WEM Clubman - EL84 bias - hot, hot, hot!

[BenHoper]

Firstly Happy New Year all.

I have a WEM Clubman Mk8 opened up at the moment. It's working, and looks all original. A bit of hum and pot crackle to clear up. I'm checking the various voltages and checking the bias. The old Mullard EL84 seems to be set at about 150% dissipation! The voltages read seem to be as per the schematic and the resistance values seem OK. I've heard that Class A EL84 amp are often run hot. Is this correct for a WEM?

Cathode bias amp, 130 Ohm cathode bias resistor, 8V across, 300V plate voltage = 58mA = 18W !!

Any experience info appreciated.
Ben

 

[JonSnell Electronic]

Sounds a bit too hot. Check there is zero volts on the grid. If it is a valve from 1974 then it will probably be soft and gassy now and that will make it draw excessive current.
DO NOT use JJs!

 

[BenHoper]

Thanks Jon.
There doesn't appear to be any DC leakage on the grid. The voltages seem to agree with the schematic so perhaps it's by design??
http://wem-owners.com/wp-content/uploads/2015/04/WEM-Clubman-Mk8.pdf

 

[Enzo]

Class A generally means the tube is running at about full dissipation.

 

[JonSnell Electronic]

Your schematic is similar to mine;
They do run hot. just like the Fenders and I tend to add a 33R resistor in series with the 130R for peace of mind and reliability.
Make no noticeable difference to the sound.
Check the grid leak resistor, I measured many at 68k not 560k!

 

[PRR]

...Cathode bias amp, 130 Ohm cathode bias resistor, 8V across, 300V plate voltage = 58mA = 18W !! ...

Not done right. Plate voltage is noted as 270V, so there is only 262V across the tube. EL84 G2 current is large, shows in cathode current, but not in plate. I don't see where "58mA" comes from.

I get 14 Watts. This improves loudness per dollar at the cost of short tube life which is acceptable to musicians.

 

[BenHoper]

Thanks, Useful input. Pushed me back to the data sheet - max dissipation 17W ! I didn't know that.

PRR - many thanks for the marked up diag. Very helpful. The size of screen current is significant so it seems.
I don't read 270V on the output transformer, seems to be 290V, as is screen voltage. But it's starting to make a bit more sense now.

Jon - I may add a series resistor in, I would like the Mullard to last a little longer! It's done 45 years, but a little more would be good.

Thanks all.

 

[PRR]

> Pushed me back to the data sheet - max dissipation 17W !

Where do you see that?

 

[BenHoper]

PRR - the first data sheet I came across:
https://www.telefunken-elektroakustik.com/wp-content/uploads/2017/06/EL84-TK-Tube-Data-Sheet.pdf

Jon - the grid leak has a 890k fitted, measuring 720k insitu.

 

[JonSnell Electronic]

They didn't manufacture 890k resistors back when this was made. Mullard recommend around 300k for the G1 leak resistor.

 

[BenHoper]

Thanks Jon, that's helpful.

 

[PRR]

PRR - the first data sheet I came across:
https://www.telefunken-elektroakustik.com/wp-content/uploads/2017/06/EL84-TK-Tube-Data-Sheet.pdf..

That clearly says Rating 12W.

The "17W" is in descriptive text and they are talking about OUTPUT of a PAIR.

Any datasheet with a color picture is very new and suspicious. This one is honest on the first page but gets overenthusiastic on the extra material.

 

[BenHoper]

Thanks for the clarification.