Bass and Treble pot... am I testing these correctly?

[HiFi-Stereo]

Bass control of this RCA 6V6 amp I’m working on does nothing when I turn it, but the Treble is working...
I’m suspecting if the pot is bad, it could be the reason the music is sounding harsh and breaking up...

I tested the Bass pot which should be 500k and it’s measuring 1.900+?
To see if I was doing it right, I also tested the treble pot, which should be 2M, but only tested around 0.53 but it does work good.
Do older pots just not measure accurately, thought they should at least be close. Am I not measuring properly?
Settings were put at or above resistance called for.

 

[tubeactive]

How about a schematic posted ? Which model ? The pots have capacitors "across" them to ground and/or through the circuitry. Thus, combined, their "reactance" affects resistance measurements. Often, when troubleshooting, you have to unsolder one or both ends of a resistor to obtain its' true resistance readings.

So, while de-soldering (and re-soldering) the connections to your bass pot, you should use heat sinks, like small, copper or aluminum alligator clips, in order to protect the associated parts. While carbon resistors and ceramic caps look secure, they can quickly change value with heat. If you find the pot is OK, then the problem might be one of those parts associated with the controls. As tube amps age, the biggest culprit is historically capacitors, being leaky, open or short. Leakage is most likely. Coupling and tone caps are supposed to "block DC and pass only AC (the signal being AC)" while power supply caps are supposed to "pass DC and block AC (hum)."

A very versatile meter is an LCR meter, which measures inductance, capacitance, and resistive impedance, respectively. The part(s) under LCR test usually must be independent of any attached circuitry or other parts for accurate readings.

Good Luck with your restos...

 

[HiFi-Stereo]

Does .5M on the 2M scale = 500K? Does 1.9 on the 2M scale = 1.9M? What could be wrong? .....? .....? .....? .....?

Thanks,
It’s actually flipped around;
The treble pot works well, at least to my ear, it’s a 2meg pot, but reads 0.53
That doesn’t seem to be even close, and I did desolder the terminals.
The Bass pot should be 500k, but reads 1.900.

 

[Tom Bavis]

Need a model number or schematic. There's a common circuit used in phonos, bass control WON'T do anything unless it's driven by ceramic cartridge.

 

[HiFi-Stereo]

Need a model number or schematic. There's a common circuit used in phonos, bass control WON'T do anything unless it's driven by ceramic cartridge.

Okay thanks, would have never known that! I’m running a CD player through the amp now, so I take it the Bass control won’t do anything.
Here’s the schematic...

 

[gadget73]

Reading across the outer ears of the treble pot should give accurate results provided nothing is plugged into the input. There is nothing across it. Same on the bass pot actually.

 

[HiFi-Stereo]

Reading across the outer ears of the treble pot should give accurate results provided nothing is plugged into the input. There is nothing across it. Same on the bass pot actually.

Thanks, so with everything off the lugs, the 2meg treble pot is measuring 0.54 on the outer lugs, and the multimeter is on the 2M and 200m settings.
The meter tests a common 2.2meg resistor fine... could be the pot is worn, but still works fine in the circuit?

 

[gadget73]

I wouldn't expect that it would read a half meg on a 2M pot. Usually they drift up, not down.

Either way, it could still work. Its a really simple low pass filter setup for the treble pot. Basically the more you turn it one way, the more HF it shunts to ground. The bass isn't going to do anything on something that is't the original ceramic cart as Tom mentioned. It simply changes the input impedance of the amp but only a ceramic cart is a high enough impedance source for that to bother.

honestly I'd ditch the tone controls entirely. Use a 220K resistor from 12ax7 grid to ground and feed the input straight to the 12ax7 grid.

 

[HiFi-Stereo]

I wouldn't expect that it would read a half meg on a 2M pot. Usually they drift up, not down.

Either way, it could still work. Its a really simple low pass filter setup for the treble pot. Basically the more you turn it one way, the more HF it shunts to ground. The bass isn't going to do anything on something that is't the original ceramic cart as Tom mentioned. It simply changes the input impedance of the amp but only a ceramic cart is a high enough impedance source for that to bother.

honestly I'd ditch the tone controls entirely. Use a 220K resistor from 12ax7 grid to ground and feed the input straight to the 12ax7 grid.

Thanks again, very good technical information that I wasn’t aware of, I’ll get rid of the pots and put in the 220k and run an input to the grid.

Always appreciate so much great help from the experts!