Tube amp volume fluctuation

[m1989jmp]

Hi all,

I've got a Carvin Valvemaster 100W head.

Recently it started completely dropping in volume, either warmed up or cold, without crackling, static etc.. It starts for no reason, gradually fading to zero. At that point I wiggle every cable, rotate every pot, tap on the chassis; nothing.

If I wait for a few minutes, the sound fades in gradually the same way like it faded out even if I don't touch the amp at all.

Time for a retube, right?

I swap power and preamp tubes, bias them and monitor for a few hours while playing guitar through here and there for 10-15 minutes constantly. The chassis is upside down at that point. As you can guess, no volume issues, although I don't play it very loudly.

After putting it in the headshell, carrying it over to the rehearsal space which is next door and playing for 10-15 minutes at similar levels like on the workbench, the volume cut happens again.

Any suggestions?

The schematic is for a Vintage 50 model because it's a first version Valvemaster, so same preamp but 4xEL34/6L6 tubes in the poweramp. I'm certain power amp works because there is sound coming out when plugging into return jack.

http://carvinimages.com/schematics/77501revI.pdf

[Enzo]

Amp fades in and out? When it fades away, look at ALL the tube heaters. are any of them going dark at that time?

Also classic problem, when it fades away, plug a cord from FX send to FX return, any help?

[m1989jmp]

Amp fades in and out? When it fades away, look at ALL the tube heaters. are any of them going dark at that time?

Also classic problem, when it fades away, plug a cord from FX send to FX return, any help?

Thanks for the reply.

Already tried patch cord during the volume cut, no change.

I'll try looking at heaters tomorrow when the amp will be back on the bench.

[Helmholtz]

http://carvinimages.com/schematics/77501revI.pdf

Wow, 100k input impedance! This effectively "equalizes" the frequency response of passive PUs. Sorry, but this is really special.

[glebert]

Looks like the channel switching is ICs not real relays, right? Could the channel switching ICs be getting sent into a middle state? I have a Crate VC50 that had dirty footswitch contacts and it would do something similar (though much different switching circuit). Also, jumping the FX loop didn't help, but if you plug straight into the return when it is faded out do you get any output? Just trying to separate preamp from poweramp.

[m1989jmp]

Looks like the channel switching is ICs not real relays, right? Could the channel switching ICs be getting sent into a middle state? I have a Crate VC50 that had dirty footswitch contacts and it would do something similar (though much different switching circuit). Also, jumping the FX loop didn't help, but if you plug straight into the return when it is faded out do you get any output? Just trying to separate preamp from poweramp.

Nope, 2 relay packages. I already wrote if I plug into return jack (power amp in) while the sound is out, I get signal through speakers.

It's worth mentioning the sound fades on both channels, so I would pinpoint it to either input jack circuit (unlikely) or somewhere between channel switching relay and return jack.

My apologies, it turns out this is more a accurate schematic.

http://carvinimages.com/schematics/v...ster77701g.pdf

I should mention I run the amp in 50W mode (2 inner EL34s) set to 8R impedance into a 16R cab. Carvin's manual doesn't mention that head's impedance needs to be halved, but I've done it anyway.

[m1989jmp]

While prodding around the power amp board, I've noticed one of the board mounted male spade connectors that supplies AC heater voltage is completely loose and popped out when I pulled it.

I'll report back when I try it again, for now I'll keep on prodding.

[m1989jmp]

As per Enzo's suggestion, I've watched heater filament while the amp was on the bench. Turns out V1 and V2 don't light up. Swapped tubes around, stays with the sockets.

Measured VDC between pins 9 and 4&5 on V1 and V2, I get 0.4-2.5VDC and it's changing if I wiggle diode bridge. V1 and V2 are the only ones with DC filaments. All AC filaments are fine, but low-ish at around 6VAC with tubes pulled out.

After swapping diode bridge, I've put 4 separate diodes as per schematic and the voltage stabilized at around 4.5 VDC without heater supply capacitors. I can see the board has heated under the diodes, I'm guessing they were under spec'd and caused a fault before.

Now I'm trying to relate this fault with the power tube spade connector's bad connection, which did cause the fault? Maybe the traces overheated because of the bad spade connector and burned the diode bridge, I can also see a few lifted traces capacitors are soldered too, maybe just bad tech work?

[g1]

If the power amp worked when you plugged into FX return, then the spade to the power tubes must have been ok. Probably not related to the cutting out but good that you found it and corrected.
The diode bridge connection for the V1 & V2 DC heaters is probably what was causing the problem. It is not uncommon with DC heaters to see the solder fail there. Resoldering would have probably fixed it, but if you replaced it that's fine too. Using a bigger bridge or standing it off from the board will help prevent the heat induced solder failure there in the future.

[m1989jmp]

If the power amp worked when you plugged into FX return, then the spade to the power tubes must have been ok. Probably not related to the cutting out but good that you found it and corrected.
The diode bridge connection for the V1 & V2 DC heaters is probably what was causing the problem. It is not uncommon with DC heaters to see the solder fail there. Resoldering would have probably fixed it, but if you replaced it that's fine too. Using a bigger bridge or standing it off from the board will help prevent the heat induced solder failure there in the future.

I've put DMM across pins 9 and 4&5 of V1 and when wiggling both rectifier (which was in a single 4 pin package) or any of the 2 filter caps, the voltage would drop. Reflowing didn't help, so I replaced everything.

The diodes that were there before the 4-pin bridge definitely failed because of overheating, but I can't say if a 'tech' lifted 3-4 associated pads while resoldering or heat was the cause.

The weird thing is that the schematics calls for 6V rated heater supply caps, and the voltage gets up to 6.5VDC, maybe that was a factory error.

On a side note, the amp has a great clean channel but gain channel sucks really bad. I'm attributing it to a lack of cathode follower, anode fed tonestack and clipping diodes. Turning the gain pot beyond noon results in a squealing mess, that seems to be a common problem with these amps.

I found this great website with a number of mods, I'll try them out before I give up and use the chassis and iron as a basis for another circuit.

Hasserl.com - Carvin VT50 / Nomad / BelAir

Thanks again for the replies, they are as usual concise and to the point.

[m1989jmp]

One question. While I was doing final measurements, I've noticed the chassis vibrates a bit, the only thing that could cause that is the mains transformer.

I've read somewhere a rectifier shorting to ground can cause that? All voltages check out fine, maybe I just didn't notice it before, although I doubt it.

Any thoughts?

[m1989jmp]

Another problem presented itself; filament tap is at 6VAC unloaded (tap disconnected from the board), as schematic states.

The preamp board heaters are supplied from the power amp board via thin ribbon(!) cable.

With power tubes and the preamp tubes that run on AC heaters it goes down to 5.8VAC. Furthermore, when V1 and V2 are plugged it goes down to 5.5VAC and 5VDC on preamp tubes. I guess that DC heater supply loads transformer a lot.

I plan on removing heater supply rectifier and caps and try V1 and V2 on AC heaters, since there is a huge mess around that area. If that doesn't bring all voltages to 5.8VAC, I'll replace the PT.

On a side note, a guy that runs a website with Carvin amp mods told me almost all of their amps from this era suffer from low filament voltages, but I think 5.8VAC will be fine, especially when mains voltage rises from current 220ishVAC to 230VAC.

[m1989jmp]

Does anyone see a problem with a 3k humdinger pot, I always see 100-300R trimpots there?