Motorboating after tube change

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

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).
02-10-15: Almarg

Al,

You are absolutely correct, my bad. LOL, I wish I could blame my mistake on looking at a small screen but I cannot. I am dumbfounded for an explanation. I will just have to blame it on a brain fart moment.

5751

Pin 1: Plate (Section no. 2)
Pin 2: Grid (Section no. 2)
Pin 3: Cathode (section no. 2)
Pin 4: Heater
Pin 5: Heater
Pin 6: Plate (Section no. 1)
Pin 7: Grid (Section no. 1)
Pin 8: Cathode (Section no. 1)
Pin 9: Heater Center-Tap

5755

Pin 1 Plate, unit 2
Pin 2 Cathode, unit 2
Pin 3 Grid, unit 2
Pin 4 Heater
Pin 5 Heater
Pin 6 Grid, unit 1
Pin 7 Cathode, unit 1
Pin 8 Plate, unit 1
Pin 9 Heater center-Tap

Jim

It happens to the best of us, Jim :-)

I note that two of the absolute maximum ratings of the 5751 would most likely have been exceeded by a considerable margin as a result of inserting it into the 5755 socket. The absolute maximum negative grid bias rating of 55 volts (presumably relative to the cathode, which had what should have been the voltage applied to the plate applied to it instead, in one section of the tube), and the absolute maximum heater-to-cathode voltage rating of 100 volts. I'm not sure what the results of that might have been, but it seems very conceivable to me that it could have caused some kind of breakdown of the tube, in turn resulting in secondary damage to something in the power supply or other circuitry.

Best regards,
-- Al

Thanks for all the responses. It's apparent to me that the fix is beyond my capabilities. Therefore, it's going in for repair. It'll take a while for an answer since the unit needs to be shipped. I will post the results of that repair here when I get it back.

Again, thanks for all the responses. I truly appreciate the information that everyone offered.

Regards,
GP

From what I saw from pin locations there could be some protection involved and maybe some resistor is blown that provides voltage drop for the offset.