6 posts / 0 new
Last post

solid state to 12ax7 driver mod?

Hi everybody!

I'm completely new to this forum as I found it ten minutes ago.
I´ve owned a 212 HD one-thirty (or 130 as I've noticed being the "lingo") for close to 25 years and it has never given me any problems.
It's got the silvery logo-plate so it's an earlier than the silver-on-black variety.
It's not early enough for the 12ax7 driver. It's a solid state driver BUT (if it's important to my coming 2 questions) theres a plate covering where the hole where the 12ax7 sat on the slightly older ones.
My questions are then:
1. Would there be an improvement/benefits to modify it to 12ax7 specs to the degree that it's worth it? What are the difference between the two versions? Is it even possible to make that mod (or have it done, as in my case)?
2. The speakers blew recently. Not the cones but the voice coils are burnt/short circuited. Since the celestions in it had BIG fat magnets I thought I'd have them repaired. After reading some stuff on the net I found out they wheren't the originals anyway and buying new ones would probably be cheaper. What speakers would you good folks out there recommend?

Kind Regards
Olof Fredrik Alexandersson

Hope I did't get too long winded and that my english is understandable. It's my 3rd language so it's far from perfekt.



I've seen the 12ax7 called both driver and phase inverter. Wich is more correct and what does it do? (Pardon someone with basic electronic knowledge.


It does both

One half of the tube is the driver, the second part is the actual phase splitter.

Lars Verholt


I once raised a query about

I once raised a query about the 12ax7 with regard to my RD50. Now I know that this doesn't specifically answer your question but it's good to know.

Courtesy of Andy Fuchs....
a two section tube like a 12AX7 can be a voltage amp, followed by what's often called a "Concertina" phase splitter, which is a stage that takes a single ended signal from the first half of the tube, and produces two equal but opposite phased signals to drive a pair of power tubes push pull. Wow, long sentance... The Music Man amps used this with the tube driver, before they designed the IC/Transistor driver in the later amps. Fender used this in the Princeton's and a few other models, but it was nothing innovative nor unusual and was used in audio and guitar amps by many companies. It's different from the Schmidt style diff-amp driver in traditional Fenders and overloads differently for sure.

As far as the RD-50's, they used the 12AX7 at a fairly low plate voltage as an overdrive stage or sometimes called a limiter stage. The majority of the RD-50 preamp circuits were solid state, but the second channel used that 12AX7 to produce a warm compressed tube overdrive. They can sound pretty decent when dialed up right, and some people have come up with mods for those amps to improve them like Ed Goforth. "

My company also mods Music Man amps quite extensively: Fuchs Audio Technology


When the SS driver board came

When the SS driver board came out, Musicman referred to that as an upgrade. They even provided retrofits to change the older 12AX7 versions to SS which 30 years later sounds kind of silly.

I have seen this question asked before on the forums (can't find it right now, but there are a lot of posts in these forums) and the general consensus was "not worth the effort". Sorry, I know that doesn't really answer your question...

EDIT TO ADD: I found the post I was thinking about. Seems complicated to change back. Mike thought it wasn't worth the effort, Ed thinks it is.. You'll need to decide. Here is the link...

mm210's picture


The problem with the early MM's is that they had a bad habit of burning up tubes. I had one of these. I had bought a brand new 2-12 130 WITH the PI tube. Burned up tubes quite regularly. Sent it back to MM under warranty and THEY converted it to SS driver to cure the problem. It never sounded the same after that and I sold it. Got another 130 later that had the tube and I converted it over with a factory supplied conversion kit. Went fine and it worked but likewise again, didn't like the sound of it. I reversed the process and converted it back. Sounded better but had a hum that I was looking for when I dropped a pair of dykes in while it was running. 700 volts doesn't bounce around very far before it eats something. My position is that if you use pedals for dirt and effects, leave it alone. If you're looking for some kind of natural born distortion, you want the PI tube. Also, if you doing it yourself, you can justify the time and expense. If you have to pay somebody 75.00 per hour to do it, it's NOT worth it. Mike.
PS-never did go back in and fix it. One of these days..........

Log in or register to post comments