power supply dropping string potentiometer

[beasleybodyshop]

In trying to obtain proper plate voltages, has anyone tried installing a 3 or 5 watt linear taper pot in place of a resistor and dialing in the value until your plates measure where you want them? Is this a good idea? I found this on mouser, it's a 5W pot rated at 500v: http://www.mouser.com/Search/m_ProductD ... IKgslg8%3d[/i]

[Stevem]

so you want to drop just the plate voltage and keep the rest of the power supply nodes at the current level of B+ ?

[Phil_S]

Plates take the supply from the first B+ node. Exactly where do you intend to install such a pot and produce a result?

[sluckey]

I don't think he's talking about dropping voltage to the 'power' tubes.

[matt h]

(deleted)

[Leo_Gnardo]

For output tubes, sacre' bleu, def. NOT.

For pre tubes, hm, maybe, if you followed your HV trim with a filter cap. Why? You know DC on pots makes scratchy noises, sometimes even when you're not moving the wiper. You don't want that in your audio - so an extra filter cap may do the trick.

IIRC one of the Ampeg SVT rackmount variations in the 90's (SVT II ?) had a control that did exactly what you describe: it adjusted plate voltage on one of the tubes, maybe the output drive. Dialed to full voltage, crisp & clean. Dialed down, kinda flubby to simulate an old worn out SVT tone, that has some charm too. Note they didn't continue with this idea for long. Mostly players looked at the control, said "what this?" didn't hear any EQ change or anything else obvious, and just left it parked at one or the other end of its travel. One thing's for sure: there was a bit of a time delay - a couple seconds - in what that control did, indicating a cap charging/discharging.

More complex to install, but less likely to be noisy/burn up, multiposition switch.

[beasleybodyshop]

Sorry I should have been more clear. I did mean the preamp plates, not the power tube plates. The idea would to be to dial in the desired plate voltage, unhook the pot and measure it, then sub in the proper resistor.

Could also be fun for testing lowered or raised preamp plates in real time.

but no, my intent would not be to keep the pot there forever. Just as a building tool.

[Leo_Gnardo]

The idea would to be to dial in the desired plate voltage, unhook the pot and measure it, then sub in the proper resistor.

Something like my old Eico resistor sub box only not so wide ranging. I'd recommend a rotary switch & selection of standard value R's that might be found in plate circuits, that will save you having to measure the pot. If you can find an old R sub box, swap in your choice of R's say from 47K to 470K, relabel the panel & off you go.

[beasleybodyshop]

The idea would to be to dial in the desired plate voltage, unhook the pot and measure it, then sub in the proper resistor.

Something like my old Eico resistor sub box only not so wide ranging. I'd recommend a rotary switch & selection of standard value R's that might be found in plate circuits, that will save you having to measure the pot. If you can find an old R sub box, swap in your choice of R's say from 47K to 470K, relabel the panel & off you go.

Thanks leo. Those older decade boxes are getting more expensive to find. Newer ones use the really small 1/4w jobbies. Guess I could just *build* one but I figured it would be cheaper to go with a pot.

[Leo_Gnardo]

The idea would to be to dial in the desired plate voltage, unhook the pot and measure it, then sub in the proper resistor.

Something like my old Eico resistor sub box only not so wide ranging. I'd recommend a rotary switch & selection of standard value R's that might be found in plate circuits, that will save you having to measure the pot. If you can find an old R sub box, swap in your choice of R's say from 47K to 470K, relabel the panel & off you go.

Thanks leo. Those older decade boxes are getting more expensive to find. Newer ones use the really small 1/4w jobbies. Guess I could just *build* one but I figured it would be cheaper to go with a pot.

Too bad there wasn't a sub box along with the other prizes you found. To build from scratch, yeah I s'pose the pot will do. I'd put say a 47K in series with a 250 or 500K pot, and hand-mark the panel to save yourself a lot of time faffing with the ohmmeter. In case you do find a sub box cheap or free, you'll know what to do. Start collecting all those 75K, 82K, 91K, 100K, 110K, 120K, 150K, 180K, ........ ed cetera.... Needn't be a "decade", a bit over complex for the purpose and yes costly. Just a 10-12 position rotary switch & insulated box.

[Smokebreak]

Cheaper to build, but Weber has a whole mess of subshttps://taweber.powweb.com/amptechtools/subs.htm

[beasleybodyshop]

Cheaper to build, but Weber has a whole mess of subshttps://taweber.powweb.com/amptechtools/subs.htm

Yeah! Those are perfect! Might order one after I snag a high voltage cap tester.

[Smokebreak]

Better yet, do the PI too! http://www.ebay.com/itm/2-x-Full-Size-4 ... 3a7ada5e06

[matt h]

(deleted)

[Phil_S]

It seems to me, all this can be figured using Ohm's Law. If you give me a schematic (as built) with a complete set of actual voltages, and provide the desired preamp voltages, I think I can get really close on the resistor values for the B+ ladder.