Hot power tubes on Silvertone 1482

[mike60510]

I recently replaced the filter capacitors in my Silvertone 1482 amp with the values on the schematic (attached). Now I get no volume and the 6V6 power tubes get very hot almost immediately. I checked the bias (thanks to the Uncle Doug videos on youtube) and get 14.9 watts from one and 12.8 watts from the other. I think the wattage exceeds the maximum plate dissipation for the tubes. Voltage at the plate (pin 3) is 320 v and 318 v. The schematic shows 346 volts.

silvertone_1482 schematic.pdfJust was hoping for some direction on what to check. Thanks

[52 Bill]

Voltage at the plate (pin 3) is 320 v and 318 v. The schematic shows 346 volts.

And what voltage do you have at the cathodes of the output tubes?

Exactly which caps did you replace?

[mike60510]

I replaced the three main caps on the power rail from rectifier, 5, 10 and 20 uF . The cathode Pin 8 holds at about 17.3 volts. I have sound now but the output tubes still get hot.

[nickb]

I replaced the three main caps on the power rail from rectifier, 5, 10 and 20 uF . The cathode Pin 8 holds at about 17.3 volts. I have sound now but the output tubes still get hot.

Are the plates of the 6V6's actually glowing red?
17.3V across 270 ohms and plate = 320V => 10.2W dissipation max is 12W. I don't see any problem with that unless...

...with the amp off measure the resistance from cathode to ground, it should be around about 270 ohms.

[mike60510]

Yes, I got that result using that method. (actually, the in-place resistor measured 272 ohms) Uncle Doug posted a second video where he measure/calculated the plate dissipation using the impedance and the voltage drop through the output transformer. I measured 150 and 170.6 ohms through the two legs and voltage drop of 6 and 8 volts, respectively. Using that, the plate current would be 0.04 amps and 0.0468 amps giving 12.8 and 14.9 watts for the plate dissipation. I watched the video twice, maybe I misunderstood.

[mozz]

Pull the output tubes and measure your plate voltage and others on all the tube pins.

[mike60510]

I read a post earlier where that was suggested. from the main transformer to the input of the rectifier i got 334 VAC. The output of the rectifier was 433 VDC and both leads of the output transformer, 433 VDC on at the 20 uF, 422 VDC at the 10 uF and 321 VDC at the 5uF. 136 VDC at pin 1 of the 12AX7 preamp and 221 VDC at the pin 1 of the 12AX7 phase inverter. One of the previous thread about a similar issue made reference to a 100 volt difference being excessive. I don't have enough experience to know if that statement is accurate.

Thanks for the help

[nickb]

Yes, I got that result using that method. (actually, the in-place resistor measured 272 ohms) Uncle Doug posted a second video where he measure/calculated the plate dissipation using the impedance and the voltage drop through the output transformer. I measured 150 and 170.6 ohms through the two legs and voltage drop of 6 and 8 volts, respectively. Using that, the plate current would be 0.04 amps and 0.0468 amps giving 12.8 and 14.9 watts for the plate dissipation. I watched the video twice, maybe I misunderstood.

I think you did it correctly .

You have to take into account the measurement errors, In the case of the transformer drop you have two meter accuracy error adding togther. In the cathode case you the resistor is probably 10% tolerance + meter accuracy or two meter accuracy errors if you measure the cathode resistor. When you now go to compare the two sets you end up with a four meter accuracy error band ( I hope you're keeping up here ).

The important point is the dissipation is not high enough to worry about. The maximum working glass envelope temperature is often very over 160C.

So, just to double check here, can you see them are red-plating?
How long to they take to get "too hot".

[pdf64]

Make sure that the meter and its leads are well away from the pre-amp when probing a power tube plate.

221 on a plate of V2 seems a bit high. Which V2 plate is that, the common cathode stage, or the cathodyne phase splitter?

I guess that the power tube cathode bypass cap may be passing dc, eg when the voltage and temperature increases; 25V seems rather underrated, as the voltage may rise above that under load.
The bypass cap should be mounted with some air space around it, not tight against the cathode resistor.
That cap is 'mission critical'; a cap with long lifetime-temperature spec may be beneficial.

Try removing the cap to see if it has been affecting things.
The amp will be fine without it, albeit with reduced audio power output.

[nickb]

..and just to flesh out the leaky cathode cap idea, you can lift one end of the cathode cap and measure the DC current thru it to see if it is leaky.

[mike60510]

What does pulling the power tubes tell you and what is an ideal increase in voltage range when the tubes are removed?

[mozz]

What i meant when pulling the power tubes was to check for the correct voltages at all the pins. Screens, grid, plate, filament. Your plate voltage will drop, i'm gonna guess 10% or less, that means they are conducting. If you replaced the filter caps, be sure they are inserted correctly.

[mike60510]

No, the tubes don't red plate. i measure a surface temperature of the power tube of about 210F. So after doing some research, that seams to be within reasonable limits. I don't think I have a problem.

Thanks for the education