Vox AC4C1 schematic and mod ideas

[PurpleStrat]

Would it cause any issue if I use a 1uf 450v in place of the 10uf 350v caps for testing? I also have 22uf 450v but they are radial and if I remember right you should not use a bigger cap after a smaller one in the power section.

 

[Ike286]

1 is kinda small, might not work as well. I would say just make a stop to the shack and see if they have a 10uf or two. And it's ok if it's radial, just make sure the leads will reach. Only problem, is I'm not sure about bigger after smaller... google it.

 

[PurpleStrat]

I'm gonnq have to wait for the power supply caps till I have a few bucks or can trade some parts.

I changed the 220k plate resistor to carbon film and with the 100ks already changed to carbon comp I am a big beliver in not using metal film for plate resistors. I also changed the first two 22uf cathode caps out and got no change. The fizzy decay lives on!

 

[Cleeve]

I listened to the fizz clip from the link and stared at the schematic a while.
I see a potential source of trouble with the cathode follower not having a stopper. As shown on the schematic, the plate of the second gain stage is directly connected to the cathode follower's grid, which is not a big deal as long as the grounded cathode stage feeding the cathode follower is near quiescent or otherwise not being driven hard.
I wonder if the grid of the cathode follower is being driven high enough on half cycles of signal to be into diode-blocking?
A way to check is to insert a 100k resistor between said gain stage plate and the cathode follower grid.

Another thing related to the cathode follower is the heater-cathode voltage.
It could be that the positive excursions of the cathode relative to the chassis-grounded heater exceed the heater's oxide coating insulation's ability to insulate, causing intermittent leakage from cathode to heater.

To check for that, try floating the heater winding?

 

[Wally]

RE: this 'fizz' trailer.... Have you run this amp through more than one sepaekr. A speaker can yield that type of problem, as well. Lsat year I bought a Super Champ that had a brand new Chinese-built Celestino G10 80th Anniverasary speaker in it that had that trailing fizz....it wasn't worth putting in one of my guitar amps.
You might have caught on to my method of analysis/diagnosis of problems by now. I eliminate the simple stuff first. ...tubes, speaker....before delving into the circuit and those PCB-mounted components.

 

[PurpleStrat]

I'll check out the ideas above soon and as for the easy stuff it's my fault I have not listed what I have ruled out.

Here is some more detail.

I have tried it through two other speakers, two difrent guitars both humbuker, multiple cords and multible tubes.

I have been using a PRS SE with a PRS#7 humbuker thats around 8.5k. The fizz is worse as the gain goes up and worse on bass notes. It really shows up with hitting the open E and A together and hates the G cord. Power cords don't sound as bad. At high gain it's bad but mixes in with the overdrive pretty well accept when hitting the notes I listed above.

I have done tons of research and tried many things and at this point the things I think it could be are "Ringing" in the OPT or the power caps. I was planning on getting a new OPT when I can anyway but can't test that yet and need some new caps to check the old ones.

I have read about the ringing in the OPT a bit and even see you can buy snubber kits but they are for push pull amps. Anyone have any info on using a snubber circuit in an EL84 single ended amp?

My modded schematic

 

[Ike286]

I think I asked once, but I don't remember. I assume this fizz wasn't around when you bought the amp, so it either had some kind of mini breakdown, or it was a mod... So you might want to think about when it may have come around.

Also, you say the fizz is worst at bass notes... Did you try replacing the tonestack bass and mid caps? (mid cap is the other cap that's not the bass cap or treble cap... lol) They say mallory on them so I assume you replaced them, but maybe one of those is the problem.. you never know.

 

[PurpleStrat]

It was always there but I tended to play clean or with the gain all the way and the treble of the bright caps helped make it blend better. It wasn't till I tried playing around with the mid gain tones that it really started to sound like a problem instead of just the amps character. Also it comes out the harder you strum.

 

[Ike286]

Oh ok... You should try to find some other guys with these, and listen to videos and see if theirs do it too... Otherwise check lead dress...

This article is great if I remember correctly..

http://www.geofex.com/article_folders/lead_dress/lead_dress_in_tube_amps.htm

 

[PurpleStrat]

I listened to the fizz clip from the link and stared at the schematic a while.
I see a potential source of trouble with the cathode follower not having a stopper. As shown on the schematic, the plate of the second gain stage is directly connected to the cathode follower's grid, which is not a big deal as long as the grounded cathode stage feeding the cathode follower is near quiescent or otherwise not being driven hard.
I wonder if the grid of the cathode follower is being driven high enough on half cycles of signal to be into diode-blocking?
A way to check is to insert a 100k resistor between said gain stage plate and the cathode follower grid.

Another thing related to the cathode follower is the heater-cathode voltage.
It could be that the positive excursions of the cathode relative to the chassis-grounded heater exceed the heater's oxide coating insulation's ability to insulate, causing intermittent leakage from cathode to heater.

To check for that, try floating the heater winding?

The cathode follower looks just like the one in any Top Boost. Is there something you see about this amp that would make any o what you said more of a concern?

 

[Cleeve]

The cathode follower's voltage exceeding the maximum heater-cathode insulation rating is not specific to the little Vox, certain Fender and many Marshall have similar arrangements, all designed and tested using western tubes of the day. I don't know for sure that the fizz is being caused by that, I can't get my ac4 to do that, but i may need to try different preamp tubes. The fizz from the clips sounds like something intermittently arcing to me, and the first thing that jumps out at me when looking at the schematic is the cathode follower. The heater is crammed inside of the cathode and the only insulation is an oxide layer. Google "marshall cathode follower heater cathode" and see the woes some are having with some preamp tubes but not others standing up to the high cathode-heater voltage. I'm not trying to be right or anything, just letting you know what I'd be looking at if my vox was making that noise.

 

[PurpleStrat]

Do you have an AC4C1?

I'm not questioning your suggestion as you obviously know your stuff I was just wondering if you saw something different in this circuit that would make the cathode follower more suspect. I have been using a combo of a reissue Tung Sol in v1 and a JJECC83S in V2. I have also mixed it up with a EHX 12ax7 and the Chinese tubes but the combo above seemed best but the fizz is there no mater what. It's the second half of the cathode follower that is the issue though right?

 

[Cleeve]

Mine is the weak TV model, so that tube with the cathode follower is not in mine anyway- no wonder!
If you swapped tube brands and the fizz stayed the same then it's probably not the heater-cathode thing.

Hmmmm

 

[PurpleStrat]

Thanks for the heads up on the cathode follower issue. I don't think it has any thing to do with my issues but I had ever heard of this but now I'm reading about it a bit. Love learning something new!

 

[Cleeve]

Seeing how much overall gain the top boosted version has, I'm now thinking it may be a ring as you suspected earlier.
Something as simple as a lead wire being nudged over a little one way or the other can create and destroy ringing in some amps.
If possible make very sure the speaker wiring in the chassis is twisted and routed well- i think the rule is you want the heavy current wires such as heater and speaker to be against the chassis metal, and control grid wires as far from the chassis metal as possible..
Also scrutinize the grounding- I think vox at least makes an attempt to observe "star" grounding, which is a good thing.

 

[PurpleStrat]

Seeing how much overall gain the top boosted version has, I'm now thinking it may be a ring as you suspected earlier.
Something as simple as a lead wire being nudged over a little one way or the other can create and destroy ringing in some amps.
If possible make very sure the speaker wiring in the chassis is twisted and routed well- i think the rule is you want the heavy current wires such as heater and speaker to be against the chassis metal, and control grid wires as far from the chassis metal as possible..
Also scrutinize the grounding- I think vox at least makes an attempt to observe "star" grounding, which is a good thing.

I did get a chance yesterday to play two AC4C1's at Guitar Center yesterday and noticed that with humbucker guitars they have the fizzy decay as well. Did not notice it with a Tele but maybe that's because they had lower output pups. I also played a couple AC15's on the Top Boost channel and they did not have it but I was playing only a Tele through them. The thing that really struck me was the amount of bass in the AC15! Floor shaking bass!

That seems to be the other big issue with this AC4C1 is that it does not handle bass well at all. Even when I have done mods like raising the first coupling cap to .022uf it just turned to mush and that was through a 12" V30. Something in this circuit or the transformers is holding it back. I know Vox intended for it to be a bright amp and I am starting to wonder if they choose an output transformer with that in mind.

Is there any other reason that anyone can see in the circuit that would keep it from letting bass through? Something about the power transformer? I really don't know much about them. I know it's not the speaker since I have tried others and I never except the idea that an EL34 amp can't have bass because I have had many that had tons.

 

[Cleeve]

The lack of bass from those little vox 4 watt amps is due to the single tube output.
The ac15 has a push pull output circuit (should probably be called pull-pull).
The output transformer in the ac4 has to deal with a magnetic field partially saturating the iron, where the ac15 has the magnetic field cancelled by it being a center tapped winding, so more variation of magnet field is able to be devoted to the signal, instead of half or more of the capacity being wasted.

The only way to fix it within reason is to do some semi extensive modding- pretty much converting the circuit to an ac15!

 

[PurpleStrat]

Not looking to do all that! I have had a Valve Jr. that I put a Weber WSE15 transformer in and it had tons of bass. Also the newer Ampeg single ender the GVT5 has only a normal sized transformer in it and the bass from the baxandall tone stack is MASSIVE. I actually have one I'm selling right now so I know it's possible to have bass in a single ended amp.

I'm at the point that I really believe it has to be the OPT and want to change it eventually when I have the cash.

I'm just wondering if there is anything else that could rob it of bass? Could the power transformer have anything to do with it? Could it be the bias of the EL84 or something like that?

 

[Cleeve]

You could disconnect the tone stack to see if it's sapping your bass strength.
Disconnect either end of that 10k resistor shown near the bass control on the schematic and turn the bass all the way up.
I guess you don't have an oscilloscope and signal generator. You're missing out if not!

Ages ago i built an amp based on vox ideas, a friend found an old radio with a couple of good transformers and i went from there.
I remember messing with cap values and that 10k resistor for quite some time.
Other fun things with vox circuits- see if you can find a schematic of an ac30 top boost and look at the "cut" control. It's the simplest but most effective control ever!
Ok maybe the volume control has it beat- but the cut control rounds off the harsh corners happening to the waves are they go into diode clipping when the power tube is pushed beyond reason, which is the very reason for a small amp!
I have not tried a cut on a single ended amp, so maybe it won't be as effective, since only positive going half cycles are due to get clipped by driving the grid of the power tube higher than the cathode.

Another super easy swell thing to try is triode mode for the output tube. Simply apply your safety skills to prevent being shocked by leftover charge in the power supply and locate the 470 ohm resistor that connects the power pentode's screen to the B+ supply. Disconnect the end of the resistor from B+ and connect it to the plate connection of the pentode. I demonstrated this to the guy for whom i was building the trash can amp and he liked it, so I had to scrounge up a switch that could deal with the voltage and make it a feature. Easy to find now, back then pre-internet, whew.

In triode mode the output impedance of the tube is much lower, and the transfer characteristic is well, that of a triode instead of the transistor like pentode curve.
The input capacitance is greater though, so the treble will be attenuated for a given tone setting, making you jones for the bright cap, but f you dime the volume the bright cap wouldn't have mattered. Also there is less power in triode, but still cool..

As to bias, I haven't messed with my Ac4 yet, but I did go through another 5 watt el84 rig, the crate v5. I found it biased hopelessly cold (too high value for cathode resistor) along with other problems. I did a thread about it here somewhere long ago, now missing it's pictures.
To bias that amp, i used alligator clip leads to parallel various resistors across the cathode resistor of the power tube.
The bias voltage decreases as the value of the cathode resistor combination is decreased, causing increased idle current. The sound absolutely gets better as the bias is made hotter (smaller cathode resistance), but so does hum and if you go too far, the screen starts to glow red.
Once you find the magic combo of resistance that has the screen not glowing when viewed in a darkened room, and you can live with the hum, you can either replace the single cathode resistor with the ohm value of the parallel combination, or add a resistor in parallel to make the value, solder the thing on top of the original if you want, that way the original is still there for the future.

With no feedback, hum is gonna be a problem with this amp, but I find with single coils, if I stand just right, the two wrongs sometimes make a right.

 

[PurpleStrat]

I've tried triode mode on a couple amps and never like the tone much. It might be a good idea just to see if it effects the fizz. I tried lifting the tone stack already and it did not help the fizz but did give some extra sounds. I plan to put in a toggle switch for that and for the bright caps after I get the OPT changed and get the sound where I want it. No use in putting them in now or it would be extra stuff that to unhook every time I work on it.

I have a lot to learn and am ashamed to say I know very little about biasing tubes. I need to do some home work. Being that it's a Vox though I am guessing it's going to be biased high.