74 Super Lead radio interference

[chadb]

Hey Guys,

I recently scored a 74 super lead on the cheap and have done my best to convert the circuit back to its original specs. Someone had hacked a bunch of holes in the chassis and added a relay switching system and a bunch of other crap...

Anyway, other than adding a PPIMV and a half power switch, the circuit is now stock. The amplifier works, biases up just fine, and sounds pretty good, but I'm getting some terrible radio interference, and a little more hum than I would like. I assume I'm getting some AC in there where it shouldn't be (yes I completely re-capped the power supply and all other electrolytics).

If anyone has any suggestions, It would be greatly appreciated.

[M Fowler]

AC hum would be power supply or bias cap.

[Blackburn]

Hey Guys,

I recently scored a 74 super lead on the cheap and have done my best to convert the circuit back to its original specs. Someone had hacked a bunch of holes in the chassis and added a relay switching system and a bunch of other crap...

Anyway, other than adding a PPIMV and a half power switch, the circuit is now stock. The amplifier works, biases up just fine, and sounds pretty good, but I'm getting some terrible radio interference, and a little more hum than I would like. I assume I'm getting some AC in there where it shouldn't be (yes I completely re-capped the power supply and all other electrolytics).

If anyone has any suggestions, It would be greatly appreciated.

That's known as the Spinal Tap mod.

[chadb]

That's what I would assume as well, but as I've replaced all 6 power supply caps and both bias supply caps with brand new ones, I wouldn't think that would be the issue.

[JazzGuitarGimp]

Not sure how large a hole in the chassis has to be to allow radio waves in, but if any of the holes you've described as 'hacked a bunch of holes in the chassis' are larger than, perhaps 1/4", you might consider covering them up with metal plates that are well-grounded to the chassis. Certainly, if there are any holes large enough to host a tube socket, I'd start there.

[M Fowler]

Radio interference maybe need to add a 33k grid on the input if there isn't one. Also check grounding of the input jack.

[jelle]

Consider rebuilding all your grounds at the chassis level.

[sepulchre]

Radio interference maybe need to add a 33k grid on the input if there isn't one. Also check grounding of the input jack.

Yep, 33K right on the the input pin of the tube socket worked a treat on mine with the radio problem.

[Reeltarded]

Turn your radio off.

[Structo]

If the interference is getting into the grid of V1 then try a 15pf-30pf cap from the input to ground.
It will shunt radio freq to ground.

Also, make sure the 1 meg resistor on the input is connected right.

[chadb]

As this circuit uses both the input grids on V1 (pins 2 & 7), should I add a 33K resistor to both grids or just one?

[M Fowler]

As this circuit uses both the input grids on V1 (pins 2 & 7), should I add a 33K resistor to both grids or just one?

Solder a 33k resistor on each tube pin (2 & 7).

[chadb]

Thanks! I'll give it a try. In theory though, I still don't quite understand how this will make a difference. On say a TW Express that does not have an input grid resistor this makes sense, but since the superlead has two 68K grid resistors and when you plug into one input the value is approximately 33-34K already. I don't see how adding another 33K in series would change this. But hey, I'm not claiming to know everything.

[M Fowler]

Your right you already have 68k grids, so forget about it.

[chadb]

If the interference is getting into the grid of V1 then try a 15pf-30pf cap from the input to ground.
It will shunt radio freq to ground.

Also, make sure the 1 meg resistor on the input is connected right.

If I'm not getting this interference without an input cord plugged in, or even with the wires to the grids on V1 removed, do you still think adding a small cap on the inputs would work? Or is the problem most likely somewhere else in the circuit?