03-04-2012 09:30 PM
03-04-2012 09:45 PM
03-05-2012 07:08 AM
Sensitive grid wires to close to previous grid wires. Just a mess of wires. Make sure the voltage divider after that gain stage is still 470k/470k and not something higher. Just in case you changed them.
Poor lead dress?
03-05-2012 11:11 AM
03-05-2012 05:33 PM
What else did you change? Honestly, I like that 10k there. There is other ways of getting more gain if you need it.
Did not change the voltage. I tried a couple different resistor combos in the v1b cathode, still the squeal and fizz with the preamp past 8. Oh well, looks like I am leaving that part stock.
03-05-2012 05:41 PM
03-05-2012 05:56 PM
The presence circuit is NFB which can cause hell with your preamp if you don't watch lead dress. Something as simple as changing the way the wires that go from the board to the tube sockets lay and cross can screw you and cause oscillation.
Presence didn't matter. Changed nothing else. Will upload pics when I get home
03-05-2012 06:17 PM
03-05-2012 07:58 PM
03-06-2012 12:16 AM
IIRC that's one of the more unstable ways to boost gain in a 2203. I've never actually modded a 2203 I didn't build from the ground up so I never had issues cause I usually just take steps ahead of time to keep things really quiet. There's usually a set level of "gain" you can squeeze into something like a Marshall preamp design with stock filtering and all that without oscillation problems and the better the amp is built with regards to lead dress and things like grid stoppers and whatnot the more you can get away with. Without seeing the amp it's hard to say. To get higher gain most people do more gain stages or they do a diode clipping stage or both which allows you to go more gain with less oscillation but at the same time you lose alot of the magic of a plexi/2203 type amp. (the two amps BTW are really close to the same thing)
Why would it not normally do it if a lead problem?
03-06-2012 03:44 AM
03-06-2012 10:25 PM
03-06-2012 10:32 PM
03-07-2012 10:58 AM
03-07-2012 11:45 AM
03-07-2012 12:51 PM
03-07-2012 02:21 PM
That 500pf cap is standard. I like to use a silver mica cap but it's all the same. If you have an old ceramic disc like the one in the pic they sound great. The new ones sound like ass IMO. One of the best places for info on Marshalls of this vintage is the Metro amp forum. Their Wiki has tons of good info as well. I'd hop on there.
I may try the wire. Besides that, does everything look right? For instance, a 500 value cap on the preamp.
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