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Re: 1970 Ampeg SVT w/6146b's tech help.


 :
1/24/2004 10:26 PM
Rob Mercure
Re: 1970 Ampeg SVT w/6146b's tech help.
Ray,  
 
Thanks for the thumbs up! On another, "ulterior motive," I kinda like tubes with plate caps - somehow they just look more powerful and "frankensteinish" :). (and I've made a few nice sounding "test" amps with TV HO tubes - always wanted to make one with a few 7193s, a single squatty, 6J5ish, triode with both a plate and grid cap pushing a quad of HO tubes). But fer some reason I just think that other techs don't - mostly a matter of aesthetics IMHO.
 
1/25/2004 4:00 PM
Ray Ivers

Rob,  
 
Yeah, I like the plate-cap tubes too - I see them at work sometimes, in RF-transmitter drive stages. I don't think the 6146 could be as compact and powerful as it is without the cap. I know that with that SVT I converted, the plate caps were definitely a factor in making the decision; back then I didn't know where to find them (Antique has two sizes, I believe) and was generally more comfortable with the 7AC-based tubes for guitar. I even followed the procedure in the Tube Amp book! :(  
 
I'm not familiar with the 7193, although I have a couple of dual-cap tubes around here someplace. Speaking of aesthetics - how about tie-wrapping some neon bulbs to the plate leads? :D  
 
Ray
 
1/26/2004 2:11 AM
Rob Mercure

Ray the 7193 is a "cute as a button" little octaal bottle about the size of a 6J5 - but it has two little "horns" - equally spaced and slightly angled from center - and makes a quite nice "martian." :) The only rub is that the tube is a single triode so it's hard to justify the chassis space. Tube Lore states that they are equivalent to a 2C22 which is described as: "VHF triode, GT Octal w/grid and plate caps, heater, 6.3V @ 300 mA, u 20, 3.3W; very nearly a double ended 6J5GT."  
 
I wound up with them, along with a bunch of 6G6s, 12SH7s, 12SL7s, and VR150s in a box of still packaged WWII tubes that Richie Hall gave me around 5 years ago.  
 
Rob
 
1/27/2004 3:40 AM
Rick Erickson

quote:
"I even followed the procedure in the Tube Amp book!"
 
 
I tried that once. The screen supply node they suggested seemed to turn the amp into a 120Hz generator/modulator. I added a rather large power resistor and an extra stage of filtering for the screens and the amp in question was much happier. I don't think I've had much success with any of the TAB mods, except for possibly the 12DW7 to 12AX7 conversion in the SVT preamp.  
 
RE
 
1/27/2004 12:41 PM
Ray Ivers

Rick,  
 
Yeah, now that you mention it I think I may have ended up using the stock 220V for the 6550 screens, where the TAB recommends tapping off the raw plate supply IIRC. Apparently the Ampeg factory conversion adds a separate screen transformer, which is probably the best way to do it.  
 
I think even the TAB 12DW7 conversion may be a little funky... I remember working it out and thinking that it traded minimal-component-changes for clean headroom (not what I would choose for a bass amp) but I've never actually done it so it may work fine. IMO they should have converted to 12AU7's or 12AT7's and traded some gain for greater headroom, but hey... ;)  
 
Ray
 
1/28/2004 4:33 AM
Rick Erickson

Ray,  
 
I think that 12AX7 conversion mod. actually came from the Made-in-Japan SVT's that came equipped with 12AX7's, so in effect it's a factory-mod.. I had used 12AT7's in SVT preamps (before I saw the TAB mod.) with good success. I remember mentioning this over the phone to a factory tech who told me "you can't do that!" to which I replied "I just did." ;)  
I figured if it sounded good to the pro's who were using them regularly then it was a "good" mod..  
Another time I had a customer who wanted stronger bass response from his SVT. I found the weak link to be the .1uF/250V coupling cap. on the output tube (6C4) cathode. Upping this to .47uF allowed much cleaner low end response through the amp which can be handy for 5 & 6 string basses.  
 
RE
 
1/28/2004 1:51 PM
Ray Ivers

Rick,  
 
Thanks for the tips about the coupling cap and the 12AX7 mod - I didn't realize it was a factory tweak. I'll have my '73 open soon to try and track down a hum problem (and put in a quieter cooling fan!), so I'll definitely replace that cap. And 12AT7's seem like a natural for a bass amp - really like those tubes. I guess you were able to just plug them right in, with no circuit mods?  
 
Ray
 

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