Converting 6V6 Plexi to an Emplexador 6V6 success and probs

[chopstuck]

I had a 6V6 Plexi I wasn't too happy with - my own fault I realized later -
though changing the bass pot ground helped a lot.

I removed the normal channel preamp and switched the brilliant channel to the single channel Emplexador values.
This just changed the whole character of the amp for the better.

Brians designs just blow me away. I like to tweak them to work with the stuff I have on hand but the designs sound so good as drawn.

When I try the vintage mode everything sounds great. I can't even hear the blow with the volume down.

When I go into into boost cap mode I just get feedback with the amp on 1.
In modern mode I get squeal at settings past 12:00.
Now my amp is laid out like a classic plexi and I'm thinking I need to run some shielded cable to cope with the increased gain.

Which are the most sensitive lines to feed back ?

I have just a little extra wire length at the mode switch but not much.

I was thinking of the input to the V1 grid, the tonestack to PI input and the V2 grid feed.

Lastly there is a switch on the schematicheaven Emplexador circuit that puts a 100K shunt resistor next to the MV on the PI input.

Is that the boost switch or is it supposed to function with the mode switch so it is always active in "modern mode " ?

[thejaf]

Did you move the 68K grid stoppers from the board to tube pins on V1?

[tubeswell]

Which are the most sensitive lines to feed back?

Typically the signal wires where the signal strength is weakest being too close to where the signal is strongest.

A thought that occurs to me is that it could be unwanted EMI coupling between xformer and signal wire? In the OT, the phase lag between Pr and core, and between core and Sec is different at different frequencies. This could possibly make the HF prone to +ve feedback if the OT was coupling somehow with signal wires at some point (esp. when you kick in a mid-HF boost?).

So maybe shielding and signal wire shortening as you say, and maybe also NFB pole slugging (as per Merlin Blencowe's 1st book)? Who knows. Needs a bit of experimenting. JM2CW

[chopstuck]

I did a wire cleanup and put the grid resistor on the V1a socket pin. I used shielded cable on the the second stage and the boost stage grids from the panel switch.
I now have a very quiet amp. I can run full volume on the pre without squeal from the cap boost switch.

All in all I think it's the best sounding amp I have built.


But I think I say that every time .

I used the Marks 6v6 Plexi power amp married to the Emplexador preamp with it's great switching ability.

The master works so well that I can get great chunk at less than clean Champ levels.

I'm just a very pleased hack.

[Reeltarded]

Neat amp!