Help with Scott 299c 233 potentiometer

[hagstrom71]

I have a Scott 299c and the vol pot has some issues on the left channel
The left channel will sound thin and weak until the vol pot is raised to a certain level, after this point it works as normal until it is maxed out where the volume then noticeably decreases.
I'm not certain, but I'm guessing the left channel of the dual gang 500k pot is worn and needs replacing.
I've tried de-oxit without much luck, though the half of the dual gang pot closer to the face of the amp is harder to get to, so It's possible I haven't gotten into it enough.. but my gut tells me the pot is shot.
I am open to other suggestions or advice on troubleshooting.

On this 299c the power switch is on the face of the amp and not part of the Volume pot like most 299c's.
Though it is labeled 299c, it shares many similarities with the Scott 233 which I understand was common on some late era 299c's.

What would be a good 500k dual gang pot replacement for this amp?

Thanks,
Chris

 

[century tek]

I see loudness taps on the volume pot. One channel tap could be the problem.
The loudness switch itself could also be problematic. Those flat disc ceramic capacitors could also need to be replaced but it seems that those things last forever so maybe that would be the last thing to look at.

 

[hagstrom71]

Thanks for the suggestions.
Would disconnecting the loudness switch bypass it and tell me if it or the taps are the problem?

 

[century tek]

You can always take an ohm meter to the switches and compare any discrepancies. Deox and work the loudness switch also. I do have the same type of volume pot for your unit but it is the one with the power switch built in where you pull for on and push for off..

 

[hagstrom71]

I was able to confirm that the Left channel on the volume potentiometer is shot.
Looking for a replacement I'm not able to find any 500k dual pots with a loudness tap.
I realize I probably have to use a pot without a loudness tap, but I'd still like to use the loudness switch feature if at all possible.
Does anyone know if Scott had any amps with Loudness switch that used resistors rather than a Loudness tap on volume?

 

[primosounds]

I was able to confirm that the Left channel on the volume potentiometer is shot.
Looking for a replacement I'm not able to find any 500k dual pots with a loudness tap.
I realize I probably have to use a pot without a loudness tap, but I'd still like to use the loudness switch feature if at all possible.
Does anyone know if Scott had any amps with Loudness switch that used resistors rather than a Loudness tap on volume?

You might be able to find one used.
You might be able to use a 250k pot, sometimes without any issues or mods. If the new pot has no taps for the loudness function then you will need to delete that. I have read about a mod to add a resistor to each channel . I have never done this so you will have to check on how some have done this.

 

[Bradbrews]

Oldradioparts.net says he rebuilds volume controls. Might want to check that option out.
No affiliation etc..

 

[Patrice B]

Oldradioparts.net says he rebuilds volume controls. Might want to check that option out.
No affiliation etc..

Yes, I confirm that Mike can rebuild and fix your pot. It was successful for me for a worn pot on a Fisher X-100-2.

 

[AJD1964]

Keep looking - one will probably pop up for sale, but tube amp parts are getting pricey. Online, I recently picked up a couple of 500K stereo pots, with loudness taps (no loudness contour on this little amp though). I saw other 500K stereo pots too, so they're out there. A new or NOS volume control will make a BIG difference though. It did on my little 6V6 powered Bell/ TRW 2418. Its sounds nice and balanced now.

Be careful with deoxit. I'm pretty sure I trashed one (the original volume control on the unit noted above) by using (or probably overusing) deoxit. I thought adding Faderlube was supposed to help prevent damage. I took the pot apart and the carbon track was completely gone on one channel. Hopefully the bass/treb/balance controls will hold up. I did replace the volume control with an NOS 500K stereo pot, but shaft is about 1/8" shorter than original, but still grabs the knob. For me, the hard part was finding one with the exact shaft length.

Old used potentiometers from a donor - Even though they still "work" they may have a large gap of uneven resistance between the two channels throughout the rotation, especially within the first 90 degrees causing channel imbalance at lower volumes. That happened to me by trying to use an old one from a parts unit first. It worked, but the pot was tired and worn out as the volume would not balance out before 12:00. A check with an ohm meter verified that - so I would try to rule that out if you do find a used one from a donor.

I also have a 299C (with power switch on the volume control), I think you'll like it - they sound fantastic. Mine's still 100% untouched and working, but mothballed for a refurb. I hope that I do not have to redo the volume control on that down the road.

 

[njcanuck]

I also have a 299C that I restored. It sounds fantastic! I’ve never heard of a Scott 299 that doesn’t have the power switch on the volume pot. Where is it located on yours?

 

[NoTransistors]

Last year, I snagged two correct volume pots for the 299D [without power switch]. I was going to be generous, and put one up on our buy/sell. Well, I just went to my parts drawer to pull one out. Can only find one. WTF? If it ever shows up, I will put it up for sale. But where oh where is it?????????????????????????????

 

[kvflyer]

With regard to the 299C having an On/Off switch on the volume control... or NOT, the 299C came in at least 7 different varieties. Scott made many changes. Some of the 299C amplifiers were pretty much the same as the 233 (which was just a "stripped down" version of the 299C).

Mark Oppat can make any volume control that you need. But make sure that components around the volume control are spot on before condemning the control. A simple comparison, front to back (or left to right if you prefer) between the two controls at the same location will tell you.

http://www.oldradioparts.net/

 

[kvflyer]

BTW, I have a 299C on the bench right now, totally original (except for one driver/phase inverter tube) playing whilst I work in the garage. That one has a "Pull On" (pull it out to turn it on) volume control.

 

[hagstrom71]

Thanks for all the informative replies.
I’m going to use a cheap alpha dual 500k pot for now as it’s all I could find and see how things go without the loudness switch. But I think I will have Mike at oldradioparts.com rebuild my original pot as some of you have recommended.
Njcanick, the on/off switch is one of the black switches above the knobs rather than built into the vol pot.