Setting bias using amp head bias probe

[frank57]

I'm confused by this amp head bias probe.
http://www.amp-head.com/product_info.php?cPath=21&products_id=44
I set it at 10k like the original resistor that was in the amp(adjustable bias pot I put in el84 tubes)
I getPV 362 and current .044 on Dc.
What exactly is the dissipation on this thing?
Is it biased cold?
1mv=1ma according to their pdf.


If I go higher I get 388 and .021.

[stratele52]

What is the 10 K doing with this bias probe ?

Anyway you set your tester to read less than 100 millivolts DC ( 0.100 mv ) and plug it to bias probe. That's it.

You read 362 volts plate voltage and you adjust bias to 44 mv (ma)  . Tooooo hot. Not more than 22 MV (ma) for class A amp . What is the amp  ?

Look there , this will help you;

www.webervst.com/tubes1/calcbias.htm

[frank57]

It's a troublesome Hiwatt custom 20 tube amp.
I have an adjustable bias pot in there which I set at 10k.
I used an autoranging meter set on dc.
So .044=44ma?
So as originally sold this amp was set too hot?
Not sure if it's class a or class Ab.
But either way it's too hot?

[stratele52]

Most Hiwatt are class AB and this is the case for your amp if it as an ajustable pot . Class A amp don't have that.

EL 84 is a 12 watts tube.

Your bias ; 362 volts X  44 ma = 15.9  watts

What do you think ,is it  too hot ?

[frank57]

The adjustable pot was put in by a tech and I subsequently moved it.
The original amp was a fixed 10k resistor.
The first tech who saw it said it was Class A.
The second tech said the bias was good.
The third guy put in a pot adjusted it super cold to eliminate the buzz problem,
 for which I got a refund.

According to Hiwatt it's class ab but that makes no sense given the voltages at 10k?
They also said it was cathode baised.It is not.
If it's class ab, you would have to adjust each amp individually right?
The fixed resistor would have to go up to something like 14k.
Otherwise the transformer is plain wrong.

[stratele52]

The adjustable pot was put in by a tech and I subsequently moved it.
The original amp was a fixed 10k resistor.
The first tech who saw it said it was Class A.
The second tech said the bias was good.
The third guy put in a pot adjusted it super cold to eliminate the buzz problem,
 for which I got a refund.

According to Hiwatt it's class ab but that makes no sense given the voltages at 10k?
They also said it was cathode baised.It is not.
If it's class ab, you would have to adjust each amp individually right?
The fixed resistor would have to go up to something like 14k.

Otherwise the transformer is plain wrong.

Frank57,

Not easy to understand what you wrtite, I'm french speaking Canadian.

Just I can tell you is with what you write before ( 44 ma X 362 volts ) bias is to hot and will blow your tubes.