Schematic Question.?

[trem]

Selmer Futurama Bassist Major Schematic

Keeping in mind I have just a basic understanding of electronics.....
From the grid of the phase inverter there is a .01 cap, then another cap symbol that says 2k Pot I believe.
What is that Pot for exactly.? I am kind of baffled by that whole circuit actually.
Do not think I have ever seen part of the PI tied up like that.
Thank You

[loudthud]

That is 2Kpf or 2000 pico Farads, not a pot. That type phase inverter is sometimes called the See Saw. It is used on some early Ampeg amps.

[trem]

10-4
Thank You
Guess I need to get my eyes, as well as my head, examined.
Even armed with the correct info, it still looks weird.
Thanks Again

[jazbo8]

2Kpf = 2000pF is a capacitor, not a pot. The PI looks like a type of floating paraphase. I think the 2000pF is used to bleed off HF, some one please correct me if I am wrong...

Jaz

[J M Fahey]

At the PI, the left triode is a conventional gain stage, and it drives directly the bottom pentode grid.
The right triode also receives this signal and inverts it, driving the other pentode grid.
So far so good, but it has way too much gain (around 50) so they add a local negative feedback resistor: the top 220K.
So it behaves as a unity gain (220K/220K) inverting Op Amp.
Agree that the 2000Pf was added for stability purposes.
Mind you, the impedance at that point is not 220K or 100K or some other "visible" R value, but a nuch lower one.
The right triode grid is behaving like a virtual ground, so the effective impedance there must be around 80K/50 , some 1K5 or 2K, so the treble cutoff must be way above the audio band.
He who wants to can calculate it, considering R~2K and C~2000Pf.
Me?: *destroyed* going to sleep .
Good night.

[Gingertube]

Drawn better on the Caravelle 15W Amp on the same Semer Schematics Site, definitelt 2KpF = 2000pF 0R 2nF

Yes its a floating paraphase splitter. If interested The Floating Paraphase Circuit

As Juan says the impedance to ground against which this 2000pF capacitor works is almost impossible to calculate due to feedback etc. but it is there for stability purposes

Off for 3 weeks after today - a Cool Yule and all the best wishes for the new year to all.

Cheers,
Ian

[Enzo]

2000pf, 2nf. FOOEY!

Call it a 0.002uf like god intended, dammit.

[Dave H]

2000pf, 2nf. FOOEY!

Call it a 0.002uf like god intended, dammit.

Why not just stick to Farads so we wouldn’t have to mess with these new fangled micro thingies. 0.000000002F it is then.

[R.G.]

Why not just stick to Farads so we wouldn’t have to mess with these new fangled micro thingies. 0.000000002F it is then.

It is NOT a 2kpf, 2000pf or 2nF, or even a 0.002. It's a 202.


I still remember the day when our department secretary (this was back when those existed) walked into the power supply lab and said "Hey guys - what color is a capacitor?"

[J M Fahey]

"Hey guys - what color is a capacitor?"

You should have answered: "esasiest in the World !!! Orange Drops are orange, Blue Astrons are blue, Mustards are ... welllll .... mustard coloured )

[trem]

2000pf, 2nf. FOOEY!

Call it a 0.002uf like god intended, dammit.

If it had been printed that way, I would not have made my fist mistake to begin with.
Without my reading glasses ON... the pf honestly "looked" like pot to me.
Floating Paraphae
Thanks Everybody
and Thank You Mr. Fahey for the details.
Not sure if I had ever seen a PI tied up like that
God Bless

[Dave H]

It is NOT a 2kpf, 2000pf or 2nF, or even a 0.002. It's a 202.

Yeah, I love those 10 ohm resistors that are marked 100

[Jazz P Bass]

Yeah, I love those 10 ohm resistors that are marked 100

And the surface mount capacitors do not have ANY markings on then at all.