Help! Looking for a potentiometer for my old Valco amp

ramonneke76

Member
Messages
184
I recently purchased a nice 1967 Valco 62-9032 (Montgomery Wards) amp. I just did a complete overhaul and it's a very nice amp. It's basically the same as the Supro S6606, Gretsch 6150. It has a volume and a tone control. Tubes: 5y3, 12ax7, 6v6.

There is one thing that I could not fix. There is some crackling especially on low notes.
I checked everything; leaky coupling caps, resistors, wiring, solder joints, speaker, etc.

I noticed when I put a (very low) single through the amp with an audio generator I am able to find the scratchiness. It's the volume knob (which is also an on/off switch). There is no dc on the pot (checked the caps too that goes to the volume pot). Cleaned it several times with Deoxit but the scratchiness won't go away. The plastic shaft is a bit loose. I can wiggle it a little bit and when turning the knob the scratchiness is there. So I think the pot is worn out.

My question is: where can I get a potentiometer? It's a 500k CTS volume pot with on/off switch (not push/pull). I can't find this pot anywhere on the web... Is there a modern equivalent of the pot?

Pot looks similar like this:
151262236600_1.jpg


This is the exact amp:
airline-supro-valco-guitar-amp-62_1_222334764d022a359a12dfeb1a3526d4.jpg
 

Tone Meister

Member
Messages
3,266
The pot you found will work fine.

Have you tried opening the old pot and servicing it? If you work carefully you can clean the carbon disc inside then lube and reassemble. If the dis turns out to be bad, many times you can replace the disc with one from a new pot. Pretty simple if you work carefully.

 

guitar007

Senior Member
Messages
482
I've done what tone-meister suggests many times over the years on vintage equipment. I also recommend.
 
Messages
6,837
Before disassembly and/or replacement, simply touch up the solder joints on those lugs. That might cure the scratchiness. IF not...proceed.
 

ramonneke76

Member
Messages
184
The pot you found will work fine.

Have you tried opening the old pot and servicing it? If you work carefully you can clean the carbon disc inside then lube and reassemble. If the dis turns out to be bad, many times you can replace the disc with one from a new pot. Pretty simple if you work carefully.



Yes, I am definitely try to service the pot. I put in the new pot just to verify the pot is the culprit. The scratchiness went away for about 50%, but it's still there. The (500K) pot measured 668K! The amp sounds different with the new pot. I think the volume decreased a bit. The amp sounds more focussed, more tight. More under control, so is the speaker. The amp reacts different on the dial of the pot. Off course it's a new pot and maybe this is exactly how it was back then when it left the factory. But it's a bit stiff. I don't know jet that it's better or worse. Maybe it's something to get used to it. It definitely had more balls/power before. Maybe it was the mojo of old drifted parts.
 
Last edited:

Tone Meister

Member
Messages
3,266
I checked everything; leaky coupling caps, resistors, wiring, solder joints, speaker, etc.

Assuming you didn't miss anything here ^^^ If your new pot still has scratchiness, you could have grid leakage from a preamp tube. Try swapping out with known good preamp tubes. If you're sure it is only the pot, then you can certainly try servicing the old pot.

You may be able to swap in the wafer from a new pot into the old body.
http://www.geofex.com/article_folders/potsecrets/potscret.htm
http://www.planetz.com/potentiometer-disassembly-explanation/


.
 

ramonneke76

Member
Messages
184
Assuming you didn't miss anything here ^^^ If your new pot still has scratchiness, you could have grid leakage from a preamp tube. Try swapping out with known good preamp tubes. If you're sure it is only the pot, then you can certainly try servicing the old pot.

You may be able to swap in the wafer from a new pot into the old body.
http://www.geofex.com/article_folders/potsecrets/potscret.htm
http://www.planetz.com/potentiometer-disassembly-explanation/


.

Thanks for the info of servicing the pot. I'll look into it.

One thing that I thought was a bit strange that there is dc on pin 5 of the 6v6 and the dc is climbing slowly up above 10 dcv or more. So I changed the .005 cap with a new sprague. Checked it again and the same thing.. dc on the grid and climbing. I tried a different preamp tube with no change.

Other strange thing is that I have approximately 200 dcv instead of 270 dcv (on the schematic bottom left @ 10uf/450v and 47k)

373409-1.jpg
 

guitar007

Senior Member
Messages
482
And clean with de-oxit.

I had to quit doing this years ago...after seeing the mess it makes over time. You can easily spot the corrosive mess it makes sloshing around a pot for years on vintage equipment. I have far better luck using canned air and blowing debris out of troublesome pots. If that doesn't work, I'll disassemble the pot and only then use de-oxit to clean the resistive element when fully removed from the pot enclosure.
 

TimmyP

Member
Messages
2,488
That's why I use CRC QD Electronic Cleaner - no residue. DeOxit leaves crap everywhere, unless it takes only a drop or two to get the job done.
 

ramonneke76

Member
Messages
184
What about the resistance of the pot? I measured 668k. It's a 500k pot. With the new pot the amp feels different. Don't know I like this or I like it with the old pot.

I am going to service the pot next week.
 

BOBSMITH

Member
Messages
87
You replaced the .005 coupling cap and still have excessive DC on the 6V6 grid. Sounds like a bad 6V6, replace it and see.
 

Dan40

Silver Supporting Member
Messages
2,097
Is the amp pretty quiet other than the crackle? I remember reading years ago that having AC voltage so close to the preamp circuitry can introduce unwanted noise. I have always veered from the schematic by separating the switch from the volume pot when building clones, but I can understand that you may not want to modify a vintage amp.
 

ramonneke76

Member
Messages
184
You replaced the .005 coupling cap and still have excessive DC on the 6V6 grid. Sounds like a bad 6V6, replace it and see.

Yes I have tried a new 6v6. The crackle was still there. Can't remember I measured the grid on that new tube for leakage.

Could I try a .022 cap?
 

ramonneke76

Member
Messages
184
Is the amp pretty quiet other than the crackle? I remember reading years ago that having AC voltage so close to the preamp circuitry can introduce unwanted noise. I have always veered from the schematic by separating the switch from the volume pot when building clones, but I can understand that you may not want to modify a vintage amp.

That could be something. I also recently purchased two Gibson Skylarks crestlines from mid '60's and they both crackle. The same as this amp on the low notes. Haven't figured that out either. Driving me crazy.
 

ramonneke76

Member
Messages
184
Is the amp pretty quiet other than the crackle? I remember reading years ago that having AC voltage so close to the preamp circuitry can introduce unwanted noise. I have always veered from the schematic by separating the switch from the volume pot when building clones, but I can understand that you may not want to modify a vintage amp.

The amp is pretty quiet by the way...
 

Dan40

Silver Supporting Member
Messages
2,097
I would think that having AC voltage next to the volume pot might create some hum, but I don't believe that it would be the cause of your crackle. It sounds like you are on the right track with your troubleshooting.
 

ramonneke76

Member
Messages
184
I would think that having AC voltage next to the volume pot might create some hum, but I don't believe that it would be the cause of your crackle. It sounds like you are on the right track with your troubleshooting.
Can I check the two speaker leads somehow?
The leads coming directly from the OT and soldered to the speaker. The leads are short, a bit stiff and wiggling them around change the crackling a bit. Is there a way to check the leads if the are damaged?
 




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