Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Charles_63 on April 23, 2022, 11:59:10 am
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Hello,
I kindly ask you for advice on a vintage guitar amplifier I'm restoring these days.
It's a 1962 Selmer Twin 30 Selectortone, blue-gray era, that my friend took from the attic after decades.
His father used to gig it with his accordions, and it sports a funny fake "Selmer Accordeon" old badge.
Apparently it hasn't never been touched or repaired. The first power on issued a strong motorboating, so I put the two chassis (preamp and power amp are separated) on the bench and replaced the electrolytics by re-stuffing the original big cans with new Nichicons.
I put also new coupling caps both on the driver and power tube grids.
The power tubes were very weak so a new pair of 6L6GC is in place now.
A strong 80KHz oscillation is present at the output of the lower driver and it starts creeping
at the OT secondary at about 20W RMS/8ohm.
The problem stays also with the 6L6 grids disconnected and with the phase inverter grids shunt to ground. Both phase inverter and driver tubes replace with no joy.
Then I installed a 50pf across the driver's plates (two 100pF in series actually) and the oscillation went away from the speaker side. It still shows up at the lower driver's output.
The amp now starts clipping around 32-35W, which is good, and the nasty distortion can barely be observed on the scope.
The picture shows the sinewave at about 23W, before installing the 50pF cap.
Should I be fine with this workaround or is it anything better I can try ? I read about "snubber tone killer", is it a real issue ?
Thanks a lot :)
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> new pair of 6L6GC
Plan says KT88, a significantly more powerful tube.
I think you could use a lot more capacitance across the driver. Like 5 times more pFd.
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Thank you PRR.
I found two Sylvania 6L6GC on the amp, so I installed two JJ's to stick with the original.
The schematic is something found on the web, although it is exactly as my amp, down to the last component (but for the tubes).
I'm trying to raise the snubber value per your suggestion.
Btw, I would avoid to introduce any modification on such a rare amplifier.
It is in very good shape cosmetically and it sports two beautiful Goodmans 30's, anyway it doesn't work properly without taming that nasty oscillation.
> new pair of 6L6GC
Plan says KT88, a significantly more powerful tube.
I think you could use a lot more capacitance across the driver. Like 5 times more pFd.