Would this work

[Bluefinger]

Hi,

I am planning to do a harp amp build and here's my plan. Maybe some of you can share your thoughts. I'd appreciate it!

I'd like to go with a simple design with few gain stages so a 5E3 based design comes to mind. For more power and punch I will probably go with 6L6 power tubes instead of 6V6 and tighter filtering and most likely fixed bias. Weber's "Proluxe" (https://taweber.powweb.com/store/5e3p_layout.jpg) comes quite close in that department. OTOH I would like to have at least a TB or maybe a TMB tone stack and I don't need the bright channel.

1) So would it work to combine the first stage and tone stack of a Princeton Reverb with the phase inverter and power section of the 5E3 like this http://www.king-bees.com/harpamp/layout1.jpg ?? Replacing the 6.8K resistor at the bass pot with a pot should turn it into a mid control.

2) If teh above is not complete nonsense, is it ok to go directly from the coupling cap to pin 2 of the PI or would I need to drop the signal with a resistor? If yes, how much?

3) Usually 12AY7s or 12AU7s work fine in the first gain stage but would it also be ok since the 2nd half of it is used to recover the signal of teh tone stack? Maybe I could use a 12DW7 but I'd have to swap the wiring of the two pentodes since pins 6, 7 and 8 are for the lower gain section.

Do you think this all could work or is there a mistake in my thinking?

thanks a lot!!!!

Herbert

[pdf64]

The grid of V2 pin2 would need a resistor to ground, to act as a grid leak. You may need to drop some gain so it would be sensible to fit a 1M preset pot as a vol/gain control there.
If using 12AX7 in V1 and V2, you may be able to miss out a gain stage.

[Bluefinger]

Thanks for the response!

Please excuse my ignorance but although I try to catch up, I am still lacking a lot of theoretical knowledge. So the obvious dumb question is: How do I calculate or find out the appropriate value for that grid leak resistor?

That pot as a signal dropper makes sense. Once I have found a good value I can replace it with a resistor again ...

I guess omitting a gain stage would mean to run the (in this case purple) wire coming from the Volume pot directly to pin 2 of the PI and not use pins 6 through 8 of V1, right? Would I still need that grid leak resistor then? The original 5E3 layout does not have one ...

edit: wait a minute ... pin 2 of the 5E3 is grounded through the two volume pots in parallell so there IS an equivalent of a 500K grid leak resistor ...

thanks!
Herbert

[pdf64]

You've got it!
Use a 1M resistor / pot as a grid leak to ensure that the grid gets referenced to ground.

'I guess omitting a gain stage would mean to run the (in this case purple) wire coming from the Volume pot directly to pin 2 of the PI and not use pins 6 through 8 of V1, right?'

If you mean the existing Volume pot, then that would cut out 2 gain stages (2nd section of V1 pins 6-8 and 1st section of V2 pins 1-3). You wouldn't have enough gain in the system then. What I meant was to take the 0.022cap off V1 pin6 to the PI input. Pete.

[Bluefinger]

ok, great ... makes sense now.

'If you mean the existing Volume pot, then that would cut out 2 gain stages (2nd section of V1 pins 6-8 and 1st section of V2 pins 1-3). You wouldn't have enough gain in the system then. What I meant was to take the 0.022cap off V1 pin6 to the PI input. Pete. '

thanks for pointing that out! I'll give it a try ...

thanks a lot!

Herbert