VOX AC30CC2 intermittently blowing main fuse

[yann]

The customer call me back today. After nearly 3 weeks of use the thing don't power up anymore... Despite they are not using the standby switch anymore.

I did not see the amp yet but I bet another GZ34 failure... The 2nd JJ. I may switch to another brand or mod the thing to go with SS rectifier

[nevetslab]

The customer call me back today. After nearly 3 weeks of use the thing don't power up anymore... Despite they are not using the standby switch anymore.

I did not see the amp yet but I bet another GZ34 failure... The 2nd JJ. I may switch to another brand or mod the thing to go with SS rectifier

If you DO mod the rectifier circuit with diodes. use a pair of series-connected 1N4007's, avoid the 5VAC winding, and now USE the Standby Switch.

[alexradium]

I've seen several of these where the rectifier tube is wired wrong. B+ is taken off the wrong tube pin and actually goes through the filament. Make sure the red B+ wire is on pin 8 and not pin 2.

Another thing I don't like about these amps is that standby works by effectively removing AC from the rectifier. Every time you go to play mode, the caps have to charge, which is harder on the rectifier. I'll often rewire the amp so that AC is always on the recifier and use the standby switch to interrupt cathode ground for the output tubes. You'll need to talk to the customer first, as it does render the standby indicator light useless. It's a rectifier saver in an amp that seems to go through rectifier tubes more than most amps.

The Fender trick to save the rectifier was to leave the main capacitor always on and the switch to the rest of the circuit,choke,OT etc. so you charge it slowly at startup and doesn't shock anything.

[yann]

If you DO mod the rectifier circuit with diodes. use a pair of series-connected 1N4007's, avoid the 5VAC winding, and now USE the Standby Switch.

Before doing the mod I need to check how much the HT voltage will increase. I will have a closer look at the schematic as there is some ICs in the amp and I need to check how they get their power supply. I also need to check the filter caps voltage rating to be sure they can handle the voltage increase.

[g1]

Before doing the mod I need to check how much the HT voltage will increase.

There are plug in SS rectifier modules that have sag built in so there is no voltage increase. I think the weber copper cap is like this.

[Chuck H]

There are plug in SS rectifier modules that have sag built in so there is no voltage increase. I think the weber copper cap is like this.

Just want to add another to this one.

[yann]

There are plug in SS rectifier modules that have sag built in so there is no voltage increase. I think the weber copper cap is like this.

Yes I know but really difficult to source in France (for now, I only found source from USA with delivery cost no less than 50$).
I will give following options to my customer :
- Replacement of the GZ34 by a NOS one and disabling the standby switch.
- Replacement of the GZ34 by a weber copper cap
- Mod the thing to use diode rectification (I have checked schematic, no issues for ICs as their supply is coming from zener diodes so no problem for the +/- 15V and +/-30V rails. I will just need to adjust cathode bias resistor a bit to avoid cooking the EL84 which are already hot biased).
He will compare the prices and make his decision.

[Enzo]

The plug in modules are essentially two diodes and a resistor. You could make one yourself on an old tube base.

[yann]

The plug in modules are essentially two diodes and a resistor. You could make one yourself on an old tube base.

That's a very good idea, I will do it as a matter of test because I really want to test this ! I guess the resistor is used to emulate the voltage drop of the GZ34.

[Chuck H]

Using Duncan PSUD2 it looks like a pair of 1n4007 diodes and a 30 ohm resistor would get pretty damn close to the stock GZ34 voltage drop. One issue might be the resistor. It'll be dissipating about 7W but if you conceal the thing it's not going to dissipate heat as well as it would mounted upright in open air. So bump the wattage rating to 20W to account for derate. But then there's the voltage rating. The typical "cement block" or "sand" type resistors, which are actually ceramic, are usually rated around 350V IIRC and the ripple peaks in an AC30 go a little over that consistently. So... Use two 15 ohm/10W resistors in series for this thing. I think that'll work. Corrections accepted.

[Dave H]

I wouldn't worry about the resistor voltage rating. It will only be dropping around 20V

[Enzo]

Agree^^^

The resistor doesn't connect to ground.

[nevetslab]

Using Duncan PSUD2 it looks like a pair of 1n4007 diodes and a 30 ohm resistor would get pretty damn close to the stock GZ34 voltage drop. One issue might be the resistor. It'll be dissipating about 7W but if you conceal the thing it's not going to dissipate heat as well as it would mounted upright in open air. So bump the wattage rating to 20W to account for derate. But then there's the voltage rating. The typical "cement block" or "sand" type resistors, which are actually ceramic, are usually rated around 350V IIRC and the ripple peaks in an AC30 go a little over that consistently. So... Use two 15 ohm/10W resistors in series for this thing. I think that'll work. Corrections accepted.

I'd remove the two 5VAC yellow heater leads from the rectifier tube socket (cap them off), which will now allow space for the vertical-mounted dropping resistors in your plug-in SS rectifier module.

[Chuck H]

I'd remove the two 5VAC yellow heater leads from the rectifier tube socket (cap them off), which will now allow space for the vertical-mounted dropping resistors in your plug-in SS rectifier module.

Y'know, that makes me think... Why leave the amp stock if it doesn't work stock? So, yeah, build the SS rectifier with the dropping resistor inside the chassis and then screw a metal plate over the rectifier tube socket so no one ever tries to plug a tube in there. Then the amp works and there's a lower probability that someone will try something incorrect with the leavens'

[yann]

Y'know, that makes me think... Why leave the amp stock if it doesn't work stock? So, yeah, build the SS rectifier with the dropping resistor inside the chassis and then screw a metal plate over the rectifier tube socket so no one ever tries to plug a tube in there. Then the amp works and there's a lower probability that someone will try something incorrect with the leavens'

That's would make sense. I know very well the owner of the rehearsal studio, he just wants a reliable solution and doesn't want to have the amp serviced every month. I will just make the mod in a way so the amp could easily be reverted back to stock in the case they want to sell it.