Vadis / Electrovox 408R Volume / Channel Bleed

[Helmholtz]

Another error in the OP's schematic is a missing plate resistor of the reverb driver.

[The Dude]

Yes. I'm doubting that we should trust that schematic at all, as g1 said. Who knows what other errors are there. That is why I tried to find another one. I'm not sure if that matches the amp we have here, though. Hopefully the OP can let us know.

[q9522678]

Without a decoupling cap on the 186V node you've got a four-input anode mixer *post* volume control, so no wonder you've got bleed. Add a 10-22uF decoupling cap at that node, should fix it right up.

Alright. So I clipped a 10uf / 500v electrolytic in initially and this fixed the volume problem but also created some popping and crackling. So I thought I’d try another cap - and the next smallest cap I had sitting around of appropriate voltage rating was a 0.47uf600v non electrolytic (PIO). I put this in and solved the volume bleed problem without cracking. Are there any issues using this value and non-electrolytic for the decoupling?

So I excitedly hooked the reverb and tremelo up. Reverb working fine. Nice and lush. Dick Dale would be proud.

Hooked up the tremelo and it’s working but I noted the effects channel sounds thin, distorted and volume drop. I thought it might be the new cap I installed, so pulled this and the issue remained. So tried pulling the tremolo tube and channel 2 came back to life. Loud and clean. I put a different 12AX7 in case it was a faulty tube, but same issue.

I can see the Tremolo draws a from V2B - could this be a case of it drawing too much to the point of starving V2? (Excuse my layman terminology here). I hadn’t noticed this issue before -I’m sure channel 2 used to be loud and strong even with tremolo. I rechecked the location and solder connection where I had disconnected, tried multiple tube and checked for lead dress issue to no avail. I note the voltage to the tremelo is a lot higher than the schematic.. over 300v. Could this be an issue?

[The Dude]

The trem circuit is basically a LFO. You'll need clean/filtered plate voltage for it to work right and so supply doesn't swing with the oscillator. You'll need a larger value electrolytic on that supply node, as with any other tube amp. The .47 isn't going to cut the mustard. There is only 186V there, so anything rated 200V+ should work. I suspect the first cap you tried may have been arcing.

[q9522678]

The trem circuit is basically a LFO. You'll need clean/filtered plate voltage for it to work right and so supply doesn't swing with the oscillator. You'll need a larger value electrolytic on that supply node, as with any other tube amp. The .47 isn't going to cut the mustard. There is only 186V there, so anything rated 200V+ should work. I suspect the first cap you tried may have been arcing.

I rummaged through my parts box and only really have relatively low voltage (50v) capacitors sitting around. However have plenty 10uf 500v, so tried putting two in series and the amp seems happy, volumes are cutting signal, and no popping and cracking. Is this suitable? If so I’ll order a dedicated higher voltage cap and reclaim my two 10ufs.

Ref tremolo - is there I can do here. The tremolo wiring is per the schematic but voltages are higher. The C+ feed on the schematic is connected direct to grid through resistor. No capacitors here either. Would dropping the voltage and or adding some caps here improve the situation?

[q9522678]

Thanks for tracking this Schematic down. I have just printed it and will work through and see what I can find.

Appears a little more complete than the one I found.

[q9522678]

Another error in the OP's schematic is a missing plate resistor of the reverb driver.

Hi Helmholtz. I’ll take a look at the reverb wiring when I run through the new schematic. From fuzzy memory I think there was a plate resistor in my amp as I recall noting there was a resistor there and not on the schematic.

[The Dude]

I rummaged through my parts box and only really have relatively low voltage (50v) capacitors sitting around. However have plenty 10uf 500v, so tried putting two in series and the amp seems happy, volumes are cutting signal, and no popping and cracking. Is this suitable? If so I’ll order a dedicated higher voltage cap and reclaim my two 10ufs.

Ref tremolo - is there I can do here. The tremolo wiring is per the schematic but voltages are higher. The C+ feed on the schematic is connected direct to grid through resistor. No capacitors here either. Would dropping the voltage and or adding some caps here improve the situation?

Two 10uf in series is fine. However, a single 10uf should do the trick. "The C+ feed on the schematic is connected direct to grid through resistor.". Do you mean the plate? Which gain stage and which resistor are you talking about?

[Greg Robinson]

The 10uF decoupling cap you clipped in place won't cause popping or crackling, I'd suspect that issue was caused by poor contact of the clips you used (croc clips?). Else, possible you revealed another problem. By adding the cap and decoupling the four triodes from each other the gain would be increased mildly as the plates aren't loading each other any more, this could have unmasked noisy tubes or resistors, which are presumably carbon composition as you noted they appeared original.

The 470nF cap you tried later is barely enough decoupling to prevent audible crosstalk between channels, but would be better an order of magnitude larger, as are the rest of the filter caps in this and other tube amps. No need to use two 10uF's in series, this drops the value to 5uF which provides less decoupling and costs twice as much, no point to it. You don't need a 500V cap, but it won't hurt. Film or electrolytic will both function fine here, but film caps in these values are enormous and very expensive, that's why we use electrolytics (despite their otherwise less-ideal nature). Just use 10uF or 22uF, smaller values in this voltage range (>200V) tend to be more expensive and provide no benefit. Note the other filter caps in both the schematic you and The Dude posted are all 22uF, there's good reason for that, and no reason to think this node is in any way special or different from the others.

Not sure what you mean about C+ having no cap, in both schematics it has a 22uF cap, just like the other nodes.

BTW, hi from Melbourne!

[The Dude]

......Not sure what you mean about C+ having no cap, in both schematics it has a 22uF cap, just like the other nodes......

.......unless someone has removed it? There should have been a cap on the 186V node and there wasn't. The amp could not have worked properly without a cap there, so I'm led to believe that someone has fiddled with the amp. Or, the caps are there, dryed up, and not spotted by the OP. Is there maybe a dead can cap that's been missed or something? We should maybe see a gutshot picture of this amp.

[q9522678]

Thanks for tracking this Schematic down. I have just printed it and will work through and see what I can find.

Appears a little more complete than the one I found.

Having has a quick look at the schematic I recall this was a project where someone tried to combine a 408R preamp and effects with a BR60 preamp ant power amp. That being said, the 408R parts of the schematic look right.

Going to have a close look at the tremolo tonight. If I can get that sorted out, should be able to stitch it up and call it a day.

[q9522678]

Alrighty, I’ll try soldering the 10uf and see how I go with the popping and crackling. It could very well be the alligator clips I used. The resistors in the amp are a mix of carbon comp and carbon film (I think) but looking at some original gut shots appears it was a bit of a mixed bag in terms of what was used. They are either original or installed a very long time ago!

Yep - C+ has a 22uf feeding it. I was referring more to no caps between that 22uf cap and the tube for decoupling purposes. in the tremolo.

Melbourne? Looking chilly down there! It’s 23 degrees here in Sunny Queensland and I’m shivering!

[q9522678]

All the power supply caps appear to have been replaced with modern electrolytics. There’s no cap can on this one. There does appear to be some differences with the power supply in the amp and the schematics but I think it is largely layout related as the smaller more modern caps could be positioned differently to the older giant ones that came it is.

[The Dude]

......Yep - C+ has a 22uf feeding it. I was referring more to no caps between that 22uf cap and the tube for decoupling purposes......

That existing 22uf is the decoupling/filter cap for the C+ node. You don't need another one.

[q9522678]

Alrighty, I’ll try soldering the 10uf and see how I go with the popping and crackling. It could very well be the alligator clips I used. The resistors in the amp are a mix of carbon comp and carbon film (I think) but looking at some original gut shots appears it was a bit of a mixed bag in terms of what was used. They are either original or installed a very long time ago!

Yep - C+ has a 22uf feeding it. I was referring more to no caps between that 22uf cap and the tube for decoupling purposes. in the tremolo.

Melbourne? Looking chilly down there! It’s 23 degrees here in Sunny Queensland and I’m shivering!

So 10UF soldered in and working fine. At least an easy fix on that one.

Channel 2 still sounds horrible when tremolo tune is plugged in. The tremolo itself sounds great - very clear and strong. But the audio is thin and distorted. Will play with that later today