Vox VT100 amp

[GRAHAM99]

My pal has one of these.It plays ok for a few minutes and then looses a lot of volume.
It will then go back to being ok and so on.
I have had a look inside but cannot see anything obvious.
Anyone had the same issue?

[Folderol]

Try giving the valve a wiggle in it's base - may be just a poor contact.

[Wonks]

Bad solder joint or a dodgy pot are also likely candidates.

Can we presume they’ve tried another guitar and lead to rule those out and confirmed it’s an amp issue?

[GRAHAM99]

Yep tried another lead and guitar.
Will have another careful poke around inside.

[Folderol]

If you need to poke around where there are unknown voltages, the only thing I'd suggest would be a plastic knitting needle.

The other hand in your pocket.

[Wonks]

The single valve in the amp, an 12AX7, is fed from +51v and -51v DC PSU rails, so whilst not as bad as an all-valve amp, it’s in that just not-quite safe area for working on live unless you really know what you are doing. But leaving it off for a couple of hours before opening it up should allow the capacitors to discharge to a safe level.

I’d first be checking for cracks in the solder around the volume control and input jacks.

[ef37a]

Yes, as Wonks says be careful. A valve amp fading down and up is almost always a problem in the heater supply.
MIGHT be the valve but I doubt it.

Dave.

[Wonks]

The valve heater voltage should be the standard 6.3v AC, (each side of the transformer output is connected to ground through a 1W 100 ohm resistor). But the heater takes a reasonable current, so that can be an issue if PCB tracks are undersized or the inter-board connectors don’t connect well.

It’s almost all a DSP amp, with just that single valve in the circuit between the digital pre-amp and the power amp.

All the front panel controls look like they feed directly into the DSP processor with the exception of the master volume control.

There’s also a series effect loop, and if unused, corrosion on those switched connectors in the jack sockets doesn’t get wiped off and the signal can become dodgy. So always good to occasionally spray some switch cleaner in those and push a jack in each one a few times.

[GRAHAM99]

Thanks for all the help on this.
I have seen a lot of criticism of the soldering on these amps so maybe that is the issue.
I used the output of the effects send into another amp and it worked ok.Also the headphones out put worked ok.
This would point to a problem in the power amp section?

[Wonks]

It would seem so from looking at the schematic. The line out is taken from after the valve section.

[GRAHAM99]

I couldn’t find a schematic could you put the link on here?

[Wonks]

I got it from here.

https://elektrotanya.com/vox_ad100vt.pdf/download.html

[GRAHAM99]

Thank you.

[jjlonbass]

That is a very odd circuit around the 12AX7. The 2 triodes are biased from a -51V supply, but the anodes' DC supply comes from the unity gain inverting op-amp circuits around IC2A and IC2B which in turn are driven from the triode anodes, IC2A by a DC blocking capacitor! I'm not sure what is going on here, but I suppose with no signal, the anode supply would be 0V.
Ah, I see that the cathodes are connected to the -51V supply via R42 and also Q3 if it is switched on. Q3 is controlled by the A/AB output of the M3803 microcontroller. So the valve bias is under the control of the micro via Q4 and Q3.

Does the fading occur in A or AB mode or both?

The power amp uses one of the Sanyo STK thick film hybrid modules, these often cause trouble - hopefully not in this case.

John

[James Perrett]

It would seem so from looking at the schematic. The line out is taken from after the valve section.

By my reading of the circuit diagram the line/headphone out is taken before the valve stage. Plugging something into that socket interrupts the feed to C31 and I think the Phone_DT signal is used to mute the input to the power amp IC and also sent to the control processor.

The headphone socket could well be causing the problem.