"Keith G" wrote in message
...

"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.




Greetings Gents!

The pic above confirms our thoughts so far:

What does the following stand for:

Va
Plate (anode) voltage - measured anode to cathode.
which, in your case it is purely academic as the cathode is less than a
volt above ground. If it were cathode bias, then the ground to cathode
potential would be considerably more, and need to be taken into
consideration. So to all intents and purposes the anode is 430V
above ground. (just for reference, my amp is 425V)


Vg2
Screen grid voltage


Vg1
Input grid DC voltage, i.e. the bias. At -36V this is mighty close to
the -35V I thought you might find:-)

There are actually three grids, but g3 (pin 1) is usually tied to
the cathode (pin 8) Take a peep:-)


Ia
Anode current. i.e. the idling current flowing through the valve with
no audio present. This is the figure we are trying to achieve when we
set the bias.

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?

Yes. They tell us everything we need to know:-)
To borrow a quote from a member of another group
"Ratings are for transistors....tubes have guidelines"

There are many sets of operating conditions for valves in push pull,
depending on the circuitry, fixed bias, cathode bias, triode connected,
UL etc etc,


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'???

Yes. Manufacturers/builders usually match power valves in pairs, but one
does not always see a label attached:-)
That's good attention to detail.

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?

Yes presumably.

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

Do you have a set of test points to measure the voltage across the 10 Ohms
cathode resistor, cathode to ground? If you have, then that is the way to do
it.
So set the bias to give you 310mV on your meter then you will have 31mA of
current flowing.

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...)

The manufacturer has been kind enough to supply that information precisely
for you on a label attached to each valve.

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

That's one of the fascinations about valve audio, it's a "hands on" hobby
(though hopefully not hands on 'lytics or anodes:-)


Cheers
Iain