[462ron]

Progress moving along! I have recapped this beast except for 4 electrolytics that are entombed in tar inside a huge armored box that I have circled in the attached photos. Also found what appears to be a mistake at the factory. On the schematic, the plate circuits for the RF amp and 1st detector are tied together as can be seen in red are on the schematic. Upon doing continuity checking and tracing wires, I can’t find continuity between the plate circuits. I fired it up to take voltage readings and both plates should be at 180v per schematic. The RF amp reads correct but the 1st detector is around 255v, clearly too high and proving both circuits aren’t being sourced from the same B+ as they should be. Drove me nuts tracking it down and I’ve come to the conclusion that it was wired wrong at the factory. It all looks original, not altered so I rewired the 1st detector so that it ties in with the same 180v source as the RF amp and it works. Need to gut that big box for the electrolytics yet and do an alignment. I will see how that goes!

Ron    

[RodB]

Ron, the voltage chart confirms your actions, 180 on both plates. It's a real mystery as to how it got that way.

[462ron]

I’m pretty much done with all the electronic work except I still need to replace the cloth covered line cord. In doing the alignment, the set started motor boating which would change with the slightest tap on the chassis. Tracked it down to a bad ground rivet on the IF amp socket. It affected how well the tube shield would stay grounded. With the bad ground the shield wasn’t working properly thus allowing for strange oscillations and motor boating. I will order some new cloth covered line cord and antique plug. All that’s really left now is to refinish the cabinet which is partially stripped but that needs to wait til spring to do outside. Then a new grille cloth! 

Ron    

[Arran]

Grigsby Grunow seems to have been like Rogers, they were not into sparing the steel, most of their chassis are made out of heavier gauge sheet metal, and lots of it. Also, given the design of some of their chassis it's a wonder how they were assembled on a production line basis. Judging by the values of the filter caps,1,  2, and 3 uf, respectively, the filter caps were probably paper rather then electrolytics, so motor run caps, or Solen fast caps, will work as substitutes, and likely outlive you. It's not really recommended to replace such caps with electrolytics anyhow as the modern types in that size will not like the ripple current, I think Terry tried some in a Radiola set, and one of them short circuited within a year.
Regards
Arran

[462ron]

Yes Aaran I replaced that 2mfd cap coming off the 80 with a Solen cap. It amazes me how there is no hum in this set with such low value caps as that 2 mfd and the 3 mfd that comes after the choke and the big speaker! As for all the steel they used on this chassis, it not only made it crazy heavy but also very difficult to trace things and work on. It’s the worst chassis I’ve ever worked on but it is a decent performer considering the age of the design!

Ron

[RodB]

Makes you realize how much work those chokes do. Especially when the correct value of inductance is used.

[MrFixr55]

Congrats on the chassis restoration, Ron! Another important piece of history saved!

Unfortunately, the spider (or lack thereof), reminiscent of the Philco 20 speaker probably detracts from the sound.

I remember finding one of these speakers (without the radio) at the local landfill as a kid in the 1960s. I didn't keep it due to the condition, but I did realize that it was from the very early 1930s. It was not common to find real early radios in our landfill, as the large majority of the houses in Huntington Township were built in the mid 1950s (Although I did find a 1929 Radiola 62 chassis once). My friend did find a Scott Philharmonic that I fixed for him. He moved away, and I lost contact. I don't know if he still has it.

[462ron]

MrFixr the sound is pretty decent as this is a 10” speaker but admittedly not as good as some radios from the 40s or even late 30s. I attribute that to possibly the type of 2nd detector, almost like my Philco 70. This set doesn’t appear to have a separate spider but instead, it’s all part of the speaker cone like it was all formed at the same time. The cone is in great shape luckily.
RodB you are correct about the choke doing a good job. This set has not only a separate choke but also use the field coil too!

Ron

[MrFixr55]

I hear you Ron.  It appears that the diode detector did not become popular until about 1934 or so when the '55 / '75 / 85/ 6Q7 double diode / triode detector, 1st AF Amp tubes became popular.  There is a big difference in sound quality between my Radiola 17, 18, and others with a grid leak detector, my Radiola 60, my RCA R7A and R28 with a "Biased Detector" and my more modern radios from 1934 (Crosley 5C2) and on.  Zenith 6S52 chassis, RCA 6T7 and so on and so on with the diode detector and triode AF Amp.

One of my retirement "experiments" will be to create a "plug in" replacement for the biased detector, replacing it with a diode / triode detector / AF Amp.  I will try it first on my R7A (2 '47s in push pull)

[462ron]

MrFixr that sounds like a good project and worthwhile too. To think they went to the trouble of designing this radio with a 10” speaker and P-P 45s giving it plenty of oomph only to have the 2nd detector be it’s Achilles heel. As I said, the sound is pretty good but not where it could be! I suspect once I get it back in the cabinet it should make some difference.

Ron

[MrFixr55]

Hi 462Ron,

I don't know why, but ever since before Consumer radio, they were using triodes as detectors, either grid leak, grid leak with regeneration (1914 Armstrong Super-Regenerative patent or Biased Detector.  The famous SE1430 and IP501 Shipboard receivers used a single triode as a grid leak with feedback.  It seems that Diode detectors were not used in Consumer radios until around 1933 or so.  The 1930 RCA Receiving Tube Manual (RC10) does not even discuss diode detectors.  This is interesting because the "Fleming Vlave", Ca 1904, the first vacuum tube ever invented was a diode, used as a detector in radiotelegraphy. 
The first Diode detector / 1st AF Amp combination tube, the RCA58 was available in 1931 according RadioMuseum but not used till 1932 - 1933

The US Navy Basic Electronics (Van Valkenberg Nooger and Neville, John Rider, Publisher) states that a Diode Detector has the least amount of distortion, followed by the Biased Detector (AKA Power Detector or Plate Detector), with the grid Leak and superregenerative detector as having the most distortion.

[Arran]

In my most recent acquisition, Phonola (Electrohome) from 1936, they practiced a bit of tube stuffing, they used a 6F5 for a 1st audio, and used a #76 triode connected as a diode for a detector, whereas they could have used a 6Q7 or a 6R7 for both. It's seems to have been a common practice in the mid 1930s to do this, but usually they used a 6F5 or 6J5, in combination with a 6H6 double diode. Other then wasting more current on extra heaters I don't know what the electrical advantage was of using separate detector, and audio tubes. 
I have heard of some people wiring in gemanium detector diodes in place of the plate detectors many early 1930s radios used, and rewiring the existing tube as a 1st audio stage, but making an adapter might be a better way. 
Regards
Arran

[MrFixr55]

Very few radios before maybe 1933 had a diode detector.  There is a Philco Model 3 Transitone Car radio Ca 1930 -1931 that used a '71A (of all things) with the plate grounded to the cathode, as a diode, a '01A as a 1st AF and another '71A as an output stage.  Power output of this radio was less than or equal to 0.7 Watts.  The radio ran off the 6V Car battery and 180V worth of B Batteries.  3Stage RF Amp used "24As.  The RCA Receiving Tube Manuals of the days Described the Grid Leak Detector as having a higher level of distortion than the "Biased Detector (Power Detector), as used in your Majestic.  Diode Detectors have less distortion than either, according to the RCA Receiving Tube Manual.

A biased detector was cheap to produce, and this was the beginning of the Great Depression.  Interestingly, even the Radiola 60 and other RCA radios produced during the "Roaring 20s" used grid leak or biased detectors, as they required fewer tubes than the separate diode detector and triode 1st AF Amp.

[462ron]

If this radio wasn’t such a pain to get at, I might want to experiment with using a 1N34 or some such diode and let the 27 just be an audio amp. Not sure what affect that would have on loading the IF transformer and impedance! Also not sure about controlling the volume, might be more hassle than it’s worth.

Ron

[MrFixr55]

Oops, sorry.  Computer shut down and when I reentered the forum, I accidentally sent 2 messages.  I am wordy enough on one message!

Yes, a 1N34 is a good try.  I have to read through my US Navy "Basic Electronics" (John Rider, Publisher), a must for anyone new to the hobby, as it was originally written in the mid 1940s.

I did this on a Radiola 16 (a much easier radio to do this with), but I may have used a switching diode.  I went from this diode to an external 2 stage amp.  Disappointing, but still an improvement.  Will try this eventually with a 1N31.

The RCA Receiving Tube Manual (I used a 1937 edition) states that both a diode detector and a grid leak detector can load down the previous stage more than a Biased detector can.  the 24 Tetrodes and 27 Triode have less gain than later pentode tubes, but given the strength of current AM stations, this may not be an issue.  The "Power Detector" can be changed to an audio amp merely by changing the cathode resistance.  Find a schematic for a radio that uses a biased detector but has a radio / phono switch and this may give you an idea.

Ron Ramirez did an article on "bridging" AF Interstage on Philco 20s whose primaries opened.  He recommends cap coupling on any Philco 20, not only to preserve the transformer but that the fidelity of the amp greatly improves. (However, the philco had a separate 1st AF and tetrode Biased Detector.

Direct injection of an audio signal will allow evaluation of the speaker, which due to a lack of compliance, is a weak link.

If you have another radio, especially a TRF, this experiment may be easier to do.

I will attach the detector section of the RCA Manual if U want, but I gotta go.