BOGEN LOM Tube mixer to 3 pre mod

OK. Well I've got a much bigger post, where I'll describe all I've done to this later. But essentially each channel has no gain control, just output control. Could I just move the pot to before the tube, or the transformer to have gain control? Or is it more complicated than that? I'm not sure what is clipping, the tube or xfrmr. What's odd about the clipping is, it seems to only be clipping the top of the waveform, and not the bottom. What does that mean? I'm trying to figure out some new transformer to get from cinemag, as well.

OK, may as well post the whole thing. I found this after I did all my mods:
http://www.clarkhuckaby.com/BogenMod/MxmMain.html

I e-mailed the guy, but he never got back. Was just looking to really tweak it out. Here's the E-Mail:


Hey man, I just came upon your page recently while I was doing some
restoration on my LOM. It's a great article! How esoteric... I'm a bit
of a novice in this area, trying to get some advice. The schematics
look pretty similar between the two models.

A couple years ago I just wired some 1/4" jacks directly to the faders
of three of the channels. The output is kind of weak though. Hence
your output amps, I gather. I might just switch my interfaces input to
-10 from +4. That should boost it heartily. (it did) Two channels share one
12ax7 though, and there was crosstalk between the channels the tubes
share, so I only am using three channels. Oh, and only 2 of the input
transformers work. They look shinier and newer than the 3 others in
there that don't seem to pass any signal. And I'm using the output
transformer as an input transformer for one of the channels. Seems to
work OK. I can't find any new 9-pin transformers though, so I'm
thinking of getting some new input transformers. Have any suggestions?
Or do you know what the turns ratio is supposed to be? So you use
channel one as a DI too, with that switch? How is that?

I replaced the coupling caps with cardas, auricap, and sonicaps, and
the filter caps with sprague atoms. Was thinking of putting in a
second rectifier socket and a switch, so I could switch between a tube
rectifier, and a solid state one. That suggestion about the pads seems
good. What if I just put the pot on the input side of the tube
instead? Then wouldn't it be a proper gain control? If I opted for a
pad instead, I could use the speech filter switches. And was also
thinking of running the output of the tubes, back into the second half
for a second stage. But if I did that I might have to use lower gain
tubes, like 12at7s. And also one of the channels I might let go
through the rest of the circuit. I may just bypass the eq though. I
should also get shock absorbers for the feet. This thing is
microphonic! I might also remove the 2 unused channels from the
circuit. Might be sucking power that could be better used?

Wondered what you thought about all this, and if you had any wisdom to share.
Thanks for listening!

End Quote

And now I got a new Amperex 6x4, and now it really shines. Sounds awesome when it's not clipping. Now about that clipping, and the input and output gain, any help? I have a schematic I can scan in later, if anyone's interested. Thanks!

Thanks for the link ! reminds me that in my 10 deep unfinished project bin is a groemes 5into 1 tube mixer that i was going to try different types
of pres in , including NYDaves one tuber with that exact idea of directouts

i thought the power supply decoupling ideas were good on that link , but
the challenge is a one tube circuit that has enough jam for mics & direct outs

regards Greg

I've got a Bogen MXM-A that I tinkered with a bit. All my comments are based on this model, so might not all apply to the one you have. It actually is a really cool dirt machine stock, but I have other, lighter-weight, dirt machines.

Caveat lector; Don't be discouraged by the following, I'm certain that the Bogen can be a cool piece to have around the studio. I've used it a few times to great effect.... with emphasis on effect.

About the clipping. You could be saturating the transformer due to it's small core not handling the low frequency energy, but the greater share of the distortion is coming from driving the tube grid positive, into conduction, or clipping. It is normal to only clip the top of the waveform, especially with sources such as vocals, where the waveform can be uneven or top-heavy to start with. These things were designed for lower output microphones used in public address situations, like the silver dynamic on a gooseneck on the store counter. Hook up a higher-output mic and sing into it and you sound like Butthole Surfers. I've used it to get that sound specifically.

Mr. Huckaby noted the need for input pads as you mentioned. Just open a few of the schematics in the Microphone Preamplifiers META to see various input pad arrangements. The trouble is, these preamps are noisy already, and a pad before the first stage of amplification just makes the noise more of an issue.

If you take a look at the schematic of the 12AX7 preamp, you will see that the mic input runs to the first triode, which has a plate resistor of 220k and a cathode resistor of 1k, a good arrangement for a lot of gain but also a very low clip point, due to the small amount of negative grid bias. The line input runs to the second triode, which has a plate resistor of 100k, and a cathode resistor of 2.7k. Less gain, more negative grid bias, better for higher level input. This latter circuit also has the benefit of a lower output impedance, if you were going to use a stepdown output transformer, but within the context of a several tube system a high output impedance will interface well with the next stage grid. This leads me to think that when you attempted direct outputs, you tapped the higher output impedance triode circuit (220k plate R in parallel with tubes dynamic plate resistance), and the tube was loaded down by the input impedance of the interface you ran it into. For instance, a 10k AD converter input would be far too low impedance for this tube circuit's output impedance. Mr. Huckaby noted that his opamp buffers took care of this by having input impedance around 1M, which is close to what the tube would see looking into the grid of the next triode (which has input impedance of 1M grid resistor in parallel with the impedance of the grid).

The HT (or B+) voltage is too low to create a nice clean pre with headroom to spare. Also the input transformers are not very clear sounding. Other members of this forum have tried them in front of well known circuits, such as the Pultec MB1, and reported that they are not as nice as the usual suspects for an input transformer, check the transformer META and CJ's page for those names.

All that said, if you can throw in another transformer to give you higher B+, like 300VDC, you could make a few of NewYorkDave's One-Bottle preamp, which sounds great.

Sorry I rambled on so long. I thought a lot about what to do with my Bogen. I just wanted to pass on some the things I learned along the way.

Here's the schematic I just scanned:




After I upgraded the tubes and caps, it's not so dirty anymore. It's pretty clean and sweet, at least when the input is low. I use my modded 57 on it, without the output xfrmr. So that has about 10 db less than a regular 57, and it clips less. I kind of like it when it clips, to a point. I'd really like to be able to control that though. Was just listening to some buttholes today, actually. Humpty-Dumpty LSD...

OK guys, thanks for the help. I've been trying to wrap my brain around all of this, and I think it's starting to make some sense, so I'll get back soon with more detailed questions and comments, but for now I've got to pass out, before I run out of run-on sentences to spew forth from my tired brain.

I was just thinking of trying to get new input xfrmers for this. Can anyone suggest any from looking at this schemo? And are there any other mods that might boost the headroom for some standard dynamic mics? What about suggestions for output transformers on the direct outs to boost the output? Thanks!