86-Special ready with noise problem

[10thTx]

Lord Preset,

THANKS for updating Mat's layout!

You had a few minor errors. Mostly around the volume pot & bright switch.

I would NOT ground the power tube's cathode resistor and cap with preamp buss wire grounding. I would ground them with the power tube filter caps.

http://www.el34world.com/charts/grounds.htm

There still may be errors, so check for them.

with respect, 10thtx

[mat]

Sorry, been ultra busy at work. No chance to play the amp at all

From the earlier rehearsal I recall the clean channel sounding surprisingly similar to fender amps. The OD channel does not remind me of the dumble style sound. More like marshall and drive cranked turns to fuzz face style break down. Nothing like any amp I have tried. I actually like it being a kind of odd ball.

Will continue tweaking and promise to record something as soon as it gets quieter at work

Lord Preset, are You sure You are after this kind of sounds

If so, good luck for the build

[lord preset]

I don't have any fixed expectations but the clips you previously posted sounded very encouraging. I just built a D'Lite which I like a lot but I have no idea if it sounds like a proper Dumble either. Variety is good

[mat]

Variety is good

Agreed !

[lord preset]

Progress on my 86 Special duet. I'm using a Ddawg 12x7x2, recycled PT and OT, various bargain parts from Apex Jr and a local electronics surplus store, and Mouser for the rest. It's a tight fit, at least by my standards and giant hands. One Impact of the space constraints is that I realized I couldn't use the nifty IEC/fuse holder unit I got from Apex. The power supply board is wedged in the back corner of the chassis and there was no room. I didn't want to move the AC supply close to the preamp tubes, hence the traditional power cord. The power light is going on the cabinet external to the chassis which is why it's dangling out there.

The power supply is built and working although I may have to play with the dropping resistors once the tubes are in. The main board is mostly populated. I will make a front plate template and do as much of the pot and switch wiring out of the amp as I can. Once this all is running, if it's good I plan to put this in a goofy robot shaped cabinet as a present for my brother. Or, if it's too good I may keep it

[lord preset]

And the front panel. Mini switches are a PITA.

[10thTx]

Looks like it's coming together just great! Thanks for sharing the pictures. Hope you will post a sound clip when done.

With respect, 10thtx

[lord preset]

My build is all together now. No problems with the light bulb test. I fired it up and it works, but it sounds absolutely hideous. I am getting pronounced buzzy distortion at anything but very low volume. On start up I measured voltages and saw they were quite low, so the 1st thing I did was lower the value of the 1st dropping resistor from 10k to 2.2k. No change in distortion but the voltages are now:

V1a: 145v Plate 1.28 Cath
V1b: 147, 1.3
V2a: 147. 1.3
V3a: 187, 61
V3b: 187, 61
V4: 269Plate, 247 Screen
V5: 269. 247

I am not seeing any wiring issues although there must be one So far the only thing I've found is that my updated layout was missing the 220PF cap across the 82k plate resistor on V2 that is shown in the schematic. I didn't expect that would be the issue and it was not. I have swapped out all tubes - again no dice.

One thing that might be significant is that the buzzy distortion doesn't start until a few seconds after the amp is taken off standby, so it sound fine for a bit then things get nasty.

Here are some gut shots. I would appreciate any suggestion of what to be looking for. I have to assume that something in the preamp is getting hit too hard because of a wiring fault or...?. Note that that the power resistor dangling in the air will get mounted properly once I figure out what is wrong.

[lord preset]

More pics:

[Tillydog]

Is it the same on both channels?

What schematic did you use in the end?

[lord preset]

Is it the same on both channels?

What schematic did you use in the end?

Yes it's the same for both channels. I played around with setting the guitar volume pot very low and various combinations of volume vs master volume. The buzz happens in all cases if the overall volume is above a very low level.

I have used the schematic from a few posts above provided by 10thTx.

Thanks

[10thTx]

Yes it's the same for both channels

it sounds absolutely hideous. I am getting pronounced buzzy distortion at anything but very low volume.

Then I would start looking for the problem in the preamp or LTPI or power amp section. Recheck component values and measure voltages to see if something is amiss? I'd also try different tubes. Maybe try a 12AY7 in V1

I am thinking there must be wiring problem or bad solder joint or a leaky cap prior to a pot? Maybe try moving some wires carefully and with a non conductive stick such as a chopstick or dowel. Perhaps there is some oscillation issue going on?

Try a 22k or 33k mid resistor instead of the 120k ............. OR put in a 100ka or 50ka trim pot there. IF that doesn't eliminate some buzz, then I'd anticipate a wiring problem or a bad tube.


On my amps, I like having a .02 for the bass cap and I typically use .03 for the mid cap with a 100ka (or smaller) mid pot. You might try that?

For the time being, I'd eliminate the EF86 OD as a concern until you get the rest of it sounding right.

IF you followed the schematic correctly and it's wired up correctly, I would not expect a horrible buzzy tone.

with respect, 10thtx

[lord preset]

IF you followed the schematic correctly and it's wired up correctly, I would not expect a horrible buzzy tone.

with respect, 10thtx

Neither would I

This is my 2nd build so I'm betting I did something boneheaded. My grasp of translating schematics into a physical layout is getting better but still pretty weak. I have double checked all component values and they all look right. I only have a DMM so I don't know how I could check for leaky caps. I will work through your suggestions until something turns up. Or until I hurl the amp into the canyon behind my house.

Thanks![/b]

[10thTx]

Is there any chance that you have a passive effects loop? And if so, is there any chance you plugged a delay or reverb pedal in backwards?

That can make a horrible buzzy noise. It's been known to happen.

With respect, 10thtx

[lord preset]

Is there any chance that you have a passive effects loop? And if so, is there any chance you plugged a delay or reverb pedal in backwards?

That can make a horrible buzzy noise. It's been known to happen.

With respect, 10thtx

Nope, I left the effects loop out of the build altogether. Thx.