Vox AC4 or something else?

[g-man]

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[DummyLoad]

building an AC4 clone without the trem channel

respectfully, i don't see a tremelo channel - only LFO and CF that wobbles cathode of EF86.

B+ to the EF86 will bump up a bit w/o the trem tube - shouldn't be an issue. 

looks like fun and doesn't get much simpler than that.

[TubeGeek]

I built an ac4 a while back out of an Epiphone Valve Junior.  I posted a few pics and words about it here:  http://www.glacieramps.com/Glacier_Amplification/AC4.html

There are a few sound clips at the bottom, looks like a few links are dead, I will have to fix them.  It sounded clean to me, not much breakup.  I ended up selling this for a couple hundred bucks.

[tubeswell]

FWIW I did a 'super AC4' with parallel SE output stage (that can run a variety of output tubes) and bias-vary trem (with an IRF820 source follower) on the output stage, and Merlin's triode/pentode morph control - here. Sounds good with a Greenback G10

http://music-electronics-forum.com/t23443/

[g-man]

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[TubeGeek]

If I remember the volume was pretty much cranked to max, tone was probably at 2 or 3 o'clock.  You are correct, I was playing a strat.  I didn't try it with humbuckers, it sold before I could really play around with it.

If I were to build another one, I'd spend more time testing out different component values until a nice balance between clean and breakup is achieved.

Changing the 5.6M to 1M added more grit.  It was very clean and sterile with the 5M6.  This amp had very low output too, I'd say around 3 to 4 watts.  Oh yeah I also installed a Weber blue pup in the final version.  I still have another blue pup to use in a future epi amp "flip".

[38Super]

One question on the AC4 ckt.  Why no screen grid current limiting resistor on EL84?


rob

[sluckey]

Cheap

[g-man]

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[jojokeo]

I've built about 5 pentode amps now and w/ the AC4 version, the single EF86 isn't enough even to drive an EL84 properly IMO. I ended up using a second one in an amp I still have and also one w/ a CF stage following it. Both of these give the EF86 mojo where the original AC4 schem was lacking by a lot.
The higher value resistor on the screen gives you cleaner operation. The standard values are 220k load w/ a 1M screen. If you get microphonics you can raise the plate voltage or drop the load resistor which helps this issue a lot. I like to install a tone stack bypass or lift and also control the cathode bypassing which also allows good control of headroom/break-up characteristics on the EF86.

Don't worry about the screen resistor on the EL84 w/ this amp right now. You can mess w/ it later if you want to. This isn't critical at all w/ your build you're considering.

[tubeswell]

I've built about 5 pentode amps now and w/ the AC4 version, the single EF86 isn't enough even to drive an EL84 properly IMO. I ended up using a second one in an amp I still have and also one w/ a CF stage following it. Both of these give the EF86 mojo where the original AC4 schem was lacking by a lot.

+1 That's why my recent variant has a triode following the EF86. This gives it enough power to drive a 6L6 into mild clipping with the EF86 in "triode mode".

[g-man]

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[jojokeo]

Sorry, I wasn't trying to discourage you. I wouldn't say to abandon the idea of using the pentode. They have a great tone, complexity, fullness, grind, and vibe that triodes don't. Also getting the hang of them is a good learning experience. Knowing ahead of time that you should plan on another stage or two will save you dissapointment, frustration, and time. Ofcourse, you can try it your own way and see what you think too? There's been many happy users w/ them being used as V1 doing the initial driving and also others that like them in V2 position being the one that's driven so to speak.