bias on sunn 200S with 6550s?

[marku52]

Anyone got a bias rec for a 200S, it's got 6550s, the schema says -55V, but that was back when line voltage was still 110V. So the bias might have to be higher now. What current should I shoot for?

I'm gonna have to recap it, as even with a GZ34 inplace of the SS rectifier someone put in it, I've still got more than the 50 year old electrolytics are rated for (525V)

TIA

[The Dude]

40-50mA is recommended. I prefer the low side, but that's just my preference.

[soundmasterg]

The -55V on the schematic doesn't tell you the actual current the tube is drawing which is the most important thing to know. Back in the day they would often just list the negative bias voltage number and call it good...but then techs back then also used to bias using the o-scope crossover distortion method too. You need to know the plate voltage and the cathode current to be able to accurately bias with the method most people use these days, and so the negative bias voltage doesn't really matter then. All you need is the plate voltage, the cathode current, and to do a simple calculation to make sure the max plate dissipation for the tube is not exceeded. Most people seem to aim for 60% to 70% of the max for Class AB amps like the Sunns. If you elect to use a can cap like the original amps, then the plate voltage today will be higher than the caps are rated for, which is why I have recommended series connected discrete caps for the first two stages and a 500V rated cap for the third filter stage in the past.

Greg

[marku52]

Thanks, that's good info. I didn't use voltage, but measured current into the tube.