Help with JCM800/Plexi volume

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andresound
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Help with JCM800/Plexi volume

Post by andresound »

Hi Guys,
I posted this build in a earlier topic in this MARSHALL thread. However, since installing the PPIMV (Lamar style), the amp (50W) seems to have less power. Prior to the PPIMV install, the amp sounded “big” and “strong”. It now sounds like a 15w amp!! It also has developed farty type bass. The coupling caps to the output tubes were .1uf. I changed these to the standard .022uf. This did not improve matters much. Here are my voltages at present. The amp is a MV 50w model. Friend asked to also install a PPIMV.

Any ideas?

B+ 360 V
PLATE 358.6 V
SCREEN 353.6 V
PIN 5. -30.5V

PI 1= 202 V
6= 193.3 V
3+9 = 33.5 V
7= 22.2 V
2= 21 V

V2 1+7= 152 V
6= 264 V
8= 153 V
3 = 0,9 V

V1 1= 229 V
3= 2.4 V
6 = 193 V
8= 1.67 V
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romberg
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Re: Help with JCM800/Plexi volume

Post by romberg »

More details about your LarMar MV would be helpful. Did you use a 250k dual ganged pot? Did you include the 2m "safety" resistors? Did you use shielded cable for the runs to/from the pot?

If all goes well, a LarMar should pretty much be out of the circuit when turned all the way up. Of course as you turn the LarMar down the amp can and will sound different. But full up it is more or less identical to the 220k resistors it replaces. So, it should be a simple fix. How exactly did you wire this up?

You mention changing the phase inverter coupling caps. Did you do this before or after the MV install? Did you change anything else along with the MV install?

Mike
andresound
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Re: Help with JCM800/Plexi volume

Post by andresound »

I used a dual 220k pot with the 2.2m safety resistors and screened cable. I will post a pic later. I just tacked in the 22n’s for now.

After extensive research, I changed the coupling caps after the MV install, thinking this might be the problem, as the PI section is the same in all the 50w versions. The older amp with the tube recto had the 100n caps and this is ss recto. I would have thought the 100n would be a “big” sound?
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Littlewyan
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Re: Help with JCM800/Plexi volume

Post by Littlewyan »

It should work fine with 100n caps. I used these when I had a PPIMV fitted. You've got the coupling caps between the PI Plates and the PPIMV circuit yeah?
andresound
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Re: Help with JCM800/Plexi volume

Post by andresound »

Yes
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romberg
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Re: Help with JCM800/Plexi volume

Post by romberg »

andresound wrote: Mon Oct 09, 2017 10:27 am I used a dual 220k pot with the 2.2m safety resistors
a 250k pot with 2.2m in parallel gives you: 1 / (1 / 250 + 1 / 2200) = 224k.

For a 220k pot you get 1 / (1 / 200 + 1 / 2200) = 200k

So, a 250k pot is 4k off from the "stock" 220k. A 220k pot will be off by 20k. This does make the load the PI sees a little different (from stock). But worst case it is only about 10%. I doubt this would be the reason for the amp sounding radically different (with the MV cranked). The original 220k bias feed resistors were removed right?

Mike
andresound
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Re: Help with JCM800/Plexi volume

Post by andresound »

EA32D105-C487-47AD-8B35-734BE518969D.jpeg
Thank you for the replies. Here is a pic of the amp. The Orange drops are just “tacked” in and therefore not mechanically secure!! I will replace with the 100n’s.
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romberg
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Re: Help with JCM800/Plexi volume

Post by romberg »

Your amp does not appear to have any grid stoppers on the power tubes. I wonder if adding the extra wire/pot has caused an ultra-sonic parasitic oscillation? If you have a scope you could check for it. Or just add a couple 5k6 grid stoppers onto the power tubes. Unlike Marshall, you don't need to pinch every penny and leave those out.

Also, your picture is really hard to tell how your MV pot is wired (black on black). The important bit is that the top of each pot is connected to the coupling caps, the bottom of the pot is connected to your bias voltage (the cable shield should also be at the bias voltage) and the middle wipers connect to the power tube grids (I'd connect some 5k6 grid stoppers right to the tube sockets). The 2m2 resistors connect from the bottom of the pots (bias voltage) to the center wiper.

If the above is true I'd check for an oscillation and or just install some grid stoppers and see if things improve.

Mike
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Re: Help with JCM800/Plexi volume

Post by sluckey »

Your B+ is only 360V??? I would expect more like 450v to 500v for a 50 watt amp.
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martin manning
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Re: Help with JCM800/Plexi volume

Post by martin manning »

romberg wrote: Tue Oct 10, 2017 8:14 amYour amp does not appear to have any grid stoppers on the power tubes. I wonder if adding the extra wire/pot has caused an ultra-sonic parasitic oscillation? If you have a scope you could check for it. Or just add a couple 5k6 grid stoppers onto the power tubes.
Good eye Mike. I was thinking oscillation, but the description of the MV install sounded pretty solid. I'd try adding 5k1 or 5k6 grid stoppers on the power tube sockets.

Voltages were lower (~400V on plates) on earlier JMP amps, so that's not necessarily a bad thing. What do you have for plate voltage with no tubes installed? Is the bias current stable?
andresound
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Re: Help with JCM800/Plexi volume

Post by andresound »

Martin, I did have 5k6 grid stoppers in and then removed them, to see if it helped. It didn't. I will reinstall them.

Unfortunately, we have been hit by a category 3 storm and my workshop got flooded 2hrs ago, so will post plates without tubes when sorted 😏
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martin manning
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Re: Help with JCM800/Plexi volume

Post by martin manning »

Sorry to hear that, good luck! Is it possible that there is a strand of shield wire grounding out the signal on one side?
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Littlewyan
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Re: Help with JCM800/Plexi volume

Post by Littlewyan »

Sorry to hear about that. Hope the damage isn't too bad!

Just to confirm, what resistance reading do you get from pin 5 of the output sockets to the bias supply turret on your board between the coupling caps? Should be around 220k.
andresound
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Re: Help with JCM800/Plexi volume

Post by andresound »

martin manning wrote: Tue Oct 10, 2017 11:20 am Sorry to hear that, good luck! Is it possible that there is a strand of shield wire grounding out the signal on one side?
Thanks Martin,
This hurricane is the first hurricane I have experienced ever, and i have lived here all my life.. all 56 years of it.!!! Extremely rare occurrence. Nature is NOT happy with us humans at the moment. Natural disasters all over

I did heat shrink all ends, but I will check. I will also check resistance to pin 5 (with the 5k6’s in place). Might be a few days I’m afraid 😔
If it sounds good, it is good! Trust your ears
andresound
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Re: Help with JCM800/Plexi volume

Post by andresound »

9784727F-FC7E-42CD-A953-3DB8F1F5B096.jpeg
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