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Reissue '63 Vibroverb tube options


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10/23/2000 8:27 PM
Tom Kiefer
Reissue '63 Vibroverb tube options
I suspect I need new tubes for this amp and have several questions on the subject. It uses two 6L6GC, five 12AX7 and one 12AT7. I run this amp totally clean and I want to maximize volume, clean headroom and punch. The less distortion the better because I run all my effects outboard.  
 
1) Given my preferences, what are my best choices for power and preamp tubes ?  
 
2) How do I know when it is time to replace them ? The amp seems OK at first but it starts breaking up too easily after it has been run hard for an hour.  
 
3) I only use the Normal channel, not the Bright channel, nor the reverb or tremelo. I discovered that the amp runs and sounds the same on the Normal channel with only the two 12AX7 tubes installed that are required to make that channel operate. Therefore, I think I will only replace those two tubes. Comments ? Besides the obvious dust and dirt that could get into an empty socket, are there any risks to operating the amp with only two preamp tubes installed ? The amp does seem to run a bit cooler. Any other suggestions would be helpful.  
 
4) I also have a copy of a tech bulletin issued by Fender about swapping a particular resistor in order to reduce crossover distortion. Apparently this had something to do with the switching to Russian 5881 tubes right from the factory. Any insights ?
 
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10/23/2000 9:56 PM
Ray Ivers

Tom,  
 
Your break-up-after-an-hour symptom seems indicative of a need for a change of output tubes, provided the amp is functioning properly otherwise. If the new tubes make no difference after about a 20-hour break-in period, there may be another problem.  
 
For the most headroom from a 6L6-type tube, I would suggest NOS 7581A's or Sylvania 6L6GC's. For current production tubes, the Svetlanas are the only ones I have much experience with (I liked them, haven't found any 'loose' ones myself), although the Teslas have been recommended here on Ampage as well. I would recommend an NOS 12AT7 driver tube as well; you could check the KCA site (http://www.erols.com/bluestat) for these tubes. RCA and GE seem to be pretty good choices for NOS 12AT7's, as well as (of course) Mullard ECC81's.  
 
I don't think running without the two preamp tubes will be a problem for you. Just replace the ones you'll be using if you can't swing the full set right now.  
 
The Fender tech bulletin probably refers to a bias circuit change. With any output tube changeout, you'll have to have your amp's bias adjusted so I don't think this change will apply to you; it's probably more of a coarse-adjustment production-line thing. You may have to make this component change to get your amp to bias up right, so it's good to know about it, though.  
 
Ray Ivers  
R.A.G.E. Electronics
 
10/24/2000 3:01 PM
joe

I'll defiinitely second the motion on the  
NOS 7581's - I put a pair in my VVRI, and the  
difference in headroom was amazing. I wouldn't  
worry too much about re-biasing them; they have a  
very high plate dissipation rating and you want  
to run them colder to increase your headroom.  
Put them in and see what they sound like. I  
also got a boost in headroom and harmonics by  
replacing the first preamp tube (V1 in your  
case) with a Chinese 12AX7.  
 
You would also do well to dump the speakers;  
the stock OXfords just blow, and for a clean  
tone, lots of headroom with minimal breakup  
I would consider a pair of Weber C10Qs or  
P10N's.
 
10/24/2000 3:26 PM
Matt

I've never tried it but I've read that some people striving for the cleanest tone from a Fender substitute a 5751 preamp tube for the first 12AX7. You could also use a 5751 for second preamp tube or any combination you want. You could also try 12AT7's in those two sockets for even less gain. Maybe using lower gain preamp tubes and a slightly higher gain PI tube (so that could be 5751's in all three sockets) would achieve what your going for. Lots of possibilities to try. 12AX7's have a gain of 100, 5751's have a gain of 70, and 12AT7's have a gain of 60. Good luck.
 

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