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.
AC4 project
Moderators: pompeiisneaks, Colossal
AC4 project
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Re: AC4 project
Here's a link to the same project on TDPRI: https://www.tdpri.com/threads/ac4-proje ... st-9627792
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martin manning
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Re: AC4 project
Resistors in line with the rectifiers will simulate the voltage drop and (nearly) the sag of the EZ81. Looking at its data sheet, I think 370 ohm would be a closer match. Those will dissipate a bit of power.Just guessing I'd make them 5W. You do have some opportunity to adjust the plate voltage with the 1k resistor between the first two caps. I would add a small screen resistor 100Ω to 470Ω say to prevent oscillation, and that will lower the screen voltage re the plate a bit.
Re: AC4 project
Thanks for the suggestions, I've made a revision with some changes...
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Re: AC4 project
As drawn the pentode volume control will also control the volume of the triode preamp. It's common to use two mixing resistors when connecting the outputs of two preamps together. 220K is a common value.
Re: AC4 project
That’s right... it will work a bit like the Tweed Deluxe volume for the ECC83 channel. Maybe that’s a good thing though... 
But the mixing resistors is probably better for this one. Maybe I should skip the 5k6 one (R16) then, not that it would make any noticeable difference in resistance...
But the mixing resistors is probably better for this one. Maybe I should skip the 5k6 one (R16) then, not that it would make any noticeable difference in resistance...
Re: AC4 project
With the mixing resistors added a new problem arises:
I guess that with 220k mixing resistors the value of R16 has to be at least doubled, otherwise half the signal would just bleed to ground.
I guess that with 220k mixing resistors the value of R16 has to be at least doubled, otherwise half the signal would just bleed to ground.
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martin manning
- Posts: 13207
- Joined: Sun Jul 06, 2008 12:43 am
- Location: 39°06' N 84°30' W
Re: AC4 project
That resistor wants to go from the EL84 grid to ground. The data sheet says 300k max, but you might get away with a bit more.
Re: AC4 project
Aah!
It should be like this then:
It should be like this then:
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Re: AC4 project
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.
(EDIT: Layout corrected.)
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.
(EDIT: Layout corrected.)
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Re: AC4 project
Board almost ready for soldering. Ruined some eyelets in the process of seating them, so I have to order some more...
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Re: AC4 project
Put the amp together yesterday evening:
So now I just have to get some blue and/or cream Tolex...
It sounds great, different from the Top Boost circuit that was in it before. The switch selecting different caps for the EF86 screen grid works fine, cutting the bass just enough when switched to the smaller cap.
The different channels have a distinctly different sound, but the problem is that the first channel (EF86) has way less gain than the second (ECC83), maybe something like 10dB less output when maxed. And with my Tele bridge pickup (7,1k A5) the first channel won't go into overdrive even when dimed, but the second channel starts overdriving with the volume about halfway up.
When I hit the first channel with a clean boost pedal I get the most lovely, aggressive and biting overdrive out of it, so it can get there, just too little gain from start.
The EF86 has an amplification factor of 185, and the ECC83 at 100, for each side... so the difference is not very strange perhaps.
Here are some voltage measurements (voltages from original/other schematics in parenthesis):
An idea I've had is to remove the R1 resistor to increase the gain, maybe that would do some difference?
Another idea is lowering the value of the R4 and even more so the R5 resistor...
So now I just have to get some blue and/or cream Tolex...
It sounds great, different from the Top Boost circuit that was in it before. The switch selecting different caps for the EF86 screen grid works fine, cutting the bass just enough when switched to the smaller cap.
The different channels have a distinctly different sound, but the problem is that the first channel (EF86) has way less gain than the second (ECC83), maybe something like 10dB less output when maxed. And with my Tele bridge pickup (7,1k A5) the first channel won't go into overdrive even when dimed, but the second channel starts overdriving with the volume about halfway up.
When I hit the first channel with a clean boost pedal I get the most lovely, aggressive and biting overdrive out of it, so it can get there, just too little gain from start.
The EF86 has an amplification factor of 185, and the ECC83 at 100, for each side... so the difference is not very strange perhaps.
Here are some voltage measurements (voltages from original/other schematics in parenthesis):
An idea I've had is to remove the R1 resistor to increase the gain, maybe that would do some difference?
Another idea is lowering the value of the R4 and even more so the R5 resistor...
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Re: AC4 project
Very nicely done and excellent job on the documentation too! Looks like a fun amp. What EF86 did you use?
Re: AC4 project
The EF86 is an Elektro-Harmonics tube.
Re: AC4 project
I have a question about this arrangement with the caps and resistors.
I wonder why it's there, and what it will mean for the mixing of channels before V2b instead of after.
I wonder why it's there, and what it will mean for the mixing of channels before V2b instead of after.
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