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6LS7 to 12AX7 adapter

7/16/1998 9:33 AM
6LS7 to 12AX7 adapter
I am working on some old ampeg amps that have the 6LS7s in them I have heard that groove tubes and I think new sensor sells an adapter so you can use a 12AX7 has anyone used one of these? I would like to build some myself so any info on wiring up the 9 pin to octal would be greatly appreciated, I have some tube diagrams, but I would like some different opinions on how I could wire this. [thanks Richie]
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7/16/1998 10:16 AM

If the tube is a 6sl7 (6ls7?). I would try to find some NOS (new old stock)6sl7s they have a very good sound of their own. Some new amp companys make a big deal about using octal preamp tubes. Try yard sales, hamfests, estate sales etc......  
7/17/1998 9:06 AM

Richie: I agree with Gus, in fact, there are NOS 6SL7s of better quality/price available than 12AX7s - check - I've replaced stock Ampeg 6SL7s with their offereings and my customer's have been very pleased.  
As to your adapters, which you will need for some tube somewhere in the future. Save all of your old octal tube bases - even those without all of the pins are good for rectifier replacement. Use the stiffest, solid, wire you can work with and sleeve EVERY piece of wire. Keep the wires as short as possible, especially for first stages, and you probably won't have to use shielding (I have at times for a 6SF5 to 6AV6 made a shielded adapter with a shielded miniature tube socket on the top). On trick that works for me, once you test the adapter and know that it works, is to fill the void between the sockets with hot melt glue - it really stabilizes the adapter and has great dielectric strength.  
Keep the 6SL7s but make a few adapters for practice/the work bench.  
7/17/1998 9:22 AM

I have checked on the 6SL7s and the gain on these are listed as 70 and the 12AX7s are 100 Thats why I was wondering if anyone had tried the adapter and put in a 12AX7,what different tone or sound change would be, or how the pick attack or response would be.I will make some adapters and let you know what I think the difference is.[Richie]
7/21/1998 6:54 AM

Dear Richie: To answer your specific question I made a set of adapters for a friend with a Super Echo Twin, 12AX7 to 6SL7, and he did not like the sound. You noted the "gain" of the two tubes, actually, the term is amplification factor as the gain of a stage is dependent on the operating conditions (Rp, Rk, Ep, Eg, etc., etc.) of the stage. Not every stage with a 12AX7 in it has a gain of 100, some Fender stages, for instance, use this tube for a gain of only 35 or so.  
While you may like the increased stage gain with the sub, you might not like the increased noise floor. As an example, a different friend brought in his 1954 Deluxe (a chassis between the 5C3 and 5E3 [5D3?] it has the 6SC7 component values but 12AY7 tubes on the chart) for reconditioning. I recapped it, added some metal film resistors, etc., but it was still incredibly noisey. I wasn't until I replaced the 12AX7 that someone (maybe me 20 years ago) had put in with the 12AY7 that the stage was designed around that the noise floor lowered to an acceptable level.  
A final note: Some Vibro Champs distort too quickly for harmonica players who use high impedance "bullet" mics through them. I often lower the gain of the first stage by subbing a 12AT7 - it doesn't lower the maximum power output and seems to give more undistorted volume variation.  
PS: Just picked up a couple of 6SL7s, 1953 vintage, from an army surplus store for $1.00 ea. - one has a nifty eagle and crossed arrows stamped on the base.)
7/21/1998 12:41 PM

I did mean amplification factor not gain, you were correct in that, I did make some and found you correct there too,I got some nos 6SL7 tugsols and used them,I'm sorry for my mixup on the gain & amp factor, My brain is not working so good, because I had my 1975 pontiac grand lemans, in showroom condition with only 9000 miles on it, STOLEN, by a bunch of joyriders,and totaled! So it's hard to keep your mind straight sometimes.[Richie]

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