Ampeg G-212 Hiss in Channel 2

[RWood]

It looks like IC 103 handles the reverb return. It is the one marked 709 CG and on the parts list it is N5709A. So I guess they have the "709" in common.
There is also a .22 cap, C121, leaving the op amp which I suppose could be leaky but it's a film cap so my wild guess is that the IC is bad.

If that's the consensus here I'll look for a replacement. Would the RC4739DB work there? I've seen NTE909D listed as a possible sub. Other ideas?

[g1]

Nothing funny with the footswitch jack or the pot?
I'd go with the NTE part (make sure you get the D suffix), they show a listing at Parts Express: https://www.parts-express.com/nte-90...inear--nte909d

[Enzo]

It is just a simple op amp. It shouldn't be hard to cobble in a TL072 or even TL071 in its place.

[RWood]

OK But how does one cobble when the pins ain't in the same places?

[Enzo]

There is always the dead bug method. TAck glue the IC to the board upside down, legs sticking up...like a dead bug. Then short wires down to the appropriate holes in the circuit board.

[J M Fahey]

4739 was a revolutionary Op Amp ... way back then, the first ever *designed specifically* for audio and not instrumentation or missile guidance analog computers.

It was used by everybody and his brother , including Kustom ans a host others, plus it was *the* Op Amp in Craig Anderton´s book.

Problem is that it stopped being made and it had its own unique pinout, incompatible with others.

Now we have many vintage nostalgia lovers and if I don´t remember wrong, at least one site is offering "converter" mini PCBs, where you mount a little modern SOIC dual Op Amp and proper pins get routed to proper DIP sized external pins.

Serch for it, I will do the same.

EDIT: here´s one
RC4739 uA739 MC1303 Drop In DIP Board Ultra Low Noise OP AMP NE5532 Upgrade
not just a replacement, but an upgrade.
personally I´s trust this fresh made adapter more than any "presumed NOS" offering, specially if coming from Asia.
Only problem is that *I think* the mini 5534 Op Amp comes unsoldered, I woukd only buy it if fully assembled, tested and ready to solder in the old PCB. So ask him.

[RWood]

Thanks for the ideas and the context. I got a lot more hits searching for RC4739 without the suffix.

I did order a pair of them from a seller in CA. We'll see (hear) what they do.

I also ordered a pair of NTE909D from a place in Seattle, Vetco, but just got an email saying they are out of stock. So it looks like I'll be dead-bugging the one for the reverb socket. Looks like the pinout on that one is different than the RC4739.

[RWood]

Wrapping this up - the NTE part fixed the reverb. I got it from NTE Parts Direct, a very good experience from chatting online on Monday to confirm they had it, to having it in my hands on Thursday. I also got some 14-DIP sockets from them and a few other things.

The RC4739DB came in today from CA and it does work. It was Made in Korea. It does not sound as good as the one working original part.

The tone controls on this thing are impressive. The midrange, in particular, provides quite a range of sounds. The reverb - - meh, it's a little tank - - but the Tremolo can go really, really slow, which I like.

Thanks, as always for the responses and the assistance. I've learned a lot about solid state amps from you guys that I would have never learned. Much appreciated!

[g1]

Is that a Firebrand SG? Nice. Maybe not, looks like the headstock is black.

[RWood]

Yeah it's a "faded". Everything I want in an SG (and a low price).

[RWood]

One other thing I meant to ask. When I bought the filter caps I also got a set of four output transistors. I got MJ15003G from Mouser.
I never installed them, though, as the output was fine after I put in the caps and set the bias. Is there any reason I should install the new devices anyway?

[g1]

Nope.

[RWood]

cool...thanks!