Fargen Blackbird on the bench

[xtian]

@Tom, how can I check out the tank? As I mentioned, tank output is good if I rap it with my knuckles. Should I scope the input, and what signal voltage should I expect?

@Phil, I have a large sheet of adhesive-backed copper for cavity shielding. I tried holding this between the trannies, with an alligator clip grounding the copper to the chassis. Didn't make any change at all.

[xtian]

Photos. Reverb socket is V2, center of closeups.

[pula58]

the wet signal looks as if it is being fed (through a white mallory 150, on right side of PI "tail" resistors) into the "other" PI input, the one nornally used (in a blackface style amp) for feedback from the output (speaker/OT secondary). I think you'll have a reverb, with this amp, that is much more in the background compared to the Fender AB763 style of reverb circuit

[Leo_Gnardo]

Low reverb? Consider the possibility that it's the wrong input impedance tank. I see this a lot when amp owners replace their own, or have it done by techs who don't know better. - - - Also possible that Mr Fargen didn't plan to have too much 'verb with the control only on 3 as on many Fenders.

Hum - I'd try a more substantial piece of metal between the xfmrs. Grounded or not, it's worth a try. Failing that, is there enough space "below deck" to mount that OT? might get it out of way of the PT field.

[Firestorm]

I don't think copper works to shield stray fields (actually I'm surprised that aluminum would work either. I know the flux bands are typically copper, but if the coupling is magnetic, the shield would have to be iron or steel (or mu metal).

As to the reverb, try a different 12AT7 (not 12AX7), and try a different tank. If that ain't it, it could be the reverb transformer itself.

[pula58]

This is unrelated to your reverb and hum woes , but, in the photo I noticed that the fuse holder connected to the standby switch looks like it's nut has come loose.

[xtian]

This is unrelated to your reverb and hum woes , but, in the photo I noticed that the fuse holder connected to the standby switch looks like it's nut has come loose.

Yes, it's broken. Thanks for having a close look!

[ampmike]

I always read about dif. Amp Co.Fargen looks very nice,thay are reasonable on ebay as well.Good luck with the repair,thanks for sharing photos,Mikey

[tictac]

Non twisted heater wires running parallel and laying on the chassis? Seems like that'd be a pretty good hum generator.....

TT

[xtian]

Non twisted heater wires running parallel and laying on the chassis? Seems like that'd be a pretty good hum generator.....

TT

In my experience, heater wires running parallel tightly together do just as well as twisted. I have had success using two-conductor ribbon cable as heater wire.

[xtian]

Just spoke with Ben Fargen. Learned how to pronounce his name: Far-JEN, not Far-GEN. He said the reverb is operating as designed, and is not meant to be a big, splashy Fender-style verb.

[Leo_Gnardo]

He said the reverb is operating as designed, and is not meant to be a big, splashy Fender-style verb.

OK, good, reverb solved. How's about that dyna mo hummmm? hmm?

[xtian]

How's about that dyna mo hummmm? hmm?

She was kneelin', I was dealin'. Ah, vintage, sexist Frankism.

I reduced the hum somewhat by pushing the two trannies around, but didn't go to heroics remounting the OT, because owner dunna wanna payferit.

[tictac]

xtian wrote:

In my experience, heater wires running parallel tightly together do just as well as twisted. I have had success using two-conductor ribbon cable as heater wire.

Your statement is contrary to everthing I've even read about filament wiring layout. Here are a few examples:


RDH4 states – “Electrostatic pickup of hum voltages by high impedance circuits from nearby leads carrying AC currents may be avoided by the fitting of electrostatic shields or, where the capacitance of screened leads is permissible (as in the case of filament wiring) the use of screened AC leads is recommended. The use of twisted filament leads tends to cancel out the magnetic field around the leads…”

OZ Amp site
Because the heater circuit is AC and at a fair current it has a considerable 50Hz magnetic field around it. This field will induce hum in any nearby signal loops….All the heater wiring should be tightly and uniformly twisted at about 2 twist/inch (1/cm) to cancel out most of the radiated field.

Valve Wizard
Layout / lead dress: The lead dress of AC heater supplies is very important for noise reduction. The AC heater wires will have significant EM radiation and should therefore be routed well away from all signal wires, and are usually tucked into the corner of the chassis. The wires should either be made from twin cable (bell-wire) or better still, should be made by twisting the wires neatly and tightly together. In this way the wires are kept perfectly parallel and close to each other, which increases opposing field density and encourages the radiated fields to cancel out. Loosely twisted wires are no use at all….

This Fargen gut shot is the first amp I've ever seen that doesn't follow the above recommended practise. Sorry but it seems like laziness instead of good layout practise...

TT[/b]

[alvarezh]

I certainly don't qualify to challenge the twisting of filament wires, I do it myself. But I have always wondered how on printed circuit boards, the traces are not twisted, yet there doesn't seem to be any issues. Can anyone explain?

Thanks in advance.