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Old 31st July 2011, 07:09 PM   #1
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Default Home build 6L6 pp amp buzzing noise

Hi, I built this amp from an old McMartin 30 watt PA amp. I completely gutted it, used the AA864 Fender Bassman schematic for the output stage and designed the preamp myself; power supply is based on the voltages desired.

The preamp starts with a twin triode 12AU7 wired parallel, feeding into the tone and gain controls. Then it goes into another 12A-7, also wired in parallel, that is set up for distortion by putting a rotary switch w/ different sizes for the cathode resistor. This changes the plate voltage and gain factor from extreme clean to extreme distortion-much like a distortion pedal sounds. In this position the tube can be changed from a 12AU7 for quieter situations w/ the same crunch, to my favorite-12AV7- that gives a really full bright breakup that is perfect for all my pickup configurations on my guitar. A 12AX7, is really nice if you want extra clean on the clean stage with high gain and headroom. Or if you need huge gain for a big stage this is the tube. After this stage the signal goes into a master pot and to the phase inverter.

The power stage has a pot for the negative feedback to control the amount fed back to the phase inverter. Anywhere from a ratio of 2.9, quite a bit of NFB, to 150- virtually no NFB. What a nice control this is. It controls the output tube distortion point that is dependent on how much NFB there is.

Issue: When the amp is in its extreme settings, full volume, tone, highest gain on the second tube (high gain is low plate voltage), and no NFB there is a buzzing sound. The level of noise is dependent on the preamp tube used; 12AU7 is the quietest. At 'stock' settings the amp is pretty quiet from buzz, only hiss. I expect some from the resistors-Allen Bradley's.

Is there a way to eliminate this buzz with the high gain settings while using high gain tubes?

The noise does not affect the playing or sound and can't be heard when playing. I've tried all the suggestions in the Aiken Amps forum which helped considerably but not completely. I am beginning to conclude that the high gain will be a trade off w/ buzz. Also, it's not the guitar-I tested it w/o it plugged in and with a shorting jack. And it's not AC hum, that is very quiet and the hum adjust works well.

The only place I notice a big improvement is in the tone control. At all extremes, the amp will squeal slightly when this is 3/4 to full.

Any thoughts, suggestions or knowledge?? Also, I would like some feedback concerning my schematic and design.

Thanks much...
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File Type: jpg Lil Mänster.jpg (118.7 KB, 184 views)

Last edited by dscottguitars; 31st July 2011 at 07:34 PM.
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Old 31st July 2011, 09:31 PM   #2
tricomp is offline tricomp  Denmark
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Hi,

I'm not really a pro at this but it seems to me that what you are experiencing could be self-oscillation.
I would certainly put the grid-stoppers on the 6L6 g1 directly at the tubesockets and attach the 220K grid-resistors away from the tube.
Also a 47K grid-stopper at the input tube seems high. 1K would be more normal.
Other than that I just wonder about the different values for the plate-resistors for the Long-Tailed-Pair? Normally they are chosen to be at the same or closer values, but I doubt it'll have any influence on the buzz, just on distortion.

The measured voltages shown on the schematic are not correct, but that's not so important.
Take f.x. the LTP g1-voltage shown to be 65V. That's simply not possible.
The reading of 105V at the 22K cathode-resistor is correct but since no grid-current flows in the LTP, g1 should read the same.
And it will, of course if you use a very High-Z voltmeter, not just your ordinary 1M-ohm/Volt Rat-shack DVOM.

rgds,

/tri-comp

Last edited by tricomp; 31st July 2011 at 09:41 PM.
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Old 31st July 2011, 11:21 PM   #3
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Here is a schematic of the Bassman, and it shows the voltages to be what I'm getting. I have a little more on the tail, 105v vs 92v. Also, Fender uses 68K grid stoppers and the 47K is the closest I had. I do have the 1.5K's on the sockets but I will try moving the 220K's, they are mounted directly with the 1.5K on the socket w/ the ground line going to the power supply star point.

That schematic doesn't show the grid voltages. When i find one I'll post it...
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File Type: pdf bassman_aa864.pdf (77.7 KB, 40 views)

Last edited by dscottguitars; 31st July 2011 at 11:33 PM.
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Old 1st August 2011, 08:20 PM   #4
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here are a couple of pics of the amp:
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File Type: jpg 100_3335.JPG (494.5 KB, 101 views)
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Old 1st August 2011, 09:01 PM   #5
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The hum pot should be a lower value - 100 Ohms is typical. 100 Ohms across a 5K pot is not the same thing... better yet would be to bias the heaters positive (cathode of 6L6s could be used, or a voltage divider from B+). You could use DC on the first stage heaters
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Old 1st August 2011, 09:06 PM   #6
tricomp is offline tricomp  Denmark
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Sorry, I didn't mean to say ALL voltages are off.
Just the obvious ones.
I suppose I'm used to Hi-Fi schematics and not that much to guitar-amps.
68K or 47K as a gridstopper will make high frequencies suffer but maybe that's not so important with live guitars?
Other than what I've already mentioned I can't see any grave errors in the schematic; it should work.
If it does is probably up to wiring, grounding and perhaps avoiding a cramped layout. With such a small space available probably wiring should strictly adhere to star-grounding and shortest possible length of wires.
Tightly wound filament-wires that doesn't pass across tube-sockets or splits around sockets etc. Not grounding both ends of screened cables; the list goes on....
I recently finished this:
CF/CCS, can it be made simpler?
On open ordinary phono inputs and max. volume you'll be challenged to hear any hum and there's absolutely no hiss, so it worked for me.
Probably because of rather strictly adhering to star-grounding but not forgetting that power-supply electrolytics should be fed directly from the rectifiers and the power-transformer. That goes for both the positive AND the negative connections.
Since the last pictures posted I've finished the amp. completely and removed the rats nest in the upper right corner of the final picture. When the picture was done the amp was still in an experimental state.

Good luck,

/tri-comp
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Old 1st August 2011, 10:58 PM   #7
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Thanks for the quick responses.

Tom: since it doesn't have any hum problems should I change that pot anyway? The other stuff is over my head.

Tri: The voltages on the PI are correct. There is a little more than 60v on the grids that comes from the 100v at the cathode resistor-grid resistor junction. When I first discovered voltage there I was concerned, but found it to be that way on another similar guitar amp schematic. The amp works really well, I am really happy with it. I've moved the 6L6 220K resistor grounds and a couple others and now the amp is not bad at full volumes, tone, low plate voltage on the 2nd stage, no NFB, and using a 12AX7. With just a little NFB, ratio of about 25 (standard is 8-15; which is more NFB) the noise goes away. I can't really play it at this level w/o having a visit from the police, it is really loud. It has great tone and versatility with the distortion controls.

Thanks again...
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