"Iain M Churches" wrote


Hi Keith and Stu,

Fixed bias actually refers to the negative voltage applied at the grid of
the power valves to prevent them trying to pass infinite current and
destroying themselves. A typical value for an EL34 might be -35V.

When this bias voltage is established, we have a quiescent current through
the valve. Some amps have a built-in ammeter, to measure the current.
When
using an external DVM it is a lot safer to measure a low voltage across a
fixed
resistor of known value between cathode and ground.

So, in your amp you have a resistor of 10 Ohms. That means, if you need a
quiescent current of 31mA you will see a voltage of 0.31V (i.e. 310mV
on your meter) when the bias is correctly set.. You probably have
four separate wirewound pots to adjust, one for each output valve.

I don't know what the plate (anode) voltage is on your amp, but 31mA
seems a little on the "lean" side to me. My own EL34 pp amp has a B+
of 425V, and is biased at 2V across a 39 Ohm resistor, and so has 51mA
idling current. You may have a higher anode (plate) voltage.

Set the bias after the amp has been switched on for about an hour, and
check it again after another couple of hours.

I have a pal who reckons he can hear the sweet-spot and set the bias by
ear. His amp sounds good, but get gets through a lot of valves:-)
Not recommended:-)



OK Iain, that's good stuff. Have a look at this pic:

http://www.apah69.dsl.pipex.com/keit.../valvebase.jpg

and could you explain what the figures represent and how they relate to your
explanation above.

What does the following stand for:

Ua

Vg2

Vg1

Ia

S

and what do the Voltages tell us? Do they give us the plate voltage. Is that
an amp design thing or a valve design thing?

My guess would be that the bias can be set at 36mA (?), but what does that
handwritten figure mean? It's obviously taken from measuring the valves. All
four valves have the same figure and are presumably 'matched'???

On the other amp the figure on each of the valves is 27mA. Does this mean
there is a different bias figure for each amp?

Other pix on:

http://www.apah69.dsl.pipex.com/keit...chineseamp.htm


Also, what is the best way to set the bias - mA or Volts?

I guess what we (me and Stu) are looking for is the *correct* figure to set
the bias at and the best way to do it? (You are correct about the adjustment
pots, btw...)

Apologies for my iggerance - I'm new to this game, know bugger-all (never
claimed any different) and only learning as I go along! ;-)

Many thanks.