AC4 project

[FullertonAmps]

Interested in this. I have brought a job lot of amp cabinets 3 years ago which included a Vox AC4C1-BL mini. Same as AC41-BL but with a 6.5 inch speaker - cabinet 13.25" by 10" tall. I have replaced the baffle and installed an 8" speaker and added a Vox appropriate grill cloth.Was thinking of putting a SF1 champ circuit but I think this circuit looks better option. Has anyone managed to complete on and able to share a demo ?
I have created a simple L fold chassis just wondering about how I will mount the tubes & sockets.
I will post a couple of photos later.

 

[voskarp]

I did finish the modifications of the 2:nd version schematic of this amp today. The thing I did missing from the schematic and layout is that I have bypassed the C8 and R12 with a jumper. So either skip the C8 and R12 or put a cap between P2 and R28 (47n).

I'm also not sure about R5, maybe a 2M2 will be better.

And I'll probably change R26 & R27 to 47k. It has very high gain, so you'll have to back down almost all the way to get a really clean sound.

I'll post a sample later.

The EF86 channel has a lot more mids (especially low mids) and sounds a bit more Marshally than the 2:nd (ECC83) channel.

 

[FullertonAmps]

Heres the original cab arrangement

 

[voskarp]

Looks like a nice little cab.

If you use my schematic for this amp maybe you should choose one of the two channels. (Or perhaps better yet, use the original AC4 schematic or the AC4TV one from the first post.) I found it quite cramped in my cab, and it's a bit bigger.

 

[voskarp]

Interested in this. I have brought a job lot of amp cabinets 3 years ago which included a Vox AC4C1-BL mini. Same as AC41-BL but with a 6.5 inch speaker - cabinet 13.25" by 10" tall. I have replaced the baffle and installed an 8" speaker and added a Vox appropriate grill cloth.Was thinking of putting a SF1 champ circuit but I think this circuit looks better option. Has anyone managed to complete on and able to share a demo ?
I have created a simple L fold chassis just wondering about how I will mount the tubes & sockets.
I will post a couple of photos later.

Are you reusing any parts or will they be all new?

I did something similar converting my AC4C1-BL to a handwired circuit - but reused the transformers and the chassis.

View media item 39718

Maybe this is the answer to mounting valves and transformers.

 

[voskarp]

Here are the voltages for the V.2

Voltages look better at the EL84, almost the same at the ECC83 and very different at the EF86, due to the change of R5 from 5M6 to 1M. I probably should go for 2M2 instead. (The AC15 uses 1M, and the plate voltage for the EF86 is at 90V.)

 

[voskarp]

Could the low headroom be because of the low plate voltages in the preamp valves?

Perhaps I should change the voltage dropping resistor R23 to 10k, or even lower...?

 

[FullertonAmps]

Maybe this is the answer to mounting valves and transformers.

I never had the full amp. I am going to have to source most parts from new. Thinking of going with a tag board. I have a Transformer set from a H&K 18 watt. Probably use the power transformer and buy a SE output transformer and have spare EL84s and 12AX7s around.

 

[voskarp]

If you look at the picture in gusfinleys post, you’ll see that he has put a shelf on the chassis for the valves to sit on.

 

[gusfinley]

If you look at the picture in gusfinleys post, you’ll see that he has put a shelf on the chassis for the valves to sit on.

Yup. If I had mad chassis bending and faceplate skills I would have loved to make this a more classic JMI style chassis.

Check out this beauty made by JMI a few years back:

 

[ChopSauce]

What amp is it?

Note that you can build this kind of structure by bolting together two Hammond boxes in an "L" shape.

Also note that Merlin "the Valvewizard" Blencowe provided all the material for anyone to calculate what he is doing when tweaking EF86 resistor values - here : http://www.valvewizard.co.uk/pentode.html - if I recall well.

That might be a nice addition to copying just part of AC15 component values, the other ones being possibly different.

 

[voskarp]

Hmm... I'll give that one a read first. Thanks!

 

[ChopSauce]

My pleasure ...

PLEASE, note that I don't recommend that you follow Merlin's advices. According to him, you should temper the gain of the EF86, and my experience with this tube - not for this circuit, though - is that you'll loose "that" sound, this way.

Rather, I meant that if you envision comparing/tuning the EF86 settings of the AC4 vs. the AC15, the proper way to do that is to do the whole calculations for both amps, cos not only one resistors matters, but instead a combination of both grid and plate resistors, plus the voltage, etc.

Most likely you already understood all that but I feel better writing it.

 

[gusfinley]

What amp is it?

Note that you can build this kind of structure by bolting together two Hammond boxes in an "L" shape.

...

That is the "modern" JMI AC4 (sold by Music Ground, but built by Steve Giles who I believe did their Hiwatt line also). I'm not sure what the circuit is. My pic aren't hi-res enough to get component values, but it looks like it could be a better constructed copy of the vintage Vox/JMI AC4.

I have modeled up a JMI style chassis from some scrap aluminum that I have laying around. I might built a AC4C1 2x8 one of these days, since I have four 8" speakers waiting for a home....

 

[ChopSauce]

Thanks for the reply! I saw some very clean looking reverb version also, but - as I understand things - this one is rather unlikely to depart much from Andy's experiments & outcome.

 

[voskarp]

Made some sound samples of my amp today. I used a free version of an app that only let you export 90 sec of sound, so some are cut at the end... It's recorded with my Telecaster with single coil bridge and neck humbucker in parallel. Cab miced with a Shure SM58 close to the speaker and a AKG P170 about a foot away. Only effect is a slap back delay.


First is the EF86 channel switching between 10n and 2n2 cap (C2a/b):




Then EF86 with the screen bypass cap out of circuit (which gives less gain, starts getting some distortion at half volume):




Next EF86 (2Cb2n2) and then ECC83 clean(ish) volume just about on:




EF86 and then ECC83 with volume at about 1/3:




And now with the volume dimed (on EF86 about 3/4, since full on makes it squeal and makes an oscillating effect like tremolo when the notes fade.):



https://soundcloud.com/oskar-pohjola/6nac4moredimedef86ecc83

https://soundcloud.com/oskar-pohjola/7nac4evenmoredimedef86ecc83



The ECC83 channel definitely has more scoped sound.

 

[voskarp]

I just tried the amp with a 12AY7, that I borrowed from the Tweed Deluxe, in V2.

It works really good, I get a little clean sweep on the EF86 channel (about 1/5), then it starts overdriving slightly.

The second channel is clean to about 1/3, and really starts overdriving at 1/2 volume. At full volume you still get that 70's Rock feeling.


If I can't get more headroom by increasing the plate voltage, I think the 12AY7 is the solution. (Still waiting for the resistors I ordered to come before I can start working on that.)

 

[LuciMan]

I'm not sure what the circuit is. My pic aren't hi-res enough to get component values

Hi Gus,

maybe you remember me from the Vintage Amps Forum where I did the VOX clones of AC4 to different AC30 ("Surrounded by clones ...") about 6 years ago.

I caught some better pics a few years ago from their advertisment - maybe this could help:


A friend of mine bought one J.M.I/MG AC15 (which I already modded with an additional Top Boost Kit at the back) & one AC4, which will be modded too onto a EF86.

Don't look for a EF86 here like on the vintage AC4 because there isn't. Steve Giles used a ECC83 circuit instead (EF86 too sensitive?). But he did an exellent job on using the L-shape even for the AC4.

@voskarp

Very nice job you did with your AC4!

Cheers

 

[gusfinley]

Hi Gus,

maybe you remember me from the Vintage Amps Forum where I did the VOX clones of AC4 to different AC30 ("Surrounded by clones ...") about 6 years ago.

I caught some better pics a few years ago from their advertisment - maybe this could help:
View attachment 717136 View attachment 717137

A friend of mine bought one J.M.I/MG AC15 (which I already modded with an additional Top Boost Kit at the back) & one AC4, which will be modded too onto a EF86.

Don't look for a EF86 here like on the vintage AC4 because there isn't. Steve Giles used a ECC83 circuit instead (EF86 too sensitive?). But he did an exellent job on using the L-shape even for the AC4.

@voskarp

Very nice job you did with your AC4!

Cheers

LuciMan! Welcome to the forum!

I don't remember you by name (if it is the same), but I do remember your "Surrounded by clones" Thread.

It was some serious eye candy!

While I don't sling a telecaster, the Shock Brothers Amp forum has a lot of great projects and some great expertise is shared.

You may recognize some other VAF members here or over at the Beat Gear Cavern ( more beatles related ) but this forum is more active in the D.I.Y area.

Thanks for the info on the JMI AC4. I've tried to steal whatever pics I can from the Internet Archive but it is tedious.

 

[voskarp]

Here are voltages after my latest tweak, haven't played the amp yet, 'cause I did it late last night (it's now 6 a.m. here) and I'm off to work in a minute.

One thing that bothers me is that the EL84 screen is higher than the plate. It has always been, and it works fine, but I don't like it. How does it effect sound? I imagine it raises the screen current.

I think I've found a good balance for the EF86 plate/screen, but maybe a bit higher voltage would be good.

And I also want to raise the voltages for the ECC83, especially the second half, as I want it as clean as possible.

I should perhaps get rid of R18 and R22 to start with, and lower R23 some more. Maybe move the EF86 to B+3 and the EL84 screen to B+2...