My take on a Vox hybrid

[pompeiisneaks]

I've got an AC30/4 yes, this isn't that amp, it's a specific amp with two preamps that were never in the same amp before (EF86 normal and top boost), There was an EF86 Normal with a tremolo (AC30/4 that I built), there was a more common AC30/6 with Normal channel with a 12AX7, trem and Top boost, but not top boost and EF86 normal. I'm taking my personal two favorite channels, making them foot switchable, for one, the other or both, and adding two more improvements, I hope. An OD channel stolen from a dumble Overdrive Special, and a dual cathode setup like the lIverpool to give it the ability to easily remove two tubes in a cathode biased amp.

Make sense? Not sure it does, but that's the goal.

~Phil

[M Fowler]

Yeah the Split load bias works well or like Ceriatone does it works well too 62R + 68R+130R

[valveandsound]

I built something very similar to this, but using a 6BR7 pentode instead of the EF86.

You can get away with pseudo tone controls on the pentode channel by using different interstage coupling caps on a rotary switch a-la Matchless. That sets your "Bass" control. You can make a treble control by toggling between 2 or 3 bright cap values on the volume control on that channel. That, in my experience gives you enough control over the lows and highs without loading the stage down with a passive EQ stage.

You can add another toggle to select between triode/pentode mode on that EF86. Again lots of different tones without a whole lot of extra stuff that loads down the signal. Done this way you don't need an extra gain stage and if you use the stock pentode circuit - which is actually aped from an old classic Phono preamp stage I believe? - It can easily drive the PI.

FWIW, in my version i did pentode preamp on one channel, trainwreck rocket on the other. I was amazed that I could get the Rocket preamp to match the Pentode one in overall tonality if I adjusted the settings between each...especially with the Pentode's channel volume nearly maxed out. if you like pentode cleans, its totally worth it. But honestly I could push the Rocket preamp up high enough that it sounds pretty much dead on to the cranked pentode mode. YMMV.

[valveandsound]

Here was the schemo I drafted of it. You only need one DPDT relay for the channel switching. I also had a switchable effects loop as well.

Edit: Well, nevermind. I can't get the image to attach to this post

[pompeiisneaks]

Thanks! What error did you get when trying to post the schematic? I'd like to know if it's something I could sort out? So you're saying that a real tone stack isn't very useful on a pentode input? I also thought the point of the second gain stage like that was to avoid loading it down and handle the tone stack, it also just wins me the fact that I then get both stages in phase and it's a bit 'easier' but I could omit it and not use half of a triode, and then send each half to the opposing grids of the PI like they do in the matchless and some later Vox as I recall? no?

I had thought about a pentode/triode switch, but why when I have a second triode input already, then it just becomes 'more' like the other input already? I guess without the second gain stage and cathode follower in the Top boost side, it would be different, but basically quieter and less output no?

~Phil

[dbeasley]

Phil,

It kept giving me "http error" whenever I tried to attach the PNG to my post.

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[pompeiisneaks]

I'm not sure what that means, could you retry and get like a screenshot of it, or at least the full content of the error? If that was it, I don't have enough info to fix ;(

~Phil

[Roe]

Keep the 100r screen resistors but use a choke with high resistance. The original measure ca 420r

[pompeiisneaks]

Keep the 100r screen resistors but use a choke with high resistance. The original measure ca 420r

The one I've been looking at is the hammond 194e that's got 30H and 517.2 ohms resistance.

~Phil

[Roe]

Classictone makes a nice ac30 choke

[dbeasley]

Ive built that pentode preamp circuit with/without EQ and makeup stages and to me, the pentode's Sonic characteristics become less apparent the more circuitry there is before the PI. I toyed around with EQ circuits before and after a makeup stage, driven by a cathode follower, etc and every time I tried to add a conventional EQ it didn't have anywhere near as much of that EF86/pentode "glassiness".

I looked at the matchless schematics and Dr Z/Vox etc - especially the handwired AC15 head that vox does and dawned on me (Also a big dose of help from Lyle Caldwell, Vox expert extraordinaire) that they kept the signal path pretty simple and avoided lossy tone shaping for a reason. It just sounds like every other preamp if you add the conventional EQ stuff into the topology. So like matchless when they use a rotary coupling cap switch, and Vox uses a handful of switches to shape the lows and highs, the circuit retains more of that Pentode grit and compression.

Long winded, my bad. But that's the conclusion I ended up with after all my tinkering and research.

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[pompeiisneaks]

Ive built that pentode preamp circuit with/without EQ and makeup stages and to me, the pentode's Sonic characteristics become less apparent the more circuitry there is before the PI. I toyed around with EQ circuits before and after a makeup stage, driven by a cathode follower, etc and every time I tried to add a conventional EQ it didn't have anywhere near as much of that EF86/pentode "glassiness".

I looked at the matchless schematics and Dr Z/Vox etc - especially the handwired AC15 head that vox does and dawned on me (Also a big dose of help from Lyle Caldwell, Vox expert extraordinaire) that they kept the signal path pretty simple and avoided lossy tone shaping for a reason. It just sounds like every other preamp if you add the conventional EQ stuff into the topology. So like matchless when they use a rotary coupling cap switch, and Vox uses a handful of switches to shape the lows and highs, the circuit retains more of that Pentode grit and compression.

Long winded, my bad. But that's the conclusion I ended up with after all my tinkering and research.

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Oh wow I think that actually made sense I may have to think about just adding a switch that skipps the second stage with EQ and goes straight to the mixing resistor and call that 'raw' mode or something

But now I'm rethinking it. Your explanation totally clicks. I also remember that the valve wizard suggested taming the sensitivity of the EF86 by putting a gain stage before it, so that it's not the most sensitive stage. Hmm.

Now I gotta rethink this thing. I am thinking, though, that the idea of switching the tone stack and second stage makes it more versatile. If they just want a more 'normal' normal channel, with tone shaping, they've got it. Or they can switch it into the EF86 pentode only mode. Then my jumpered idea isn't going to work as well, as the phase would be a problem (if the tone stack is 'in' then it's in phase if it's 'out' then it's out of phase and jumpering would be bad)

~Phil;

[dbeasley]

Look at the Matchless Clubman for a topology that puts a gain stage before the EF86. Honestly though you could pretty much achieve the same effect by pushing the front end with a clean boost IMO.

As far as "taming" the high gain of the EF86, you can do that by lowering the screen voltage. Honestly though I liked the sound with the stock 1M screen resistor.

The stage as it's represented in classic schematics works well as long as you use a good tube. That's a tall order considering the EF86 is pretty inconsistent from tube to tube, especially modern ones. A good noise free vintage one ain't cheap either.

That's why I ended up using 6BR7s. Electrically identical but slightly different noval pinout. Cheaper tube, and naturally less microphonic than the EF86. Just something to think about.

I enjoy all the technical info Merlin goes on at length about regarding small signal pentodes but man I've tried most of his little tricks and at the end of the day I felt the basic preamp as done sounded so much more dynamic and glassy. The natural compression from the tube is very nice and plays well with OD pedals and delay/reverb. Probably why all those Dr. Z guys love those amps. Between a few bright cap values, switchable coupling caps and a cut control to really take the highs you get a really unique, flexible preamp stage.



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[dbeasley]

I'll attempt again tonight to upload my schemo. Hope that helps give you some ideas.

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[pompeiisneaks]

Cool thanks so much for the input.

~Phil