Motorboating after tube change


I posted this on Audioasylum tubes, but I'd like to increase the knowledge base, so here I am.

I own an Aikido preamp that was built for me in 2007. Since I don't use it that much, I haven't replace the tubes till now. After I changed them out, there was clearly audible motorboating emanating from both speakers. I returned each old tube to the unit but the motorboating didn't stop. The only screwup was that when initially replacing the tubes, I accidentally installed a 5751 into the spot for a 5755.

Any ideas?

Thx,

GP
Convert?fit=crop&h=128&rotate=exif&w=128garibaldi
The only screwup was that when initially replacing the tubes, I accidentally installed a 5751 into the spot for a 5755.

The pinout for the two tubes are not the same.

5751

5755

You need to let the guys on AA know about the tube mix up.
I am sure you will get a different set of answers and hopefully some new answers.

You say motor boating? Is the music cutting in and out at a regular pattern?
.
Motorboating is an indication of a power supply problem.

My theory is that the different tube somehow was able to do some damage, but it is not obvious how upon looking up the two different tubes. So it could be coincidence, but at any rate you are experiencing a power supply problem.

My theory is that the different tube somehow was able to do some damage, but it is not obvious how upon looking up the two different tubes.


Atmasphere,

I was wondering the same thing as neither heater of the tubes would have been energized. But what if the designer of the amp connected one lead of the filament secondary transformer winding to chassis to save wire or trace and at each of the tube sockets connected one heater socket pin terminal to chassis? If that were the case I can see a completed circuit where plate voltage would have been directed through one section of a triode through the heater to power supply B- chassis. Not sure what that load/s could do to the power supply. Just a thought.

On another note, as a kid I remember if a coupling/blocking capacitor was bad it would not block the high plate DC voltage from a preceding tube stage from entering the grid of the next tube along with the signal. Just going from memory the music was heard through the speaker only in very short segments of on and off repeated sequences repeatedly. Back then that was also called motor boating.
Jim
Thanks, Guys.

Based on your responses and those on AA, I have a coincidence on my hands. Hard to believe, but there it is. Thanks for diagnosing this over the interwebs.

GP
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

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.
I agree with Bjbcab, that's the only reason why I read this thread :)
Lou, b aware that Bj-bcab already sounds wierd!
Be nice Czar, he doesn't have a hyphen in his screen name.