Marshall plexi built treble issues

[pureoldsound]

Hey all,

I built a Marshall plexi style amp 50w about 1yr or so ago. After the built process , for some reason I was getting DC voltage into my tone stack so I added a 500pf cap (tone cap), however, it seems that this value was too small as it cut all my lows and mids. So I added a 0.47uf and all my frequencies came back. That being said, I am getting too much highs, to the point that if I max the treble (10) I get this crashing whooshing sound the volume sort of drops. That is with the treble on 10 only with the other controls at 0. If I bring the mids up and the bass the issues sorts of disappears, there might be a slight whooshing sound at the end. If I lower the treble it goes away. The amp sound fantastic, tight, articulate, with a great growl. I checked all my voltages and I am where I want them to be. Plate voltage on my El34 are about 454v. No red plating . Using current Mullards EL34 . Preamp tubes are NOS . I did switched V1 with V2 and I did notice that it helped. So I thought it was tube issues. These tubes are old. I’ve had them for about 20yrs.

I also thought it could be that tone cap causing the problem. The question is why was I getting DC on my tone stack. Marshall plexis don’t need a dc tone cap . My TS is coming from V2 cathode?

Any thoughts? BTW i am using NOS Mallory mustard caps and piher resistors. Yes I went all out with this one. It sounds great though, well worth it. At least for me.

Thanks

[Reeltarded]

DC on your controls still?

Uhh I want to see what you think is the schematic you used and a picture of your amp inside around the DC BLOCKING TONE CAPS.

Rge TONE CAPS block DC. You have made a critical error.

Parasitic oscillation. Don't turn that control there. Everytime the volume dumps.. your output is being HAMMERED hard enough to wear the tubes a year per minute.

[pureoldsound]

Thanks for the reply.

No I am not getting DC anymore after the tone cap was installed.

I used a mix of schematics , but mainly a 1968 50w schematic.

Thought about oscillation. But it is stranged that the problem goes away after I add the mid and bass. I will chop stick V2 and see if it helps. I’ll try to post a clip pf the problem.

[sluckey]

No I am not getting DC anymore after the tone cap was installed.

You don't need a cap leading into the tonestack. That's just a Band-Aid fix. One (or more) of the three tonestack caps are leaky.

There is one more way for dcv to get on the tone controls. That's through a leaky cap for the PI input. But since your band aid fixed the dc problem, I would not suspect the PI cap.

[Reeltarded]

He sent a pic. His work looks good but the layout and dress seem to be all the issues. Be back later.

Post pics.

[pureoldsound]

No I am not getting DC anymore after the tone cap was installed.

You don't need a cap leading into the tonestack. That's just a Band-Aid fix. One (or more) of the three tonestack caps are leaky.

Ahhhh I was afraid of that with these old cap. I thing I may be able to narrow it done. I think is the on the bass pot.

[pureoldsound]

This is the amp

[sluckey]

That's a whole lot different from my Plexi! I don't know if what I said applies to your amp or not. Can you provide an as-built schematic?

[pureoldsound]

That's a whole lot different from my Plexi! I don't know if what I said applies to your amp or not. Can you provide an as-built schematic?

It has a few bells and whistles but it is basically a Plexi.

[Reeltarded]

It looks excellent but I bring terrible news. You re never going to get away with all the unshielded long runs of brights and I guess whatever else is on those multi-position switches. That is a bummer. You should go look in Trainwreck files at Liverpool and Express layouts. See those tiny wires from bright switches to volume? Still hardly short enough. What you have is basically Arecibo. You can hear the inception of the Universe from your amp!

[pureoldsound]

What you have is basically Arecibo. You can hear the inception of the Universe from your amp!

Hehehehe, well, if the problem is shielded wiring I can work on that. I use to built a model that had TW express pre-amp and I remembering using shielded wiring. Thought this amp will not have that issue. I am not getting any interference or radio station waves though .

I’ll work on it and see it shielded wiring solves the issue.

[pureoldsound]

Just an update. I made my lead dress shorter going from volume pots to board. Still the same issue. Took My hook up wire off installed shielded wire on volume pots, switch and cap routing. Still the same problem.

I have an internal jumper of channels, disconnected those wires. Still the same issue.

I’ve read that adding a snubbber cap of 7pf 1000v between plate and grid helped with oscillation. How ever I am not sure that should be the way to fix the problem.

The problem is only in the treble pot. Is like if the treble goes beyond the audio frequencies when is maxed out by itself . Is not a squeal, or noisy. Is the 1st time that I’ve seen this happened. I agree that it must be oscillation, the question is where?

[pureoldsound]

Alright, so I tried the snubber approach with a 50pf cap I have here (I don’t have a 7pf). It looks like everything boils down to V2 pin 1 and 2. Tried the snubber in all other pins (V1 and v2) and it cures it in V2 pin 1 and 2. Since I have the issue on pin 6 and 7. The culprit could be that I have unshielded wire coming for the 470k mixer (from volume pot) resistor to grid. I am going to shield that and see what happens. I have no wires coming on top of my plate wire. So the issue must be coming from there.

[bepone]

where is the pic? why so much of dry text? DOS mode will not solve anything?

[pdf64]

Removing the photo has made subsequent posts something of a nonsense.