From bruce@ashbysolutions.com Thu Jun 20 05:53:07 2002
Subject:Re: Swapping Out the H&K Rotosphere Tube

All,

>Seems to me there was another tube with similar characteristics to the 12AX7A
>that had a gain around 60 -- 12AT7A, I think. I no longer have a copy of the
>RCA Receiving Tube Manual (that book used to be a constant companion when I
>was in High School), or I could look it up. Man, those were the days!

There are several tubes in the "12A" family:

12AX7 -- gain = 100
8025 -- gain = 90 (???)
12AT7 -- gain = 60
12AV7 -- gain = 42
12AU7 -- gain = 20

This is a huge oversimplification, because as Steve H. mentioned, there are other parameters of importance in a tube spec besides the gain.

As far as, "Do they work?" this is an area where it's difficult to pre-predict the results -- particularly since success is in the ear of the user. Stevie Ray Vaughn substituted 12AV7 tubes for the 12AX7s in at least one position on his Fender amps -- the urban legend end of this basically true story makes it hard to get all the details right -- and it allowed him to get a warmer sound without so much overload. Put the same tube in another amp, though, and you might get 1) noise; 2) low output; 3) funky tone; or 4) self-oscillation (microphonic issues). I sell a NOS 12AV7 that seems to work well to heat up the first stage of a Leslie 122/147 if your clone's output is a little weak, but was highly microphonic when I tried them in my Peavey Classic 30.

In most of the old-school tube circuits, swapping within the family, so to speak, might not give pleasant results, but it won't destroy anything. I can't be as certain of newer designs, like the Rotosphere, especially since many new designs use "starved plate" circuits where the tube is run at tens of volts (or less!), rather than the higher voltages it was intended for.

If you want to experiment with the Rotosphere, I would stay with various 12AX7/8025/ECC83 models for a starter. Bear in mind that if the design is starved-plate, then the tubes the H&K use may be hand-selected for a certain response, so even a direct-brand substitution may not achieve the desired results.

Regards,

-BW

--
Bruce Wahler
Design Consultant
Ashby Solutions™ http://consult.ashbysolutions.com
CloneWheel Support Group and HiNote moderator
978.386.7389 voice/fax
bruce@ashbysolutions.com