Thread by @Nash076

New project time!

A long while back, I picked up one of these: a Vox AC4TV. Little 4 watt amp with an attenuator and a 10" speaker.

And it was just ... bad. Boxy sound, broke up way to fast, too much high end.

So I resolved to make it better.

The first thing I tried was chasing it from a closed-back cabinet to an open back, by cutting some of the material off.

That helped a little, but it was still way too bright.

It also had some nasty hum.

Part of the problem is, well ... it's a very cheap PCB-based tube amp. Sometimes you can do those well with no issues, but this thing was way too compact. It needed a revamp.

I tried a lot of things, including moving the transformers, shielding components, etc, etc. Nothing worked to tame the noise, and it still sounded wrong.

I tried converting the schematic into point to point wiring, but it never worked.

Finally I got fed up and just turned it into a Blackface Champ clone.

But now that I've re-housed that amp in a proper cabinet, I have what's left of the AC4TV ... and I still want a British, Vox-y sounding recording amp.

I stumbled upon this: an upgrade kit from Mercury Magnetics. New transformers and major circuit redesign. Promised Vox tone while taming the amp.

$345.

Noooooooope.

But ... maybe I could do it myself?

I found copies of the instructions online, which contained both the original and modified schematics.

So from that, I came up with this. I'll be keeping the transformers, but I'll be using a better speaker I had laying around.

The circuit's fairly simple (simpler than the Champ I just made, even).

It does have a *ridiculous* amount of power filtering, though. With luck that'll mean it'll come out much quieter.

Time to get started!

Step one: circuit board.

I still had the rest of my bargain basement phenolic sheet, so I cut a new circuit board, drilled it and installed turrets.

And yes, I know the cuts are rough as hell. I'm using a dremel FFS.

Going for function, not form.

Next I need to retrofit the retrofit.

When I put the Champ circuit in the cabinet, I added a bigger metal chassis that could be enclosed for shielding.

It needed 3 tubes, but now I'm only going to be using 2. Also, they're both the smaller 9 pin style so I needed to fix a hole.

And yes, I realize it's ugly as sin but it's going to be inside the cabinet. It'll be fine.

Next I'll populate the circuit board. BBL ...

All right, circuit board's populated, and ... hmm.

I'm 90% sure I translated the schematic properly, and there's three additional resistors I'm going to mount directly to the tube sockets and one more capacitor that's going on the volume control, but ...

That's a tiny circuit.

Everything on the left is power stuff. Rectifier (silicon, not tube) and power filtering. The right hand side is the main "amplifier" itself.

I mean, compare it to my Champ board. I'm kind of confuzzled.

I'm fairly certain a good portion of it comes from the fact I'm using better rated components than the original PCB.

Despite being very busy, this is also a pretty simple circuit.

Ran into a problem: with the attenuator that comes in the amp, I have to have a 16 ohm speaker. Anything else could damage the output transformer.

So I can use my spare speaker. I *have* to use the stock one.

Phooey.

GOOD NEWS EVERYONE

I came up with a workaround.

I'll try to explain without taking a year:

The output transformer that sends the signal to the speaker needs a load. It's kind of like a counterbalance for a ship, only in this case it's measured in resistance (Ohms).

If the speaker's resistance matches what the output transformer is designed to expect, everything's golden.

But a mismatch is a problem. It can work for a time, but if you hook an 8 ohm speaker to a 16 ohm output transformer or something, you're unbalancing the load.

Eventually this can damage the output transformer which can end up stupid expensive.

The problem here is, the speaker that came with the amp suuuuuck. It sucks donkey dick until the donkey dies then it keeps on sucking.

Now I have a slightly better speaker, but it's rated at 4 ohms. The transformer that came with the amp is rated for 16 ohms. Mismatch, imbalance.

Solution? I have a low powered 4 ohm output transformer that should work.

BUT THERE'S A COMPLICATION

This is a simple attenuator that came with the amplifier. It's a circuit that lets you dump some of the amp's power into heat, making the amp quieter overall for when you don't need full ear-splitting.

But it's part of the transformer/speaker system, and thus part of the load.

It was originally designed to fit into a 16 ohm transformer/speaker match. Moving to a 4 ohm setup would cause a mismatch in the attenuator, or as I like to call it, "OH SHIT FIRE."

Solution? Re-balance the load.

Fortunately @KHMakerD stepped in and did the math, and told me what resistors to replace to sort it out.

Trouble is, I didn't have the right resistors. But here's another neat electrical trick for you: wiring resistors in parallel.

If you put resistors end-to-end, they're cumulative. 2 ohm plus 2 ohm = 4 ohms. You increase the resistance.

But if you wire two resistors in parallels, you cut their resistance in half. 2 ohm and 2 ohm = 1 ohm.

You also increase their power rating to handle more wattage.

I didn't have the 45 ohm 3 watt resistors I would need, so ... I cludged some.

Three 220 ohm resistors and one 120 ohm resistor, all 1 Watt, wired in parallel = 46 ohm, 4 watt resistor. Attenuator is ready to go.

SUCK IT, ELECTRICITY

And finally, here are the transformers mounted on the steel backplate I added to the internal rear panel of the amp for just this purpose.

The original chassis was so small, this gives me more room to properly orient the components.

It's not the best output transformer, but it's the right rating and it sounds okay. It'll do just fine.

The next hitch is one of the tube sockets I had was used and old and finally gave up the ghost. Had to order a replacement.

$10 and it won't be here until Wed.

(grumble)

Okay, yes, this is more accurate.

https://twitter.com/mithrasangel/status/1318050995604107265

Done some more odds and ends while I'm waiting.

So this part here is the stock chassis. This was the entirety of what held the amp, the transformers, the circuit board, and the dog.

... the dog?

My idea was to add this enclosure to the unit, for a number of reasons.

One, it allows me to build in a more Fender-like layout, which is good for accessibility, upgrades, repair and reducing noise.

Two, I can reverse just about all of the changes I've made and put the entire thing back to stock should I ever want to for some reason.

And three, I got so fed up with the damn thing I went overkill to try to chase out the bugs and noise.

Here's where we are so far. Compare it to the stock layout.

I have more room to place the transformers, I can move them further from the signal path and I can isolate them from the circuit by closing it up, forming a shield.

Again, very like a Fender Amp.

Also, here's what it looks like from the back.

Compare it to a stock unit.

From the outside, you can't even tell any changes have been made just by looking.

So far, I've wired up the power inlet and the pilot light.

This is pretty much all it can do for the moment.

Just waiting for that damn tube socket to arrive ...

It's a nice idea and I've considered it, but it would require a lot more modification and retrofitting, plus additional parts.

I'm pretty much just optimizing and bulletproofing what I already have.

https://twitter.com/BleatsSinodai/status/1318197777868988416