Bogen MX6 Trade and Mod

[jmccanna]

Hello,

I recently did a local Craigslist trade - a turntable for a 1956 tube Bogen MX6 which was in good shape and had its full complement of tubes. All I knew was the other guy said it turned on but he did not hook it up to anything. After the trade, I pulled the 12AX7 tubes and they were original 14mm Bogen tubes made by Mullard.

I got it home and put it on a variac and brought her up really slow - over an hour - and then plugged in a Strat to Mic ! and the MX-6 into my amp. Lo and Behold, clean, full sound, pleasant tone and very little noise. I tested all four mic channels in the same way and got largely the same response and tone. I then plugged in a Calrad 36 microphone I picked up years ago and it worked too and sounded pretty neat using my harmonica.

In viewing the schematic it shows that all channels go through a single stage of a 12A before going to the 12AT7. A very simple circuit. I noted that the plate voltage on the 12AX7 tubes was only 75v and the main output of the transformer appears to be 125v and it gets stepped down by a couple of dropping resistors in the B+ path.

It sounds sweet now but would it be better for guitar if I were to modify/remove the dropping resistors and get more plate voltage on the 12AX7 tubes? I was surprised that the tubes sounded so good at this low voltage.

Would it make much difference if I were to change the input resistors from 470K to 1M to voice it for guitar? Two thoughts (and yes they hurt!)

Option 1: Reroute the output of Mic 1 to the second half of the first 12AX7 with the pot in the middle leaving the output of the second half to the 12AT. I would hope I could get some decent overdrive tone. However, with the low voltage I would not have much headroom.

Option 2: Perhaps even dumber would be to rewire Mic 1 for an EF86 to drive the 12AX7 - again the plate voltage would be a factor. Schematic attached.

If I do these mods but leave the other two channels alone, I still have them available for microphones etc.