VOX AC30CC2 intermittently blowing main fuse

[yann]

Dear MEF members,

I have been given a VOX AC30CC2 that made a buzz followed by main fuse blow.
I have replaced the fuse, removed all power tubes, started the beast with the limiter and checked HT. Everything looked fine.
So I have put a new set of EL84s (I also checked screen resistors) and played the amp with the limiter in place (150W bulb, partially lighting). Sounded as expected.

So I removed the limiter, re tested the amp and it blew the fuse again (with the buzz).
So changed the fuse again, put the limiter back and start to power cycle many time the amp.
I have noticed that sometime the limiter get full bright with sound produced in the HP. This happend also with all pre-amp tube removed. But this is not easy to reproduce...

Now I try to go through power cycles without power tubes but for now nothing happend, so it seems that the amp is intermittently drawing too much current but only with power tubes inserted.
This amp is cathode biased, I have checked voltage on the bias resistor which gave me an average of 25mA through power tubes at Idle, which seems good to me.
The HT fuse never died and do have the right value.

Any idea about what could cause intermittent shorts when power tubes are in place ? I am also a bit worried about repetitive attempt to reproduce the problem (with the limiter) that could damage the output transformer.

Thanks for your great help !

[Pedro Vecino]

Have you tried a new GZ34?

[yann]

No, not yet. The GZ34 has been replaced 2 months ago but I can give a try. Is it possible for it to intermitently fail only when power tubes are drawing current ?
I think I have a couple of new ones in stock (hopefully I will not kill it )

[Pedro Vecino]

Yes, it can happen.
Problems with GZ34´s are common in that model (at least with new production tubes).

I imagine that those 25mA per tube corresponds to a bias setting with 82 Ohms. "Warm" position on the rear panel switch.

[yann]

Yes, this is the 82 Ohms "Warm" position.
I was aware of this common issue but I was thinking the GZ34 will just die like the last time. 2 months is so quick, and the amp is heavily used (running all day long in rehearsal studio).
I will test with a new one and post results accordingly. If it work fine, maybe I can suggest to not use the standby switch anymore ? When I look the schematic I understand that the GZ34 is taking a huge current surge each time this switch is flip to play mode !

[Pedro Vecino]

Yes, it is advisable to leave the standby always in the "on" position and use the "power" switch exclusively.
Bonne chance.

[yann]

When the problem is happening I can see sparks in the GZ34.

[yann]

New GZ34 in place, i have done 3 power on directly from main without blowing fuse (and no sparks within the rectifier). I need to run this test several time as it was not easy to produce the problem with the suspect GZ34.

[Leo_Gnardo]

When the problem is happening I can see sparks in the GZ34.

Sparks? Proof that it's kaput. Throw it in the rubbish bin so you're never tempted to try it again.

Good report on your new tube. Be aware, once the GZ34 has heated up, it takes an extended cool down period for the slow warmup feature to work. I'd say give it 5 minutes to cool before re starting, otherwise you'll be bringing on the lightning storm again.

[yann]

Sparks? Proof that it's kaput. Throw it in the rubbish bin so you're never tempted to try it again.

Yes, this one will go directly to recycle bin.
I was asking myself, does the spark happen only if the GZ34 is defective ? I mean, if something is shorting for example the GZ34 output, could it produce sparks also in this case ?

Be aware, once the GZ34 has heated up, it takes an extended cool down period for the slow warmup feature to work. I'd say give it 5 minutes to cool before re starting, otherwise you'll be bringing on the lightning storm again.

Yes I will, thanks for the advice.

[The Dude]

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.

[nevetslab]

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.

I agree and have modified all of our rental AC30-6/TB amps in that fashion. It was the 6/TB version that was known for mis-wiring the rectifier tube's output. I never stopped to work out re-wiring the standby indicator light thru one of the two DPDT switch contacts so it's off when muting the amp by opening the cathode circuit of the power tubes.

[The Dude]

......I never stopped to work out re-wiring the standby indicator light thru one of the two DPDT switch contacts so it's off when muting the amp by opening the cathode circuit of the power tubes.

I never did either. It probably wouldn't be that difficult to do, but most of the customers I've discussed the mod with don't care about the light as long as they can mute the amp on breaks.

[yann]

Hi Guys, thanks a lot for sharing this. I have double checked and B+ is coming from pin 8 so no problem with this one.
Regarding the standby switch, this one is opening the ground line, so it clearly get current surge when switched to play mode.
I will talk to the customer to know if we wants me to modify it (the way you proposed seems to be fine, opening the cathode ground of output tubes).
If he doesn't want, then I will suggest him to not use the standby switch anymore and keep it in play mode.

[yann]

No more fuse blown since I have replaced the GZ34, after several cold starts. Thanks again for your great help