biasing an AC30

[JC@]

Is there a prooved and safe point where to bias these amps?
I mean how many mA per tube without loosing the pseudo class A character of the amp without burning tubes in six months.
Thanks

[mytco]

Not too technical an answer but... I once worked on an AC30 for a guy who was concerned about tube life so I increased the value of the bias resistor to cool it down a bit.

The guy was instantly complaining that the amp lost some magic or as he put it "kraaang". I put it back to stock and he was most happy. FWIW.

[Liam]

The Vox schematic calls for 10 volts at the cathode resistor, which is a bias current of 0.2 A overall, or 50 mA per tube. I have just sold my AC30, but with JAN Phillips NOS tubes in it I gigged regularly for years on the same set of power tubes.

I always ran with the standard cathode resistor, in fact it might have even been a 47 Ohm. I was told the tubes wouldn't last, they didn't. Then I connected the speakers to the correct windings on the OT (Doh!) and had no further problems.

Liam

[JC@]

The Vox schematic calls for 10 volts at the cathode resistor, which is a bias current of 0.2 A overall, or 50 mA per tube. I have just sold my AC30, but with JAN Phillips NOS tubes in it I gigged regularly for years on the same set of power tubes.

I always ran with the standard cathode resistor, in fact it might have even been a 47 Ohm. I was told the tubes wouldn't last, they didn't. Then I connected the speakers to the correct windings on the OT (Doh!) and had no further problems.

Liam

Liam, I had 344 volts on the plates with the stock 47 ohms resistor.
So the power transformer in this amp I worked in had a few more windings I guess.
I have changed the cathode resistor to 60 ohms and now is more or less where it should. Hot, but not that much.
This amp has two 8 ohm speakers, that I connected in series to the 8 oHm tap. The other way would be 4 ohms but the transformer only has taps for 8 and 16 ohms. Do you think this is ok?

[Liam]

I'm never all that sure about mis-matching speaker taps on OTs. I'm kind of hoping that you meant you connected two 8 ohm speakers in series to the 16 ohm tap. As far as I can recall that was standard AC30 wiring and worked just fine for me. If you mis-match the speaker impedance you will be giving the OT and the valves a load of work they don't need. How much bias current is it running at the moment? (Old AC30s take a LONG time to fully warm up. I always check cathode voltage after 15 minutes. They are a 25 watt amp for the first 10 minutes after turning on IME.)

[JC@]

Yes that's right, two speakers in series to the 16 ohm tap. Blue wire.
I'll double check what you mention about measuring current after 15 minutes or so.
Many thanks for your advise.

[TD_Madden]

sorry to hijack....bit it's related! (somewhat)

I've got a '66 AC50 coming from the West Coast and I'm interested in a similar pursuit...best bias range. I've even purchased some NOS Tesla (NOT JJ) EL-34s and sure want to take advantage of the under-rated TONE of this baby.

Recommended bias, given this scenario? Problem areas to look for? I was thinking about an Avatar 2x12 cab with Celestion Golds....

Thanks in advance.

[Liam]

Also related but not the same - had a fixed bias Hiwatt Custom 100 until recently, and with modern tubes in it (Sovtek EL34) it would "run away" really badly. Lost a couple of power tubes that way. I put in a voltage doubler and a pot for bias voltage adjustment, and biased it at 36 mA per tube IIRC, and it behaved fine after that. It was the hottest I've ever biased EL34s, but it sounded quite good. I sold it about a month back, and the new owner is going back to the original bias circuit. He's got some NOS Mullards, which I think is the only way you'd get away with it.

If you're interested I can get him to check what the actual bias current was on that amp. With adjustable bias it's quite interesting to bias as cold as you can, and then bring up the current until it sounds sweet. You'd be amazed how low that can be on some amps.

[JC@]

I've got a '66 AC50 coming from the West Coast and I'm interested in a similar pursuit...best bias range. I've even purchased some NOS Tesla (NOT JJ) EL-34s and sure want to take advantage of the under-rated TONE of this baby.

I don't have much experience with this amp, although according to the schem it seams a bit on the outer safe edge for modern production tubes.
You will see 470 vots on the plates and 450 on the 2nd grids.
According with vox you should bias to measure 22 volts dc across each 47 ohm cathode resistor.

*Real* nos Teslas sound nice but you shouldn't bang them while they are hot. Nos Teslas didn't have a rounded glass top like JJ's and such. *Real* nos Svetlanas are more sturdy and stand high voltages better.

[Alex R]

I see a lot of older AC30s and imo not to mention ime they are running too hot at 50ma per valve for modern production valves to cope easily - even JJs. 65 ohms tames them a little without too much de-kranging. Change the bypass cap too if it's old; even at 11 volts I once found one conducting DC and heating things up.

While I'm at it - if something isn't sounding quite right, and you see any little black cylindrical Hunts caps in there, suspect them first. I rarely have checked one good, and now change them on sight.

[JC@]

I see a lot of older AC30s and imo not to mention ime they are running too hot at 50ma per valve for modern production valves to cope easily - even JJs. 65 ohms tames them a little without too much de-kranging. Change the bypass cap too if it's old; even at 11 volts I once found one conducting DC and heating things up.

While I'm at it - if something isn't sounding quite right, and you see any little black cylindrical Hunts caps in there, suspect them first. I rarely have checked one good, and now change them on sight.

Yes I did put a 60 ohm resistor and it was still a bit hot but as I had 344 volts on the plates I didn't loose much kraang if any. This amp's power transformer was definetely hotter than others.
All cathode bypass caps changed including of course the 220/25 one.

[TD_Madden]

think that was 2.2vdc across each 47 ohm reistor on the AC50 :-)

Received it today and it runs HOT HOT HOT, even with the 2.2v bias....and with "real" Tesla EL34s.

[Liam]

But more importantly, what does it sound like?

[TD_Madden]

LOUD and CLEAN

[TD_Madden]

have to track down some excess hum in the "brilliant" channel as well......