AC4 project

[voskarp]

I’ve decided to gut my Vox AC4C1TV (blue and beige one with a 10” speaker and Top Boost preamp), and build a hardwired one closer to the original, using an EF86 preamp valve.



Not because I don’t like how it sounds, more like because -why not?



I’ll be keeping the original circuit in a box so I can put it back if the project goes south… I’ll just be using the cab, speaker and transformers in the new build.



Looking around for schematics I’ve found one for the original AC4 and the AC4TV, so I thought it would be fun to ad a second channel with the AC4TV preamp which is more like the AC30/6 preamp.

This is what I’ve come up with so far, but I have some thoughts about some things:

I’ve measured the power transformer putting out 271V ac in circuit (288V out of circuit), so it’s about 21V higher than the original one (and the AC4TV one), so I thought the 220R resistors would get it closer to 250V. Have I got that about right or have I thought/calculated it all wrong? (It’s mostly guestimations…)



I found a EZ80 valve is hard to find for not too much money, so I went with a solid state rectifier (as the AC4C1 and AC4TV). I guess that won’t affect the sound very much, or does it?



In the original AC4 schematic, the screen grid is connected directly to the +270V and since the anode HT goes though the output transformer first, shouldn’t that make the screen grid more positive than the anode? Shouldn’t there be a limiting resistor between the B+1 and screen grid?



I’d be happy for any help with this, or please mention if you think I’ve missed something important.

 

[Squawker]

Go with an EZ81. Should be much easier to source, it's pin compatible, and it will handle more current which could be handy for your mods. It will probably stiffen your B+ a bit more, so you might need/want to drop a few more volts on the first wirewound.

Solid state will also work fine, but then you'll be dropping way fewer volts in your rectifier. Single-ended amps don't really induce rectifier sag, so solid state won't affect the tone noticeably.

 

[Squawker]

It's a very good idea to add a screen grid resistor. It was fairly normal to run the screen straight off the B+ back in the day on these simple amps, but the screen resistor is good engineering practice.

 

[tubelectron]

Hi @voskarp,

Your last schematic is a good start option.

Yes, I would insert a 100, 470 or 1K resistor in the screen grid of the EL84. By my experience, a higher value tend to give a smoother clean-to-overdrive transition : you'll judge by your ears.

Choose at least 10W resistors for the 220R in the HV taps. You may end with lower R values, possibly. It's not an exact science here...

Or as mentioned by @Squawker , the EZ81 would be fine too, despite a lower internal resistance though.

You will have to tweak more or less these values to obtain what you expect, you guess it...

But it's me, OK ?

-tbln

 

[voskarp]

Thanks for the replies.

Since the cab is quite small I thought that a SS rectifier might be a better choice, thinking a bigger rectifier won't give any sag anyways, so...

Wow, maybe 10 watters are required in the HV taps. Better safe than sorry, of course! As to the value of the resistors -I figured I just have to build it and try some out...

 

[gusfinley]

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

 

[voskarp]

I’ll be reusing transformers and cab+speaker, but I’m planning on making it a open back, like the original.

I think I’ll use a Tweed style of chassis, as it seems most simple/cheap way to go.

 

[LightningPhil]

Looks like a great project.

I have the flu, so a schematic and circuit to mull is a good distraction. Here’s some nonsense:

Suggest considering building the hand wired AC4 amp and achieving a top boost effect by making c3 much smaller and adding a switch to it. 0.68uF may be a good place to start. Would only give a moderate amount of boost, so perhaps lower C12 to 10uF to assist. Again, the effect would be a mild taming of the lows.

A switch on C6, dropping it by a factor of 10 when enabled would certainly have a pronounced VOX type, bin all the lows effect.

C4 has an effect on tone too. Smaller caps will lower the gain in the bass region. Easiest to swap some about than guess value. Removing it would decrease gain somewhat - useful if more headroom is desired (are you using humbuckers?)

lastly, since we live in a world of rf interference, it may be a good idea bypassing R4 with 100pF or so to attenuate above audio frequencies from sneaking in. The EF86 doesn’t have the helpful Miller capacitance the a triode does and so is more an issue here. Adding capacitance here can allow R1 and R2 to be reduced too, as these were presumably 100k to roll off RF. 10k would be lower noise but potentially more problematic RF wise, so perhaps 33k?

Thanks for the distraction. Now time to go and poke the real world with a stick. Just had a storm pass, so plenty of sticks to choose from and a suspicious bit of someone’s roof in our garden to go look at!

 

[voskarp]

About the voltage dropping resistors after the PT, I thought 3 watts would be enough since it's a parallel pair (the 1k after the rectifier is 5W), and that's what the AC4TV uses. (As far as I understand about 95% of the power goes to the El84, so the added EF86 don't ad much current.)

 

[voskarp]

Looks like a great project.

I have the flu, so a schematic and circuit to mull is a good distraction. Here’s some nonsense:

Suggest considering building the hand wired AC4 amp and achieving a top boost effect by making c3 much smaller and adding a switch to it. 0.68uF may be a good place to start. Would only give a moderate amount of boost, so perhaps lower C12 to 10uF to assist. Again, the effect would be a mild taming of the lows.

A switch on C6, dropping it by a factor of 10 when enabled would certainly have a pronounced VOX type, bin all the lows effect.

C4 has an effect on tone too. Smaller caps will lower the gain in the bass region. Easiest to swap some about than guess value. Removing it would decrease gain somewhat - useful if more headroom is desired (are you using humbuckers?)

lastly, since we live in a world of rf interference, it may be a good idea bypassing R4 with 100pF or so to attenuate above audio frequencies from sneaking in. The EF86 doesn’t have the helpful Miller capacitance the a triode does and so is more an issue here. Adding capacitance here can allow R1 and R2 to be reduced too, as these were presumably 100k to roll off RF. 10k would be lower noise but potentially more problematic RF wise, so perhaps 33k?

Thanks for the distraction. Now time to go and poke the real world with a stick. Just had a storm pass, so plenty of sticks to choose from and a suspicious bit of someone’s roof in our garden to go look at!

Your Rf-eliminating cap suggestion is a good one, and that's what's in the AC4TV schematic of course, C1 (& C2?) in the second schematic.

Otherwise I guess you're referring to the topmost schematic (Vox AC4 V-1-1), right?

 

[voskarp]

If this is one of the things you're talking about I had it in my earlier drawings, but with a pot instead of the switch, like a bass cut control.

But when I added the second channel I had to take it out, because of the limited space in the little cab.

It would be a nice thing to have though... may be able to squeeze it in.

 

[voskarp]

My thinking around the two channel thing is to have one old school (like the original) one, and one more modern (like the AC4TV).

 

[LightningPhil]

Your Rf-eliminating cap suggestion is a good one, and that's what's in the AC4TV schematic of course, C1 (& C2?) in the second schematic.

Otherwise I guess you're referring to the topmost schematic (Vox AC4 V-1-1), right?

Yes, C1 and C2 in the AC4TV schematic are certainly to do with shorting RF to ground. But so is the Miller capacitance in the ECC83. That's probably around 150 pF, and forms a low pass filter with R4.

And also yes, all the nonsense was with regard to the topmost schematic (Vox AC4 V-1-1).

A further random idea would be to have different values for R1 and R2. Say 10k and 250k. Though experiment a bit. The higher resistance will work with the capacitance suggested capacitance to form a low pass filter and roll off highs more - better for rhythm and better noise rejection. The lower resistance would allow a good deal more chime. (probably)

 

[LightningPhil]

OK, nerded out further.

Have you got a copy of Micro-Cap? It's awesome for modelling valve circuits, and free.

Anyhow, for a very VOX top boost type gain curve, drop C4 down to 1n for an experiment. 2.2n might be more sensible.

 

[voskarp]

A new revision:

Mixing resistors added and more...

 

[voskarp]

OK, nerded out further.

Have you got a copy of Micro-Cap? It's awesome for modelling valve circuits, and free.

Anyhow, for a very VOX top boost type gain curve, drop C4 down to 1n for an experiment. 2.2n might be more sensible.

Seems like a good application, but I'm on a Mac, so it wont run without a bunch of hassle.

C4... are you still on the original Vox AC4 schematic?

 

[LightningPhil]

Yes, C4 from the original one with the EF86. The reducing the capacitance on the screen grid effectively gives more local negative feedback at low frequencies. So it can be used to similar effect as small coupling caps. But, by not amplifying and then attenuating, which seems to be the usual practice in guitar amps, much noise can be avoided.

 

[voskarp]

OK. I think I’ll build it as stock as possible to begin with, but that’s a neat mod to know about if I find it has too much lows.

Actually, having the original cap and the smaller one on a switch is a good idea for normal/bright.

That will of course not work in the second channel, but it already has a smaller coupling cap.

I’m also planning to build an AC15 later on, so all these tricks are good to know.

 

[voskarp]

Finally finished the Layout for this little amp, and it's messy, but it will be a blast to solder!!!

Might have tried to put a bit too much in this little chassis, the turret board will almost fill the whole back of it.

When I get the parts I'll see if it's really doable... but I guess it should be.

 

[alathIN]

Thanks for the replies.

Since the cab is quite small I thought that a SS rectifier might be a better choice, thinking a bigger rectifier won't give any sag anyways, so...

Wow, maybe 10 watters are required in the HV taps. Better safe than sorry, of course! As to the value of the resistors -I figured I just have to build it and try some out...

I have a notion to try something like this with my AC4 C12. My "other" amp is also Vox-inspired and I love the EF-86 channel. It uses a Matchless-style six position rotary tone contour switch, which I also like - in addition to the different favor of tonal control, it also saves a panel spot compared to Treble/Bass.

I was also thinking solid state rectifier, for the same reasons you mention here.

My possibly foolish variant is to put a short tank reverb in it. I just love spring reverb.