Mods etc for the Mark 1 style Boogie.

[mark1406]

I own a SOB amp which I converted to a Mark 1.

That is to say I changed the phase invertor (which had the limit control) the the Mark 1 reissue PI. Which worked much better with this amp.

I use to own an early Mark 1 amp many years ago, and I seem to recall the PI was different from the circuits I've seen drawn. The grid leak resistors were 330K instead of the normal 1M. The circuit diagram that I received from Mesa was different from my amp too.

I would be interested in what you guys think is the best Pi stage in these amps as well as the best gain stage version. Memory tells me that there have been four different versions of the gain stage. Currently I am using the Gil Ayan mod version, it's not too bad and Gil's mod does reduce the output of this first stage which does lead to an undesireable type of distortion.

On my original Mark 1, I loved the sound of the fetron and I wasn't too impressed when a friend who borrowed it returned it with the fetron destroyed!

As far as valves go in this amp, I like the EI elite golds, a Philips NOS 12AT7 and the Svetlana/SED 6L6's.

I'd like to hear more from you guys on pulling tones from these amps.

Yours Sincerely

Mark Abbott

 

[mark1406]

Gil Ayan's mods were quite popular with the reissue amps.

There are quite a few different Mark 1 circuits. The first mark 1 amps did have a reputation for being a bright precussive amp. When I bought my Mark 1 it had more midrange to the tone.

I have often wondered why certain amps were very bright, does anyone have an answer to this question?

Thanks.

Yours Sincerely

Mark Abbott

 

[John G]

Mark,
If you do a Google search for Mesa Lonestar and Heartbreaker schematics then what you will see are two more updated versions of the old MK1 albeit with switching options and all the modern bell and whistles of their later models.
There is a ton of options you can use from those schematics.
John G

 

[mark1406]

Dear John

Thanks for the tip. I have had a look at the Heartbreaker circuits and what you say is true. The circuit is a variation on the Mark 1 theme.

I did try and look for a Lonestar circuit without any luck. Should you know of a site that has the circuit, I would like to see it.

I will have to post up a few things I've found out about modding the Mark 1 amp.

Firstly, don't touch the 220K resistor across the second input, it is there to minimise noise and hum in the amp (found this one out the hard way.)

Everything said about a master volume before the PI stage effecting the tone of an amp (Fender or Mesa Boogie Mark 1) is also true. I've found the clean tones are better without the master volume however the distorted tones seem uneffected.

Turning down the master volume does seem to reduce the bass response. I have found doing the Kevin O'Connor mod where a 100K resistor is placed on the wiper of the master volume pot and the other end going to the input of the PI stage does eliminate this loss of bass response.

Changing the value of the cathode resistors in the pre-amp does also effect tone. I seem to recall higer values such 2.2K to 2.7K did result in a more Marshall like tone.

I seem to have gone off on a tangent here, again John thank you for your assistance.

Yours Sincerely

Mark Abbott