Odd interaction between amp and guitar pot

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4/26/2006 2:07 PM

Carlo Pipitone

Odd interaction between amp and guitar pot I have just restored a 1482 Silvertone amp.   I did a complete job: caps (coupling, tone, filter), a few drifted resistors, rewound power tranny, three-prong cord...   A bad hum was defeated installing shorting input jacks and shielded cable from input 1 to the grid of V1.   Now that the amp is dead quiet I can observe an odd behaviour:   when I plug my guitar into the first ('mic') jack and turn the guitar's volume pot I get a loud scratching noise. This doesn't happen neither in the other input jacks ('instrument') of this amp, nor in any other amp. I have a cap+resistor in parallel across the pot's lugs as a treble bleed, and it works perfectly anywhere other than in that jack..   It seems that the simple contact of my bare fingers with the volume knob cause a slight scratch, which becomes loud when I hold and turn the pot.   I have checked and resoldered all the joints in the 1st jack, and have added a 68k resistor in series from the hot lug of the jack to the grid of V1 (like in the other two jacks) to no avail.   The shielded input wire is grounded only at the jack's ground lug.     What else should I check?     TIA,   Carlo

 

4/26/2006 3:05 PM
Jag	Is there a 1M resistor from hot to ground on that input? If so, have you checked that it is good?     Have you tried moving the wires to the associated tube?

 

4/27/2006 1:51 AM
Carlo Pipitone	Jag,   I have replaced that 1M resistor (more than 10% drifted) with a new one, and also moved all the wires associated to V1 while turning the guitar pot.   Tha scratch is still there.     Thanks,   Carlo

 

4/26/2006 3:10 PM
Enzo	For whatever reason, could there be some DC on that input?

 

4/26/2006 8:24 PM
loudthud	If the plate voltage is too low on the first stage, this will cause grid current to flow and a slightly negative voltage at the guitar. Unplug the guitar and measure the voltage at the tip of the cord. It should be real close to zero volts.     Possible causes:   B+ is too low.   Plate or cathode resistor off value.   Bad tube.   Conductive flux on tube socket.

 

4/27/2006 8:20 AM

Carlo Pipitone

loudthud:   these are the plate voltages on V1:   95V on pin1   47V on pin6.   Is such 'unbalanced' plate voltage on V1 normal?     This is a partial schematic:       (full schematics at http://img.villagephotos.com/p/2005-9/1080251/Silvertone1482.jpg )     The above values remain very close with different tubes on V1.     If I plug only the guitar cord, I have -.43VDC on the tip. Is this suspect?     For the rest:   plate and cathode resistors are good as well as the tube. The socket looks very clean.

 

4/27/2006 11:39 AM

Satamax

Hi Carlo!     If you say you've got -0,43 vdc on the tip, i would just do a temporary trick, put a cap in series with the grid, to see if that cures the problem. I would do another check, touch the chassis, with one hand, guitar plugged in it, and touch that pot, turn it etc. May be touch a grounding point in yar house and do the same. May be make a temporary ground on that chassis, test between a remote ground and the tip.     Bye.     Max.

 

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