EF86 vibroverb

[valvetones]

Hello friends. So I’ve got the board for the next build laid out almost completely. I had considered that my main issue with the last VV build was that the normal channel is a waste of parts. I never use it. And you can’t jumper channels because it’s out of phase.
So the solution in my mind to put the channels in phase would be to use an EF86. Since it’s only one gain stage, it would now be in phase with the vibrato channel, and thus the normal channel would now have purpose, and actually, incredible potential I would assume.
This will be the 6G16 brownface circuit for everything, except the normal channel of course.
Now I have a few questions
1. I have real-estate to fill up, a volume, a bass and a treble knob that need to function as such. I considered just keeping the tone stack as it was in the 6G16 and driving it with the ef86 but there will be no recovery gainstage so I wonder if that would suck. It would be twice as much going into it, so I kinda feel like it may balance out. If I did this, what values if any should I change in the tone stack to allow this?
2. After those changes have been implemented, what would an appropriate value be for the mix resistor going into the .0005 cap, then into the phase inverter?
In the normal set up, it’s 220k.
3. The vox ac4 set up uses a 5.6k screen resistor, I’ve seen people changing it to 1 Meg. What effect does this have?
I want to get as much gain from this as possible and keep the tone stack.
I think the idea here will be to use much smaller caps in the tone stack for the ef86. I would want it to be mid and treble focused, that way when channels are jumpered, your essentially using the normal channel to mix in a better overdrive sound.
Fenders tend to break up in a cool, yet distinct bass heavy way. The idea here would be to have the option to dial that bass heaviness out by jumpering the channels and  dialing in the tone.

I feel this set up could be glorious, 100% fendery when only plugged into the vibrato, and have the option to go to other places by jumpering the EF86 side.

I really need help with this, I assume just building the vox stage/keeping the tone stack, may not be the best thought without tailoring the idea and making changes where needed.

Ideas please , thank you all.
I’ll post pics and videos when it’s done.

[tubenit]

Some thoughts ..............


My understanding is EF86 tubes are prone to be microphonic?  I enjoy using a 5879 pentode in two of my amps and think they sound great.  My understanding is that traditional tone stacks like a Fender or Marshall don't pair well right after a pentode, but that using a cathode follower then one can use more traditional tone stacks?

My experience with mosfets as a cathode follower is they don't sound any different to my ears than a tube triode in that position.

Mosfet cathode follower (el34world.com)

If I were doing this, I'd want the reverb to be on both channels, so I drew the schematic with the insertion point prior to the LTPI and after the preamp gain stages.  This is how I always insert reverb and have been happy with it.

One option is the 5879 or an EF86 into the LTPI with whatever tone stack you want?  To balance the volume between the channels, you might want to consider a 5E3 tone stack after a pentode as it has less volume loss.

Another option is the psuedo Marshall topology using a mosfet CF also. 

Regarding how I drew the AB763 preamp channel,  I'd be prepared to experiment with dropping resistors to manage the degree of gain you want.  You could consider using a trim pot on board the layout board to do that.

Just ideas to consider.  I am NOT saying you should use any of them.

With respect, Tubenit

[thetragichero]

i have used 12au6 (6au6  or the more ruggedized versions for 6.3v heaters) to good effect and they're dirt cheap

[valvetones]

Thanks for that Tubenit.

So yes I understand it is an impedance issue with the EF86 driving the tone stack.

As for the microphonics, I’m not scared. I’ve got nice 9 pin sockets with runner mounts.
It’s going on a head cabinet not a combo.
And these are no ordinary EF86
I’ve got 10 Genalex Z729 tubes.

These were constructed under very strict standards and selected for low noise.

So with all the above I just have to try the them. Now is the time.

The cathode follower for a lower impedance source!!! Genius!!

That would require an extra socket and but is not out of the question.

I wonder if I could use a 6C4 for that.

I’m pondering.

The other idea I had was to use the normal ac4 vox tone control in the treble knob position and then for the bass control, try to figure out how to taper between different cathode bypass caps with a potentiometer.

I’m scratching my head though..

Thinking extensively on it.

As for the mosfet, I’ll need to read up on that, I’ve not ever known what that was exactly, though I’ve heard it said before a few times.

Thank you all.

[bmccowan]

My understanding is that traditional tone stacks like a Fender or Marshall don't pair well right after a pentode,

My experience on that is the same. I tried doing exactly the same thing with a blackface Concert chassis I have. Channel 2 I left stock (well with a Sluckey Termonator mod, which works great) and channel 1 I did the pentode thing with a 5879. I tried dozens of combinations of caps and resistors and tried an EF86 before I gave up. What did work was bypassing the tone stack or using 1/2 of the trem tube for an additional gain stage. Of and on I experiment with that circuit. What you find out may inspire me to pop that open again.

[sluckey]

You may get some ideas from Smoky. Schematic at bottom of page. The 5654 is a rugged military tube. Microphonics is not a problem. It has a pot on the cathode and a simple low loss tonestack. Probably need a CF or SF to drive a TMB tonestack.

[thetragichero]

The other idea I had was to use the normal ac4 vox tone control in the treble knob position and then for the bass control, try to figure out how to taper between different cathode bypass caps with a potentiometer.

i couldn't get the cathode-bypass cap on a pot to work right (regardless of pot value or taper, it was basically an on/off switch) on my single 12au6/6au6 into long tail pair amp but what i did end up doing was a three-way rotary switch with 330nf 1uf and 25uf as my 'bass control' and that seemed to work nicely ('treble control' on that amp is just a simple vox cut control)

[tubenit]

The cathode follower for a lower impedance source!!! Genius!!  That would require an extra socket and but is not out of the question.

Using a mosfet, you would not need another tube socket. 

Sluckey is correct, the 5654 is a great tube and tone wise is similar to the 5879 pentode.  I understand you're wanting to use the EF86's you have on hand.  That may be a good choice for you? 

with respect, Tubenit

[valvetones]

You guys ROCK. I really appreciate having a community of builders and doers that will actually consider my thoughts, folks to bounce ideas off of.
We’re in it together.
The real world trial and error experience of the community of builders on this site is an abundant resource that I feel blessed to have.

Let me study up on mosfets, and pentodes with cathode followers, and I’ll get back.

So , this treble knob, bass knob, it’s labeled as such on the faceplate, and it goes 1 through 10. I like the idea of the rotary switch, but can’t do it on this build, due to the face plate being what it is, and my stubbornness of not wanting a bass control that says 1-10 but only clicks three times.
Kind dumb I know. It’s gotta be a pot though. I had considered the cut control for the bass knob, but it’s just such a long run of wire from the normal input all the way over to the other side of the chassis. I may do that. But not before exploring every option I can think of.
There’s this other idea I have.
So the vox tone control is a 1 Meg pot. It’s a .047 off the EF86 plate and at the junction of the .047 cap is the volume control and the .001 cap that is going to the tone control. If I’m thinking about this right, all frequencies that get passed by the .001 cap get shunted to ground when the tone pot is in its darkest sounding / bassy position. Turned down all the way.
So, could I take two 500k pots, wire them is series, but only ground the bass pot. Then have the .001 cap go to the bass control, and a 500pf cap going to the treble control.

There would still be 1 Meg resistance to ground, so the impedance shouldnt change , and I’d simply be putting a finer sweep on the frequencies using two pots.

Am I thinking about this right?

The junction of the .047 cap, volume pot, .001 to bass, 250pf to treble,
500k treble and bass pots wired in series, only bass pot grounded.

I get that the treble will only really be at max if the bass control is also at max, but it would kinda do what I eat right? Wrong?

Whatcha think?

[valvetones]

I’m not gonna try to eat it!
I swear it was a typo. 

[valvetones]

The cathode follower for a lower impedance source!!! Genius!!  That would require an extra socket and but is not out of the question.

Thank you my friend, I’m considering this as an option.
Using a mosfet, you would not need another tube socket. 

Sluckey is correct, the 5654 is a great tube and tone wise is similar to the 5879 pentode.  I understand you're wanting to use the EF86's you have on hand.  That may be a good choice for you? 

with respect, Tubenit

[valvetones]

The other idea I had was to use the normal ac4 vox tone control in the treble knob position and then for the bass control, try to figure out how to taper between different cathode bypass caps with a potentiometer.

i couldn't get the cathode-bypass cap on a pot to work right (regardless of pot value or taper, it was basically an on/off switch) on my single 12au6/6au6 into long tail pair amp but what i did end up doing was a three-way rotary switch with 330nf 1uf and 25uf as my 'bass control' and that seemed to work nicely ('treble control' on that amp is just a simple vox cut control)

Valuable ground covered. Even though it didn’t work out, it’s helpful because now I know to get that out of my head.

Thank you.

[valvetones]

You may get some ideas from Smoky. Schematic at bottom of page. The 5654 is a rugged military tube. Microphonics is not a problem. It has a pot on the cathode and a simple low loss tonestack. Probably need a CF or SF to drive a TMB tonestack.

Thank you Slucky, I’ll take a look.

[valvetones]

The cathode follower for a lower impedance source!!! Genius!!  That would require an extra socket and but is not out of the question.

Using a mosfet, you would not need another tube socket. 

Sluckey is correct, the 5654 is a great tube and tone wise is similar to the 5879 pentode.  I understand you're wanting to use the EF86's you have on hand.  That may be a good choice for you? 

with respect, Tubenit

Thank you Tubenit.
I’ll be considering this option. Certainly beats drilling another whole.
Also even though a single triode may only draw 150ma
I think I’m at maximum budget on my 6.3v winding.
So it really would be advantageous to not have to use another tube.

Definitely considering it.