Spite Build - Marshall JCM 800 2204

[JD01]

I saw one of mine and Shans favorite bands right before covid. The bassist had a fucking mighty sound! He went through his rig with me in a lot of detail afterwards: a P bass, a couple of pedals, an SVT into an 8x10 and a massive orange into a 4x12. It was glorious... I was deaf the next day.

[Greg_L]

Well the spite 2204 got used and abused last night for the first time and it is fucking awesome! Wow, so perfect. I love it. I'm thrilled with myself.



Seriously though it is exactly what I hoped it would be. A perfect 2204. Meaty and grindy but with amazing sensitivity and clarity. I'll probably get some tones up today.

[JD01]

Cool, glad its working out.

[einstein magoo]

Well the spite 2204 got used and abused last night for the first time and it is fucking awesome! Wow, so perfect. I love it. I'm thrilled with myself.



Seriously though it is exactly what I hoped it would be. A perfect 2204. Meaty and grindy but with amazing sensitivity and clarity. I'll probably get some tones up today.

Did you carry a spare head in the truck just...case if?

[Greg_L]

Did you carry a spare head in the truck just...case if?

No this was just band practice and there are other amps there. But I had full faith that it was gonna work.

At gigs I do always bring a second head for backup.

[Lt. Bob]

[Tadpui]

Nice! And congrats on another awesome build!

[rayc]

Well done.

[Greg_L]

Thanks gents. It's a ripper!

[CrowsofFritz]

Noiiiice. I didn’t realize you had so many Marshall Amp heads Jesus Christ!

[Greg_L]

Noiiiice. I didn’t realize you had so many Marshall Amp heads Jesus Christ!

Yeah I have a problem.

[Greg_L]

so sorta like a bias current in a stereo amp that the signal rides on top of

Yeah, I suppose.

Here is an expertly crafted visual aid. The main thing to understand is that AC and DC can live in the same wire.

Let's pretend the red wave is the 6.3v AC filament voltage riding on a 0v reference. With the transformer's filament winding center tap grounded, that center tap provides a 0v reference.
heater 6.3v - 0.jpg

So what can be done to reduce heater hum getting into stuff, sometimes an elevated voltage reference can be created and that's where the heater voltage gets it's reference point. It's very common in really high gain amps. What you do is take a feed somewhere off of the high voltage B+ supply and create a voltage divider network which is really just two resistors. Without getting into all the math, you choose two resistors that will yield the voltage you want - say 50 volts DC. At this 50v point you created is where the filament center tap will be attached. Now instead of riding on 0 volts, the heater voltage will be riding "elevated" on a 50v DC reference point. It's still 6.3 v AC for the filaments, but it's not referenced to 0v ground anymore. It's riding up above everything on a 50v reference.
heater 6.3v - 50.jpg

This is a very loose and sloppy generalization, but it's the basic point.

Just an update...I went and did this today ^^^^^^

I created an elevated voltage reference for the filament center tap. The amp has been rock solid, sounds awesome, and I'm using the hell out of it. It's become my #1 amp. I truly love it. It's so good.

Since it is getting used and abused so much I wanted to do this preventative mod to protect the tubes....mainly V2. Without getting too far into the technical weeds....sometimes some tubes don't like their life in the V2b cathode follower position. That's a rough spot in these amps....or any amp, really, that uses a 12AX7 as a cathode follower. The elevated heater reference is just some insurance to protect the tube in that position. And as a side benefit, it makes the amp less susceptible to heater hum. So it's all good.

[rayc]

Je ne comprends pas BUT I believe you.

[Greg_L]

Je ne comprends pas BUT I believe you.

Lol it's funny...every single time I take this amp out, which is pretty much every time I gig, other guitar players start barraging me with questions about it. I've even had my own bandmates come up to me, "Hey Greg, so-and-so from so-and-so band is looking for you to talk about your amp". I have to explain, it is just a Marshall 2204. I didn't invent this thing. I did optimize it though. I tell them it's story, why it exists, and how I built it out of spite and rage and anger. That's why it sounds so good. Cook food with love, build amps with hate.

I have about a half dozen "orders" from people for me to build them one. Lots of talks and phone numbers and hand shake deals. I tell them all the same thing...show me some $$$$$. That part hasn't happened yet.

[Lt. Bob]

Somehow I missed this .....
When I get a minute I'll read the thread but I already know the quality of teh gerg's work so I know it's awesome.

[WhiskeyJack]

That is awesome man. I don't understand the language but i get the jist of what you are going after. very cool. even cooler that it is getting notcied and generating a good buzz. You can't rewire a good buzz mi hermano

[vomitHatSteve]

So am I understanding this that by raising the ambient DC voltage on some of the wires that carry AC current as their actual signal, you reduce hum/buzz?
And this is non-standard, so your Marshall sounds way quieter than normal when muted.

[Greg_L]

So am I understanding this that by raising the ambient DC voltage on some of the wires that carry AC current as their actual signal, you reduce hum/buzz?
And this is non-standard, so your Marshall sounds way quieter than normal when muted.

Yeah that's sort of how it goes.

This is a generic power transformer and it's windings

pt.jpg

The green box is the filament (heater) winding. It is a center tapped 6.3VAC winding. The white/brown wire is the center tap, the two brown wires on either side of it carry 3.15v each for the total of 6.3 VAC. It is what lights up the tubes and gets them warmed up to operate.

That white/brown wire center tap is usually grounded somewhere to provide the balance point for the two halves of that winding. That puts the center of that winding at 0 volts and the 3.15v arms of that winding swing around that center tapped 0v reference. That's fine, no problem. It works great. But you know what else grounds at 0v reference? Everything else. The entire circuit and all of the little sub-circuits are all trying to climax at the same time right into that 0v ground hole. It gets messy! What can we do? We can get that center tap off of 0v and "elevate" it to some other DC voltage. The winding will still only make 6.3 VAC but instead of pivoting around a 0v reference, we'll move it up to like 40 VDC. That gets the heater AC voltage up and away from the delicate stuff that we actually want to hear and actually needs a 0v ground reference. Nothing really changes, the tubes still only see the 6.3VAC for the heaters, but that noisy heater voltage is now riding up on a 40VDC elevated highway instead of the surface streets with all the other people.

Now the question is...where do you just find a 40v spot to attach the filament winding center tap? Well you make one. Ideally it will be a clean, filtered spot with enough DC present to make a little connection point for the center tap. I used the screen supply filter node which has around 440 volts DC. I created a 220k/20k voltage divider. That gave me a nice little 39ish VDC spot to connect the filament center tap. I added a little filter cap to the voltage divider for extra smoothing and it worked great! Like this...


The green wire is the filament center tap and it's going to the junction of those two resistors coming off a high voltage node at the big filter cap. The junction of those resistors is the voltage divider and it sits at 39 VDC. Now I gots the elevated heaters. This is pretty common practice on modern high gain amps that are super sensitive and susceptible to lots of noise and hiss. Some modern amps even use a dedicated DC heater supply for the first few tubes. But the old style amps, like mine, don't have any of that.

But more importantly...now the cathode follower tube will not arc from heater to grid or heater to cathode. That's a whole 'nother explanation and it's actually the main reason to do this little tweak. The reduced heater noise potential is a side benefit.

[rayc]

On topic @ 1.02min.

[Lt. Bob]

Very nice explanation sir!

I am seeing a little excess burnt flux on some of those solder joints though.