Bias voltage/ma problem

[loupy31]

High guys/gals,

I am having some odd voltage/milliamp readings on my TWEx.
Please see attachment, this is the schematic I have used for my build,

All voltages are within a couple of volts on average, my bias voltage with R29 150k, is -33.8
the milliamps per tube is about 12 ma with the bias pot turned up full.

Now, if I change R 29 to 220k, I get a bias voltage -24 and a reading of 31milliamps bias pot up full.

That's it, I am hoping to sort this out, any help will be appreciated

Cheers Peter

[Littlewyan]

What output valves are you using and what is your B+?

[Guus]

Hi,

I think there is nothing wrong. If you change R29 you will get a different negative voltage range in the trim pot.
If the negative voltage is rising to possitive you get automaticly more mAmps running through your output tubes.

Measure the plate voltage. devide the wattage of the tube (EL34 is 25Watt) by the Plate voltage, multiply by 0,7 or 0,6 and you get the mA your have to dial in.
If the dail range is not sufficient (you'll need about 30 Volts negative voltage on the grid of a EL34, change R29. You can verify the range and the amount of negative volts with the amp on standby.

grtz
Guss

[loupy31]

B+ is around 397 volts,

EL34's

All voltages are with in a couple of volts of the schematic dia,

R29 = 150k, negative V is -33.8, Milliamps is around 12

R29 = 220k, negative V is -24, milliamps is around 31.

Both of the above readings are with the bias trim pot all the way up.

(what are the pin numbers of the plate and grid)

cheers peter

[Littlewyan]

12mA is awfully low for -33V Bias. My EL34s draw about 48mA each at this bias level. Have you got any other EL34s you can try? Also are both EL34s matched? Also have you checked the actual plate voltage on the EL34s?

[loupy31]

Thanks for the relies,, what pin do I check the plate voltage on.

Yes Both are matched

I did also try a pair of 6V6's, But same problem

Regards Peter

[Littlewyan]

Pin 3 for the plate voltage. Also check pin 4 for the screen grid voltage.

I'm guessing this is a new build, have you got any photos of the insides? Particularly the valve sockets.

[loupy31]

Some pics sorry about the quality

[loupy31]

OK, Plate V = 390.8/392
Pin 4 = 356.5/356.5

Cheers Peter

[Littlewyan]

Very nice build, I'm jealous!

Your screen current seems to be about 17mA at idle which is a lot for idle screen current. Have you got another set of EL34s to try? How are you working out the plate current by the way?

[loupy31]

Thanks for that,
Ok, I don't have another set of el34's , the current I built a pair of sockets from this site http://el34world.com/charts/BiasChecker2.htm and plug in my Multi meter and away I go

Peter

[Littlewyan]

Where have you measured the bias voltage? On the grids of the output valves? (Pin 5)

[loupy31]

Yes, On the actual pin

[Littlewyan]

Can you put the 6V6s back into the Amp and measure the plate and screen voltages again please? And provide us with the bias reading at the same time with the bias voltage?

Edit: Before that lets measure the current a different way. Turn off the amp, drain the caps and then measure the resistance from the B+ OT Winding to both plate windings. They should be similar readings. Then post the readings on here along with the plate voltage and B+ voltage.

[loupy31]

No Problem, give me about 30 minutes ,

BTW you said 17millamps was high for idle current, I thought low 30's was idle, or are you referring to something other than the bias current?

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