[Phreon]

I know asking questions related to a guitar amp is quasi-off topic on this board, but I couldn't think of a group of people better suited to answer my question.

I have a 70's hybrid amp (solid state front end and valve final output) that's loaded with LM307Hs. Not only are they starting to crap out (one pooped and took my reverb circuit with it), but they sound AWFUL when pushed to distort.

The LM307Hs are old style, metal can, circular pin pattern chips (is the "H" package the same as a TO-99 or is there a difference). I'd love to upgrade and have seen a few people mention Burr-Brown OPA627's as a plausible replacement, but the price of each chip plus and adapter (digikey A330-ND) is cost prohibitive considering this amp has SEVEN of those dreadful LMs. Plus, I'm not quite sure what modifications to the amp's circuitry would have to be made to accommodate the OPAs. Somehow I doubt they're a one to one swap.

Ideally, a direct replacement would me the preferred solution, but I'm game for just about anything at this point.

Thanks,

Doug

P.S. Luckilly, the LM307's are in sockets

 

[Megaptera]

What kind of voltage does your amp run, and what kind of gain does it have? That's going to be the first thing you need to look at in order to decide what kind of opamps to swap in.

 

[tangent]

Ugh...this looks like the chip that gave op-amp audio a bad name. They don't even give a straight bandwidth spec, but I found a graph in the datasheet that basically says you can't expect more than +/-2V peak-to-peak out of this thing at 20kHz. Ucky poo.

You're going to have a hard time finding chips in the metal can. They still exist, but you'll probably pay more for them than you would for a DIP chip plus an adapter.

Luckily for you, you shouldn't have too much trouble finding chips that will work in that design. As it was made so long ago, the newer, nicer chips should work happily in the circuit. The main worry would be if the circuit counts on the slowness of that 307 chip to get away with bad layout or poor power bypass. In that case, you'll have to stay away from the faster chips. Anything from, say, 3 to 10MHz should be tame enough to work in most any circuit, unless it's pathologically bad.

For some chip reviews, see my op-amp notes.

 

[Phreon]

From the schematic (here), I'm pretty sure the LM307s are running on 16V. I am not, unfortunately, currently learned in circuit design enough to interpret it's intricacies to determine it's "wonkiness".

From what I can gather from your op-amp notes, the OPA227 (cheap) sounds like it would fit the bill as a replacement for the interstage 307s. An AD843 (quasi-cheap) might be a nice choice for the final gain stage, as you mentioned it has a pleasant "tube like" sound and smooth distortion (unlike the @^@# 307s). Sounds like a good match as the pre in a tube driver amp.

If you do look at the schematics, you might find it interesting that this amp (by design) runs it's 4 6CA7s at a B+ of over 700 volts!

Thanks,
Doug

P.S. the schem. pertaining to my particular chassis is page 3.

 

[tangent]

Quote:

I'm pretty sure the LM307s are running on 16V

That's +/-16V, or 32V. Beware when using that voltage, since it's over the recommended voltage for many chips. You may be able to replace D5 and D6 with a different zener value to get +/-15V, which a great many chips will tolerate happily.

While examining this schematic, I ran across another scary thing: a 1458! This is the dual version of the ancient uA741...it goes back to the early 70s! I'd replace it, too.

Quote:

An AD843 (quasi-cheap) might be a nice choice for the final gain stage, as you mentioned it has a pleasant "tube like" sound and smooth distortion

You must have missed this part of the review: "the sound gets very nasty very quickly when it starts clipping." The smooth sound I reported elsewhere in the review is when it is not clipping. Generally, you shouldn't ever run op-amps into clipping, so I don't bother to mention that behavior in the reviews except to warn of noteworthy behavior.

Not that I think any of this worrying about clipping matters. It looks like the output stage is the tube circuit in the upper right corner of the schematic. Probably the only reason you're able to drive the current op-amps into clipping at all is because they're so horrid. With 30V supplies, you can get about +/-13V signals out of most modern op-amps. Compare the +/-2V I think you're limited to with your current chips!

 

[ppl]

Brown dog has an adaptor that will adapt those to-99 Metal can IC's to an 8 PIN DIP part # 020601 http://www.cimarrontechnology.com/in...WPROD&ProdID=8