Peavey VTM 60/120 DC heater mod - Illustrated

[Guitarist]

This Boogie-style, DC heater mod seems to working great. Quieter for sure. I notice whenever doing this to an amp it seems to bloom, just like cleaning your glasses making the tone transparent



To lose the last bit of hum, I tried lifting the output's heaters stock center-tap (after lifting Peavey's 220uF cap) and it worked (balance control now ineffective)! But is this OK for the amp?

For crunchy tones this is the best amp I've ever owned or worked on. The best I played through was a VHT Pitbull.

(There might be a schematic error, as the LED Voltage circuit shows tied to one side of the heaters, but prior to this mod there was no DC floating on them. I may've read/printed this wrong).

[Enzo]

You are not reading the schematic correctly, it is fine. The LED circuits are not running off one side of the heater voltage, the DC supply the LEDs use is also used to elevate the AC heater supply. And that is why there is a e-cap at the hum balance control instead of a ground. If there was no DC on the heaters before, that was probably why it was humming in the first place. F3, CR6, C8 form a small + supply, I forget how much, probaby 20-25v or so. Then R10 connets that DC offset supply to the heater AC.

[Guitarist]

So DC injection doesn't have to be through a "center tap" either side works? That could make things easier down the road on other amps. I called Ho here in Vancouver and he mentioned that it should be fine to use the amp without the tap. The LEDs still work too.

thanks for the reply

[Enzo]

There are MANY sources of hum in amps. Each source has its own cures. And for example weak filter caps cause hum, but all the filter caps in the world will do nothing to cure hum from AC heaters. So hum is not generic.

One POSSIBLE source of hum - and not all causes show up in all amps - is electrons boiling off the heater and finding their way to the cathode nearby in a preamp tube. This can only happen if the cathode is more positive than the heater. Electrons have a negative charge and are attracted to positive things. In that case, what matters is that the heater be made more positive than the cathode. No, you don;t have to use the center tap, as long as your DC offset is large enough to keep the heater more positive than the cathode through the entire cycle of the heater AC.