60Hz buzz after tone pot of a 50L6 PP amp.

[filmdiary]

Hi,

Just finishing up a conversion project using the chassis, output transformer, and pair of nice RCA 50L6 power tubes and I'm looking for ideas about how to quiet a 60Hz hum in the preamp. I probably should have avoided this build but it's for a friend that has been super generous so I'm doing the best I can!!

The amp is built into a RCA model 400 16mm projector and my friend wants to use the projector for projecting silent films while he rips on guitar :)... The main issue from the beginning was that the original amp was a widowmaker so the heater filaments are in series and were juiced from the mains 120VAC wall supply... VERY BAD!!!

First thing I did was install a 120:120 50VA isolation transformer and a bridge rectifier for the HV. I also completely gutted the original circuit and looked at the push pull 50C5 designs by Garnet that sold as 'stencil' amps as I've worked on a couple and loved the way they sound. The adjusted schematic is posted below...

The amp actually sounds great BUT I'm getting an annoying 60Hz buzz when the tone control gets cracked. The buzz disappears when the tone control is at zero. My guess is that there is something up with the preamp filament. I've tried a few different 12ax7's and the problem remains. I've tried chopsticking the heater wires and it doesn't seem to have any effect. I have shielded wire running from the input to the grid of the 1st triode... I've double checked ground connections from the input and everything seems tight. I've connected the series filament line to pins 4 and 5 of the 12AX7 and I get 11.8 volts between them. The 50L6's are at 47 Volts each. My guess is that there is 60 cycle noise leaking onto the grid :(... 

I'm considering installing a 2nd bridge rectifier and running the heaters off DC but I'm not sure this will actually make a difference. Before I go down this road i wanted to appeal to the brains here to see if there might be other methods for busting the buzz before the tone pot! I'm just assuming that if I try running the heaters off the same DC as the audio circuit that it might cause other issues... The thought behind adding a 2nd bridge rectifier would be to use much higher capacitance (1000uF) for the heater string to minimize ripple... I'm open to ideas!!! Did the American 5 radios have some magic bullet for dealing with hum and buzz from the tube heaters?

Thanks in advance!!!

John.



 

[PRR]

Layout. I bet that tone pot and network run near rectifier wiring. Maybe heater but maybe not. Picture?

[octal]

This doesn't quite fit with the "it changes when you adjust the tone" scenario in the same way that PRR's suggestion does, but it would be interesting to fully bypass the cathode of the first 12ax7 and see what happens. Put a really large cap across that that cathode resistor, maybe 220uf or so. With the filaments not being referenced to ground in the normal way, I'm suspicious of heater to cathode leakage.

[filmdiary]

I've been thinking a lot about the fact that the heater has no ground reference so that makes sense to me! Heater connected to pins 4 and 5 and the cathode of the first triode is pin 3 so indeed that could explain things... looking forward to trying a bypass cap to see if anything happens :)... have none in stock so it will have to wait for tomorrow :) Thanks for your responses!

The bridge rectifier is on the back side of the chassis... it's all jammed petty close together but I did rotate the iso transformer to see if there was much of a difference in hum but there wasn't. I tried to make sure the windings were oriented at least 90 degrees opposite the OT...

The output transformer CT is connected to the positive, the red wire goes directly to the 47uF filter cap.

[filmdiary]

here is the other side and a close up of the preamp socket... it's tighter than it needs to be :)... but I tried to keep the heater pair away from the grid!

[WimWalther]

"The main issue from the beginning was that the original amp was a widowmaker so the heater filaments are in series and were juiced from the mains 120VAC wall supply... VERY BAD!!!"

Oh, come on now.. no, it's not an ideal design, but over the decades literally millions of radios, phonographs, etc. were built with series-string filaments and cutthroat AC/DC power supplies.

These things are not nearly as dangerous as many (often younger folks) make them out to be. Simply adding a polarized plug improves things a lot - as does verifying that your AC wall sockets are wired correctly.

[PRR]

> millions of radios, phonographs, etc.

But NOT with the user holding bare chassis.

Radios are etherwaves in, soundwaves out. Phonos are needle in, soundwaves out. Generally you can't touch bare chassis. This was a film projector so same thing except a photocell, lightwaves.

In guitar we never got away from the user holding bare metal, because most metal on the guitar (and user also) must be bonded to amplifier common to reduce buzz.

Most forums think it is poor form to encourage hot-chassis projects. YOU may know how to do it, but it isn't something that lurkers always get right from reading the internet.