mace which has 6 6L6GC's in the power section and my amp does have those 100 ohm 5 watt screen resistors which I just happened to fry one of the other day. From what I gathered, Peavey ran these tubes cold to help them run longer and supposedly make it easy so any nontechnical guitar player could just plug in new tubes and not have to worry about messing with the innards to get it to work. Anyway if you could help me out with how to install a bias pot or something like that that'd be great. Thanks man.

RnFR · Post by **RnFR** » 28 Oct 2008, 15:17

i've been a live sound engineer for the last 15 years, and i hear a lot of amps. i've always thought that the peaveys sound especially brittle, and it's always hard taming their harsh sound coming off a loud stage. it's good to know that now i have something to suggest to the guys that come in with these amps that will not only make them sound better, but run better as well. one amp at a time...

iwantmypie · Post by **iwantmypie** » 29 Oct 2008, 01:48

Yeah so I went on Peavey's tech forums to see if I could get any help and they seem to think there is nothing wrong with the way they biased one of their amps. One of the admins stated in numerous posts that if bias was such a big issue Peavey would stick a huge knob on the front of the amp right next to the volume. He also mentioned that people should stop worrying about biasing and worry more about how they set their EQ and pickup height on their guitars. I honestly can't believe those guys anymore no matter what they say.

soulsonic · Post by **soulsonic** » 29 Oct 2008, 03:58

Those Peavey guys don't want you playing with your bias because if you set it way wrong it can burn up the tubes and cook stuff in the amp. But that doesn't change the fact that they have their factory bias so cold that it's pretty much running the amp in Class B; which explains both why they're so loud, and why they sound like shit... just like a bad 60's-era Class B transistor amp. If they really cared about those things sounding good, they'd put some kind of a slick auto-biasing circuit in it so you wouldn't have to worry about adjusting it, but it would always be biased at least in a good ballpark; I've seen newer Traynors that have that.
I'm convinced that all Peavey cares about with those big amps is a super-loud power amp and super-saturated preamp distortion. That's all those 5150s are; they copied a Soldano and then strangled all the soul out of it with that awful power amp design.

I'm checking that MACE schematic; I'll get back to you with some drawings to show where to add the stuff for adjustable bias.

bajaman · Post by **bajaman** » 29 Oct 2008, 04:30

they copied a Soldano and then strangled all the soul out of it with that awful power amp design

AND added two (that's right - TWO) more stages of gain, so you could always here when the surf was up!!

ME - I modify the power stage to the same as a Fender or Soldano specs - they sound a whole lot better then.
bajaman

Nyquist5 · Post by **Nyquist5** » 29 Oct 2008, 16:43

soulsonic wrote:I'm convinced that all Peavey cares about with those big amps is a super-loud power amp and super-saturated preamp distortion.

Having been part of Peavey for some time, I can say that this is correct. However, the intention on their part is good (depending on how you look at it). The idea is to give the customer the "best bang for the buck" as Hartley would say: hence extract maximum power out of the amp. They could have chosen to maximize tone but that was not their choice. The company is first and foremost a PA company: they don't try to make guitar amps have better "tone" as much as they try to give them the highest technical specs. Although, this has and is changing these days.

They have been spending more time on producing better tone since the 90's. Remember those eighties Peavey guitar combo amps with Public Address power amp modules, shallow cabs (for reduced bass

), and Public Address speakers (Scorpions)? These ones were bad.

soulsonic · Post by **soulsonic** » 30 Oct 2008, 01:35

I certainly agree about the "bang for the buck". All my friends who own Peaveys bought them because they were cheap and super powerful. And I'm always impressed by the huge transformers and indestructible cabinets, but I wish they were more reliable on the inside. They break down all the time and it's always because of something that could be avoided by building it differently, and not necessarily doing something that's more expensive, just doing something in a more reliable way. Like the VTM I repaired; it had a cracked solder joint on a coupling capacitor - it happened right on a spot where the board flexes when you put a tube in the preamp socket. There are two solutions for this: either mount the socket to the chassis (more expensive because of increased labor), or relocate the board mounts to a place closer to the socket to minimize the flexing (no increase in cost, just relocating a part that's already in the design).

It's alot of things like this that bother me about how Peaveys are made. Like these 100 ohm screen grid resistors; there's no reason for it - all they do is cause trouble and using a different value wouldn't cost any more. The Butcher I modified to have different resistors is just as obnoxiously loud as it was before, so I don't think the power specs would "suffer" by taming down the screen current. And, why do they put hot glue all over everything? It doesn't help; all it does is gunk up the insides with crap you have to remove when you need to replace anything. That cap with the broken solder joint in the VTM was covered in hot glue; the glue didn't do anything to keep it from breaking. If they're concerned about the larger electrolytic caps breaking off when the amp gets thrown around, they could just include a couple extra holes in the board and secure them in place with a zip tie - that shouldn't take any more labor than having someone squirt glue all over it, and zip ties are probably cheaper than glue anyway.

I should write a book: "How To Modify Your Peavey Guitar Amp So It Will Sound Good and Not Die". I'm open to any suggestions for what to include.

bajaman · Post by **bajaman** » 30 Oct 2008, 03:36

tales from the workbench - yesterday, I had a Peavey 5150 MKI combo in for repair (do not sit on top these if you are overweight

).
This particular amplifier had one of those lovely intermittant faults that we al love (NOT)

After a couple of hours shredding, it would suddenly get quieter and quiter until the sound completely fade away to silence.
After switching it off, an hour later and back to the shredding - no problems!!

Okay - out comes the chassis and guess what I found??? (don't forget to include this in your forthcoming book Martin

)
There is a 5 way molex crimped molex connected flat ribbon cable between the power supply and output tubes printed circuit boards - The output tube heater cables had been arcing nicely at the connection to the power tubes board and the solder was crystallising nicely on the molex pins. I have seen exactly the same on those awful Triumph 60 and 120 combos, that Peavey produced in the late '80s. I am sure many Peavey service techs have seen and know what I am talking about here - the same stupid construction techniques being used years later!! (and I would not mind betting they are still built like this).
The solution is obvious - remove the heater section of the ribbon cable and hard solder two wires from each board - no more problems, next please!!
PS: have you ever seen Fender, Bogner, Soldano, Mesa, DrZ, Matchless, BadCat etc. use a similar output tube heater connection??? Of course not - they are constructed properly - come on Peavey - pull finger!!!!

chromaticdeth87 · Post by **chromaticdeth87** » 30 Oct 2008, 21:54

But the most bang for the buck does really resonate with me. I mean I have had to buy completely new tubes and 8 to 10 dollar caps to fix my triple xxx. Not why and how would that be considered bang for the buck? If I am out of a gig and have to fix something in my rig because of some engineering flaw. And as Bajaman said two extra gain stages, when if you have heard a real SLO the things sound amazing to begin with, that just adds to the cost and the only reason they are cheaper is because they use less reliable components. So I get a voltage gain that results in a frequency that for one, It's hardly perceivable to the human ear, and two introduces excessive amounts of crossover distortion into my signal. Cool. Oh and let's not forget the II's they have six triodes and supposedly a "better" clean channel, bullshit the things still sound about as good as the triple xxx's cleans which suck, and I have to pay for more pots to control a suck ass clean. Now why the hell would a company like Bugera want to copy and market these designs?

Nyquist5 · Post by **Nyquist5** » 03 Nov 2008, 10:24

What can I say? Your remarks are perfectly acceptable: this is just not the way they seem to see it at Peavey. Especially the molex connectors

.
Nevertheless, I don't think you can compare Bogner, Soldano, Dr Z, etc.. to Peavey, sound-wise, quality-wise AND price-wise. And I have seen comparable Fender or Marshall amps built (and failing) the same way as the Peaveys... I wouldn't tour with one (I gig with my own builds!) but I know many bands who do and who have been pretty lucky with reliability. Also, as far as PA equipment or bass amps are concerned, I think Peavey is great (or were anyway).

Don't forget many musicians are more interested in price, number of controls or hype than the sound they get out of their amplifiers.

bajaman · Post by **bajaman** » 03 Nov 2008, 19:07

this is just not the way they seem to see it at Peavey. Especially the molex connectors

Molex connector for power tube heater supply = ticking time bomb. I am sure it would be CHEAPER and far more reliable just to solder four connectons, than to insert four push pins and crimp (not solder) the molex connector block and ribbon cables, but as you say , Peavey cannot be told - they have their way of doing these things and this is what gives some of their products a bad name.
Yes, I use my own hand made and soldered tube amps - you will not find ANY crimped telephone or molex connectors like Marshall DSL and TSL and all the peavey tube amplifiers have, and that is why they are 100% reliable in use.

bajaman

iwantmypie · Post by **iwantmypie** » 04 Nov 2008, 00:52

So I'm just about read to "tame the nasties" and I have a few last questions.

1st: I found some information online that the Mace VT series' resistor to change in the voltage divider for bias is 68K. So something along the lines of a 47k resistor and 25k pot should do the trick, right?

2nd: If I change these screen resistors to 470 ohm, would I probably have to change the values again for the adjustable bias control?

Lastly: Should I change the value of the big 400 ohm 10 watt power supply resistor or leave that be as long as I perform the screen resistors properly?

Thanks again

bajaman · Post by **bajaman** » 04 Nov 2008, 08:08

Should I change the value of the big 400 ohm 10 watt power supply resistor

Yes - put a fender twin reverb or similar choke in there instead of that resistor.

your ears will thank you.

bajaman

iwantmypie · Post by **iwantmypie** » 04 Nov 2008, 15:19

Sorry for my ignorance but I'm not too sure what a choke does. From the sounds of it and as you said it quiets the amp but is there anything else I should know about it? As long as its a simple swap out the resistor stick this puppy in sort of deal I think I should be alright with it. Thanks.

Fuzzer · Post by **Fuzzer** » 04 Nov 2008, 16:26

http://www.aikenamps.com/Chokes.html

chromaticdeth87 · Post by **chromaticdeth87** » 04 Nov 2008, 22:34

what aiken said. Chokes are just big inductors that help filter out unwanted ripple after the rectifier, it's another stage of filtering that offers some pretty positive effects on the resonance frequency of the bass and lower mids.

soulsonic · Post by **soulsonic** » 08 Nov 2008, 08:54

Mercury Magnetics sells a choke intended for doing this mod to a 5150. Since all these amps have pretty much the same power supply, it should work in most of them. At least I'm assuming it's a good value to use.

iwantmypie · Post by **iwantmypie** » 03 Dec 2008, 18:28

sorry to dig this one out of the grave so to speak but i finally received the screen grid resistors in the mail and low and behold i ordered 470 ohm 1 watt resistors instead of the 5 watt ones. would it be alright to use these or should i probably return them and try to get those nice big 5 watt cemented wirewound ones like the ones already in there?