Valve amp bias

[karma67]

Anyone near Brighton that can do this please ?

[Firebottle]

Might be helpful to say what amplifier.

You might be able to learn how to do it yourself, are there accessible controls and test points?

[karma67]

Hi Alan,it’s a kt88 as in my sig, John wood told me the bias is set at 30 me but I can’t for the life of me remember where he showed me to put the meter probes, I know roughly, I’ve emailed him to ask but thought I’d see here as well. I will post a close up pic later if that helps.

I’d like to check the bias because I seem to have a slight drop in gain and after he adjusted the bias last time it sounded great, the power valves according to him were about 63% but I wonder if some new ones might help?

[karma67]

I should add it’s a fixed bias and it’s adjusted by a small screwdriver .

[Firebottle]

The little blue 'trimpot' is where the adjustment is made, as you say.

I am assuming there is just one adjustment as you have said it is fixed bias, as opposed to individual bias adjustments for each valve.

The only drawback with fixed bias is that as the valves age they can become unbalanced, that is have different conduction.

[karma67]

Each valve has the blue adjustment pot, it’s knowing where to put the red and black bolt meter probs that I’m unsure about.i will post a better pic when I get home, I’m aware of the huge voltages hence why I’d like some advice first

[karma67]

here's a better photo,


and close up.

[Mixc]

Typically you measure the millivolts across the resistor, this gives the current in the circuit.

[karma67]

thanks,you mean the brown one or the blue one just behind the capacitor?

[Firebottle]

Second photo shows the sense resistor. It is 1 ohm, Brown, Black, Gold, Gold.

Connect the meter across each of these in turn and adjust the respective trimpot.
For 30mA you will measure 30mV (millivolt) on the most sensitive voltage range on the meter.

Job done