Vibro Champ build advice for a first timer?

[Johnny_March]

Hi Everyone,
This is my first time posting on this forum and I would really appreciate some advice.

To start off, I got screwed on an amp trade. I traded an working 90's Ampeg amp of mine for a "hand-built" Fender Prosonic from an unknown builder from Fender. Or so said the person who traded this amp to me. When I tried it out at his place, I thought it sounded good and I took it. When I got it home and inspected it, I saw that it was the most horribly put-together ticking time-bomb of an amp. The worst part of this amp was that there was no protective cover on the filter capacitor section; I could have been blasted by high voltage while I was getting the chassis out of the cabinet.

Anywho, I'm not salty; I'm just going to build this amp back, from-the-ground-up and do it right. At the very least, I'm going to make it an amp that isn't a safety hazard to use. I ordered a set of transformers for a SF Vibro Champ and I'm going to try to use as many parts from what's currently inside this chassis. There's a bunch of filter caps and other orange and brown drop caps, 3 preamp tubes and 1 6L6 tube.

I could use some advice on a couple of questions: Firstly, I've gone ahead and liberated all the big filter caps from the existing boards. I have two 22uF caps, two 16uF caps and two 47uF caps. I see that the filter caps of the Vibro Champ uses a 40uF and two 20uF caps for filtering. Will my build be way off if I use a 47uF and two 22uF's since that's what I have on hand?

Second, there's 3 preamp tubes on this chassis, but the vibro champs only uses two. Would a vibro champ sound "bad" or be otherwise sour if there's another gain stage or two added by using a third preamp tube? Maybe someone could share some stories or advice.

I'm including the schematic I'll build off of for reference.Thanks so much in advance for your advice and help.

1974-fender-vibro-champ.jpeg

[pompeiisneaks]

First off, I'm not sure how much experience you have building amps. If you've got little to none, then building a known amp is going to be a lot better than trying to wing it with mixing up this and that etc.

The amp you're referencing is a single ended amp, but the prosonic should be push pull.

Using an extra gain stage requires knowing how to modify a schematic and then create a custom layout.

All these are doable, but just asking these questions w/o the above knowledge may mean it's a bit out of your skillset for now.

I'd say look for a push pull amp with the same power tubes etc. Why did you buy new transformers? Are the ones on this amp not operational?

There are quite a few amps you could pull off with a setup like this that uses all three tubes and is a known amp, etc.

Hope that makes sense.

~Phil

[Johnny_March]

Hi Phil,
Fair questions/comments. I wanted to include pictures of the guts of this amp, but I keep getting errors when I try to upload. For full disclosure, this amp is pretty far from an actual Prosonic. It's a single ended amp with a 6L6 and 3 preamp tubes.

About the transformer: I measured the voltage coming out of the high voltage secondary, and I was getting about 470V. There was a lot of extra circuitry there to step down the voltage (I got a total of 9 high voltage capacitors out of the circuit and a choke) and I concluded that the power transformer that was there was not a good match for the amp. I bought a set of Champ transformers because I know it's a good design and has a good sound. Furthermore, I wasn't able to really track down what transformers I actually have here. Searching the numbers stamped on the transformers didn't yield good results.

As far as my experience goes, it's true that I have no experience in amp building, none whatsoever. My experience comes from repairing analog guitar pedal circuits and other appliances. I know how to follow a schematic and how to troubleshoot. I also have a pretty good understanding of how tubes work and what function different components have in circuits. Building this amp is kind of a necessity or I would otherwise be stuck with an amp that might eventually burn my house down if I don't do something about it. I would benefit a lot, educationally, from the experience instead of paying someone a ton of money to redo this amp from the ground up.

Anywho, the most important question I have is about the filter capacitors. Would using filter cap values that are slightly off from the original design, e.g. 47uF and 22uF instead of 40uF and 20uF, respectively, really hurt the design of the amp? I could always get caps that are the values per the schematic, but I'd like to reuse as many components as I can from this circuit. I'm trying to make lemonade out of the lemons I got.

[pompeiisneaks]

Hi Phil,
Fair questions/comments. I wanted to include pictures of the guts of this amp, but I keep getting errors when I try to upload. For full disclosure, this amp is pretty far from an actual Prosonic. It's a single ended amp with a 6L6 and 3 preamp tubes.

About the transformer: I measured the voltage coming out of the high voltage secondary, and I was getting about 470V. There was a lot of extra circuitry there to step down the voltage (I got a total of 9 high voltage capacitors out of the circuit and a choke) and I concluded that the power transformer that was there was not a good match for the amp. I bought a set of Champ transformers because I know it's a good design and has a good sound. Furthermore, I wasn't able to really track down what transformers I actually have here. Searching the numbers stamped on the transformers didn't yield good results.

As far as my experience goes, it's true that I have no experience in amp building, none whatsoever. My experience comes from repairing analog guitar pedal circuits and other appliances. I know how to follow a schematic and how to troubleshoot. I also have a pretty good understanding of how tubes work and what function different components have in circuits. Building this amp is kind of a necessity or I would otherwise be stuck with an amp that might eventually burn my house down if I don't do something about it. I would benefit a lot, educationally, from the experience instead of paying someone a ton of money to redo this amp from the ground up.

Anywho, the most important question I have is about the filter capacitors. Would using filter cap values that are slightly off from the original design, e.g. 47uF and 22uF instead of 40uF and 20uF, respectively, really hurt the design of the amp? I could always get caps that are the values per the schematic, but I'd like to reuse as many components as I can from this circuit. I'm trying to make lemonade out of the lemons I got.

Gotcha, make sure to be safe with the electricity in tube amps, they're not like pedals that work between 9 and 18V as you yourself noted of the 470VDC.

That level of differences (47/40 or 20/22) is within most electrolytic capacitors tolerance range so no it's not a big deal. And slightly higher does help with filtering. The only thing you'd need to watch for is if you're using a tube rectifier, some need a bit lower first stage capacitors or it can tax them too much.

As for having an extra tube, I'd just shoot for using the two, and later you might want to do some kind of mod like 1 tube reverb etc, but stick to the simpler stuff for now.

Not sure if someone has a Vibro Champ layout you can crib from or not, but that's a pretty straightforward design. You could, also look for any other single ended tubes that maybe have two channels so you can have a bit more tonally on tap, but basically, it's best to stick to minimal designs at first before pushing into too deep of water.

What was the error you're getting uploading? Maybe you're too new and it needs you to have a few posts in your belt before posting pics? I can't remember if that's a limitation or not.

~Phil

[thetragichero]

depends on the transformer set if you are using tube rectification. that being said, depending on the current rating of the choke you could probably build the amp with a CLC filter every single-ended amp should have (in my opinion) before the plate supply. you could use a smaller value capacitor if the transformer impedance is not high enough to support 47uf with the 5y3 champs run and then a larger value after the choke

all THAT being said, i refuse to reuse electro caps. the few times i've tried (like overpriced sprague atom caps from my first 5e3 build that i eventually scavenged for parts) have resulted in issues (luckily not safety-related, just motoroboating and other annoyances). i'd say that goes triple for electro caps of unknown vintage from an amp that wasn't well put together

what transformers were in this amp originally?

[Johnny_March]

Hi Phil,
For some reason, I couldn’t upload from my laptop, but it worked on my phone. Here’s some pics so you can feel my pain...

34B6F96F-1157-4EB8-8B8E-CF84C795C677.jpeg

394E8931-49D3-45FF-8F49-1079C5FE273A.jpeg

A0673738-5B30-4815-9F61-D11444284A40.jpeg

296A9E8F-B62E-4E7C-AD67-3CC966BD68D2.jpeg

019D5461-AE3B-4D75-9E56-FB3C20AECFC4.jpeg

715640E5-9BD2-40AF-B39C-27CF06F7D998.jpeg

[thetragichero]

i would definitely trash those ic brand electro caps

choke is a schumacher 50ma 4h choke that likely came out of a fender amp
got pictures of the stamps on the other transformers?

[Johnny_March]

Oh, so you recommend getting rid of the gray capacitors?

[Johnny_March]

Here are pics of the stamps on the side of the transformers:

CE43E916-09D7-4547-AC90-2FD05507DE9D.jpeg

D2FC6F29-C25D-4BA6-BB9A-42862CDF3B00.jpeg

[thetragichero]

i would. those gray ic capacitors seem to have a bad reputation and the five bucks or so you'll save isn't worth it. to be honest, if you value your time/frustration trying to desolder the rest of the components at more than minimum wage you'd probably do well to just order new axial capacitors and resistors whenever you get your board making supplies/board. welcome to the world of "oh crap i need more parts," although i'm sure you're familiar with it from building pedals (about the time my tayda order ships i realize the five different parts i forgot to include)
i'm interested in what the other transformers might be as they could be useful (couldn't make out the ot but it appears like a 606 shumacher eia code.... might already be a champ ot as it looks a little big for the reverb driver). i use a lot of organ/pa salvaged iron so i'm often working backwards: this is what i have, what can i build from it?

edit: okay that smaller transformer is the reverb driver from a fender vibro king. the hammond equivalent has these specs "Output transformer, guitar amp, 5482 ohm pri., 8 ohm sec., 10W"
would work nice for a 1x6l6 single-ended

other transformer is coming up as the output transformer for a fender hot rod deluxe. my guess is the bell wasn't originally on there, looks as if the rectangular cutouts were done rather haphazardly (laydown style that would have two circular cutouts on the bottome.... there's something about sharp metal edges and high voltage wires that makes me QUEASY). leads don't appear to be hot rod deluxe though... anything on the other side?

this is a mystery lol

[thetragichero]

this ALMOST looks like it could be the the hammond replacement for the vibro king power transformer but there are too many secondary taps!
https://www.hammfg.com/files/parts/pdf/290SX.pdf
hammond would've been a sticker on the side... time to do some more looking in the hammond sheet. the primary colors appear to match up

[Johnny_March]

That’s very interesting... so the reverb transformer was being used as an output transformer. When I typed in the other transformer code, I did get that it was an output transformer for a hotrod deluxe, but then OT’s don’t have high voltage secondary windings, so that led me to conclude that I couldn’t really work with that transformer for now. I have a variac coming to me and I’ll use it to figure out the different secondaries. Judging from the relatively high voltage B+ that can come from it, it’s probably more appropriate for a Marshall-type of amp.

So the smaller tranny is for the reverb of a Vibro King? Doesn’t the Vibro King have an actual Fender Reverb Unit circuit in it? If so, then maybe I can build a Reverb Unit with it. That would be cool.

[thetragichero]

vibro king has a three knob reverb like the standalone units. early versions used an el84 for the reverb driver which seemed to eat em up, later used a 6v6

if you feel like getting a wrench out, carefully removing the bell cover that the power transformer leads are poking through, there may be more information under it. obviously i like a good mystery

[Johnny_March]

Sure, I can give opening that PT a try. I’ll post more pics of that when I get a chance. And so far from this thread, I gather I should just get new filter caps of the correct values. At least I’ll get to use one of the eyelet boards I salvaged from this wreck

[Phil_S]

Not wanting to interfere with what appears to be good advice, however, if using a 5Y3GT, I wanted to point out the spec sheet says there is a 32uf upper limit on the filter cap. I suppose 40uf isn't too awful breaking the "rule" but I'm concerned that 47uf may be a bit too much for that rectifier.