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PostPosted: Mon Mar 28, 2005 12:02 am 
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I'm debugging a single channel AC15 with an EF86 preamp.I like the sound, but I'm dealing with some squeal at high volume. I"d be interested in the 6AK5 - did you use the standard pentode preamp circuit?


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PostPosted: Mon Mar 28, 2005 2:26 am 
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Mark,

I used the same component values that the DC30 has around the EF86, except the screen cap to ground is 3.3uf instead of .1uf (it was on hand.) The gain is probably less than the EF-86, but still more than enough to get the amp singing.

Is the squealing from microphonics? Do you have a cut control across the outputs of the PI?

I don't know if 6AK5's are inherently less microphonic than EF86's. I do know they are cheaper, so you can buy a bunch to test. I have a dozen of them, and a couple were too noisy to use. The rest were fine.

-neil


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PostPosted: Mon Mar 28, 2005 8:22 am 
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MarkB wrote:
I'm debugging a single channel AC15 with an EF86 preamp.I like the sound, but I'm dealing with some squeal at high volume.


I never built an AC-15 but Is/are the input grid resistor(s) right on the tube pin? Can you try reversing the transformer leads - long shot.

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PostPosted: Mon Mar 28, 2005 9:13 am 
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Neil. Lice looking amp. Can't wait to hear mp3's. BTW, is that a 70's RCA 12AX7 you have tucked in there? I really like those tubes.


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PostPosted: Tue Mar 29, 2005 12:13 am 
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The grid resistor is on the socket. The squeal only occurs when the volume pot is turned up past half way. I don't think it's microphonics, because it happens when I have the strings muted. I tried putting a cap across the PI plates, but that didn't help. I assume it's a lead dress issue - it's my own layout. It did seem better when I put a piece of aluminum foil-covered cardboard over the open top of the chassis. I'm not sure why that would help the squeal. I was working on some buzz, but it seemed to raise the squeal ceiling.
I have no cut control, but turning the treble pot way down does get rid of a lot of noise. I'm assuming that the buzz I'm getting is related to the squeal, so I really need to find the root of the problem, rather than band-aid it out. When I turn it way up, before it starts to squeal the buzz changes frequency/level, and I feel like by brain cells are getting vibrated.

Any way, the amp sounds very nice at low volume. I used the Root 666 schematic for the preamp component values and the VTB tone stack.


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PostPosted: Tue Mar 29, 2005 12:54 am 
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Have you tried moving the ax7's around? try one at a time, to see if one in particular is causing the problem.

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PostPosted: Tue Mar 29, 2005 1:49 am 
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MarkB wrote:
It did seem better when I put a piece of aluminum foil-covered cardboard over the open top of the chassis. I'm not sure why that would help the squeal. I was working on some buzz, but it seemed to raise the squeal ceiling.


By open top, do you mean circuits and guts exposed, or the top above the tubes and transformers? Maybe you need some additional shielding. Aluminum does a good job of blocking EMF. Copper and steel work well also.

Does the squeal happen when nothing's plugged into it. if not, your guitar's pickups might be magneticly coupling with the transformers of your amp. In which case, the cheap cure is a longer guitar cord, the expensive cure is mu-metal shielding: the first step down a dangerously slippery slope.

I've had the best luck chasing down such things by using a wooden chopstick to nudge and tap components and solder joints while the amp is running and making the offending noise. Rather unscientific, but it works.

Good luck.
-neil


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PostPosted: Tue Mar 29, 2005 2:44 am 
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Coco

The amp goes EF86 - tone stack - PI - power tubes, so there's only one 12AX7. The treble pot cuts out the buzz/oscillation,, so I assume it's originating before that. The EF86 socket has two pairs of lugs that get tied together, with one pair going to ground, so the wiring is kind of busy.


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PostPosted: Tue Mar 29, 2005 2:53 am 
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neilium wrote:
MarkB wrote:
It did seem better when I put a piece of aluminum foil-covered cardboard over the open top of the chassis. I'm not sure why that would help the squeal. I was working on some buzz, but it seemed to raise the squeal ceiling.


By open top, do you mean circuits and guts exposed, or the top above the tubes and transformers? Maybe you need some additional shielding. Aluminum does a good job of blocking EMF. Copper and steel work well also.

Does the squeal happen when nothing's plugged into it. if not, your guitar's pickups might be magneticly coupling with the transformers of your amp. In which case, the cheap cure is a longer guitar cord, the expensive cure is mu-metal shielding: the first step down a dangerously slippery slope.

I've had the best luck chasing down such things by using a wooden chopstick to nudge and tap components and solder joints while the amp is running and making the offending noise. Rather unscientific, but it works.

Good luck.
-neil



I have the chassis - it's a head - sitting guts-up, tubes/transformers down in a cradle. so the board is exposed. I get the same effect with the guitar across the room - about ten feet. When I plugged in my Jazz bass, I did get a big variation in the buzz by moving/turning the bass body around. I think I've got more than one source of noise, so I'll have to isolate them to figure it out. It's a rebuilt Marshall 18 Watt, and I didn't have the squeal with that circuit.


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PostPosted: Sun Apr 03, 2005 11:23 am 
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How is the amp grounded? I know this is not a Lightning, but I found that you need two ground points on that. One for the Power side, another, opposite side of chassis, for everything else. See the layout in the Lightning project section

Do you use a PS choke? Another "must have' for a lightning/AC15. This will reduce the hum.

Did you use shielded cables from Volume control to board? May affect squealing.

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PostPosted: Sun Apr 03, 2005 2:26 pm 
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Last night I redid the EF86 socket wiring. I tied both 2+7 and 3+8 together over the top of the socket. Before I had 3+8 tied together with a wire running around the base of the socket. I also moved the ground wire for 2+7 from the top of the board to under it.
I couldn't play loud last night, but when I turned up the volume pot I was able to get it to 3/4 with no squeal - that's an improvement. I'll be able to crank it up tonight and test it properly.
I have a single chassis ground and no choke. There is some buzz, but not much hum. I needed to get the squeal taken care of before I try to lower the general noise level. It's not bad, but I'm sure I can get it better.
I can recommend the Rt66/Traynor TB tone stack. It has a nice range, but not so much that you can't use a lot of it.


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PostPosted: Sun Apr 03, 2005 4:20 pm 
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Sounds like you're making good progress - finally. The squeal would appear to be related to that EF86.

I strongly recommend that choke to reduce the hum, and then if it's still annoying, I would try to separate the two ground points and/or shield the inputs.

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