Just birthed a new ODS - and a preliminary problem SOLVED!

[mlp-mx6]

Hello friends!

I have just this past weekend fired up a new 2-6L6 ODS I built. She's non-HRM. I tried a slightly different presence circuit. I am using the 5F6-A Bassman circuit as an experiment.

The odd behavior is that when I turn the presence cap up toward the top of the range I get an odd crackling sound that trails the notes. If I turn the pot down it goes away. Been thinking about it and I'm wondering if I have a high-freq oscillation. Have not yet put it on a scope (just too many things going on lately!), or even checked the idle current when I turn the presence all the way up (will try to do that tonight). If I see the current go up I'll DEFINITELY suspect an oscillation problem.

Nonetheless, I was wondering if anyone else has seen/heard/solved such an issue.

Thanks!
Michael

[Bob-I]

Yes, I've had that and you're correct. I had to use a 22pF snubber from plate to plate on the PI, same as Marshall.

[mlp-mx6]

WOW! 2 minutes to a good answer!

Thanks, Bob-I. I will give that a try tonight! (though I may have to use 47pF, a la 5F6-A or earlier Marshall.)

Regards,
Michael

[Funkalicousgroove]

I tried that same PI and use a 47pf snubber, but I get a SQUEAL when I turn up the input volume past 1 o'clock, or hit the bright switch in the OD mode....I've tried a few different things without luck, any ideas?? The tone is really fantastic, but that squeal kinda rattles my cage.

[heisthl]

Bright switch squeal can usually be cured by soldering the blocking resistors as close to the grid pins on the tube as physically possible and in the case of clean channel squeal increase the 22K to 47k or even 68k on the first input. You could try an increase in the OD grid resistors too...and better (or more) shielding of any of the runs to the v1 and v2 tube grids. Does the squeal change when you cover any of the signal caps with your fingers? If so try reversing the cap's lead orientation to reverse the polarity.

[mlp-mx6]

I had to reroute the wire from the effects loop return to the master volume, and all was well. The oscillation was NASTY! I could evoke it at will with certain settings of the master volume, irrespective of the presence. The presence just made it more noticeable on the notes. What was strange, though, was it was from about 2 o'clock to about 3:30 on the dial. Past that, the oscillation went away. Nonetheless, it rocks now, and it's nice and quiet when it is supposed to be!

Thanks, all!

[Funkalicousgroove]

The only thing I did differently on this one from the other 10 I've built is the different PI,

The squeal is only with the OD and Boost Engaged, and only with the input volume at 12: o'clock or above. I don't think it's parasitic oscillation, it sounds like something is microphonic. The amp is EXTREMELY liveley. I think I'm going to put the regular PI back in annd see if that makes the difference. I've already changed the snubbers on V2 up to 300pf.

[ayan]

The only thing I did differently on this one from the other 10 I've built is the different PI,

The squeal is only with the OD and Boost Engaged, and only with the input volume at 12: o'clock or above. I don't think it's parasitic oscillation, it sounds like something is microphonic. The amp is EXTREMELY liveley. I think I'm going to put the regular PI back in annd see if that makes the difference. I've already changed the snubbers on V2 up to 300pf.

For what it's worth, I built an amp -- with the regular PI, mind you -- that exhibited this problem: with the presence control dimed, some high pitch noise would make its way into the audip path when I switched channels. I think the OD channel would also display some oscillation type behavior with everything dimed. Same layout, same components and same wire as before, but this problem. What I did was as follows: started trying different caps on the OUTPUT side of the output caps of the PI until the problem was gone. In my case, the regular tone of the amp -- as seen on a scope -- was not changed, but that funny thing was gone. You may try that approach and see if it works for you.

Gil

[Tdale]

Same thing happened to me. Squeeling when I turned the precense up. I moved the loop wires away from the OT primaries and speaker jack, and it disappeared.

Tommy

[Funkalicousgroove]

Mine was the Wires going to the tone controls of all things, they were a bit more spread out than usual, and seemed to be coupling themselves either to my FET board or the input jack. I moved them under the Channel Switching Relay, and changed the snubber on the PI plates to 100pf and it's fine......Actually it sounds REALLY good!