JCM-800/2204 Build

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ampgeek
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Re: JCM-800/2204 Build

Post by ampgeek »

Getting there slowly but surely!
I have received all parts and will start drilling/staking the circuit board next week. Chassis punching and drilling shortly thereafter.
I have had a flurry of business travel lately that has held this project up a bit.
Cheers,
Dave O.
ampgeek
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Re: JCM-800/2204 Build

Post by ampgeek »

Starting to pick up a bit of steam now that I am at home a little more frequently.

Main board loaded and chassis drill template nearing completion.

Just noticed that I bypassed the third stage (just up-stream of the CF) with a 0.68 uf. Is that going to be a bit over the top?

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

Post by Reeltarded »

Weird! hah

No, bypass all good. Hurry and hear it.
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john l
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Re: JCM-800/2204 Build

Post by john l »

Wow dave Your an animal! Whats the the story on those resistors?
John
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Re: JCM-800/2204 Build

Post by ampgeek »

Thanks mates!

The resistors are Precision Resistor Products (PRP) metal films in both 1W and 1/2W ratings. I built an 18W mutation with them earlier this year that sounded pretty good and had some extras that were useable for this project.

They are a tad pricey but not completely over the top for what appears to be a very decent product. Only semi-audiophoolish in my book. :lol:

http://www.partsconnexion.com/resistors ... _watt.html

I spent some time on the DC distribution approach. A friend gave me a 12VDC/1.5A switching power supply that is pretty compact. I used Merlin's approach to a circuit for regulated DC pre-amp filiments and came up with the attached PCB design. I also included 5VDC and 10VDC take-offs for relays and an LED strip light "accent" that the ultimate owner of the amp inquired about.

Hoping that the solid state gurus can give that a quick look. I am not convinced that the first 4700uf filter is needed..but...I can't see it hurting either.

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

Post by john l »

Alright Dave report already. I cant wait to see where your at.
John
john l
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Re: JCM-800/2204 Build

Post by john l »

Also an update on where Im at with mine. I added a couple of toggles and played with the filtering a little.

-toggle 1 is a pre gain master defeat. A great tech friend of mine brought to my attention that with it in circuit all the time it loads down the tone stack and that the Jose is a different animal without it. He was right there is a significant change in tone (a lot more sparkle and chime along with improved clarity), the catch is that you loose the crazy versatility and ridiculously good tone at whisper levels. Putting it on a switch keeps both.

-toggle two (on off on) switches a .002 and a .0047 over the second treble peakers 500pf. Its cleverly labeled thick and thicker.

-Push/pull gain pot throws a 700pf cap across the 1st treble peaker. I found this value is perfect for putting just the right amount of air around lead work, adds some nice definition to thing

-Ive also gone with 1uf cathode bypass caps throughout over the .68ufs... Somethings happening there that I like.

-Last I raised the mains filter to 200uf! Sounds crazy but I love it. Keeps everthing punchy as all get out and not the least bit stiff.
Last edited by john l on Mon Nov 04, 2013 2:43 am, edited 1 time in total.
John
ampgeek
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Re: JCM-800/2204 Build

Post by ampgeek »

Hey Jon,

Doing the catch as catch can gig between family and work time. Third quarter business travel took its toll on me. But.. it is getting there little by little.

Populating the sub-boards right now. DC power supply module pretty much done. 12VDC switching power supply and homegrown regulator board for pre-amp filiments, relays and lights. Tone board in progress...and I already do not like the thumbwheels!! :evil:

Front and back faces fully drilled and faceplates on order (BNP Laser).

No fatal flaws so far!!

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

Post by Reeltarded »

Nice stuff, both of you guys!

I can't see John's removed pics.. wtf :P
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john l
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Re: JCM-800/2204 Build

Post by john l »

God bless the wifes phone. Also I don't think I had added the infamous Miles squish pot in the last pic I posted so that's also been added and is right under the rotary switch.

[IMG:800:600]http://img.photobucket.com/albums/v299/Johnspics/photo_zps16ad9411.jpg[/img]


Dave yours is awesome looking! Your buddy is going to be a pig in shit with that thing!
John
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Re: JCM-800/2204 Build

Post by ampgeek »

Thanks guys. Nice and clean looking build there Jon!!

Too bad I am trading it away for a 81 Explorer...but...I can always build another one. :wink:

I like the idea of being able to switch out the pre PI MV. Are you lifting the tail of the pot from the circuit ground? What brand switch was to your liking?

Rock on!!

Dave O.
john l
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Re: JCM-800/2204 Build

Post by john l »

ampgeek wrote:Thanks guys. Nice and clean looking build there Jon!!

Too bad I am trading it away for a 81 Explorer...but...I can always build another one. :wink:

I like the idea of being able to switch out the pre PI MV. Are you lifting the tail of the pot from the circuit ground? What brand switch was to your liking?

Rock on!!

Dave O.
Thanks dave
Heres a pic
Both brown wires apex at the PI entrance. This is the only way I could get it completely out. You only need a DPDT but all my parts store had were 3 poles. Its a cool option.
[IMG:800:600]http://img.photobucket.com/albums/v299/Johnspics/photo_zps2d4460c8.jpg[/img]
John
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Re: JCM-800/2204 Build

Post by ampgeek »

Got this thing all wired up and running and have identified an issue that I can't seem to sort out.

The build currently sits at typical JCM800 specs save a 56K on the cathode of stage 3, a CF volume and a Presonence circuit (which is awesome BTW!). I have never used a JCM800 and wanted to see how the "stock" configuration sounded before starting all of the cool mods discussed throughout this thread. All voltages are nearly spot on to the reference values.

Everything is behaving VERY nicely and I can hear the ballzy Marshall goodness....except for an "un-natural" distortion after the coupling capacitor of the normal triode when in high-gain mode (so...the second gain stage in this mode).

It isn't there in any audible way when in low-gain mode and doesn't show up on the grid pin in high-gain mode. It seems to pop up only when being driven by the preceeding stage in high-gain mode.

It sounds very much like what you hear in this clip. However, in my case, I am hearing it early on in the chain. It gets progressively amplified later on and sounds pretty bad when played through the rest of the circuit and out to a speaker.

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


I drew up the location of itnerest and added a few notes in the image below. The amp is firing into a dead load and I am using the amp stethoscope feeding a nice and clean, low gain tube amp for signal tracing/trouble shooting purposes. I am confident that the "listening amp" system isn't a part of the observed problem.

Did all of the normal troubleshooting stuff that you would start with (e.g., tried MANY tubes known to be good, re-flowed solder joints, replaced plate load resistor and coupling cap multiple times, re-placed the tube socket, different guitars, etc..etc..etc..) and nothing has changed the character of the distortion in any way.

I am starting to look at the filter caps as possible sources and note that the on-board uF readings vary from #1 to #4 but know that "in-circuit" readings can be suspect. Oddly, #2 and #3 never reach a fixed reading. The both jump back and forth between the values cited in the sketch. I did slap in a temporary unit at position #4 but heard no change. Any thoughts/experience with this possibility?

Also starting to think that the underboard wiring (God forbid!!) should be at least considered. Seems unlikely that there could be "interplay" between two similar level plate signals....but...there isn't much left to tinker with at this point.

Input and thoughts GREATLY appreciated!!

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

Post by Reeltarded »

hrmmmm I thinking but my brain gets no traction. :/
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ampgeek
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Re: JCM-800/2204 Build

Post by ampgeek »

LOL! I have been "brain slipping" on this one for a lot longer than you my friend!!


I was thinking about slapping in a grid stopper just for shits-n-giggles. Not sure if that makes sense given that I hear nothing offensive on the grid

I have already tossed in a plate-t0-cathode cap on a whim. Didn't make a bit of difference as I suspected.

Thanks as always,
Dave O.
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