Discuss the stuff that makes your ears bleed.

Moderators: Slowy, Capt. Black

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BY handsoffmatt
#856021
edited..
Last edited by handsoffmatt on Tue Mar 24, 2020 9:41 am, edited 1 time in total.
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BY griff7628
#856022
Any EL34 recommendations for this amp? I'm not too clued up on the ins and outs of 34's..
I will be using this with an attenuator so there will definitely be a cooking power section in this amp when I use it.
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BY handsoffmatt
#856029
handsoffmatt wrote:
Optical wrote:
handsoffmatt wrote:
JCM900 mkIII 2100(100W) / 2500 (50W) models. These are essentially identical circuit to 2203/2204 but with a diode clipper added for more gain and footswtichable dual master volumes. Later on, the 2100 / 2500 became known as the SL-X, which removed the diode clipper and added an extra preamp tube gain stage.



?? that's not right at all...
It has less gain stages than a 2203/4, and the SLX has two more gain stages than the 2500 and shares almost nothing in common circuit-wise...




Just rechecking schems....and although I'm not what you would call an electronics expert.... it still looks to me that the following is true, unless I'm sorely misunderstanding something.

The 2100/2500 have the same number of valve gain stages as a 2203/2204 (3 stage + cathode follower), plus an added diode clipper. Take the diode clipper out and the preamp design is pretty darn similar to a 2203/2204, albeit with some value changes here and there.

The 2100/2500 SL-X versions ditch the diode clippers, and add an additional tube gain stage - so in the preamp section is very much like a hot-rodded 2203.

There are however marked differences in the poweramp/NFB circuits in the JCM900's as opposed to the JCM800's.

Schematic sources here:
https://www.thetubestore.com/marshall-schematics
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BY Optical
#856060
Ah yes I see your point. The second stage has local out of phase mixing and some NFB, so really a frequency shaping stage rather than a gain stage. however there are two gain controls which i hadnt noticed, so the gain reduction can be dialed out, interesting.
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BY jeremyb
#856061
griff7628 wrote:Any EL34 recommendations for this amp? I'm not too clued up on the ins and outs of 34's..
I will be using this with an attenuator so there will definitely be a cooking power section in this amp when I use it.


Always had good experience with JJs in the past, dunno what else is available these days!
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BY godgrinder
#856071
jeremyb wrote:[
Always had good experience with JJs in the past, dunno what else is available these days!


I had more brand new JJs die on me in a few months than Shuguangs for some reason.
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BY jeremyb
#856080
godgrinder wrote:
jeremyb wrote:[
Always had good experience with JJs in the past, dunno what else is available these days!


I had more brand new JJs die on me in a few months than Shuguangs for some reason.


They probably implode when you tune to drop Z!
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BY godgrinder
#856085
jeremyb wrote:They probably implode when you tune to drop Z!


More like they dropped the balls in the last 2-3 years. I found that if they don't die during the first year they'll be fine for a while, but some of them can't.

If I buy an used amp with JJs I wouldn't swap them out but I no longer buy brand new ones.
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BY willow13
#856088
I think a lot of the boutique amp makers (friedman etc) stopped using JJ's a while ago due to unreliability
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BY godgrinder
#856108
And they are not even cheap which makes it even worse. I don't mind Shuguang's dying since I can get their power tubes for $10 a pop (but funnily enough I hardly ever had Shuguang's die on me other than from old age).
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BY higainer
#856186
willow13 wrote:I think a lot of the boutique amp makers (friedman etc) stopped using JJ's a while ago due to unreliability


This

Svetlana’s have done right by me.
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BY godgrinder
#856272
griff7628 wrote:Are KT77's worth considering for this amp?


If you want more 6L6 flavour then yes.
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BY willow13
#856273
griff7628 wrote:Are KT77's worth considering for this amp?


I think you might need to swap out some resistors to be able to run them??