Incarnations of the Mark 1 amp?

Hi

There are various versions of the Mark 1, the earliest Mark 1 amp were quite bright whereas the later ones were quite mellow sounding. The S.O.B. amp was another version of the Mark 1 as was the Heartbreaker amp. The Mark 1 was also reissued some years ago too which differed from the original amp.

What component differences were there in early amps that made them sound different.

Thanks for your time and assistance.

Regards

Mark

I've owned 5-6 1970's era Mark I's and none of those amps would be considered bright. On the contrary, they were some of the fastest sounding amps that I've ever heard, clean or overdriven. One of them even had a JBL K-120. I think that I read somewhere that the Mark I R.I. was based off of Robben Ford's Mark I. The Mark I reissues that I've owned were definitely on the dark, wooly side. I don't know exactly which component spec changes happened during the original Mark I's lineage, though. Sorry.

The MKI that got away.

I have a friend who factory ordered a 1x15 Mark I back in the 70’s. He said one of the preamp tube sockets had some sort of solid state device called a “Fetron” and that’s what gave the early Mark I’s their sound. The amp came from the factory with the Fetron. Anybody heard of this thing? I presume the newer Mark I reissues just use a 12ax7 type tube.

A "fetron" is a solid state replacement device that was made for a variety of vacuum tubes. They were made by TRW and employ one or more JFETs as amplification and switching devices. They plug right into the tube socket but you need to be sure that you have the RIGHT fetron for the right tube replacement. They're definitely not all the same.

https://www.scribd.com/document/88749765/Fetron

Thanks for your replies I appreciate your input.

I liked the sound of the Fetron. It sounded better than a 12AX7. I was really annoyed when I loaned my amp to a friend's brother who returned it not working with the Fetron blown and without an apology.

I bought a new Fetron but I couldn't get it to work properly.

Regards

Mark

Are fetrons still in production somewhere?

I think the answer to that would be no. The model to look for is TR1009. I was able to find this link.

https://el34world.com/Forum/index.php?topic=12710.0

Regards

Mark