> fellows that said it couldn't be done.
Correct. ANY audio interface can be balanced/unbalanced *except* a 6G15.
Baloney.
> I do not have the knowledge or the skill
> to construct a schematic
> I have searched the internet
> I think there must be a way to do this.
> I hope one of you experienced guys can
You are dumbfounded, flummoxed.
You may not have all the info needed, but if you sit and study the problem you can get a lot of info pretty quick. Enough to fend off ideas that don't cut the mustard.
And first.... don't reference a Guitar amp on a studio-ish forum without a link. Even guitar freaks can't remember all the toys available. Studio guys may have seen a 6G15, or not, but wouldn't have much call to open one up and memorize the guts.
http://el34world.com/charts/Schematics/files/fender/Fender_reverb_6g15_schem.pdf
> rack mount a reverb unit but I want to be able to connect it after mic pre and compressor
So studio line level. Around 1 Volt in and out, 10K impedance in and low impedance out.
The 6G15 is made for Guitar levels. More like 100mV than 1V. The input impedance is high (500K), and the output impedance is much higher than studio-guys ever see (about 70K).
Without a wholesale re-design, what you want to do is:
Unbalance a balanced input. Transformer is simple and period-appropriate.
Knock the signal down from 1V to 0.1V. Exact knock-down unknown.... depends on what you run for studio levels and how much flavor you want from the 6G15. An input pot is appropriate.
10K:10K transformers are readily available and will not notice the 500K loading. Put a 10K or 50K gain pot on the secondary. Since we do want a large knock-down, and perhaps a wide range of knockdowns for different flavors, a Audio taper is appropriate. The 10:1 loss will be near "5" on the knob.
This goes right to where the 6G15 input jack is.
At the output you want to build-up what you knocked-down. Also buffer from the high impedance to a studio-friendly low impedance. And you may want to re-balance.
Personally, I'd throw a TL072 on the end, gain adjustable from 1 to 20. This low-Z output "should" drive any studio input despite not being balanced. However if you have fussy inputs, use the 2nd half of TL072 to give the inverted output. If you truly have true 600 Ohm inputs, well, they should have their own buffers; but you can use 5532 to push 300 Ohms each side of a 600 Ohm balanced input. Or use 5532 to drive a 600:600 output transformer.
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With a deep-thought re-design, we see that V3b cathode-follower can carry quite high levels, V1a is not needed if we have high levels, but we need gain after V3a to deliver high output levels. And we need buffering between Mixer pot and studio interfaces. So the re-design merely moves V1a function over behind V3a, and still needs a TL072/5532 at the end. With careful computations we might eliminate the input and output gain knobs... but hey, knobs are good, the more the merrier, more ways to twist tone.