Odd behavior regarding variable NFB, presence, and volume in a Plexi clone

[pdf64]

… if the presence knob is not cranked to maximum (or slightly below maxium), I have some pretty obnoxious 120hz buzz. Additionally, switching in the additional gainstage in front of the plexi topology leads to some strange oscillations unless the presence knob is at MAX.

How about if the NFB is disabled, loop broken, eg 27k leg lifted?

… rewired the presence pot to the modern 25k pot with 4.7k parallel resistor). Disconnecting the presence pot ground from the circuit and attempting to ground it at different spots has lead to changes in the buzzing tone wise, but never gets rid of it.

Is the 4k7 NFB shunt resistor grounded to the same point as the presence control?

Disconnecting the ground COMPLETLY however still allows the buzzing to stop if the presence knob is at maximum again,

Disconnect the ground where exactly, eg is the LTP still passing DC, do its anode voltages change?

Sketches of the schematic and layout, and build photos, would be very helpful, may save pages of to and fro.

[jarrodthebobo]

Everything from v2 on is just a straight up superlead; only difference being a switchable cathode bypass cap over the 820ohm cathode resistor and the now changed presence circuit (more like the jcm800).

When I'm disconnecting the ground on the presence pot, I'm also disconnecting the 4.7k resistor as well technically since they share the same ground point.

I'll be messing around with the amp more today when I get the time; I work 12 hour shifts on 3rd shift so my playtime with this toy is at odd times of momentary freedom whenever I get my daily nap in

[cdemike]

Do you have pictures of the guts? I suspect something is still going on with the lead dress at the input stage, especially with that JCM800 high gain section. From my perspective, changes in bad behavior depending where the presence pot gets grounded and the fact that things are worse in the higher gain input both point to lead dress issues. Also, what grounding approach did you land on? When you tried the star ground, was it a strict star ground or a modified star system? Keeping gain stage grounds discrete especially early on and with respect to using low-noise grounds for shielding, would make a difference I predict.

Are you using shielded cable for the Lar/Mar leads as well? If so, where are they grounded?

[jarrodthebobo]

PROBLEM SOLVED!!!

The phase inverter had a very loose tab on the tube socket! I had noticed when I was chopsticking around, every now in then if I bent one of the output wires on the phase Inverter the oscillations would subside ONLY if I kept bending the wire pretty far... however relieving pressure on the wire would cause the oscillations to begin again.

Going to the tube pocket itself, the tab that wire terminated at looked like it was nearly pulled out of the socket... so using some pliers I just pushed it back in for a temp solution until I get a new socket (or hot glue everything in place ).

Now I can goose the amp to dumb levels of gain ( 820ohm/.68uf into 820ohm/.68uf into 820/.68uf, all cascaded) with both gain pots maxed! No weird noises, oscillations, or hum! Just some hiss.

Thanks for all the help guys!

[Colossal]

( 820ohm/.68uf into 820ohm/.68uf into 820/.68uf, all cascaded)

This is a great combination, by the way.

[bepone]

i was sure that is some mistake