I recently bought a 1982 Champ (AA964) that arrived with a JJ 5Y3 rectifier, an E-H 6V6 power tube and a GT 12Ax7 pre-amp tube. Everything else looked to be original so I figured it was time to replace the electrolytic caps and give it a general checkup.
First, I went through the amp and recorded the baseline voltages. The readings showed that the 6V6 was seeing about 20 watts of dissipation. I substituted some older U.S. tubes without any significant change. I proceeded to replace the caps and also changed the cathode bias resistor from 470 ohms to 1k. This brought the 6V6 dissipation down to a more reasonable 13.5 watts.
Then I put in a 1kHz signal and saw on the scope that the output waveform wasn't nicely shaped even at low levels. I replaced the U.S. 6V6 with the E-H that came with the amp just to see if anything was different.
Within about 5 seconds of turn-on there was a loud POP followed by considerable smoke from the chassis. Yikes. After I settled down I found that the new 25uF/50V cathode bias cap had blown up.
I replaced the cap and tried a JAN 6V6GT that I bought years ago and hadn't seen much use. Now the output looks great on the scope, I'm getting 5-6 clean watts of output, and it sounds great.
I'm wondering if the first cap I installed was defective, or if the E-H tube suddenly failed?
It could have been either. With a suitable wattage bulb, a light bulb limiter (LBL) turns any amp into a tube tester. So my suggestion is to try out the suspect tube with the amp powered via your LBL, fitted initially with a low power bulb, eg 25 watts. If the amp voltages check out to indicate no shorts then try a 40 watt. Recheck and repeat with a 60watt then 100 watt. By this stage, after the initial flash the LBL should stat dim, and the amp should be receiving almost the full wall voltage. If the amp checks out fine then it’s reasonable to assume all is well and the LBL can be dispensed with.
Regarding your process, are you sure that the modern 5Y3 is putting out the correct HT voltage for the amp? Generally they don’t drop as much voltage as vintage types, and the amp runs too hot. Whatever, doubling a cathode bias resistor value seems an excessive degree of change to the tube’s operating point, ie if that degree of change is required to bring plate dissipation into line then something significant may have changed. But 20 watt (plate + screen grid) idle dissipation seems to be the intention for post CBS champs el34world.com/charts/Schematics/files/Fender/Fender_champ_cbs.pdf With the amp idling, what’s the PT primary and heater Vac?
Thanks for the reply and the information. I don't have all the values (such as heater voltage) but I recorded other parameters as I went through the process. I put the amp back together yesterday so I don't plan to get into it again for a while.
As received, the original power supply cap had 398, 387, and 363VDC at each section. With the new cap and all other changes the readings are 438, 434 and 406VDC. These are certainly higher than the schematic that shows 360, 350 and 330. However, there is a handwritten note on the schematic that says "later models use 022772" for the PT which is the one I have. Perhaps this explains some of the difference? My wall voltage is a consistent 120VAC.
There wasn't much change in the plate voltage whether I used the original JJ 5Y3 or the older Sylvania 5Y3. In fact, the older rectifier was slightly higher.
At this point the amp sounds great and I feel better knowing that the 6V6 is at or below the 14W maximum from my RCA Tube Manual.
I'll add the light bulb limiter to my to-do list. It sounds like very useful thing to have!