AC30/4 build

[Colossal]

I've got a Vox AC30 transformer set and have been wanting to build a single channel EF86-based AC30/4 type amp for while. It's a neat amp and has a unique sound. There are some noteworthy modern derivatives such as the Dr. Z Stangray (EF86) and DB4 (5879) (both amps now discontinued) and of course numerous Matchless amps. Here is the basic schematic:

AC304.jpg

The AC30/4 had a simple switch for Brilliance which changed the coupling cap for a bass cut. A cut control on the back end did the rest. Simple. Given the relatively high output impedance, EF86s don't appreciate heavy loading from a tone stack. Dr. Z incorporated the low loss tone control from his Carmen Ghia and changed the values to match the EF86.

Here is the circuit from the Carmen Ghia:

tonecontrol.jpg

For fun, I made a model. The doctor calls this the 3kHz Brilliance control. It's a rather interesting control that will scoop mids (to the left), and add mids (to the right). Here is the sweep from 0 to 10 on a 1MA pot with 330k,120p, and 4700p in the filter, and 1MA Volume on 10.


[EDIT: freq sweep removed]


Matchless, Trinity, and others that have an EF86 front end, use the Tone Contour control which is a 6-way switch that substitutes in different coupling caps which rolls off bass as you click down.

TC15EF86.jpg

I am going to build mine as a head to keep the EF86 away from the speakers. I plan to keep it simple and soup up the power supply a little to reduce hum. I have some nice Mullard GZ34s saved. I am debating to use either the six way tone contour control or maybe try the bridge filter.

PT is a Haddon 285-0-285 (unloaded) with 117VAC primary
Choke is a Radiospares 460ohm 20H 250mA
OT is a Haddon with 4k primary and 8/16 ohm secondary

Of note with the AC30/4 amp compared to AC30/6 derivatives are the PI and output coupling caps:

AC30/4, Stangray: 10nF for PI coupling, 10nF PI slave bypass, and 10nF output coupling, 220k grid leaks
AC30, TW Rocket, and Songwriter 30: 47nF for PI coupling, 47nF PI slave bypass, and 47nF output coupling, 220k grid leaks (they don't call it Fat Bottomed Girl for nothing)
Marshall 18W (and 36W Lite IIb): 10nF for PI coupling (470k grid leaks), 10nF PI slave bypass, and 10nF output coupling, 470k grid leaks

[pompeiisneaks]

I've built an exact clone of the circuit of the AC30/4 and love it. Basically found with sluckey's help that it's identical to the old original AC15 with the added 2 power tubes. I definitely thin the EF86 sounds glorious, BUT I like one of the suggestions I've heard here on the forum that it' often works better with pentodes in the preamp to have at least one gain stage before them to help them be less microphonic... not sure how well that would work here, but I did have to get some silicone rings on mine to quiet down some of the ringing I was getting.

~Phil

[Colossal]

I've built an exact clone of the circuit of the AC30/4 and love it. Basically found with sluckey's help that it's identical to the old original AC15 with the added 2 power tubes. I definitely thin the EF86 sounds glorious, BUT I like one of the suggestions I've heard here on the forum that it' often works better with pentodes in the preamp to have at least one gain stage before them to help them be less microphonic... not sure how well that would work here, but I did have to get some silicone rings on mine to quiet down some of the ringing I was getting.

~Phil

Yeah man, I do remember your build and high praise of it. I am going to build a light version, no tremolo. I was thinking of adding a little trampoline mount for the tube socket as well as using the usual temp silicone rings for dampening. An EF86's character can change a lot depending on its B+, so I am going to explore that as well. I know the Stangray ran a little higher rail voltage (more like a Marshall 18/36) and had stiff filtering with the dropping resistors in series down the line, rather than 22ks branching from screen supply like the typical Vox AC. That thing sounds like a piano; just a killer amp. I am going to use a pot to dial in the plate voltage.

[ChopSauce]

I look forward to read about your experiments with the EF86, eventually.

... one of the suggestions I've heard here on the forum that it' often works better with pentodes in the preamp to have at least one gain stage before them to help them be less microphonic...

That's the suggestion of Merlin Blencowe himself - if I'm not mistaking.

At least in its article "the small signal pentode" he recommends setting the pentode for moderate gain.

Not sure how an EF86 with mild settings (for reliability) sounds vs. a "pushed" one, though...
(nor if its so different it being followed by - rather than following - one or several gain stages)

[pompeiisneaks]

I look forward to read about your experiments with the EF86, eventually.

... one of the suggestions I've heard here on the forum that it' often works better with pentodes in the preamp to have at least one gain stage before them to help them be less microphonic...

That's the suggestion of Merlin Blencowe himself - if I'm not mistaking.

At least in its article "the small signal pentode" he recommends setting the pentode for moderate gain.

Not sure how an EF86 with mild settings (for reliability) sounds vs. a "pushed" one, though...
(nor if its so different it being followed by - rather than following - one or several gain stages)

My understanding, (partially from the above merlin book stuff, and partly from discussions here and at hoffman forums) shows that basically the EF86 and any small signal pentode provides too much gain for a first stage. The signal is so weak and prone to noise/etc, that amplifying that is asking for trouble, but once a simple triode stage amplifies it to a solid signal, that's enough that the pentode stage can amplify it and give you the tonal characteristics, without the sensitivity to oscillations/microphonics and the like.

~Phil

[sluckey]

It would be pretty easy to modify Phil's board for a Normal channel only if that idea appeals to you. LMK

[ChopSauce]

I look forward to read about your experiments with the EF86, eventually.

... one of the suggestions I've heard here on the forum that it' often works better with pentodes in the preamp to have at least one gain stage before them to help them be less microphonic...

That's the suggestion of Merlin Blencowe himself - if I'm not mistaking.

At least in its article "the small signal pentode" he recommends setting the pentode for moderate gain.

Not sure how an EF86 with mild settings (for reliability) sounds vs. a "pushed" one, though...
(nor if its so different it being followed by - rather than following - one or several gain stages)

My understanding, (partially from the above merlin book stuff, and partly from discussions here and at hoffman forums) shows that basically the EF86 and any small signal pentode provides too much gain for a first stage. The signal is so weak and prone to noise/etc, that amplifying that is asking for trouble, but once a simple triode stage amplifies it to a solid signal, that's enough that the pentode stage can amplify it and give you the tonal characteristics, without the sensitivity to oscillations/microphonics and the like.

~Phil

That looks good but thinking twice about it, it seems to me that whatever the noise is, amplifying it (lets's say) 30 times, then 100 times or 100 times and then 30 times, you should end with 3000 times the noise in both cases, isn't it?
(most likely I've been missing something - again)

[pompeiisneaks]

I look forward to read about your experiments with the EF86, eventually.



That's the suggestion of Merlin Blencowe himself - if I'm not mistaking.

At least in its article "the small signal pentode" he recommends setting the pentode for moderate gain.

Not sure how an EF86 with mild settings (for reliability) sounds vs. a "pushed" one, though...
(nor if its so different it being followed by - rather than following - one or several gain stages)

My understanding, (partially from the above merlin book stuff, and partly from discussions here and at hoffman forums) shows that basically the EF86 and any small signal pentode provides too much gain for a first stage. The signal is so weak and prone to noise/etc, that amplifying that is asking for trouble, but once a simple triode stage amplifies it to a solid signal, that's enough that the pentode stage can amplify it and give you the tonal characteristics, without the sensitivity to oscillations/microphonics and the like.

~Phil

That looks good but thinking twice about it, it seems to me that whatever the noise is, amplifying it (lets say) 30 times, then 100 times or 100 times and then 30 times, you should end with 3000 times the noise in both cases, isn't it?
(most likely I've been missing something - again)

no not from my understanding. The normal triode gain stages are better at rejecting unwanted noise so amplifying 0 noise is still 0 (or next to 0) Whereas the pentode is very good at amplifying these small noises if already a bit over sensitive like the EF86, but when given a strong signal, it overrides it's tendency to pickup weaker noises.

Or so my understanding goes. It's all theoretical and non practical

~Phil

[Mark]

Looking forward to seeing what works and doesn’t work.

Reducing the plate resistor to 100K would reduce the gain too, and thus the noise, but would it affect the sound of the circuit?

[Colossal]

It would be pretty easy to modify Phil's board for a Normal channel only if that idea appeals to you. LMK

That's a beautiful layout, Steve. Thanks for the offer. I am going to do something similar, with the power supply caps distributed on the board and local grounds tied back to their respective cap.

At least in its article "the small signal pentode" [Blencowe] recommends setting the pentode for moderate gain.

Not sure how an EF86 with mild settings (for reliability) sounds vs. a "pushed" one, though...
(nor if its so different it being followed by - rather than following - one or several gain stages)

I went through the exercise once of casting a load line for a 6SJ7 for a single-ended amp after being dissatisfied with the stock values and rather dark presentation. It sounded good and changed the amp for the better, but not for reasons of there being too much gain from the previous configuration. For this amp, however, I want to stick to the British sound, warts and all. I do have a few small mods in mind:

1) Tie the screen bypass cap to the cathode, instead of ground, to preserve transients
2) Make a shock absorber for the EF86 tube socket (although this amp will be a head, not a combo, so will be away from the speakers) and use high-temp silicon rings for damping
3) Use a high impedance divider on the screen supply to keep the screen voltage from wandering all over the place when driven hard
4) Keep B+ a little higher to give tight bass and more pronounced midrange.
5) Tweak the input network, reducing the grid resistor value and adding capacitance from grid to cathode (~20kHz rolloff) to reduce Johnson noise
6) Use metal films for both plate and screen loads
7) Maybe use a low value grid stopper on the screen for some degeneration

Of interest, here are the plate, screen, and cathode values for various amps:

PLATE / SCREEN / Rk / Ck
MERLIN 100k / 560k / 680Ω / 10µF
VOX AC30/4 220k / 1M / 2k2 / 25µF
TRINITY TC15 220k / 1M / 2k2 / 25µF
DR. Z STANGRAY 220k / 1M / 2k2 / 25µF
MATCHLESS 330k / 2M2 / 2k2 / 25µF
BROWNNOTE 150k / 1M / 2k7 / 25µF
65AMPS LONDON 220k / 1M / 2k2 / 25µF

Looking forward to seeing what works and doesn’t work.

Reducing the plate resistor to 100K would reduce the gain too, and thus the noise, but would it affect the sound of the circuit?

I am going to keep this real close to the traditional circuit. Changing the values changes the sound. Also, I am leaning towards using the tone control shown above, although still considering the 6-way contour control, the latter being lower loss. But the magic of the Stangray is that it is a mostly clean amp with slightly scooped mids at lower tone control settings, which make for killer cleans. It soaks up a bit of that gain without neutering the amp. It does not break up as early as the ACs. I went back and read every thread on the forum that discusses AC30/4 type amps and there were some interesting ideas and observations.

Mark Fowler has had one of the original Stangrays for many years and says it is the loudest EL84 amp he has, but to quote him, "the tone is glorious". He runs a pair of Blues or I think he may have changed one of the Blues for a Gold in his 2x12. I love the extremely articulate and 3D swirly, reverby quality of those speakers with AC30 type amps.

[pdf64]

Merlin’s Bone Ray tone control looks very interesting, I’m going to give it a go in my next project.

[M Fowler]

Dave, yes my Stang Ray is the loudest EL84 amp I own. I gigged with that amp for 4 years into a Dr Z 212 open back cab loaded with two Celestion Blues and because of Brad Paisley I changed one blue to Gold. It is loud after all so I was worried about the blue speakers but they never failed me. One of those blues is in another 212 open back cab I have my Blue Zwreck amp and cabinet I built.

The Stang Ray does have an effects loop but I never used it, ran all effects into the front end, this included, Boss Blues driver, DS-1, MT-1 and RV-5 using my Fender Lone Star Strat with Pearly Gates Humbucker and two Texas Specials single coils.

Mark

[M Fowler]

My GZ34 rectified voltage 347vdc to plates and 345vdc to screens from 120v mains.

PT: 290-0-290

If I ever get my stuff out of storage from home moving I will tell the bias info Dave.

Mark

[Colossal]

My GZ34 rectified voltage 347vdc to plates and 345vdc to screens from 120v mains.

PT: 290-0-290

If I ever get my stuff out of storage from home moving I will tell the bias info Dave.

Mark

Thanks, Mark, much appreciated. That amp has got to be running 130% with 50R/250uF bias!

[Colossal]

I've been doing a bit of reading and digging on the AC30/4s. Of interest is the Haddon PT was spec'd for 285-0-285 with the bias set at 80R/250uF. The change, dated mid 1963, saw the PT go to 280-0-280 and the bias resistor to 50R. This would have affected the later PTs; Woden, Parmeko, and Albions.

Here is the modification note showing the bias and PT change on the AC30/6 OS/065 schematic:

HV change.jpg

Here is a photo clearly showing the 80R Welwyn bias resistor and Hunts 250uF cap in an early black panel AC30.
Photo credit: https://www.voxac30.org.uk/vox_ac30_4_black_panels.html

AC304bias.jpg