Silver Jubilee 2555 replica - humming

[djseban]

Hi,
Not long ago I've bought tube amp with trannys and tubes in the intention to build Silver Jubilee 2555. Earlier it was some modification of JCM 900 with some strange specification. It was made without Turret Board, in my country we call it simply: spider's web. But on earlier specification (JCM900) amp wasn't humming. Amp has symetrized heater for pre-amp tubes (12V), EL34 are heated by standard 6,3V, but I was forced to make an artificial center tap (2x100Ohm), because Power Tranny has not center tap. Output Tranny is new, but not shielded. One "feature" of this amp, that pisses me of is arrangement of transformers and pre-amp tubes - ECC83 are more or less on center of chassis (tube sockets are under Turret Board), PT on left, OT on right. I've added choke (shielded), but it's near to pre-amp tubes. For about a week, I am trying to eliminate the hum. I've desoldered Turret Board and applied Larry Grounding Scheme.
http://i42.tinypic.com/2jaztxt.jpg" onclick="window.open(this.href);return false;
unfortonately it didn't have effect. When I take out third preamp tube (input) hum is gone.
Few photos
Inside of Chassis:
http://i10.photobucket.com/albums/a132/ ... 8fceb7.jpg" onclick="window.open(this.href);return false;
Top of chassis:
http://i10.photobucket.com/albums/a132/ ... _01841.jpg" onclick="window.open(this.href);return false;

One and only thing, that has changed is hum frequency. Samples of hum:
Before Larry Grounding:
http://soundclick.com/share.cfm?id=12432088" onclick="window.open(this.href);return false;
After Larry Grounding:
http://soundclick.com/share.cfm?id=12432089" onclick="window.open(this.href);return false;

Can someone explain, what is the reason of this hum? It's driving me crazy -.-

P.S.: Of course this long cables at FX Loop jacks and at grounding of BIAS circuit are temporary

Regards

[Haze13]

Does the wire that runs to the input cap or after cap/resistor to the tube socket is shielded? Do you hear the humming when guitar cable is unplugged? There is a Post Phase Inverter Master Volume, is it shielded?

[djseban]

Does the wire that runs to the input cap or after cap/resistor to the tube socket is shielded? Do you hear the humming when guitar cable is unplugged? There is a Post Phase Inverter Master Volume, is it shielded?

Yeah, signal cable is shielded. About PPIMV - for sure it's not reason of hum. I've done it in the same way in my DIY JCM800 and it's dead silent. PPIMV cables in this amp are too short for real shielding, so I've twisted it.

For now I want to change the OT/Pre-amp tubes arrangement like in original Marshalls, maybe this is a way to solve a problem.

Regards