'65 Fender Champ No Power

[ca7922303]

Replace the resistor from 6V6 pin 8 to ground. Usually 470 Ohm 1 or 2 Watt.

Ive got some 470 ohm 5 watt cement resistors and 560 ohm 1/2 watt. Either if these ok to use? If not, will have to order. Thanks.

[Chuck H]

With power off, ground to pin 8(6v6)(0.8k ohms).

Does your meter not have a setting that reads below thousands of ohms?

Ive got some 470 ohm 5 watt cement resistors and 560 ohm 1/2 watt. Either if these ok to use?

Use the 470 ohm 5W. UNLESS, and this seems possible, someone put a higher value in there to mitigate over dissipation due to high plate voltage? Increasing current will bring the voltage down. As it is, with roughly 800 ohms and 31V on the cathode, subtracting 31V from the plate voltage to get "working" voltage it seems the 6V6 is currently dissipating 16 watts at idle.

[ca7922303]

[QUOTE=Chuck H;n957059]

Does your meter not have a setting that reads below thousands of ohms?



Use the 470 ohm 5W. UNLESS, and this seems possible, someone put a higher value in there to mitigate over dissipation due to high plate voltage? Increasing current will bring the voltage down. As it is, with roughly 800 ohms and 31V on the cathode, subtracting 31V from the plate voltage to get "working" voltage it seems the 6V6 is currently dissipating 16 watts at idle.[/QUis there any risk of using the 470 ohm 5w if what you described has been done previously? Thanks for the help.

[ca7922303]

[QUOTE=ca7922303;n957077]

Does your meter not have a setting that reads below thousands of ohms?



Use the 470 ohm 5W. UNLESS, and this seems possible, someone put a higher value in there to mitigate over dissipation due to high plate voltage? Increasing current will bring the voltage down. As it is, with roughly 800 ohms and 31V on the cathode, subtracting 31V from the plate voltage to get "working" voltage it seems the 6V6 is currently dissipating 16 watts at idle.[/QUis there any risk of using the 470 ohm 5w if what you described has been done previously? Thanks for the help.

My meter is autoranging.

[Chuck H]

You should put in the 470 ohm 5W. If this creates an operating condition that is dangerous to the tube you may not have much time to take measurements. Watch for red plating so you can shut down at the first sign. You'll want to get a DC voltage reading at the cathode and the plate. If you manage to get these measurements shut down and report.

[glebert]

Do we know that the power transformer is correct? Trying to figure out why the voltages are so high.

[Helmholtz]

Check heater voltage for reference.

[ca7922303]

You should put in the 470 ohm 5W. If this creates an operating condition that is dangerous to the tube you may not have much time to take measurements. Watch for red plating so you can shut down at the first sign. You'll want to get a DC voltage reading at the cathode and the plate. If you manage to get these measurements shut down and report.

Installed 5w 470r, now 6V6 pin 8(27vdc)
pin 3/4(439vdc).

[ca7922303]

Check heater voltage for reference.

6V6 pin 2 to pin 7(7.1vac)

[Chuck H]

Installed 5w 470r, now 6V6 pin 8(27vdc)
pin 3/4(439vdc).

Ok. That's what I was afraid of. I had hoped that there was some slop in the testing and you would land somewhere around 420V at an appropriate current. But this is not the case. The 6V6 with the 470 ohm cathode resistor now looks to be dissipating over 23 watts at idle. So I think it's true that the higher cathode resistance was an attempt to mitigate current. You should not fire the amp up again with the current operating conditions.

Does the 5y3 rectifier tube happen to be a Sovtek? There was a run of Sovtek 5y3's some years ago that were not on spec and had a much lower voltage drop than a proper 5y3 tube in rectification. If the PT was replaced with a later model AND the hinky Sovtek 5y3 were put in this might explain the high voltage. It's just as likely that the PT was replaced with a completely unsuitable model. At any rate...

The voltage has to be brought down. You will not be able to bias the power tube properly as things are. Since the preamp in that Champ doesn't produce any overdrive clipping it's probably changes to the power supply and maybe the power amplifier circuit are causing the anemic tone. Once the power tube can be biased properly and the drive circuit checks out you should be able to get some clipping at the amps output. Though it would still be less clipping than something like a 5F1 Champ because of drive signal losses in the AA764 circuit. These Champs always sounded a little weak compared to the earlier models.

[Helmholtz]

6V6 pin 2 to pin 7(7.1vac)

This tells us that the PT was designed for 6.3V/7.1V = 0.89 times your actual mains voltage and all other supply voltages will be at least 13% higher than intended.

[Chuck H]

This tells us that the PT was designed for 6.3V/7.1V = 0.89 times your actual mains voltage and all other supply voltages will be at least 13% higher than intended.

For me, questioning the PT is even the right one for that amp, this only speaks about the filament winding spec. It's possible that if this isn't even a Champ PT in the amp that the filament winding could be spec'd for higher current than the Champs tube compliment.

[ca7922303]

For me, questioning the PT is even the right one for that amp, this only speaks about the filament winding spec. It's possible that if this isn't even a Champ PT in the amp that the filament winding could be spec'd for higher current than the Champs tube compliment.

I played guitar thru amp for a moment and did notice increase in output and there was some breakup at 8-10 that wasn't there before. Won't test that way again until voltage issue is addressed.

[Chuck H]

There are a few ways to drop voltage with the existing PT. But first, is the 5y3 a Sovtek brand tube?

[Helmholtz]

For me, questioning the PT is even the right one for that amp, this only speaks about the filament winding spec. It's possible that if this isn't even a Champ PT in the amp that the filament winding could be spec'd for higher current than the Champs tube compliment.

Yes, I assumed the PT to be original (the transformer code might give some clue).
In any case secondary voltages are too high.

But high supply voltages won't cause low output.