Phase Inverter: Plate resistors and tube types

[Darkbluemurder]

I noticed that most non-HRM schematics do not specify the PI tube. Is it a 12AT7 like in Fenders or is it a 12AX7? I tried both and both sound good.

My clones have 82k and 100k for plate resistors. How would the sound change if I moved to 100k/110k or 100k/120k?

Thanks for your input
DBM

[glasman]

I noticed that most non-HRM schematics do not specify the PI tube. Is it a 12AT7 like in Fenders or is it a 12AX7? I tried both and both sound good.

My clones have 82k and 100k for plate resistors. How would the sound change if I moved to 100k/110k or 100k/120k?

Thanks for your input
DBM

The 12AX7 is the standard for that position. As far as plate resistors I have tried 82/100, 100/100 and 100/110. All with a balance trimmer. I am not sure that I ever heard a difference .

Gary

[Darkbluemurder]

Gary,

Thanks a lot! So I can rest my mind by leaving the PI plate resistors at 82k/100k and just swap in a 12AX7.

Cheers,
DBM

[dogears]

You could, but, I have never seen a Dumble with an 82k. Many have much higher values. The last one I saw had a 100k/110k with a 25k trimmer. I have seen as high as 120k/130k and no trimmer. Some had 91k/110k. If I were you, I would raise the plates up. For accuracy sake.

Gary,

Thanks a lot! So I can rest my mind by leaving the PI plate resistors at 82k/100k and just swap in a 12AX7.

Cheers,
DBM

[seulb-rd]

cheers! long time lurker, first time poster.

i've got a clone up and running. i'm using the hybrid rev. A schemo. much tweaking left to do. i have 91/100 pi plate resistors. my question is how important to "the sound" is getting the plate voltages close to the schemo voltages of 317 and 303? i'm getting 303 and 299. i've tried 100/100 and still got only 309 and 304. i'm not using a trimmer at present. should i? i'll try swoping positions for the 91/100 and see what i get.

please advise.

[swt]

I think that the tail setup...820 and 24k makes the voltage goes higher at the plates, that's why some amps use larger resistors...to make the voltage go down, and have a more organic tone. My best sounding clone is aprox 290/280 and sounds terrific...

[dogears]

Hi seulb-rd,

Measured voltage means nothing (unless it is REALLY off). Don't pay any attention to the voltages written on the schematic. Especially the 230v on V2......

My suggestion is to raise the resistors to 110/120 which would simulate the addition of a 20k trimmer to a 100/110 setup. This is quite common..... Last Dumble I checked out had measured resistance of 113/122 on the PI plates (measured with trimmer). So, something in the 100/110 or 110/120 area should be good.

Fwiw, different tubes will show different plate voltages.....

If you want, PM me the layout and I'll check it and make some suggestions.... Hybrid A schematic has a few bad mistakes/confusing alt values on it.

cheers! long time lurker, first time poster.

i've got a clone up and running. i'm using the hybrid rev. A schemo. much tweaking left to do. i have 91/100 pi plate resistors. my question is how important to "the sound" is getting the plate voltages close to the schemo voltages of 317 and 303? i'm getting 303 and 299. i've tried 100/100 and still got only 309 and 304. i'm not using a trimmer at present. should i? i'll try swoping positions for the 91/100 and see what i get.

please advise.

[pazzo63]

Ciao,what is the real benefit to raise the values of the plate resistors?in some amps were 100k and 120k on the bottom.Thanks