Elevating heater via 100K resistors...

[FUCHSAUDIO]

The values should stay high, so the resistors draw no current (and will not heat up) and will not load the point from which you are feeding them. The time constant means if you have a 1-Meg on the top, and too big a cap down below, the voltage takes too long to ramp up. You don't need much filtering to clean up a voltage source coming through a 1-M resistor, so a .1 UF 400 mylar works fine....hope that makes sense.

[Cliff Schecht]

Later on in the circuit you have a lot more resistance and capacitance on the B+ which means it will take longer for the elevated heaters to reach their quiescent operating point (less current is available later on in the supply chain). Plus the preamp voltages tend to jump all over the place, even at the B+ rail, when you have an amp that sags under heavy playing. Using a higher voltage would make this effect less noticeable.

[FUCHSAUDIO]

I usually grab it post choke at the screen supply. The time constant of the lower cap prevents it from hopping around much. A fairly rigid voltage source. You can mount the whole thing on the cap lug and ground lug or add a terminal strip if need be.

[Cliff Schecht]

So you add this circuit to every amp that has a CF? A couple of my recent builds are a tad buzzy (120 Hz) and I was thinking they would all benefit from this. The buzz I'm getting isn't more than your typical amp would get FWIW but I'd like to knock the noise floor down as low as possible, these are supposed to be studio amps.

[FUCHSAUDIO]

It's a mod that costs pennies, and it cannot hurt....

[Gaz]

The values should stay high, so the resistors draw no current (and will not heat up) and will not load the point from which you are feeding them. The time constant means if you have a 1-Meg on the top, and too big a cap down below, the voltage takes too long to ramp up. You don't need much filtering to clean up a voltage source coming through a 1-M resistor, so a .1 UF 400 mylar works fine....hope that makes sense.

Thanks for the explanation, appreciate it. I guess I still don't understand why the voltage taking too long to ramp up would be an issue. Bias voltage I get as it endangers the output tubes, but not the DC heater elevation.

Later on in the circuit you have a lot more resistance and capacitance on the B+ which means it will take longer for the elevated heaters to reach their quiescent operating point (less current is available later on in the supply chain). Plus the preamp voltages tend to jump all over the place, even at the B+ rail, when you have an amp that sags under heavy playing. Using a higher voltage would make this effect less noticeable.

And Cliff, are you concerned about the elevation voltage jumping around, or it somehow effecting the tone of the amp? Since I normally elevate to 60-70vdc, I never really was concerned about the elevation voltage sagging a bit. I guess that's what you were saying at the end of your post.

I also read somewhere that the heaters 'don't like getting hit with a DC voltage, which is what happens when you flip the standby with they typical elevation off the B+ rail method'. I don't remember the reasoning behind it however. I've heard of a lot of folks elevating heaters this way, but no one complaining of increased tube failure, so I never really gave it much thought.

[Cliff Schecht]

You are still running the tubes on AC, you are just adding a DC offset to the AC. They will still be seeing an AC signal even if the DC moves around so this is different than straight DC heaters. The issue you are thinking of is DC heaters causing hot spots on the filament wires which is a legitimate concern but hasn't ever been a problem for me either. I think some tube types are more susceptible to this than others.

I think the problem of the heater DC moving around a bit is a non-issue. This is why we add a decoupling cap to the bottom resistor, it counteracts quick changes in the voltage. This doesn't hurt the tubes but if the amp sags too much, you might notice some goofiness like the heaters inducing more noise as you play the amp hard and cause the voltage to droop too much (more potential between the c-h inducing more hum). Or if the voltage dropped enough to hurt CF or normal inverting gain stages (this shouldn't be a problem in most any guitar amp).

[Gaz]

Cool, just make sure I got what you were saying. Thanks.

[Guy77]

Didn't mean to disturb this old thread but couldn't resist!
I used this trick on an amp (ODS) and it worked like a charm! Removed the unwanted hum I was getting from the heaters and got it nice and quiet!

I created a 1M - 100K voltage divider (1 watt resistors) right after the choke and before the screens and used a .1uf 630v Orange drop cap. I set everything up on a small terminal strip.

The blue wire is coming from my 100 ohm resistors and the beige resistor is coming off the power tube screen supply, then the a 100k resistor goes across the orange cap and everything to ground.

20230127_112100.jpg

Happy Friday everyone!

Guy