Vox AC30CC2 tremelo circuit

[NoisyIndividual]

Well there is clearly something I'm not seeing here and it is doing my head in.

VoxAC30CC2 which is working except for the tremelo, which causes large currents the moment the Depth control is moved off zero.
I have it on a dim bulb so nothing can blow up.


What I don't get, is that there is a tremelo AC voltage of around 200v pk-pk on the Depth pot.
This pot then puts a portion of this straight to the EL84 grids, causing mayhem.

With the EL84's removed, there is something like 60v pk-pk's worth of guitar signal on these grids, and 60v pk-pk of tremelo AC if turned up full.
This can't be right.
I changed the Mosfet MF3 just to rule it out and it is still the same.
The tremelo frequency control works as normal, everything else seems to be in place.
There is about 300v pk-pk tremelo on the Drain of the Mosfet.

I'll probably kick myself for not seeing the problem.

 

[MagnetoSound]

There should not be any DC reaching the DEPTH pot.

Check C27 for leakage.

 

[NoisyIndividual]

No, there is no DC on the depth pot, just humungous amounts of AC tremelo.
With the tremelo oscillator turned off, there is also no DC.

 

[MagnetoSound]

What you see as AC on the DEPTH pot is actually pulsating DC.

C27 should be blocking it.

Instead, you should be seeing a pulsating AC impedance to ground, via C27, MF3, C26 and TR1.

 

[NoisyIndividual]

I've just replaced C27 with a 0.1uF 600v cap, and there is still 200v pk-pk of more or less triangle wave on the Depth pot.

I think I'm totally misunderstanding this circuit, I assumed TR1 was to stop the oscillator via the footswitch.

 

[MagnetoSound]

Yes, it looks as though TR1 needs to be conducting for the oscillator to work.

C24 might have failed open. Whatever the cause, you should not be seeing large voltages on the DEPTH pot or beyond.

To be fair, I mistook the principle too. Looks like it’s actually a bias trem circuit, not a variolosser. So there should be some pulsating voltage across the DEPTH pot and into the output stage, but nothing like hundreds of volts.

There might be a grounding issue. Perhaps worth checking you have 0v at the bottom of C24, etc.

 

[NoisyIndividual]

Yes, I've been assuming its a modulated bias design, just can't work out why there are hundreds of volts of tremolo flying around.

 

[tinn]

Can't it be that everything is fine?
You don't have to dial up the tremolo pot all the way.
I would guess there should be 300 Vpk-pk at the Drain of the FET and 90V pk-pk at the tremolo pot
(10 Hz Tremolo speed)

 

[NoisyIndividual]

No it is not fine. If you put any tremolo at all in, the mains transformer will draw so much current it will blow the mains fuse.
There is 200v pk-pk at the tremolo pot, and up to 60v pk-pk on the EL84 grid pins if you remove them. The Cathodes sit at around 2.4v so effectively the grids are at -2.4v bias.

I just don't know what a normal working amp looks like, but all that tremolo looks way too much.
There may be a fault other than the tremolo circuit, but this definitely triggers it.

The amp works fine otherwise. Bias currents are a bit high, maybe 80 - 90% from memory, but then that's pretty much par for the course IME. I've seen far worse from the factory.

 

[radardoug]

So how much bias do you have on the EL84s, should be about -15 volts at 300 volt plate. Driving the grids positive from the tremolo would not be good.

 

[NoisyIndividual]

Ok, just got in and have been thinking I need to check the bias again.
The bias IS high, but as I said, I have seen worse on EL84 reissue builds straight from the factory that just copy the original circuit.
I normally reduce this.

So my measurements from my notes:
345v Anode
69R 70R OPT
4.2v 4.8v OPT
30mA 34mA
11W 12.3W

So 100%, needs to come down really, and the OPT looks OK.

The Cathode measures 10.4v, not 2.4v as I stated yesterday, Grid is zero, giving an effective grid bias of -10.4v.
This is on the 82R Cathode resistor "Warm" setting, the "Hot" setting is obviously worse.

 

[NoisyIndividual]

OK, so I went back to this amp today, and gingerly fired it up with a dim bulb. It didn't light the bulb much or try to blow up.

Last time(s) it was intermittent - it would be fine, and then suddenly even a slight amount of tremolo depth would do something VERY bad. The kind of bad that even with a dim bulb you would not want the amp to experience, or you might have no amp left.

So I got a scope shot of the grids, with the EL84's in place driving a speaker, so now I know what I am expecting to get there with Depth on max, about 30v pk-pk with the top clipped at a couple of volts less than the bias.

I have a feeling that replacing C27 actually fixed the problem, and there WAS +ve DC getting onto the grids, so thanks MagnetoSound if you sussed that correctly.

More gingerly testing is required though.
I hate it when disaster strikes suddenly.