Circuit design and tone impact

[Leonc]

I've got two Magnatones (M10 and M13) with some circuitry that is quite similar on one channel. Because they have different power sections, cabs and speakers, I can't quite tell how much of the difference I'm hearing is strictly a result of these other factors as opposed to the differences in the input/tone circuit.

So the question is...what differences would you expect from these two different circuits?

http://www.patchwerkz.com/LeonsGear/MagnatoneM13/ch2_schem.jpg
http://www.patchwerkz.com/LeonsGear/MagnatoneM10/ch2_schem.jpg

I think these are the differences. In the second schematic, there's a 33pf cap taking some input signal to ground; this isn't in the first circuit. There's a .01mfd cap off the plate, rather than a .022 in the first circuit. And the mellow selection on the contour switch uses a .001mfd cap rather than .0047.

So, noting these differences...what tonal differences do you think would result?

[VacuumVoodoo]

The first circuit has an error. Control grid of the tube is permanently shorted to ground. As drawn, this channel should be dead & silent. I'm assuming it's a draftsmans mistake. The wire to the left of ground symbol under the cathode resistor should be removed or replaced with a 33pF cap as in the 2nd schematic, this one looks correct.
This cap is there to cut the RF and prevent Albanian propaganda AM transmissions from interfering with your playing.

The plate coupling cap and mellow filter cap. 22nF vs 10nF should give you some more low end.
4.7nF vs 1nF in mellow position (which is basically a hi cut filter) will begin to attenuate treble at lower frequency than the one with 1nF cap.

Then we have an interesting bias scheme, not very common in guitar amps. It is a combination of fixed and self bias. The 220k resistor from "D" to the cathode has significant impact on the tubes operating point and thus its linearity aka distortion characteristics. You may want to experiment with this resistor, 150k to ca infinity is a good range. I'd refrain from putting a 150k resistor in series with a 1Meg pot there. High voltage on pots isn't in the least funny. Simplest mod: remove it and listen:)

[Leonc]

Cool. thanks - That is exactly the type of info I was looking for...and yes, the 1st one definitely has more low end on that channel.