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|previous: Dave Rice That sounds like a cool project. Wh... -- 9/3/2004 7:55 AM||View Thread|
|9/4/2004 6:34 AM|
|Greg Simon||Re: Supro Reconditioning|
Actually Dave, I meant the coupling cap between the last stage of the preamp and the input to the phase inverter. I can see where putting it on either chassis has its advantages and disadvantages, but it would be cool to know how others did it. If you put the coupling cap on the preamp chassis, then you wouldn't have high voltage DC in any cabling between the chassis, but then you also have a very long grid wire. Putting it on the power amp chassis makes the gird wire much shorter but then you have high voltage DC between chassis.
My project actually is a 3 in 1. The original chassis is the power amp chassis, and it was for a Conn organ made in 1964. It had 3 seperate power amps in the chassis, each with its own OT. There were 2 7027 sections with a 7199 cathodyne inverter for each. I think these ran a pair of Leslie speakers. There was also a 7868 section with a 12ax7 in a paraphase inverter. I think that one was intended for an internal speaker.
I'm seperating the sections as much as possible electrically so I get a minimum of crosstalk, but they are all still in the power chassis and that is where they will stay. I will be modding each section, but as of right now, I haven't done anything beyond adding an on/off switch, changing to 3 prong AC cord, and putting in standby switches for each section. I also had to change the common screen resistor as it had blown, and I got rid of the selenium rectifiers and put in some 1N4007's.
The preamps will be on a seperate chassis, 1 preamp for each power section. All of it will be in the same head. The head ends up being about as big and heavy as an Ampeg SVT! The first preamp chassis I made wasn't big enough, so I barely got started before I realized it wouldn't work due to lack of space. So the second time I planned it all out on paper, and then had a metal shop make me a chassis. My layouts will be much better now too than they were with the first try. Going from right to left when looking at it from the front, the amps will be as follows:
1. An AA864/AB765 Fender Bassman circuit with parts from each circuit, and also with bias modulating tremelo like the Princeton Reverb. The 7027's will be replaced by a 6L6GC pair, and the splitter will be just like Fender's rather than the cathodyne that is in there.
2. The 7868's will stay, as will the 12ax7 paraphase inverter. The preamp will have an EF86, into a cathode follower, into a 2nd Marshall Plexi stage, into a Vox/Fender tonestack with values from each.
3. The 7027 pair will switch to EL34's and the preamp will be a Marshall 2204 JCM800 style circuit. I'm using Jfets to switch between channels and using some mods from Kevin O' Conor's books. The splitter will revert to the standard Marshall LTP.
I'm sure once I get it all done, there will be lots of tweaking to do, but its a fun project! Its actually my first one except for rebuilding a Sunn 200S bass amp. There are 450 volts on the plates of the 7027's, and about 440v on the 7868's currently, and every section is currently cathode biased, though the 7868 pair will be the only one cathode biased when I am done. The pwoer supply is a voltage doubler. So far, this project has taught me a LOT, and the main problem is lack of space, so my layouts are really critical.
Thanks for your help and interest!