AC4TV - Circuit board differences and mod question.

[stonerose]

IMG_6118.jpg

Hi Everyone I've recently started to look into mods for this amp, this is my 1st tube amp mod.

after a bit of research I've came across the following mods

C5 - Increasing to 22nF to open up the low end
C4 - Increasing to 470pF to improve the range of the tone control
C15 - decrease to 220pF to "open up" the voicing of the amp

When researching theses mods I came across a refence for cap C6 = 22nf but upon inspection I've noticed My board has no C6 under the volume pot.
It does however have 22nf cap with no board number next to R12 and R8.

when checking circuit gut shots others has posted online I noticed My board was not the only one like this
and Some people had C6 and nothing next to R12 and R8 apart from a jumper wire.

so it appears there are 2 slightly different versions of the lay out of this board.

will the above mods still be applicable to my board which is missing C6 and the the unnamed 22nf cap added next to R12 and R8 ??
also why do you think they are different - have they just relocated C6?
Thank you

[ChopSauce]

I had a look at a schematic for this amp and there seem to be several things that didn't make much sense to me, including the points you reported.

That C6 seems pretty weird, notably and I wouldn't be troubled that my amp hasn't it.

If I were you, I think I'd go one mod at a time, starting with C4, as noted.

For C15, I'm not even sure there's something needed there, but I think usual values are closer to 47pF. Maybe you can start testing just C15 clipped?

I don't think 22nF for C5 is a good choice, also. It may make your amp sound muddy. By the way, C18 = 100nF seems to much, in contrast.

I'd rather increase C16 to something closer to a more common value, as is 22uF.

NB (as I seem to see isolated gloves) how about any discharging resistor?

[bepone]

this is great amp to throw everything out and build some fender champ type, with the new board using the same mounting holes, i did one in the past.

after changing of the speaker to some better,amp will be usable.

[ChopSauce]

Of course, we both know that we would rather gut it and build something else, whatever that is - even though an EL84 powered circuit would possibly be prefered - but that is not the typical "first tube amp mod" thing ...

Anyway, I was just passing to post the schematic I found for the amp. Is it the attached one which you (stonerose) refer to?

Vox_AC4TVH.jpg

[dorrisant]

FWI... Those rubber gloves are a false sense of security around high voltage circuits!

A cheaply made amp like this will cut them to shreds leaving you in danger. Learn safe practice instead of trying to outsmart danger.

Also, I agree with bepone. This amp would be fun to remove the PCB and replace it with a turret board. You could rebuild it as stock or mod it to your hearts content.

PM me if you'd like to go that route. I can help.

[stonerose]

Hey thanks for the info everyone ,

In regards to the mods- I like the sound of this amp and don't want to change it away from what it is, I would rather maintain the VOX sound , just slightly tweak it to be the best it can be.
This is my Kitchen amp , its the mini version so I also use it as a head connected to a 10 inch cab for recording.
The caps have been discharged using a resistor and checked with a meter. - you have to take the board out completely to discharge the caps.

I had a look at a schematic for this amp and there seem to be several things that didn't make much sense to me, including the points you reported.

That C6 seems pretty weird, notably and I wouldn't be troubled that my amp hasn't it.

If I were you, I think I'd go one mod at a time, starting with C4, as noted.

For C15, I'm not even sure there's something needed there, but I think usual values are closer to 47pF. Maybe you can start testing just C15 clipped?

I don't think 22nF for C5 is a good choice, also. It may make your amp sound muddy. By the way, C18 = 100nF seems to much, in contrast.

I'd rather increase C16 to something closer to a more common value, as is 22uF.

NB (as I seem to see isolated gloves) how about any discharging resistor?

ChopSauce - C15 was chosen due to this information found here - https://www.diyaudio.com/community/thre ... ct.314517/
" I used a frequency generator playing test tones through the amp as I alternated different capacitance values for this circuit - C15 was decreased from 470pf to 220pf; this is because the capacitor was actually draining all frequencies from about 1000-1500 and above. Now C15 only starts to drain frequencies above 2-3KHz. This has the efffect of "opening up" the voicing of the amp"

ChopSauce - I don't think 22nF for C5 is a good choice, also. It may make your amp sound muddy. By the way, C18 = 100nF seems to much, in contrast"

22nf was recommended by an amp tech who had worked on a few of these amps for customers.
"Cap C5 is unusually small, at 4.7nF. Upping it to 22nF opens up the low end nicely, without spoiling the trademark Vox chime. C5 is just to the right of the volume control, and a 400 volt Sozo mustard cap fits nicely, stood on end. Parts cost is only a couple of buck" - https://www.thegearpage.net/board/index ... 524/page-2

Of course, we both know that we would rather gut it and build something else, whatever that is - even though an EL84 powered circuit would possibly be prefered - but that is not the typical "first tube amp mod" thing ...

Anyway, I was just passing to post the schematic I found for the amp. Is it the attached one which you (stonerose) refer to?

Vox_AC4TVH.jpg

ChopSauce - yes that's the one , its better than the one I found but also differs as it has C6 at the volume pot as mentioned before , is there a better resolution version? I cant see what voltage the Red LED resistor, which I assume is R6 would be getting ? I'm going to swap with a purple LED so might need to change R6 which is currently 560 ohm

I have attached a pic of the back of the board ( I have removed some components)
"A" is where C6 = 22nf is installed on some board versions at the volume pot, but NOT on mine
and "B" is where I have the unnamed 22nf cap
Have they just moved C6 for some reason ?
I have also noticed that C2 in the diagram you provided is also missing on my board , but this seams to be common across the 2 apparent board versions

[ChopSauce]

You should be able to find the pdf as well as more competent feedback (including from member dorrisant, likely) on the EL34 World forum.

Have fun!, but ... manage to stay alive ...

NB (as I seem to see isolated gloves) how about any discharging resistor?

Learn safe practice instead of trying to outsmart danger.

(PS : As long as it doesn't harm someone else, I don't care much about what other people feel like doing or claim should be done, because to each one his taste ... )

[ChopSauce]

I'll take a few more minutes to answer you more precisely, so you possibly know better what I meant ...

"A" is where C6 = 22nf is installed on some board versions at the volume pot, but NOT on mine
and "B" is where I have the unnamed 22nf cap
Have they just moved C6 for some reason ?

Possibly something along the lines of what I have written before - out the top of my head: "C6 doesn't make much sense to me and I wouldn't bother with it" (in first approximation, as making too much mods make less sense than gutting the whole amp and start with a fresh & unobfuscated design).

Let me rephrase my point of view: What's the point in having a relatively "huge" (C15 ten times greater than usual) snubber cap to throw out high mids and above, for keeping the tone balanced after having cut so deeply (C16 twenty times smaller than usual) into the low mids and below?

How balanced the outcome, the tone will be poor - as far as my understanding goes - and there I stand waiting to be corrected ...

... but if anything I'd just change that(*) - in doubt - and see how that sounds.

Quite franckly: I don't trust much your sources. I can only recommend that you operate as I wrote, changing only one component as a time, testing the amp to see if that results as expected, managing to understand why in both cases and then operating another component, etc.

One step at a time, you risk less going nowhere - as you can step back more easily, plus you'll gain a (much) finer understanding of the mods ...

That doesn't preclude you finally gut the amp and build something else at all, as you'll be more advised at what you're doing.
____

(*) See my first post

[stonerose]

Hey,

Ok I will wire up some leads so I can swap cap values.
will try C4 as noted.
Probably wont bother with C15 as you suggested.
I will try C16 at 22nf - Any reason why C16 is electrolytic? can it be replaced with polyester/polypropylene or Mica?
So you recon I should keep C5 at 4.7nf ?
You mentioned C18 was to much at 100nf would some thing lower be more suitable or is that need to maintain the vox sound?
Thanks

[ChopSauce]

Well, afterwards I am not sure I would start messing with C4, plus I feel like I should have moderated myself. For example snubber cap values close to 500pF may not be so uncommon.

So, to start at the begining, blindly :

A typical value for C16 is 22uF, not 22nF. It indeed is an electrolytic. Using a smaller value (5 or 10 uF) would cut some lows, but there's already the 4n7 couping cap after that (1st) stage, which does the same job, so I would tend to rely on the coupling cap only and let the "full" signal be amplified firstly. Relying on some sort of dichotomic iterative method, C16 = 10uF (between 5 and 22uF) might be a fair first guess, in doubt(*) ?!.

Now, how does that sound?

NB : my next move would be towards C4 and C15 for I feel like C5 and C18 (for which a more common value would be between 22 and 47nF) shouldn't harm (much) in first approximation.

One more thing. If you want to check where components connect, with respect to the schematic, whatever their position on the PCB, it is as easy as trying to measure the resistance between poles. That way, you should be able to see where that 22nF connects.
_

(*) as I recommend you don't take my words as gospel and go manage to figure things by yourself (e.g.) by comparison with schematics for other small single ended amps.