Fender 5c5 Pro RF Pickup

[Ted]

Fender-Pro-5C5-Schematic.pdf

Got 5C5 Fender Pro here. It's picking up cellphone radiation like crazy. Adding foil screening has reduced the problem slightly. Schematic enclosed.

I'm inclined to fit some 68K stoppers on the input grids as on later Fenders.
My question is:

The input 6SC7s are grid-leak biased. Would this make any difference to the operation of a grid stopper?
I'm assuming that the correct place would be between the cap and the top of the 5M resistor.

Thoughts?

[Helmholtz]

Adding foil screening has reduced the problem slightly. Schematic enclosed.

Is all foil connected to amp ground?

I'm inclined to fit some 68K stoppers on the input grids as on later Fenders.
My question is:

The input 6SC7s are grid-leak biased. Would this make any difference to the operation of a grid stopper?
I'm assuming that the correct place would be between the cap and the top of the 5M resistor.

Grid stoppers should be directly soldered to grid socket pins.

[nosaj]

Dumb question here.....Is it all cell phones or just a certain cellphone. If it's all cell phones carry on. If it's just one could be a problem with spurious emissions.

nosaj

[Helmholtz]

Are all input jacks shorting?

[Enzo]

How close are the cell phones? MANY amps react if you set your phone on top of the amp. With poor grounds, many amps pick up AM radio stations in the area, could this be the "cell phones" you hear?

Note the 68k grid stopper resistors Fender added to the amp in later versions were mounted on the eyelet board for a couple models - midway between tube and jack, and then later they were moved to the jack, not the tube. There may be "better" places for those resistors, but if the topic is "what Fender did" then not so much.

You have grid leak bias here, you could very easily convert it to cathode bias like the 5D5 or other later versions.

[Ted]

Some 68K resistors at the grid pins of the 6SC7s fixed it. It didn't totally get rid of pickup but it's massively improved.

In answer to various questions:

1. Yes the input jacks were shorting.
2. Yes it was happening with every cell phone that came within a couple of metres of the amp.
3. Yes I did ground the screening.
4. I don't think my customer would particularly go for "may I change the circuit topology of your 1954 amp. It won't affect the tone". You've met guitarists, right?

I went for the tidiest looking solution - on the grid pins of the 6SC7s. This does slightly change the operating point of the tubes, since the grid leak is now 5.068M but the plate voltages are still in sensible places.

[mozz]

Doubt the operating point changed, how far out of tolerance are the 5 meg resistors?

[Helmholtz]

^^^^That.

The grid stoppers will increase total grid leak resistance by a mere 1.4%.
Generally grid leak biasing is a rather crude method, prone to considerable variation of plate current among individual tubes.

[mozz]

Old carbon comp resistors go up higher in value as they age. Usually the higher values are the worst in that respect. Being that amp is getting close to 70 years old right now, i would check those resistors.

I just checked some old 5.1 meg i have, they are all over 6meg. I would also check the 250k plate resistors, i can bet they are all high too. Just pull the tubes and check, no need to unsolder. If the plate resistors are high, and the grid leaks are also, your gain is probably maxed out and that's why it's picking up any near transmissions.

I built a tweed deluxe with the 6SC7 front end. Problem i had was microphonics from the metal tubes. After swapping many tubes, i found i had a pair of Tung-Sol 6SC7GT glass tubes. They were much better but i didn't realize how expensive they are selling for when i wanted to buy a backup pair. I ended up converting it to cathode bias.

[Helmholtz]

Higher grid leak resistance causes more negative grid voltage and thus less plate current. That may partly compensate lowered plate voltage by increased plate resistance.