Hifi Tube Amp to mic preamp

So I have this old calrad ap-15 hifi mono tube amplifier and I've been told it would make a good mic pre with some work. I understand I would need a input transformer for balanced input and I would like to have a proper balanced output transformer as well. Attached is the schem.  I was looking for a schematic of something similar to what I would like to convert this into and found the Gyraf G9 schematics.  Would be cool to integrate some of the features of the g9 into this project like 1/4 Di input, gain control, output control, low pass filter. Was thinking about using a Cinemag B11178 for the input

No schemo attached.

oh yeah oops

it's saying the file is too big ever tho I resized it to 800x500

put it on a host site http://s000.tinyupload.com/index.php?file_id=06925822685844541603

SunkenCity said:

oh yeah oops

it's saying the file is too big ever tho I resized it to 800x500

put it on a host site http://s000.tinyupload.com/index.php?file_id=06925822685844541603

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No. The server is acting weird, you have to try again. I don't know when it's gonna be fixed but it did the same a few days ago and it worked a couple of hours later, then broke again.
The link you gave doesn't work anymore, it's a temporary thing. Reducing a schemo to 800x500 is generally a sure way of making it illegible, unless it's a very simple one.

https://s18.postimg.org/kmhg53qix/Calrad-_AP-15.gif

I don't see any obvious mic-pre in there. 250V DC and 3 sockets... that's the cheap stuff. As you say, input and output transformers will be needed. There's potentially a stereo mic-pre; but a lot of work.

PRR said:

https://s18.postimg.org/kmhg53qix/Calrad-_AP-15.gif

I don't see any obvious mic-pre in there. 250V DC and 3 sockets... that's the cheap stuff. As you say, input and output transformers will be needed. There's potentially a stereo mic-pre; but a lot of work.

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I would turn it into a guitar amp and make some kid happy.

It would be easier too start from scratch.

Nice power supply
...even a hum pot ??

Fancy

> ...even a hum pot ??

It is early Japanese. They were trying to lose the "cheap" image. When they could add a feature, with a part they could make cheaper than US/EUR makers, they liked to do that.

Agree it is not far from a geetar amp.

PRR said:

> ...even a hum pot ??

It is early Japanese. They were trying to lose the "cheap" image. When they could add a feature, with a part they could make cheaper than US/EUR makers, they liked to do that.

Agree it is not far from a geetar amp.

Click to expand...

ありがとうございました for the schematic...... All I ended up with when I tried to get it before you posted it was a bunch of spam emails to fight off for the next who knows how long....lol

I guess that it could be used as a mic pre, probably more like a character toy than a clean one. I'd use a cheap input transformer as a start (OEP? or anything lying around), get into the mag input, make sure that there is no RIAA filtering and use V3 as a cathode follower for the output. No output transformer, lose the power amp (pull the tubes). If it turns out to be useful for something you can still experiment with expensive input transformers, beefier output with iron and so on. It might be good for drums, egits and maybe loud voc mics. Just don't think it might become a V76  Yet it might sound cool and it even has a passive eq stage!

Of course you could also make a 'serious' attempt, redesign all the stages, build a 'proper' output stage and so on - but then it would really be easier to build something new from scratch.

Michael

Sorry bout the spam emails I don't use image hosting that much I just kinda picked one.

I've used it for a guitar amp a few times sounds good but not really anything special.

I wasn't expecting anything super nice and clean from it just something different from anything else.

How would I make sure there is no RIAA filtering?

How do I set up v3 as the output?

And pulling (v4 or V2?) out would disable the power amp stage?

I was thinking I could bypass the input selector switch  and hard wire the mag input

Is there a way to rebuild  the input select rotary switch to control the gain like the rotary in the G9 schem?

What would be the first step to redesigning the stages?

Cheers!

SunkenCity said:

Sorry bout the spam emails I don't use image hosting that much I just kinda picked one.

Click to expand...

It wasn't yours. I couldn't pull up anything with that. I went and tried to find a schematic myself and got tricked into using my email to get it..... Guess I wasn't careful enough.....

But it's cool to have that schematic....

Hopefully you'll get some ideas for it......

SunkenCity said:

Sorry bout the spam emails I don't use image hosting that much I just kinda picked one.

I've used it for a guitar amp a few times sounds good but not really anything special.

Click to expand...

Which input did you use? None of the existing ones  is adequate for guitar. The line input is not sensitive enough and the mag input has an RIAA equalization, which cuts the highs and boost the lows.

How would I make sure there is no RIAA filtering? 

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But there is one!

How do I set up v3 as the output?

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The output signal is avalable at the rightmost terminal of capacitor C16, but it cannot drive a low impedance; it would probably drive a 10k load though.

And pulling (v4 or V2?) out would disable the power amp stage?

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No, you need to disconnect the red wire from the output transformer and the 230V source from the screen grids of the output tubes (pin 7).

I was thinking I could bypass the input selector switch  and hard wire the mag input

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Yes you could but first you need to disconnect the RIAA EQ (C7, R9, C6).

Is there a way to rebuild  the input select rotary switch to control the gain like the rotary in the G9 schem?

Click to expand...

No. The topology doesn't lend itself to variable gain.

What would be the first step to redesigning the stages?

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You must learn the fundamentals of tube operation.

SunkenCity said:

Sorry bout the spam emails I don't use image hosting that much I just kinda picked one.

I've used it for a guitar amp a few times sounds good but not really anything special.

I wasn't expecting anything super nice and clean from it just something different from anything else.

How would I make sure there is no RIAA filtering?

How do I set up v3 as the output?

And pulling (v4 or V2?) out would disable the power amp stage?

I was thinking I could bypass the input selector switch  and hard wire the mag input

Is there a way to rebuild  the input select rotary switch to control the gain like the rotary in the G9 schem?

What would be the first step to redesigning the stages?

Cheers!

Click to expand...

You would be much better off modding this first into a decent guitar amp. Much easier than a preamp. Very little of this circuitry lends itself to a simple preamp conversion.

Yeah I used the mag input and hooked it up to a 4 ohm speaker. Sounded pretty good for clean jazz/twangy country. plugged in a overdrive and sounded right.

I guess Ill just gut it use the chassis and tube sockets to build a preamp from scratch based on another tube preamp. unless just a heavy mod of whats already there will do the trick but it's not sounding like the way to go.

And start learning bout tube circuits.

Thanks for the tips and info

before you hack up the amplifier, consider this:
remove R5, R6, and C6.
get a small 1 gang "handy" type electrical box from a home improvement store.
they have knockouts that allow you easily mount a chassis XLR.
insert your step-up mic transformer in the box, wire primary to the XLR and the secondary to grid (pin 2) of V1.
fasten the steel box to the side of the AP-15 furthest from the power transformer.
if it functions decently, you can mount polarity and pad switches in that box.
take the output from the 16 ohm tap, yes unbalanced, should not be a problem with your interface.
18 watts from the 16 ohm tap is about +26 dBu..

Just straight remove them or remove then jumper?

No interface, what about into the my consoles line in?  or my little portastudio 414 cas-shit-te machine?

SunkenCity said:

Just straight remove them or remove then jumper?

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Shunt R6, remove C6, R5 can stay but better replace with 1Meg.
I really would do that for a guitar amp, sell it and buy what is right for your project.

No interface, what about into the my consoles line in?  or my little portastudio 414 cas-sh*t-te machine?

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If you use the speaker output, you need to provide a dummy load; a 10W wirewound resistor should be fine. Beware that the speaker output, although from a transformer, is not balanced nor floating (i.e. one side is connected to chassis earth.

Nathan your PM box is full  ;D