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Old 29th November 2010, 03:07 PM   #1
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Default Bogen MX60A as pre-amp

Hi. I'd like to use a Bogen MX60A as a tube preamp for music recording, but there are a couple of things I want to check. I know just enough electronics to get in trouble. . .

The Bogen MX60A is a 60-watt PA, popular with churches and schools in the sixties. It has, among other things, four high-impedance mic inputs. Each mic preamp uses half of a 12Ax7A. The preamp outputs are bussed to a 7247, which I presume is used as a mixer. The output of the 7247 circuit goes on to the tone control circuits, but it's also available at a pair of RCA jacks marked "Bridging input/output." I think the idea was to connect them to more Bogens so that they could share inputs and drive multiple sets of speakers.

Question 1: I've tried feeding the bridging output to a standard line-in connection on a low-end DJ mixing board (Nexxtech, if you must know). The MX60A has no speakers or other load connected, and the master volume is turned all the way down. Everything sounds fine, but is it safe?

Question 2: I'd like to send the 12Ax7A outputs out on individual RCA jacks (so I could run a guitar to one recording channel and a bass to another, or chain one preamp to another for an overdrive effect). The mod looks easy: The MX60A has a "speech filter" switch on each channel, immediately before the bus to the 7247. The switch just cuts in or bypasses a series .0018uF cap. My plan is to remove the cap so that I can pick up the isolated preamp output and connect it to an RCA jack. There's no DC at this point in the circuit, but I'd put in a decoupling capacitor for safety. I was thinking 0.1 uF, because it's often used between stages in this amp. Does this plan sound reasonable?

I have a schematic. I can scan and post the relevant sections later today if anyone can help.

[EDIT] The schematic is already posted!: Bogen MX60A Schematics

Last edited by AJOwens; 29th November 2010 at 03:17 PM. Reason: add link to schematic
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Old 29th November 2010, 06:46 PM   #2
Gilgy is offline Gilgy  United Kingdom
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As someone once said when asked for directions - if I was going there, I wouldn't start from here.

If I'm reading you right, what you want is either a simple tube-based mixer or summing amp (which seems to be what you have in the Bogen up to the first 7247 tube) OR you want a series of tube buffer stages to colour your recording source with that indefatigable "tube sound".

The problem is that everything after V5 on the schematic is a power amp therefore surplus to your requirements and no, running a tube power amp without speakers is never a great idea.

Wouldn't it be simpler to simply build a copy from scratch and make whatever modifications you need?
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Old 29th November 2010, 07:53 PM   #3
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Just pull the output tubes and put them in the closet for another day. The supply voltages will be a bit higher that way so you may want to drop them a little to get longer life from the components.
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Old 29th November 2010, 08:47 PM   #4
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Thanks for the replies!

For me, it's definitely not easier to build a preamp from scratch using the parts -- or to tweak the power supply voltages. I'm a musical hobbyist, not so much an electronics hobbyist. I love my old Bogen, but if this is not under a dozen or so solder joints, I'll just drop $200 on an off-the-shelf two-channel tube preamp with phantom power.

I have some big old power resistors bundled up as a dummy 8-ohm 60-watt load, so I'll throw those on the speaker taps and leave the 8417 power tubes in. The basement needs more heat anyway. :-) Thanks for the heads-up not to run the amp unloaded.

As for my mod, it's dirt simple. I'm just not sure if output impedance is an issue, or whether .1uF is anywhere near an appropriate value, or I need a series resistor as well (as in the bridging input/output connection). To make this real easy, I'm posting a link to zipped PDF of what I have in mind. The red mod would apply to MIC1 through MIC4.

http://www.storm.ca/~jje/music/mx60a_preamp.zip
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Old 1st December 2010, 06:41 PM   #5
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I have actually been studing this for the last few weeks. Using a small power amp to drive a power amp driver stage. Why. Well, power amps with a half dozen to a dozen power tubes per channel typically use small power tubes as driver tubes. the design I'm looking into would use el84 power tubes to drive 6 to 12 kt-88's, el34's, 6l6's, etc. Standard driver tubes do not have the drive capability to drive that many tubes.

so how have they been using small power tubes to drive power amps. some use a resistors to change the gain ratio, some use small transformers, none use capacitors. using capacitors would not change the gain ratio, it would just change the bandwidth.

I like the small transformer approach the best, which allows the flexibility of using the circut in ultralinear mode.

in the case of the original question, the bogen manual shows using a resistor across the speaker terminals as well as a resistor in line with the signal path. I can understand the resistor across the speaker terminals, it provides a load to the output transformers. The resistor in line is probally a current limiter to protect the input stage. 8 ohms is not enough of a resistor across the terminals, I think i recall somewhere between 60 and 600 ohms, but read below about using an autoformer instead.

what would I recomend that you do. I would recomend that you use a pair of wall mountable autoformer base volume controls. The autoformmer base volume controls take care of the amp output load issues. the autoformer steps would proivde tweaking of the signal strenth so you have some control over the gain. most of these autoformers are 11 step. Start out at the middle step and decide from there what the levels should be. Unlike using resistors, using an autoformer will reduce distortion.

now, unless you are directly driving a power amp stage, with out a driver stage, becareful, you might wind up with a lot of input distortion.
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Old 1st December 2010, 07:29 PM   #6
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Thanks for your reply. I can't comment on your plans because I'm not that knowledgeable, but if they involve anything after the "speech filter" switch on the 12Ax7A preamp stage, they don't relate to my question anyway. I'm not going anywhere near the driver or power stages.

When I talk about putting a resistor across the speaker terminals, it's just because I don't have any speakers connected to the power stage (I'm just interested in the preamp) , and I should provide the power stage with a load of some kind. Come to think of it, I should probably just hook up a speaker and keep the master volume turned down.
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Old 1st December 2010, 07:41 PM   #7
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Re-reading the replies so far, I don't think I've been clear.

What I want is four separate tube preamps, so that I can send each one to a different input of a multi-channel sound card -- or, if I feel like it, send the output of one tube preamp to the input of another one and drive the hell out of it. (People who have modded the MX60A to chain two or three input channels say it sounds just like a Marshall guitar amp.) The trouble is that the four channels are bussed to the 7247 driver. I just want to break them out and use them separately -- safely!
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Old 1st December 2010, 08:33 PM   #8
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I'll add this, just to be quite clear: Tube overdrive distortion is considered a good thing by Marshall guitar amp fans. In fact, in case anyone doesn't know. you can buy guitar distortion pedals that have a 12Ax7A in them.

You can also buy clean tube preamps for that mysterious tube warmth, but you probably knew that.
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Old 10th December 2010, 07:53 PM   #9
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"but if this is not under a dozen or so solder joints, I'll just drop $200 on an off-the-shelf two-channel tube preamp with phantom power."

good idea. sell what you have...buy what you need.
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Old 11th December 2010, 12:09 PM   #10
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Quote:
Originally Posted by speakerfritz View Post
"but if this is not under a dozen or so solder joints, I'll just drop $200 on an off-the-shelf two-channel tube preamp with phantom power."

good idea. sell what you have...buy what you need.
Thanks for responding. I don't plan on selling the Bogen, if only for sentimental reasons (although it's still a pretty good medium-power amp for a musician).

I'm replying to express my hope that someone will tell me if my proposed modification is unwise for any reason.
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