'61 Gibson GA30RV ... power problems

[capnjuan]

I bought it a couple of months ago; when it arrived, it had a 100uf/450V cap at the rectifier output and a 20uf/450V tied to one leg of the rectifier's 5V heater supply ... the leg where the DC taken.

Schematic: http://www.schematicheaven.com/gibsonamps/ga30rv.pdf

I straightened out the caps, put in a known-good 6087/5Y3 in, and a new 6V6 cathode bypass cap. Turned it on for a few minutes and the bypass cap blew ... put it on the bench and, through a variac, I get 355VAC on each secondary and a whopping 425VDC out of the rectifier; 4.7VAC on the heaters. The schematic secondary is 320VAC and the expected rectifier output is 330VDC. The PT looks original; says 23-P on top. Have drawn the actual power supply and wiring is as the schematic. I've tried 3 different rectifiers; the one that shipped with the amp, the 6087/5Y3, and another known-good bench 5Y3.

At one point, the rectifier was glowing bright orange and the 6V6s glowing blue; ????????

This 5Y3 data sheet: http://www.tubebooks.org/tubedata/HB...t_1/5Y3-GT.PDF seems to suggest that chokes and capacitors at the filter input are mutually exclusive ... one or the other but not both ... the GA30 schematic has both ... as does the amp.

The NJ7P 5Y3 data here: http://www.nj7p.org/Tube4.php?tube=5Y3GT indicates that at 350VAC, the expected output is 360VDC ... I have way more than that.

The 6.3V and 5V windings are ok; maybe this particular amp was late in the production cycle and got a bumped-up transformer but if a 5Y3 can live with 355VAC, I'd just as soon put a dropping resistor at its output but don't know how to calculate what value would yield about 330VDC. Even if the 5Y3 can survive the higher AC, it would mean that all the other taps on the DC rail are higher than design. If I'm stuck with the PT, I think it would be simpler to drop the rectifier's output than it would redesign the bias and all the rest of it.

Hum:

In either standby or on, the amp hums ... not volume dependent. The polarity switch is gone, 3-wire cord in, ground wire landed under a transformer bolt, and both HV and heater center taps grounded together at same point as filter caps. Schematic has odd way of shorting (?) the output at the power switch; anybody see anything wrong with running a wire to ground from the X/Y connections on the switch?

Thank you; John

[sluckey]

Schematic has odd way of shorting (?) the output at the power switch; anybody see anything wrong with running a wire to ground from the X/Y connections on the switch?

Yes. Don't do it. You will kill the drive to the output tubes. X and Y are shorted together on the off and standby positions. This effectively kills the drive signal to the power tubes. The short is removed when the switch is in the on position.

Does the amp make any sound other than hum? Your voltages may be high because the amp is drawing very little current. Take some voltage readings on the power tube pins and report.

[capnjuan]

I guess it would have helped taking the readings with the 6V6s in the amp; early info was with them out. 

I get 318V on the screens, 308V on the plates which I think will be fine.  The blue glow was before finding/fixing blown cathode bypass cap.  Thinking that it was okay to test voltages without the 6V6s, was gun-shy about putting them back in.  Glow at rectifier and 6V6s normal; all that's left is some hum but that will be lead dress or something.  Thank you for your reply.  John

[PRR]

> This 5Y3 data sheet.... seems to suggest that chokes and capacitors at the filter input are mutually exclusive ... the GA30 schematic has both

No.... what's at the filter INPUT?

The input is a plenty-big CAP. Rectifier current flows in pulses and charges the cap to nearly peak AC voltage with small sag.

Yes, there is a choke further on, but the cap "hides" it from the rectifier. The choke takes sorta-clean DC and makes it cleaner, it is not the INPUT filter.

Use the Capacitor Input chart.

> whopping 425VDC ..... test voltages without the 6V6s

Then total current is not 85mA but closer to 13mA. Read the 5Y3 chart for 320V-350V 15mA. Looks like 440V-480V.

You had a shorted (backward?) cathode cap. The tubes pulled MAXIMUM current, were VERY unhappy, and the excess load caused excess ripple and thus hum even in standby. OTOH, with-OUT the 6V6es you were pulling far less current than normal, UN-sagged to scary voltage.

> 318V on the screens, 308V on the plates

Nothing wrong with that in self-bias 6V6.

[capnjuan]

Thank you PRR: I'll have to <ahem> speak to my tech about the backward cathode cap ... I have a 20uf/450V cap at the filter input IAW the schematic.  Voltages are stable and it's running pretty quietly. 

Thanks again.  John

[Steve_P]

Maybe you should talk to that guy that showed you how to restring your saxophone, no wait, he was the same guy you bought the amp from?