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Virtual Center Tap

4/11/2006 11:34 AM
Virtual Center Tap
Hi Guys,  
I completed my first 5E3 build over the weekend. After checking voltages and all that during the first start up, I had quite a bit of interactive hiss when turning the volume and tone controls with nothing plugged into any of the inputs (always something...). I used Mission Amp tranny's (they sound great!). On a hunch, the first thing I tried was to do the virtual center tap and ran two 100-ohm resistors to ground off of the heater circuit at the pilot lamp. The PT does have a center tap which was grounded with a soldered and crimped ring terminal bolted down to the PT with the power cord AC ground and the 1st two filter cap grounds. I grounded the virtual center tap here as well. The inputs are not shielded - for what ever reason I left them unshielded as per most layouts to start with. Anyway, that totally resolved the noise and the amp is quiet as a mouse. The voltages all check-out great and I'm satisfied that the amp is running optimally.  
My question is - why does this work? I can follow the circuit well and generally understand how the signal flow works, how each cap and resistor effect the signal and so forth, but I've never really been able to understand why this method will cure a noise problem like that when the PT already has a center tap that is grounded.  
BTW - I followed Al's lead on beefing up the circuit for 6L6 use and got just great results. I'm starting to think I may be more of a 6L6 man - I tend to keep a 6L6 and 5V4 in my 5F1 build and so far I'm liking the 6L6's in the 5E3 a bit better than the stock set-up!

Allen I too would like to get a good gras... -- 4/11/2006 3:48 PM