TG amp and rf interference

[shay1510]

Hey there,
I know the subject has been discussed a lot but I still need your help.

I've built a tiny giant amp with a valvecaster (as a preamp), and I'm getting a lot of fm radio signal.
It happens with the cable (without guitar) connected (functioning as an antena) . different cables tested.
Tried to put a 50-500pf cap on the input jack - didnt work.
Tried different wall sockets - didnt work either.
the radio signal starts when I turn the amp on, then amplified when the tube starts working.
BTW - the negative and the ground are connected together.

Suggestions?
Thanks.

[GibsonGM]

Well, the 'usual' suspects, Shay...

Are you using shielded cable from jack to input? With just one end grounded?  How about to the different stages?  Is your input resistor right on the tube socket?  Have you tried the small cap from tube socket to gnd right there where it's mounted?
   
Is your input jack 'floating', or grounded to the chassis?  That can create ground loops...you need a 'star' setup, with ONE ground point for the amp, and one for power supply.

I'd seriously look at how everything is grounded in the amp!!   Can you audio probe the 1st stage and see if that's where it's entering the circuit?

[GibsonGM]

Also....if possible, lay the grounds out like this:
chassis ground >> power supply >> power amp gnd >> preamp gnd - input floating

Since currents flowing on the bus wire will be larger at the PS, then power amp, and smaller at the preamp, you want current to flow this way, if that makes sense....the preamp is very susceptible to noise.  I still think it's your grounding scheme...

[shay1510]

GibsonGM,
Thanks for the help,
I took the amp to a rehearsal room and it was perfectly silent.
I guess there is a grounding problem in the electric circuitry in my home, or maybe the apartment itslef is just a rf magnet
Thanks again.

[GibsonGM]

That could be, TG!  Some rooms are 'noisier' than others, some outlets have crappy grounds - maybe use an outlet tester to make sure it IS grounded?
Also - we overlook our GUITARS as a source for noise input...strats etc., notorious for that!    RF gotta go somewhere, ha ha.

I'm glad you at least know it is working well in another location, which suggests any issue inside it is probably minor.   Tweak on!