Bias pics

Hamilton Barnard · Post by **Hamilton Barnard** » 10 May 2001 5:06 pm

After reading the interest in biasing a tube amp, I thought that I would snap this picture of a '68 Marshall plexi that I swapped the power tubes (EL34) in today, then proceeded to set the bias. You can see the bias probe on the right power tube.

The Fluke 8000 meter at the right is hooked to pin #3 of a power tube and ground; it is reading 436 vdc to the plates. The next meter is a military Simpson analog that is hooked to the tube cathode and ground; it is reading the amount of amperage the cathode is drawing from the chassis.

I bias Marshalls to 17 watts :

cathode current = 17 (watts) / 436 vdc (plate voltage), or 39mA.

I adjust the trim pot, which is in the middle of the tag board, to achieve this while watching the mA reading on the Simpson.

And that's it!

In the visible speaker jack, a 16 ohm resistor is plugged in just to keep the output transformer stable and happy -- volume controls are set at '0'.

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[This message was edited by Hamilton Barnard on 10 May 2001 at 06:21 PM.]

Chris DeBarge · Post by **Chris DeBarge** » 11 May 2001 4:19 am

mmm...I can almost smell it! Great picture Hamilton. Have one question: you say you measure cathode to ground for current? I always thought it was the plate.

Hamilton Barnard · Post by **Hamilton Barnard** » 11 May 2001 7:02 am

Hey Chris, the smell of burning dust! *L*

The bias probe ties the meter in series from ground to the cathode at pin #8.

Matt Farrow · Post by **Matt Farrow** » 11 May 2001 7:28 am

Chris, to answer your question, the cathode current is equal to the plate current plus the screen current. In an old 50W Marshall especially one with no screen resistors, the cathode current will be off from the plate current by perhaps 8-10mA. As long as you take this into account when biasing the amp you'll be fine. I bias with the "transformer shunt" method usually, which measures the current going through the output transformer. I usually bias Marshalls to be at the very edge of class AB, somewhere around 36-42mA per tube depending on the plate voltage.

Remember, the "correct" bias point for YOUR amp is the one that gives you the tone you want, as long as it doesn't make the plates glow "cherry red-" you don't want your amp to damage itself. If you want clean headroom, you might like your amp biased colder than Hamilton's Marshalls. Hell, if you want clean headroom you would probably not play a Marshall at all! Try a variety of settings, take good notes, and see what gives you the tone you like! That's what matters. The only "correct" way to bias your amp is to get it sounding like you want it.

Matt Farrow
Pharaoh Amplifiers

Patrick Smith · Post by **Patrick Smith** » 11 May 2001 7:54 am

Hamilton,

What's a "plexi?"....LOL

Pleebus

Hamilton Barnard · Post by **Hamilton Barnard** » 11 May 2001 9:14 am

I'll go find out...

I should be back by midnight. If I'm not, wait right here, I mean 'riat cheer'.

Hammus

Brandin · Post by **Brandin** » 11 May 2001 9:29 am

Hamilton, does your Marshall have the 'Le May'(spelling?) mod?

Hamilton Barnard · Post by **Hamilton Barnard** » 11 May 2001 9:55 am

ROFLMBO

Uhh...I'll go find out...wait right here.

Chris DeBarge · Post by **Chris DeBarge** » 11 May 2001 3:56 pm

I do bias it by ear, but I always check the plate current afterwards to make sure the tubes aren't about to start working for NASA.

Yup, the musty, dusty smell of an old tube amp. Takes a few days to get it off of your fingers!

Hamilton Barnard · Post by **Hamilton Barnard** » 12 May 2001 6:52 am

This is worth reading, IMO. Click on "Tech", then "Advanced", then on "the last word on biasing".

http://www.aikenamps.com/

[This message was edited by Hamilton Barnard on 12 May 2001 at 08:29 AM.]