Sorry, Claus, I forgot to specify that Mr. Turner is right when he says that
it would be better to use a lower dissipation ( - a higher, more negative)
bias in a guitar amp. In a push-pull pair of valves operating in AB1 class
You actually get MORE power out if You reduce the idle current (increase the
bias), because efficiency increases. The counterpart is an increase in
cross-over distortion, because the "overlap" between "push" and "pull"
phases becomes smaller and the output transformer has its troubles to tie
the 2 halves together without leaving a "notch" in between. This is why in
HiFi use the PP pairs work closer to class A, while Marshall amps dedicated
to heavy metal headbangers use nearly class B biasing.
A further advantage of a higher bias is that the tube runs cooler and last
longer.
Anyway, I'd plug in 2 new tubes, 'cause 26 mA @ -14V bias and 290V are a bit
low
By the way, are the tubes connected in UL or in pentode? And if they're
pentode, how high is the G2 voltage? If it is lower than specified it will
reduce dramatically the tube efficiency! Maybe there's only a higher R in
the G2 supply line...

Ciao again

Fabio


"Claus Misfeldt" ha scritto nel messaggio
...
I'm about to bias one of my guitar amps. By now I used the "crossover
notch"
method (...and it worked for me), but as this is said to be inaccurate I'm
willing to improve on that.

I've read

http://www.tonelizzard.com
http://www.aikenamps.com
http://aga.rru.com

on this issue, but still have a question. The reason on biasing output
tubes
is afaik to operate the tubes in the appropriate range and not drive them
beyond their limits. The important parameter is the max. plate dissipation
of the tube which must not be exceeded. I've read that in a class AB amp
the maximum value of the plate dissipation is "normaly" reached at an
output of about 1/2 of the nominal wattage. So it is considered to be a
good rule of thumb to adjust the idle current so that about 70% of the
max.
allowed dissipation is reached.

But: Is it really correct to multiply plate current and plate voltage to
determin that bias point?

In my case the amp has:
- dual 6V6GT tubes
- 305V Plate voltage
- 26mA Plate current per tube
- a kathode resistor of 270 Ohm

The date sheet for the 6V6GT says: max plate diss. 12W; idle current 70mA
at
a plate voltage of 285V in a class AB amp (Bias at -19V). Now 70mA x 285V
are about 20W!?!

What I read in my amp is 26mA / Tube at -15V bias (26mA x 305V = 18,3W)

That confuses me...
Claus



--
Claus Misfeldt
www.frozenfrog.de