Bias in 70's Vox AC30

[Jimmy74]

Working on a 74 Vox AC30 (no reverb), changed out the old PT for a new MM and converted to tube rectifier, at first tried the 220v tap but was getting around 7vAC on the heaters (all tubes inserted except the power tubes) and around 425vDC off the first filter cap. I fitted in the power tubes but noticed that once powering up one of the tubes started to redplate so I opted for using the 230v tap hoping to calm down voltages. This change gave me the possiblity to keep the amp on for a while, getting 344vDC off the first filter cap and 11.2vDC off the bias, but there was still one tube that was redplating. I changed the position of this tube, but this didn't change much, the tube I swapped it with started to redplate too, together with "first" redplating tube. All eletrolytic caps are new, I used 32uf+32uf can caps and 33uf axial caps for the filter stages. The bias resistor is stock 50r 10watt(gives me 51r) and the stock 100r screen resistors give me 114r and the stock 1.5k grid resistors give me around 1.3k. The power tubes are new Gold Lion EL84's. Any ideas what's to be done? Raise the value of the bias resistor or go back to the 220v tap, or both?

thanks
J.

[g1]

Check that you don't have any leakage through the coupling caps from the phase splitter. This would appear as DC on the grids of the power tubes. There should not be any DC voltage on the power tube grids, you can check with the power tubes removed.
If there is none, raise the value of the cathode resistor. With the 50R there, and 344V on the plates, the power tubes are idling around 18W each, you want to get it down around 12W.

[Jimmy74]

Checked the heaters with the power tubes in and I'm getting around 6.4vAC, and I'm getting 0.3vDC off pins 1&2, is this a problem? If not would going to 100r bias resistor be correct?

[Jimmy74]

well I had a 120r 10W resistor left over so I tried that, now I'm getting 370vDC off the first filter cap and around 15.3vDC bias, no more redplating, is it in the safe zone now?

[g1]

Yes. How does it sound?

[Jimmy74]

well I tried it for about half an hour, volume and power seem to be there but I do get hum, which may be heater balance, and I can't get the tremolo\vibrato to work but that may be because without the footswitch it is set to off. The power tubes still get quite hot.

[Alex R]

They do run hot. The hum is likely power tube imbalance. Try pulling out power tubes one at a time, then plugging them back in again after 10-20 seconds. Listen to the hum lessen as the tube heats up. If it goes right down to silent then a slight hum builds after the silence, that's a tube that pulls more current; if the hum decreases but never quite goes away, without coming back up again, that's a tube that pulls less. Do it with all four and get a rough idea of the current draw of each tube, then try to arrange them to balance better. I usually find that I can get something close to hum-free balance from more or less any set of four mixed tubes that way. They are arranged in pairs, right and left.

NB this will heat up the bias on the remaining three tubes each time so watch for redplating.

[Mick Bailey]

I usually find that I can get something close to hum-free balance from more or less any set of four mixed tubes that way.

Spot-on; the irony with some AC30s is that a well-matched set of tubes can hum whichever way you order them, yet a moderately mismatched set can be ordered for almost zero hum. Some amps are more prone to this than others.

[Enzo]

And alternatively, hum comes from many sources. As anyone who has wondered at an amp that hummed more at zero volume than at a setting of 3, if any of those sources are out of phase, they cancel. So if the mismatched power tubes cause a bit of hum one way, if it is out of phase with some other hum in the amp, they will cancel. Thus slightly hummy power tubes results in a less hummy amp.

[Jimmy74]

I think Enzo is right, I tried pulling the power tubes 1 by 1 and the effect was the same with all 4 so I believe they are fairly closely balanced, I notice that the normal channel volume does increase hum and also the bass control

[Alex R]

I think Enzo is right, I tried pulling the power tubes 1 by 1 and the effect was the same with all 4 so I believe they are fairly closely balanced, I notice that the normal channel volume does increase hum and also the bass control

that's cool, though you can tell more from how the hum acts when you plug them back in and listen to them warming up.

[Jimmy74]

I've given it a thorough tryout today, the cleans are perfect especially on the brilliant channel, though I notice that power tube distortion starts up at around three quarters up on all channels, fuzzy sort of distortion that doesn't sound nice, well not to me anyways.

Is this normal on Vox AC30's?

[Zouto]

I've given it a thorough tryout today, the cleans are perfect especially on the brilliant channel, though I notice that power tube distortion starts up at around three quarters up on all channels, fuzzy sort of distortion that doesn't sound nice, well not to me anyways.

Is this normal on Vox AC30's?

It might be some speaker or OT issue, some AC30s tend to harsh fuzzy distortion when driven hard. You also could try a set of 6P14P-EVs (sort of Russian mil specs EL84s) for power tubes, which have a nice round sound and are quite reliable. I put them in different AC30s, from early 60s to contemporary models for real good sound.