Modding Champ 5C1 Tweed for less feedback

[ardarvin]

Hey guys,

I've got a 1955 Champ 5C1 (actually 5D1 but same circuit), using a 50s green bullet mic. Sounds good, but I'd like to reduce the feedback.

The guy from greenbulletmics.com recommends using a lower gain preamp tube. But the champ uses a 6SJ7. Isn't this already pretty low gain?

Any suggestions of tubes to use, or mods, would be greatly appreciated!

[Jazz P Bass]

I know it sounds stupid but can you try to play with the amp in front of the mic?
Like that it is pointing away from the mic.

[ardarvin]

>I know it sounds stupid but can you try to play with the amp in front of the mic?

Not stupid, but yes the thought had occurred to me. I tend to want to use the amp as a monitor, otherwise I can't hear what I'm playing over the sound of the drums, marshal stacks, and the hari krishnas banging on their tambourines in the corner.

I'm guessing this is a common ailment. Has anyone tried to tackle it with some amp mods? The mic has had it's 39K roll-off resistor removed -- I'll try putting that back on, but I understand it doesn't make much difference.

[Tom Phillips]

Have you heard of / considered using a notch filter to kill the feedback?
Google "Behringer Feedback Destroyer" and "Behringer Shark" for info. The feedback destroyer may not be appropriate for your setup but I believe that the shark is used in line between the mic and the amp. If one of these don't work out you will still trip across lots of discussion about how other harp players are dealing with feedback problems.
Regards,
Tom

[Tom Phillips]

I forgot to add that there may be other, less fancy, adjustable notch filters available. There was someone building a harp amp with the feature built in. The player had control over the notch center frequency and the notch depth. Every stage setup will require different settings to optimize the performance.

[arnenym]

You could cut out the 6V6 bypass cap.
Or:
You could buy a Kinder AFB+ but it's about 395$ and a new harpgear amp cost 500.
A better choice would be a Lonewolf Harp Octave pedal. At the lowest effect it have just a pretty good anti feedback funktion

[Rillo]

If your feed back is high pitched, another simple remedy may be a treble bleed cap. You can simply add a .01mf from the signal path to ground between the gain stages. You may need to play with the value of that cap, perhaps a .0047mf is all you need. I was able to get these from Radio Shack. If you want to get fancier use a 250k pot followed by the cap to ground. You can see an example of this at lonewolfblues.com. Look for Mark Burness' schematic under the Forum Amps. I've used the simple cap to ground in my Champ type amp and am quite happy with it.

[Bruce / Mission Amps]

In the spirit of experimentation, as a start, with the 6SJ7 as your first and only preamp stage, ....
I'd replace it's 250K plate load resistor with two 120K resistors and attach your .022uF coupling cap (to the volume pot) right between the two 120K resistors.
Then I would put a 1000pF to 1500pF cap across the 120K resistor that is being fed the high voltage B+.
I'd also use a .047uF to .068uF cap to drive the volume pot instead of the .022uF cap.

[soundguruman]

Place a 1000 volt 6 pf silver mica capacitor between plate and grid of V1 input stage.
Place a 39 pf 50 volt silver mica cap between grid and ground of V1 input stage.
place a 250 pf 500 volt silver mica capacitor across V1 plate resistor.
Install a tube damper on the V1 preamp tube, not a silicon O ring, a real tube damper please.

But of course the best would be to install an EQ between the mic and the amp. Use the EQ to drop out the frequencies that are ringing. That would be easiest method.

[MWJB]

Also try backing off the plate current, see how you go around 30mA on the 6V6 plate.

You say the 5D1 & 5C1 have the same circuit? Do me a favour & check whether the 6SJ7 is grid leak biased or cathode biased, if cathode biased try adding a NFB loop, start about 15:1 ratio?