Motorboating after tube change

I had a bad internal ground which caused motorboating. It was causing a cap to overload. The cap was fine after the ground was fixed.

Thanks, Guys.

Based on your responses and those on AA, I have a coincidence on my hands. Hard to believe, but there it is.
02-09-15: Garibaldi

Garibaldi,

Not necessarily a coincidence.

I suggest you reread my posted message to Atmasphere. It is possible if you powered up the amp with a 5755 tube accidently plugged in the tube socket for a 5751 there is a chance you could have damaged something in the power supply. Of course that is if the designer of the amp bonded, connected, one lead of the tube filament secondary winding of the power transformer to the chassis where also the B- of the power supply is connected. Without a wiring diagram of the amp it is purely speculation on my part.

At any rate if the above scenario is the case I can see a circuit provided if the 5755 tube was plugged in the tube socket wired for the 5751. Section #2 of the 5751 triode tube pin #6 terminal will have B+ voltage present for the plate of section #2 of the 5751 tube. Note pin #6 of 5755 is one side of the heater filament. If a completed circuit is provided through the heater out pin 4, of the 5755 tube to B- chassis then DC current will flow through the heater filament of the tube to the B- chassis. Would it be enough current to damage something in the power supply of the amp? I don't know....

Maybe if Atmasphere is still following the thread he may chime in. LOL, he could also tell me I am all wet.

I guess you will find out for sure when you get it repaired. Post back and lets us know the results afterwards.
Jim

Sad. I thought this would have some type of wedding crasher like story. :-)

Yeah, I don't think those boobs are interchangeable...I mean tubes aren't interchangeable.

Jim, :-) I suspect that you looked at the tube datasheets either with an iPad or some such device having a very small screen, or without putting your glasses on :-)

The two tube types have identical pinouts for their filaments (pins 4 and 5, with the center tap on pin 9). Also, note that while he said that he "installed a 5751 into the spot for a 5755," he did NOT say that he also did the opposite, as your last post seems to suggest.

The differences between the two pin-outs are:

For the triode section on the left (lower pin numbers):

1)Grid and cathode are interchanged between pins 2 and 3.

For the triode section on the right (higher pin numbers):

1)The connection on pin 6 that is intended for the grid of the 5755 would have been connected to the plate of the 5751.

2)The connection on pin 7 that is intended for the cathode of the 5755 would have been connected to the grid of the 5751.

3)The connection on pin 8 that is intended for the plate of the 5755 would have been connected to the cathode of the 5751.

I suppose that various damage scenarios could conceivably have resulted from those differences, and it's hard to be more specific without a schematic. But I would feel pretty certain that the problem wasn't a coincidence.

My speculation would be that an electrolytic capacitor was damaged. It might pay to look under the chassis to see if any capacitors appear to be bulging, leaking, etc. Of course, be careful not to touch any circuit points where a residual high voltage charge might exist.

Good luck. Regards,
-- Al