Hot Cat 30 out of higher voltage EL34 amp

[Colossal]

So with the schematic I marked up above the PPIMV won't shut the volume all the way off. You still have the lower Rk forming a divider with the pot element, putting the pot wiper a few hundred ohms from ground. It won't be loud, but to cure that you'd need to put caps on both sides of the MV pots.

Thanks for the review, Martin. I will update that schematic to include the caps on the wiper and CCW lugs.

[martin manning]

So with the schematic I marked up above the PPIMV won't shut the volume all the way off. You still have the lower Rk forming a divider with the pot element, putting the pot wiper a few hundred ohms from ground. It won't be loud, but to cure that you'd need to put caps on both sides of the MV pots.

I will update that schematic to include the caps on the wiper and CCW lugs.

And put the garter resistors back in after the cap on the wipers.

[Colossal]

So with the schematic I marked up above the PPIMV won't shut the volume all the way off. You still have the lower Rk forming a divider with the pot element, putting the pot wiper a few hundred ohms from ground. It won't be loud, but to cure that you'd need to put caps on both sides of the MV pots.

I will update that schematic to include the caps on the wiper and CCW lugs.

And put the garter resistors back in after the cap on the wipers.

Revised schematic showing modifications:

Garter Bias.png

EDIT: Added suggested revisions from Helmholtz and Manning

[Helmholtz]

The garter cross-coupling works by positive grid voltage.
In the schematic the grids have no DC reference at all, so can't work.

[Colossal]

The garter cross-coupling works by positive grid bias.
In the schematic the grids have no DC reference at all, so can't work.

So, C6, C8, C9, and C11 would need to be removed then. This would put the grids at 24VDC (the potential at the divider).

The Hot Cat 30 circuit is very simple, just one gain stage into a PI (much like an 18W Marshall or Brian May AC30) so a PPIMV while nice to have, would not be strictly necessary, but would be useful as it would allow the preamp to be pushed hard. With no negative feedback, it's going to be loud too.

[martin manning]

Like this.

[Colossal]

Ah, I see now. The voltage reference is fed back to that added grid leak downstream of the PPIMV. I was looking at the PPIMV as replacing that grid leak.

[Helmholtz]

With cathode bias the grid circuit resistance of an EL34 can be up to 700k.
To not unnecessarily load the PI I would either use 680k grid leaks or use a 470k pot with 470k grid leaks.

[Colossal]

To not unnecessarily load the PI I would either use 680k grid leaks or use a 470k pot with 470k grid leaks.

This is exactly what I was wondering, since with the PPIMV and garter bias, we now have a C-R, C-R in series with the grid which, at 33n/220k, 33n/220k would be load the PI more. Thanks, Helmholtz.

I updated schematic above with these changes for the record.

[bcmatt]

This is an interesting discussion on the PPIMV for this circuit. I have been thinking about it as well since it might be nice to scale only my power tubes with the VVR and that would require one.
Right now it is easy to dial in great tones for either channel at any volume, but if I want the two channels to match in volume for footswitching, I have to have the voltage at almost half or higher, or else the gain channel is too starved to work very well and both the Master and Level controls have to be cranked to match the clean channel volume. I guess a PPIMV would also allow me to just keep the Voltage at half or higher on it's own.

Anyways, this is my current schematic I'm working with. I am still waiting for my 360r cathode resistors to arrive, and then I can measure all my voltages again.

Garter Hot Cat.png

[Helmholtz]

That "Brilliant" control is essentially the same as the Vox "Cut" control.
All it can do is lower treble response.
As I didn't need it with my AC30 I converted it to a quick and dirty cross-line PPIMV (around 1970) by increasing the cap value to 0.47µF.

[Colossal]

That "Brilliant" control is essentially the same as the Vox "Cut" control.
All it can do is lower treble response.
As I didn't need it with my AC30 I converted it to a quick and dirty cross-line PPIMV (around 1970) by increasing the cap value to 0.47µF.

I have not found the Cross-Line Master to be as effective as the Post-Phase Inverter Master. The Cross-Line seems to have a very abrupt transition. With your AC30, is it a Top Boost with Bass and Treble controls and so you don't feel the need to cut any high end on the way to the power amp?

[Helmholtz]

I have not found the Cross-Line Master to be as effective as the Post-Phase Inverter Master. The Cross-Line seems to have a very abrupt transition. With your AC30, is it a Top Boost with Bass and Treble controls and so you don't feel the need to cut any high end on the way to the power amp?

Well, it is a post PI MV working by loading down the PI output. Certainly not perfect (that's why I call it quick and dirty), but might be improved by a different pot value and taper.
Amp was a Top Boost AC30 bought new in 1970 (trading in my AC50, which I didn't like), which could use every bit of treble. Later I found out that its silver alnico speakers were rather dark sounding, though incredibly efficient making this amp louder than a 100W Marshall.

That "Cut" control is essentially the opposite of a presence control.

[bcmatt]

Ya, it is funny that Bad Cat didn't call it a cut control.... Maybe because it's wired in the more intuitive way where it gets brighter as you go clockwise. I find that this amp definately needs it... I'm always running it noticeably below noon.

[bcmatt]

Hi Matt,

Ok, thanks, that is helpful! If I use the Songwriter bass circuit, I would probably choose 1n for the base value and 47n in series with a 500kA pot. I found that with a 3M-RA pot, most of the useful travel and bass was from 12pm to 3pm on the pot anyway. Yes, that bass control and the 330k/220p treble peaker that follows in the Songrwriter circuit does soak some gain. A 6-position rotary switch might be a good choice for bass as well in this amp.

I also added a similar bass control to a single channel 5E3 once and it was useful. I used a 100k pot with 47k in parallel between the second and third lugs for a custom taper. It worked well and you could hear the sweep in the bass. I used a 22n cap in that amp and it was full but not wooly or bloated sounding.

Thanks for this Colossal.
So for your songwriter bass circuit you would use this same layout, but swap the pot for a 500kA, the 500pf for 1nF, and the .1uF for a 47nF?

Bass Control.png

I just took my songwriter build back from my friend to double check the bias due to some paranoia. I may be interested to try this as well.