PI anode resistors

[lowell]

I'm looking at two different Fender Bassman schematics. One has 2 47k resistors on the plates of PI and one has a 100k and 82k... what gives? I've read about needing to offset half of the PI in order to balance the two signals coming out, so wouldn't the 2 47k resistors lead to a distorted output?

[Old Tele man]

1) the 82K/100K pair represents the "original" Leo Fender design.

2) the 47K/47K pair represents the infamous CBS-era "quasi-cathode/fixed" bias revision.

[jrfrond]

With the long-tailed PI scheme, where you have one common-cathode amp and one common-grid amp, all is never equal. You can choose between best signal symmetry (different resistors) or best DC balance (same resistors). There's also the scheme that Fender has been using which consists of using two 100K resistors and splitting the common-cathode side plate resistor into 91K and 9.1K to form a voltage divider for the output signal, in order to maintain DC balance AND symmetry. The plate still sees 100K as far as DC is concerned, and the output signal is dropped by 9% to compensate for the differing gain structures. However, the two sides STILL have differing impedance characeristics, so it's NEVER equal.

The two 47K resistors drop the gain of the PI, which, in a bass amp, is not a problem. But, the symmetry suffers. I use the term "suffer" loosely, because asymmetry is a fact of life in most tube amps, unless they have fully independent and adjustable PI halves, e.g. the Ampeg SVT Classic.

[lowell]

wonderful info thanks!

[hasserl]

There was discussion on the old Ampage about these two different resistor values (the 82k/100k and dual 47K values), and the point was made how the 47k value is better suited to the 12AT7 tube Fender uses in the PI slot, that the 82k/100k values would be better suited for a 12AX7. Doc reported how he changed a later silver face Bassman form 47K to 82k/100k and was not pleased with the results. The 47k value was louder and stronger. He also mentioned a Bandmaster he worked with and had similar results. Ray Ivers added that 82k/100k resistors have no place on a 12AT7 tube, unless you wanted a squashy compressed sound. There was even some experimenting by KB (Amp Kat today) dropping those resistors down to 33k and trying a 12AU7 tube for interesting results, but also using them with the 12AT7. I have the discussion saved on a pdf file.

In my own experience with a SF Super Reverb I've left the PI alone, though most of the rest of the amp is close to black face, though with a few of my own tweaks. It's a screamin amp, would make many a bf SR run and hide.

[Rick Erickson]

Ray Ivers added that 82k/100k resistors have no place on a 12AT7 tube, unless you wanted a squashy compressed sound.

Ah yes, everyone knows that BF Fenders were famous for their squashy, compressed sound! Thank God for the CBS era engineers to straighten out their "tonal issues" <LOL>

RE

[hasserl]

I'm just reporting what was posted back in the day from a couple of guys that used to hang out around here, and contributed a lot to what made Ampage the best amp message board on the net. I'm sure you remember them. I didn't make it up, and since you made your comments, well, here I posted the thread up here: http://members.dslextreme.com/users/...65modstalk.pdf

Ray is pretty sharp on electronics theory, snide remarks aside, maybe you could offer a rational and educated explanation of why he was wrong; and why Doc's experience was off the mark.

[Rick Erickson]

I was just having a little fun, not trying to start a flame war or anything. When I think of 12AT7's and 82K/100K plate resistors though, I immediately get a picture of a Pre-CBS Twin Reverb in my head. Not my idea of a squashy sounding compressed amp. However when I think of a more squashy compressed sound, a vision of an early 4 input Marshall 50 or 100W type amp comes to mind, (or a CBS era Fender with all their "improvements") Wouldn't you know it - the Marshall's use a 12AX7 and 82K/100K plate resistors. There is usually more involved in creating a tone than one little part of the circuit. I reserve the right to change my mind at any time and may even find that I do like the 47K plate resistors with a 12AT7. Stranger things have happened. Not all the engineering changes were bad ideas in the CBS era, they just over-did it on many models and ended up creating a lot of very squashy, compressed amps.

RE

[hasserl]

Fair enough Rick, thanks for the honest reply.