Replacement bias trimmer pot for Hot Rod Deluxe

[glebert]

Just replaced power transformer on this amp. Powered up and found no bias voltage at the socket. After turning the trimmer pot a little the voltage kicked in. Is operating now but since I have it taken apart I think I should replace the pot with something that I trust more, maybe multi-turn for smooth adjustment. Anyone have a favorite that fits nicely? I don't mind bending some leads but don't want to have to do stuff like dead-bugging the pot and blue wiring all the leads.

Pot is this one: https://e-amptone.com/p/170/3459/fen...ear-bias-.html

Schematic here: http://www.thetubestore.com/lib/thet...-Schematic.pdf

Thanks,
Greg

[Enzo]

Why make life complicated? Just replace it with the same item. Tens of thousands of them are out there working fine. There is no part you can install that can never fail.

A multiturn pot is false precision. You can multiturn your bias to exactly -49.862 volts or whatever, but if the mains voltage shifts three volts - and it does that sort of thing ALL THE TIME - then your B+ shifts over 10v. And then so much for your careful precision. yes, the bias shifts a little too, but good luck hoping they track. It is just a guitar amp, the little trim pot is plenty precise enough. Imagine setting the tire pressure in your car to exactly 34.862 PSI.


I wish they hadn't taken bias voltage off the wiper though.

[xtian]

Just hit the exiting trimmer with DeOxIt and work it a few times. Should clean up.

[nevetslab]

And, if that doesn't restore the pot, Digikey has the part: CTS 201UR253B

https://www.digikey.com/products/en?...s=201UR253B-ND

[SoulFetish]

Why make life complicated? Just replace it with the same item. Tens of thousands of them are out there working fine. There is no part you can install that can never fail.

A multiturn pot is false precision. You can multiturn your bias to exactly -49.862 volts or whatever, but if the mains voltage shifts three volts - and it does that sort of thing ALL THE TIME - then your B+ shifts over 10v. And then so much for your careful precision. yes, the bias shifts a little too, but good luck hoping they track. It is just a guitar amp, the little trim pot is plenty precise enough.

I remember reading that Bob Pease used to argue this point as well, daring someone to prove they could get values from a multi-turn that couldn't be dialed in with a single turn.
I uses multi-turn pots for biasing my amp, because they are panel mounted and adjustable on the back of the amp. The shafts have friction locking hardware, but the multi-turn is just an added layer of protection against someone being able to knock the bias out and redplating the tubes. Which still shouldn't happen, because there is a stopping resistor which should clamp the bias at the highest end of the range. Hopefully 3 layers of redundancy is enough?
But that's off topic, you guys already cracked the case on this one. Replace it with the existing part. Chances are you'll see this amp again anyway with a blown plate resistor (or 2) in the PI.

[Enzo]

And cracked solder on the octal sockets.

[SoulFetish]

And cracked solder on the octal sockets.

word.

[glebert]

Man, the ONE time I think to go above and beyond on a repair and everyone's like "no brah, it's cool." I actually was not thinking multi-turn for precision but for the mechanical stability that SoulFetish mentioned, but my post made it sound more like it was for precision. Oh, and if the pot goes open on the wiper it will red plate the tubes no problem (at least it did for me). That is why I was thinking of doing something more reliable.

[Enzo]

Oh there is nothing wrong with a multiturn pot, but almost always, the reason for it is to dial it in exactly.

I never put bias trimmers where they will be jostled. They really do not need to be on the panel. If you want them accessible, I would recess them and hide them under a plate. But that is just my opinion. I have never had a problem with pot settings getting knocked awry.

Any adjustable bias supply is going to be a voltage divider of some sort. The typical "volume control" circuit used here has the disadvantage of losing bias completely if the wiper opens. The way to correct this, at least during design, is to make the pot a variable resistor wire the wiper to one end. Take the bias out at a junction between resistors. Now if the wiper fails, the resistor part is still there, and your circuit reverts to the maximum voltage - the coldest bias. It doesn't matter if it is single turn or multiturn, the same consideration applies.

[SoulFetish]

Oh, and if the pot goes open on the wiper it will red plate the tubes no problem (at least it did for me).

No doubt, can't have the grid go open.
My bias setup is different (but that's another story), none the less, there are still fault conditions which could develop (ie tube short, or the entire pot going open). But, that's what the secondaries where fused for.

Hey, not trying to discourage you. I say, if the amp is yours, you can put in a 20W rheostat if you like. No harm. But if its a commercial repair, time is money and so are multiturn pots. Often, its difficult get the customers to see enough value in that, where you can not only pass the cost off to them, but make it profitable for you as well.

[glebert]

It's my amp, but was bought with intention of flipping it.

Enzo, thanks for the comment about cracked octal socket joints, I think that may explain the problem that started today.

[g1]

For the reliability issue, I like Enzo's option of altering the bias circuit so the pot is used as a rheostat and a fault condition puts the tubes to coldest bias. That's pretty much the traditional Marshall arrangement.

[drewl]

I only use the multi turn pots because I have a bunch, usually for amps that don't have a pot like the Blues JR's.
What is ridiculous are this flimsy loose pots like on some of the 5150's and other more recent Peavey's.
Just breathe on it and it moves.
Plus it's located where idiots can get to it easily.
I get them in for repair alot with bias set to "melt plates" setting.

[Enzo]

My experience with the 5150-2 and other VERY similar PV amps with adjustable bias is that the range of the adjustment was designed to be very limited. I am not sure how to get a 5150-2 into melt the tubes land.