Vox AC30 Breaking up Too Soon

[The New Steve H]

A while back I asked for tips on fixing my church's Vox AC30 amps so they break up earlier. Now--do not ask me why--I have been asked to bias them so they break up LATER. The guitarists want as much clean headroom as possible.

It's my understanding that these things are cathode-biased, so you don't mess with the bias. If biasing is not what's making them break up, what is? Is there any way to change it without hacking the amp to pieces?

It's highly possible that there is nothing wrong with the amp, and that nothing can be done to improve it, but I said I would ask.

As long as I have the amp, I think I should add diodes to the rectifier tube to make the amp more reliable.

Unlike the last time I asked questions about these amps, I now have one sitting in the garage, so I will actually put the info to use.

[Firestorm]

Since you have one to look at, which model is it? How old?

[The New Steve H]

AC30CC2

Hecho en China.

[The New Steve H]

I have read that 6p14P-EV tubes could increase the clean headroom.

I don't know how old this thing is, but probably only a few years.

[selloutrr]

due to the PCB it's probably easiest to google AC30CC mods and follow a pictorial of how to, then try to address the schematic. as with any amplifier the tube compliment goes along way toward voicing the amp.

NOS tubes will often give you more headroom then newer production.

how is the tube life of the current tubes? a failing or dying tube will / or can cause pre mature break up.

[xtian]

I have read that 6p14P-EV tubes could increase the clean headroom.

I don't know how old this thing is, but probably only a few years.

Also replace 12ax7 for lower gain. It's probably the preamp that's distorting, right?

[The New Steve H]

RE where the distortion comes from, you got me. I haven't even turned it on yet. Tomorrow, I hope.

I can't even guess how old the tubes are.

[Gaz]

You can't get any more real headroom out of that amp without increasing the power output of the amp. If it puts out 30 watts clean, that's all your gonna get. Now what you can increase is apparent headroom by reducing preamp gain, so that the guitarist can turn up the volume knob more before the amp starts to break up. I think you should explain this to the guitarists, and how it is counter-intuitive to making the amp quieter, which was their first request, right?

[Reeltarded]

They want more gain, and later breakup.

Attack them in the dark with a hammer. Never see you coming if you dress like a ninja, and it's dark.

Make sure it's dark.

[azatplayer]

They do have 2 bias settings. Little switch on the back, run cool or run hot.
If i recall they also have a filtering switch, tight and loose. Theres a few tones in amongst that switching might get them somewhere.

[The New Steve H]

Maybe I should stick a 12AY7 and two of those Russian tubes in it and see what happens.

Are 41N007 diodes a good choice for the rectifier? I think I made a mistake when I put them on my Bassman clone. I read somewhere that they were a little small for the job, so I should use two, but then I realized that voltage was the big issue. I don't think reducing the current through each diode by paralleling them is going to have any effect on what high voltages do to them.

Found the manual online, so I'm reading it now. I have a feeling these guys have never seen it.

[Reeltarded]

What amps are they playing away from Church?

[The New Steve H]

One of them has a Morgan AC20 Deluxe. Don't know about the other guy.

[Weathered]

Unless you're used to working on a PCB amp, I strongly recommend against doing too much work in them. I had a CC2, and replaced a few components in the time I owned it, and the PCB is not the heaviest duty thing I've ever worked on.

As far as increasing headroom, how are they using the channel volumes versus the MV? They can get plenty loud and clean if you open up the MV. It actually sounds better when the MV is over halfway up. It honestly sounds to me like there is some user error going on here - if they're blending channels, they're reducing headroom there.

[The New Steve H]

The amp has a blown fuse. I am suggesting they get familiar with the filter switch and bias switch before they start messing with tubes. I will pass on all the info I get here.

I assume I can examine the old rectifier with an ohmmeter to see if anything has shorted, right? If there's a short, surely it would be obvious on a meter.

I have a good 5AR4 I can stick in there to see if the amp is okay, but I'm not eager to screw up a good rectifier before I know what's going on.

I am suggesting they get replacement tubes to keep on hand; I don't see any point in waiting until old tubes blow.

I probably won't be working on the amp until tomorrow.