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Peculiar Fender Driver Problem...

6/1/2000 12:20 PM
andyfuchs	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 12:20 PM

andyfuchs

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 ?

Replies:

Carl Z Andy;Depending on t... -- 6/1/2000 1:43 PM
Ray Ivers Andy,It does sound ... -- 6/2/2000 2:55 AM
John Stokes Biased at 450 ma?? Huh?? Do you m... -- 6/2/2000 4:43 AM