Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: AmberB on August 31, 2021, 01:10:28 am
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Hello all!
I just started working on another Bogan CHB-20A last week. I got 2 of them from the same guy...
The rebuild of the first one was pretty normal, and it ended up working quite well, I wrote about that amp a few weeks ago.
When I opened up this latest amp project, it was obvious that someone had been inside this amp and had done some funky things with the power supply. The standard voltage doubler was no where in sight, and the rectifier diodes each were paralleled with three 470 Kohm resistors in series. I still have no idea what that was about. After looking at the amp, I realized that it has a replacement power transformer and a replacement power supply 3 section filter cap. I removed all the stuff in the power supply circuit when I removed the parts in the preamp and tone control section. I'm rebuilding this one to the Ampeg preamp/tone stack, same as the last one.
My main question this time around is about the high voltage of the new power transformer. I measured the AC voltage from each side of the secondary to the center tap, and it comes out at 333 volts. Once through the rectifier diodes, that's going to be in excess of 400 volts. I'm guessing that when loaded with the tubes it will be somewhere around 410-420 volts.
The original Bogan schematic shows 400 volts going to the primary winding of the output transformer, so I don't know how much of a problem it will actually be for the 6GW8 power tubes, but I think 400 volts is above the max rating for that tube. So, that makes me wonder if that will be a problem...
Another thing, the replacement PT looks like it may be a Bogan part. It fits in the mounting area without modification, and it has numbers stamped in the bottom bell that look the same as the way other Bogan PTs are stamped. It's a different number than the one on the other CHB-20 with an original transformer, as would probably be expected.
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well ain't that an interesting little bugger!
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I did an internet search last night to try to identify the replacement power transformer in this amp. I had limited luck in the search.
The transformer does not have the standard configuration for the high voltage secondary winding for a diode rectified amp. All of the Bogan amps with diodes seem to have a voltage doubler circuit, and no center tap on the B+ winding. This transformer has a center tap on the B+ winding. It only has 1 other sceondary winding, the 6.3 volt heater winding, which does not have a center tap.
The only Bogan amps I found schematics for that had that PT configuration used an EZ-81 rectifier tube.
Of course, most Bogan schematics don't show the part number of the power transformer or output transformer, so I couldn't identify it that way. Also, most Bogan schematics don't show the AC secondary voltage going to the rectifier, and with the ones that do, I don't know if it's voltage under idle load, or unloaded, so that doesn't help much either.
Without being able to identify the transformer, I have no way of knowing what it's capable of handling for heater amperage as well as main load amperage.
Just in case anyone has knowledge of Bogan transformers, the part number stamped in the bell is 83-566-000
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Bogan designed some good stuff. And since they chose to run the 6GW8s at 400V I wouldn't worry about it. Just run it as is. I think whoever replaced the PT knew what they were doing. It's a fixed bias amp so it's probably not gonna be running at 100% dissipation. 'Course when that amp was built you could buy a 6GW8 at the drugstore for under $3.00. :icon_biggrin:
Having said that, my only 6GW8 amp, a Hammond AO-44, has a 250-0-250 PT. B+ is 300V and amp is cathode biased running very hot.