How To Increase Headroom In 50 watt JMP Style Amp

[whalecalls]

Hello All,
     Aside from the usual tube swaps (AX7 to AT7, etc) what would y'all recommend to obtain the maximum clean headroom out of a 50 watt Marshall JMP  master volume type circuit? I am trying to maximize my clean headroom to make my amp more of a pedal platform. My band plays insanely loud and my amp simply cannot compete with my bandmates' Hiwatt DR504.
      I have tried running my amp with el34s and 6l6gs. What circuit mods would you guys recommend to increase the headroom in the preamp, phase inverter and power amp?  I have about 235 volts on the first preamp tube and somewhere about 414vdc on the 34's. 
Also, does anybody know of any "small bottle" kt66s? I would like to try out a pair but my sockets are spaced to close to put the standard sized kt66 in next to one another.
Any input is appreciated. Cheers

[Latole]

I like more 12AY7 than 12AT7 in V1 and in Phase Inverter too

https://www.premierguitar.com/articles/8_Guitar-Amp_Mods_for_Newbies

Swap Preamp Tubes to Adjust Headroom

One of the most common things guitarists request from us at our shop (schroederaudioinc. com) is the ability to get more or less headroom—either cleaner tones at higher volumes or more overdrive or distortion at lower volumes. Let’s begin by looking at some simple ways to alter your amp’s headroom.

The first preamp tube (aka “valve”) in an amp’s circuit is used in its first gain stage(s) of an amp. It’s usually a 12AX7 (aka an ECC83 in Europe and abroad), and it’s the small tube located farthest from the larger power tubes. Typically, a 12AX7 has a gain rating of 100. One simply way to achieve more headroom in your amp is to replace this tube with a 12AT7 (aka ECC81), which has a gain rating of about 70 and will yield cleaner sounds than a 12AX7. Conversely, players who have an amp with a 12AT7 in the first gain stage can get more gain and overdrive from their amp by swapping it for a 12AX7.


You can further alter your amp’s headroom by simply changing its phase inverter, which is the preamp tube located right next to the power tubes. It sends the signal from the preamp into the power amp, and swapping it with one that has a higher or lower gain rating (i.e., a 12AX7 vs. a 12AT7) will also adjust the amount of gain being sent to the amp’s power tubes.

[whalecalls]

Hey,
    Thanks for the quick response. I am fan of your amps or at least Nels makes them sound very good! I have tried swapping out the preamps from Ax7s to AT7s in V1, in the PI and pretty much every position. It helps a bit, but not a ton. I was more seeking if there are any circuit mods one would recommend to clean up an amp, whether it be cathode bypass mod, a way to give the PI or the power amp more headroom. I will try out a AY7 once I get my hands on one.
     I know that a big part of Hiwatt's massive headroom is their really high plate voltage, but I wonder if there is a way to make my amp compete with the output of the DR504 given that they are both 2xEL34 amps. I know Hiwatts are also very different circuit wise than Marshall or Fender type circuit and that there is something to be said about the transformers.
Thanks again for the response.

[whalecalls]

Does anybody think that modding a Marshall 2xel34 circuit to the Hiwatt specs as shown in the two schematics would increase headroom? Also, would there be any problems that might happen? Thanks again for the help.

[Colas LeGrippa]

More headroom can be obtained by rising the preamp and power tubes plate voltage. On the power tubes it may appear tricky to achieve that but easily done on the preamp section by swapping  B+ supply resistors with lower values, paralleling the two halves of the first 12 ax7 oe eliminate one half, removing a cathode bypass cap on the first preamp tube as well as beefing up the B+ supply cap up to 50uF if not already done. Take a look at a clean amp schems to clearly see the differences  with your amp.


Colas

[dunner84]

If a 50W JMP isn't loud enough, lord have mercy. Mine is ear splittingly loud at 3 on the master. When opened up, it even seems louder than my mark v, and dual rec. Even the low input is crazy loud when set to the onset of clipping. The low input also take pedals much better.

Everything already noted will increase headroom, but probably not by a large margin. The headroom difference between my 50W, and my buddy's 100W JMP is actually quite small in terms of volume.

Tube swaps are obviously easy enough to try, and maybe coupled with more efficient speakers you will achieve your desired result, but if you are looking to increase plate voltages, you are really saying you need a different amp.

**Edit**
I will also note that if your MArshall is stock, or a DIY build with a "replacement" style power transformer, it is likely only 150mA. The internet says that is pretty light for a 2xEL34, but that is how they were spec'd. I don't know about the Hiwatt iron, but if they used hefty transformers, it could account for some of the difference.

In my experience though, the fastest way to make your amp louder is with more efficient speakers. Also, perceived loudness is a function of frequency, so sometimes an amp will "seem" louder than a similarly spec'd amp just based on the voicing.

[jammied]

One of my friends has a 100watt master model that he set up loud and clean. Then pushes it with a boogie v-twin.


The 6550's he is running makes for ear splitting clean through a 4×12 .


I think 6550's Kt88's give the best clean headroom and volume when pushed with pedals. Just my opinion


Also I think his cabinet is loaded with g12-75's

[brewdude]

I think a more efficient speaker(s) is a wise place to start. 


Also, how do you use the MV?  I would think you have MV at max. 

[AmberB]

Modifying a Marshall amp from EL-34s to run 6550s will probably increase headroom slightly because they require a bit more drive to reach distortion.  However, you'll have to increase the bias voltage to them by about 10 volts or so, according to the article in the book that I have.  I did the modification to a 50 watt amp, but that was a long time ago, so I don't remember what what was involved.  Apparently, the heavy metal players in the 1990s and 00s preferred the 6550s because of the hard edged distortion when pushed, as opposed to the softer edged distortion of the EL-34s...

[High Voltage]

Unfortunately I dont think you're ever going to get close to a Hiwatt's volume or headroom. They are the king of the hill in that respect. And I don't think theres any easy ways to mod your 2204 into a DR504 BUT if I was to going to try I would get a Hiwatt-spec OT (Mojotone sell them) and try to copy the Hiwatt phase inverter - but thats seriously non-trivial.

Wish I could give you some better ideas, but I think the only real viable option is to put an 12AU7 in the PI as that is the first to break up in the 2204.

[glass54]

+1 with dunner84 and brewdude.
Changing speakers to more efficient driver will/could help you. For example if your speaker sensitivity is 96dB SPL and you find a speaker with 99dB sensitivity, you have effectively "doubled" you power amp output without rebuilding your amp.
I don't know what speaker(s) you are using, but you would have to make a comparison to see if its worth the effort and MOST IMPORTANT if you change speaker brands/types, is this the tone you are used to or want?
I'm a bit of a fan of EVM12L drivers, but some people complain that they are too flat (too HiFi-sh) and I have one as my Laboratory Test Speaker. (Love him or hate him Alexander Dumble was a huge fan/user of these drivers.) The sensitivity is 100dB SPL (1m at 1Watt).
The EVM12L is good for 200W (or 300W for the ?Black series) and have also been used by Mesa Boogie and others, as a premium speaker and price to match.
Other popular speakers imported into Aussie are the Webers, but you would have to research suitability.
Kind regards
Mirek

[whalecalls]

     Thank you all for the responses. The fundamental problem is that my band just plays too f#cking loud during practice and whilst on tour. I have rather efficient and wonderful sounding speakers that the good chaps at warehouse guitar speakers hooked me up with. I have their Invader-50s and C/12s loaded a 4x12 loaded X-pattern in a Hiwatt cab and they are pretty efficient speakers. Much louder and less compressed than the original Chinese "fanes" that it came with (this is probably a 2000s cab). I have also run the amp with 6l6gc and holy grail 7581a GEs. The 7581a did increase the headroom a bit and sounded real good, but I want to keep those in my other amp and they're $$.
      Brewdude, most of the times I run my master on full volume and use my pre-vols for volume and gain. Dunner84, yeah, we are a loud band. I won't lie, it's fucking fun! Colas, you're right. I have raised B+ though the entire amp. I will be poking around comparing different amp schematics and note differences. I am curious how a Hiwatt PI would sound, but to be honest it seem radically different the LTP PI that we also know via Marshalls and fender.
      I do think the the phase inverter is probably the place to be looking at in terms of increasing headroom. I am considering trying a Silverface AA769 Fender Twin reverb PI. I believe that is a large part of why they're so clean. Anyone ever experimented with putting one of these PI in a Marshall style circuit or has anyone radically altered a Marshall PI? Does anyone know of any other clean/high headroom PI LTP PI schematics besides the one mentioned before?  Something that might make an amp more of a pedal platform and increase dynamic range?
      I have a new and strong power transformer that I believe is rated around 200ma. I actually had to replace it recently as I believe my Heyboer started to develop an internal short over time. It was putting out lower voltage over time and getting hotter and hotter with each use.
      I guess it just blows me away that the DR504 which is another "50 watt" 2xel34 amp could be SO much louder. I actually think I have about 440 VDC on the plates of my power tubes which I would not call low voltage.

Again, thank you all for your input. I look forward to dusting off the old soldering iron once I figure out a plan of attack.

[Bieworm]

A bogner shiva pushes a pair of EL34 into 80 watts. That thing is LOUD !!! Plate voltage is 500V
That was the only amp that could keep up with the loud drummer I played with at that time.

[pdf64]

...I have rather efficient and wonderful sounding speakers that the good chaps at warehouse guitar speakers hooked me up with. I have their Invader-50s and C/12s loaded a 4x12 loaded X-pattern in a Hiwatt cab and they are pretty efficient speakers. Much louder and less compressed than the original Chinese "fanes" that it came with (this is probably a 2000s cab)...

Maybe so but your WGS speakers only have 35oz magnets, so probably 97dB/watt (it's regrettable that WGS don't publish this data). Whereas speakers with 50oz magnets, eg based on the Celestion G12H30, would put out 100dB/watt.
In regard of the sound pressure level (SPL) that the cab generates, that's equivalent to doubling the input power.
Hence by a long shot, the most effective thing you can do to make your rig able to generate a higher SPL is to fit more efficient speakers, the ultimate being the EV EVM12L previously mentioned, as they won't exhibit power compression in the way the lesser speakers inevitably will.

...I have about ... somewhere about 414vdc on the 34's...

...I have a new and strong power transformer that I believe is rated around 200ma...I actually think I have about 440 VDC on the plates of my power tubes...

What's your HT Vdc, ie at the standby switch, at idle? And what plate or cathode current are the power tubes drawing, at idle?
If the HT V is about the same as the plate voltage, and it's down around 415V, then you've got a Marshall that will probably put out about 35 watts clean, 55W overdriven.
What does your HT voltage drop to when the amp is pushing out full power?

Marshall HT voltages varied wildly through the late 60s and 70s, ranging from under 400V to over 500V, see
https://el34world.com/charts/Schematics/files/Marshall/Marshall_jtm45_lead_45w.pdf
https://el34world.com/charts/Schematics/files/Marshall/Marshall_ampchart.gif
Because the OT specs stayed at about 3k4 per p-p pair, HTs down around 390 get about 30W clean, 50W overdriven.
Whereas HT around 450V get the full 50W clean, 75W overdriven.
Up at 500V it'll be around 60W clean, maybe nearly 90W overdriven.
Above that just gets silly, even Mullards and Siemens tended to have a short, red platey life, and modern EL34 can't cope.

Things settled down in the very late 70s, HTs in the high 400s, and the JCM800s were very definitely able to meet (and exceed) the 50W nominal per EL34 pair, see p5 of the pdf of the JCM800 range spec sheet https://drtube.com/schematics/marshall/jcm800-mv-ld-manual.pdf

If you want more power output, you need to supply the power amp with a higher, stiffer HT voltage; it's as simple as that.
Decent power tubes may help a tinsey bit, but all the stuff about preamp and phase splitter tube swaps and circuit mods won't make diddley squat difference once the rest of your band kicks in.

If you fit a power transformer that gives you a solid >450V HT, and load your 4x12 with EVM12L, I promise you that your bandmates will be begging you to turn down a little 

[Bieworm]

To my little knowledge about EL34 tubes I was always told the original svetlana winged =C= is the toughest EL34 known to mankind. Bogner says it's the only tube that would do the 500V plate voltage longer term..

[High Voltage]

       I do think the the phase inverter is probably the place to be looking at in terms of increasing headroom. I am considering trying a Silverface AA769 Fender Twin reverb PI. I believe that is a large part of why they're so clean.

That's the same PI you already have in your 2204, but they use a 12AT7 in there which has a little more headroom and lower gain.

[PRR]

... I guess it just blows me away that the DR504 which is another "50 watt" 2xel34 amp could be SO much louder....

So steal/borrow his amp.

First you find out if it is just the amp or the speakers too.

Second: he can't be louder if you got his amp.

(Unless he IS louder using your amp, which means "it is all in the fingers", which is sometimes true.)

[glass54]

+1 pdf64 
Higher/Stiffer power supply is a worthwhile alternative. (Dumble did it)
Dynacord did it (750V DC +B1 and 425V for G2s!!) using EL34. That was wild and 100W Output/pair  and yes I have worked on them in Aussie.
Now that we are all intrigued, if you ever visit Aussie (Canberra), you are welcome to bring you Amp+Speaker and your mates DR504 to my lab to do a full test. This includes static and fully driven with my APx525 Audio analyser and full electroAcoustic test of both speaker boxes, all for free.
We would also compare the iron in both amps and of course plot the PS sag and PI characteristics under test.
I'm sure we could learn something from such an investigation. It is a pity that we are so far away.
Also speaking of compression, when Speaker Drivers are used at high output SPLs for extended time, depending on Driver design, the voice coil is subject to heating/overheating? (sometimes very dramatic) and further depending on design/material choices, the resistance (and impedance) rises causing poor dynamic range, etc.... And most important "Don't let the Smoke out"  Thus some people will pay more for good driver performance under challenging/extreme conditions.
I have attached the Dynacord Sch for fun.
Kind regards
Mirek

Edit: When I do the sums its approx 80Wrms into 8R Load. Still not bad for EL34s

[whalecalls]

Hey y'all,
     Thanks for all the detailed and informative responses. I will consider all of your post. I have been a little occupied elsewhere as I just scored a '69 Sound City L100 for cheap the other day. I am currently about to undergo a full recap (had a leaky PS cap etc...) I might see how this amp works with my playing situation. After getting the amp running properly and safely maybe I will consider a master volume if needed. Anyone ever try out a Trainwreck "Type 2" Lar-Mar master volume in a Hiwatt style circuit?
      I am sure I will be in touch with more questions. Again, thanks for all the help. Keep soldering and picking'.

[mresistor]

I think a more efficient speaker(s) is a wise place to start. 

A speaker or speakers that handle more power, like a Texas Heat, and are more efficient like over 100db spl would surely help if speakers are breaking up early.

[Bieworm]

Trade the amp for 100WJMP.  I think lots of people are willing to do that. I think It's silly to turn an amp with a great natural overdrive into a clean pedal platform..

[Mike_J]

+1 on all who mention speakers with a higher sps rating and EV EVM12L speakers. They do loud clean very well compared to other speakers I have used.