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


 
1/24/2004 2:27 PM
Rob Mercure
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Re: 1970 Ampeg SVT w/6146b's tech help.
Hey Mike,  
 
Noticed your response but I have to disagree for one reason. Higher quality NOS 6146Bs are available for much less money than current production 6550s. Now I will admit that current production tubes quality is quite high compared to the recent past. Still why replace good available tubes with tubes that are more expensive even if the 6550s are now up to NOS standards (and I'm not still convinced)? And, from another perspective, there are few of the 6146B equipped SVTs around - hell, there are few of the "real" SVTs around as is - so why not leave these "originals" alone as they are part of history. There really is nothing wrong with using 6146Bs at audio frequencies and Ampeg changed for availability and cost reasons not any inherent problem with the bottle. Don't believe information provided by popular gurus stating that the 6146B is unstable 'cuz it was designed for radio frequencies - the 6550 was also used in some lower radio frequency linears so the same mis-rational for not using the 6146 should also apply to the 6550.  
 
Rob
 
1/24/2004 3:27 PM
Ray Ivers
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Rob,  
 
I agree completely. I converted one of these to 6550's a long time ago (total basket case/ant farm/spider's nest) and looking back I wish I hadn't. A while ago someone was selling NOS 6146W's in bulk on eBay, and there's even a current-production Chinese 6146B IIRC (quality unknown). The PT's are also different in the early ones, with a lower 220V screen voltage which is a bit low for 6550's (and much better IMO for the positive CF driver voltage), and also less filament current (the 6146B's draw @ 1.1A each vs. the 6550's 1.8A - this may be the best reason for not converting, I never noticed it before). Apparently the OT's are identical; at least the Ampeg part numbers are the same.  
 
I've got a '69 (6146B's) and a '73 (Svet KT88's), and the '69 has a slightly more midrangy (vintage?) tone, while the other one I think would be more suited to modern bass styles, with a bit more deep bottom and top end.  
 
Ray
 
1/24/2004 10:26 PM
Rob Mercure
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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
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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
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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
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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
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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
 
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