Magnatone M13 - Easy Gain Boost

[Diablo]

Channel 1 on the M13 has a complex tone circuit with the contour switch. This channel also has the pitch shifting vibrato. Unfortunately, channel 1 volume is a quite a bit lower than channel 2 or 3. Channel 1 has three triodes for signal amplification, and channel 2 and 3 have only 2 triodes each. The schematic shows that channel 1 throws away most of the signal voltage in the contour tone circuit. There is a voltage divider with 470K and 22K resistors connected to the normal setting on the contour switch. All I had to do was swap out the 22K resistor with a 33K resistor. This provides plenty of volume boost to fully drive the power amp. The only downside is that the volume boost is only available for one position of the contour switch. However, that position is the only one with useful tone for me. At low volumes, the amp sounds the same as it always did because the signal voltage is set by the volume control. It's now like my volume control goes to "13".

http://www.magnatoneamps.com/schemat...natone_m13.jpg

[isaac]

That's cool. You could tweek the other stages to do this too. Like on mellow there is the same voltage divider that you could switch out the 22k(actually looks lie a 27K) for a 33 or 50k something like that. The other thing is if you did that, you could also change the .01 cap on the mellow side for a .001 or a .002 and it would still mellow but would give more treble. (no experience with this amp, just saying that if that mellow position is too dull, or bassy etc... you could lessen the value of the .01 cap and that would help. You could go as small as you'd like until you eventually could even remove it, and then have a different gain setting than the normal position. You could have the voltage divider set for a lot of gain or a little bit of gain between normal and mellow. etc.....

[Diablo]

Good idea. I think I'll do some tweaking on the mellow circuit too. Right now, it's not much use because it's so MELLOW.

[olddawg]

I love old Magnetones and in my experience they really grind stock. Just out of curiosity, how does it sound with simply using a boost pedal without modifying the amp?

[isaac]

I like the idea of not modifying vintage gear, but if he already did it, I figure I might as well help him out. Out of all the amp threads, I like the ones that are "Tweak for more brightness" "How to make my (so and so amp) sound like ..." etc.. I just like the tweaks and mods. Adding switches for different stuff. THAT is the part that gives the most addiction to the hobby for me.

[Diablo]

I love old Magnetones and in my experience they really grind stock. Just out of curiosity, how does it sound with simply using a boost pedal without modifying the amp?

I'm really not a pedal guy. The modification I made so far doesn't change the tone of the amp at low volume. It just allows me to get more volume and drive out of channel 1 when I want it by turning up the volume control. I could get channel 2 and 3 to grind, but channel 1 didn't have enough gain when stock. The M13 is kind of the mac daddy of suitcase Magnatones, with the fixed bias push pull EL34s, Jensen C15N speaker, four varistor vibrato, three channels with reverb, + 16 knobs.