Fender FSR '65 Princeton Reverb

[dpm309]

Have a Fender FSR '65 Princeton Reverb on the bench with a smoking problem. Replaced the PT in it a couple of months ago and everything fired up just fine. Owner said after it was running awhile, it started smoking and getting very hot on the face plate. Before he brought it in the first time, he did not mention it started smoking and then just quit. I did not notice any evidence of a fire or extreme smoke in both cases except there is a little soot on the wires coming out of the replacement PT (Hammond 290AX). Voltages on the power tubes seem low, 312 VDC on the plate and -10 on the grid (should be getting 390 and -37). Checked the bias and was reading about 90mv using my bias checker and the bias pot seemed to have no effect. Having a hard time getting a schematic for this amp as I want to check the bias resistor, etc. Could a bias problem cause the OT to fail? The rectifier tube seems fine (checked out good on a tube tester) but am only getting a B+ of about 317VDC. I have included a picture of the rectifier tube and PT.

[dpm309]

Finally found the correct schematic. Checked bias using the method listed on the schematic, Pin5 on the power tubes should read 23m. I am getting over 90mV. The bias pot and associated resistors are reading within tolerances. Going to pull all of the tubes and check voltages again. I have attached the schematic for reference.
Thanks,
Dan

[dpm309]

Pulled the preamp and power tubes and checked voltages. Getting high voltage on pins 1 & 6 on the preamp tubes and pins 3 & 4 on the power tubes. I am only getting between -9.5 and - 11VDC on pin 5 of the power tubes (should be getting at least -50V). With the bias pot rotated CCW, I get - 9.5V and and when it is rotated all the way CW, -11V. These readings were made when the vibrato intensity control was on 10. When I turned it all the way down to 1, I am getting between -24 and -29V.  According to the schematic, the intensity pot controls the bias if I am reading it correctly. Tried swapping the rectifier tube and am still getting the same results. Tested all of the components in the bias circuit, cap, diodes, trim pot, and resistors, and they all checked out fine.  Looks like the power tubes may have red-plated due to the brown discoloration on the stickers on the tubes. Not sure of where to look next.

[sluckey]

The power tubes are burning up because there is not enough bias voltage on pin 5 of the output tubes.

With the bias pot rotated CCW, I get - 9.5V and and when it is rotated all the way CW, -11V. These readings were made when the vibrato intensity control was on 10. When I turned it all the way down to 1, I am getting between -24 and -29V.  According to the schematic, the intensity pot controls the bias if I am reading it correctly.

The intensity pot should not change the bias voltage. Disconnect C22 from the intensity pot. (Lot easier said than done). C22 may be the culprit so check it for short or leakage. With C22 disconnected does the bias voltage jump up to the  -30 to -40v range?

[dpm309]

Sluckey, disconnected C22 (I hate these PCBs) and am getting the same readings and the intensity pot is still changing the voltages. Checked the cap and it is reading 100n (.1uf).

[Ed_Chambley]

I had a big problem once with bais voltage not stable in a PR.  Went through all kinds of things, checking and rechecking.  By mistake I picked up another rectifier tube and plugged it in.  First time I had ever seen a rectifier tube vary voltage.  Probably not you problem, but I thought I would mention it.

[dpm309]

Ed, I did put another rectifier in it and was still having the same problems. Thanks for the suggestion though. While I had the board pulled, I checked for cold joints and re-floated a couple of suspicious solder pads with no luck either.

[shooter]

When I turned it all the way down to 1, I am getting between -24 and -29V.

what happens when you unplug the pot P18b-3? change the range to -40?
Did you swap the zener in the bias supply?

[silverfox]

I would try disconnecting the pot connections completely and jumper the bias directly to the bias resistors at the grids. See attached. That way, and this is perhaps what you were after shooter, that way you can attempt to isolate the problem between either the bias or the output section and eliminate the Trem circuit.

I have to wonder if the transformer is seriously damaged now though since the wire is so badly cooked. There used to be a growler test you could do on windings but I don't know that they make that device anymore.

silverfox.

[dpm309]

Did a search on the internet about bias problems with this particular amp and it seems to be a common problem related to the intensity pot. Silverfox, thanks for the suggestion, I will try that next. Someone with this problem noticed that the intensity pot was leaking to the ground. He pulled the pot apart, cleaned it up, and reinstalled it and the amp was now able to bias properly. Don't know why Fender designed the bias circuit to interact with the intensity pot. Even thought he PT smoked a bit, I think it is OK since I am getting good readings on the HV and heater taps.

[shooter]

the intensity pot was leaking to the ground

That is how I saw your issue, make sure you check that the solder tabs aren't touching under-board, Also the connector that the pot plugs into

[Willabe]

Don't know why Fender designed the bias circuit to interact with the intensity pot.

Because the trem is (grid) bias vary. It works by varying, fighting against, the grid bias.

All Princeton amps with trem work that way and so do all output stage (grid) bias vary amps.

[dpm309]

Disconnected the intensity pot and jumpered the bias directly to the grid, P18-3 to P18-5 and am now getting -40VDC on pin 5 of the power tubes. Will check out the intensity pot and worst case is I will have to replace it. AE does not carry a Fender 250KA snap in pot (only 1M and 100K reverse). They do carry a Peavey 250KA snap  and a generic 16mm snap in but not sure if either of these will fit.
Thanks for everybody's help in getting to the bottom of this.

[dpm309]

UPDATE: Removed the PCB board with the pots on it and cleaned the intensity control pot and am now getting -40 VDC +/-  on pin 5 and the bias is reading 25 to 30mV. Everything seems to be working now but I will keep it running awhile to make sure it's OK.

[dpm309]

Noticed that the bass pot was not working. Pulled the boards again, cleaned the bass, treble, and volume pots and removed a couple of solder bridges on the pot PCB board. Reinstalled everything and all is working like it should. Looks like a quality control problem with amp. The owner who got this on a trade-in, said it never worked right from day one.