If you make a virtual center for the filament winding you must disconnect the connection to V2 cathode. The, "usual" value is a pair of 100 ohm resistors.

Sounds like it's worth a go. 1/2 watt resistors should suffice, correct? Thanks!

While I'm in there, I'm thinking one other mod. The amp doesn't have a master volume control (and there's not really any room on the front panel for another potentiometer). I'm thinking of simply creating a "2-step" master volume with a small switch (already installed) so I can lower the input into V2 and turn the overall level up with the normal volume pot. The idea is turn up the volume/gain/distortion on V1 without raising the total volume. That way I can get a nice crunchy "tube tone" at a reasonable volume level, so my plan is split that 220k resistor before V2 into 2 resistors, such as a 160k and 56k, and use a small SPDT toggle to select either the full output of V1 or a smaller value from the voltage divider. my understanding is I need to cut the voltage by about 4 in order to reduce the volume by half.

 

This is how you're, "supposed" to do it, but your method will work.
One of my tricks is to put a pot in series with the 100 uf on the V2 cathode.

 

This is how you're, "supposed" to do it, but your method will work.
One of my tricks is to put a pot in series with the 100 uf on the V2 cathode.

If I had room for another pot, that's precisely what I'd do.

Another idea I had was to replace one of the tone controls, Bass or Treble, using it's spot for a Master Volume, and using the other knob as a simple Tone control, but I'm not sure what exactly to mod in the tone stack to make that happen.

 

That's a baxendal tone stack (if I spelled it right). You would have to replace it with a single knob type of tone control. Center boost and cut is one way. The Supro uses a HF cut only type.

The Internet needs an acronym for, "If I spelled it right". IISIR?

Edit: Baxandall
Edit: I think a center boost and cut would be right for this one, but it's YOUR ear that needs to be pleased.

 

The Internet needs an acronym for, "If I spelled it right". IISIR?

Agreed. Personally, I use "(sp?)".

It's something my school teacher's wrote on my papers after a misspelled word to say. "Ya sure that's how it's spelled? Perhaps you should double-check."

 

Many ways to skin the cat.

 

It worked!!!

I went ahead and performed the mod to the filament heater using a virtual center tap. I set up a "hum dinger" by placing a 200 ohm potentiometer across the filaments with the wiper to ground and with a slight adjustment, knocked out most of the noise! Note: ModKits replied to my e-mail and said the noise was "normal".

I also went ahead and repurposed the BASS adjustment knob as a Master Volume control and simply wired up a couple of micro toggles and resistors to modify the tone stack. Pretty much kept the Treble control as is and created a Mid Cut and Bass Cut control. No, it's not a 31-band parametric EQ, but for such a simple amp, it does the job.

So glad I went ahead and made the changes. I figured once I placed it inside a protective cabinet, I'd be less likely to take it out and tinker with it. Plus, I did build the thing to learn and gain experience... which I did.

Now, the only remaining issue is the amp still plays fairly clean, even with the input to the second stage of V1 maxed out. I guess I could get more distortion by turning up V2 all the way, but that's just too loud for most situations. What this little amp needs is the classic response to this classic problem... add an Overdrive pedal! Which is my next project. Stay tuned.

Thanks again everybody for all your help and input. I just may gig with this little amp one day.

 

Thanks for coming back to tell us. I know I learned something.

 

It worked!!!

I went ahead and performed the mod to the filament heater using a virtual center tap. I set up a "hum dinger" by placing a 200 ohm potentiometer across the filaments with the wiper to ground and with a slight adjustment, knocked out most of the noise!

That is very valuable to me.

Still...wouldn't DC for the filaments accomplish the same thing?
Maybe that's why my little design worked so flawlessly. I used DC for the filaments just to be really cautious.

 

Still...wouldn't DC for the filaments accomplish the same thing?
Maybe that's why my little design worked so flawlessly. I used DC for the filaments just to be really cautious.

I've been reading up on filament noise and reduction. I didn't really understand elevating the heater AC voltage by tying one side to the 5.5 VDC at the V2 cathode, but now I have a better grasp on it. Apparently, by making the heaters more positive than the cathodes, it reverse biases the equivalent diode that exists between the cathode and filament. However, many designers and builders used much higher DC, such as 40-70 volts, not a piddly 5.5.

That being said, now I'm trying to understand why the center tap, real or artificial, works? This causes the heater to swing negative for half the cycle (even more so if the cathodes are biased slightly positive), which should cause some current flow between the cathode and filament resulting in hum. However, I've seen more than one person, other than myself, state the center-tap worked better than DC elevation.

Lastly, DC current for heating is touted as one of the best ways to reduce hum, but only if done "correctly", otherwise the noise is even worse. Not sure what the incorrect way is, but I'm still studying.