balanced in and output in a 6G15 fender reverb unit

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johnheath

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This has been on my mind for some time now and I do not have the knowledge or the skill to construct a schematic to get balanced in and output in the Fender stand alone unit (6G15)

I have searched the internet for info but all I could find was a small chat on audio diy between two fellows that said it couldn't be done.

I think there must be a way to do this. My intention is to rack mount a reverb unit but I want to be able to connect it after mic pre and compressor….not in the beginning as it has to be done right now.

I hope one of you experienced guys can give some ideas on this.

Thanks

/John
 
Input & output transformers would do it.

Easy enough to experiment with the idea by sticking a couple passive DI boxes around it. Assuming functional success, you can then upscale the transformers to whatever quality you feel is needed for your signal path.
 
johnheath said:
Yes, input and output transformer yes… only question for me is … how? :)

... With two tube microphone input (let's say 600R to 20K) transformers in reverse. Alternatively, buy two passive DI:s or even two of those cheap Shure line matching pieces. Voila!
 
> 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.
________________________________________

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.

 
Wow… now we are getting somewhere… I wanted this reaction :)

First … there is a overall problem… everything is in english and I am from Sweden, and even though I know my way around english there are sometimes problems when it comes down to terms and expressions concerning facts and industrial term and so on.

Secondly I know that the reverb unit is a typical guitar mojo in this case… but I just had an idea to incorporate it into the studio recording world :)

I will remember a link in the future thanks for the input on that. ( new to the whole forum thing really)

So… PRR… what you explain is that it would take a near complete inside out mod to fix this? Anyway… very much thank you for the reply and all the facts… I guess it is a bit too big project for my knowledge anyway :)

I also guess that I still can plug in a Gretsch through the reverb into a tweed deluxe and just rock away? hahaha :)

Thanks

/John

 
> near complete inside out mod

I don't see it as "major". Certainly less complex than the 6G15 itself.

Add a couple transformers, a couple volume pots, and an op-amp chip.

Power for the chip can be taken from the 6K6/6V6 cathode.

I've probably left some errors but here's the general idea....
 

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Hahaha - wow, thanks… It would be stupid to explain in details but I envy you who can write down this out of knowledge… excellent! I have this kind of knowledge when it comes down to gardening (lifetime gardener) and for me it is easy to give advise and explanations… I see the comparison :)

Besides building  two stomp boxes (tube screamers) a few years back I have no experience in handling OP amps but they seem to be fairly easy to assemble.

I will give it a try :)

/John
 
I don't want to disregard PRR's suggestions (I wouldn't dare, trust me), but I would (as suggested by me and others) start out a tad simpler with a couple of impedance matching transformers, even though there will be some mismatch here and there. Considering your own knowledge, I'd say that makes it a very good suggestion.

What kind of gear do you plan to be using before and after the 6G15? Seems like good info to contribute with.
 
Hej

Well I guess that there are many ways to fix it and from my point of view any suggestion is welcome.

Gear before and after? Well I guess it would be a mic pre and a comp before... and then maybe the reverb in the end if it is useful :) Preferably for live performance.

/John
 

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