screen supply pot?

lowell · 11-14-2009, 11:11 AM

anyone try a 2w pot in series w/ the screens after the supply node for power scaling? I have a 2 6v6/2 6l6 amp and I'd like to try this. The screens/plates are around 460vdc. I'll leave the 470ohm screen resistors in there so when the pot is essentially dialed "out" of the circuit the 470s will still be in. Thinking of using a 2watt 5k pot. Any thoughts please contribute, but I might just order the part, try it and see.

guitician · 11-14-2009, 06:51 PM

You could try those tapped WW power resistors. They have a clamp that slides across an uncoated section of the resistance. If your just going to "set-and-forget", why use a pot?

Old Tele man · 11-14-2009, 08:03 PM

...two things to consider when playing around with screen voltages:

1) source VOLTAGE...it comes from the power supply and has a very low series resistance.

2) source RESISTANCE...it's any impedance between the source voltage to the screen and the screen itself, ie: screen grid resistor.

...putting a "pot" in the path would lower voltage, but also introduce a "changing" screen resistance in the screen grid path, which would likewise change both tubes' operating characteristics as well as the screen grids' voltage "sag" characteristics.

...is that what you want?

mooreamps · 11-14-2009, 08:12 PM

Quote:

Originally Posted by lowell

anyone try a 2w pot in series w/ the screens after the supply node for power scaling? .

You can install it and run it, but it's not power scaling.

-g

lowell · 11-15-2009, 01:41 AM

I'm not thinking "set and forget." hence the pot for manual adjustment as needed.

I figured it'd at a sag effect as well, not sure if I'll like that or not.

I guess yeah it's technically not as involved and compensational throughout the entire amp as power scaling is.

guess I try it and see how much sag it adds and if it's the effect I'm going for. I want to reduce power output, w/o an attenuator and after or in the PA.

booj · 11-19-2009, 09:56 AM

Sometime back a guy gave me a '55 Twin to fix. He was unsatisfied with the 20 or so watts it put out and ask me if I could do something about it. I noticed it had 10k between the plate supply and the screen supply. I put a 1k resistor in place of the 10k and got about 40 watts out of the amp, but the tone went from sweet and buttery to bright and hard edged.

11-14-2009, 11:11 AM	#1
lowell Senior Member Join Date: Jun 2007 Posts: 1,114	screen supply pot? anyone try a 2w pot in series w/ the screens after the supply node for power scaling? I have a 2 6v6/2 6l6 amp and I'd like to try this. The screens/plates are around 460vdc. I'll leave the 470ohm screen resistors in there so when the pot is essentially dialed "out" of the circuit the 470s will still be in. Thinking of using a 2watt 5k pot. Any thoughts please contribute, but I might just order the part, try it and see.

11-14-2009, 08:03 PM	#3
Old Tele man Senior Member Join Date: Oct 2007 Location: Tucson, AZ Posts: 430	...two things to consider when playing around with screen voltages: 1) source VOLTAGE...it comes from the power supply and has a very low series resistance. 2) source RESISTANCE...it's any impedance between the source voltage to the screen and the screen itself, ie: screen grid resistor. ...putting a "pot" in the path would lower voltage, but also introduce a "changing" screen resistance in the screen grid path, which would likewise change both tubes' operating characteristics as well as the screen grids' voltage "sag" characteristics. ...is that what you want? __________________ ...and the Devil said: "...yes, but it's a DRY* heat!"*

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11-14-2009, 06:51 PM	#2
guitician Senior Member Join Date: Aug 2008 Location: Phoenix Posts: 334	You could try those tapped WW power resistors. They have a clamp that slides across an uncoated section of the resistance. If your just going to "set-and-forget", why use a pot?

11-15-2009, 01:41 AM	#5
lowell Senior Member Join Date: Jun 2007 Posts: 1,114	I'm not thinking "set and forget." hence the pot for manual adjustment as needed. I figured it'd at a sag effect as well, not sure if I'll like that or not. I guess yeah it's technically not as involved and compensational throughout the entire amp as power scaling is. guess I try it and see how much sag it adds and if it's the effect I'm going for. I want to reduce power output, w/o an attenuator and after or in the PA.

11-19-2009, 09:56 AM	#6
booj Senior Member Join Date: Jun 2008 Posts: 333	Sometime back a guy gave me a '55 Twin to fix. He was unsatisfied with the 20 or so watts it put out and ask me if I could do something about it. I noticed it had 10k between the plate supply and the screen supply. I put a 1k resistor in place of the 10k and got about 40 watts out of the amp, but the tone went from sweet and buttery to bright and hard edged.

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