Princeton Reverb AA764 conductive board?

[Willabe]

I have a BF Princeton Reverb, tube chart says AA764 and OC, March 1965.

And it's making very loud pops and crackling noises. More than intermittent, can't play through the amp. Sounds very similar to, if not the same, as touching a meter prob to a tube socket grid pin with the amp on and the volume up a little.

It will also do it with the amp just warming up and the volume on 0. But not as bad and often as when playing through it. Setting the volume to 3 ~ 3.5 is plenty to cause it to go goofy.     

I opened it up and was poking the components/solder joints/eyelets/wires, nothing. But when I pushed down on the (original) fiber board it would happened. I could get it to make the noises when I pushed on the board around the PI and trem section. So I thought maybe a bad component or solder joint? Replaced all the R's/C's on the board for the PI input/output. I also cleaned the pots and tube sockets. No success.

So I unscrewed the 3 fiber board screws and lifted the board up off the lower blank insulator board. Then no problems poking the board, but, tapping the board gets a pop, like it's micro phonic. Not as bad as a push, but it's clearly popping when I tap the board, single tap, single pop, almost a pinging sound.

It also pops badly, most of the time, but not always, when I turn the amp off, even with the volume on 0. This amp has no stand by switch.

I had removed the trem circuit years ago, so I stuck the meter prob into an empty trem LFO cap eyelet, meter set for DCV and watched the meter climb up to ~ +180mdcv. This happens with the eyelet board unscrewed or screwed down.

So I'm thinking the board has gone conductive?

Any sense in me taking the knobs and nuts off the pots and input jacks to lift the board up to see under it? 

I don't think it's the power tube grid wires that run under the board. But I guess it could be? But the noises are random and continue happening. If a grid wire was broken, I would think it would stay open and not keep opening and reconnecting? It happens with the head out of the speaker cab, so no speaker vibrations to cause that.

  Thanks, Willabe   

     

[pdf64]

Those solid core wires can fatigue. If it happens inside the sleeving, the circuit can become intermittent.
Also they’re prone to dry solder joints, especially on valve socket lugs.

But yes, that fibreboard can be a pain.

[Willabe]

It only happens at the board, not happening when I poke the tube socket pins or their wires.

That board is 57 years old, guess I can't complain too much. 

[sluckey]

Just check for voltage on the board. One probe on ground (negative lug of cap can) and the other probe at different locations on the board. A needle sharp probe is best. Check the board near an eyelet that you know has B+ voltage or plate voltage. Readings may vary depending on how conductive the board is in different locations. There should never be any volts or even milivolts on the board. If you find any voltage, replace the board.

[Willabe]

First, with my Klein MM2000, set for dcv, connected to meter, but not connected to amp, it reads ~22dcmv. Short the probes, it goes to zero, then climbs back to ~22dcmv.

With all tubes in, board screwed down, black meter prob grounded to PT bolt, cap can ground lugs are soldered to the chassis. I get ~22dcmv most places, so probably 0dcv? The only other place I get any reading for dcmv is right next to, 1/8", B+ dropping R's eyelets, on the end of the eyelet board, I get ~65dcmv. So, maybe 65dcmv - 22dcmv = 43dcmv?

No noises with only the rectifier/power/PI tubes in. Pull the PI tube, no sound, no noises.

I'm going to lift 1 side, pot side, of the board and look underneath.   

This amp is modded, stole the trem triode and wired in LTPI. I did this 15 years ago or more, has worked fine with no problems.

[Willabe]

Lifted the board, 1 side, looks fine. With the meter hooked up to both ends of the power tube grid wires and pushing/moving the grid wire with a chop stick the 2 grid wires seem fine. 

 

[pdf64]

…
This amp is modded, stole the trem triode and wired in LTPI. I did this 15 years ago or more, has worked fine with no problems.

That’ll probably add a big chunk of gain?
I wonder if that, combined with ageing ecaps, have made it more prone to (ultrasonic) oscillation?
eg the crackling may be an audio artefact of the amp flipping in and out of oscillation.

[sluckey]

…This amp is modded, stole the trem triode and wired in LTPI.

That’ll probably add a big chunk of gain?

Yes, just like all AB763 amps. Would be a good idea to include a 47K resistor just prior to the LTP PI input cap to simulate loading of the missing INT pot. Of course you may like the extra gain sound. I didn't. Look at Hoffman's single channel AB763 schematic. I wouldn't expect this to cause any problems though.

[Willabe]

I wonder if that, combined with ageing ecaps, have made it more prone to (ultrasonic) oscillation?
eg the crackling may be an audio artefact of the amp flipping in and out of oscillation.

Thinking back, I did the mod probably 20 years ago or a little more. At the time I changed all the caps, OD, didn't know better. I still have the Fender blues around here somewhere.

   

[Willabe]

Yes, just like all AB763 amps. Would be a good idea to include a 47K resistor just prior to the LTP PI input cap to simulate loading of the missing INT pot. Of course you may like the extra gain sound. I didn't. Look at Hoffman's single channel AB763 schematic.

Yes I had that 47K R in there.

The mod is in a Gerald Weber book and at the time I'm sure I was just looking to mess around in an amp, get my hands dirty.   

I've always played on the cleaner side of amp tone. So that amp stock was probably fine.

I'm leaning towards replacing the board, but were getting new windows and a couple new doors, then the painters are coming to paint the whole house, be a couple weeks, walls need a lot of prep work. And I'm painting my shop myself, faux finish on the walls.

So..... I don't know when I can get to it.   

I bought a new Strat, Pro II, and I have some Strat and HB PUP's I need to swap to hear them. A BF Fender is my 0 plumb line for my ears. 

    Thanks guys,   Willabe

[Willabe]

I've decided to redo the whole amp with a new fiberglass board and new tube sockets. And set it back to the original PR circuit, including the trem circuit.

With only 2 mods?

1.Probable add a 25K mid control, put it in the extension speaker jack. 

2. Add a little more B+ filtering.

I've been looking at Doug's diy Princeton Reverb file, very nice work as always. It has the components on it already. I might be able to use it.   

He shrank the length of the board a little, Doug's is ~ 10 1/8" by 3 1/8" and the board in my PR is ~ 11 1/2" x 2 1/2". And Doug uses 3 1/8" height for all his boards. That will fit in the PR chassis. If I slide/move a few components down toward the tube sockets, I can get the small radial ecaps on the pot side of the board.   

I'm playing around with it. I have to measure where the mounting holes are on the old PR board and where Doug has placed the parts. I need to make sure any parts won't block where a mounting screw/bolt needs to go. I'd rather not drill any new holes in the chassis to mount the board. 

I'm thinking about wiring up the 4 x 20uF cap can as 40uF plate, 40uF screen and then adding 3 (?) small radial ecaps to the board. Then I'd have a power section ground and a preamp section ground.

I'll post what I come up with, might be a while though? 


      Thanks guys,  Willabe 

[sluckey]

I've been looking at Doug's diy Princeton Reverb file, very nice work as always. It has the components on it already. I might be able to use it.   

I don't think Doug sells any boards with components already loaded. Better double check.

Doug also sells a PR eyelet board that matches the original Fender layout. Board is still 3-1/8" wide though.

[Willabe]

I don't think Doug sells any boards with components already loaded. Better double check.

Sorry, I meant the diy file has the components on it, not just the eyelets/turrets. Easier to see what's going on with it and to move things around if needed.   

Doug also sells a PR eyelet board that matches the original Fender layout. Board is still 3-1/8" wide though.

Thanks for reminding me. I'll look for that, but I think he has the screen grid R's on the board?

       Willabe   

[sluckey]

Hoffman did several upgrades to his version of the PR... screen resistors, grid stop resistors, adjustable bias supply, 1Ω cathode resistors. None of this stuff is on the original.

The only reason I mentioned the original board was just to be sure you were aware it's available in the event that you may want to retain the original circuit and looks. Not trying to sway you either way. If I was doing a scratch build I'd def use Hoffman's improved circuit. If I was restoring an original, I'd probably forego Hoffman's improvements and use the original style board, cap can, and bias board.

[12AX7]

Those solid core wires can fatigue. If it happens inside the sleeving, the circuit can become intermittent.
Also they’re prone to dry solder joints, especially on valve socket lugs.

But yes, that fibreboard can be a pain.

So true.  have no idea why anyone would use solid except maybe for heaters which are such a huge gauge it's probably not a issue.  Even with caps and resistor leads it's enough of a worry which is a sort of bonus when i decided i liked orange drops better then mallory with hair thin leads. None of it matter if you don't intend to use the amp hard (vibration) by transporting it to gigs a lot and playing loud. But solid core wire is just one more reliability issue no one needs.  Just MO.

[Willabe]

So true.  have no idea why anyone would use solid except maybe for heaters which are such a huge gauge it's probably not a issue.  Even with caps and resistor leads it's enough of a worry which is a sort of bonus when i decided i liked orange drops better then mallory with hair thin leads. None of it matter if you don't intend to use the amp hard (vibration) by transporting it to gigs a lot and playing loud. But solid core wire is just one more reliability issue no one needs.

All the years I has a BF SR and gigged with it in the Chicago blues clubs, other bars and out on the road a bit, I never had a wire break. The only thing that happened was I had a faux CT R for the heaters burn out. And that's it. 

And I never saw a Fender amp go down on a gig, never. And none of the players that hung out every told me their Fender went down on a gig.

There were some guys that left their BF Fender amp in the trunk of their car all winter. They'd go to a gig, pull it out of the trunk, plugged it in, wait a 1/2 hour or so for starting time, played the gig, everything was fine. And it gets down to -20F in Chicago.   

            

[Willabe]

I found the original board version in Doug's store. I think I'll get it and the bias board. 

I need to get a BOM together. Doug has that listed too. I only need part of it. 

Thanks Sluckey.

       Willabe
 

[Esquirefreak]

Solid core wire is much more easy to work with and if one just avoids bending it multiple times and weaken it - it most often hold up just fine.

But I've learned from welding that stranded wire has one benefit - multiple points of contact. That ensures proper contact to a higher degree. And although it's probably generally not an issue, I think it's a reasonable approach to use it where the connection doesn't have mechanical support. I try to use stranded wire when soldering to a chassis or around a bus wire.

Just throwing it out there :)

/Max