I have modded my Marshall Class 5!!!

[ron vogel]

OK, y'all are going to pissed...as this is a tease sort of.

I could not get a good recorded clean sound out of this amp without the "fizz"...and dimed the amp was too muddy...but I found an easy (and cheap) fix for it.

Change R1 to a 220K resistor, and R18 to a 2.2K resistor. Effectively reduces gain on the first stage. Amp stays pretty clean up to the halfway mark, and is just gobs smoother. It also helped the muddiness a bunch.

I am waiting on parts, but there are 7 caps in the signal path I will be replacing, as well as changing c12 to 33uF from 47uF (this will allow the EL84 to reproduce more bass)

Well why would I do that? Because the 1963 Jensen c12r sounds a little thin compared to the stock celestion.

What's that?....Yes, I made a baffle for a 12" speaker...and found a way for it to fit. I'll post pics of the completed baffle in a few days, but have been playing with the mock up, and it just blows away the stocker in every possible way. THIS amp is no more a "one trick pony".

Also...the stock speaker is nowhere near 16 ohms. Lots of room for improvement there. I measured 11.2ohms, and the Jensen I'm using measures 11.8ohms.

Marshall was kind enough to float me a clear and detailed schematic to mod the amp...I'd be lost without it! It is a sound design, but room for improvement!

[J M Fahey]

I think you should sue those b*st*rds, they stole you 4 ohms each.
They did never imagine you would actually *measure* them, did they?
Now I would ask not only for what they stole, but, say, an extra 4 ohms *each* for damages.
I forgot .... plus interest !!!













Now on to serious stuff: what you are measuring is the DC resistance of the voice coils; he actual speaker impedance will always higher than that, and varying with frequency; so to make life easier, a "conventional" value is used, some 20% to 30% higher t5han VC resistance, so those almosd 12 ohms indicate a real-world speaker of around 16 ohms.
The exact value is imprecise, or, better said, complex.

[ron vogel]

Yeah, it's tube electronics too...so it's like horseshoes and hand grenades where "close" is OK. Although the lower ohm load would lose headroom. I know the amp is just supposed to be rowdy, but the tone is good stock...so it's such a waste that 3/4 of the dial is distortion.

I did find the PCB is VERY fragile, I may hardwire the tube mounts and use mounts that attach to the chassis. I'm always changing tubes, and will probably kill the stock mounts in no time. Plus (especially with the 12" speaker) the EL84 is almost right on the speaker...probably wouldn't hurt to relocate the tubes up a bit higher.

To make chassis removal easier I disposed of the stock aluminum screws, and put in more durable hand screws. I personally don't care about the warranty...so I'm making the amp fit MY needs. I did find a nice combo of tubes for it. I have a pair of '62 RCA 12AX7's, and a late '50s Amperex/Holland EL84. The stock JJ's were totally microphonic in that amp.

[olddawg]

Yeah, it's tube electronics too...so it's like horseshoes and hand grenades where "close" is OK. Although the lower ohm load would lose headroom. I know the amp is just supposed to be rowdy, but the tone is good stock...so it's such a waste that 3/4 of the dial is distortion.

I did find the PCB is VERY fragile, I may hardwire the tube mounts and use mounts that attach to the chassis. I'm always changing tubes, and will probably kill the stock mounts in no time. Plus (especially with the 12" speaker) the EL84 is almost right on the speaker...probably wouldn't hurt to relocate the tubes up a bit higher.

To make chassis removal easier I disposed of the stock aluminum screws, and put in more durable hand screws. I personally don't care about the warranty...so I'm making the amp fit MY needs. I did find a nice combo of tubes for it. I have a pair of '62 RCA 12AX7's, and a late '50s Amperex/Holland EL84. The stock JJ's were totally microphonic in that amp.

Why don't you make a new turret board while you're at it and use ceramic tube sockets?

[ron vogel]

Ehh, maybe someday, sounds like a lot of effort!

[J M Fahey]

Hi Ron.
You will, eventually, because as you correctly guessed, those relatively thin/narrow tracks have the nasty habit of cracking just where they join the soldered pad .
Removing tubes often won't exactly make them last longer, so someday in a not too distant future, you'll find rebuilding on turret or eyelet board the "easy"solution, relative to troubleshooting repetitive so called "cold solder" problems.

[Enzo]

As JM points out, your ohm meter is not measuring the impedance of the speaker, only the DC resistance. If you want to know the impedance for real, use a 1kHz test signal and AC measuring meters and determine the impedance with Ohm's Law.

It might be instructive to measure a lot of your speakers. You will find that the DC resistance of most 8 ohm speakers is around 6 ohms, that of 4 ohm speakers is around 3 ohms, and that of 16 ohm speakers is... about 12 ohms. The amps do not produce DC output, at least we hope not. SO the amp does not face whatever resistance your meter shows.

You cannot measure speaker impedance with a regular ohm meter.

[ron vogel]

Just wanted to show off the final product...stuffed a 12" speaker in there with a custom baffle. Just got the grill fabric on today, put on the piping, and put it all back together:

[J M Fahey]

Hey, It's nice.
That grill cloth and piping sure look professionally applied.
Wouldn't have believed that your C12R would fit there, and even with a little room to spare.
Now we deserve some MP3's.
Can't you place the original speaker side by side or close by to the new one?
I'm curious about what they fit as an original.
A couple sideways, back and front pictures wouldn't hurt either.
The Jensen, I know by heart and don't specially need more info about it.
What difference do you hear, in actual use, between them?
Thanks.

[ron vogel]

Hey, It's nice.
That grill cloth and piping sure look professionally applied.
Wouldn't have believed that your C12R would fit there, and even with a little room to spare.
Now we deserve some MP3's.
Can't you place the original speaker side by side or close by to the new one?
I'm curious about what they fit as an original.
A couple sideways, back and front pictures wouldn't hurt either.
The Jensen, I know by heart and don't specially need more info about it.
What difference do you hear, in actual use, between them?
Thanks.

Thanks!
The grill cloth and piping are original Marshall. The grill cloth is $$$ ($70 a yard!), I put the unused pieces in my ebay store. The Marshall logo was pretty pricey too, I should have just bought the plain white 6" logo. I have done a few cabs, so I have a little experience under my belt. The fabric is very heavy duty, and not easy to work with. I probably removed more staples than I put on when I was working with it!

I wouldn't want to put in the original speaker in there with it, I'm already pushing the cab volume with the larger speaker. I made the new baffle like a soundboard so I can still get a nice "bloom" out of the amp...which fortunately worked out very well.

The magnet has about 3/4" to spare installed...so it is a very tight fit.

I also forgot to mention I replaced the hard plastic feet with large rubber ones.

I'm really not trying to whore out my ebay store, but I added several of the items (like the gold corners, fabric, rubber feet, and felt) I used that were hard to find.
eBay Stores - Fogelsang Studio items on eBay.com

I have a recently done clip with the Marshall BEFORE the mods:
Start Player

I am starting a new tune this week that has to have a very defined tone, and will be using this amp. I wanted to already start it, but I have 4 kids...so recording time is a bit limited!

[stratele52]

You are right Enzo, ron vogel dind't have a 16 ohms but a 8 ohms.

I knew a guy who is testing is ouput trans, at the secondary if he has 8 ohms.
Of course not, after mesuring with an ohnmeter he was sure that is transf was bad.

[dido__15]

Ron,

Would you mind sharing that schematic? I can only get hold of the blurry copy thats been doing the rounds.

It seems like you are aiming for the same goal as me, cleaner/less muddy, so I will give the mods you've documented a go. I will however be keeping the stock speaker as I like the bass and punch it gives (unless I come into a Celestion G10 Gold by some stroke of luck).

One mod I've started, is replacing LK23 (Link between mid pot and ground) with a foot switch to bypass the tone stack and provide a boost. Sound like a good Idea? I've got the jack socket in place just need to wire up a footswitch. Using a switching jack so the amp is stock (with tonestack) when the footswitch is unplugged.

Also had thoughts about removing C8 (10uF). I believe it will increase headroom and increase upper mid/treble response. Am I on the right track?

Cheers,

James

[ron vogel]

Screw it...here's the schematic!

I don't know about the boost switch, but I am still making changes as I go (when I have time to).

I put the 100K resistor back in r1 (it dropped too much bass), but kept my change of 2.2K in r18. I don't think removing c8 is a good idea, but upping the value a touch will lower bass.

The design is a fine balance, and the trick is to cascade the gain to retain the touch sensitivity, and still drive the output tube. I still can't get a good overdrive sound out of the amp recorded. It just sonds fizzy, and I think the output tube is not being driven hard enough, and the preamp tubes are driven too much...although it's much better.

I find myself turning the mid pot down to get a tone that isn't too muddy. I'm going to swap in a lower value in to see if it works.

I'm no expert, just an eager tinkerer. So if anyone more knowledgable can chime in about how to increase headroom in the preamp, and either lower the voltage to the EL84, or push the input (el84) on it harder that would be great!

(and help me get the mid pot frequency to a higher range)

[J M Fahey]

Going step by step.
1) Ron, thanks for posting the schematic, it's very useful and it answers a lot of questions.
2) ¡¡¡ Oh My God !!!!!!!!!!!!!!!!! ¿What's that? ¿What were they trying to do?
That's not a Marshall !!!!!!!!!!!!!!!! Totally amateur design, no wonder it's muddy and fizzy.
Drop that board in the nearest trashbin and build some of the AX84 versions. Any of them will sound much better. Do not waste your time polishing a turd.
Just the simple fact that a track is broken in up to 6 pieces joined by jumpers shows the amateurishness.
The headphone out is incredible. That bass lifting net they use is too high impedance for headphone use and too poor to compensate for fizziness if used as a direct recording line out.
3) Consider building something else there, an eyelet or turret board will allow you mod it to death at some later date.
Sorry and good luck.

[ron vogel]

I jumped the jumpers with push-back wire...and I gotcha on the design...might be why Marshall was reluctant in letting it get public.

I truthfully have had a lot of issues with this amp...mostly from trying to record it. It sounds absolutely great in person, nice cleans, and the amp dimed is unbelievable. However, I think the lack of 2 output tubes hurts it for harmonic content...maybe I just like the 6V6 sound better I dunno.
For recording, I put a modded DS1 in front of it and it helps a bunch for tone. I also have good luck with running a wah in a set position to bump the mids.

I like the amp, but I can only have one...so this may go in favor of a nice little champ...I made up my own design (a ripoff cross between a Champ/Supro)...maybe I'll just build from scratch and be done with it!