Biasing pots and where/how to measure

[martinswimmer]

Would anyone know how would I go about biasing the valves of the amp as per photographs. I believe that the area with little pots is what I'm looking for, just not sure what to connect the multimeter to.

Cheers!

[Blackdog]

First of all be very careful. LETHAL voltages inside this amp. If you don't know what you are doing, find someone who does.

Now that the friendly warning is done

If you look near the output tubes you will see 10 ohm resistors connected to pins 1,8, and to ground. That is measurement point. You can us any common ground. A black binding post will work, or the bare copper wire. Measure from the pin1 side. Your measurement in Volts DC divided by 10 (10 ohm resistor) will give you the current through the tube. Adjust the associated trim pot ( they appear to be labelled) to get the manufacturers recommended voltage or current.
If you don't have the recommend bias information then it will need to be calculated. For that more information is needed.

[martinswimmer]

First of all be very careful. LETHAL voltages inside this amp. If you don't know what you are doing, find someone who does.

Now that the friendly warning is done

If you look near the output tubes you will see 10 ohm resistors connected to pins 1,8, and to ground. That is measurement point. You can us any common ground. A black binding post will work, or the bare copper wire. Measure from the pin1 side. Your measurement in Volts DC divided by 10 (10 ohm resistor) will give you the current through the tube. Adjust the associated trim pot ( they appear to be labelled) to get the manufacturers recommended voltage or current.
If you don't have the recommend bias information then it will need to be calculated. For that more information is needed.

Wow, what a great answer! Straight to the point! Many thanks for this. I don't have the values, but can at least compare them. It's 4 EL34s that I have in there.
Cheers!

[Blackdog]

You can't just compare to another amp, but we can calculate a reasonable value. First thing we need to know is the plate voltage. This is measured at pin 3 of one of the EL34's. This could be as high as 500VDC, so be very careful. Once we know that then we can calculate the required safe current.

What make is the amp? Or do you have a schematic?

[martinswimmer]

You can't just compare to another amp, but we can calculate a reasonable value. First thing we need to know is the plate voltage. This is measured at pin 3 of one of the EL34's. This could be as high as 500VDC, so be very careful. Once we know that then we can calculate the required safe current.

What make is the amp? Or do you have a schematic?

Hi Dan, The make is Diva Audio by TS Lim - the model is Blue Sky - this is one reference http://www.echoloft.com/soulofmusic/divafaq.htm

I'll measure the plate voltage and report back ;-)

Thanks!

[Blackdog]

Boy, they're not much on information on the that website are they?
You could also just email them and see what the recommended bias setting is.

[CornishPasty]

I used to have one of Lim's original AVC EL34 amps and the output sections looks pretty much like the OP's Diva. Bias was stated as 39mA but I used to set mine at 36mA. If I can find my original circuit diagram I may be able to confirm the anode voltage.

[martinswimmer]

I used to have one of Lim's original AVC EL34 amps and the output sections looks pretty much like the OP's Diva. Bias was stated as 39mA but I used to set mine at 36mA. If I can find my original circuit diagram I may be able to confirm the anode voltage.

Thanks for this. I haven't had a chance to do the measurements this weekend, but will confirm what are the findings ;-).

[martinswimmer]

I used to have one of Lim's original AVC EL34 amps and the output sections looks pretty much like the OP's Diva. Bias was stated as 39mA but I used to set mine at 36mA. If I can find my original circuit diagram I may be able to confirm the anode voltage.

BTW - Are you sure that T S Lim of Singapore has anything to do with AVC?
http://www.audiosalon.com.au/Diva_Audio.html
http://www.enjoythemusic.com/magazine/equipment/0601/divaseduction.htm

[CornishPasty]

BTW - Are you sure that T S Lim of Singapore has anything to do with AVC?
http://www.audiosalon.com.au/Diva_Audio.html
http://www.enjoythemusic.com/magazine/equipment/0601/divaseduction.htm

Thats a good point and I'll admit that when I saw the name Lim I just made an assumption. However, looking at your amplifier it looks pretty much like most Chinese made EL34 amps. I'll still have a search for the diagrams tomorrow though.

[bumpy]

Anyone owning amps containing hum pots should give them occasional attention. Once pots are set then they are often not touched for many years. Indeed mine were a bit scratchy in the speakers when being adjusted.

Treatment with Deoxit did the trick. A huge benefit is that the residual hum in the speakers can now be adjusted to almost zero.

[martinswimmer]

Finally got round to measuring plate voltage and the bias - it seems to have been waaaaaay off!
I just hope that the resistor is indeed 10ohms! The amp is AB.

Plate voltage 470V
I re-biased to 31.6mv

The values were:

1 was 41mv
2 was 45mv
3 was 44mv
4 was 35mv

[martinswimmer]

31.6mV across a 10 ohm resistor means there is 3.16mA flowing through it. I have no idea what the circuit is but 3mA quiescent current sounds a bit lower that what is typical.

This bit makes no sense to me:

41mV plate voltage?? :scratch:

No, I took off the plate voltage because it was 470V and forgot to delete "plate voltage" bit.
CORRECTED!

I used a calculator for biasing - so picked 3.16mA - would you say that's too low?

[martinswimmer]

Yes, round about 30mA sounds like a typical value. 3mA? Sounds very low.
I suspect the OP is measuring something else entirely, though I could be wrong. Without a circuit diagram, or some brave soul with a lot of spare time to trace out the circuit, there's too much guesswork here.

I divided the value by 10 - so 316mv

[martinswimmer]

Common ground.

[martinswimmer]

31.6 / 10 = 316 :scratch:

316 / 10 = 31.6 :eyebrows:

In any case, sounds much better now.

[martinswimmer]

I was trying to be helpful but you seem unwilling to cooperate with any useful information, so I'm finished now.

?? Not sure how I offended you? I measured 316Mv, divided that by 10 and got 31.6Mv.
The same when I measured the initial voltage. This is the first time I've done it, so probably not using right terminology.

[Radford Revival]

It looks as if that amp has no protective earth connection. If this is the case this is a far more urgent issue to rectify. Any wiring fault or insulation breakdown in the transformer will make the chassis live.

It also looks like the fuse is located between the two 120V windings which are in series, and not on the live. This is less of an urgent issue but still a problem.

It also looks as if the power switch is implemented with a TRIAC which is a bit dodgy especially given the fact that it's hanging off the live pin of the IEC inlet with no other support than solder. If am correct on both these counts the amplifier shouldn't really be used in its current state.