[morzh]

Ed,

I think you put a comment in Ron's topic that was meant for this one.

[Ed Locker]

No, the comment I put in Ron's topic was meant for Ron's topic.

[Radioroslyn]

Sorry a little late to the party but I've had in the back of my head the idea to get rid of the 24A and swap the socket to a 6 pin. With this done a 2A6 can be used for a diode det, avc diode, and triode 1st audio amp. The 2A6 is a 2.5v 75 or 12SQ7 tube. Haven't done it yet so I don't have all of the values worked out but not too much of a big deal. The early 70 series (70, 270, 370, and 570) are beastly to use w/the blasting on strong stations and distortion from the power detector. I've got a couple of 370's but just haven't gotten round to it.
I suppose if you wanted to go modern on it you could reconfigure the 24A as an audio amp and use a coulpe of 1N34's for the detector and avc diodes.

GL

[Ron Ramirez]

Terry

Ed's mod does change the last 24 (power detector) to a 35/51, configured as a straight audio amp.

The 2A6 is a good idea as well, although I like the idea of a tetrode audio amp instead of a triode.

It was mentioned that Ed's mod was loosely based on the late 70 circuit, but retaining the separate 27 oscillator which is good, I think. The late 70 uses the same wonky 1st detector circuitry of the infamous 89/19. The separate oscillator of the early 70 will deliver better, more reliable performance.

[Ron Ramirez]

Mike and Ed

I saw the comment you were both referring to. I think it does make more sense in this thread, and I have moved it here (it's now post #59, page 4, this thread).

Ed - hope you don't mind.

[rfeenstra]

Ed, that AVC voltage came from tuning into my local transmitter so it has a very strong signal.  On strong BC stations, I have measured as much as 9 volts and am just starting to hear distortion.  I suspect the 35's will clean it up.  It is so nice to not hear the distortion from the detector!

[rfeenstra]

So, everyone hears the "power detector" distortion.  Didn't the Philco engineers hear it? Didn't they care? Or, were transmitters of the day so loaded with distortion that they may have assumed the distortion to be coming from the source?  Although, they were experimenting with "Hi-Fi", so I would assume they could send out a clean signal.

[morzh]

The Plate detector is known to distort, especially at low signal level (it is quite OK at larger levels). Not just Philco, everyone used it and it is a known problem.
The Plus of that: only one triode tube is used to both detect and amplify, no need for diode and then a triode, even if in one tube (at the time those dual diode-triode tubes might not even exist).
The minus is obvious.

The remedies are few:
- remove the large value feedback cap in the Cathode and replace it with a small (under 1000 pF) one. This noticeably improves the sound quality but sacrifices the gain, so the sensitivity drops.
- simply remove the existing tube, change the socket (if needed) and use a tube like the 75 to detect and amplify the signal. This will remove the problem altogether, but the authenticity of the radio will be violated.
- someone might think of something else.

[rfeenstra]

Thanks morzh. Ed's modification certain takes away the authenticity but it sure makes the radio more useful/enjoyable.  So in this case, nuts with authenticity.  I like the results.  I never listen to my 70 cathedral cuz of the distortion.  I'll probably listen occasionally to the 570 with the mods.

[morzh]

Same goes for 20, 90 and 50.

[Jake Blake]

"I never listen to my 70 cathedral cuz of the distortion. "
"Same goes for 20, 90 and 50. "

I appreciate this info.  So , one should avoid these models, ( for listening to), unless he's willing to make this modification ?

[morzh]

No. I did not say so. The distortion is not too bad so one wouldn't listen to them. It is more pronounced on low volume. And the better spider in 70/90 makes it more palatable.
20 is the worst but still listenable.

Plus it is music where it is more noticeable; the speech is good.

Again, no, those are not my daily players (in fact nothing is nowadays - do not have time), but if I listen to something, it is 37-116 or Zenith 9S262, or my 38-690.

Collection is there to collect and keep good old radios in working shape so every radio could be listened to, but to listen to good music one needs something better than Philco 20.

[Ron Ramirez]

...to listen to good music, one needs The Fisher.

Or McIntosh.

[rfeenstra]

I like my HH Scott! It's on for several hours a day.

[morzh]

Well.....I do have McIntosh preamp and Eico HF-22 monoblocks to listen to stuff......but when playing pool, I use my SSTRAN from youtube playlist through 38-690 and it sounds great.
In fact if I were to use a good source like an iPod with high quality recording and the 37-116 or 38-690 as an amp, it would sound really really good. Maybe not as good as an Eico or Mac with Klipsches but still very nice. Radio signal has inherent limitations but the amps in these radios are excellent.