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

12/5/2003 11:17 PM
1970 Ampeg SVT w/6146b's tech help.
I am trying to figure out a problem with my svt. I bought this off of ebay a few months ago and had to do a lot of work on it to get it playable again. I had to replace all the components on the power amp driver board and I did a total cap job and a lot of cleaning. Now, when I hit a loud note with the volume up not super loud I get this nice crashing type sound out of the speakers. I am only running 4 power tubes right now waiting to get new ones. If I only run 2 tubes I can turn it up as loud as I want until the power tubes go into distortion and that sound doesn't happen. I thought it might be the weird ext. speaker output jack so I have a seperate 4 ohm jack and 2 ohm jack now. Anything else I should try? I tried different tubes in the whole power section, 12bh7's and 12dw7 and tried an external preamp so I know it is in the power amp. Thanks for any help.
12/6/2003 1:00 AM
Tom Phillips
Does it work OK with any combination of 2 power tubes plugged into any combination of the 6 power tube sockets? (Given that one is on the push and one is on the pull side of course)  
Could be:  
1) One bad power tube  
2) One bad socket. Loose or corroded tube socket contact.  
3) Bad solder joint on one of the power tube sockets.  
4) Bad component in the circit to one or more of the sockets.  
Check around and don't assume that anything is right. Sometimes it's a combination of things. I once worked on a 20+ year old SVT in which the plate lead on one power tube socket had never been soldered.  
Good luck. Hope these ideas help.  
12/7/2003 3:14 AM
Don Symes

and as always see for Mr. Keen's Tube Amp Debug page.
12/8/2003 3:54 AM

Doesn't kg have one of these? I thought I recalled him speaking of it.
1/19/2004 12:51 AM
Mike @ Vintage Amplifier Service

While your in there, why not convert to  
6550 outputs? Much better. Safer too!  
A very Good DIY Project.  
1/24/2004 2:27 PM
Rob Mercure

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.  
1/24/2004 3:27 PM
Ray Ivers

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.  

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