50W plexi build questions

[Bieworm]

Hey y'all

Somebody asked me to build him a 50W plexi type amp to use in his AC/DC coverband.
He plays a TSL 100W and that is not the sound he's after. He's the Malcolm Young of the band , not Angus.
Been looking into the type of amps mostly used by Malcolm.. turns out he plays on a super bass. Now I'm thinking of building a 1987 or a 2204. The 2204 has a pre PIMV , the 1987 has none. Is it a crazy idea to add a pre PIMV and a PPIMV on 1 amp? Should I go for the 2204 and add a PPIMV ?  Or is it a better Idea to ho for the 1987 with some MV?
Anybody ever did a VVR on this kind of amp?
He's doing clubs AND larger stages...
Thanx for you input

B

[dude]

I'd go with 2204, as is. The normal input gets an extra gain stage, the Bright input is like a 1987 but not as clean, can jump the inputs too. the pre MV is good on the 2204, IMO. Do a 6L6 version of the 2204, maybe.

[Willabe]

Is it a crazy idea to add a pre PIMV and a PPIMV on 1 amp?

Anybody ever did a VVR on this kind of amp?

With a PPIMV you can get the PI to distort. With a PIMV only the preamp tubes distort. So, do you like the addition of the PI distortion or are the preamps tubes enough? Little bit different sound, not just more distortion.   

I  ~think~  some guys use both a PIMV and a PPIMV with power scaling. Gives you options.

[Bieworm]

I try to avoid fizz with attenuating. Isn't the VVR the way to preserve the most of the amp's original tone? But I know VVR works great in cathode biased amps.. Isn't fixed bias more complex?

[Auke Jolman]

It is indeed. You have to match the biasvoltage.

I have build a plexi with only the bright channel and via a switch add the second stage to it like a 2204 at will. That amp has both pre- and post MV. If i was to build it again, I would implement a pre MV voor the bright channel and one for the 2204 channel along with the PPIMV.

[Willabe]

I try to avoid fizz with attenuating. Isn't the VVR the way to preserve the most of the amp's original tone? But I know VVR works great in cathode biased amps.. Isn't fixed bias more complex?

Master volumes and VVR you can only dial them down so far before they sound funny. I think people often ask too much from a MV and or VVR. A 50w amp with 4 x 12" cab, try and take it down to 10w or 5w? Can you even bring a 50 Plexi down to 25w without it sounding different or odd? 

Much more realistic to build a 20w Plexi with a single 12" cab and use MV and power scaling to go down further if needed.

Like Auke said you have to have a way to track the -bias when turning down the VVR.

Kevin O'Connor's London Power power scaling has a -bias tracking circuit that is supposed to work very well. 

[Bieworm]

It's not the purpose of bringing the volume down drastically,  just some to be more flexible at different venues.
It will be a gigging amp anyway. Put on top of a 4x12 G12h30 (original '67, that he just got for free!!!! 55Hz too!!!!)
Maybe the JMP50 is the way to go.. with the pre PIMV.
I just finished a superlite TMB 18W this week and that has the pre MV too.. quite nice crunch. Guess I will build the 50W JMP 2204 and keep some options open on the turretboard for a PPIMV .

[High Voltage]

You can get a really decent Malcolm tone on a 2203/4 with the preamp volume low and the master wide open. Malcolm actually used one on the '78 tour. Of course the absolute authentic tone is a Super Bass or JTM 45/100, but as you say the 2204 is more flexible and sounds very good even at low master volumes. You might want to put the bright cap on a switch.

[Bieworm]

In preparation for the 2204 build I find a lot of schematics. It puzzels me that there are some serious differences in the PT secondary voltages between those schematics, all 50 watt amps. One says 345-0-345, the other 310-0-310. This has a lot of influence on the B+ voltage. Which one is the one to go for the early 2204?
I've been assigned to get ACDC tones, not slayer 😀

[pdf64]

I In preparation for the 2204 build I find a lot of schematics. It puzzels me that there are some serious differences in the PT secondary voltages between those schematics, all 50 watt amps. One says 345-0-345, the other 310-0-310. This has a lot of influence on the B+ voltage. Which one is the one to go for the early 2204?
…

Transformer regulation is the elephant in the room here.
Voltage specs for the secondary windings are meaningless without the relevant related current draw.

[Bieworm]

I In preparation for the 2204 build I find a lot of schematics. It puzzels me that there are some serious differences in the PT secondary voltages between those schematics, all 50 watt amps. One says 345-0-345, the other 310-0-310. This has a lot of influence on the B+ voltage. Which one is the one to go for the early 2204?
…

Transformer regulation is the elephant in the room here.
Voltage specs for the secondary windings are meaningless without the relevant related current draw.

I feel like it was a stupid question,but don't understand why...

[dwinstonwood]

I feel like it was a stupid question,but don't understand why...

It confuses me.

Somewhere I read that when a transformer manufacturer states: 275-0-275 @ 100mA - for example - it means that the secondary supplies 550VAC under a 100mA load.

Other times I've read that it means not to exceed a 100mA load.

Or, maybe it means one or the other, or both, depending on the manufacturer. 

[shooter]

550VAC under a 100mA load.

that's how I've always done it.  If you want to get in the weeds, there is probably a temp over time spec, a max "instantaneous" current over time and on and ......
if it last's 90 days at gig strength it'll probably last for years, otherwise - upsize 

[sluckey]

There's more than one 1987 and 2204. Maybe you could post a schematic of the one you have in mind.

[Bieworm]

There's more than one 1987 and 2204. Maybe you could post a schematic of the one you have in mind.

Hey Steve

I'm thinking about the more vintage sounding 2204 (mid 70's). Ceriatone has a layout for one, but still looking for that schematic. I think I saw one on RobRob's threads. That's why I was so confused. One schematic says  470VDC B+ and the other says 367VDC B+ (not even mentioning lots of other schematics online)
The Ceriatone layout marks a 325-0-325VAC power transformer
I'm just afraid if I use the 345-0-345 I'll end up with that 80's metal sound.. I want the looser feeling version.
I thought maybe somebody could point me in the right direction...
http://www.ceriatone.com/british-style-jcm800-2204-hw/

Maybe this is the safest way: https://mhuss.com/MyJCM/JCM800_2204.pdf  it' s Mark Huss' MK2 2204. It say a 320-0-320. Might be off well with the Hammond 260J (325-0-325 @173mA, 4A filament currect, should do 2x EL34 and 3x ECC83)

[sluckey]

The JCM-800 2204 (Huss and Ceriatone) is the most popular 2204 amp. This JCM-800 2204 (or it's big brother 2203) is the amp used by the big hair boys in the late '70s and '80s. The earlier JMP 2204 uses the same topology as the JMP 1987 (commonly called Plexi) but has a bit more gain.

There ain't a nickel's worth of difference between a 320-0-320 PT and a 325-0-325 PT. I'd use the 325 just because I like Hammond. Huss shows 320-0-320 but he also shows the B+ at 468V, which is impossible. In theory 320 can only make 452V max! It would take 330-0-330 to make 468V.

I don't recall seeing any official looking schematics that show 470VDC, or 367VDC, or 345-0-345 PT. Could you post a link?

I'm just afraid if I use the 345-0-345 I'll end up with that 80's metal sound.. I want the looser feeling version.

I don't think of any of the 50 watt Marshall amps as having a "looser feel".  You should at least study the JMP 1987 (Plexi) before committing.

However, your opening statement

Somebody asked me to build him a 50W plexi type amp to use in his AC/DC coverband.

makes me think "somebody" wants a JCM-800 2204.

[Bieworm]

I think so too about the 2204. He plays a TSL100 right now, so the 2204 will definitely give him the ol' marshall feel. And that pre PIMV will make it a useful amp for all venues.

Here's the 345VAC reference, by robrob, somewhere down that page:
https://robrobinette.com/How_the_Marshall_JCM800_Works.htm


BTW, the 367V turns out to be a 6V6 power amp.. nit sure what that's doing on that page?

[pdf64]

...Huss shows 320-0-320 but he also shows the B+ at 468V, which is impossible. In theory 320 can only make 452V max! It would take 330-0-330 to make 468V...

Transformer regulation results in a secondary winding voltage being higher when it's open circuit, lower when its full load current is being drawn.
My reading of M Huss's schematic is that 320-0-320 is the rated voltage of the Drake 1202-324/3. I think it's for historical reasons that Marshall '50W' PT HT windings are rated at a 150mA current draw, even though the full load current will be somewhat higher than that (perhaps reach twice that level).
Hence a 320-0-320@150mA winding may well measure 330-0-330 at an idle current draw, which will be somewhat less than 150mA.
See the Hammond 290MAEX link below - full HT voltage 690V@150mA, max loading 213mA (voltage not noted), unloaded / open circuit voltage 729V.

... I want the looser feeling version...

A looser feel may indicate poorer transformer regulation hence higher HT winding resistance.
I suggest the Hammond 290MAEX https://www.hammfg.com/files/parts/pdf/290MAEX.pdf
The orange HT wires provide an option for a lowered HT voltage.
The 5V winding provides an option for a valve rectifier, if even more looseness is desired.
Here's a schematic with voltage chart from the JTM45 to JTM50 transition era https://el34world.com/charts/Schematics/files/Marshall/Marshall_jtm45tr.gif

Here's the mid 70s, 'non-cascaded', lower gain JMP2204 https://el34world.com/charts/Schematics/files/Marshall/Marshall_jmp_mastervol_50w_2204u.pdf

I think this Marshall voltage chart dates from the early 70s, HT around 400V on the '50W' models https://el34world.com/charts/Schematics/files/Marshall/Marshall_ampchart.gif

Later, by the mid 70s, I think they may have dropped down even lower, maybe 360V.
The changes may have been due to their combo models using a pair of greenbacks. Given their power handling limit, and the Marshall rep for overdrive, a full 50W output may have been too much, resulting in excessive warranty claims for blown speakers.
So it may have been that the clean output was lowered to 30-35W by reducing the HT voltage (30W clean results in about 50W when heavily overdriven, given typical Marshall levels of HT sag).
Here's a Marshall 50W schematic from 1981 with a 360V HT https://el34world.com/charts/Schematics/files/Marshall/Marshall_jcm800_bass_50w_1986.pdf

By the mid 80s, higher power Celestions were available, and the 50W PTs seem to have been respecced, allowing the amps to put out a full 50W clean (90W @10% THD!), see p7 of https://www.scumbackspeakers.com/faqs/jcm800specs.pdf
M.Huss's 2204 was from that era (1983).

[Bieworm]

Are there sound samples for the lower gain 2204 you know of?
It will surely sound more vintage, leaning towards the late 60's super lead I guess? This will definitely sound more like Malcolm Young. I wonder if the pre PIMV is effective enough like the cascaded version.

[pdf64]

Are there sound samples for the lower gain 2204 you know of?
It will surely sound more vintage, leaning towards the late 60's super lead I guess? This will definitely sound more like Malcolm Young. I wonder if the pre PIMV is effective enough like the cascaded version.

The non cascaded 2204 didn't last long, I suspect that most of them got modded 
When I've experimented with it, the degree of overdrive is somewhat underwelming, some sort of external boost pedal is needed really.
I suppose that with good pickups and a heavy right hand,  it may be just about ok for Malcolm Young style 
I suggest to build a regular 2204, but add a switch for the bright cap, and DPDT to 'true bypass' the 2nd stage.
By that, close approximations of both versions of the 2204 would be user accessible

[sluckey]

I've never seen any talk about the original 2204. Everything seems to relate to the JCM-800 2204 high gain amp. In fact, it's so popular that most folks just refer to it as JCM-800. I suspect many folks don't even realize that JCM-800 is a series of amps, not any particular amp.

Dr. Tube website is gone, but thanks to the wayback machine, you can still see much of the content. He has the Marshall amp schematics listed chronologically, dates and a brief description. It's worth a read...

https://web.archive.org/web/20201112010824/https://drtube.com/en/library/schematics/69-marshall-schemas

[pdf64]

Here's a nice Marshall history course  https://solodallas.com/the-vintage-marshall-guide

[Bieworm]

I think that late 70's, early 80's is the sound he's after... and more.  For smoothness that cold clipper is a nice feature anyway. I have it on one of my amps, minus the CF, because I have a single tone control a la 5E3 in it. But pretty nice sounding.. well  not the 10k, but I settled for 5k there.

If the standard 2204 isn't what he likes, the components and tube layout are there to easily convert it to a lower gain version.

[sluckey]

I'm gonna throw this in the ring just to add to the confusion... 

     http://sluckeyamps.com/misc/Marshall_Dual_50.pdf

[SILVERGUN]

Here's a nice Marshall history course  https://solodallas.com/the-vintage-marshall-guide

And then this, from that same site:
SoloDallas.com | High Wattage – AC/DC’s Marshall amps through the years (Work in Progress – UPDATED: Back in Black era uncovered)


 
People get a little obsessive about this stuff

[Bieworm]

I'm gonna throw this in the ring just to add to the confusion... 

     http://sluckeyamps.com/misc/Marshall_Dual_50.pdf

Now I wasn't going to throw that thought in here... but that's what I was initially thinking!!! Dang Steve!!! Thanks!!! Will investigate this one thouroughly!!!

[dude]

Wow!, all that Marshall info, thanks all for posting.

[PharmRock]

Is it a crazy idea to add a pre PIMV and a PPIMV on 1 amp?

I recently built a switchable 1987/2204, primarily based on "Neil's Special" (1959 modified for switchable 2203) that has both a pre-PIMV as well as post-PIMV (Lar/Mar).  I think the combination of MV's does allow for a little more "tweakability" on not only the loudness but also the tone/quality of the overdrive, and the Lar/Mar PPIMV was very easy to install. 

[Bieworm]

Is it a crazy idea to add a pre PIMV and a PPIMV on 1 amp?

I recently built a switchable 1987/2204, primarily based on "Neil's Special" (1959 modified for switchable 2203) that has both a pre-PIMV as well as post-PIMV (Lar/Mar).  I think the combination of MV's does allow for a little more "tweakability" on not only the loudness but also the tone/quality of the overdrive, and the Lar/Mar PPIMV was very easy to install.

Did you directly install the lar mar? I once did one a d found it too noisy, even with shielded wires... how is yours?

[PharmRock]

Its remarkably quiet.  Its on the front panel, right between my indicator light and presence control.  I followed the tutorial made by Curt Granger on Youtube, except for the placement of the pot. I used the 2-conductor shielded wire (red/black) for the cable runs like he shows in the video.