Akai M7 into two REDD 47

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jusmu

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Joined
Oct 22, 2016
Messages
3
Hello.

I'm starting a project to convert Akai M7 tube tape recorder into tube preamps.
Power transformer gives 250v-ct-250v(60ma), 6,3v twice and 5,7v which was used to heat up EF86 in the original.

Seems that since I plan to have +48v option in this preamp I need to produce it somehow.
I have an extra toroid tranny that gives 12v twice so I guess tripling and regulating that I can archive wanted phantom power that?

- It already has EZ90 rectifier, go or not?
- Wondering does it give me any trouble to have to powers, where another is producing +48v?
-  Is there any point to use those oa2 tubes for regulating?
-  Sowter or Lundahl?
- How to utilise original VU meter?


regards
 
jusmu said:
at up EF86I'm starting a project to convert Akai M7 tube tape recorder into tube preamps.
Power transformer gives 250v-ct-250v(60ma), 6,3v twice and 5,7v which was used to he in the original.
Seems that since I plan to have +48v option in this preamp I need to produce it somehow.

You can use multiplier followed with reg at 5,7V AC winding to produce phantom power.

- It already has EZ90 rectifier, go or not?

Maybe. The problem is high internal resistance of the tube and significant voltage drop. So the real question is could you get B+ higher than +300V at cca 40mA if the power supply gives only 500Vct AC.
If not, just use two UF4007 or such instead.   

-  Is there any point to use those oa2 tubes for regulating?
IMO, not really.

Sowter or Lundahl?

I like Sowters. I didn't try Lundahl there.

How to utilise original VU meter?

Just connect the VU meter via 3k6 resistor to secondary of the output transformer and see.

 
...and since its cheaper to go diode rectified. Maybe I'll do that.

Well, what components to use for regulating?

Does anyone have schematics for attenuator setup
 
Yeah, rectify it with diodes because OA2s can be problematic quite often and will become only more expensive, beside voltage drop.
I built quite a few 47s and now use RC filters, then zeners and cap multiplier for stiff supply. Noise is extremely low, can't really say real regulation sounds better compared to zener voltage reference. Seems like there is a bit of difference between zeners and OA2s, zeners sound better to me (zeners and cap into pass transistor base). I used G9's HT regulator in one or two, works well, although needs some load/current through it to regulate when testing out of circuit. Otherwise it is easy one if you want it for some reason.
Signal transformers give a lot more difference, so you might not want to bother and rather spend a little more on them because it is pretty clean high end preamp.
PM me if you need that complicated stepped attenuator, simple -20dB ala Jensen works well in almost every situation, also without output level control. Again, i would spend money for transformers instead of any kind of expensive stepped attenuator switches, difference is that big here.
 

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