PI mod help...

[HTH]

ok, I'm still tinkering with my 6V6 AC30 amp. I've changed the PI over from the usual 'Vox' values of 1k2 and 47k to the values suggested by Bruce (820 and 22k).

it's hard to tell what effect this has had on the tone of the amp by the time I soldered in the new resistors.

what effect 'should' this have both tonally and from a design standpoint?

[Matt T.]

It changes the operating point of the PI. I believe it gives it a bit more headroom (or rather 'footroom') and gain at the expense of balance. If you want to take it a step further...try the Marshall values of 470/10K. Go to Aiken's site and read the Long-tail PI paper if you haven't already, it explains each component's function in a LTPI.

[Bruce / Mission Amps]

I think I sort of answered that question in this reply:

http://music-electronics-forum.com/s...2&postcount=10

The main idea being, the ability to drive the 6V6s into power tube clipping with more phase inverter swing.

[HTH]

I think I sort of answered that question in this reply:

http://music-electronics-forum.com/s...2&postcount=10

The main idea being, the ability to drive the 6V6s into power tube clipping with more phase inverter swing.

cheers Bruce, the penny is (slowly) dropping... so using 820ohm/22k will give more breakup than the 1k2/47k setup (?).

the 470ohm/10k setup suggested by Matt would dirty thinks up further (?)

[Bruce / Mission Amps]

In a nutshell... yes.
Even though the balance is worse with a 10K then a 47K, if the PI is biased up like that, sitting on a 10K tail resistor.... and if the B+ voltage supplied to the plates of the PI tube is high enough, the PI will have more drive and swing a larger signal
It takes a much larger signal swing to drive fixed bias 6V6s into clipping then a pair of cathode biased EL84s.

[HTH]

Bruce, thanks for that - I tried the 470ohm and 10k now the amp has grown a new set of balls

with the 820ohm and 22k it sounded great for a powerfull crunchy Tele kind of tone, but now with the 470ohm and 10k I've got the gain I need.

I like aspects of both tones - maybe 680ohms and 15k would be perfect

Lastly - I've seen Fender amps where they've got a 47ohm resistor between the bottom of the LTP and ground. Can you (or anyone) please explain what effect this has both tonally and technically?

[novosibir]

That's the lower voltage divider resistor of the negative feedback loop of the combo amps with one speaker and 8 ohms. The upper R is the 820 ohms.

Larry

[HTH]

That's the lower voltage divider resistor of the negative feedback loop of the combo amps with one speaker and 8 ohms. The upper R is the 820 ohms.

Larry

Larry, just ignore that last question from me - I was having (another) brain fart. Was sure I'd seen a schem with a 47ohm resistor to ground on the bottom of the PI but with no NFB.