Mesa Mk1: Intentionally unbalanced PI? More balanced?

[NorCalTuna]

I'm committing heresy and converting a 67 bandmaster into an mk1 boogie. The topology is very similar and simply needs to be re-arranged for the most part.

http://tubefreak.com/mkirga_1.jpg

http://www.turretboards.com/layouts_...ster_aa763.gif


The driver/PI is also quite similar, save for a few touches: a .003 input cap; the plate loaded triode has an 82k plate resistor, which I think is to optimize balance between the two halves; a 15k resistor in the long tail, instead of a 27k, I assume to give the driver more gain; and finally- a 330k grid load on the plate loaded triode instead of 1M.

Wouldn't this effectively lower that triodes gain and either: closer balance the phases gain or make it intentionally out of balance?

thanks forum!

[Wilder Amplification]

If I remember correctly, decreasing the tail resistor value increases the symmetry between the two stages while increasing makes it more asymmetrical.

[octal]

Actually, it's the exact opposite. :-) A larger tail resistor promotes balance.

[NorCalTuna]

So, Mesa seems to be intentionally introducing imbalance into the pi? Any speculation on the effect? I'm tempted to try it fendery and then remove the balance and see if can hear or feel it dynamics wise.

[octal]

Actually, due to the way the LTP works, the plate resistors should be different values to improve the balance- but that can be blown off if the tail resistor is really large. You will notice that Fender also went to the 82K/100K resistor combo on the AB763 Bandmaster circuit, and most of the later BF Fenders have the 82k/100K combo. It's not all that critical.

Nathan

[NorCalTuna]

so, pardon me, Mesa is balancing it.

But what of the grid resistor? It's really the only one thats radically different, ie 330K compared to 1M?

thanks!

[Robert M. Martinelli]

The first triode's grid resistor also sets the stage's input resistance, in this case it's possible that they chose a 330k one to load the previous stage a little more heavily. The other resistor can be (and indeed is) 1M as usual.

As to the "tail vs. balance" issue, remember we're not dealing with hi-fi amps here, so perfect balance is not essential (as a matter of fact, it could even be detrimental in a guitar amp). A little imbalance helps in promoting even harmonic distortion. Ideally speaking, to achieve perfect balance the tail resistance should be made infinitely large, something that's obviously impossible in the real world. I see the tail resistor is rather low in value, this results in the stage being capable of a greater output voltage swing, but at the expense of some balance, and that's the reason why in older (Fender) designs the first anode load resistor is somewhat lower (82K vs. 100k). With a higher tail resistor, the two anode resistors can be made the same value.

Hope this helps

Best regards

Bob

[Merlinb]

so, pardon me, Mesa is balancing it.
But what of the grid resistor? It's really the only one thats radically different, ie 330K compared to 1M?
thanks!

Mesa has used a small tail resistor, probably to obtain mucho output swing.
This makes balanced worse, however, so they have lowered one anode resistor to 82k to improve the balance again (not perfect, but good enough for rock n roll). I doubt they even calculated anything, they probably just copied the values out of an earlier Fender.

They obviously tweaked the input cap and grid-leak to something they liked the sound of. The actual input resistance of the PI is about twice the grid-leak, because it is bootstrapped, so about 660k. They have still used 1M on the other side of the circuit though, because it allows a small decoupling cap to be used without losing bass (0.1u as usual). The two grid leaks are independent, they don't always have to be the same value.

[Bruce / Mission Amps]

And to add to Merlinb's reply... I have found to suit my taste, with the 330K grid load, the driver coupling cap has to be a little larger to get a fatter tone... many old Fender amps using 1M resistors sound fine with a 1000pF to 3300pF driver coupling cap but start to thin out as you drop the input grid load resistor below 1M.
So, how does that negatively relate to the Mesa .003uF coupling cap after the MV wiper and the poor high pass filtering using the 1M MV pot?
Seems to me a builder/player would be a little better served with a 100K to 220K resistor after the MV wiper followed by a .0047uF or .0068uF driver/coupling cap.
No math from me... just my ears.