Best High gain Mod/ Master Volume for a JMP 50 MK II

[xtian]

I had great luck removing the two ceramic caps on the inputs, cleaning up the screwey wiring, and running shielded cable to from inputs to first stage. Also changed R5 (low input resistor) from 470K to 220K. MUCH better tone overall, especially without those damn bright caps. Finally, added an 1-ohm to the power tube cathode, and biased at 30mA. Sounds awesome.

[img747]http://www.monkeymatic.com/pub/ebay/marshall800f.jpg[/img]

[img747]http://www.monkeymatic.com/pub/ebay/marshall800g.jpg[/img]

[Decko]

Hi All,

Sorry about the other link. I pulled it from my IPAD.

Here is an updated link from my PC.

Here is the tone I am after:

High Gain Shootout 6 Push slider to 6:42 minutes into the video.

http://www.youtube.com/watch?v=NzrgBF7v9fo

There were a ton of contenders these guys sampled Mesa, Bogner, Marsha, Soldano, to name a few. All sounded bad ass...but the one that sounded awesome to my ear was the Marshall 100 watt JA modded.

Nice low end, tons of gain but good separation and that midrange shimmer. That is what I am after that midrange shimmer! My mod would attempt to inject a bit more low end.

Cheers,
Danny

[Roe]

this works for me: jose gainstage into stock 1959 with larmar.
k=2k7/.68u
a=320k, 2n2 cap into 68k/68k voltage divider, then to 1959
also, add a snubber over the anode resistor

[Decko]

Hi Dave, and All,

I did the mods today.

.68 bypass cap on v1a.
Sounds good.

.47 bypass cap on v2a
Sounds good.

I tried the slope resistor at 47k but the amp lost some low end bass that I liked so I put the 33k back.

I will not JA md ths amp. The JA mod will be anther project. It was difficult or more trouble that it was worth to consider exactly how I old stuff an AX7 into the chassis.

Overall the tone is good. The low end is not as tight and focused as I would like. Would this be the filters?

The 5k6 grd stoppers - on the power tubes...what will these do?

Danny

[Structo]

Higher value grid stoppers on the power tubes add stability but can also choke high frequencies if made too big.

A value between 1K and 5K is usually ideal.

[Roe]

5k6-10k is standard for el34s and highly recommended

[Decko]

What values are best for 6550's?

[Colossal]

Hi Dave, and All,

I did the mods today.

.68 bypass cap on v1a.
Sounds good.

.47 bypass cap on v2a
Sounds good.

I tried the slope resistor at 47k but the amp lost some low end bass that I liked so I put the 33k back.

Ok good start. Are you happy with the level of gain? Is it any closer to what you are after?

Overall the tone is good. The low end is not as tight and focused as I would like. Would this be the filters?

Were you able to go through the amp and measure the B+ at each node? It would be useful to know the voltages (plates, screens grids, cathodes, input grids) as well as your bias voltage (and cathode current). What size filter caps do you have? Can't quite make out what values they are from your picture.

One thing that comes to mind is the (PPI) master volume. With the master volume all the way up, it is setting the grid leak resistance at 250k, or about 224k if you strapped a 2M2 across terminals 1 and 3. If you look at the tube manuals for a 6550, the recommended grid leak resistance for fixed bias is a mere 50k and 220k in cathode bias. 220k/220k is the standard values for EL34s in Marshalls run with fixed bias. I've seen JMP 50 watt amps with 150k for grid leaks when running 6550s.

You might consider lowering the overall resistance the PPIMV presents to the phase inverter. So if you strapped 180k across each side of your 250kA-dual PPIMV pot, the resistance would drop to 105k. Still double the published recommended grid leak value for a 6550 but it would load the PI nicely. This should give a bit more preamp gain by driving the phase inverter. It will also change the time constant for recovery in the power amp.

You could also add a fixed resonance circuit (or variable) which will help you dial up the low end in the power amp. There's other things that can be modded, but you don't want the low end to get muddy so important not to change too much at once and have a plan.

The 5k6 grd stoppers - on the power tubes...what will these do?

I would leave as they are for the time being.

[Roe]

What values are best for 6550's?

68k bias splitters, no higher than 150k. the standard 5k6 grid resistors works fine

[john l]

Hi Gents,

Thanks for the recommendation.

PPIMV is installed. Please see picture. Any comments on the installation? Wiring? I based it of Ken Fisher PPIMV...

Works great!

What can I do to tighten up the low end?
Sounds a bit muddy.

Looking for the "chunk, chunk" effect.

Cabinet is a vintage 4X12 with g12m rolas.

Would you recommend EL34's over 6550's for a heavy rock sound?

Danny

Hello everyone, new to the forum. Just got a 74 jmp 50 MKII that has a pre PIMV already installed that Id like to rewire to post master such as the one in the OPs pic. What value dual pot did you use here? The lar/mar sugests 250k but the TW pages have a dual 1meg and dual 100k version.

[Decko]

Hi there, I used a 1 meg dual pot because that is what I had in stock. That said, 250k should be fine... Read through the earlier posts and you see a few comments on the different PPIMV's...

Danny

[Colossal]

Dudes,

Something to keep in mind when selecting a dual-gang pot to replace the two grid leak resistors as a PPIMV, it is important not to select an arbitrary or on-hand value. The value of the pot (and any resistors in parallel) should total a grid leak resistance that either matches the original circuit or some value below. The maximum values for grid leak resistance can be found in tube manuals and are different from tube to tube and whether fixed or cathode bias. Throughout guitar amp history, designers often bent the rules making these values larger than recommended (e.g. 100k is the maximum recommended grid leak resistance for a 6V6 in fixed bias; Leo Fender used 220k here), seemingly with no technical consequence.

If you only have a 1M dual-gang pot hand, you can parallel 284k to reach 221k, pretty much spot on for an EL34 based Marshall and the 220k grid leak resistors you are replacing. 6550 based Marshalls should run at 50k (according to the RCA tube manual), but historically ran at 150k.

If you hit the power amp with too much signal by making the grid leak too large driving the grid voltage positive, you can push your screen voltages well past maximum dissipation (meltdown), create conditions for crossover distortion (and ugly distortion products), slow bias excursion recovery even more, etc. Sometimes the resulting sound is a happy accident, but screen safety must be considered and is usually the limiting factor.

[Roe]

the important thing is resistance to ground. on a 50w with 220k grid resistors, this means 220k plus bias circuit (usually 25k pot plus 47k-82k resistor). on a 100w it means 220kx2 plus bias circuit

[Colossal]

Hello everyone, new to the forum. Just got a 74 jmp 50 MKII that has a pre PIMV already installed that Id like to rewire to post master such as the one in the OPs pic. What value dual pot did you use here? The lar/mar sugests 250k but the TW pages have a dual 1meg and dual 100k version.

Hi John,

Please see the enclosed drawing. I made the PDF pretty large so you can zoom in to see the details. If you are running EL34s, you'll want a dual 250kA pot and a pair of 2M2 1/2W (or more) resistors and some dual conductor shielded wire.

[Roe]

resistors need not be half watt really