JCM800 input/preamp volume help

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nevusofota

nevusofota

Well-known member
I bought a 1984 vertical input 2204 a few years ago. Messed around with it at home and played live with it a few times. I noticed that when I plug into the high sensitivity input the preamp volume knob doesn't do anything, like it's stuck at mid volume. When I plug into the low sensitivity input it's beefier and now the preamp volume knob functions properly. It may have been like this since I purchased it but not sure as I've always automatically plugged into the high input and max'd the pre volume. I tried replacing each preamp tube but no change. I thought I read somewhere that there was a mod where you could bypass the first tube which takes the pre volume knob out of the equation, but I really don't know why you would want to do that, and my memory on the topic is very fuzzy. My guess is that this is a new problem as I think I would have noticed this sooner.

Everything else with the amp seems to be fine. It's quiet (even on start-up and shut- down) and there are no scratchy knobs or inputs. The caps look to be in great condition and appear to have been replaced not too long before I purchased it.

TIA. :rock:








 
The JCM800 is setup such that the signal from the Hi input goes into the first gain stage and the output of that stage goes 'through' the Lo input jack to get to the second gain stage. Plugging into the Lo input bypasses that first gain stage. The signal from that first gain stage that goes 'through' the Lo input jack goes through connections inside the Lo input jack. Those connections are broken when you plug into it. But those connections inside the jack can go bad. This might not be it but this is where I would look first.

Seems like if the jack was bad you'd get virtually no sound but sound could till get through (from other coupling in the amp). It would be really weak and thin though.

EDIT: Cool amp! On second thought, it's ruined. Sell it to me cheap. :D
 
Great amp! Beauty is, it's very simple to fix. Probably the high input jack going bad (like the post above says). If not, take it to a tech, and for $100 or so, you'll have a lifetime amp to play forever!
 
SpiderWars":1t8dpu7p said:
The JCM800 is setup such that the signal from the Hi input goes into the first gain stage and the output of that stage goes 'through' the Lo input jack to get to the second gain stage. Plugging into the Lo input bypasses that first gain stage. The signal from that first gain stage that goes 'through' the Lo input jack goes through connections inside the Lo input jack. Those connections are broken when you plug into it. But those connections inside the jack can go bad. This might not be it but this is where I would look first.

Seems like if the jack was bad you'd get virtually no sound but sound could till get through (from other coupling in the amp). It would be really weak and thin though.

EDIT: Cool amp! On second thought, it's ruined. Sell it to me cheap. :D
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:lol: :LOL: This is one I bought to have forever. Pass it down to my kids one day.

Thanks for the info!

The high input does have moderate sound. It's about as loud as the low input at a preamp volume about noon.

I think I'm just going to take it to my tech.
 
AndyK":2ufhv4fy said:
Great amp! Beauty is, it's very simple to fix. Probably the high input jack going bad (like the post above says). If not, take it to a tech, and for $100 or so, you'll have a lifetime amp to play forever!
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Yup, just posted that :lol: :LOL:

Probably going to drop it off today.
 
Bad jack or bad tewb ?

Try spraying contact cleaner on a 1/4 inch plug and fuck the jack hole in & out ...
 
if you have a tester you can easily check the connections; but yes, probably a tech it's the best option :) have fun!
 
Ok, so I got the amp back yesterday from the tech. He said that the solder connections on the inputs were reversed. He also cleaned some pots and replaced the power tubes with new el34's. The amp sounds good but I still think something is wrong. The high input is now significantly louder and gainier than the low, as it should be. However, the preamp volume knob has no effect on the high input. It does control the low input though. :confused: This can't be correct, any suggestions?
 
The tech shouldn't have given it back to you like that. The High input just puts a gain stage in front of the Low input so the Gain control should work for both. Seems like whoever modded it did more than just reverse the inputs.
 
SpiderWars":1giw0zaj said:
The tech shouldn't have given it back to you like that. The High input just puts a gain stage in front of the Low input so the Gain control should work for both. Seems like whoever modded it did more than just reverse the inputs.
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Yup, kinda pissed. I almost want to have someone else look at it since my trust in this dude is shot. I just don't want to pay someone else again.
 
So I spoke to the tech over the phone. He insists that the pre-amp volume should NOT work when using the high input and only works with the low input. I know this is incorrect and actually can't believe this is even a topic of discussion. Does anyone have a link I could send him or a phone number of someone he can call to prove to him he is incorrect?

Any help would be greatly appreciated as I feel my back is against the wall with this dude.
 
nevusofota":1c3avwgg said:
So I spoke to the tech over the phone. He insists that the pre-amp volume should NOT work when using the high input and only works with the low input. I know this is incorrect and actually can't believe this is even a topic of discussion. Does anyone have a link I could send him or a phone number of someone he can call to prove to him he is incorrect?

Any help would be greatly appreciated as I feel my back is against the wall with this dude.
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Do you have another tech you can take it to? I have a 2203 and a 2204, and the pre is both high and low input. My local guy is blind in one eye and can't see out the other and he knows that.
 
nevusofota":17sk3co8 said:
So I spoke to the tech over the phone. He insists that the pre-amp volume should NOT work when using the high input and only works with the low input. I know this is incorrect and actually can't believe this is even a topic of discussion. Does anyone have a link I could send him or a phone number of someone he can call to prove to him he is incorrect?

Any help would be greatly appreciated as I feel my back is against the wall with this dude.
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This individual is a straight up moron. Wow. NEVER take anything to him again.
 
Dude I feel for ya. I've had to argue with guys that worked on my gear, and I don't have the technical ability to fix it myself. It's frustrating to say the least.
 
cardinal":2mt1m0kd said:
Don't bother trying to take it back to him, but I might try to get your money back. He's completely wrong.

This is the 2204 schematic, I think:
http://www.drtube.com/schematics/marshall/2204u.gif

And here's the original handbook:
http://www.ikebe-gakki.com/ec/pro/disp/1/492067

It doesn't explicitly say the preamp volume works for both inputs, but come on.
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That 2204 schematic is wrong (however the Gain control would still work for either input). Here's one that is correct (the preamp at the bottom left). http://www.drtube.com/schematics/marshall/jcm800pr.gif

If you can follow: Look how the Hi Gain goes into a 68k resistor then into the grid of the preamp tube then comes out of the plate (the element at the top of the triode symbol) and then goes into the Lo input jack (thru that .022uF cap). So regardless of whether you use the High Input or Low Input, the signal ultimately comes out of the Lo Input jack, then to the 470k//470pF then to the 1M Gain control.

I don't think that tech is this stupid, he's just bullshitting you imo. Either way, what a POS.
 
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