5F1 low hum filament mod

[Telecaster65]

Hi everyone,

Could kinda angel confirm if the two green wires of the secondary tightly twisted goes straight to the pilot light tangs, and from there another two twisted wires to the pins 2 and 7 of the 6V6 tube socket, from
there, other two twisted wires to pins 4+5 and 9 of the 12AX7 tube. Yet if there's nothing to move from the stock schematic except the green wire (long) that goes
from the pilot light tang to the filament pins.

I would also like to know then if the two 100 Ohm resistors (1/2 Watt is ok?) must be "V" connected to the light tangs at one end (each) and then together at the other side and then to ground

And also: what about stock grounding of 12AX7 pin #9?

...Lot of questions, I know...

Thanks
Fabio

[Gord K.]

The schematics that I have seen that have the 100 ohm hum reduction resistors have them connecting between the filament wires and the cathode end of the cathode resistor. On a single 6V6 type tube amp, you could install one 100 resistor between pins 2 and 8 and the other between pins 7 and 8.

BTW, I built an amp like the Mini Amp project on the STF Electronics site and found that these resistors, though not shown on their schematic, made a significant improvement.

Just my $0.02.

Gord.

[mark norwine]

hang on....

5F1 uses an unballanced filament structure. unless you completely rebuild the string, all of the above mentioned data is irrevelant...

[Gord K.]

Originally Posted by mark norwine

hang on....

5F1 uses an unballanced filament structure. unless you completely rebuild the string, all of the above mentioned data is irrevelant...

Not necessarily. Looking back at a previous post it looks like he is building a Marsh 5F1 kit which may not be wired like the original.

[Telecaster65]

Originally Posted by Gord K.

Looking back at a previous post it looks like he is building a Marsh 5F1 kit which may not be wired like the original.

Indeed... The filament wiring that I choose is totally different from the stock one. I have connected the two 100 Ohm resistors from on side each to the pilot assy tangs, the other side, tied together, to ground.

The problem now is that I don't have power at all... I think I've messed something with the input transformer.

[mark norwine]

Please accept my apologies for interfering. I have no idea what a "marsh" is and see no reference to such in the previous posts.…I only saw the 5F1 reference, and the only 5F1 I know is an old fender, which, indeed, employs an unbalanced filament scheme. furthermore, the comment about "grounding pin 9 on a 12AX7A only confirmed in my mind that an unbalanced circuit was under discussion.

I'm sorry to have caused confusion…..over & out,

mn

[Gord K.]

Hey Mark,

No need to apologize, your information is correct for a Fender 5F1. If I make a mistake here, I hope people will point it out and correct me, I know there are many who visit this site that have much more knowledge and experience with amps than I do. Thank you for that.

Marsh Tubes and Amplifier Service in Clifton, NJ offer a 5F1 kit, as do several other tube places. In the tech section on their web site the filament wiring scheme that Telecaster65 referred to is among their suggestions for reducing hum in the 5F1 circuit.

To Telecaster65,

I can't help you with your power problem, I just don't have enough information. What I do suggest though, is that you contact Marsh Amps. If you are building their kit following their directions, they should be able to help you sort out the problem. And they may need to revise their instructions if you followed them and something doesn't work.

Standard disclaimer: I have no affiliation with Marsh Amps. I am aware that they exist, and I have read reviews either on this web site or the 5E3 forum that their kits use good quality components and their support is good as well.

Gord

[Telecaster65]

Thanks Gord, I'm in touch with Michael and I think I have the layout of this mod clear enough now. The problem I'm having now is with the input transformer wiring which must be adapted for 220V (Italy)

I have the Mojo 760 set this way: two reds to the rectifier tube pins nr 4 and 6 (checked) and two yellow at pins 2 & 8. Black/red tied with black, black/yellow to the switch, white to the fuse, orange and red/yellow to ground, black white alone. I have tested both with or without the yellow/red to ground but the 250V-2A fuse pops after 1 second. I have tried with a 2,5 A and for a few seconds I've heard a nice buzz (veeery low hum) but after having checked the fuse I've noticed that it was hot a lil bit.

These are the groundings I've made:

to the two lug tml strip screwed to the hole under the 6V6 socket:

-Board grounds (black cloth wires)
-The cap off the fuse and the two resistors off the pilot light tangs (I really don't like this, could I have other grounding points?
-The white wire of the power cord (mistake? needs an individual grounding point?)
-The Faraday shield of the Tranny

Other issues: I have a red/yellow and a black/white wires off the power Tranny, going nowhere: should I ground'em? Where? It is recommended to use different grounding points? (example: a solder lug under one of the Tranny mounting screws could serve to the cap off the fuse and make room also for the 100 ohm resistors to be grounded on the pilot light assy...)

Thanks to any saint replying...

Fabio