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cyclecamper 17th September 2012 10:36 PM

guitar amp transformer upgrades
 
This isn't any Marshall or SVT or boutique amp, it's a cheapo. Seeking an affordable really good-sounding practice (& occasional recording) bluesy amp, I picked up a beat-up Peavey Classic 50 head with 4 EL84 outputs and two 4X10 bottoms with stock Peavey blue-marvel 10" speakers. I pretty much know what these amps are like, they get a decent clean sound, and break up pretty early, not really very powerful... but playing across the line between clean and dirty makes these amps very "emotive" IMHO for a bluesy style. Their distortion is OK but a bit treble and brittle rather than "creamy" (partly due to the 10" guitar speakers but I like the sound of 10's, even knowing how one 15 would make a nice delta-blues style sound even for guitar). I'm sealing the cabinets so they will even work for bass guitar practice as long as I don't try to get loud...after all that really is 8 10's much like an SVT for lead-style bass though of course it's not going to go very low or loud or clean and sealing the cabinets pushes resonance even higher. I don't have any delusions that this is much of an amp, but could be fun; it's probably going to end up in my bedroom. It's so beat up that I'm rebuilding it before I even turn it on. I'm replacing all the pots, changing the input network between the input jack and the grid of the first tube to my personal favorite design, adding a traditional Fender-style "bright" switch across the first volume pot, adding small modern fender-style "DEEP" switches to double the coupling capacitors between stages for a little more bass when I practice bass, upgrading the small-size Accutronics reverb tank to their larger long-decay 3-spring unit with the same input & output impedances, doubling up all the power supply electroytics, putting in a half-power switch that disconnects the cathodes on two output tubes, and building a larger cabinet for the head, and covering it with new fake-tweed patterned Tolex to match the bottoms. So far, it's been REALLY cheap for what I'm ending up with, as it's been recently re-tubed with Mesa EL84 tubes (I didn't check the 12AX7 tubes). I might add a potentiometer to the feedback circuit. And a back-panel potentiometer in series with the zener limiter on the reverb drive (reverb send & return are IC op-amps so they didn't need a trasnaformer). Maybe even install bias adjustment pots and a built-in meter.

What I'd really like to do is upgrade the cheap output transformer to the Mercury Magnetics upgrade items made for this amp. But they're overpriced IMHO. Even with a typical 10% discount below list they're still so exepnsive they cost more than the entire amp. But I'm still considering it, and lusting for their upgrade power transformer too.

I don't see a model-specific upgrade output transformer from other manufacturers. Anybody got a good recommendation for a cheaper upgrade output transformer for 4 EL84's?

Pretty amazing they can squeeze 50 (admittedly dirty) watts out of this thing with only 4 EL84's, when the Peavey Classic 30 also has 4 EL84's and only makes 30 watts.

But I really don't know if I could squeeze much more out of it. I look at it this way: the Classic 50 is a Classic 30 without the cheap transformers; but I don't know how much better it might be with even better transformers.

Since this doesn't have 5150-type gain, I would also like to get rid of ALL hum and hiss. Sealing the cabinets makes them a bit less efficient, but it also reduces the amount of audible noise.

Of course, like I said, this is all conjecture since I haven't even heard it yet. But looking at the little output tubes, little trannys, little reverb tank, little potentiometers, I sure wasn't impressed.

cyclecamper 18th September 2012 02:11 AM

How about a Heyboer output transformer meant for 2 EL34s, wouldn't that be pretty close to the right impedance for 4 EL84's? And less than half the cost of the Mercury Magnetics upgrade tranny? What about the neg feedback winding?

cyclecamper 18th September 2012 06:19 PM

What about adding chokes to the power supply in a guitar amp? This seemed to be a common practice in the days of tube rectifiers when caps were large; especially adding inductors between the various power supply sections, instead of just the series resistors between parallel caps?

Besides the advantages of a higher-order filter, how does the charge density of inductors compare to capacitors; I imagine caps are lighter but which stores more energy for a given physical dimensional size?

These bluesy amps are sometimes intentionally built to have some power supply sag, but I prefer a more punchy clean and some dynamics to my dirt.

cyclecamper 18th September 2012 07:17 PM

Of course, I'm talking about smaller chokes for just the grids and preamp sections, like typical Marshall or Fender circuits...

Enzo 19th September 2012 10:07 PM

No offense, but I see a lot of "this oughta be..." thinking.

Closed back cabs just don;t sound like open back cabs, you prefer whichever one you like. As to reducing noise, I think that is more a matter of the closed cab being more of a beamer, and you aren;t standing in the beam. The open back spreads the sound around more.

Hiss is an artifact of gain and noisy parts. You MIGHT reduce hiss some with different resistors, or might not. Less noisy tubes maybe, but you'd have to try all different types in each socket to see how to manage that. But between a scope, knowledge of circuits, and even trial and error, you should be able to localize sources of hiss.

Hum is not generic, ther are a hundred sources of hum and a hundred ways to cure it - one per cause. Point being that if you have grounding issues in the preamp, no amount of larger filter caps will have any effect on it. And likewise, if you have a little ripple hum coming from the filter cap grounding, no amount of shielding in the preamp will have any effect on it. You propose adding chokes, OK fine. The choke then is a power supply filter, specifically B+. The question must be asked, DOES your hum come from power supply ripple? Not only that, even if your hum is 120Hz (meaning it comes from ripple), it MAY need larger filter caps, or it MAY be a matter of an improperly shared bit of copper trace on the pc board. I sure wouldn't just throw chokes at the amp without a reason up front.

" It's so beat up that I'm rebuilding it before I even turn it on."

This scares me. The C50 is a great little amp, and tearing into it and throwing a bunch of parts at it without even knowing how it sounds or plays, just seems counterproductive. In my humble opinion, you should assess the needs of the amp and then address them.

The Classic 50 drags 50 watts out of four EL84 while the Classic 30 draws only 30 watts. Tubes don;t make the watts, they only control the power from the power supply going to the speaker. They are called "valves" in much of the world for a good reason - they act as valves for current. SO the fact the C50 and C30 has the same number of power tubes is like a car engine having the same number of cylinders. How can my kid's hot car drag 350 horsepower from 8 cylinders when my wifes car also has 8 cylinders but only makes 150 horsepower?

My old buzz-kill thinking is that you ought to get the thing and play through it to finmd out what good qualities it has and what you hate about it, then go about addressing THOSE things.

And I;d also recommend going to a web site like Blue Guitar and reading up on the various modifications people have done to the Classic 50 (and the Classic 30 for that matter, I even got in on that one), you may find some of them are exactly what you are looking for.

The Blue Guitar

counter culture 19th September 2012 10:26 PM

It's a Peavey.

Don't fiddle with it.

Most importantly, don't **** with the transformers, you'll ruin it.

Read the Hartley Peavey whitepapers, with the emphasis on #3.

Peavey.com

cyclecamper 20th September 2012 02:37 AM

SO nobody's tried the Mercury Magnetics transformers for the C50; no genuine first-hand knowledge at all... I can't find anybody who's tried it. Like I said, it's mostly the transformers that make the c50 a 50 and not a 30; they're obviously not too shabby. I have liked the Mercury Magnetics output trannys for some Fenders, those suit my tastes with nice clean highs and tight bass, and smooth gentle transition into subtle creamy harmonics from the tubes. But each model and application is different... Just too darn expensive to try on a lark.

Anyway, of course you're all correct that I should try it first. But "if it ain't broke don't fix it" is not my motto; "nothing ventured nothing learned". I got it running last night. No surprises, except that it's in better electrical condition than mechanical.

What it really really needs first is a quieter fan, this one's shot and really noisy. Fan in series with a resistor runs off of 120AC according to the schematic. I expected a computer fan driven off filament supply. Might just get a new one from Peavey. When I build the new case I'll build in two opposed-flow air paths to muffle, like the new IBM mainframes use. The small pots, true to their nature, are predictably noisy to operate but sound OK when not being touched, so I was able to evaluate what I had.

Oh yes, I read the 'blue guitar' mods before I even bid on this amp on eBay. Not much there, but he's absolutely correct about the dirty "channel" C18 and C1 inter-stage coupling caps values being very small. I suspect Peavey wanted to limit bandwidth in dirty mode so that low bass in pick attack didn't make the tubes really compress the output way too much and then have the note volume bloom louder in sustain. I don't want more than just a little compression, not much power supply sag either. I know that's bluesy stuff that customers for these amps want, but not quite what I'm after. But I want the bass back, even in dirty mode.

I'm definitely going to make the circuit from the input jack to the first grid just like I did on my Alembic FB2, cause it's what I like. But I might consider making it somewhat adjustable to accomodate guitars with a lot more output than what I usually play.

There was some site where they converted the phase splitter to a long-tailed pair (might have been blue guitar again in the advanced mods?), but that might be more than I'm willing to pursue.

The dirty channel needs some voicing, but this amp has potential. I like the way it relay-switches out the 'clean' volume control and switches in two more gain stages with different volume controls before & after the additional stages (rather than actually switching between entire channels). Very economical use of resources. Now if only the relay also switched between alternate master volume controls so you had two complete alternate settings. I used to run my favorite Alembic FB2 preamp (which I've owned a few of, and modded subtly) similarly, either using one channel (2 stages) or relay-switching its two channels in series (4 stages). I'd like to replace R8 with a 1M audio taper volume pot, giving the dirty mode a volume before, between, and after the 2 stages that get insterted by the relay. I will install a Fender-style " Bright" switch across the hot to wiper of the "clean" volume pot and the dirty "pre" volume also.

I'll probably add a small switch to add a second cap to the presence control.

One thing I liked about the two Alembic Fender-ish preamp channels in series was the two sets of tone controls; I could change the 'tone' of the distortion independently from the 'tone' of the guitar. For intance, I would usually boost the treble in a very early stage, then generate some distortion in the middle stages, then cut the treble in a later stage. The real guitar's treble was boosted & cut, but the distortion generated between the tone controls only had its treble cut. This made for a really great creamy distortion that wasn't irritating, but chords had wonderful complexity and beats.

This preamp does (predictably) have more noise than I can tolerate considering it doesn't have a ton of gain, and despite a decent grounding scheme and intelligent layout that keeps the power tranny etc. over a foot and a half away from the high-gain sections. Of course, the Alembic benchmark had a dedicated power supply for the preamp, and a LOT more big filter caps for the preamp B+. Filament transformer is center-tapped but there's no hum balance pot, might be worth installing one.

The first stage in clean mode (first two stages in dirty mode) are driven from their own resistor-isolated B+ supply caps. The B++ supply for the other preamp stages, driver, and splitter also has its own resistor-isolated caps. No voltages indicated on the schematic though. I'll probably do a bit of measurement and simulation and get some cheap chokes to replace the resistors and double up the caps in the power supply. But for the outrageous price Mercury Magnetics charges for their upgrade power tranny for this amp I could build multipple completely isolated dedicated power supplies using multiple stock power trannys LOL.

The reverb needs some work to approach Fender standards. I don't think the op-amps and transistor driver and recovery amp are a problem at all. Decent current drive (as opposed to voltage). But there's a pair of zeners which limit the input level. I'll experiment there just a little bit and a little at the recovery amp too, and replace the mini-size medium-decay tank with a full-size long-decay Accutronics with the same input and output impedances.

The good news is that it's going to work OK for me for practicing bass too.

A lot of high-fi tube fanatics use DC for the filament heaters. But NOBODY I know of does this with guitar preamps. My dad says he suspects the tube life suffers when the filaments run on DC. Anybody got any real knowledge to share about using DC for the preamp filaments? How much hum originates with the heaters compared to the B supply anyway?

Enzo 21st September 2012 12:47 AM

Stop and think about those zeners on the reverb input. The tube circuits can handle tons of signal level, and if you add even more all they do is clip. The reverb is driven by an op amp running between -27v and ground, the input is referenced to -14v. You can;t feed the op amp more than 14v peak of signal or the inoput will exceed the rails on the IC. That will damage the IC. That is why the zeners are there, they prevent any signal excursion past the rails.


I'd wager a squirt of Deoxit into each control will cure just about all their noise.



What do you mean NOBODY runs preamp tubes on DC heaters in guitar amps? This very amp has the two first preamp tube heaters wired in series across -27vDC. They have been making this amp that way for 20 years. The 5150 is still in production - now named 6505 - and has been also for 20 years, there are four preamp tubes with heaters wired in series between +24vDC and -24vDC. The 1991 Rockmaster preamp has all its tubes running on DC heaters, it is a preamp unit. The 1988 Peavey Triumph ran V1,2,3 on DC heaters. In looking, I see even AC heaters are often lifted by DC voltage, like the 1983 Rock Master Head or Encore 65 from the same year. SO peavey themselves have been running DC preamp heaters now for decades.

Peavey is not alone, Marshall runs the first couple preamp tubes on DC heaters in many amps. A quick look and I see it in the TSL/DSL series, and others.


Tube life suffer? Nah. I run a pro audio repair shop, I can count on my fingers the number of tubes with failed heaters I have replaced in the last 10-15 years. Tube heater failure is just not an issue. The tubes will go microphonic or noisy or quit working, long before the heater dies.

Hum through the heaters is POTENTIAL hum, you don;t always see it. Heaters have more than one way to let hum into a circuit. B+ ripple will certainly cause hum as will lack of good filtration on the bias supply. But also QUITE common is grounding issue causing hum. If a preamp ground shares copper with the ground return of a B+ filter cap, you will get ripple in that preamp. That is a separate issue from lack of enough filter cap. There is radiated hum from any AC carrying wires into sensitive circuits, ther is poor grounding, especially around jacks. There is a huge list of potential hum sources, and each has its own cure.

counter culture 21st September 2012 01:03 AM

Did you read the Peavey whitepaper?

If you want a different amp, buy a different amp and sell this one to somebody who appreciates it.

Printer2 21st September 2012 03:09 AM

The amp is not loud enough and breaks up too soon. Get new speakers. The ones in the cab are suppose to be 96dB, not bad but not great.

Now, this amp is not loud enough for your bedroom?


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