JCM-800/2204 Build

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Reeltarded
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Re: JCM-800/2204 Build

Post by Reeltarded »

Yeah the grid storeper was my only idea but because it wasn't hitt..

HEY that IS it.. grid stop on your test signal and tell me what you have there.. lmao.. It HAS to be, right?
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Re: JCM-800/2204 Build

Post by martin manning »

Could be blocking distortion, which a larger grid stopper will help cure. The resistor you want to increase is the one in front of the stage that is producing the distortion.
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Re: JCM-800/2204 Build

Post by Reeltarded »

yes careful with me.. I will work at a problem on either side of the problem for a week..

:oops:
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Re: JCM-800/2204 Build

Post by martin manning »

What does this probe look like and how is it connected when tracing? Are you sure that it is not causing the problem?
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Re: JCM-800/2204 Build

Post by Reeltarded »

take the signal from the signal side of the next divider even

Played the amp straight yet?
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Re: JCM-800/2204 Build

Post by ampgeek »

I clipped in grid stoppers from 33K to 100 K on the fixed stage's grid and it progressively reduced the distortion along with the volume and some of the desirable "umph" character. Not much different than lowering the volume pot setting just upstream. So...that doesn't appear to be a magic bullet unfortunately.

The previous, switched stage, does have a 68K grid stopper like the original 2204 plan. It also has a 1Meg A volume pot just upstream of it. I need to study up on the blocking distortion phenomenon to understand how increasing that value might assist. This is a new concept to me.

Again, the signal on the fixed stage's grid when being fed by the switched stage is big, phat and glorious. Wouldn't I be hearing the distortion on that grid if blocking from the previous stage was occuring?

The amp sounds wonderful when fully cranked-up without the switched stage engaged. Kind of like a tighter, punchier 5F6A plan with just a touch more desirable "hair"...if that makes sense!

I am convinced that the probe isn't a part of the issue. Here is a link to the background on that marvelous device. Scroll down ~2/3rds for a description. The volume pot is a must IMHO. I frequently "calibrate" it by probing the "test amp's" speaker jack output and comparing that to the test amp's output directly to a speaker. The sound/character is nearly identical in both cases so I don't think that the probe is falsifying anything here.

http://www.el34world.com/Hoffman/tools.htm

A picture of my stethoscope assembly is attached.


Any thoughts on if the under-board wiring shown in dotted lines in the previous drawing might be problematic? The wires very closely follow the shown path.

Thanks as always for the valued input guys!!!

Sincerely,

Dave O.
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Re: JCM-800/2204 Build

Post by martin manning »

The thing about the probe is that you are paralleling a 1M load with whatever exists in the amp, as well as adding a lot of lead length. I don't think that is an issue here, but it will affect what you are hearing when you listen. One of the mods my 2204-ish amp has (thanks to Jana) is a 1M to ground from the wiper of the volume pot (it's on the pot from wiper to CCW lug), which takes out some muddiness when the volume pot is turned up. You might give that a try. I do not have a grid stopper there, but I do have a 27k on the third stage (the one driving the CF).
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Re: JCM-800/2204 Build

Post by ampgeek »

Thanks for tips/thoughts once again Martin.

A grid stopper on the stage feeding the CF sounds like a great idea. I will definitely give that one a shot.

Does your 2204'ish build still employ the peaker network (470K ll 470p + .001uf) upstream of the volume pot?

Yep....I amost always go through the..."how could lowering the impedance here impact performance/tone?" thought process everytime I am about to land the probe somewhere in the circuit. Can't say that I have it all figured out with theory/math...but...have come to a reasonable practical understanding of the effect (which is most always minimal).

I would truly appreciate your (and others) thoughts on possible oscillations caused by the underboard wiring. Again, seems unlikely given that they are both plate signals. But....there is a possible 360 deg phase shift interaction going on around there. In hindsight, I should have run the cap ---> common plate B+ well away from the plate resistor path to reduce the liklihood of an issue.

BTW: Your resonance circuit performs marvelously in this plan. I etched up a daughter board for the componants which worked very nicely.

Thanks again!

Dave O.
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Re: JCM-800/2204 Build

Post by Reeltarded »

Dave, in all my Marshall stuff there is no beef between the signal and the B+ and in a couple it's routed over and in between and on top and..

You know what does something like that and hides easy? Shitty 1M pots. I have had a handful of really troubled ones in the last couple years where suddenly towards the top of the range they do bad stuff. Alphas here.

The Martinizer is fantastic where it works, but 4 out of 5 here went crazy into oscillation on Marshalls at settings more towards the res side.
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Re: JCM-800/2204 Build

Post by ampgeek »

Thanks Miles! Yep...I wasn't holding out much hope that cross-talk there was the issue. Although, it was the very last place in this area of the amp that my feeble mind could envision a cause for what I am experiencing.

I guess that I am a bit stunned by the fact that such a eff'ing awesome signal on the fixed stage grid (from the switched stage via 1M pot) can get so fizzed-out from such a cold biased (10K cathode resistor) stage. That even with consideration for a reduction in impedance when probing that grid with the stethoscope.

After a 1/2 lb of solder, dozens of replacement components and a couple of hours of listening to JCM800 sound samples, I am beginning to think that a small/controlled amount of this type of distortion is a part of the character of the amp. Tweaking in or out the desired amount subject to the listener's desires.

Got it on the potential pot problems! I will slap in some alternates and see what happens. I am contemplating reducing the values of one or both of the 1 Megs I am currently using to see if it helps in the fizz control quest.

Ahhhh.....I now see what was behind Bob S's comment regarding potential instabilities with the Martinizer circuit earlier on in this thread! Wondering if I got lucky with my PCB implementation? I will offer up some samples of that to those interested in playing with it in their JCM800 builds to see if it is useful.

Onward!!

Dave O.
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Re: JCM-800/2204 Build

Post by Reeltarded »

The throaty thing that bothers you doesn't bother me at all. I try hard to make them do that. The lingering and growing growly thing.
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Re: JCM-800/2204 Build

Post by ampgeek »

I hear ya Sir Miles. We all strive for different things which makes places like TAG Forum so cool!!!

It is oh so difficult putting what we are hearing into mere words for obvious reasons. Subjectivity rules supreme on this topic. I would say that it is the "throaty" thing that I do like!

It is the too fuzzy, too un-natural sounding (both IMHO) "buzz" that you hear in this clip that I am trying to tame.

https://www.youtube.com/watch?v=y1osT39OSjs

It pops up in spades after the coupling cap of the fixed pre-amp stage when being driven by the switched stage and ends up being too prevalent for my liking at the output stage.

It is not quite as dominant as you hear in the clip...but...not too far off. I can knock it down to a tolerable level with the volume knobs (any or all of the 5 available) but the overall "ballzy character" of the amp suffers (subjective and in IMHO again!).

Because that character pops up at a very specific spot, my (potentially flawed) approach was to eliminate/tame it at the point of origin. I am now thinking that I need to adopt a more holistic perspective!!

What are your (and others!) thoughts on the character of the amp in that video clip? The "base" tonal and sustain character sounds, generally, OK for the classic, harder rock (not quite into metal zone territory) result I am envisioning. But that fuzz, buzz, etc..etc..whatever we choose to call it...character just ain't doing it for me.

Thanks as always,
Dave O.
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Re: JCM-800/2204 Build

Post by Reeltarded »

Instead of playing it soft like that put on Back in Black and play along with Shoot To Thrill.

100p fizz cap instead of 47p. Defeat brite cap... Turn up treble and moderate pres a little..

TURN it UP!! :)

Oh.. Also 100p cap over inverted side of the PI on the plate. That alone might take you there, maybe then the 100p swap from 47p.
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Re: JCM-800/2204 Build

Post by ampgeek »

LOL! I rattle the window panes when doing a "critical", end-to-end listening test and I know exactly what you mean. It must be breathing very deeply to bring out the circuit's real character. I just can't seem to be able to get my wife and three daughters to understand that concept!! :roll:

That video clip was intended to provide an example of the distortion character that I am hearing at...like...200 dB. :wink:

Funny you should mention Angus. It was a couple of those examples that made me realize what I am hearing is a part of the overall character of the sound that I am looking for. I had never disected it until now. A number of Thin Lizzy solos are pretty heavy with that character as well.

Thanks for those more holistic suggestions!

Cheers,
Dave O.
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Re: JCM-800/2204 Build

Post by ampgeek »

Spent the little quality time that I have had over the last month or so in clipper mania!

Etched up a "universal" board, laid down a few rails of sockets, scrounged/bought numerous diodes/transistors and the like (not the least of which was an LED assortment bag from RaT ShaK!) and had at it!

I was also armed with the guidance received here and other internet technical data and experiences by others to get the ball rolling.

All in all...I never tried anything that I completely hated. Almost every funky combination that I could come up with had its sweet spot and possible uses. Some just seemed better than others for no reason that I can describe objectively.

I began with a certain skepticism that they would all sound, generally, the same but was delighted to hear that there were distinct differences in tone and feel with each arrangement that I couldn't totally "blame" on other amp adjustments to find it's sweet spot.

It became apparant fairly quickly that I tended to like LEDs early in the chain. They just seemed more "natural" to my tin ear and playing style.

In general, the sweet spots seemed to all fall into the following range of settings:

Low gain mode (input-->gain stage-->vol-->gain stage-->CF)
Vol: nearly dimed to fully dimed
PPIMV: dimed
CF vol: Miles' favorite-"just backed off a smidge" (BTW: that seems to be universal no matter how this beast is set-up and played)
Pre-PI master: to taste and the desired overall volume
Squish: 2:00 to full on (25K Lin pot)

I ultimately ended up with the six shown in the attached for their broad range of coverage and tonal appeal. Still dinking around with the transitor based option a bit though.

One final takeaway for me is that if I could have one, and only one, of the arrangements that I tried in this amp....it would be the Marshall Jubilee clipping circuit. Ballzy, thumpy, rocking goodness over a very wide range of settings.

Cheers,
Dave O.
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