Old Masco to custom hifi

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Old Masco to custom hifi

Postby chosborne » Sun May 25, 2008 2:55 pm

I just got some old tube amps that are in disrepair. I am in the planning stages of rebuilding and/or redesigning them. The models I have are:
Masco: MA-25N and MA-8N
Executone: PB-325

I have a general idea of what I'd like to do with them. I think I will restore the MA-8N to use as a nice little guitar/mic/whatever amp, mono. I will also restore and upgrade the PB-325 to probably use as a sub amp, mono. Does anyone know of schematics for the MA-8N? I will scan and post the PB-325 schematics.

The MA-25N I have bigger plans for however. I want to gut it and turn it into a nice hifi stereo amp. It appears to have a nice power transformer that i can use. After cleaning up the metal case and painting, it should be nice. So the deal is, now I am entertaining all sorts of hifi push pull circuits.

Chris
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Postby Gingertube » Sun May 25, 2008 9:12 pm

In-case you don't have it - MA-25N Schematic can be found here, with some pictures as well.

http://www.angelfire.com/vt/audio/masco.html

Cheers,
Ian
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Postby chosborne » Sun May 25, 2008 11:47 pm

I like this design because it fits well with the original design and it also captures exactly what I am trying to do.

http://diyaudioprojects.com/Tubes/KT77- ... -Tube-Amp/

What is self inverting push pull? The schematic loses me with the LM317. Someone help!
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Postby erichayes » Tue May 27, 2008 12:23 am

What the designer has done is move the phase inverter to the output tubes, themselves. The 317 clamps both of the output tube cathodes to a(n) (arbitrary) current. By essentially grounding the grid of the "lower" output tube, a rather unique differential amplifier is created in the output.

I'd be interested to see what kind of distortion numbers are generated with this topology.
Eric in the Jefferson State
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Postby erichayes » Tue May 27, 2008 12:31 am

Oops; forgot to mention that Mark A Simpson (MASCO) was primarily in the PA/intercom biz. Output transformer frequency response is a little better than telephone quality (200~5000 cps), so don't expect any sonic miracles.
Eric in the Jefferson State
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Postby chosborne » Tue May 27, 2008 8:55 am

erichayes wrote:Oops; forgot to mention that Mark A Simpson (MASCO) was primarily in the PA/intercom biz. Output transformer frequency response is a little better than telephone quality (200~5000 cps), so don't expect any sonic miracles.


I figured that. I will be stripping these down all the way, cleaning them up, painting, and ordering new components. I plan to get all new edcor outputs.
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Postby chosborne » Fri May 30, 2008 6:17 pm

Demolition is complete. I've since worked down to bare metal and painted the case and the power transformer.

I've decided on a design from one of my more recent tube books, "Modern High-End Valve Amplifiers" by Ir. Menno van der Veen. I've decided on the VDV-6040 which is a 30W 12AU7/EL34 push pull setup. The circuit is straightforward and is something I think I can handle.
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