Trimmer Replacement

[WntrMute2]

I am building a 300B amp with a CCS on the driver tube and the Coleman V-9 regulators for the 300Bs. I would like to be able to adjust these from outside the chassis instead of the small pots on the circuit boards. Are there any pitfalls in using 1KΩ (for the CCS) and 500Ω pots (for the V-9 regs)mounted a few inches away from the circuit boards?

 

[rayma]

If you do this, for each pot keep all of the wires short, and twist them together.
The wires should have a HV rating larger than the supply voltages.

Bear in mind that the pots could have HV on the wires (depending on the circuit), and thus on the pot element.
Verify the pots' voltage isolation capability from the pot case and ground, since the metal body of the pots
should contact the chassis for safety, grounding, and shielding purposes.

 

[WntrMute2]

Getting the boards mounted in such a way that the adjustment protrudes the correct amount through the casework is challenging for me.. Rod Coleman suggests his regs be adjusted when changing tubes is why I want to do this.

 

[euro21]

I would like to be able to adjust these from outside the chassis instead of the small pots on the circuit boards.

Why?
This is a very bad idea.

CCS loaded driver may part of rapid oscillation -even at MHz range if you use high gm trioded pentode- if the CCS far from tube's anode. Sometimes anode stopper is necessary even so CCS near to tube.
I have several CCS loaded amplifiers VAS stages/preamplifiers. If you set the CCS, years pass before it has to be set again.

So this is also true for R.C. regulators (I built few bunch of them). After preheated and warmed up quite a bit, after several initializations setting is became permanent. I check it every year, but in my 801 preamp never had to set it up.

 

[WntrMute2]

I understand what you are writing. The thing is that my Coleman regulators in my 3C24 amp seem to need tweaking every few months (maybe I built them incorrectly) AND the instructions are to adjust if the tubes are changed.

 

[dubadub]

Don't put knobs on the outside of the amp you don't want kids messing with. Just a personal rule.

It also means I can't build amps with plate cap connections. Don't poke the bear.

 

[WntrMute2]

No kids or dogs that can reach. No worries.

 

[euro21]

adjust if the tubes are changed.

In every few months need to change output tube?

 

[russc]

Position the board so the trimmer is below a strategically positioned hole in the chassis.
Add a length of tube (brass, rigid plastic) to act as a guide for a screwdriver blade, through the chassis hole and surrounding the trimmer screw.
No slipping blades, sparks or bangs.

 

[WntrMute2]

In every few months need to change output tube?

No, but I do check them occasionally and the filament voltage drifts. I do not wish to disassemble the amp every time I want to roll 300bs. This is for fun after all.

 

[TR000]

My one 300b amp with Colemans regs drift too, another 300b amp are stable. Maybe bad trimmer pot?

 

[TR000]

I replaced trimmerpot and no more voltage drift.

 

[WntrMute2]

I have decided to stick with the board mounted trimmers but another question arises. I can find the form factor I want in either Bourns Series: 3296, 3299 or for about a third the cost, the PV36. Trimpot Information

Any reason to select one series over the other? One set will be used for adjusting a CCS and the other sets will be for Coleman regulators.
Thanks in advance.

 

[rayma]

The 3296 appears to be a mil spec version of the 3299. Otherwise they appear equivalent.
The P36 has few listed specs, and is an economy pot. I would stay away from it.

 

[euro21]

Both application use trimpot as variable resistor, so accuracy or monotony is unimportant.
Use 50ppm 25 turn versions with appropriate power rating.