"Keith G" wrote in message
...

"StuH" wrote in message
om...
Hi, first post!

First off welcome!


You'll like it here - it's a regular 'home from home' for UK valvies!!
:-)


Im new to valve amps and decided to purchase one of
the afore mentioned as a cheap intro (ok, have a play with), Im
pleasantly surprised with what it offers.


So far, so good then! ;-)


Ive made a decent input
selector and had a good snoop around inside.


Heh heh? What is it with valve amps that makes people want to get the lid
off as soon as they've got them??


My only prob is how do I
bias the valves?? Ive no instructions on this. There are test points
at the side of the valve, I tried to get a DCV (350mV), and a
resistence (10ohms, I think) but TBH Im stuck, can someone help me
with this one???


Right, I've been studiously avoiding facing up to one this ever since I
got into valves - due to having mostly cathode bias amps and a pair with a
test socket which clearly states 1.56VDC!! :-)

I don't yet fully understand the relationships between the voltage and
current readings myself yet. (I'm working on it, or will be in the v. near
future!) I have 2 of these amps and measured them as follows - both amps
have a resistance of 10R at the test point, power off and Amp No. 1 (with
valves marked '1a: 31 mA') seems to have a bias voltage of about 0.31V
(-0.31V with the leads the other way round I guess) and Amp No.2 (with
valves marked '1a: 27 mA') seems to be at the 0.33V mark - although this
is yet to be checked properly.

Someone here (Wally?) offered some 'correct settings', but I'm going to
have to do this all from scratch and try to understand completely that
which I only know in the broadest terms atm. So I'll be posting here as
well as worrying the arse off 'someone' offlist....!!! ;-)






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:-)

Iain