amp volume pots linked and have odd ohms- are they bad?

Steve Waltz · From: USA

This is the preamp section of a Sano amp. 6sn7, 6sn7 and 6sl7 preamp tubes.In short...volume pots have highest ohms in middle position and then go down in ohms.

Two inputs. One 1/4 goes to a 500K pot. Signal leaves the center lug of that pot, hits a 100K resistor and then enters the grid of the center 6sn7. The other input is a TRS with two signal wires each going to a different spot. The "treble" side leaves the TRS and goes to a 500K pot and leaves the center lub then through a 100K resistor and connects at the same place as the prior mentioned pot just after or at it's 100K pot. I won't get into where the other TRS is going.

My issue is that when I use an ohm meter on either of those pots, they read about 24 ohms either full on or full off and go up from either on or off in resistance until the middle of the travel and peak out at around 82K at middle of travel. In use this middle point is the loudest location of the pot. There is a master volume later in the circuit and it's ohms are normal.

Question: 1. both these pots can't be damaged in the same way, right? If they aren't damaged, then I don't get what or why they are doing it this way. 2. Is the 100K resistor on the pot a grid resistor like the 68K on Fenders. 3. I'm not sure if a pot directly after input prior to a gain stage is a great idea, I could just go directly to the grid?

The TRS is related to the original use in that input to accordian with a left and right pickups. The amp tone is great so I don't really care to alter the tone, just chasing noise and wondering why these pots are working like this.

BTW the 1/4 input in the picture is just the foot pedal spot for Tremolo.
Thanks,
Steve

Michael Brebes · From: Northridge CA

Turn both volume controls to zero and measure the resistance on the outside legs. That should be the actual resistance of each pot. Since the resistors off the wiper are tied together (and going to an undisclosed location with other possible resistors), reading the resistance between the wiper and ground will never give you a true representation of the actual resistance of the pot as it turns. Hope that helps.
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Michael Brebes
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Steve Waltz · From: USA

I'll double check that method tonight. I did run a signal generator through the input and looked at on the scope it as it entered the grid of the first tube and the pot was acting exactly like it should. So, yes, I might be using the multimeter incorrectly. The other connected "treble" pot was still causing some changes to the instrument input so I cut that off and that stopped, which is good.

I'll put the amp back together and see how it sounds now. I'm hoping the noise is gone since the other input has been removed.

I noticed the preamp heater wires are twisted but with the B+ being twisted around the heater wires. Seems like that would cause some noise?

Donny Hinson · From: Glen Burnie, Md. U.S.A.

Jim Sliff · From: Lawndale California, USA

What Donny said. You can check the static resistance by "zeroing" them, but not whether they are operating properly.

One note - as a few parts of your post are rather basic as far as electronics knowledge goes - you DO know how to safely work inside an amp - right? AND you discharge the filter capacitors before doing so - right?

AC voltage wires - such as most filament wires - are tightly twisted to reduce hum. They are also kept as far away as possible from signal wires and the power rail (high DC voltage.

BUT - some small makers use DC voltage for the filament supply, which could explain it being twisted with the B+ wire. You'd have a separate filament transformer running DC (easy to identify or have to measure the voltage to identify it as AC or DC.

Other noise reduction parts of lead dress are crossing different types of wires - like signal wires and plate wires - at as close to a 90 degree angle as possible, and (as Donny mentioned) never running them adjacent to each other.

It's very hard to help you figure out the pot issues or how to reduce noise without:

1) a picture of the full chassis

2) If possible, a copy of the schematic

3) a description of WHAT noise you are talking about! There are two types of hum, hiss, popping/crackling, pot scratchiness (often caused by use of contact cleaner, which destroys pots - only cleaner/lubricant should be used), physical noises (like pops from loose tube sockets`) and microphonic tube sounds.

Without knowing what type of noise you are chasing we're throwing darts in the dark!
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No chops, but great tone
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Steve Waltz · From: USA

Jim, I drain caps and double check voltage. I do appreciate the concern. I did change how the input was wired enough that the volume and master are working correctly for my use now. Its an accordion amp with odd inputs and I wasn't familiar with that set up.

Heaters are AC which is why I thought it was strange that the heater wires were twisted with the B+ twisted around them. The amp section didn't do this, just in the preamp section.

I know noise is hard to find so I wasn't trying to waste anyones' time asking for help on that. There is no schematic.

The list of what I have done to this amp is long. The chassis is used as ground but they also ran a separate ground wire to ground points on many but not all tube sockets and RCA input/output locations. I'm thinking to change those RCA jacks to isolated in case it is creating a ground loop or somehow getting noise into the Jack because it's in contact with the chassis near a point that might induce noise.

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