# Researching Bogen M330 PA Amp Conversion For Guitar



## keto (May 23, 2006)

I can't find a schematic online other than for sale, so I bought it off ebay for $5. It's NOT an M330*a*, so don't point me at that one (it runs 7868 power tubes).

The M330 I have, I bought dirt cheap off ebay a number of years back strictly to rob the tubes out of it. It runs [email protected], [email protected] and [email protected] (or is it 7242?), rated at 30W/50Wpeak, 2 knob tone stack, 4-8-16ohm taps, and so would seem to me to be ideal for conversion. But I never really thought about it that way until now, and just threw it on a shelf in the garage until I rediscovered it today.

I'm good with an iron, but have no experience inside an amp. I can read the basics of a schematic, but know nothing about stuff like feedback loops, or how to split the halves of a preamp tube - and probably a ton of other theoretical stuff. I do know how to tweak a tonestack, or figure that part out, (from pedal building) but that's probably about it.

Things I know to watch for:

-change all electrolytics and coupling caps
-add 3 prong plug with strain relief
-the speaker outs are on a tagboard on the back panel, gotta change that system

Anyone gone through this? Things to watch for besides my list? My first question is, if I wanted it to run 6V6's (probably my first choice) or EL34's *and/or* drop the overall output to more in the 15-20W range, how would I go about those things?


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## keto (May 23, 2006)

Pics

Front panel - 3 @ input vol, bass, treble. Treble is also an on-off switch.









Guts, with tubes removed obviously. Big old transformers, though no coding on the bigger one. Smaller one has 606941 on it, I'm guessing 1969. Are the 5 big silver things cap cans?









Put these 2 pics side by side and you have the circuit. Lots of stuff I'm not sure about here. Some of the caps look like candle wax. There's a big striped thing with about half of it faded to white, in the middle of the bottom pic - is that a big honkin resistor?


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## keto (May 23, 2006)

Above the knobs, Mic1 - Phono - Tuner - I'm guessing for changing among multiple sources. Filter, not sure if that's a tone/presence type control or what.









Rear panel - lots of extraneous extras to be done away with. At least there will be lots of pre-drilled holes for mods lol. I was warned that the tagboard strip for output connections is a live voltage source, will be taking that inside the box with just an input jack showing outside, if I proceed.









I lied above, it's the BIGGER transformer that has a code on it, none on the smaller one that I've found yet. How do I source manufacturer, is there a database to decipher the code?









Isn't that cute, someone replaced one handle (the blue one). Seems to be an 'off the shelf' item, was this handle configuration pretty common??


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## washburned (Oct 13, 2006)

Converting Integrated/PA Tube Amps into Guitar Amps.

You will need to do a bit of research, but this will walk you thru the big steps.


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## mbielman (Aug 18, 2010)

I have one of these as well. Using it as PA until I get around to modifications. As I recall, the tone "stack" is a simple cut-only (aka "tone control") set up. Not Fender/Marshall or even Baxandall (hi-fi) type. 1st thing I would change! Also, you need to defeat/remove the low-Z input transformers. I suggest a shorting plug just in case you want to revert back to mic inputs. (but that's just me)

Changing the output tubes could be tricky - EL34's might work... probably not 6V6's as you would probably have to replace the power and output transformers. 

UPDATE - checked my tube "manual" (Duncan Amplification): heater current for EL34's is significantly higher than for 6L6, which could be an issue for plug-in replacement.

Like you, I never found a schematic on-line, and never ordered one. Tube amps are all basically the same and I can just create my own schematic in an afternoon, if I ever get serious about an alteration.

One time I plugged my (electric) bass into the MAG input and the sound was FANTASTIC provided I kept the volume (control on the bass) low as not to overdrive the input. Really had that old Fender Bassman sound.

I hope to follow this project of yours... pls keep us all posted!

PS - yes, that's a big honkin' resistor.


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## Tyler Savage (Nov 16, 2009)

I have a Bogen CHB50 - Let me tell you, converting these aren't for the faint of heart. I was lucky enough to have a schematic for mine, but the PtP wiring is so confusing to follow.

I bought it initially for $20 to rob the black RCA tubes, but I'm thinking I might just drop a turret board into it and take advantage of the transformers too


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