Amp thinks it's a receiver ??

[bereanbill]

Ya know...something tells me I know the answer to this, but it's buried back in the deep recesses of my mind...or what's left of it anyway !
One of my guitar amps...a Gregory MK X if that means anything to anyone...has this annoying habit. It thinks it's a receiver...TV or radio I'm not sure. I don't watch TV anymore...and it definitely ain't the Yankee game...but I digress. Anyway, it picks up stray signals from somewhere. None of my other amps does this...not even in the same spot. The building I live in is old with masonry walls at least a foot thick ..hard to believe TV would penetrate. Any ideas? When I shut it down it seems like the signal gets stronger ???

 

[ray_moth]

It's common to use a high value stopper resistor (22k or more) on the input grid of a guitar amp to prevent this sort of thing. The guitar can act as a radio antenna.

 

[bereanbill]

A very common value is 68K to the grid, and 1M from the input jack to ground. Of course a common way to boost gain is to cut these resistors' value...I've done it a couple of times. Can't remember if I did with this guy...I'll have to open it up and look. So will a larger value help correct this, Ray?

 

[ray_moth]

I don't see how reducing the value of a grid stopper is going to increase the gain except, maybe, at high frequencies because of Miller.

I would expect 68k to be easily enough to stop any radio signals picked up by the guitar.

Other sources are possible, such as the speaker leads (if there's global NFB), or the PS (unlikely).

 

[Wavebourn]

It seems to me, that 68K is across the jack's switch, in a voltage divider, so one jack works for high-Z pickups, another one for low-Z pickups.

I may be wrong, since I did not touch guitar amps last 25 years.

 

[ray_moth]

There may possibly be a voltage divider using a 68k resistor, but this is different. It is connected exactly as a stopper should be, i.e. between the upper end of the grid leak resistor and the grid pin. It does not attenuate at low to middle frequencies, but it may well do so at higher frequencies because of the tube's input capacitance (Cgk + Cga x gain (=Miller)).

It serves the dual purposes of preventing parasitic oscillations and acting, together with the tube's input capacitance, as a high-frequency filter to block RF, such is bereanbill has been describing.

 

[Enzo]

In the two-jack input configuration, the typical old Fender input circuit used the jack cutout contacts to control those resistors. There was a 68k from each jack to the input grid. In the high gain jack, the two wound up in parallel for a 34k resistance as a stopper. If you plugged into the #2 jack, then the two resistors became a divider, cutting signal level in half.

This is an extremely common input jack configuration in guitar amps.

Another reason guitar amps turn into radio receivers is when the ground at the input jack has no integrity. SOmetimes backing off the mounting hardware on the input jack then retightening allows the toothed washer to find a fresh contact surface.

 

[pointy]

maybe put a mu ring choke on the earth (ground) if it uses one and the inputs.
all you need is a 1" mu triode ring and then pass the wire through it 4 or 5 times. it can help to block RF

 

[bereanbill]

Enzo was on the money. My Gregory has an aluminum chassis, and I guess it got oxidized in it's 44 years on this planet. I took it out, cleaned it and the mounting surface, and voila ! No more radio. Thanks Enzo !