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FDP Forum / Amp Mods, Repairs, and Projects / Static Noise that is subject to position of volume control

wrnchbndr
Contributing Member
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New Jersey

I'm back with the otters again
Jan 23rd, 2019 03:28 PM   Edit   Profile  

I'm working on a 1965 Supro 6498 VR. If you happen to know where I can find a schematic, it would be helpful. The amp is totally original and has no evidence of ever being worked on. The tubes are all OEM RCAs. It had been sitting for years. I brought the power up slowly using a variac.
Its a 2 channel amp. Channel one is totally clean with no complaints. Channel 2 has both vibrato and reverb. Channel 2 has a noticeable noise that is like AM radio static and hiss. Its not bad enough to prevent the amp working at a bar gig but it is enough to say there is a problem. The noise is totally subject to the position of the volume control. Fully CCW, there is no noise and the louder you set the volume control, the louder the noise whether or not there is something plugged into the input.
Everything is working 100%. The reverb, the vibrato, the tone controls... everything works and the amp is loud as hell in both channels. Like I said, channel 1 is wonderfully quiet with the volume fully CW, a guitar plugged in and is loud and clean so the filter caps are holding on despite their age.
I'm looking for a schematic and won't get into this until I have one. The layout of the amp is totally weird compared to a Fender with the 2 6L6 power section being a separate chassis on the bottom of the amp. Its cool.
Generally speaking, would one assume that the problem would be in the circuit prior to the volume pot in a situation like this?
I've swapped the 12AX7s between channel 1 and 2 with no effect.

willie

Too Near Atlanta GA

Amp Tech Emeritus
Jan 25th, 2019 09:38 AM   Edit   Profile  

Those are great amps! I thought I might have a schemo in my archives but alas, I no longer have it for some reason. You might have to use similar amp schematics as a guide, Valco, Gretsch, etc.

willie

Peegoo
Contributing Member
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Just beyond Mars

there's a world of fools
Jan 30th, 2019 09:42 AM   Edit   Profile  

This is the closest I can git'cha:

Old Amps

DrGonz78

United States Phx,AZ

Jan 30th, 2019 12:29 PM   Edit   Profile  

I have to wonder how close the Supro 6499vr is related to the 6498vr model? Maybe this can be confirmed here if they are close?

Supro 6499VR Schematic

Peegoo
Contributing Member
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Just beyond Mars

there's a world of fools
Jan 31st, 2019 10:29 AM   Edit   Profile  

Possibly a bad tube or shot noise from a bad resistor in the power section.

These are pretty simple amps. Desolder one end of each resistor to test it for spec. Check all caps in the came manner.

If it's a hiss or static noise, it's usually a component that's bad. If it's hum or buzz, it could be a component, lead dress, or a shielding issue.

willie

Too Near Atlanta GA

Amp Tech Emeritus
Jan 31st, 2019 11:11 AM   Edit   Profile  

The 6499VR is close...very close if memory serves, which is not saying much really. :) I would check the cathode and plate circuit of V1 quite carefully.

w

Leftee
Contributing Member
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VA

Toxic Humility
Feb 20th, 2019 06:45 AM   Edit   Profile  

Did you ever get this sorted?

rfrakes331K
Contributing Member
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IL USA

RonHalen Jokingly He Says
Feb 24th, 2019 07:46 AM   Edit   Profile  

Since it set, could it be a dirty pot? Then the capacitors possibly went bad.

(This message was last edited by rfrakes331K at 10:40 AM, Feb 25th, 2019)

FDP Forum / Amp Mods, Repairs, and Projects / Static Noise that is subject to position of volume control




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