How do you increase headroom in a 50w JMP?

[paul-e-mann]

I have a 50 watt jmp with about 440v HT and was wondering if I can raise it anyhow to get more headroom? I’m sorta tryna get the 50 to sound more like a 100w.

If there are other ways of increasing headroom as well please do tell.

Plug into the low input?

 

[Pete Farrington]

You could try increasing the filtering.

I think that up till the mid 80s, the 100W models tend to have comparatively lower reservoir capacitance than the 50 watters.
Bear in mind that the 100s have twice the current draw, so just using the same capacitance as the 50s is effectively a smaller reservoir.
The July 70 schematics show 100uF reservoir on the 50Ws, 50uF reservoir capacitance on the 100Ws.

https://el34world.com/charts/Schematics/files/Marshall/Marshall_jmp_lead_50w_1987.pdf

https://el34world.com/charts/Schematics/files/Marshall/Marshall_jmp_superlead_100w_1959.pdf

So to get the same HT response on a 50W as a 100W, the reservoir cap should be around 25uF.

When a new thread is started on this amp, it would be helpful to include an up to date schematic.
As it’s pointless to devise and suggest stuff that you’ve already done.
I was thinking of the HT dropper and feedback resistor values, and V2 cathode bypass, but dunno where you are with that

 

[sdn25]

I think that up till the mid 80s, the 100W models tend to have comparatively lower reservoir capacitance than the 50 watters.
Bear in mind that the 100s have twice the current draw, so just using the same capacitance as the 50s is effectively a smaller reservoir.

When a new thread is started on this amp, it would be helpful to include an up to date schematic.
As it’s pointless to devise and suggest stuff that you’ve already done.
I was thinking of the HT dropper and feedback resistor values, and V2 cathode bypass, but dunno where you are with that

This is the Layout from the website. My amp is exactly as the layout shows except for the fact that I have 50+50 filtering.

 

[mallcorn]

I have a 50 watt jmp with about 440v HT and was wondering if I can raise it anyhow to get more headroom? I’m sorta tryna get the 50 to sound more like a 100w.

If there are other ways of increasing headroom as well please do tell.

That's a "no-brainer"... Power Station 100 (PS-100).

 

[Pete Farrington]

If it’s getting too mushy, consider the possibility that your just overdriving it too hard, and turn it down a bit

I suggest to tinker with the HT cap values, nfb and the HT dropper.

eg move the 50uF to the phase splitter and preamp nodes, 32uF back to the OT CT and screen grid nodes.

NFB try 100k fed from the 4ohm tap.

Link across one of the 8k2 HT droppers.

 

[Browneyesound]

You’re already at 1kohm screen grid resistors…I wouldn’t go any lower on those, would have suggested 1k if you had something higher, like 2.7k.

If, a big if, you had some very robust tubes, you could try some lower value screen grid resistors, but at your own risk.

 

[sdn25]

Link across one of the 8k2 HT droppers.

What do you mean by that?

Apart from the circuit changes, would having kt77s be a good solution?

 

[mcnedley]

Have you considered trading for a 100 watt model? Probably a few old geezers like myself that would consider trading their 1959s for a 1987.

 

[Jackco]

I have a 50 watt jmp with about 440v HT and was wondering if I can raise it anyhow to get more headroom? I’m sorta tryna get the 50 to sound more like a 100w.

If there are other ways of increasing headroom as well please do tell.

I dont want to sound crass, but if you want the 100w sound, you need the 100w amp.
You can Mike it and adjust the eq ?

 

[neikeel]

EVH brown sound is about working the whole amp hard into distortion
Clean headroom is not that.
I think that you need to consider what you want and expect from a 50w amp.
KT77s are probably not going to work. KT88 might.
If you want more tightness try more NFB.

 

[sdn25]

Have you considered trading for a 100 watt model? Probably a few old geezers like myself that would consider trading their 1959s for a 1987.

I have a 1987 clone which I built and is much cheaper than a regular 1987, can’t afford the 100w ones unfortunately, they cost about $2700 here. My clone was about $900 and that itself took a lot of saving I’m only 18 haha.

 

[sdn25]

EVH brown sound is about working the whole amp hard into distortion
Clean headroom is not that.
I think that you need to consider what you want and expect from a 50w amp.
KT77s are probably not going to work. KT88 might.
If you want more tightness try more NFB.

Right, I get that, not exactly looking for clean headroom, sorry I’m not really the best at tryna explain or describe things but bare with me here, could you tell me exactly why the 100 watt sounds punchier and more aggressive when compared to the 50 watt (more so when cranked)? We’ll work from there.

 

[Trapland]

I have a 50 watt jmp with about 440v HT and was wondering if I can raise it anyhow to get more headroom? I’m sorta tryna get the 50 to sound more like a 100w.

If there are other ways of increasing headroom as well please do tell.

What DoD said. Except you can get good results staying with EL34s if you use a "10" set from Groove Tubes or any of the other tube resellers that grade based on distortion character. A 10 set or similar, biased for tone will give you more volume before break up. I have a Vibrolux that breaks up at 2-3 with most 6L6 but a set of 10 rated groove tubes go to 8-9 before any serious crunch. I also bias them a bit cool so I get a more distinct point where it sounds clean them changes to crunch, rather than a gradual change at a more standard bias setting.

And then for even more headroom, go with the KT88 as suggested by @Dogs of Doom

 

[_Steve]

Right, I get that, not exactly looking for clean headroom, sorry I’m not really the best at tryna explain or describe things but bare with me here, could you tell me exactly why the 100 watt sounds punchier and more aggressive when compared to the 50 watt (more so when cranked)? We’ll work from there.

Yeah what you are asking for is definitely not headroom thats why youre getting the suggestions for KT88s etc.

Are you sure 'punchiness' is even the right word for what you're looking for? Im no EVH expert (far from) but Ive never thought of his sound as very punchy? I've heard it described on amp forums as the 'brown sound' and he famously/apparently ran his amp through a Variac to reduce the voltages significantly - you might want to look into that?

One super EASY and FREE thing you should try before doing anything too complicated is simply playing with your bias setting. Its amazing how much difference it can make to the way an amp feels.

I do have a mod to add what I consider punchiness, but i dont think that's what you want..

 

[Gene Ballzz]

@sdn25 ,
You may want to find a time when you can get away with running your amp at it's "sweetest spot" without that attenuator and see if that gives you what you're looking for, even if much louder than you want and/or need! While the Suhr is one of the few I've not tried, every passive attenuator I've tried, since the early '70s, has totally destroyed tone, dynamics and feel, all except for the @JohnH design. There really is a very significant difference! I won't bore you with the details as they can get fairly technical. Here's a link to the thread that covers it all and gives as much detail as one could ever want:

Simple Attenuators - Design And Testing

Background Passive attenuators are wired between the amp output and the speakers. Their function is to absorb most of the output power of the amp, feeding a smaller amount to the speaker itself. This allows the amp output stage to run at higher power, letting the glorious tone of a good valve...

www.marshallforum.com

Although the thread is massive, all the important build info is in the first post. A lot of technical stuff and useless chaff is easily skimmed past and practical layout/wiring diagrams show up at page #111.

Simply Attenuatin'
Gene

 

[LoudStroud]

I have a 50 watt jmp with about 440v HT and was wondering if I can raise it anyhow to get more headroom? I’m sorta tryna get the 50 to sound more like a 100w.

If there are other ways of increasing headroom as well please do tell.

Which model 50W JMP do you have? Lead or Bass? There are simple minor circuit changes that can be made to allow for more headroom, if tubes don’t get you there.

Reading everything you’ve posted and knowing what you’re sayin about 100watter’s, I agree with the suggestion of trying KT88’s. They still have sweet smooth mids but with punchier low’s and crystal highs. Heck, Jimmy Page used KT88’s in his 100 watter’s. Also NOS Sylvania 6CA7’s can get you there.

 

[len_zwerf]

I don't know what 100 W you mean but I think the 50 has the same pre amp section as the 100. If so, you will have to get your settings right. When you dime the amp with the channels jumpered you get a mess. When you balance the bright and normal volume so the amp doesn't fart out you'll probably have better results.

 

[Pete Farrington]

What do you mean by that?

Apart from the circuit changes, would having kt77s be a good solution?

Currently there’s a series pair of 8k2 HT droppers, between the screen grid and phase splitter supply nodes.
I’m suggesting to try it out with a single 8k2 dropper.

Dunno about KT77
EVH supposedly used Sylvania 6CA7. I’ve got a old, well used pair of those, I’m not sure they sound much different to the vintage Mullards I usually use.

 

[sdn25]

Currently there’s a series pair of 8k2 HT droppers, between the screen grid and phase splitter supply nodes.
I’m suggesting to try it out with a single 8k2 dropper.

Ah! Okay I will try the single 8k2 and let you know my findings. Thanks.

 

[sdn25]

Yeah what you are asking for is definitely not headroom thats why youre getting the suggestions for KT88s etc.

Are you sure 'punchiness' is even the right word for what you're looking for? Im no EVH expert (far from) but Ive never thought of his sound as very punchy? I've heard it described on amp forums as the 'brown sound' and he famously/apparently ran his amp through a Variac to reduce the voltages significantly - you might want to look into that?

One super EASY and FREE thing you should try before doing anything too complicated is simply playing with your bias setting. Its amazing how much difference it can make to the way an amp feels.

I do have a mod to add what I consider punchiness, but i dont think that's what you want..

Yes I do mean punchiness, I’m not talking about VH1 tones because I already achieve that sound. I’m more so talking about fair warning, women and children first kinda tones where it’s thick and gainy but has low end clarity and the pick attack comes through.

The variac lowers all the voltages in the amp which makes the EL34s break up earlier and become more ‘spongy’ as they say and isn’t exactly what I’m looking for.

I do agree on the bias though, i ve tried different settings and ended up with biasing them at 26mA per tube as the cold setting seemed to give a bit more punch for some reason.