biasing an AC30

[guitaraid]

Mike Kropotkin sent me a note stressing that the NOS Teslas should be biased for 15-16 watts MAXIMUM each.....

At 47mA, they were putting out almost 24....and their maximum is 25.....pretty dangerous to the plates!

Hi guys,
Just found this forum. I'm new here, so Hello to all of you.

47mA through cathode resistor means one valves takes roughly 11-12mA.
So 12mA and 470V gives less than 6watts for each valve. Am I right?
It's OK for A class, I guess. No worries you don't bias them at their maximum power.


Regards,
Greg
www.guitaraid.co.uk

[slideman82]

Hi guys,
Just found this forum. I'm new here, so Hello to all of you.

47mA through cathode resistor means one valves takes roughly 11-12mA.
So 12mA and 470V gives less than 6watts for each valve. Am I right?
It's OK for A class, I guess. No worries you don't bias them at their maximum power.


Regards,
Greg
www.guitaraid.co.uk

Hi Greg! I've just read this and it seemed interesting to me, I mean, your analisis. But, the fact is, the AC50 maybe was biased it just to work near class A operation, and that's why the primary impedance of the OT is 3,5K not 5,6K or similar, like others. Take a look at the resistor values, they are not quite common.

So, the 47 mA through EL34 could be a correct value, but maybe not very safe. Some guy told me that EL34 in class A get very hot.

[guitaraid]

Hi Greg! I've just read this and it seemed interesting to me, I mean, your analisis. But, the fact is, the AC50 maybe was biased it just to work near class A operation, and that's why the primary impedance of the OT is 3,5K not 5,6K or similar, like others. Take a look at the resistor values, they are not quite common.

So, the 47 mA through EL34 could be a correct value, but maybe not very safe. Some guy told me that EL34 in class A get very hot.

Well I was referring to AC30, not AC50, where there is only one 50 ohms resistor for all valves. Not one per valve as it is in AC50.
And you are right - there is much higher idle current in class A, as valves are always 'on'.

Cheers

[slideman82]

LOUD and CLEAN

Yeeeeeah! that's what I'm talking about! In MY theory, amp must sound clean, and you make distortioned sounds from the floor ----> stompboxes!
unless you want output tubes distortion. If you want more dirt in your AC50, just remove the 220k resistor from the whipper of the treble pot (ok, remove it, and then solder that cable to the whipper). That resistor atenuates the signal from the preamp, so the amp sounds clean. The use of a 12AU7 as 1st stage contributes to that, a 12AX7 will make it dirtier, try it, also 12AT7, etc.

[slideman82]

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.

My bad, dog! I took a look to the 1976 AC50 schematic, and it says 1.8V over the 47 ohms resistors, so, that's about 39 mA, about 18W of dissipation. I think that's the right vlaue, not 2.2 V. Regards

[TD_Madden]

I had to drop the wall voltage down to 115 to get the heater voltage to be 6.3vac (supposedly critical with these amps.), In doing so, I can't get the output tubes to bias at 2.2v anymore, just 1.7v..bcause I run out of adjustment on each bias pot.

What can I do to get the adjustment back...add a resistor to each bias pot?

[TD_Madden]

Problem solved. Replaced R23 (12Kohm) with two paralleled 15Kohm (about 6.5Kohm in-circuit)

Uisng the Browner, the amp is getting about 115vac, I have 6.3v on the heaters, AND can get the 2.2vdc bias per spec.