SVT fan coming through speakers

[WholeToneMusic]

I am working on an SVT and can hear the fan noise coupling through the speakers. It is quite annoying to say the least. Anyone have any tricks for decoupling the fan noise from the speakers without disconnecting the fan from the circuit?

Recently I had to replace a blown output tube and one of the 5 ohm 5 watt resistors it took out with it. I re-biased it to .073vDC for both K1 and K2 and balanced the PI along with adjusting the hum balance All caps are original.

[Mars Amp Repair]

I think most of the SVT fans are 110VAC motors. I'm surprised that any interaction would take place between a 110V fan & the amp.

I'd try moving the fan physically away from the amp to be certain is isn't inducing the noise directly into the amp from the motor coils.

Perhaps it's just a bad fan.

glen

[Don Moose]

The schematic I have (6550 outputs, by Joe Piazza) says the fan is a 120VAC unit. If it's original, it's probably a pretty old one and the brushes may be worn and noisy.

I'd suggest a good-quality replacement. Hmm - neither digikey nor mouser stock a 120VAC 5.25" (133mm) fan. Is that the right size and voltage?

Ah - newark has 287 available at ... $78.80 + shipping . The good news is that Papst is one of the better brands, and these should last at least as long as the old one did.

Hope this helps!

[WholeToneMusic]

Thanks Glen,
I looked at the SVT schematic and it is wired to the AC. I will do as you suggested. I have another SVT i am also working on that I can swap fans also to see if it is the fan.

Best Regards,
Brian

[Gtr_tech]

No brushes in an induction fan motor.

I'd maybe poke around with a scope to see exactly where the noise is getting in, but I'd sooner just replace the fan with a different one and see if the noise goes away.

[olddawg]

Are you sure that you just didn't move the wire dress and AC is being picked up by proximity? A microphonic preamp tube could be picking it up also. Induction motors are not known for being noisy unless it is bearing noise.

[metalangst]

I'd look at a microphonic tube amplifying the fan vibrations.

[WholeToneMusic]

Your going to love this, I hooked up the spare fan and still the same noise. I unplugged the fan and started hitting on the top of the head ( not out of anger and frustration.) and noticed the whole head was resonating through the head. I grabbed the trusty Weber BBQ glove and started holding the power tubes one at a time. Then I grabbed hold of one of the 12BH7's and the noise went away somewhat. I put in a spare pair of the 12BH7's and voila. The tube that came out had a white powder residue inside, so it had lost it's vacuum and when i shook the tube it had some loose parts inside. Thanks again for all the replies.