78 Recording Speeds.

[Coffee Phil]

Phil, I'm on SBC Yahoo! and they want me to pay them to up-load photos and scans, but I think I can "work-around" this using my cousin's cable DSL service. I may take you up on your offer for help. My new 12J5 may be "iffy" or defective too. I know a couple of repair shops with the excellent B&W tube testers/analyzers, but I hate to take their time on free testing.

You can upload images to the gallery on this site and after doing that include them in a post. The latter part is tricky and I have to struggle each time I try it.

If the 12J5 is on the way out there is what is probably a lifetime supply of new old stock of them on eBay for a good price. I'll link it.

Phil[url][/url] http://www.ebay.com/itm/3-NOS-1953-GE-1 ... 33698a3948

[Whitneyville]

I got all NOS Zenith tubes from Joe's Radio for more than a good price, but the new 12J5 is still very microphonic after replacing the old paper caps on the tube base. Zenith must be using it as the 150 Khz oscillator for the phonograph system, as that's the only thing that makes sense. It's "on" all the time, of course with 50C5, 35W4,12BE6,6AQ6, and 6BJ6 on the radio side. There's a couple of trimmer variable caps on the tube base but they don't seem to be the source, nor do the "6 dot" silver-micas. The sound is good from the phonograph except the tonearm "rings" when I move it or when I set it down onto a record manually or automatically, and it's independent of volume. One shop here in town thinks he might have a repair manual he'll let me copy, so I'm working on that now. I finally got enough Lubriplate worked into the main bearing (bronze bushing) to smooth it out. The platter would "chatter" above 66 2/3rds, but now it doesn't. A half of a 7 gram stick-on tire weight helped out too. The Zenith phonograph mechanism is more similar to a Dual than a Garrard, but uses fewer parts, so wear, lube and adjustment is more critical. Simpler isn't always better. Stupid sounding, but the "ringing" sound is more similar to an old regenerative radio set with the "regen" control just almost too high than "feedback". I'm not gonna mess with them until I have a service manual to look at, but after typing this, and thinking about it, I wonder if that's what the trimmer caps are for? One tunes the grid of the 12J5, the other tunes the plate. Works for a driver or final on a tube ham rig.

[Coffee Phil]

I got all NOS Zenith tubes from Joe's Radio for more than a good price, but the new 12J5 is still very microphonic after replacing the old paper caps on the tube base. Zenith must be using it as the 150 Khz oscillator for the phonograph system, as that's the only thing that makes sense. It's "on" all the time, of course with 50C5, 35W4,12BE6,6AQ6, and 6BJ6 on the radio side. There's a couple of trimmer variable caps on the tube base but they don't seem to be the source, nor do the "6 dot" silver-micas. The sound is good from the phonograph except the tonearm "rings" when I move it or when I set it down onto a record manually or automatically, and it's independent of volume. One shop here in town thinks he might have a repair manual he'll let me copy, so I'm working on that now. I finally got enough Lubriplate worked into the main bearing (bronze bushing) to smooth it out. The platter would "chatter" above 66 2/3rds, but now it doesn't. A half of a 7 gram stick-on tire weight helped out too. The Zenith phonograph mechanism is more similar to a Dual than a Garrard, but uses fewer parts, so wear, lube and adjustment is more critical. Simpler isn't always better. Stupid sounding, but the "ringing" sound is more similar to an old regenerative radio set with the "regen" control just almost too high than "feedback". I'm not gonna mess with them until I have a service manual to look at, but after typing this, and thinking about it, I wonder if that's what the trimmer caps are for? One tunes the grid of the 12J5, the other tunes the plate. Works for a driver or final on a tube ham rig.

It looks like your radio is a varation on the All American 5 AC/DC superhet. The 6BJ6 is used in place of the more usual 12BA6 and the 6AQ6 is used in place of the more usual 12AV6. These 6 volt tubes draw 150 ma heater current rather than the more usual 300 ma for 6 volt small signall tubes. They no doubt used them to come up with the 12 volts for the 12J5 heater.

I have two quesses for the circuit of the 12J5 RF modulator. I think you mentioned that the output of the modulator is fed to the receiver IF. I'm quessing that the 12J5 oscillator is frequency modulated by the pickup and is tuned to the slope of the receiver passband so the FM can be demodulated by slope detection. Here is my guess of two circuits which they may have used. One is a Colpitts oscillator and the other is a tuned grid tuned plate oscillator.

Phil

[/img] [album]18611[/album]

[Whitneyville]

I guessing the good old reliable Colpitts. Cheap and simple and very stable at low frequencies. The tiny coil in the phono cart is probably working as an A-Modulatorer on the grid of the 12J5, whether or not they run RF thru the coil or not (probably not) so it's a moving coil cartridge as there's no iron inside it, at all. Zenith may just be using the 12J5 to bring the voltage up to a level where they can inject it to the IF stage and modulate it there, because there were some healthy paper caps on the grid and plate that I replaced with "Orange Drops", value for value. Unless they used a heck of a tank circuit, which they might have, but it looks like an RF choke rather than a coil under the 12J5, now I'm wondering if the 12J is an oscillator. Part of my brain expects to see a 1-2" diameter RF coil 3-4" long under there, but then a ferrite core coil could be tiny at a few mW's too. "I'M SOOO CONFUSED!"

[Coffee Phil]

I guessing the good old reliable Colpitts. Cheap and simple and very stable at low frequencies. The tiny coil in the phono cart is probably working as an A-Modulatorer on the grid of the 12J5, whether or not they run RF thru the coil or not (probably not) so it's a moving coil cartridge as there's no iron inside it, at all. Zenith may just be using the 12J5 to bring the voltage up to a level where they can inject it to the IF stage and modulate it there, because there were some healthy paper caps on the grid and plate that I replaced with "Orange Drops", value for value. Unless they used a heck of a tank circuit, which they might have, but it looks like an RF choke rather than a coil under the 12J5, now I'm wondering if the 12J is an oscillator. Part of my brain expects to see a 1-2" diameter RF coil 3-4" long under there, but then a ferrite core coil could be tiny at a few mW's too. "I'M SOOO CONFUSED!"

I am going to understand this Zenith Cobra RF phono pickup or die trying.

I found a schematic of a Zenith Radio/ Phono on the Nostalga Air website. It is free and you can google it. The Radio I found wa a pretty high end peice. It has a transformer power supply and in addition to AM has both the present FM band and the old FM band.
The tube in the RF modulator is a 6J5. My guess is that since they had already designed this circuit they wanted to use it in your radio so they used the 12J5 to do the series heaters.

I redrew the circuit into what I think is a more understandable form.
I was assuming the pickup was capacitive since I seem to remember some mention of a metal vane attached to the stylus. Since you mention that the pickup is a coil and I have seen other references which seem to suggest that I am now assuming it is a coil. The Ryder schemo does not show that.

The Oscillator looks more Hartley-esq than Colpitts with the pickup coil and the other coil forming a tapped inductor.

The RF signal from this modulator is fed to the control grid of a receiver IF tube and the recovered audio is recovered from the screen grid of the same tube. The magic by which this happens is not explaned by Ryder.

Phil

[img][album]18615[/album] [/img]

[Coffee Phil]

Here is a link to that site:

Phil

http://www.nostalgiaair.org/

[Whitneyville]

Phil, I'll try and post my "before" photos with the big paper caps on the tube socket base. It looks more like a gain amp than a modulator or oscillator the longer I look at it. I couldn't get the URL to work, but I'll try and type it in, rather than click on it. This should be a 1951 built Model J665 AM radio/phonograph, as it was my sister's 2nd birthday present (she was born in Nov, 1949. I can find 1000 times more information on 1930's 4 tube super-regenerative radios from the group here in town that restores pre 1955 radios (including the BIG multi-band Zeniths and Philcos and RCAs) than on this Zenith. I've Googled and searched "Old-Radios" and "Joe's Radio" and other sites 'til my fingers ached. The group here in town had the Russians make a special run of #22, 23, 73, 83, and 86 tubes (non-keyed octal based tubes the size of an 8 oz. Coke bottle upside-down) for their Philco's and Tru-Tones and Regen's, but they looked at it and they've scratched their collective heads. Mr. Boyd thinks he has a service manual I can copy next week, so I'll try that. I know Telefunken used a similar system in the 1930's, and in the US in the 1930's when "sound trucks" playing "hit parade" music were banned (like Tulsa), The radio stations had low-powered low frequency broadcasting trucks drive thru the neighborhoods and everyone tuned their radios to 150Khz (my Mom and Dad wound a coil for their crystal radio just for KVOO , KTUL, KOME, and KRMG) just for this.

[Coffee Phil]

Phil, I'll try and post my "before" photos with the big paper caps on the tube socket base. It looks more like a gain amp than a modulator or oscillator the longer I look at it. I couldn't get the URL to work, but I'll try and type it in, rather than click on it. This should be a 1951 built Model J665 AM radio/phonograph, as it was my sister's 2nd birthday present (she was born in Nov, 1949. I can find 1000 times more information on 1930's 4 tube super-regenerative radios from the group here in town that restores pre 1955 radios (including the BIG multi-band Zeniths and Philcos and RCAs) than on this Zenith. I've Googled and searched "Old-Radios" and "Joe's Radio" and other sites 'til my fingers ached. The group here in town had the Russians make a special run of #22, 23, 73, 83, and 86 tubes (non-keyed octal based tubes the size of an 8 oz. Coke bottle upside-down) for their Philco's and Tru-Tones and Regen's, but they looked at it and they've scratched their collective heads. Mr. Boyd thinks he has a service manual I can copy next week, so I'll try that. I know Telefunken used a similar system in the 1930's, and in the US in the 1930's when "sound trucks" playing "hit parade" music were banned (like Tulsa), The radio stations had low-powered low frequency broadcasting trucks drive thru the neighborhoods and everyone tuned their radios to 150Khz (my Mom and Dad wound a coil for their crystal radio just for KVOO , KTUL, KOME, and KRMG) just for this.

I'll try again. I have a whole lot of trouble with hyperlinks on this site. It seems to want to add extra characters. OK now it works. It puts the [/url} in. I just added a space and now it works.

Phil

http://www.nostalgiaair.org/

[Whitneyville]

Phil! Success! Look under the Model H665 and you'll find the schematic and I found why I'm getting the oscillation from the phono section (C16). The 12BE6 is the "converter tube" and it shows the phono oscillator coil next to it, and it actually makes sense. The 50C5 puts out a thundering 1 watt into the speaker, so Zenith was "Hi-Fi" conservative with their power output, where the 50C5 is typically rated at 3.5 watts RMS. I know without a HV tranny and regulation that limits what you can get from the 50C5, but most other brands went for more power (and lots more distortion) especially pushing an 8" speaker. I may actually put a HV transformer and regulator circuit into "mine" to "clean-up" the sound. "Mousering" around, looks like I'm at about $25 to do that.

[Coffee Phil]

Phil! Success! Look under the Model H665 and you'll find the schematic and I found why I'm getting the oscillation from the phono section (C16). The 12BE6 is the "converter tube" and it shows the phono oscillator coil next to it, and it actually makes sense. The 50C5 puts out a thundering 1 watt into the speaker, so Zenith was "Hi-Fi" conservative with their power output, where the 50C5 is typically rated at 3.5 watts RMS. I know without a HV tranny and regulation that limits what you can get from the 50C5, but most other brands went for more power (and lots more distortion) especially pushing an 8" speaker. I may actually put a HV transformer and regulator circuit into "mine" to "clean-up" the sound. "Mousering" around, looks like I'm at about $25 to do that.

I looked at the schemo and I think I am begining to understand what they are doing. The phono oscillator is pretty much what I drew from "translating" the schemo from the big Zenith. The !2BE6 pentigrid converter has power removed during phono operation so it is out of the picture. The FM output from the 12J5 is fed to the IF tube via a first order low pass filter to make AM from FM (known as slope detection). The IF tube is triode connected (in phono mode) and serves as a grid leak AM detector. I'll draw up a schemo of the equivalent phono signal path within a few days. I'm sleep deprived now and for the next two days I have a lot on my plate. I do believe the mystery is solved and I did not die trying.

Phil

[Whitneyville]

That would account for the phono pick-up design being called a variable reluctance in some literature. The moving coil phono cartridge makes a frequency modulation of the local ocsillator, which can be slope detected because of the direct coupling in the IF stage. Ten minutes of "tweeking" of trimmer C16 killed 99% of the microphonics this morning (4AM). With a bigger, better output transformer, and a bit more B+ on the 50C5, I think I can approach a clean 2 watts RMS output. I have my eye on a modern 8" "ceiling" background music/PA speaker with a whizzer cone and 40-16.5Khz responce +/- 3dB, good for about 15 watts, a whopping $8. It's shallow so it should fit in the space, too. The idler wheel is soaking in Re-Grip to soften it.

[Coffee Phil]

That would account for the phono pick-up design being called a variable reluctance in some literature. The moving coil phono cartridge makes a frequency modulation of the local ocsillator, which can be slope detected because of the direct coupling in the IF stage. Ten minutes of "tweeking" of trimmer C16 killed 99% of the microphonics this morning (4AM). With a bigger, better output transformer, and a bit more B+ on the 50C5, I think I can approach a clean 2 watts RMS output. I have my eye on a modern 8" "ceiling" background music/PA speaker with a whizzer cone and 40-16.5Khz responce +/- 3dB, good for about 15 watts, a whopping $8. It's shallow so it should fit in the space, too. The idler wheel is soaking in Re-Grip to soften it.

Hi Ricky,

I have printed the schemo of your radio / phono and I have been glancing at it from time to time. I am sticking to my last story of how this thing works. One thing which is interesting is that the older Zeniths which used this phono pickup did not have the trimmer cap in the oscillator. I am guessing they put it there to fix some aberation. In my model of the phono circuit the IF transformers ideally play no role and it would be better if they were shorted out. Running wires from the transformers to the function switch to do that would degrade radio performance so I think they added the trimmer to adjust the oscillator frequency to not interact with the tuned IF transformers. I'm guessing a reed relay accross the secondary or the transformer from converter to the IF stage and across the primary of the IF transformer from IF stage to detector would be RF "sanatary" and just might get rid of the tweekeyness of the trimmer.

All of the pictures I see of the Cobra look to be some sort of propritary mount. Is the receptical in the arm a fairly standard 1/2" mount? I am seeing these cartridges appear on eBay. If they can be mounted in a standard headshell I may just buy one to play with. I read comments ranging from they were about the most gentle cartridges to records in their day to they chew up records. I see they were made with three mill styli for 78s, and one mill styli for Lps, and two mill styli to use on either. I'm guessing that the one and three mill styli mated to the correct records were good and the two mill styli offered equal opportunity record chewing.

I have an Idea for a simple (fairly) circuit done in CMOS to use these cartridges.

Phil

[Whitneyville]

My arm uses the propiretary Zenith tiny cartridge (I'm working on photos, really) that has bizarre VTF and anti-skate adjustments. Later Zenith used more standard cartridges, but this was over a decade later.

[Coffee Phil]

My arm uses the propiretary Zenith tiny cartridge (I'm working on photos, really) that has bizarre VTF and anti-skate adjustments. Later Zenith used more standard cartridges, but this was over a decade later.

I am wanting to try my CMOS circuit for the Cobra pickup but if I can't mount one in an SME type headshell this will have to wait. I see some Cobramatics going on eBay for ~$50 but if I buy one while I have my Kenwood KD500, two B&os, and now the Rek-O-Kut coming my wife may have me commited to the looney bin.

Phil