Modified Magna M192-5 blocking distortion

[Delta362]

I was recently given a Magnatone M192-5 amp that had previously been modified and wasn't in great shape. From reading, it seems this is similar to the Fender Model 26 Woodie Deluxe. The owner asked me to rebuild it and make some mods to it. I have attached the schematic of the modified design. It's basically replacing the original 6SJ7 first single triode preamp tube and replacing it with a 6SL7 where I switch in the 2nd stage with a footswitch (or switch on the amp if no footswitch is installed), and adding a new Hammond 1760E OT capable of 4/8/16. The paraphase phase inverter circuit was unchanged.

I had some high pitched oscillation I could hear on the first preamp tube so I added 100pF across each plate resistor to tame that. I've got it where it's working fine, up until I have both the volume and gain knobs above 3 o'clock. Then I encounter some nasty blocking distortion. From reading about blocking distortion on a similar amp (https://robrobinette.com/Fender_Deluxe_Models.htm), it seems it's inherent to this.

To address this, I temporarily added 5k6 resistors on the 6V6 grids, but that didn't help so I removed them. My coupling caps to the power tubes are now 0.01uF instead of 0.1uF. Still not solved. I'm wondering what else I should be looking at? Do I need to add a grid stopper or grid leak to the phase inverter input pin 4? If so, recommended size?

I just looked at my voltages and perhaps my A and B+ voltages are too high for this 15W amp when looking at the original schematic.

Any suggestions on next bit of troubleshooting or recommended changes? Thanks!

[mozz]

Make sure it's not the 6sj7, grid leak bias you are asking for trouble.

Edit, did not see the modded schematic. I have seen 6sl7, 6sn7 be microphonic.

[Delta362]

Just realized I missed the 1M on the input jack to ground.

[Delta362]

Make sure it's not the 6sj7, grid leak bias you are asking for trouble.

Are you meaning on V1 pin 1 when I add the gain stage in? Perhaps I need to add a 1M grid leak there?

[Helmholtz]

PI input grid has no grid leak resistor.

[Delta362]

PI input grid has no grid leak resistor.

Would you recommend trying a 1M on the phase inverter V2 pin 4 to ground? Or something smaller?

[Delta362]

Would removing the 0.01 cap to V2 pin 4 be a viable solution? Seems I already have blocking caps in the tone stack for starters. Then if the 2nd stage is switched in, there's a 0.022 cap before the gain knob. It seems the 0.01 cap would then not be needed. Is my thinking incorrect here?

[Helmholtz]

Would you recommend trying a 1M on the phase inverter V2 pin 4 to ground? Or something smaller?

Original uses 220k, but 1M should also work.

Also wondering why the paraphase circuit was changed to non-feedback type. (http://valvewizard.co.uk/paraphase.html).

V1 pin1 should also get a 1M grid leak.

Not quite clear how the relais is supposed to work.

[Delta362]

I don't see the original schematic as having feedback? I based the phase inverter circuit and output section off of the one from this page https://sluckeyamps.com/pila/pila.htm


I'll add the 1M grid leak to ground for V1-1 and a 220k grid leak to ground for V2-4. Thank you! Is the 0.01uF to V2-4 still needed?

[Delta362]

The relay is either enabled by pulling the gain knob or by having the gain knob pulled and using a footswitch. The footswitch jack has a switched tip and switched sleeve.

When the relay is de-energized, the signal passes from the Volume pot thru 1NO of the relay, then to 2NC via the jumper on the relay, then out 2C to the PI.

When the relay is energized, the signal passes from the volume pot thru 1NO, out 1C to V1-1 grid input picking up the 2nd triode. The output of that triode goes thru the cap and gain pot to 2NO, then passes thru 2C onto the PI.

[Helmholtz]

I don't see the original schematic as having feedback?

Original circuit uses local NFB around the lower paraphase triode (plate to grid) for improved (self-) balance.
Check the valvewizard link.

[Delta362]

I redrew the circuit to make it clearer, made a few circuit changes and tweaks, and added voltages:
Added 1M grid leak to V1-1
Added 220k grid leak to V2-4
Removed the 100pF across V1 plate resistors
Added 330pF across V1 plate
Changed power tube coupling caps back to 0.1uF
Changed power supply first stage resistor to 4k7 to drop screens

Now I can dime all the pots, channel switching works well, bright switch is usable, no high frequency oscillation and no more blocking distortion. Thank you for all your help!

[Helmholtz]

The added 100k resistor in the paraphase should connect to the grid pin 5 and not to ground.
Otherwise it can't have a positive effect.

BTW, how did you know you had blocking distortion?

[Delta362]

I did not change the 100k resistor from my schematic in post 1 vs post #12. I just redrew it. This is one of the images from the valve wizard page you linked. I just marked it up to show my values. The pot can be considered 100k on top section and 4k7 on the lower part of it.


The blocking distortion was when I was attempting to play thru it. If everything was dimed (or even nearly dimed), my sound would cut in and out and sound like farting. But backing off the pots a bit cleared it up.

Hope that helps clarify.

[Helmholtz]

Oh I see, the lower 100k is just used as grid leak for V4.
I thought you wanted the self-balancing as with the original circuit.