BF Showman choke

[deci belle]

Hope that is enough resolution, Tom. I tried 4k7 gridstoppers, but the ringing when I play F and A and sometimes D on the little string is still there.

I disconnected the Marshall input at the PI, jmaf— no difference.

I'm going to increase the filter caps after the choke after all— this is why:

Instead of taking the feed for the 5f6a's node off the 763's 22uf cap, I tried feeding it off of the driver's filter cap in order to isolate the two channels, but when I did that, the ringing got much worse— even affecting notes played on the B string.

So then I increased the 5f6a's node from 10uf/500v to 16uf/475v.

Now it's back to sucky normal. hehe.

I might even consider a new choke and increase the screen and driver caps to 47uf too.

Does anyone see anything really dumb on my lead dress?

[Tom Phillips]

Mar',
I see from your photo of the power amp section that you built in an additional board to mount bias sense resistors for each power tube. That new circuitry is in close proximity to the phase inverter wiring and the leads that connect the PI output to the power tube grids. There is heavy AC current flowing in that circuitry and it may be coupling a positive local feedback into the grid drive wires and/or the PI.

An easy test to determine if this is your problem is to install a short connection between pin 8 of each power tube to chassis ground per the original wiring. I suggest that you try that and determine if it solves the problem. Note that you don’t need to remove the new circuitry. It will be bypassed when you install the ground jumpers.

Regards,
Tom

[deci belle]

Hi Tom,

I've completely removed the whole bias checking board in the past and resoldered the cathodes to chassis GND like stock, and there was no difference in the ringing sound. So I re-installed it as you see it now. Thanks for to checking the fotos and offering that possible scenario.

I also tried upping the 6L6 gridstoppers to 4k7 yesterday.

What I did today as a result of seeing the gut shot is re-route the primary wires on the PT. Now they wrap around the perimeter of the chassis.

I've also pulled the "noisy" X-input from the dirty channel and just played the clean channel. Also, I've pulled the clean channel and dirty channel tube(s) alternately.

It's still there, haha! Here's why I'm going to put bigger caps under the dog-house:

I separated the pwr lead to the dirty channel from the clean channel's node and attached it to the PI's node. Then I raised the dirty channel's pwr resistor at the filter cap on the board to compensate. The oscillation got really bad, in that the "sheen " got much louder and affected notes on the B string too. So I re-wired that connection.

I'm getting bleed and feedback through the pwr section.

[deci belle]

I'm going through the filter section on my amp and I measured the resistance of mine today. It's 93.7Ω.

Does that sound about right?

I don't have any reason to believe anything is wrong with it. I've never measured another choke before.

I read a part of Torres' amp book today and it said most of the old fender chokes averaged about 94Ω. hehe!

[Enzo]

Oh my god, you're three tenths of an ohm LOW.

[deci belle]

Oh my god, you're three tenths of an ohm LOW.

Eeeeeeeeeeeeeee!!! What do I doo!!

[Enzo]

Right about now is when you separate yourself from the rest of the group and walk out into the woods alone. The plot will take care of you from there.