20 watt JCM 2204

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joeboo88
Posts: 400
Joined: Thu Mar 28, 2013 8:51 pm
Location: Way up North

20 watt JCM 2204

Post by joeboo88 »

Hi all. I have a PT that's 275 0 275 6.3v @ 6amps and a 20watt 6.6k OT
I was wondering if I have to modify anything for 6v6s?
Less Filtering? Or the 5.6k grid resistors, ok? Smaller choke?
Thanks in advance, Joe
Badside
Posts: 90
Joined: Wed Mar 24, 2010 12:40 pm
Location: Montreal, Canada

Re: 20 watt JCM 2204

Post by Badside »

That sounds like a good set of iron for such a build.

I built mine using also a 20W 6k6 OT (a Hammond 1760H, aka a Deluxe Reverb OT) and a 300-0-300 PT, but I'm using the 125V primary tap to lower B+ (effectively I get about 385V on the plates).
My PT has a 173mA rating which is more than enough.

For the choke, I went with Hammond's Deluxe Reverb choke (194B?) which is rated for 40mA I believe (there's less than 20mA going through it IIRC).

The circuit is pretty much bone-stock 2204, I did change the PI to match the "Plexi 6V6" schematic floating around, because the 6V6s will require more drive than the EL34s (less gain). The cathode resistor is a 470R and the tail resistor a 10K. Before these changes it lacked headroom and it didn't feel like the power amp was really working (distortion was buzzier and lacking dynamic)

Some tweaking of the negative feedback loop is required too, in the end I preferred it with a massive 156k (a 56k + a 100k) resistor, but I can't remember if I tapped it off the 8 or 16 ohm output. Too much NFB neuters the amp.

Also, I found the amp to be a bit top-end heavy and lacking in low-end, so I upped the bass cap to 0.047uF in the tone stack, not a huge difference but gives it more range, and the treble cap is a 330pF. The slope resistor is a 56k. This part is up to personal preference. I do believe I used 5.6k grid stoppers, but it's tempting to put 8.2 or 10k in place, because of that tendency to be very bright.

In terms of filtering, I paralleled a dual 32uF cap can for the main reservoir cap, so 64uF (compared to 100uF on my 50W build). Amp would work fine with anywhere from 32 to 100uF as the main reservoir cap, it's up to personal preference. A 2204 is not a very saggy amp, but it felt to me like the typical 100uF would have been too much and I had the 32/32 cap can already so I went with that.
Got 50uF on the screens, 50uF on the PI (a from a 50/50 can) and 22uF on each preamp node (they fit nicely on the board).
You might want to use lower dropping resistors, especially from the screen supply to the PI supply, because of the lower voltage. Ideally you want to match the preamp voltages from a full sized 2204, so some tweaking is required (keep 4.7K, 5.6K, 6.8K, 8.2K and 10K 2-3W resistors on hand and experiment)

Note that I made mine cathode biased, which is an unusual choice for a 2204 inspired build, but since I wanted to keep volume down it made sense to me. I have a pretty big 470R cathode resistor bypassed by a massive 470uF cap. This biases it to about 85% (cathode bias usually lets you get away with 100%, but then it didn't sound like a 2204 anymore).
If I was to redo it again, I would use a lower voltage PT like yours and stick to fixed bias.

As is though, of all the builds I've done, it's the one that gets the most praise from other guitarists who hear or play it. It's a great gigging amp (good headroom but not stupidly loud) and also a great recording amp (very versatile and manageable volume). Sounds better in a 1X12 or 2X12 IMHO (in a 4X12 the speakers are barely moving).

Note that it will still be very loud, will hang with a drummer easily and impossible to crank in an apartment. But 20W is a great power point for gigging as you can get it working harder than a 50W would while still having enough headroom to play clean-ish. My 50W never gets past 9 o'clock on the volume dial, even on festival gigs. The 20W manages to go past 10 o'clock!

I've been working on a Superlead inspired 8W build (using 6K6GT power tubes) recently, that one is significantly quieter, needs to be mic'ed for the drummer to hear it, but still unusable at home if the wife is present (it doesn't have a master volume).
Amplifiers built:
Marshall 2204 head with some mods
Low-power 2204 (cathode biased 6V6s)
Single-knob dual-6K6GT amps using a Wattkins uPCB
AC30 clone with Plexified preamp section
AX84 Firefly
joeboo88
Posts: 400
Joined: Thu Mar 28, 2013 8:51 pm
Location: Way up North

Re: 20 watt JCM 2204

Post by joeboo88 »

Wow. Thanks for all the info, Badside. GLad someone did this and broke ground for me.
I will have lots to digest, and am still learning what you guys are talking about when you guys say stuff like "tail resistor". Lol
But I made a diagram of all the jargon for the different nodes of an amp.
Yeah 20 watts will be loud enough, I have a homemade badcat30 and an Express and a 15 watt el84 hybrid amp and Princeton, and I find I play the small amps more.
1 question : does the MV on the 2204 work good? Can I get raunch at low volumes?

I will keep you posted. I will be ordering a couple of chassis from
Seasidemusic up here in Canada.
Again thanks......Joe
Badside
Posts: 90
Joined: Wed Mar 24, 2010 12:40 pm
Location: Montreal, Canada

Re: 20 watt JCM 2204

Post by Badside »

joeboo88 wrote:Wow. Thanks for all the info, Badside. GLad someone did this and broke ground for me.
I will have lots to digest, and am still learning what you guys are talking about when you guys say stuff like "tail resistor". Lol
But I made a diagram of all the jargon for the different nodes of an amp.
Yeah 20 watts will be loud enough, I have a homemade badcat30 and an Express and a 15 watt el84 hybrid amp and Princeton, and I find I play the small amps more.
1 question : does the MV on the 2204 work good? Can I get raunch at low volumes?

I will keep you posted. I will be ordering a couple of chassis from
Seasidemusic up here in Canada.
Again thanks......Joe
Seaside Music, that's the name!!!! Thanks a ton, I bought 2 chassis from this guys years ago (dude even made me a custom chassis with a specific angle on the front to fit an old Koch combo cab I had) and I couldn't remember the name. Had to order from turretboards.com, which makes great chassis but after exchange rate, shipping to Canada and customs, cost me 130$CAD for a basic undrilled chassis!

Used a Hammond for another project, more affordable, but they're thin and I wouldn't trust them with heavier transformers (this was an 8W build so iron was small and light).

By the way, if you're not already aware, a1parts.ca has the bestest prices on Hammond iron

Regarding the MV, like a full size 2204 sure you can play quiet, it just sounds better when you crank it up. I tried a PPIMV on mine, and I really didn't care for it, just adds more PI distortion which I don't like that much (not as controlled and tight, so not what I'm trying to get from a 2204). So I stick with the regular MV.

I find that amp with more preamp gain work better at low volume than simpler amps, so the 2204 is a good candidate.

As for what a tail resistor is, let's take the Plexi 6V6 schematic for example:
http://mhuss.com/Plexi6V6/Plexi6V6c.gif

V3 is your PI, and the 10k resistor that leads to the Presence control is what I call the tail resistor. Now, it appears I remembered wrong, and 10k is the classic value for EL34 Marshalls as well (as is the 470R shared cathode resistor), so turns out I built it exactly like a full size 2204 except with smaller dropping resistors (the typical 10K 2W resistors between the nodes) in the power supply to counteract the lower voltage. The tweaks in the tone stack are more personal preference and my EL34 build has roughly the same tweaks.
Amplifiers built:
Marshall 2204 head with some mods
Low-power 2204 (cathode biased 6V6s)
Single-knob dual-6K6GT amps using a Wattkins uPCB
AC30 clone with Plexified preamp section
AX84 Firefly
joeboo88
Posts: 400
Joined: Thu Mar 28, 2013 8:51 pm
Location: Way up North

Re: 20 watt JCM 2204

Post by joeboo88 »

hey Badside:
I just ordered from Terry @seaside, 2 chassis with IEC cutouts, shipping and
chassis cost $85 cdn, I just ordered yesterday

I ordered 2 PT's form A1 parts a few months ago.
I ordered 2 Hammonds, 1 the 1 I mentioned 275 0 275 (270hx) and another one 272 hx - 300 0 300. I was going to use this one to build an orange retro 50, but Mark Fowler said he didn't really like the Orange he built.
but I might change my mind although I a have 3.4k 50 watt OT on hand
so I wanted to build one more 50 watter and a 20 watter.

I only play in my basement and only for me, but I would like 2 use up the iron I have, still not 100% sure what to build... of course.
Maybe I'll build a full 2204 and then something else? with the 20 watt stuff?
not sure. I have pairs of el34's 5881's el84's and 6v6's
looking for suggestions? Sometimes too many choices is too many choices

these 2 last amps will be my last...yeah right

Thanks for the info and the time
Badside
Posts: 90
Joined: Wed Mar 24, 2010 12:40 pm
Location: Montreal, Canada

Re: 20 watt JCM 2204

Post by Badside »

Those are some big transformers! I only used a 272FX for my 20W 2204 and it's already a bit overkill with its 173mA current handling (it pulls maybe 115-120mA at full tilt).

The HX though would allow you to build something I've been thinking about for a while though: cathode-biased EL34s!
Per the datasheet, at 385V into 4000ohms in cathode bias is good for about 35W, full on it will pull 175mA IIRC + 10-15mA for the preamp.
The 272HX on the 125V primary tap will give you 375-380V, the 270HX on the 115V primary tap will give you something similar (I know from having a dual voltage switch for the 272FX on my 6V6 2204).
The great thing is you can swap the EL34s for 6V6, plug your cabinet into the lower impedance tap (a 16 ohm into the 8 ohm tap, or 8 into 4) to reflect double the load and boom you now have 15-20W to work with (needs a bigger cathode bias resistor though).
This actually works with fixed bias as well, just have to rebias (and make sure to increase bias voltage noticeably before you switch back to 6V6), that might bring it to 40W-ish with EL34s.

Your OT at 3.4Kohms is a bit on the low side for this (recommended is 4K), but it's close enough. You can always bias just a little colder (like 80-90%) to compensate.
This gives you the option to run "real" Marshall tubes for a more authentic sound. I love the 6V6, but it doesn't have the same midrange as the EL34.

Or look at it this way: if you build a 20W 2204 with one of those two PTs and use your 50W with the cabinet plugged into the lower impedance tap to reflect 6.8Kohms, you have the option of using EL34s with a rebias (as long as you connect Pin 1 and use big enough screen resistors). It will be louder, but not as loud as a 50W 2204 because of the lower voltage.
Amplifiers built:
Marshall 2204 head with some mods
Low-power 2204 (cathode biased 6V6s)
Single-knob dual-6K6GT amps using a Wattkins uPCB
AC30 clone with Plexified preamp section
AX84 Firefly
User avatar
Littlewyan
Posts: 1911
Joined: Thu Sep 12, 2013 6:50 pm
Location: UK

Re: 20 watt JCM 2204

Post by Littlewyan »

I turned a Pignose G40V into a JCM800.

http://ampgarage.com/forum/viewtopic.php?t=29375

Here is the thread for it.

To summarise I used an 8K OT with a PT that gives me 340VDC. The output is 10W with EH6V6s. It has a 1K resistor instead of a choke, 56K feedback resistor on the 8Ohm tap and 50uF Filtering across the board. Sounds great.

The reason for less feedback is there is much less output and so less voltage going back as NFB. So I had to lower the resistor and change tap to compensate. With these values it has roughly the same NFB as a JCM800 with a 100K resistor on the 4Ohm tap.

I could probably squeeze a bit more output from it by changing the 1K resistor to 100Ohms and using JJ 6V6s (I think the EH6V6s I have are a tad weak) but 10W is pretty loud. Loud enough for band practice anyway.
joeboo88
Posts: 400
Joined: Thu Mar 28, 2013 8:51 pm
Location: Way up North

Re: 20 watt JCM 2204

Post by joeboo88 »

Thanks Badside for the info, I will think about this for a bit.
My badcat build is cathode biased, and it does everything I want in an amp. With the 2 channels, and my homemade a/b box the amp does it all for me.

Thanks Ryan, I will look at your build thread. Good thing is I have lots of info
I also just built a 6g3, that I'm trying to tweak.

Thanks again guys
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