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


 
1/28/2004 4:33 AM
Rick Erickson
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Re: 1970 Ampeg SVT w/6146b's tech help.
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
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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
 
1/30/2004 1:32 AM
Greg Simon
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Ray, that 12ax7 setup is on my MTI made SVT from the early 80's. Its sounds ok to me, but then I've never heard a "real" SVT from the Ampeg or Magnavox days except on recordings so maybe it doesn't sound as good. I did read somewhere that the transformers and components on the MTI ones weren't as good quality as the American ones. At some point I'll probably sell this and try to find a real one for a reasonable price. Maybe once I get my Sunn 2000S up and running!
 
1/30/2004 3:24 PM
Ray Ivers
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Greg,  
 
The MTI SVT is the 'hi-power' one (350W), right? I've never played thru one of those; only the real old (pre-'75) and real new (SVT Classic) ones. I didn't realize that MTI scrapped the 12DW7's; it's quite likely that would make for a noticeable difference in tone from the older ones. Hey, you could always convert to 12DW7's :D or better yet, 12AT7's as Rick mentioned.  
 
My Sunn Model T sits in my front hall, right by the door. It used to be a basket case, but with parts of that quality it's hard to truly destroy an amp, and now it works/looks perfect (the reissue Sunn Tolex is an exact match).  
 
Ray
 
1/31/2004 8:20 AM
Greg Simon
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Ray, if you have the Ampeg book, it shows the MTI era SVT's in there. I believe it is the same power as your older ones or maybe slightly less. Everyone I have talked to says the 12DW7 sounds better in the SVT but since the rest of the compnents (resistors and caps and transformers from what I hear) are of inferior quality I wonder if it would be worth it to do the mod or sell it and look for a real SVT. The MTI ones have the same white rocker switches that the older SVT's have, and the same channel setup, but the face is black with white lettering. To be honest, I haven't even looked in mine yet to see if it has the 12ax7 or the 12DW7. I just assumed it did since its an MTI SVT. Maybe I'll check tomorrow and let you know for sure.  
 
I've got a '69 2000S which works fine from what I can tell after firing it up for 5 minutes when I got it. The price was right.....free! But I need to go through it and really have a look at it, but am waiting until I get some other things off my plate first. I also have a Sonic 1, which is essentially a 200S, although the voltages seem to be quite a bit too high....576 on the plates instead of the 500 the schematic calls for. That one works too but beyond that I haven't had a chance to realy look. Those Model T's are REALLY loud! But I agree with you on the quality of parts int eh Sunn's. The transformers in my 2000S are as big as a baby's head and there is TONS of room in it to add a channel or to fix any problems. The only problem is it weights as much as my SVT head!  
Greg
 
1/27/2004 2:58 PM
Rob Mercure
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Rick,  
 
I second your experience with the TAB - Mr. Pittance just doesn't give a rats ass about correct technical information and I know that several techs, my self included about 10 years ago, have sent him letters and e-mails offering to correct his mistaken technical advice. In my case I even offered to help his grammar, phrasing and "flow" for free if he could supply me with an electronic copy - it's not just technically incorrect it's poorly written and edited.  
 
Ah, the optimism and folly of youth.  
 
The internet has somewhat eclipsed TAB as a source of misinformation for musicians but it used to seem that every week I had to correct some customer's misconception that Mr. A had caused. Now it's the stuff from AGA and other sources.  
 
Rob
 
1/25/2004 3:28 AM
moe
I like the conversion myself.  
I just find another screen supply.  
The next one I get will be sent to  
the Smithsonian. I promise.
 
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