Advice please - No sound VOX AC10C1

Gomers

Hi all,

After extolling the virtues of my Vox AC10C1 in another thread, mine has decided this morning not to output any sound, through my pedal board or guitar straight into amp input.  I've done the, "am I being stupid ?" checks but to no avail.  

Powers up fine but that's it and does make the shutdown thump if any gain/master is up from zero.  Any advice would be great, thanks.

Danny1969

I serviced a couple of these recently with issues. I found one had a valve hanging out and one had a spade connector that wasn't gripping the connector. And both needed the pots servicing. 

Take the back off and have a look. Be careful as you remove the back as from memory there's wires attached. 

Gomers

Danny1969 said:

I serviced a couple of these recently with issues. I found one had a valve hanging out and one had a spade connector that wasn't gripping the connector. And both needed the pots servicing. 

Take the back off and have a look. Be careful as you remove the back as from memory there's wires attached. 

Cheers, I'll have a look.  I did have the back off it a couple of weeks ago to have a general look around, don't think there were any wires attached from memory.  It has worked fine since then, I might add 

Amp last used on Fri night without issue.

ecc83

Did not know the amp but, all valves, push pull EL84s it seems? No sound but evidence that the OP stage is working, the pop, as is the speaker. 

If not a valve fallen out as Danny says check the heaters are alight in the PI and first valve. 

Dave.

ICBM

ecc83 said:

Did not know the amp but, all valves, push pull EL84s it seems? 

Not quite, there are a couple of solid state gain stages. One is before the gain control, so a problem there is a possibility.

A loose or failed preamp valve is still more likely though.

ecc83

ICBM said:

ecc83 said:

Did not know the amp but, all valves, push pull EL84s it seems? 

Not quite, there are a couple of solid state gain stages. One is before the gain control, so a problem there is a possibility.

A loose or failed preamp valve is still more likely though.

Heaven forfend IC! You cannot mean I have been told porkies on a mnftcr's site? Might the SState supply have popped? 

Dave.

ICBM

ecc83 said:

Heaven forfend IC! You cannot mean I have been told porkies on a mnftcr's site? Might the SState supply have popped?

No, the transistors (FETs, can’t remember which type) run from the same HT chain as the valves.

As far as I can see they don’t describe it as all valve, it’s very sneaky - they say “a full valve compliment of 2 × 12AX7 preamp values and 2 × EL84 power valves.” They just don’t mention the solid-state bits… which *implies* it without actually lying .

Gomers

ICBM said:

ecc83 said:

Did not know the amp but, all valves, push pull EL84s it seems? 

Not quite, there are a couple of solid state gain stages. One is before the gain control, so a problem there is a possibility.

A loose or failed preamp valve is still more likely though.

Had a quick poke around, when powered up, I can't see any glow from the preamp ECC83 in V2 position.  Valve isn't loose.

jca74

Try swapping it with the valve in V1 and see if the problem follows the valve or the socket...

LPManic

You definitely sure you plugged it in?

Jeremiah

ICBM said:

ecc83 said:

Heaven forfend IC! You cannot mean I have been told porkies on a mnftcr's site? Might the SState supply have popped?

No, the transistors (FETs, can’t remember which type) run from the same HT chain as the valves.

As far as I can see they don’t describe it as all valve, it’s very sneaky - they say “a full valve compliment of 2 × 12AX7 preamp values and 2 × EL84 power valves.” They just don’t mention the solid-state bits… which *implies* it without actually lying .

Just as bad (for the pedants) - this is incorrect use of the word "compliment" - it should be complement.

ecc83

Jeremiah said:

ICBM said:

ecc83 said:

Heaven forfend IC! You cannot mean I have been told porkies on a mnftcr's site? Might the SState supply have popped?

No, the transistors (FETs, can’t remember which type) run from the same HT chain as the valves.

As far as I can see they don’t describe it as all valve, it’s very sneaky - they say “a full valve compliment of 2 × 12AX7 preamp values and 2 × EL84 power valves.” They just don’t mention the solid-state bits… which *implies* it without actually lying .

Just as bad (for the pedants) - this is incorrect use of the word "compliment" - it should be complement.

That bloody Dent woman gets everywhere!

Dave. 

Gomers

Dropped off with the tech, will see what comes back.  I've also asked for all valves to be swapped out if he has stock of decent replacements, they appear to be the factory spec.

ICBM

Gomers said:

Dropped off with the tech, will see what comes back.  I've also asked for all valves to be swapped out if he has stock of decent replacements, they appear to be the factory spec.

You only need to change them if they’re faulty, the factory ones are fine. If he’s changing V1, make sure he uses a Chinese or NOS valve, it’s a cathode follower and Russian and JJ valves have a high failure rate there.

Gomers

ICBM said:

Gomers said:

Dropped off with the tech, will see what comes back.  I've also asked for all valves to be swapped out if he has stock of decent replacements, they appear to be the factory spec.

You only need to change them if they’re faulty, the factory ones are fine. If he’s changing V1, make sure he uses a Chinese or NOS valve, it’s a cathode follower and Russian and JJ valves have a high failure rate there.

Oh ! 

Gomers

ICBM said:

ecc83 said:

Did not know the amp but, all valves, push pull EL84s it seems? 

Not quite, there are a couple of solid state gain stages. One is before the gain control, so a problem there is a possibility.

A loose or failed preamp valve is still more likely though.

Spot on with that thanks, preamp valve kaput.  The 2 EL84s were changed too, matched EH valves, so Russian

ICBM

Gomers said:

Spot on with that thanks, preamp valve kaput.  The 2 EL84s were changed too, matched EH valves, so Russian

They’ll be fine - it’s the V1 valve which is the problem, the second half of it is a cathode follower. Russian and sometimes JJ ECC83s have a high failure rate due to poor cathode-to-filament insulation - but Chinese ones are fine, as are all genuine NOS (ie western European, not 90s Chinese or Russian!) types.

If you want to put an expensive valve in it, a Mullard or especially Brimar (which I’ve found less microphonic than Mullards) would be a good choice.

Gomers

ICBM said:

Gomers said:

Spot on with that thanks, preamp valve kaput.  The 2 EL84s were changed too, matched EH valves, so Russian

They’ll be fine - it’s the V1 valve which is the problem, the second half of it is a cathode follower. Russian and sometimes JJ ECC83s have a high failure rate due to poor cathode-to-filament insulation - but Chinese ones are fine, as are all genuine NOS (ie western European, not 90s Chinese or Russian!) types.

If you want to put an expensive valve in it, a Mullard or especially Brimar (which I’ve found less microphonic than Mullards) would be a good choice.

Is the V1 not one of the EL84s ? https://www.voxshowroom.com/uk/amp/ac10c1_hood.html

Gagaryn

Gomers said:

ICBM said:

Gomers said:

Spot on with that thanks, preamp valve kaput.  The 2 EL84s were changed too, matched EH valves, so Russian

They’ll be fine - it’s the V1 valve which is the problem, the second half of it is a cathode follower. Russian and sometimes JJ ECC83s have a high failure rate due to poor cathode-to-filament insulation - but Chinese ones are fine, as are all genuine NOS (ie western European, not 90s Chinese or Russian!) types.

If you want to put an expensive valve in it, a Mullard or especially Brimar (which I’ve found less microphonic than Mullards) would be a good choice.

Is the V1 not one of the EL84s ? https://www.voxshowroom.com/uk/amp/ac10c1_hood.html

Odd they have numbered the valves that way. V1 is the first pre amp valve - almost always the one closest to the input jack - V2 in that diagram. Their numbering makes no sense!

ICBM

Ah, I forgot that - their numbering scheme makes no sense at all, it’s not even fully backwards with V1 and V2 as the power valves.. V1 in any sensibly numbered circuit is the first preamp valve. I’m guessing it’s this one that failed. Using their numbering, it would be V2.

Airmiles

ICBM said:

Gomers said:

Spot on with that thanks, preamp valve kaput.  The 2 EL84s were changed too, matched EH valves, so Russian

They’ll be fine - it’s the V1 valve which is the problem, the second half of it is a cathode follower. Russian and sometimes JJ ECC83s have a high failure rate due to poor cathode-to-filament insulation - but Chinese ones are fine, as are all genuine NOS (ie western European, not 90s Chinese or Russian!) types.

If you want to put an expensive valve in it, a Mullard or especially Brimar (which I’ve found less microphonic than Mullards) would be a good choice.

Is this the same issue for which Valvewizard advocates a diode? Or something else? https://www.valvewizard.co.uk/dccf.html