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Re: Question Dutch

11/9/2004 8:48 PM
Greg Simon Re: Question Dutch
Thanks Ken! I'll have to take a look at that and compare to various tubes.  
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11/6/2004 6:03 AM
Chris @ CMW amps
Re: Paraphase inverter questions
Hi Greg  
Sometime ago I did build a small amp with a ParaphaseInverter, slightly based on an early 50s Deluxe and a Supro with a custom pre-amp.  
It's really cool for oldschool bluessounds and very Zep/ZZtop when dimed.  
Don't know how your configuration will sound but be sure to have a look at some old Gibson schematics too like the GA20 and the GA40.  
Have fun!  
11/6/2004 12:16 PM
Greg Simon
Thanks Chris! I'll take a look at them and see what they're about. I'm used to playing a couple Deluxe Reverbs down at the club I go to. They are mostly stock, but have the negative feedback loop disconnected, so they are really brown sounding and gainy, but often have too much bass too. I'm hoping for something which can stay articulate but still have some nice overdrive tones when full up, but also back it off and have usable clean.  
11/6/2004 2:18 PM
Chris @ CMW amps

Hi Greg  
Just did get in an old GA40 for servicing, hope to report about the sounds asap. I also hope to build something GA20/Cooder-ish when possible.  
Regarding the DRs:  
You can try some lower value coupling and cathodebypasscaps caps after blackfacing, a Celestion G12H30 works very well in bf DRs, tighter bass and a bit more volume.  
If I were you I should 1st start with a GA20 or "simple" GA40 style amp and use some smaller coupling caps ( 0.047 or 0.022 ). ( imho! )  
Hope this helps/ have fun!  
11/6/2004 9:07 PM
Greg Simon
Thanks Dutch, I like your suggestion about the 12au7 and plan on going with that to start. I also think I'll try what you recommend with the 50k pot. The balance would change with a different tube in the PI slot however right? I may have room to fit a balance pot as you suggest permanately on the power amp chassis, so I could take care of that if the balance goes off when I change a tube. I thought about using the cathodyne. Thats what is on this amp in the other 2 power amp sections on the power amp chassis. Those will be changing to LTP's ala Fender/Marshall. I think I'll try the paraphase inverter first with your suggestions and see how it will work before possibly going to a cathodyne.  
Chris, thanks for your suggestions also. I'd love to hear anything you can tell me about the GA40 when you get a chance. I played an old GA40 with the pentode front end and it was really nice.  
With the DR's, they are someone elses that I use at a blues jam, so I don't think they would want me to mess with their amps!:) They sound pretty good as is, but you have to get the tone controls set right or there is too much bass. One of them is a '64 without negative feedback, and the other is a '73, that has been modded to blackface specs, again without negative feedback. The '73 has a Weber speaker in it, and the '64 I'm not sure what he has now because he blew the original Jensen a couple weeks ago. The '64 also is using 6L6's.  
I'll take a look at the GA20/40's schematics and see what I can use from them. I was thinking about using smaller coupling caps, but I'll probably end up trying the circuit as is first.  
This is a complicated project I'm doing for my 2nd one. The power amp chassis is an old ('64) Conn organ chassis with 3 seperate power amps and transformers in it. Originally it was two identical 7027 pairs, each with their own output transformers, and their own cathodyne 7199 inverters, and the 7868 pair with the paraphase inverter and its own transformer. The 7027's had about 450v on the plates and screens, and the 7868's had more like 440v, with the screens around 405v. I decided to add 3 preamps, in a seperate chassis, and put it all in one head. I know I'm crazy! When I get done, there will be a modified blackface AA864 bassman with bias vary PR trem, with 6L6GC's using the LTP into one transformer, the 7868 pair into another, and a modified JCM800 2203 circuit into a LTP and its own transformer. So 3 amps in one head! The 7868 section is the one I know I will probably change the most since I'm not following any existing schematic for that. They used some good components in the Conn, like the blue 'molded' caps for instance, and they used a real ground buss scheme only touching the chassia at one point. So far, all three sections are cathode biased. I'll have to figure out some way to get the bias voltage on the 2203 section and the bassman sections to track the plate voltage as one section or another is put into standby as they will al lhave seperate standbys, but thats down the road a bit I think. I'll just test it will all of them on and going into my load box.  
Thanks again for your help and let me know what you find out about that GA40!  
11/7/2004 9:51 AM
Bruce /Mission Amps

I'll probably start a flame war, but... every GA40 I've worked on and refurbished still sounded like warmed over crap when done and half of them have leaking caps on the under side of the board that kills the vibrato. Good luck replacing them in an afternoon.  
Even when working right they are very puny sounding, not very loud and extremely dull/uninteresting.  
Sort of like a sick 5E3 with trem.  
OK for low power harp players but that's it.  
Another one of those amps that are "legends" in someone's mind but in reaity... **YAWN* ho hum.  
11/10/2004 6:29 AM
Greg Simon
Hi Bruce. I played an older GA40 with the pentode front end at a guitar show and it was very nice. Had an original Jensen in it, and I was using an old Les Paul. Had a good blues tone. I've played some 5E3's that are nice too...a different sound however. I really liked a '49 Pro I played once upon a time too. In the end, I just view them as different tools to get different sounds.  
I don't much like working on old Gibsons, and greatly prefer to work on an old Fender, but thats besides the point! The only Gibson amp I've played that I REALLY liked was that GA40. Every other Gibson amp has been like a GA15RVT with the EL84's that my dad used to own. It was VERY dull and lacked high end. It needed some serious mods to be moderately playable.  

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