Vibro Champ build advice for a first timer?

[Johnny_March]

Interesting... I think I’d like to have a look at that data sheet. From advice on this thread, I decided that I’m going to put new filter caps in this amp. But before I buy them, I’ll have a look at the specs and make sure the caps will play nice with the rectifier tube. I did think of maybe doing a diode bridge circuit instead, but what the heck... I’ll just stick with tube rectification just for kicks.

[thetragichero]

i would not suggest a FWB with the standard 325-0-325 champ power transformer unless you like exploding filter caps (over 900vdc). even a solid state full wave rectifier would be iffy (455vdc)

[Stevem]

It’s a very good thing in terms of minimal 120 hz in the output stage to run a higher V+ level and then add another filter node and dropping resistor to get the V+ back to a much more class A 6v6 happy 390 to 400 volts.
I use a 5V4 recto in my VC set up like this with the added benefit of the slow warm up this tube has!

[pdf64]

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.

I think that about 20uF is a typical limit, when noted, eg https://tubedata.altanatubes.com.br/she ... /5Y3GT.pdf
But really it's governed by several factors, eg VAC and source impedance to each anode, DC load current at the cathode.

[Stevem]

These recto ratings many times are on the very safe side, for instance I have a Uniox amp that uses a 6X4 recto and I have had it pumping into 20 uf for years now which is 10 uf more then its rated for !

If the recto is not being called upon to output all the current that its rated to handle, then it will stand up to more uf.

[Johnny_March]

Just an update... my new champ transformers should be arriving in a couple of days. I’m getting a bill or materials to together for all the other components I’ll need for this build. I think this build will really put my skills to the test and give me an opportunity to level up.

I’ll be posting to this thread for quite a while. I’m sure it will take me a month or more to get my build off the ground.

[thetragichero]

slow and steady with constant comparing to the schematic/layout helps keep the after build troubleshooting to a minimum (and you'll know from pedal building that putting it together is the relatively easy part.... getting the $%#%%$# thing to work right is a whole other ballgame)
i work extremely slowly so ain't nothing wrong with that

[Johnny_March]

My transformers came in. Mercury Magnetics is an hour from me so I bought mine from them. Now I’m working on getting the rest of the circuit components.

[Johnny_March]

New update: I got about 98% of all the components I need for this build. I'm also in the middle of moving to a new residence, so that's slowing my progress, but I'm going to try and build the filter capacitor board this weekend. I know fender champs use the can capacitor for the filter section, but I'd rather use an eyelet board instead of drilling another big hole in this already swiss-cheese chassis I have. I'll post pics soon!

[thetragichero]

hey those are just speed holes

[Johnny_March]

If they're speed holes then this chassis is lightning-fast

[Johnny_March]

New Update: I got myself sidetracked in a good way. I got struck by lightning again. I bought someone's used Fender Champion 600 for pretty cheap. I took it home and discovered that it had been modded in a pretty bad way along with hacking up the baffle to take a 8" speaker. The amp sounded really bad and was pretty unusable. Again, I have no choice but to rebuild it. I figured this project would suit me better since it is my first rodeo.

The previous hacker/modder put some F&T capacitors and they looked to be in great shape. I took them out and I've gone to work on this amp. I'm rebuilding it to be a 5F1 champ, but with the 25uF and 10uF cathode bypass capacitors in the preamp section as in the silver face circuit.

I wired a new power section with 4 1N4007 diodes, an F&T 20uF cap and two 8uF caps all rated at 350 watts. I also wired in a new lamp with 1w 100 ohm resistors to make up for the lack of a center tap on the filament secondary winding.

Everything is coming along, I fired it up and I got my high voltage and filaments working without blowing up my house. I'm now wiring up the pre-amp section.

New question: Can I ground the cathode bias resistor and bypass capacitors to the ground lug of the input jacks since they are close and are connected to the chassis or is this a bad idea?

I'll post pictures of this new project shortly. Thanks for the support !

[thetragichero]

can you? of course
i do prefer keeping preamp grounds connected to input jack and power amp/power supply grounds connected to the chassis elsewhere, but everybody has their own preferences and as long as it works who am i to judge?
a lot of fender amps were built with poor grounding schemes in my own personal opinion

[Johnny_March]

Great, thanks for the advice. Right now I have a star ground where the power supply section and pilot light get their grounds from. I'll be wiring up the cathode resistor on the 6v6 to go here. Grounds is what I'm most worried about with this circuit. The last thing I would have wanted was to run a long ground wire from the preamp section to where the power supply is grounded. Everything is coming along pretty neatly so far. Hopefully this amp is quiet when I finish it. I'm really close!

[Johnny_March]

Ok, new update. This is a big one. My first build is about complete. I'm really happy to say that everything appears to be working. I have some new questions:

1: The filter cap section is a little less secure than I would like. Most amps have a "doghouse" around this part of the amp as its very dangerous, but on this amp, it's not an option to put an enclosure around it. Is there something anyone could recommend as some kind of a cover to go over the top of this section to protect it from my hands and my hands from it?

2: The first time I hooked up my oscilloscope, I thought the waveform looked a bit odd. I was expecting to see the waveform eventually get chopped on the top and bottom and resemble more of a square wave, but instead the peaks and valleys sort of dagger out to make a triangular waveform. Is this normal? Could someone explain the waveform to me? For reference, it' s a normal sinewave from about 1 to 3 and then it gets more and more distorted after that. I used an 800Hz signal at 150mVpp. Then I tried a guitar and it sounded good.

3: What does everyone think of this first build? Is it pretty good or am I a terrible hack? I owe Uncle Doug and Terry from D-Lab a great big virtual hug, I got all my ideas and references from them and everyone here on this thread who has helped me with advice.

Next up, I'll start on the vibro-champ build...

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