SWR Basic Black preamptube

[andre van ham]

I just bought a used SWR Basic Black. It's in perfect condition however I noticed that the preamptube is microphonic.

I tried to replace it with a brand new 12ax7WA from Sovtek and I got this low noise (somewhat like a 50/60hz cycle noise). When I put the old tube back everything is ok, except for the microphonice problem ofcourse.

I also tried that 12ax7WA Sovtek in my other preamp (Hevos) and there it functions ok.

So : do these SWR preampsections need a specific 12ax7 type? SWR's website says "any 12ax7" will do....

 

[IvanMike]

nope, any one will do just fine. The only reason to use a specific one is if you like it better, but swr seems to be less dependant then some upon the tube brand in terms of the overall tone.

if you have issues with one tube being microphonic and one humming there may be an issue with the amp itself. Try a third one, and try the microphonic tube in the other amp.

 

[AlembicBill]

nope, any one will do just fine. The only reason to use a specific one is if you like it better, but swr seems to be less dependant then some upon the tube brand in terms of the overall tone.

if you have issues with one tube being microphonic and one humming there may be an issue with the amp itself. Try a third one, and try the microphonic tube in the other amp.

I like the JJ Tesla's - I put them into my Basic Black and my Alembic F-1X.
I bought them from Butch Ammons at Link Removed

 

[andre van ham]

nope, any one will do just fine. The only reason to use a specific one is if you like it better, but swr seems to be less dependant then some upon the tube brand in terms of the overall tone.

I wrote an email to SWR tech-support and got an answer within a few hours :

"When changing the brand of tubes, you will need to have your amp re-biased. You can have this done at an authorized service center. "

Sounds credible I think.... But is it as simply adjusting a little trimpot on the board or do I really have to go to a dealer and have it done there (expensive!)?

 

[AlembicBill]

I wrote an email to SWR tech-support and got an answer within a few hours :

"When changing the brand of tubes, you will need to have your amp re-biased. You can have this done at an authorized service center. "

Sounds credible I think.... But is it as simply adjusting a little trimpot on the board or do I really have to go to a dealer and have it done there (expensive!)?

I'm no expert by any means, but from what I remember, an underbiased tube may be damaged, but the amp will be OK; an overbiased tube won't sound as good, but again no amp damage. The Teslas are $10-15, so I wasn't worried; as it turns out, it sounded better with the new tube, so I left it as is. If I spent big money on a NOS tube, I might feel differently.

I could be completely wrong, but I think preamp tubes mated to a solid state amp are less finicky than an all tube power amp section.

As always, YMMV.

 

[thumpbass1]

The only time I've been aware of having to reset bias
in a tube amp is when you replace all of the power tubes
in the power stage of a complete amplifier. I've done
preamp tube swaps countless times with no complications and
I've never heard of having to bias any preamp tube.

Biasing sets the idle current of for instance two power
tubes in a class a/b push pull style power amp. I could be
wrong but I wonder if your second tube might have a defect
of a mechanical nature that was reacting to the voltage draw
of the preamp.I'd try another 12ax7 and see if it was just
a bad tube that doesn't like your preamp.

 

[thumpbass1]

I checked 'The Tube Amp Book' by Aspen Pittman the
founder of Groove Tubes; and the book states that preamp
tubes are cathode biased as part of the design parameters
of the given preamp.The book also states that preamp tubes
aren't usually rebiased as part of routine maintenance.

I can't speak for SWR's design and maybe your amp might
need to get the preamp bias reset. I still find it odd and
wonder if your second Sovtek might have some internal defects.I'd find a freind who has a tube guitar amp and
ask them to let you try one of their 12ax7's in your amp.
It won't hurt their tube and at least you'd know.

I hope this helps.Good luck.

 

[andre van ham]

I just ordered a JJ 12AX7 / ECC83S, I'll try it in my Basic Black when I get it this week.

I also got a SWR Studio 220, if I encounter the same problem with the JJ tube in my Basic Black I'll try it in my Studio 220.

Thanks for all your answers guys! I 'll keep you posted.

 

[andre van ham]

ok, my JJ 12ax7 arrived yesterday and I installed it. No problems anymore!

Great sound with that JJ tube, sound more tubelike than before. Much warmer sound, specialy with my fretless. Love it!

AlembicBill : thanks for mentioning those JJ Tesla's!

 

[AlembicBill]

Just trying to help....glad you like them.