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Peculiar Fender Driver Problem...

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6/1/2000 12:20 PM

Peculiar Fender Driver Problem...

I have a bandmaster for service, with a strange driver instability: It produces a 'spitty' distortion when pushed hard, due to momentary oscillations. If you measure plate to plate on the driver and set voltage to 0 (300/300)(with trimpot), then try to measure each plate alone one of them makes the driver oscillate. It's pin one (with the feedback and 91K plate load. The other plate won't make it oscillate. This is measured into a speaker or test load resistor. It will also oscillate into an open load, with the input cap grounded (so it's not coming from someplace else. Filter caps tested (by shunting) they are fine. You also can get pops and oscillations when trying to measure either grid voltage. Output tubes are NOS 6L6, Driver is Sovtek 12AX7-WXT, outputs pentode connected, biased at 450 ma each. It will stop oscillating if you add a 'presense' shunt cap, which is also weird. A plate to plate cap can also kill this, but it seems I shouldn't need this, as the model never had it before. I've checked phase, and voltages, and lead dress etc. I'm stumped.

Anyone have any thoughts ? Ray I, Gil A, J Stokes, A Ruhl ?

6/1/2000 1:43 PM

Andy;

Depending on the circuit, the driver tube in these amps are supposed to be ...

blonde: AX7s with 100k plate resistors. This has a supression cap across the plates!

BF: AT7 with 100K resistors

SF: AT7 with 47K resistors

I think one of your problems is that you've got too much gain. If you'll notice, all the amps that have this hot a driver circuit uses at least a 47pF cap across the plates. The WXT tube generally has higher gain than NOS as well. Return it to NOS specs and the problem will probably go away.

Carl Z

6/1/2000 9:27 PM
andyfuchs	Thanks Carl. I guess these might be areas to look at. I've done quite a few Fenders like this before and not seen this happen, so maybe it's the output tranny being a little rolled on the high end. I'll try the cap and/or reducing FB or gain a bit. Thanks !

6/2/2000 2:55 AM

Andy,

It does sound like a feedback-related problem to me, and I think the first thing I would do is to disconnect the feedback loop entirely and see what happens (the presence cap you mentioned adding that helped seems like a good indication of this type of problem). You might also check your speaker jack grounding. You said voltages and phase response are normal, so it would probably not be something weird like the PI tail resistor going way off spec on you, or the grid grounding cap on the PI opening up.

Ray

6/2/2000 4:43 AM
John Stokes	Biased at 450 ma?? Huh?? Do you mean 45 ma? Still kinda hot, try 35. Does it oscillate with the proper 12AT7 PI tube? Is the feedback loop open? Try adding a 47 to 100 pf cap from plate to plate on the PI. Wire the cap directly on the tube socket. Clean up the lead dress.

6/2/2000 12:58 PM
andyfuchs	Thanks guys. I changed to an American 12AX7 and added the cap, and it cleaned up nicely. I guess Carl's suggestion that the WXT Sovtek is more gain may have been on track. BTW John its 45 ma per tube not 450 (g). I'll take them down to 40 each, just to be safe. Thanks !!

6/2/2000 5:13 PM
John Stokes	Sometimes when you do a BF mod on one of those messy-assed rat's nest SF era amps, it can become unstable. Adding the cap between PI plates usually calms it down without killing the tone. The key is to wire the cap directly on the tube socket, not on the board. 47 pf usually does the trick. It's the rare extreme case that requires 100 pf or so.

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