Plate voltage trimmer pots

[pine]

I was browsing through some older posts, when I came across a post by BobW in which he suggested using trimpots to dial in the sweet spots on the preamp plate voltages.

"67_Plexi, thanks for your research. Based on your observations using the Mullard standard at 201 VDC and 1.2 mA, it would be interesting to further determine what Vp and Ip works best based on the manufacturer. I know that's a lot more research, but even with a 10 to 15% rejection, it may provide a window to adjust plate resistor values using trimmer pots located at each plate. I assume your Ip @ 1.2mA is based on a no input signal? Thanks again for the research. "

Given the wide discrepancy in the actual voltages for 12ax7s, even made by the same manufacturer, is there any merit in using trimpots for this purpose rather than swapping resistors to get to that narrow window when preamp tubes are changed? Has anyone incorporated this idea in their design? Tried a search, but did not see much.

[Structo]

As far as I know most guys here like to use the 67:1 ratio of plate to cathode K ohms.
The resistor to change would be the dropping resistor on the power supply as that is what determines the actual voltage at the tube.
I suppose you could fine tune a specific tube in the manner you speak of but I think most of us just roll a few tubes through each spot and listen to them.

The phase inverter is one spot where fine tuning the AC voltage can bring sweetness.
Most of use tune that by ear but if you have a scope and a signal generator you can also do it that way.
Same deal roll some tubes through there and choose the one that balances out nicely and sounds good.

[pine]

Thanks for the reply; that is what I was getting at; if you could add a sufficiently stout trimpot in series with a dropping resistor, couldn't you just dial in the voltage to the tube? That way if you had a really nice NOS tube you wanted to use, you could dial in the target voltage. Perhaps I'm over-tweaking though...

[markusw]

In a D-style preamp I use a 10k 3W cermet potentiometer in series with 2k2 resistor that together replace the 22k resistor in the power supply dropping string (the one feeding the V2 node).
The potentiometer allows to adjust the plate voltages for V1+V2 from something like 210VDC down to about 160VDC for NOS Philips Holland/Mullard 12AX7s. With current production tubes the voltage range is slightly higher.
With it's 3W it obviously also stands the current inrush after switching from stdby to on. At least I didn't experience any troubles so far.

Edit: I think the pot is a nice addition since one can easily check the influence of the plate voltage on the sound.

Peace,

Markus

[thyx]

The cathode resister is a great place to tweak. For example, go from 1.5k to 1.2k with a 500 ohm trimmer in series. Adjust the trimmer to give you a total of 1.5k and go from there. There's usually a sweet spot to find.

[hans-jörg]

The cathode resister is a great place to tweak. For example, go from 1.5k to 1.2k with a 500 ohm trimmer in series. Adjust the trimmer to give you a total of 1.5k and go from there. There's usually a sweet spot to find.

Hi,
and which cathode you mean within the ODS?

Hans-Jörg

[pine]

In a D-style preamp I use a 10k 3W cermet potentiometer in series with 2k2 resistor that together replace the 22k resistor in the power supply dropping string (the one feeding the V2 node).
The potentiometer allows to adjust the plate voltages for V1+V2 from something like 210VDC down to about 160VDC for NOS Philips Holland/Mullard 12AX7s. With current production tubes the voltage range is slightly higher.
With it's 3W it obviously also stands the current inrush after switching from stdby to on. At least I didn't experience any troubles so far.

Edit: I think the pot is a nice addition since one can easily check the influence of the plate voltage on the sound.

Peace,

Markus

Hi Markus;
Where did you find the 10K 3 watt cermet? I'm having trouble locating them.

[markusw]

Hi Pine,

I bought it from rs-comp here in Austria.
In US you could get it here
http://www.alliedelec.com/search/produc ... KU=9700381
or from Mouser.

Not cheap but it does it's job.

Regards,

Markus

[Zippy]

In a D-style preamp I use a 10k 3W cermet potentiometer in series with 2k2 resistor that together replace the 22k resistor in the power supply dropping string (the one feeding the V2 node).
The potentiometer allows to adjust the plate voltages for V1+V2 from something like 210VDC down to about 160VDC for NOS Philips Holland/Mullard 12AX7s. With current production tubes the voltage range is slightly higher.
With it's 3W it obviously also stands the current inrush after switching from stdby to on. At least I didn't experience any troubles so far.

Edit: I think the pot is a nice addition since one can easily check the influence of the plate voltage on the sound.

A tweaker's delight!

Sweet.

[pine]

Hi Pine,

I bought it from rs-comp here in Austria.
In US you could get it here
http://www.alliedelec.com/search/produc ... KU=9700381
or from Mouser.

Not cheap but it does it's job.

Regards,

Markus

OK, now that I've looked at it you have me thinking of mounting it on the rear panel

[scotto]

I put trim pots in place of my dropping string to dial in what sounds best to my ears. after I was dialed in I replaced them with appropriate resistors with the exception of the last one. Left the pot in there so I could still do some treaking if i change tubes. before I powered up the first time with pots in i dialed the pots to typical string values. Couple of the pots had enough range to be way off if I didn't. Obvious safety precautions since you are in while it's hot..meter with clips to attach while cold and one hand in pocket while tweaking. Worked well for me.

[markusw]

OK, now that I've looked at it you have me thinking of mounting it on the rear panel

In my preamp I also have it panel mount. I like it there