I have modded my Marshall Class 5!!!

[Alex/Tubewonder]

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)

Well....the changes you did, or plan on doing, actually have opposite effect to what you intended. So to get the amp where you want it go in opposite direction with component value changes.
tip: change R11 & R13 to half their present values, change C8 to 1/10th it's value.

[dido__15]

Well....the changes you did, or plan on doing, actually have opposite effect to what you intended. So to get the amp where you want it go in opposite direction with component value changes.
tip: change R11 & R13 to half their present values, change C8 to 1/10th it's value.

I thought C8 seemed high, hence why I had thoughts of removing it. I will definately try 1/10th the value if I have it in my parts drawer.

One thing though, will halving R11 and R13 not increase the gain of the last 12AX7 stage? I know this isn't a high gain stage but will it not add distortion?

Also, increasing R1 to 220K seems to have lowered the headroom. Am I right in thinking that increasing the value of this resistor lowers the voltage heading to V1 causing it to distort earlier? I didn't think about it before the mod. I'm a tinkerer too but trying to learn the theory behind it. If I am correct, and this increase in R1 has increased the distortion, I will put it back. Would it be safer to go lower than 100K?

There is a lot of potential in this amp, and I used to love it at my folks place where I could crank it, but since getting my own place and using it with a weber minimass you can notice it gets very fizzy when the gain is turned up, this fizz was filtered out by speaker breakup before so not as bad, but without the speaker breakup at low volumes it's very prominent. I think pushing the EL84 harder should go some way to improve it as ron vogel suggested.

I think that lowering R25 from 2.2K to 1.5K will also increase gain going into the EL84 and bring a little more "sparkle" into the power section.

Anyone got any other ideas?

And Ron thanks very very much for the schematic, certainly much easier than squinting at the fuzzy version, you've made it much easier for everyone.

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.

Decreasing the value of the mid pot will mean more mids I believe, I've heard that changing the mid pot from 20K linear to 100K LOG A will get rid of a lot of the mud, and give more range on the pot. I might be wrong though, just passing on my research, haven't tried it yet. Does anyone know where I can get marshall pots like the stock ones to replace it?

James

[guitarmike2107]

Thanks for posting the schematic, I wonder what Marshall were thinking too, with all the tubes they could easily have made a mini jcm800 or mini 1959 type amp…. Which his probably what more people wanted..

Here’s some of my thoughts,

I don’t think blasting a SE amp power section super hard is a great idea, particularly if you want to get rid of fizzy distortion.
I would lower the grid leak resistor on the power valve to 220k (R26) and raise grid stopper (r25) to 220k-330k too which will help smooth the high end a little.
The input capacitance of power valves is really low so you can raise the grid stopper with out loosing high end, but it seems to help performance.
Lowering R26 will also increase your headroom…

I am surprised at how much bass they are putting through the amp, the only bass limiter is the tone stock and the coupling cap before the power valve, your muddiness may be attributed to the power section farting out with all the bass. The small speaker probably helped with this and having upgraded to a bigger speaker might have made it more evident? Anyway you can re-enforce this somewhat if you want to help it handle the bass better by increasing C12 to 1000uf – 22000 uf
Also increasing C4 to 100uf may help the bass response too a little.

If you want to loose the low mids you can change R28 to 82k or 100k, you will also loose some drive by doing this but the low mids will be reduced..
Increasing the mid pot size will not reduce the amount of mids.. it will only increase them.

[dido__15]

OK seems like I was quite a way out with my thinking. I'm obviously not quite ready to be let loose on my own yet. Thanks a bunch guitarmike that's a very detailed explanation, I will do some more studying then give it a go.

[J M Fahey]

with all the tubes they could easily have made a mini jcm800 or mini 1959 type amp…. Which his probably what more people wanted..

Fully agree.
Yet it's not too late to make a PTP/eyelet/turret board with , say, a JCM800 preamp, driving that single power tube.
Look around for the layout, it's really simple, and you need nothing special, toolwise.
The sound will be killer.
Someday you can also make a speaker emulator board, to record straight into your PC soundcard with no-fizz killer sound.

[ron vogel]

I'm sure Mouser or Digikey would...I didn't look at it real close, but it looks a lot like a bourns pot.

Good info on here.

Also, the fizziness really shows up during recording. I did change the 220K back to 100K, but left the 2.2K in the V1 stage. My intention was to lower supply voltage to V1.

[J M Fahey]

Can you detail how do you record it?
Thanks.

[guitarmike2107]

Can you detail how do you record it?
Thanks.

yes that would be helpful.

are you attenuating for the recording? if so speakers ( and your ears) appear to react differently at different drive levels, i.e. a g12h30 celestion is very fizzy at low levels.

There are some recordings of this amp on youtube that are not overly fizzy....??

BTW changing the first stage will not really effect the fizz, it will effect the drive to the rest of the amp, but if you end up driving it harder it will just fizz more

Mike

[dido__15]

Mike, I think I am going to have to concede that using an attenuator is what is giving me the fizz. But I could still do with some more headroom anyway, and just cant resist tinkering so I'm gonne have a crack at what you suggested. Any improvement is still an improvement right?

[ron vogel]

I'm curious if you tried out any of your own mods yet. I just sold my Class 5 (I put it back to mostly stock)

I really was spending too much time trying to get my guitar tones with it...and would rather be composing!

Anyway...I got lucky and picked up a '72 Champ for $200 locally. It was really poorly taken care of, and the seller probably thought he was selling me junk...but an evening of scrubbing has it looking pretty good...ordered all the caps today for it even though it works.

I know the Champ will work for me, I got the class 5 on a bit of a whim...when I really was going to buy a champ originally...so it was a bit of a detour.

It's still a nice little amp, just not for me.

[ron vogel]

I don't use anntenuators. I had a Princeton Recording amp with the built in one last year...it sucked the tone dry while it was in use. It really turned me off of those things.

I have tried all sorts of micing set-ups with the amp. Mostly an SM57 or SM7B on the grill, and a condenser a little out in the room. I have also made several mics out of old vintage speakers...I got a nice big sound out of one of them on the grill, and a condenser back in the room.

[HTH]

I wonder what Marshall were thinking too, with all the tubes they could easily have made a mini jcm800 or mini 1959 type amp…. Which his probably what more people wanted..

Thats what I thought too - I hear people asking for low-watt JCM800/plexi amps all the time. I wouldn't mod this amp, I'd just gut it and convert it to a PTP JCM800/plexi.

[dido__15]

The story so far.

OK, so I've now done the following changes.

R1 back to 100k
R18 from 1.5k to 2.2k
C8 from 10uF to 1uF
R26 from 470k to 220k
R25 from 2.2k to 260k

The tone is getting much closer to where I want it. The fizz is all but gone now, I'm getting a much better overdriven tone, even with the attenuator on.

However I've now reached a problem whereby if I play a chord with a loud attack (Particularly at the lower end of the neck) the amp suddenly drops volume. Happens occasionally with notes at the bottom end of the low E as well. Would this be the power amp "farting out from the bass" as you described? I haven't tried increasing the bass handling yet but is this likely to be the resolution to my problem?

Any help is greatly appreciated

James

[Guitarist]

Unless PT really sounds better then why waste a well designed PCB? This baby has start grounding and DC filamants, which is rare. It is close to optimized and just needs a few more tweaks, mainly de-"squashing". Kudos to Marshall!

MV at R12 and R14 right? Or might it be better after the PI subbing out R24 and R26?

Also with the four 470K resistors around V2 it might be an idea to drop in a 12ay7 and drop these all to 150K (ish). So as not to drive with the parking brake on. You'd think the distortion would disappear but it is actually a revealing of quality over/and quantity.

A: If you're getting a drop in sound, add some beef to the filter caps, anywhere.

[danzilladrummer]

I tried putting a 12" Eminence Tonkerlite 16 into my Class 5 by borrowing from Ron's baffle design. It sounded great until I turned the volume to about 3 o'clock. After a few moments of playing at the higher volume level, the amp made a crunchy sound and lost all of it's volume. I immediately turned the volume down on my guitar and waited to see if it would recover. After about 30 seconds, the sound returned. It does this whenever I have the amp turned past 12 o'clock. Has anyone run into similar issues or knows what may be causing the problem?