79 2204 - Factory V1B Plate Snubber?

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kdmay
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79 2204 - Factory V1B Plate Snubber?

Post by kdmay » Fri Jan 19, 2018 4:33 pm

I recently got this 1979 JMP 2204 Master Volume.

Has received some mods from a tele player, so dialled out a lot of top end:

V1a Cathode Bypass cap (in addition yo 0.68uf) was 10uf
Treble Peaker snipped out
PI entry cap 47nf (instead of 22nf)
PI cap 47nf (instead of 100nf)
Also for some reason the 4k7 PI tail resistor on the presence pot has been replaced with a MF in addition to the presence cap.

I have done the following mods:

V1a Cathode Bypass Cap - removed 10uf so we have stock 0.68uf
Added Treble Peaker cap. Only had a Mica, so will replace with a ceramic at some point
Replaced 1nf bright cap with 220pf styro
Parallelled 10k cold clipper with another 10k for 5k cathode resistor

My future plans are to dial out some bass in the preamp (replace 22nf couplers with 4n7), put PI caps back to stock and increase output couplers to either 47nf or 100nf. Possibly add a fixed depth 220k/2n2 circuit.

One interesting thing I have noticed is that the V2a plate resistor has been bypassed with a 220pf snubber. Now this looks factory stock, given it is the same RS type as the PI plate snubber.

I've never seen something like this stock from the factory. Has anyone else come across this? Is it possible that there was oscillation during the build process and they put the snubber in to address this?

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Re: 79 2204 - Factory V1B Plate Snubber?

Post by kdmay » Wed Jan 24, 2018 5:27 am

Does anyone still post on here these days??!!

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Re: 79 2204 - Factory V1B Plate Snubber?

Post by Roe » Fri Jan 26, 2018 8:27 am

Marshall introduced snubbers on the jcm800 2204s and 2203s. Perhaps they experimented a little?
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Re: 79 2204 - Factory V1B Plate Snubber?

Post by jarrodthebobo » Fri Jan 26, 2018 9:15 am

Perhaps there were concerns of Oscillation in some of the JCM800s of the era? Other than that the only reason I would think to believe that Marshall would do this would be to tame some top end in the circuit. I know most people love these amps for that snarl-y bite and high treble drive but, maybe not everyone was as satisfied with that tone back in the day.
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Re: 79 2204 - Factory V1B Plate Snubber?

Post by herbvis » Fri Jan 26, 2018 10:11 am

I dont think that the cap over the plate resistor looks like a factory job. The soldering job on it looks similar to the other added components.

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Re: 79 2204 - Factory V1B Plate Snubber?

Post by kdmay » Fri Jan 26, 2018 3:45 pm

herbvis wrote:
Fri Jan 26, 2018 10:11 am
I dont think that the cap over the plate resistor looks like a factory job. The soldering job on it looks similar to the other added components.
Not sure I agree with this. It's exactly the same type of cap as the PI snubber and the soldering doesn't look fresh at all.
Roe wrote:
Fri Jan 26, 2018 8:27 am
Marshall introduced snubbers on the jcm800 2204s and 2203s. Perhaps they experimented a little?
Interesting - I've never seen this before and I've looked at plenty of 2203 and 2204 guts!! Would love to see a photo of this.

In any case, it works in this amp.

Flipped the PCB yesterday (really very simple to do if you unmount the pots and jacks) and tweaked the coupling caps to dial some bottom end out of the preamp and bring it back in to the power amp. Wow, what a killer amp now. Got some ghosting or nasty trails off the note I am chasing, but I'm sure I'll sort this. Overall, really killer rock sound.

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Re: 79 2204 - Factory V1B Plate Snubber?

Post by Roe » Sat Jan 27, 2018 3:31 pm

kdmay wrote:
Fri Jan 26, 2018 3:45 pm
herbvis wrote:
Fri Jan 26, 2018 10:11 am
I dont think that the cap over the plate resistor looks like a factory job. The soldering job on it looks similar to the other added components.
Not sure I agree with this. It's exactly the same type of cap as the PI snubber and the soldering doesn't look fresh at all.
Roe wrote:
Fri Jan 26, 2018 8:27 am
Marshall introduced snubbers on the jcm800 2204s and 2203s. Perhaps they experimented a little?
Interesting - I've never seen this before and I've looked at plenty of 2203 and 2204 guts!! ...
Look at the 100pf snubber between the cathode and anode at v1
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Re: 79 2204 - Factory V1B Plate Snubber?

Post by herbvis » Thu Feb 01, 2018 7:31 pm

kdmay wrote:
Fri Jan 26, 2018 3:45 pm
herbvis wrote:
Fri Jan 26, 2018 10:11 am
I dont think that the cap over the plate resistor looks like a factory job. The soldering job on it looks similar to the other added components.
Not sure I agree with this. It's exactly the same type of cap as the PI snubber and the soldering doesn't look fresh at all.
My bad, after a 2nd look I think you are correct. :hide:

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Re: 79 2204 - Factory V1B Plate Snubber?

Post by kdmay » Fri Feb 02, 2018 3:38 pm

herbvis wrote:
Thu Feb 01, 2018 7:31 pm
kdmay wrote:
Fri Jan 26, 2018 3:45 pm
herbvis wrote:
Fri Jan 26, 2018 10:11 am
I dont think that the cap over the plate resistor looks like a factory job. The soldering job on it looks similar to the other added components.
Not sure I agree with this. It's exactly the same type of cap as the PI snubber and the soldering doesn't look fresh at all.
My bad, after a 2nd look I think you are correct. :hide:
No worries - I've never seen one like it!
Roe wrote:
Sat Jan 27, 2018 3:31 pm
kdmay wrote:
Fri Jan 26, 2018 3:45 pm
herbvis wrote:
Fri Jan 26, 2018 10:11 am
I dont think that the cap over the plate resistor looks like a factory job. The soldering job on it looks similar to the other added components.
Not sure I agree with this. It's exactly the same type of cap as the PI snubber and the soldering doesn't look fresh at all.
Roe wrote:
Fri Jan 26, 2018 8:27 am
Marshall introduced snubbers on the jcm800 2204s and 2203s. Perhaps they experimented a little?
Interesting - I've never seen this before and I've looked at plenty of 2203 and 2204 guts!! ...
Look at the 100pf snubber between the cathode and anode at v1
Again, never noticed the anode to cathode snubber on an 800.

I guess my original point remains, has any else ever seen a 2203 or 2204 with a plate snubber cap stock from the factory or do I have a unicorn here??

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Re: 79 2204 - Factory V1B Plate Snubber?

Post by neikeel » Fri Feb 02, 2018 4:38 pm

No I have not, other methods yes, small ceramic between cathode and plate, yes, different shielding , methods, yes, never seen that type of cap used in PI or across the plate, only small ceramic discs.
Maybe a rare one may be a clean modded. Certainly by this era those square mica caps had gone in Marshall production. they were using small ceramic discs or the tubular ceramic Lemcos, but one can never be definitive.
I find the method you are using regarding shelving he lows in the preamp interesting as it works very well indeed in tweaked 100w Superbass type amps in my experience, although strictly speaking you do not bring the lows back in the power amp, you simply conserve what you have and do not shelve any more as you cannot bring back what you have lost :wink: .
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Re: 79 2204 - Factory V1B Plate Snubber?

Post by kdmay » Sat Feb 03, 2018 2:49 pm

neikeel wrote:
Fri Feb 02, 2018 4:38 pm
No I have not, other methods yes, small ceramic between cathode and plate, yes, different shielding , methods, yes, never seen that type of cap used in PI or across the plate, only small ceramic discs.
Maybe a rare one may be a clean modded. Certainly by this era those square mica caps had gone in Marshall production. they were using small ceramic discs or the tubular ceramic Lemcos, but one can never be definitive.
I find the method you are using regarding shelving he lows in the preamp interesting as it works very well indeed in tweaked 100w Superbass type amps in my experience, although strictly speaking you do not bring the lows back in the power amp, you simply conserve what you have and do not shelve any more as you cannot bring back what you have lost :wink: .
It's very effective. Don't forgot the fixed depth control in the NFB, so in a sense I am actually reintroducing some bass into the PI and letting this through the power section through the PI couplers.

I ended up settling on 56nf as I had some old mustards sitting there not doing anything. Comparing 22n to 56n was a huge difference.

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