Tube Amps / Music Electronics
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|10/23/2000 8:27 PM|
||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|
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.
|10/24/2000 3:01 PM|
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
|10/24/2000 3:26 PM|
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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