Bias problem on old Fender Princeton

I am having problems on a recent acquisition, a '62 brown Fender Princeton.

Schem for reference.

I 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, and I've oriented it correctly with positive to ground. Still no luck. Then I checked my negative voltage supply for the bias and saw that I ain't got no negative voltage.

I can't figure it out. Anybody know what I should check next? I am at a loss. Sorry, I am still a newbie here, I am still trying to figure this stuff out... Can anyone give me a clue?

Chris

I'm in doubt, but I don't think I should "crank" it in this case. :shock:

Are you sure on those current readings?
That would make those guys glow red I am afraid.

I mean, a 6L6 draws about 40 ma .
The low plate voltage tells me the bias is too low.
You might have a shorted turn in the bias supply winding.
I think 30 ma is more normal for a V6.
Disconnect the bias winding and measure the ac on the winding.
Take out the tubes first.

Never mind, I see no seperate bias winding. Must be in that trem circuit.
Make sure all the tubes are plugged in.

I used to be a guitar amp tech (Fender Gold), but I haven't had a chance to exercise my chops in a while. I'll take a crack at this one, though.

First of all, I really dislike the idea of shunting the output transformer to measure the plate current. A much better (and safer) idea is to insert 1 ohm resistors in series between the 6V6 cathodes and ground. The voltage drop across the resistor corresponds directly to the cathode current of the tube. It will not affect operation of the amp.

OK, on to troubleshooting. Remove the output tubes, to keep them from burning up as you track down the problem. The B+ will rise due to the reduced current draw; so if you have a variac, use it, to avoid overvoltage on the filter caps. Disable the trem oscillator. Is there still no negative bias voltage at all? Be sure to check right at the bias filter cap as well as at the wiper of the trem pot. If bias is present before the pot but not after, suspect a bad pot, or a bad .05uF cap to ground at that point. If you read a POSITIVE voltage at that point, suspect that the 0.1uF coupling cap from the trem oscillator is leaky. If you don't have bias voltage right at the output of the diode that feeds the bias filter cap, the diode or the 100K resistor might be open. Of course, check for any bad connections throughout the whole circuit, from the bias tapoff point on the power transformer primary, right through to the power tube grid pins.

Once you get it sorted out, I suggest adding a high-value (say, 220K) resistor between the trem intensity pot wiper and point "Y" on the schematic. This will prevent loss of bias if the pot goes intermittent, but will have little effect on the action of the pot.

[quote author="NewYorkDave"]The B+ will rise due to the reduced current draw; so if you have a variac, use it, to avoid overvoltage on the filter caps.[/quote]

What do I run the voltage on my variac at?

[quote author="NewYorkDave"]Disable the trem oscillator.[/quote]

So is that just a matter or yanking the 12AX7? Sorry, you have to remember you are talking to a newbie... :sad:

I will run the checks tonight and get back to you with what I find.

Thanks for pointing me in the right direction...

I hope my PT is not on its way out... :sad:

I need a beer! :guinness:

[quote author="cjenrick"]Are you sure on those current readings?
That would make those guys glow red I am afraid.

I mean, a 6L6 draws about 40 ma .
The low plate voltage tells me the bias is too low.
You might have a shorted turn in the bias supply winding.
I think 30 ma is more normal for a V6.
Disconnect the bias winding and measure the ac on the winding.
Take out the tubes first.[/quote]

Yeah, I know. I was trippin' last night when I read it. At first, I thought it was due to lack of sleep, until I triple-checked everything! :shock: I didn't think to check the plates for glowing red. D'oh! The current readings tipped me off, though... I was expecting 30-40 mA, tops.

I really hope that the PT is OK... Dude I bought it from had no f'in' clue on the amp. Tried to pass off the cabinet as original. :roll: It came in a pathetic homemade enclosure wrapped in blue naughahyde from a '66 Mustang! :shock: That's OK, I only wanted the chassis as I have a cabinet for it. :thumb:

I'll check on the PT tonight. I'm crossing my fingers that the PT and the bias winding is OK. Reason being is because I've heard (according to Steve M. at Angela) that the replacements that Fender makes now have significantly higher secondary voltages than the oldies, probably because more people are replacing PTs in blackface amps, which I believe have higher voltages than their brownface brethren.

Go here and scroll down to "Genuine Fender Power Transformers"

You could just slap a cathode resistor in there and be done with it. A little less power, but a smother sound...

Chris,

There should not be a bias winding on the PT - it's taken from the main secondary. It sounds like you just need to trace the bias voltage from that tranny tap along to wherever that voltage disappears. Be careful.

These guys are supposed to be the best for repro transformers, very expensive though. (The tone-clone series.)

http://www.mercurymagnetics.com

Hey guys! Thanks for all your help.

It was the bias diode. It was open. Replaced it with a 1N4007 (I hope that's alright). 6V6s are now happy at 34 mA / 299V. :green:

Again, thanks much for helping a newbie learn more about his equipment!

Congrats! Yes, a 1N4007 is fine. Enjoy your "new" amp!

You might try some new pwr tubes, if that diode was bad for a while, the tubes might be a bit tired.
cj

Yeah, already got some Brimars, some Mazdas and some Philips matched pairs already. The Nationals sound pretty good in there, actually. I kinda want to use those up and then go on to my NOS stash... :green:

My source says he's nearly out of the Mazdas... :sad: