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eddiejg
Senior tube assistant
Posts: 253
(4/8/04 11:19 pm)
G40V High Pitched Squeal
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I recently built 2 Pignose G40's- 1 with JJ 6V6's & 1 with EL84's.
My only problem with both is that when the volume & master volume are both high, I get a high pitched squeal & also a little bit of sound like radio interference.
I did put a 68K resistor on pin 2 of the 1st preamp stage on both amps (even though it wasn't on the schematic).
I tried using a probe to move the wires around to see if I could isolate the problem area, but I can't find any wire that affects the noise.
The circuit is basically a Bassman 5F6A with 1 input with the 1st 2 stages in series (instead of 2 separate inputs) & a pre-PI master volume
Can anyone point me in the right direction?
Thanks.
-Ed
Tiny Daddy
I will work on all amps
Posts: 602
(4/9/04 7:48 am)
Re: G40V High Pitched Squeal
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It's oscillating, possibly ultrasonic. Try disconnecting the feedback from the speaker jack.
eddiejg
Senior tube assistant
Posts: 254
(4/9/04 10:31 am)
G40V High Pitched Squeal
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That didn't work.
-Ed
bnwitt
Senior tube assistant
Posts: 320
(4/9/04 11:05 am)
Re: G40V High Pitched Squeal
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Eddie,
what type of grounding scheme did you follow? I had this same problem with a 5F6-A that a regrounding to Doug's scheme cured.
eddiejg
Senior tube assistant
Posts: 255
(4/9/04 11:22 am)
Re: G40V High Pitched Squeal
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Bnwitt-
Actually, I used 2 different grounding schemes for both.
The 1 thing that they both have in common that deviates from Doug's scheme is all preamp & power amp filter caps go to the same lug.
I will try splitting the preamp caps off & route those to the preamp lug.
Thanks.
-Ed
Tiny Daddy
I will work on all amps
Posts: 604
(4/9/04 11:24 am)
Re: G40V High Pitched Squeal
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You may need to add another 10K/10uF section to the power supply, just for the first stage, so the first 2 stages don't talk to eacher through the power supply.
Also, what tube are you using for the first stages? maybe try a lower-gain tube to see if that helps.
Or the power supply filter cap may be too far from the first gain stages. This can be helped by adding an additional power supply filter cap near to the first stages. Usually .1 or .22uF is all that's needed, provided you keep the remote electrolytic.
eddiejg
Senior tube assistant
Posts: 256
(4/9/04 11:43 am)
Re: G40V High Pitched Squeal
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OK, guys.
I tried changing the ground scheme- no affect.
I did start to focus in on the 1st stage, though.
Even though moving the wires had no affect on the squeal, when I tap on the wires to & from the preamp volume pot to the 2nd preamp stage (pin 7), the 3 wires involved are microphonic whan tapped.
What would be the most likely thing to eliminate this- another filter stage or shielded wires?
Thanks.
-Ed
bnwitt
Senior tube assistant
Posts: 322
(4/9/04 11:49 am)
Re: G40V High Pitched Squeal
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Eddie,
I would definitely replace the first stage signal wires (the ones from the jack(s) to the V1 grids with shielded wire. Ground the shield(s) at the jack(s) but not on the tube end. Did you use buss wire grounding on the back of the pots and jacks or are they just grounded through their bodies to the chassis? Have you reversed the OT secondary wires to see if it is a phase issue there?
punkykatt
Junior tube assistant
Posts: 2
(4/9/04 4:11 pm)
Re: G40V High Pitched Squeal
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Try putting a 47pf cap across the plates of the PI. It did the trick on my plexi/bassman homebrew.
eddiejg
Senior tube assistant
Posts: 257
(4/9/04 5:56 pm)
Re: G40V High Pitched Squeal
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Bnwitt-
I replaced the microphonic wires with shielded.
No improvement.
I have the jacks & pots grounded with buss wire.
I had originally had to reverse the OT wires because it was motorboating, so now they should be correct, right?
Punkykatt-
I tried the 47pf cap.
It actually made it worse.
What would that indicate?
And would a resistor & capacitor as a conjunctive filter be in order?
Thanks guys.
-Ed
bnwitt
Senior tube assistant
Posts: 327
(4/9/04 7:02 pm)
Re: G40V High Pitched Squeal
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Eddie,
I've not had a reverse phase OT cause motorboating before only squeal. You've made a lot of changes to this amp now, why don't you try to reverse the leads one more time and see what happens. If it gets worse you can return it to the way you have it now. Have you tried the amps with 6L6's in them?
ganzonimx
Hey get your own solder
Posts: 461
(4/10/04 11:00 am)
Re: G40V High Pitched Squeal
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Did you try with a different tube as PI?
Chris
Lucid Alice
Senior tube assistant
Posts: 192
(4/11/04 10:15 am)
Re: G40V High Pitched Squeal
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You mentioned that this circuit runs the first two preamp stages in series. This sounds similar to Doug's hotswitch circuit. Are you sure this design isn't too hot when cranked wide open? You may need to tame the gain factor here.
The hotswitch circuit works fine if you start from zero and dial up to the gain you want, but it is unstable wide open without mods to reduce the max gain.
Try a resistor in series followed by a resistor to ground between the two stages. Make the two resistors the same value. You will have to try different values until you reach the optimum gain transfer without squeal.
If you have a dual 500K or 1M pot handy it would be ideal to dial in the optimum value. Then take the pot out of circuit and measure the values.
eddiejg
Senior tube assistant
Posts: 258
(4/12/04 9:35 am)
Re: G40V High Pitched Squeal
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Bnwitt-
With the wires reversed, I still had the squeal.
Also, with the NFB disconnected, still squeals.
I haven't tried it with 6L6's, but I will later.
I'd be surprised if this works as both amps with 6V6's & EL84's have the same problem.
Chris-
Two different amps with 2 different tubes in the PI socket, both squeal, even with the tubes switched around.
Alice-
It actually may be too hot, which may be causing the squeal.
But, it shouldn't be too hot as I copied the Pignose G-40 circuit & the preamp & PI voltages are all extremely close to the original.
If I make the series resistor going into the pre-PI master volume larger, it reduces the problem & the gain.
But, what would cause the amp to have more gain than the original?
All component values check out on both amps.
I may experiment with 1 of the amps & re-wire the 1st preamp tube in parallel a la 18 watter or Spitfire.
I know it will still have plenty of gain & that design is more managable & easier to control the gain level.
But, I really want to get to the root of this, too.
Thanks guys.
-Ed
Lucid Alice
Senior tube assistant
Posts: 193
(4/12/04 10:15 pm)
Re: G40V High Pitched Squeal
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In another thread Funkalicious stated "a perfect marriage of math, physics, and VOODOO!" and he is totally right on.
I've never seen the insides of a pignose. Is it a point to point or printed circuit? It's not completely surprising if you are getting more gain than the original circuit had. You would think electronics was more exact wouldn't you. There are more factors than you can shake a stick at. Maybe your ptp wiring is more efficient than their circuit layout. Maybe you used better sockets or tubes. Maybe one of your tubes is slightly microphonic at high gain. If your layout varies at all from theirs it may be introducing the squeal. Who knows?
Their design may have been pushed right to the edge. If anything is more efficient in your build, it may have pushed it over the edge. It may be that if you tried 5 different sets of tubes in the preamp, one of them would work perfectly.
(As an example of what I mean: If you go over to the 18-watt marshall site and build an exact replica of the original circuit, the tremelo circuit may not behave properly. The solution is to get a lifetime supply of various 12ax7's and swap the tube out, and again, and again, and again... until you find one that makes the original circuit work properly. Why? As I stated above, Funkalicious nailed it.)
eddiejg
Senior tube assistant
Posts: 259
(4/12/04 10:39 pm)
Re: G40V High Pitched Squeal
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Alice-
It's a cheap chinese PCB.
Sounds really nice though.
I used to own 1 before I started building amps & I thought it might be nice to build 1 PTP with quality parts.
It's very closely based on the 5F6A with the 1st 2 stages in series & a pre-PI master volume.
I used the 5F6A layout as my starting point & th
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