Sunn 200s B+ too high?

[bcreekmur]

Hi - Hoping someone might be able to help me trouble shoot. My uncle has a Sunn 200s that started motor boating after he replaced the power tubes.  What I'm seeing is that if I have the amp on standby, with no tubes except the tube rectifier, I get ~507 volts on the standby switch, but when I take the amp off standby I'm seeing the B+ jump up to ~612volts. I thought due to the amps age(1968) that the filter caps probably needed replacing anyways, so I replaced them as well as converted the amp to use a 3 prong power cord. Unfortunately replacing the filter caps didn't seem to do anything as I'm still seeing the B+ increase about 100v when taking the amp off standby. The heater supplies both 5v and 6.3v appear to be fine as. I did some searching but wasn't able to find what might cause an amp's voltage to increase when taking the amp "off" standby. This isn't normal is it? 


Thanks,
Brian

[sluckey]

What I'm seeing is that if I have the amp on standby, with no tubes except the tube rectifier, I get ~507 volts on the standby switch, but when I take the amp off standby I'm seeing the B+ jump up to ~612volts.

This is perfectly normal. You have two issues involved. First, you don't have any tubes in so there is no current draw on the power supply and the voltage goes to some maximum (no load) value. Second, with the STBY switch open there are no filter caps connected to the rectifier. What you are measuring with your meter is the average of the rectified positive AC pulses. Your meter thinks it's seeing smooth dc but is being fooled. So it's lying to you with that ≈507. When you close the STBY switch the filter caps will charge up to the peak value of those positive AC pulses and hold that value to the ≈612v that you see. This is no load condition. Plug in all the tubes and that ≈612v will drop to about 500v to 525V.

The 525v filter cap can on those amps is really underrated. 600V would be better.

[bcreekmur]

Thanks for the quick response Sluckey! You're explanation makes sense.
I did away with the cap can and made the 1st and 2nd stage (2 100uf/350v caps in series) and the 3rd and 4th 22uf/500v.
I'll get those tubes put back in and measure everything again.
Thank-you!
Brian

[PRR]

> motor boating after he replaced the power tubes.

A badly UN-balanced push-pull stage may throw enough unbalanced crap down the B+ line to cause a motorboating condition. Could be one very weak power tube. I have seen it happen when the "matched" resistors in the phase splitter drifted 10% and 80% (and new resistors were a complete cure).

So it may take more than "a cap job", though healthy filter caps ARE a key point.

[SoundmasterG]

Good call going with the series connected caps bcreekmur. The B+ voltage with tubes in for the later Schumacher transformer amps is higher than for the earlier Dynaco transformer amps. The later ones can be up around 560 V for a 200S and the earlier ones are more like 535-540 V. The can cap is under rated either way and going with series connected caps on the first two stages is a good idea.

Greg