Biasing a Musicman Amp

[Bob M.]

Hi,

I have a Musicman amp, model 2275-130. It has 4x 6CA7/EL-34 power tubes, a tube 12AX7A driver and an adjustable bias pot. I'm retube-ing the amp. What is the correct procedure to bias these amps? There is about 725Vdc at B+.

Thanks,

Bob M.

[Enzo]

According to the factory spec, set the bias for a reading of exactly 0.5v at the cathodes of the power tubes. There is a 10 ohm resistor to ground there.

[Bob M.]

Musicman, EL-34s and Bias

Hi Enzo,

I did read that on the schematic and also on line and I did set the bias control for 500mV across the 10 ohm current sensors. This pulled down the B+ from about 725 to 697Vdc. The grids then measured -32.0 and the transformer shunt measurement was 39.7mA for the blue pair (left) and 40.5mA for the brown pair (right), or about 20mA per tube in the quartet. The amp sounded fantastic (4x10 model), one of the best Musicman amps I've ever heard. I was replacing the old 6L6-shaped 6CA7 tubes that were spent/shot but probably original to this amp with current manufactured EL-34s. How will the today's current production EL-34 tube hold up in a 700 volt environment?

Do these numbers seem about right? It certainly felt right. Thanks for you input,

Bob M.

[Enzo]

The proof is in the pudding, as they say.

[Bob M.]

And thanks again!

Bob M.

[Euthymia]

How will the today's current production EL-34 tube hold up in a 700 volt environment?

In my experience, some brands do better than others.

My first choice for a Music Man is a Winged C EL34, then a JJ E34L, then a JJ EL34.

I would not again put Electro-Harmonix EL34's in a Music Man. I've tried it and they toasted pretty quickly.

I have not tried the Valve Art EL34, but I would not be surprised if it were quite good, as is their 6550 (my secret weapon for SVT's!) and GZ34.

[Bob M.]

Thanks for your tube choice comment!