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VOX AD15 valve modelling amp fault...

Started by noddyspuncture, February 27, 2014, 02:59:41 PM

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noddyspuncture

Hi guys,

I was wondering if anyone could help me with an amp I have been struggling with for a while now, please..? I have a faulty one and also another a good one to compare to - so in theory it should be really easy... but I'm still struggling....!

I've attached the PDF... it has two power amp schematics... one is the 15 and the other is the 30. Both look the same.... although mine are the 15's.

What I am getting on the faulty amp is basically low level... (and an apparent lack of bass - probably just a side effect of the low level?) It's just not a loud and sounds compressed when turned right up.

My scope confirms that the signal level is much lower on the grids of the valve in the faulty amp. (Everything up to that point is identical on both amps) So, basically on the left side of R23 & R27 (both of which are shorted out by the IC switch) I get a full, identical level on both amps. But the faulty one has a much lower signal level on the grid sides of these resistors. I originally suspected the IC which I changed. There was no difference. Even with the valve removed it is the same. The scope confirms a normal level going into the switch but a lower level coming out of the switch. On both amps, good & faulty, the control feed to pins 8, 16 (and 9) of the IC looks the same on my scope (and elongated square wave) so I can discount that I suppose. I even tried shorting out those IC 'switches' temporarily - the level improved slightly but still wasn't what it should be. This has been baffling me for sometime now.

Does anyone have any ideas please?

Cheers,
Tom

Enzo

The AD15 has 20v rails while the AD30 has 30v rails.

The AD15 uses an LM1875 power amp IC, the AD30 uses an LM3886 power amp IC.  The AD30 has the variable power feature.

Look past the switch,  Instead of the switch being bad, maybe the switch simply dumps the signal into some low impedance it doesn't like.  Is that -15v on those grids?

noddyspuncture

Thanks for the reply Enzo,

Yes it is -15volts on the grids.

I was looking past the 'switch'... it's only the valve grid next... and with or without the valve I get the same 'smaller' signal there on the faulty unit.

This is what has had me stumped.

Cheers,
Tom




Quote from: Enzo on February 27, 2014, 06:08:27 PM
The AD15 has 20v rails while the AD30 has 30v rails.

The AD15 uses an LM1875 power amp IC, the AD30 uses an LM3886 power amp IC.  The AD30 has the variable power feature.

Look past the switch,  Instead of the switch being bad, maybe the switch simply dumps the signal into some low impedance it doesn't like.  Is that -15v on those grids?

Enzo

I was concerned about the control pins for the switch IC, you said there was a slow square wave???  Ought not the voltagr sit at one level or the other and stay there as long as the channel is selected?


noddyspuncture

I'm not quite sure... this is a 'modelling amp' - and those switches just seem to short out a 15K resistor feeding the grids of the valve...! I was comparing what was on the control to the IC (pins 8 & 16) on other 'good' amp I have - and that looks identical... an elongated square wave, so I just assumed it's correct, as the other amp is working perfectly.

Basically it's the fact that the signal level just drops on the grid side of those IC switches that is stumping me. Even with the valve removed... the signal is much lower on both grids and then there is only a capacitor and that -15volt feed left to investigate on both lines...?

Cheers,
Tom


Quote from: Enzo on February 27, 2014, 08:55:28 PM
I was concerned about the control pins for the switch IC, you said there was a slow square wave???  Ought not the voltagr sit at one level or the other and stay there as long as the channel is selected?

teemuk

#5
QuoteI'm not quite sure... this is a 'modelling amp'

The Valvetronix amps are hybrids of digital and analog modeling. The preamp employs digital modeling, DSP and algorithms, the power amp employs analog modeling aided by digitally-controlled switches: different preamp patches also control switches in the power amplifier circuit. The DSP models preamplifiers and associated effects, the analog circuit integrated to the power amplifier models the operation of a push-pull tube power amplifier.

Quote..and those switches just seem to short out a 15K resistor feeding the grids of the valve...!

Well, they basically toggle an attenuator: resistive voltage divider formed by R23 & R24 and R27 & R28. Resistances R20 and R21 are low enough to be somewhat insignificant.

Why attenuate? Follow that switching logic a bit forward: the same switching "bus" also drives a switch that toggles the negative feedback loop that goes from tube stage's output to inverting input of the differential input stage. When feedback loop is disabled the gain is higher and input is respectively attenuated to compensate this.

Follow a bit further and you notice that the whole "bus" is driven by inverter built around transistor Q8. From it's base circuit you find another switch logic "bus", which in addition to aforementioned inverter also controls the A/AB bias switching circuitry (circuit around Q4 & Q3 varies cathode resistance of the dual triode, Q3 is basically a switch that connects R41 in parallel to R42).

I'd start by verifying that this entire logic circuit works as intented and that ALL switches in it work. e.g. permanently "stuck" negative feedback could cause excessive attenuation of the signal.

noddyspuncture

Thanks teemuk.... you gave me quite a bit to go on there.
I shall try a few of your pointers and see what happens...!
Cheers,
Tom