Top Boost Single Ended

[leadfootdriver]

I'm sure this has been done a bunch, but this one is mine.

It works pretty good, but I'm open to anything better.

Here's the basic schematic, what do you think?

[Chuck H]

Seems like a great idea to me. I might have used a grid stop resistor on the EL84 grid. But if your getting what you want then it's all good. The actual AC30 was designated a class A amp after all. Though they usually operate in AB1 when clipping. But I think the character of a single ended EL84 should suit the TB design just fine. How about a clip???

[yunger]

maybe for fun, you could experiment with a colder bias of the final triode.

[Diablo]

I like the idea of the single-ended EL-84 to the top boost Vox. To change the subject a little, some folks claim that Vox made a mistake on the bass pot. They say you should remove the ground connection on that pot to make it not interactive with the treble control. I really don't know if they're right or wrong.

[Chuck H]

Lifting the ground entirely renders the tone stack almost useless. Just wire it like the "VOX" stack in Duncan PSU.

[leadfootdriver]

THanks for the replies fellas. I would like the last gain stage to resemble the response of a long tailed pair of the AC15/30. Should I just build half of it like this? http://www.webphix.com/schematic%20h...s/ac3093pa.pdf

[Bruce / Mission Amps]

THanks for the replies fellas. I would like the last gain stage to resemble the response of a long tailed pair of the AC15/30. Should I just build half of it like this? http://www.webphix.com/schematic%20h...s/ac3093pa.pdf

A problem with this is type of circuit is that a single class A, cathode biased EL84 can be driven super hard with a very small grid signal.
It means that your preamp stages must be limited in gain or the amp will sound like a hand full of bumble bees in a paper bag.
So watch the amount of voltage gain coming from the two gain stages and it should be good...
I've built a similar circuit to this with one 12AX7 and a power mosfet for the cathode follower...no final triode stage as you added.
I was surprised to find that circuit still needed gain limit tweaking to make it fully useable.

[Diablo]

Lifting the ground entirely renders the tone stack almost useless. Just wire it like the "VOX" stack in Duncan PSU.

Thanks Chuck. I used the Duncan tone stack simulator with the standard Vox circuit and it looks very good to me.

[Chuck H]

Good call. Indeed that's too much gain for an EL84 grid. I would probably feed the tonestack from a split load off the cathode follower and then straight to the EL84. That way all your preamp mojo is still the stock circuit. You could argue that lowering the impedance driving the tonestack changes it's knoee frequencies. But as low Z as the cathode follower is anyway, and as high Z as the tonestack is, the difference will be tiny. But then...

Unless you use a SS device for the cathode follower you'll have an extra triode floating around. What to do with it??? I might use it for a trem circuit OR another input channel with a 1000p coupling cap and a little top end bleeder for a fatter OD tone. Then you could use an A/B pedal for psuedo channel switching.

[leadfootdriver]

I dumped the 3rd gain stage. I experimented with different blockers and cathode resistors, but I couldn't get it to sound open and clear like the stock design. I did change out the OD caps I was using for Mallory 150's. Their slightly blurred topend seems to compliment this sound. I didn't want to get crazy and try to reinvent the wheel. I just wanted to see if I could use the extra triode I had.

I put a 1.5k blocker on the EL84 because it would squeal with the MV all the way open. But it's cool, I got a little Vox practice amp, which is what I was going for.

[Austin]

You could use it in parallel with one of the other triodes like maybe the gain side of the dc cathode follower..

[leadfootdriver]

Yeah I tried that, but I thought that you lost some sparkle. Thanks tho.

[Austin]

How about using the unused triode as a ccs for the first gain stage? You would get more gain than just the resistor that way, but not so over the top.

[leadfootdriver]

Never tried one. Do you have an example? I can't seem to find one.

[yunger]

Yeah I tried that, but I thought that you lost some sparkle. Thanks tho.

you could parallel the first stage. if you change ra to ~100k, rk to ~820, both rg's to ~33k i doubt you'd lose any sparkle. i'm pretty sure you would drive the second stage more efficiently too.