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guitarmon2 8th January 2013 10:39 AM

Need Help diagnosing an OT
 
I have a 65 Fender Bandmaster. Both the power tubes and preamp tubes do not get hot, cold as an ice cube. I'm trying to figure out why the don't light up.

I have made the following readings on my power tube 6L6GC:

pin 2 is the Plate (anode), 3 is the Screen Grid, and 5 is the Control Grid all measuring 480 VDC. Pins 2 &7 are Heater (filament) and both measure 3 Vac (the layout says all heaters 6.3 volts) Pin 8 is the Cathode going to ground

At the rectifier I have on the left side I have 2 red wires coming from the transformer (power I think) measuring 387 V AC and 50 at the left side of the 470 ohn resister. On the right the top 2 rows of diode is 480 VDC, diode in line with the 470 ohm resistor is 60 as well as the 50 50 cap.

Does any of this indicate I have a problem with my OT?


Thanks for any input

bobrown14 9th January 2013 12:06 PM

Quote:

Originally Posted by guitarmon2 (Post 3316225)
I have a 65 Fender Bandmaster. Both the power tubes and preamp tubes do not get hot, cold as an ice cube. I'm trying to figure out why the don't light up.

I have made the following readings on my power tube 6L6GC:

pin 2 is the Plate (anode), 3 is the Screen Grid, and 5 is the Control Grid all measuring 480 VDC. Pins 2 &7 are Heater (filament) and both measure 3 Vac (the layout says all heaters 6.3 volts) Pin 8 is the Cathode going to ground

At the rectifier I have on the left side I have 2 red wires coming from the transformer (power I think) measuring 387 V AC and 50 at the left side of the 470 ohn resister. On the right the top 2 rows of diode is 480 VDC, diode in line with the 470 ohm resistor is 60 as well as the 50 50 cap.

Does any of this indicate I have a problem with my OT?


Thanks for any input


To measure heater voltage - connect DMM to pin2 AND pin 7, that should get you close to 6.3Vac. Voltages on the other pins not withstanding you should have tubes lit with 6.3 or so Vac across the heater pins. Be CAREFUL not to short your probes to any other pins or you will fry your heater wires - it wont take long either. If you have 6.3Vac across the heaters and no lit tubes, maybe your tubes are done. Have you tested them?

Cheers,
Bob

guitarmon2 9th January 2013 04:16 PM

Thanks Bob. I was getting 3 volts at ground to 2 and also 3. I got 0 at 2 & 7. I found that 2 solder failed. I re-soldered and all is good. Thanks once again.

kevinkr 9th January 2013 04:18 PM

This thread really belongs in Instruments & Amplifiers since it is guitar amplifier related, but since the issue is resolved I'll leave it here.


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