Can someone explain the Mesa Mark I input circuitr

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mtotheb
Junior tube assistant
Posts: 1
(2/9/04 9:00 am)
 Can someone explain the Mesa Mark I input circuitry?
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 www.tubefreak.com/mkirga_1.jpg
www.tubefreak.com/mk1.gif
www.tubefreak.com/mk1b.gif

Those are various schematics for the preamp. I'm confused as to what's going on at the input of channel 1. I'm not sure if a special input jack is required. Assuming it is a standard stereo jack I cannot decipher what's connected to the tip, ring, and sleeve. To me it seems the sleeve is grounded, while the tip goes to the grid of the first gain stage and the ring is connected to both the wiper of the first volume control and the dc blocking cap connected to the plate of the first gain stage. Can anyone explain what's going on here?  

Thanks!
 
ganzonimx
Hey get your own solder
Posts: 354
(2/9/04 10:26 am)
 Re: Can someone explain the Mesa Mark I input circuitry?
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 I think it's a jack with a transfer circuit (Switchcraft 13A). The coupling cap is shorted to the grid with no plug inserted. With a plug in, the cap connection is transfered to the following stage.

I think the idea behind this is, to avoid noise coming from the V1A when input 2 is in use.

You probably could change that to a "normal" setup without any noise problems. Disconnect the cap from the jack and connect it to the wire that goes from the jack to the 270K resistor.

Cris
 
mtotheb
Junior tube assistant
Posts: 2
(2/9/04 11:19 am)
 Re: Can someone explain the Mesa Mark I input circuitry?
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 Thanks for the reply. That seems logical. I had thought about the noise issue and some sorta double-pole double-throw kinda jack. I guess if the first volume pot is turned all the way off when only the second input is in use, any noise could be reduced, if not eliminated anyways.

And now the reason I'm interested:
I have a Bassman head that I'm tinkering with and observing this schematic gave me some ideas. Essentially this preamp is has the normal channel of a bassman wedged between two other gain stages, with even the tone stack reflecting Fender values. I'm sure this is nothing new to most of you, but being a novice at amp modding it seemed kinda neat.

At the moment I've got the bass channel of my bassman removed, anticipating that I will build something more interesting in it's place. First thing I'm gonna try is to just add the Mark I input circuitry and first gain stage before the stock normal channel and see how it works out. I'm thinking I'll need to add a voltage divider, taking note of the Mark I schematic. At the moment the normal channel is actually 3 stages as my bassman had the AB165 circuitry with the third gain stage common to bass and normal channels. I will add the voltage divider just prior to this channel. If it's not appeal I will continue to mod it until it does sounds good or until I complete the Mark I circuit, whichever comes first. Hopefully in the end I will have gained something (yes, pun intend).



 
ganzonimx
Hey get your own solder
Posts: 355
(2/9/04 12:35 pm)
 Re: Can someone explain the Mesa Mark I input circuitry?
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 If you are working on a AB165 so at least have a look to this. It talks about converting the AB165 to the AA864 BF Bassman circuit.

Cris
 
mtotheb
Junior tube assistant
Posts: 3
(2/9/04 5:32 pm)
 Been there, done that...
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 Thanks, but I stumbled upon that site a couple months ago when I first started modded the amp. It's been an ongoing process. That sites description of the third gain stage is why I kept it in there for now. I've already got the phase inverter blackfaced and all the snubbing caps have been removed from plate resistors. All the nasty feedback loops have been removed, with the exception of the globak, which has been converted to a normal circuit. I've also got the bias balance circuit converted to a bias supply circuit and the filter and electrolytic caps have been replaced with new Sprague ones. The preamp stills leaves something to be desired though. I can get a nice clean sounds, but it doesn't seem to interact with my fuzz/distortion pedals as well as my Ampeg VT-40 does so that's why I wanna try to coax natural tube distortion out of it. I'd like to be able to just plug in an have some high gain sounds opposed to relying on my Big Muff, which has grown stale on me lately. I also like the fact that I can leave the stock normal circuit in there so I still have those nice clean sounds. Thanks again for your help.
 
 
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