AB to A?

[a91gti]

I've been working with electronics for some years, mostly hobbying but I spent three years fixing microwaves down to component level. I've a fair ability with an iron and do indeed have a meter.
Outside of replacing components with similar I'm good at following instructions but the theory side escapes me.
I've just remembered the note on the copies I have of the circuit that include the new bias pots.

"Bias controls RV1 and RV2 are adjusted to give 0.35v across monitor resistors R66 and R65 respectively. This equates to a valve current of 35ma."

[Richard]

Hi Andy, well that's excellent then! You should be fine so long as you respect the voltages in there

That extra info is good too, possibly the resistor numbers have been changed when he added the bias pots. That R66 and R65 will be R86 and R85 I think. It is a common way of checking the current through the valve,
Ohms Law so, 0.35V divided by 10R = 35mA

Have a look at those extra pots you mentioned. I guess they will be to set the amount of negative bias; you'd wind them up or down whilst monitoring that nominal 0.35V across the 10R resistors to set more or less current flow through the valve.

Meter VDC from pin 3 of the EL34 to chassis to find the HT or plate voltage, probably around 400V. Multiply that by the current to find the plate disippation, 400V x 0.035A = 14W

If that is the case the valves are running pretty gently and given the low driver signal voltages likely from that triode wired EF86 I would guess it's running class A already.

If you've a meter put fairly large approx 8R resistors across the output terminals and fire it up on the bench upside down, base off, no signal voltage input, so you can check voltages. Mark these on the circuit.

[a91gti]

Thanks Richard.
I'll be back to this once I've got hold of some resistors I guess or would I be as well leaving my speakers connected and taking the voltages?

[a91gti]

Just had a stroke of luck. I hadn't noticed before but the copies of the circuit I have (post new pots) have the voltages on.
I'll transfer them onto the ones I have in pdf while at work tomorrow.

[Richard]

Originally Posted by a91gti

Thanks Richard.
I'll be back to this once I've got hold of some resistors I guess or would I be as well leaving my speakers connected and taking the voltages?

You could but probing can cause some pretty un-nerving noises, maybe use some old speakers if you have any rather than risk your good ones. Load resistors are always useful if you want to check/test/set up in future, Maplin do 10W wirewound in 8R2.

Originally Posted by a91gti

Just had a stroke of luck. I hadn't noticed before but the copies of the circuit I have (post new pots) have the voltages on.
I'll transfer them onto the ones I have in pdf while at work tomorrow.

Great stuff, almost there now then, should be quite straightforward if you have those.

[a91gti]

I'll save my Kan's then!