1976 Traynor YBA-4/YBA-1 Squealing When Bias Adjusted

[plumcrazyfx]

Been doing some work on a 1976 YBA-4 (combo version of the YBA-1).  I recapped it, replaced the dropping resistors and diodes in the power supply, added additional screen resistors to drop voltages slightly more (since this uses the shared 1K 10w on the board, I added an additional 1K 5W to each tube), disconnected the grounding switch and installed adjustable bias for the power tubes.  It seems to work, makes sound, etc. but using a Bias Rite-type unit to adjust the bias to normal levels - it squeals.  Replaced the tubes (both power and preamp) and it still does it.  It is sitting about 460v on the plates and as long as the bias reads under 20ma per tube it seems stable - the second it goes above that I it goes into a high pitched oscillation.


Any ideas where to start?  My first step after replacing all of the tubes was to replace the 1.5K grid resistors on the board with 5.6K ala Marshall.  Any ideas where to go next with this?

[shooter]

I'd put a 1 ohm resistor power tube cathode to ground, use that in place of the bias probe.

installed adjustable bias for the power tubes.

WITHOUT power tubes in, what "adjustable range" do you get at the grid pins of the power tube sockets?

[sluckey]

Was the amp squealing ***BEFORE*** you worked on it?

Did you possibly swap the grid wires or the OT primary plate leads?

[plumcrazyfx]

Didn't squeal before.  I didn't touch the output primaries.  Would taking out the ground phase switch impact something?

[sluckey]

Didn't squeal before.

Then it's logical to assume that you messed it up when you worked on it. Could be something you did purposely or could be collateral damage. Examine your work closely.

[plumcrazyfx]

I checked all of my work and it seemed fine.  A previous tech had been in there and I noticed the primaries were reversed from every pic of other YBA-1/4 amps from around that time.  Once I reversed them the squealing stopped and very stable at 445v 34ma on the EL34s.  I don't think it was squealing before because it was biased really cold.

[Latole]

I  at 445v 34ma on the EL34s.  I don't think it was squealing before because it was biased really cold.

60.5 % Power dissipation, this very good.