Oscillation Problem Vibrolux Reverb

[roadkingbluzs]

I would like to reinstall suppression caps on friends '74 Vibrolux Reverb. I have the 2000pf caps and 150ohm resistors. Schematic calls for a single no polarized 25/25. Can I use two electrolytics per tube instead without any negative issues. Thanks in advance BTW I have tried swapping tubes playing with lead dress etc.

[Leo_Gnardo]

Well I s'pose you could, with say back to back 47 uF caps. But before you put in those 150R's & cross-cathode cap try just the 2000 pF caps.

Another common fix is a cap of 100 to 1000 pF across the plates of the output drive tube. Fender commonly put one between the 100K and 82K resistors. Use the smallest value that will quench the ultrasonic spikes. Rare to need to go the full 1000 pF, usually 220 to 470 pF does the trick.

[pdf64]

I'm not sure that the mixed bias system was devised to mitigate oscillation.
For free running oscillation, if the tubes are set to the same static point, it's unlikely to make any difference whether fixed or mixed bias.
I can see that a mixed system may affect parasitic oscillation, by limiting cathode current (compared to full fixed bias) but it would probably be marginal.

Rather the mixed bias system was probably to done to make the biasing a bit more foolproof and to reduce stress on the power tubes.
Pete

[roadkingbluzs]

Well I s'pose you could, with say back to back 47 uF caps. But before you put in those 150R's & cross-cathode cap try just the 2000 pF caps.

Another common fix is a cap of 100 to 1000 pF across the plates of the output drive tube. Fender commonly put one between the 100K and 82K resistors. Use the smallest value that will quench the ultrasonic spikes. Rare to need to go the full 1000 pF, usually 220 to 470 pF does the trick.

Found some newer tubes and seems to have stopped the oscillation. I will run it for awhile because I do not want to return amp to old wiring. It really does sound great after recap. My feeling is it is in need of new tubes throughout. Presently a mis-mash of preamp tubes. LeoG ,thanks for the insite of suppression caps. I am afraid if they are installed it will change dynamics it now has. . Thanks again to all for info.
PS Learning proscess never ending

[Leo_Gnardo]

Found some newer tubes and seems to have stopped the oscillation. I will run it for awhile because I do not want to return amp to old wiring. It really does sound great after recap. My feeling is it is in need of new tubes throughout. Presently a mis-mash of preamp tubes. LeoG ,thanks for the insite of suppression caps. I am afraid if they are installed it will change dynamics it now has. . Thanks again to all for info.
PS Learning proscess never ending

Good to hear that news. Installing small ultrasonic-suppression caps won't change dynamics, but may roll off some high frequency response. The larger the cap size the more hi end is lost. That's why you select the smallest necessary to do the job - then you might not notice any hi frequency loss at all.

Putting 150 ohm resistors in the cathode circuit - that would do some harm to the dynamics all right. Not to knock self-bias circuits. Self-bias (resistor-bias) OK, fixed bias also OK. Both? Naaahhh.... And I've done it & learned the hard way.

Don't worry about a Heinz 57 variety of pre tubes. As long as they're not making noises of their own, that's good. Clean pins count for something too. You can remove noise-making corrosion with super fine sandpaper 600-800 grit.

[roadkingbluzs]

I do need to work on my general deep cleaning routine, service. Thanks