Schematic sanity check for a 9W Plexish w/Reverb

[Lectroid]

This is a design from tubenit for a 9W version of an 18W Marshall circuit with a one-tube reverb bolted on.  He inserted a MOSFET cathode follower, then I added a pre-PI master volume.  I've started a first-pass layout so I'd love to hear any comments or suggestions. Thanks!

[Removed bad schematic, see below]

[pdf64]

The reverb control is arranged to short out the reverb signal.
I can’t make head or tail of the 6bm8, the pin numbers don’t match up with those of the type, eg the pentode section doesn’t use pin 9, and the triode sections don’t seem to be used?
https://frank.pocnet.net/sheets/164/6/6BM8.pdf

[sluckey]

The pentode pin assignments match an EL84. Maybe the schematic was prepared in haste?   

[Lectroid]

Guilty.  I copied the power amp section and forgot to change the pinouts.  I've corrected the schematic.

@pdf, yes the triode sections aren't used.  I can't judge whether they might have been useful in the circuit so they're just along for the ride.

As to the reverb section, what is the issue you see?  I had questions about the reverb circuit myself (see circled area) but I believe I copied tubenit's schematic correctly.  Link to that schematic:

https://el34world.com/Forum/index.php?action=dlattach;topic=30283.0;attach=105814;image

I also added the footswitch option from my DR, but if there's an issue with it I'm happy leaving it off.  I don't use a footswitch. 

Thanks for looking.

[sluckey]

Footswitch should connect to the center pin of the return jack, not the shell.

You must have a cap between the MV wiper and the grid of the LPT PI to prevent the MV from messing with the bias on the grid. There will be more than a few volts on that grid.

I suggest using the triodes in the 6BM8 for the LTP PI. Here's one proven example...

     https://el34world.com/Forum/index.php?topic=27966.0

[Lectroid]

Thanks for the alternative, sluckey.  It looks pretty straightforward.  I've posted a v2 schematic below incorporating those changes and fixing prior mistakes. 

Can I assume the current component values will work for the 6BM8 triode sections?  If not, would you have any suggestions as to what values might need to be changed?

Why is the grid leak resistor on the reverb recovery stage bypassed?  It seems like AC should be there by design.  Won't it be shunted to ground?

[sluckey]

Can I assume the current component values will work for the 6BM8 triode sections?  If not, would you have any suggestions as to what values might need to be changed?

The schematic on the link I posted has been built by tubenit. Component values have been proven.

[pdf64]

Just look at the reverb intensity control; how can any reverb signal make it past there?

[tubenit]

There is no ground wire below the 330k resistor off the wiper of the reverb pot.


I think the PPIMV is a better choice based on my experience for this amp design but that is simply a preference choice on my part.


Having said that, IF you stick with a more tradition pre-phase invertor master volume.  I would have the insertion point for the reverb be after the pre-phase invertor master volume and prior to the .01 cap going into the LTPI.


I don't know about the values you have selected for the NFB?  I've not used those before.


With respect, Tubenit

[Lectroid]

@sluckey, 
Thank you for clarifying that.

@tubenit,
Thanks for explaining pdf64's objection.  I hadn't  seen that.  My DR does have a path to ground there, guess I assumed it was a standard thing.  I will take it off.  Ditto the NFB, you had said it probably isn't necessary here.  I'll give some thought to your advice re: the PPIMV. 

I should learn to stop tinkering with designs until after I build them. 

IF you stick with a more tradition pre-phase invertor master volume.  I would have the insertion point for the reverb be after the pre-phase invertor master volume and prior to the .01 cap going into the LTPI.

With that change, would I still need the 150K resistor?  Also, why is the reverb recovery stage grid-leak resistor bypassed on your 9W Plexi?

[tubenit]

I looked to see if I had a file somewhere with this type of idea, and I did.  This is how I would approach it.


The bypass cap on the 220k resistor to ground on the reverb recovery is simply a preference thing for me to lessen some high frequency. I also have built amps without that.  It's not necessary.


With respect, Tubenit