Fargen Blackbird on the bench

[surfsup]

Have you tried chopsticking the yellows on V1? Seems there is quite a bit of run parallel to the heaters on that guy.

[Leo_Gnardo]

The OP's OP - original problem - was hum induced into the OT from the PT. Amp sent hum to speakers even with all tubes removed.

Yes there's possibly some room for improvement in the preamp hum figure by dressing wires more carefully - and to cut hum even more Mr. Fargen also could have put his reverb drive circuit at a little distance from V1 - but all this has nothing to do with the OP's OP.

Amp owner apparently is going to settle for the hum level that's inherent in the amp so that about wraps it up. Maybe he plays loud all the time - there's your signal to noise ratio. Add a drummer and the s/n gets very acceptable. Hum, what hum, I don't hear any hum. Drowned out by tinnitus.

I'm all for "perfectionizing" but sometimes there's a practical limit.

Earlier in the thread there was a mention of "the Fargen factory". Somehow I doubt it's a smokestack industry. Probably a one man operation like everybody else here. Having repaired the disastrous Fargen serial # 7, my opinion is: it's a learn-as-you-go situation. Hey he's had what, about 12 years making & selling amps, should have it down pretty well by now. Sing along now "Getting better all the time."

[Structo]

Yeah, I don't care for the lead dress to the preamp sockets either.

It may be better to run the wire that goes to pin 9 across the socket rather than looping around the socket.

As noted by Merlin.

http://www.valvewizard.co.uk/heater.html

[brewdude]

I have had no problems with heater hum on an amp I made where I ran the heater wires parallel and heat shrinked together untwisted and laying flat against the chassis. It looks very clean and is quiet. However, it was meticulous time consuming work.

[xtian]

I really want to try a practical experiment. Put a working, quiet amp on the bench and fire it up. Use a spare transformer with 6.3v heater tap and connect some long, individual (i.e., not twisted or paired) wires, and use these wires to TRY TO INDUCE HUM in the subject amp.

Question: would you have to create a bogus load on your external PT? Because increased current creates a bigger magnetic field, right?

[pula58]

I really want to try a practical experiment. Put a working, quiet amp on the bench and fire it up. Use a spare transformer with 6.3v heater tap and connect some long, individual (i.e., not twisted or paired) wires, and use these wires to TRY TO INDUCE HUM in the subject amp.

Question: would you have to create a bogus load on your external PT? Because increased current creates a bigger magnetic field, right?

Yes, it is current that creates a magnetic field, and the magnetic field (since changing in time, i.e., 60Hz) can create voltages in a loop of wire that the magnetic flux "flows" through. So, you would need to load the external 6.3V transformer with the appropriate load to make the right amount of current flow for the experiment to be valid..

[xtian]

OK, so Ohm says if I want to run 3 amps at 6.3 volts I need a 2R1 resistor that will dissipate 18.9 watts. Sound right?

I don't have that resistor!

[Bob S]

I'm not convinced about twisted filament wires.
It takes more wire, which can't be a good thing methinks.
I run the wires parallel - tightly sleeved together, down on the deck.
For the pre / pi tubes anyway.
My last build was silent - no hum whatsoever.

[Leo_Gnardo]

OK, so Ohm says if I want to run 3 amps at 6.3 volts I need a 2R1 resistor that will dissipate 18.9 watts. Sound right? I don't have that resistor!

Anything near that will be plenty good for your "bogus load."

How's about a power soak or two. Or whatever resistor you sub for a 4 ohm speaker when you're power testing & like that?

Ya got me laughin'. thinking of the car guys Click & Clack: "Bo - o - o- gus!"

[tubeswell]

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.

Good PC board based amps run the AC traces in lineal pairs (that are close enough to ensure that the opposing phase EMF is cancelled out), or use DC supply for the heaters

[alvarezh]

Thank you tubeswell, I have seen the use of DC heaters on a few amps but only on the pre, never on the power tubes.

May we conclude that as long as the wire pair is really close together they will cansel out the EMF? If so, no need for twisting then?

Thanks again.

[Phil_S]

I have had no problems with heater hum on an amp I made where I ran the heater wires parallel and heat shrinked together untwisted and laying flat against the chassis. It looks very clean and is quiet. However, it was meticulous time consuming work.

I have read and seen pictures of heaters done with zip cord (lamp cord) that were reported to be hum free. Apparently, the perfectly parallel wires are just as good as twisted pair for hum cancelling. I think the other consideration is a routine one, just make sure to keep signal wires away from heater wires. There are many good solutions for lead dress. It isn't always clear why they work. I tried zip cord one time, but didn't like such a fat wire gauge and I found it impossible to manage the stuff on a noval socket. I've often wanted to try using a single piece of uninsulated copper with sleeves between sockets.

[pula58]

In the knowledge base of electromagnetics there is the concept of the "right hand rule." It goes like this: If you have a wire with current flowing through it, You orient your right hand such that your thumb points in the direction of positive current flow. Your reminaing fingers curl around the wire and show the direction of the magnetic field "lines." So, the Magnetic field curls around the direction of current flow. Now imagine you have another wire with the same magnitude of current, but with reversed current flow. Now, your thumb points in the oppositie direction compared to the first wire and the field lines curl around in the opposite direction as before. If these two wires could occupy the same physical space (not physically possible) the field lines, since in opposite directions and equal i nmagnitude, would cancel-out exactly.

Twisting the wires together is an attempt to have the wires with opposite current flow to occupy, on average, the same physical space. The farther apart the wires get from eachother the less cancellation of magnetic field occurs. So, the closer the better.

But how close is close enough....I would like to try runnin gtwowires in parallel one of these days to see how well, or not, that works.

[Cygnus X1]

Two of my amps virtually identical built back to back.
Inspired by seeing a Laney that had parallel filament wiring (AC)...I decided to try it myself.
High gain preamp design with three gain stages and cathode follower tone stack.

The parallel wired amp is quieter than the twisted!

The wires a lying flat against the chassis as the first layer, and opposite the remaining wires that run to the preamp board.
I need to get pics of this to be clear.