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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
128x128garibaldi

Showing 2 responses by jea48

jea48
4,266 posts
 
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?
.
jea48
4,266 posts
 

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
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