Weird op-amp question--theory

[nashvillebill]

My old Kustom 250 bass head (see the schematic) uses a uA739 (crosses to a NTE 725 http://www.nteinc.com/specs/700to799/pdf/nte725.pdf ) in one of the preamps. This is an unusual, at least to me, opamp. What's the purpose of the input/output lags on pins 2, 3 and 4?

My amp is working fine, but I have toyed with the idea of making a bass preamp based on the Kustom design. However, since this is an odd opamp, I'm not sure what Kustom was accomplishing with that opamp, and what I would lose if I made a similar circuit but with a standard opamp (forgoing the stuff connected to pins 2 3 and 4)

Thanks!

Schematic.pdf

[Jazz P Bass]

The UA739 was a good IC, back in the '70's.
The "lag pins" are for compensation.
You really have to study the datasheet to ascertain exactly what the engineers were compensating for.
Bob Pease, bless his soul, would know!
Compensation link: Design; Op-Amp Compensation
I believe that newer IC's, for the most part, have dealt with the compensation issues with better designs.
That is not to say that newer IC's do not need comp, it is just easier to apply it.

[nashvillebill]

Thanks, that datasheet is helpful. One day I'll try to breadboard that preamp circuit but using a regular NE5532 (omitting the compensation) (or if I get really motivated, draw it in LTspice first).

And one day I'll figure out why they used the weird FET Q3 (a 40841) on the preamp PC5066 circuit when it looks like a regular FET would've sufficed....

Thanks again!

[R.G.]

And one day I'll figure out why they used the weird FET Q3 (a 40841) on the preamp PC5066 circuit when it looks like a regular FET would've sufficed....

It is a little odd. It's an Insulated Gate FET, which was what they did before they got good at making MOSFETs. At the time this was designed, dual gate IGFETs were used for super-high input impedances, gain modulation with that second gate, and voltage variable resistances. I thought at first it was for a variable resistance, but from the circuit, there's no switching. Maybe they were worried about needing a super high input impedance - that they then threw away with that 100K bias resistor. Or maybe the designer just thought they were cool.

It's the return for the "fuzz circuit" which doesn't seem to be in the schemo package. Given that - who knows? It could probably be tinkered with a JFET if you arranged the biasing just right.

IGFETs were insanely fragile with static electricity.

Sorry. Can't tell more than that.

[nashvillebill]

Thanks! I suspect the transistor salesman might've convinced the circuit designer that this would be a cool device to use; on the bass head, there is no fuzz, "fuzz in" is jumpered to "fuzz out" on connector J1.