Old Fender fiber boards

[frankeg]

I'm recapping a 67 BFPR and replacing resistors when I find them out of tolerance. The fiber board had several eyelets with multiple components and lots of solder. I carefully removed the old components and used a spring loaded solder sucker to remove the solder. I am worried however about what I cannot see; the under side of the board. As you know, the board is actually two boards sandwiched together and I'm wondering how clean it is under there. Anyone ever have a problem with this? Should I try to lift it up a bit to inspect it (it's wired pretty tight) ? Thanks, frank...

[MWJB]

If the boards are relatively flat, not bowed, and sit pretty flush together I'd say that you are very unlikely to have a problem. Work carefully & keep using the solder sucker. Don't drown the eyelets with masses of new solder.

Some old Fenders (particularly, it seems, 70's SF amps) can have very warped boards, with a lot of air between the circuit board & insulating board, then you can sometimes have drips shorting eyelets, but if the board is very warped then this is easy enough to see & remedy.

If it's easy enough to lift the board then do so, but mostly I wouldn't worry. There's more chance that the board itself can become conductive and bleed voltage between eyelets. In either case, if there is a problem it will be audible - ticks & pops as voltage leaks from areas of different potential, sometimes a wooshing sound like waves on a beach..

[frankeg]

The boards are relatively flat, not bowed, and sit pretty flush together. There is a little bit of warp but not much. Thanks for the advice. I really appreciate this forum and the ability to bounce ideas off of more experienced builders and techs!

[frankeg]

Oh the JOY of turning on an amp the first time after major work and NOT having it blow up, smoke or otherwise deflate your fragile ego!!! I fired up the Princeton last night (slowly) - I used a light bulb short circuit (homemade) tester and started with all tubes pulled. It works! After several minutes, I plugged in and wasn't exactly happy with the sound. It had a heavey, flabby base to it. I turned it off and went for a walk. I didn't get far when I realized that I had connected the reverb pan incorrectly. I had connected the rca cords from the amps reverb out to the pans output and the reverb in to the pans input - out to out, in to in right? Go figure... Well thankfully, I didn't hurt anything and all was well when properly connected. Being "new" to reverb circuits, I did notice and wonder why the 12at7 reverb tube glows more brightly than the other preamp tubes. Is this because it is more of a current amp verses a voltage amp or am I missing something? As always, I appreciate the wise responces. frank

[Amp Kat]

It's got 400+ sitting on the plates and the others have about 180.

[MWJB]

The 12AT7 may simply glow more (we are talking yellow points of light, not the usually grey/black plates glowing red/orange) because the internal structure allows the heaters to be more visible? It is usually run off a much higher voltage supply than the preamp tubes, and "yes", it draws more current than a 12AX7 would do in the same circuit. Essentially the 12AT7 is acting as a little SE power tube, driving the reverb tank, rather than a speaker.

In short, 12AT7 reverb drivers run hotter than many other preamp tubes & I doubt that the glow you see is a cause for concern.

[frankeg]

Yes, we are talking yellow points of light and I do see what you mean about the higher plate voltage and SE driver. Thanks....

[frankeg]

BFPR bias adjustment

One of the things I noticed after removing the vibrato circuit and tying the bias wire directly to the pwr tube grid resistors is that the bias voltage went to -36.7 vdc from the -34 vdc before the change. I didn't worry about it because the idle current seemed fine at 31 ma. However those values were obtained with the adjustable cathode resistor (see earlier post) maxed out. I was wondering if anyone knows what value of bias resistor would bring that back to -34 vdc. It is currently stock at 100k. I have 120k and 150k on hand. The "removed vibrato pot was 250k linear. Thanks

[JoeM]

One of the things I noticed after removing the vibrato circuit and tying the bias wire directly to the pwr tube grid resistors is that the bias voltage went to -36.7 vdc from the -34 vdc before the change. I didn't worry about it because the idle current seemed fine at 31 ma. However those values were obtained with the adjustable cathode resistor (see earlier post) maxed out. I was wondering if anyone knows what value of bias resistor would bring that back to -34 vdc. It is currently stock at 100k. I have 120k and 150k on hand. The "removed vibrato pot was 250k linear. Thanks

Does the amp sound OK with the bias voltage at the -36.7? I wouldnt worry about it, your runnning the output tubes a bit cooler. It isnt like there's an exact voltage it has to be.

[MWJB]

Bringing it back to -34v will bring the plate current up (hotter than 31mA). I would only run 30mA per tube with sturdy NOS tubes or maybe JJ6V6. I wouldn't be keen to run much more than 30mA per tube with any 6V6 tube. I don't know why you want -34v specifically, negative voltage is only a byproduct of plate current (which is the critical perameter).

I would adjust the plate current by changing the value of the 27K grounded resistor after the bias diode (add a 10-20K trim pot, wired as a variable resistor, in series with original resistor to ground), rather than change the 100K.

[frankeg]

The amp sounds fine where it is. I was just wondering. I certainly could be wrong here (have been before) but I thought you could bias 6V6's between 30 and 33 ma. Does that assume nos tubes?

I would adjust the plate current by changing the value of the 27K grounded resistor after the bias diode (add a 10-20K trim pot, wired as a variable resistor, in series with original resistor to ground), rather than change the 100K.

I like this idea!!!

Thanks for the quick responces... I appreciate it.

[JoeM]

How did you measure the idle current? 30ma is pretty hot for 6V6.

[MWJB]

"I certainly could be wrong here (have been before) but I thought you could bias 6V6's between 30 and 33 ma. Does that assume nos tubes?" You can bias them to 50-60mA if your plate voltage is low enough - DON'T try it in a Princeton though, the voltage is way too high!

Fixed bias amps are usually biased anywhere from 50-80% of plate dissipation (14W in this case), 70% is considered the typical max, unless experience shows otherwise. At say 425v (?) @ 31mA you will see that your plate dissipation is around 13W per tube (92%), that's pretty hot already. Plate voltages over 425v may be detrimental to NOS 6V6, so it's up to you whether you bias cooler & run the tubes way over voltage (not an issue with EH & JJ6V6, they can take more volts), or run high current at lower volts.

Cathode biased amps may be biased more like 80-100% plate dissipation, sometimes higher for SE 6V6 designs like Champs.

[frankeg]

Transformer shunt method with a Fluke 189...

[MWJB]

"That's what I thought, so the idle current is for both tubes. Maybe a little cold." No, shunting the OT will give you full current at that end of the primary, so it is 31mA per tube & hot.