Volume drop at 10 (DIY clone)

[BlueX]

Did another quick measurement, and there is oscillation after C3, but lower than after C8. Looking at the schematics again, I can see now how this can happen when HT volume is at max with no resistance. Lifting one leg on C6 should cut the "free flow" of high frequency.

Next session has to wait until the weekend (cliff hanger).

 

[BlueX]

Did some more testing and measurements. Put a steel metal sheet, covered with insulating PVC tape, between V1 and selector switch for Lead/Bass tone stack (TS), see last pic. This eliminated the oscillation when nothing is plugged into any of the input jacks.

However, when I plug in a guitar there is still some oscillation, but with smaller amplitude, with HT channel volume maxed. Just before oscillation starts there is a tremolo effect.

The tone stack wiring, and switch, is very close to V1, much closer than other builds I've seen.

Right now thinking about two options:
1. Make a metal cage/house for the TS switch, grounded to chassis
2. Remove the switch and move TS resistor and cap as close as possible to the pre-amp filter cap.

Any comments?

 

[Pete Farrington]

Get that V1 pin2 grid wire away from any of that CF / tonestack stuff. That's the most sensitive / vulnerable wire in the amp. Extend the screen plate across to the board, to prevent V2 stuff coupling with V1 stuff as much as possible.
Also a screen plate between input sockets and the board / TS wiring.

 

[BlueX]

Get that V1 pin2 grid wire away from any of that CF / tonestack stuff. That's the most sensitive / vulnerable wire in the amp. Extend the screen plate across to the board, to prevent V2 stuff coupling with V1 stuff as much as possible.
Also a screen plate between input sockets and the board / TS wiring.

Thanks! CF?

Replaced (temporarily) the green V1p2 wire with a shielded one, shield covered with green shrink sleeve and grounded at V1 socket. Tested with the steel shield between V1 and TS switch, and between inputs and board (see pic).

Still the same: No oscillation with nothing plugged in, but some oscillation with guitar plugged in and still the tremeolo effect before oscillation visible on scope screen. Power consumption into amp about the same as in post #10 (both with and without oscillation).

Might be difficult to screen this away. Any other comments?

Bonus question: Connected the oscilloscope across speaker outs. Volume at max, without anything plugged in but with attenuator and speaker as output load, there is an irregular wave form on the scope screen, less than 1 V (about same for each channel volume). Any concern?

 

[Pete Farrington]

CF = cathode follower, the V2 triode that drives the tonestack, I think it's pins 6-8.

My guess is that 2 screening plates may be necessary here, between the i puts and the board, and around V1 wiring.

Can you get more separation between the V3 grid wires and the NFB wires?

 

[BlueX]

CF = cathode follower, the V2 triode that drives the tonestack, I think it's pins 6-8.

My guess is that 2 screening plates may be necessary here, between the i puts and the board, and around V1 wiring.

Can you get more separation between the V3 grid wires and the NFB wires?

Thanks, again!

Yes, I will try to arrange more screens and separations. Now it's quite easy to experiment.

I built this amp also as a test-bench, to experiment and learn. This process is interesting, as long as it's not too expensive (or dangerous).

 

[playloud]

Some good troubleshooting here.

Have you considered moving the boards a bit to the right?

Here's a pic of mine for reference (not relative positions of input jacks, V1/2 and output sockets):



I would also try removing the tonestack switch entirely for the purpose of debugging.

 

[BlueX]

Up-date (it's been summer and nice weather, so time spent on other things).

By-passed the NFB switches with a green "air-borne" jumper (from 27k, 16 Ohm). Together with the shields it takes away the oscillation, also with a guitar plugged in when chassis mounted on metal sheet.

Problems are almost gone, but amp still sounds harsh with High Treble volume all the way up. I will bypass the tone stack switching also, to see what effect that has.

Amp's mainly working as it should, but switches for tone stack and NFB should have been implemented differently (if at all). As already mentioned, I build amps also to learn. Reading about oscillation is one thing, but at least for me it's more educational to experience first hand.

Thanks for the input!

 

[Marcomel79]

hope you get it sorted soon