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Biasing problem with '62 Princeton 6G2

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6/25/2004 3:09 PM

Biasing problem with '62 Princeton 6G2

Hi,

Just got a '62 brown 6G2 Princeton. Plugged it in the other day and noticed that the PT ran really hot. I decided today I should go through the amp anyway and change out the cathode bypass caps and the capacitor can.

After having done that, I measured the bias using the transformer shunt method and noticed 83 and 77 mA on the 6V6s. I measured the voltage on the plates at 198 and 199 V, respectively. This equates to 15-16 watts dissipation, and I think the max for 6V6s is 12 watts. So, I thought, no problem, I'll sub in a 50k pot across the bias cap and try to adjust the bias. But I can't get the bias to change. I changed out the bias cap thinking it was shorted. Still no luck. I can't figure it out. Anybody know what I should check next?

Help!
Chris

6/26/2004 8:20 AM

Chris, 198 volts on the 6V6 plates doesn't really sound right! You really should go to the Fender Field Guide and d/l the schematic. The factory voltages are almost double what you found.

The only way you could get such low plate voltage is if when you wired up the can you crossed something somewhere. Double and triple check it - especially the bias tap on one plate of the 5Y3.

If you didn't have enough negative bias voltage in the first place tacking in a pot won't help. If there's not enough there's not enough!

I'd first try pulling the output tubes and measuring the voltages along the filter cap string. If they aren't starting off at nearly 400 volts with no 6V6 loads I'd definitely want to know why!

Then I'd check that bias voltage. The print says about -35 volts - how much do you have? If it's too low the 100k dropping resistor from the 5Y3 could be damaged or maybe even the bias supply diode.

Check the bias stuff AFTER you find out what's wrong with the B+ voltages! The low voltages might be screwing up the bias supply and after things are corrected the bias might come back to normal.

HTH

---Wild Bill

6/27/2004 6:22 AM

>Chris, 198 volts on the 6V6 plates doesn't really >sound right! You really should go to the Fender >Field Guide and d/l the schematic. The factory >voltages are almost double what you found.

I know and I have the schem. Hence my post as I didn't know what to do...

>The only way you could get such low plate voltage >is if when you wired up the can you crossed >something somewhere. Double and triple check it - >especially the bias tap on one plate of the 5Y3.

Nope. Octoplet (!) checked it.

>If you didn't have enough negative bias voltage in >the first place tacking in a pot won't help. If >there's not enough there's not enough!

I know, I did that *before* I noticed no neg. voltage...

>Then I'd check that bias voltage. The print says >about -35 volts - how much do you have? If it's >too low the 100k dropping resistor from the 5Y3 >could be damaged or maybe even the bias supply >diode.

Bingo. Bias diode was open. Replaced with 1N4007 and all is well.

Thanks for your help!
Chris

6/27/2004 6:22 AM
Chris	Forgot to add: 34 mA @ 299VDC

6/27/2004 7:47 AM
Wild Bill	Great! And cheap too! I guess the lack of bias put the idle current up so high it pulled the B+ low. No wonder the PT got hot. So how's it sound when you crank a George Thorogood riff through it? ---Wild Bill

6/27/2004 1:05 PM
Chris	Bill, I haven't played any G. Thorogood riffs thru it, but I will in your honor... The amps sings, though. My favorite of the brownies! Thanks, again, Bill!

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