Preamp tubes in a Fender AB763 circuit

Cliff Swanson · Post by **Cliff Swanson** » 31 Dec 2000 6:06 am

I'm about to retube a 1967 Dual Showman amp that has the highly desired AB763 (black face) circuit. I'm giving some consideration to the idea of using a 12AY7 tube rather than the 12AX7WA as a replacement for the original 7025 at the front end. Reasons for doing so are basically that nonmicrophonic 12AX7s are hard to find (I've been told this by tube gurus...), and also that the 12AY7 may be less prone to harshness because of their gain characteristics. Has anyone tried this in a Twin or Showman amp? Results? Opinions?

BTW, I'm putting a matched quad of JAN Phillips 7581As in for power tubes, and will run this amp head into a Jenkins Sound Shop 1x15" cabinet loaded with a Weber California 15 speaker.

All opinions and advice appreciated,

Cat

Chris DeBarge · Post by **Chris DeBarge** » 31 Dec 2000 6:19 am

Cliff,
Experimentation will cost you next to nothing, so go ahead and try. I think you're barking up the right tree, because a steel has higher output than a regular guitar and hits that first tube pretty hard. A lower gain tube will certainly help.

BTW, any comments about that speaker?

Cliff Swanson · Post by **Cliff Swanson** » 31 Dec 2000 7:23 am

Chris... I haven't gotten the cabinet yet so can't comment right now about it in my application. I can say that I heard one a while back in a cabinet being used by a blues player friend of mine and was really rich. If there is any difference between it and a D130F I think anyone would be hard pressed to find it. I'm getting a 4 ohm with the paper dust cap.

On the tube issue...I've also heard from Mike Kropotkin (web tube guru) that a 5751 might be a good alternative to the 12AY7 as the 5751 has the same gain reducing effect as the AY, but a bit less so. I'm going to experiment a little bit I think and see what my ears tell me.

Cat[This message was edited by Cliff Swanson on 31 December 2000 at 09:39 AM.]

Jay Ganz · Post by **Jay Ganz** » 31 Dec 2000 8:17 am

I have a matched set of 7581A output tubes
in my old '65 Twin Reverb & they're an
excellent choice! Mine are made in West
Germany though.

------------------
<img src=http://www.geocities.com/ganzpics/Cowboy.gif>Keep Steelin' !!! 

[This message was edited by Jay Ganz on 31 December 2000 at 12:18 PM.]

Len Amaral · Post by **Len Amaral** » 31 Dec 2000 8:55 am

A good quality 12ax7a preamp tube shouldn't be microphonic from the git go. The Sovtek 12ax7WA or 12ax7WB preamp tubes are reasonable and very clean sounding. However, as some else suggested, play musical chairs with the various preamp tubes and see what works best!
Len Amaral

Cliff Swanson · Post by **Cliff Swanson** » 31 Dec 2000 9:41 am

Jay...Oops! I misposted above. I have, in fact, ordered a 4 ohm speaker from Weber, not the 8 ohm as previously stated. Thanks for pointing to it for me.

Cat

Tele · Post by **Tele** » 1 Jan 2001 3:02 am

I often use 5751 preamps tubes. They are really quite, you could get them much cheaper than good 12AX7 or 12AY7. I once got 10 NOS GE for $10 on ebay. I think you will be surprised how it changes your sound. Go for it !!

Andy

Michael ODriscoll · Post by **Michael ODriscoll** » 2 Jan 2001 12:29 pm

Check out "Antique Electronics Supply" they have a great select of tubes at fair prices and they stand behind what they sell... Antique Electronics Supply

They also sell tube kits for Fender Twin Reverb... Twin Reverb tube kit

Hope this helps.
Mike[This message was edited by Michael ODriscoll on 02 January 2001 at 12:31 PM.]

Michael ODriscoll · Post by **Michael ODriscoll** » 2 Jan 2001 12:39 pm

Check out "Antique Electronics Supply" they have a great select of tubes at fair prices and they stand behind what they sell... Antique Electronics Supply

They also sell tube sets for Fender Twin Reverb.

Hope this helps.
Mike

Bob Metzger · Post by **Bob Metzger** » 2 Jan 2001 1:52 pm

I don't subscribe to the theory that most 12AX7A's are microphonic. To me, that sounds like a salesman talking, not an amp tech. However, having said that, some circuit positions in certain amplifiers are much more sensitive to this than others. The good news is that I wouldn't include the '67 Showman in that category. The common preamp tubes have a hierarchy as far as gain is concerned, with the 12AX7A with the most gain and the 12AU7 with the least. The 'AT's' and 'AY's' are in between. Some folks feel that the 12AT7 is not a good choice for a first stage preamp tube, because of tonal issues. Some techs who prefer the 12AX7A like to instead increase the cathode resistor on the first stage preamp say from 1.5K to 2.7K, or thereabouts for steel guitar usage, where others prefer to drop the bypass cap (25 uf/25 volt, in parallel with the cathode resistor) and prefer the sound of an unbypassed 820 ohm or 1K resistor. If I were swapping preamp tubes to enhance this amp for steel guitar usage, I would not consider my experimentation complete until I tried the Sovtek 12AX7 LPS and the Sovtek 12AX7WB and gave either/both a serious long listen. At any rate, what sounds right to your ears is the correct decision. Remember different tubes sound different in different circuits and in different amps. One size fits all does not apply here.

Bob[This message was edited by Bob Metzger on 02 January 2001 at 01:55 PM.]

Ricky Littleton · Post by **Ricky Littleton** » 3 Jan 2001 10:01 am

Cliff:

You might want to consider the 12AX7R from Groove Tubes. They area good clean tube, but their gain characteristic is a little lower than other tubes. If you want a good solid lower gain 12AX7 then these may be what you want plus they are warranteed pretty well from GT.

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Emmons LeGrande - 8x4
Session 400 Ltd
Alesis Microverb
Dan-Echo, E-Bow

Rich Paton · Post by **Rich Paton** » 5 Jan 2001 11:02 pm

The experimentation & actual sonic results cited is the best "answerer" to the 12AX7 vs. 5751 vs. 12AY7 vs. 12AZ7 vs. "whatever tube" question.
I do remember a comment from Red Rhodes concerning Fender preamp tube selection, which I believe is quite true based on what I hear...
When I got a new Twin Reverb reissue amp back around 1992, I thought it was harsh & lacked ample headroom compared to "the real thing".
I took it to Red's garage shop in North Hollywood on the way to a weekend session in L.A., where he replaced the cheesy original tubes with Groove tubes, and re-biased the amp.
His comment was to use only 12AX7 tubes vs. the 12AX7/7025 types for a smoother, warmer tone. In any case, that was the end result I got.
Red was a most gracious and interesting host, and obviously an expert on Fender amps.
Although our association only lasted several hours, I miss talking to him after that most enjoyable visit.

Ricky Littleton · Post by **Ricky Littleton** » 6 Jan 2001 6:47 am

The above post is 100% right. Especially if Red Rhodes said it. He was Groove Tubes resident expert for many years. The 7025-type tubes are good if you want a really bright tone and high gain in the preamp section. Same goes for ECC83-types. The 12AX7 and some of the other variants like the 12AX7R have a much warmer tone, mostly because they are a little lower in gain than the 7025 (which is built to a MIL-spec). The GT series 12AX7, the X7R and the X7R2 are the best recommendations.

Love those tubes!!!!!!!!!!

------------------
Emmons LeGrande - 8x4
Session 400 Ltd
Alesis Microverb
Dan-Echo, E-Bow

David Biagini · Post by **David Biagini** » 6 Jan 2001 8:13 am

I have two Victoria amps made by Mark Baier. These are exact copies of Fender tweed amps. I talked to him about using them for pedal steel. Because pedal steel pickups are generally hotter than 6-string guitar pickups he recommended using a 12AY or 12AT for the first preamp tube in order to lower the gain and get more clean headroom. I tried it and it does make a slight difference.

Jay Ganz · Post by **Jay Ganz** » 6 Jan 2001 9:32 am

If you really want a treat...try any one of
the following tubes(if you can locate 'em):

Mullard, Brimar (both from Gt.Britian),or Telefunken(West Germany).
Their tone speaks for itself. Of course they're no longer made, so they can get a
little pricey.

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