Model 70 progress - Printable Version

Model 70 progress - Printable Version

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+--- Thread: Model 70 progress (/showthread.php?tid=12812)

RE: Model 70 progress - Radioroslyn - 08-01-2015

Looks Good! It's too bad that the 70's don't have a better spkr and detector circuit. They don't sound great, good but not great.

Terry

RE: Model 70 progress - KCMike - 08-01-2015

Thanks Terry. Yea have to agree about the sound. I have a 90 and 20 and the audio is about the same. Of course none of them I own have the push pull output.
Its been a fun restore so far but don't know when I will get to the cabinet its so hot and very humid here. Probably the fall.

RE: Model 70 progress - Radioroslyn - 08-02-2015

Well it's not really the fact that it doesn't have push pull audio output ( the 20 does). If you ever hooked up a pm spkr across the output transformer of the 20 but you'll hear a big difference. Sound really good. Those brown "pie pan" spkrs have a very stiff spider and cone this limits the low frequency response.
The 70 and 90 use a thing called a Power Detector. It has a fairly high output enough the drive the 47 but also generates a high lever of distortion. I suppose you could replace the the 24A with a 2A6 and rewire it as a diode detector/triode 1st audio and add avc then you'd have something. You could also lose that dual gang volume control that is difficult to find a replacement.
Single ended audio output can sound very good it's all in the design.

Terry

RE: Model 70 progress - codefox1 - 08-02-2015

Agree with all, a single ended output and a reasonably high efficiency PM speaker can really sould very good, especially if we're talking about A.M. radios. That being said, original equipment, if still up to speed can also be quite pleasant to the ear. As always, "it depends."

RE: Model 70 progress - Ron Ramirez - 08-02-2015

Only the early 90 with push-pull 45 output tubes used the power detector. The 90 with single 47 output and the 90 with push-pull outputs used a 27 wired as a diode as the second detector, producing cleaner sound.

Likewise, the early model 70s used the power detector - late versions did not.