Jcm800 ef80 MV change...need advice/help!

Blueslover88

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I know, another master volume thread LOL bare with me I'm still pretty new at this. So I thought the master volume in Rob's jcm800 ef80 was a type 3 crossline MV. But after doing some research and looking the layout over I noticed it's actually a pre phase inverter master volume. So could I just land the wire coming off the coupling capacitor to middle lug on treble pot and then add in a tru type 3, by running two wires from grid stoppers to the 1 meg MV pot? Tell me if I'm way off base here please. The problem is this MV goes from zero to really loud with just a nudge and not a very good sweep. The schematic is for the 12au7 version because I couldn't find the true schematic for the ef80 version
 

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Lowerleftcoast

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That should work.
The only concern I can imagine is the extra 1M to ground (the 1M pot) after the treble pot will be missing so, the signal could be a little hotter there. I doubt it will be an issue.
 

Ten Over

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I know, another master volume thread LOL bare with me I'm still pretty new at this. So I thought the master volume in Rob's jcm800 ef80 was a type 3 crossline MV. But after doing some research and looking the layout over I noticed it's actually a pre phase inverter master volume. So could I just land the wire coming off the coupling capacitor to middle lug on treble pot and then add in a tru type 3, by running two wires from grid stoppers to the 1 meg MV pot? Tell me if I'm way off base here please. The problem is this MV goes from zero to really loud with just a nudge and not a very good sweep. The schematic is for the 12au7 version because I couldn't find the true schematic for the ef80 version
This one will give you a better sweep than using a 1M pot.

PPIMV 2204 Micro EF80.png

You might want to replace the pre-MV with a fixed voltage divider in order to get some cathode follower "quack" before the PI gets totally swamped.
 

Blueslover88

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This one will give you a better sweep than using a 1M pot.

View attachment 1035260

You might want to replace the pre-MV with a fixed voltage divider in order to get some cathode follower "quack" before the PI gets totally swamped.
So your scheme. Shows type 3 with 100k grid leaks instead of the 56k used and a 250k pot, correct? Thanks a ton for that!!

I have been reading others posts on here with ef80 builds and type 2/lar-mar seems to be the winner from their views. They are using dual gang 250k with 2M safety resistors but that's in the bassman micro. If I wanted to opt for a type 2 what value pot would I use in this jcm800 micro? I only ask because in the bassman it uses 220k grid leaks as opposed to 56k in mine.
 

Ten Over

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So your scheme. Shows type 3 with 100k grid leaks instead of the 56k used and a 250k pot, correct?
Yes.

A crossline resistor appears to be one half its nominal value with respect to ground. When the wiper is all the way up, it appears as though there is 125k in parallel with the 100k grid leak resistor for a total of 55.6k. Pretty close to 56k.
 

Ten Over

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I have been reading others posts on here with ef80 builds and type 2/lar-mar seems to be the winner from their views. They are using dual gang 250k with 2M safety resistors but that's in the bassman micro. If I wanted to opt for a type 2 what value pot would I use in this jcm800 micro? I only ask because in the bassman it uses 220k grid leaks as opposed to 56k in mine.
Here is one way:
PPIMV 2204 Micro EF80 #2.png

The safety resistors have to be 220k in this one so that the maximum signal at the EF80 grids is about the same as it was without the PPIMV.
 
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Blueslover88

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Here is one way:
View attachment 1035326

The safety resistors have to be 220k in this one so that the maximum signal at the EF80 grids is about the same as it was without the PPIMV.
Wow! Thanks so much for figuring that out for me. I'm still learning and definitely don't know the equations to figure out the proper values needed. I have 220k resistors so I just need to order a 100k dual gang pot and hope it will fit in the angled front face of Princeton reverb chassis!
 

King Fan

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I love the Lar-Mar in my Bassman. Still trying to understand it better... :)

I like the idea from @Ten Over about dual pot size. And I think you were onto the reason when you noted the 56K instead of 220K grid leaks. Let's see if *I* understand.

Rob says about the JCM800 EF80 version: "The power tube grid stop and grid leak resistors were rearranged to form an attenuating voltage divider that dumps 90% of the signal to keep from overwhelming the little power tubes." This is obviously different from the 470K/220K divider in the EF80 Bassman. Is that why we can use a 100K dual pot here?

Now for the dumb and dumber part. I think I understood sizing the safety resistors a few months ago, but poof, that's gone.
 

Ten Over

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In this instance you changed the grid stoppers from 470k to 560k. Was that on purpose?
Yes. It makes the voltage divider nearly the same as the non-MV 470k:56k divider when the MV is at max. So if you were getting +75Vp out of your PI (a plausible voltage), then you would get 8Vp at the EF80 grid without the MV. You would get 8.2Vp with the MV at max.

If you used 470k instead of 560k, then you would get 9.6Vp at the EF80 grid. This wouldn't be any big deal to me since I could turn the MV down so that it put 8Vp on the EF80 grids. It could well be a good thing to have the ability to overdrive the EF80's a little more than without the MV. Once you discard the strict adherence to the original design, all sorts of possibilities open up for the MV.
 

Blueslover88

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Yes. It makes the voltage divider nearly the same as the non-MV 470k:56k divider when the MV is at max. So if you were getting +75Vp out of your PI (a plausible voltage), then you would get 8Vp at the EF80 grid without the MV. You would get 8.2Vp with the MV at max.

If you used 470k instead of 560k, then you would get 9.6Vp at the EF80 grid. This wouldn't be any big deal to me since I could turn the MV down so that it put 8Vp on the EF80 grids. It could well be a good thing to have the ability to overdrive the EF80's a little more than without the MV. Once you discard the strict adherence to the original design, all sorts of possibilities open up for the MV.
Yes I realized this after I read your answer in my "other" post. Hence kingfans great advice to keep thread numbers limited (sorry y'all).A If I knew how to merge the two I would. Thanks again. I ordered the pot from AES yesterday and a few more silver micas caps. Would lower number value caps for the treble caps mean les treble or is it the other way around? Ex. I have 500pf silver micas in the treble peakers spots, would 330-220pf caps cut out some of the highs?
 

Ten Over

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Would lower number value caps for the treble caps mean les treble or is it the other way around? Ex. I have 500pf silver micas in the treble peakers spots, would 330-220pf caps cut out some of the highs?
Lower value caps will shift the treble peak to higher frequencies. Whether or not you perceive this shift as more treble I really couldn't say because the higher mids will decease, also. Best just to try it and see how you like it.

Make sure that you heatsink those silver micas when you solder them.
 

Blueslover88

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Moving forward with this MV change I will only be posting in this thread as to avoid any confusion or double posting.

Pot will be here the end of this week, I noticed as others had said about this particular build the presence control didn't have much effect......well it wouldn't because I missed the wire going from NFB resistor to the output jack...DUH.

After adding the wire the output seemed to decrease and I now have presence control, who woulda thunk it!

Anyways I think I may leave the original 470k grid stop resistors in at first as @Ten Over was discussing earlier. If it sounds totally different I will go with the first suggestion swapping them for 560k! More to be revealed.
 

Blueslover88

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I received the dual gang pot las night, it was smaller than I intended ( not the full size dual gang). I'm am going to attempt to use it as is. Does anyone think I should leave the current MV in and add this type 2 as a third volume after the presence knob? I'm leaning more towards replacing the pre-phase inverter MV with this one. Any thoughts?
 

dan40

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I would leave them both if you have the space. It's nice to have control over the signal level going into the phase inverter because when you dial down your ppimv and raise your preamp volume to get a bit of preamp overdrive, your PI begins to distort from the large signal it's receiving from the preamp. In certain amps, this PI distortion sounds good and adds an extra layer to the overdriven sound. In other amps, PI distortion can sound unpleasant and ratty. Having the pre-pi volume allows you full control over the signal being fed into the PI. You can push your preamp volume up to get the first few gain stages cooking and then reduce the level before it hits the PI. The ppimv will be the final control just before the signal enters the power amp. Having both MV's allows you to dial up just the right amount of the preamp gain, PI gain and lastly the power amp volume.
 
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Blueslover88

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I would leave them both if you have the space. It's nice to have control over the signal level going into the phase inverter because when you dial down your ppimv and raise your preamp volume to get a bit of preamp overdrive, your PI begins to distort from the large signal it's receiving from the preamp. In certain amps, this PI distortion sounds good and adds an extra layer to the overdriven sound. In other amps, PI distortion can sound unpleasant and ratty. Having the pre-pi volume allows you full control over the signal being fed into the PI. You can push your preamp volume up to get the first few gain stages cooking and then reduce the level before it hits the PI. The ppimv will be the final control just before the signal enters the power amp. Having both MV's allows you to dial up just the right amount of the preamp gain, PI gain and lastly the power amp volume.
That makes total sense and makes it easier because I stupidly bought the smaller version of the dual gang pot. Next thing is if I want to drill a new hole in the Front or install it in the back?
 

dan40

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I normally choose the back because I'm limited for space but it's a headache to have to constantly reach around the back to adjust the volume. If you have the space for the pot on the front panel, I would definitely go with that.
 

Blueslover88

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I normally choose the back because I'm limited for space but it's a headache to have to constantly reach around the back to adjust the volume. If you have the space for the pot on the front panel, I would definitely go with that.
I'm replying to you because you asked for a sound clip after I put it in. This is quick and dirty from my phone because it's all I got. I took your advice and left original two volumes in. I'm glad I did it sounds great. This is at "TV" volumes you can still hear my strings and I think it sounds great. Lots more tones and volume in tap as well!!! Thanks @Ten Over for your schematic I left on the original 470k grid stoppers everything else is per your drawing.https://photos.app.goo.gl/AXA4Y4vscwbL76uy8
 




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