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 Post subject: Megadyne and other questions
PostPosted: Nov Mon 06, 2006 9:24 am 
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Joined: Nov Mon 06, 2006 8:36 am
Posts: 79
hi everyone!

this is my first post. has anyone created a "Megadyne" receiver? Gernsback's design avoids a seperate AF amp stage. it looks like a Hiker's but the normal grid-leak has been replaced with...

Image

what glass tube (1.5 Volt or less filament) has the lowest filament current? metal tube? my 6418 only needs 10 mA (1.25V) but i prefer a real base; and i cannot find a source for a "sub-miniature" socket.

what kind of results are obtainable by using a Bogen transformer and low-impedance earbuds instead of the normal 2k ohm phones?

thanks,
phil


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 Post subject:
PostPosted: Nov Mon 06, 2006 3:56 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Livermore, CA
Hi Phil

There is a series of 7 pin miniature tubes which draw 25 ma filament current. Check 1AF4, 1AF5, 1AH5 and others are those numbers.

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 Post subject: Re: Megadyne and other questions
PostPosted: Nov Mon 06, 2006 7:27 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 11100
Location: Vieques, PR, USA
philwashere wrote:
what kind of results are obtainable by using a Bogen transformer and low-impedance earbuds instead of the normal 2k ohm phones?


I find the earbuds to be rather deaf. But, if you have enough audio they'll do fine. You need darn near speaker volume for them.

-Bill


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 Post subject: Re: Megadyne and other questions
PostPosted: Nov Tue 07, 2006 12:22 am 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 459
Location: Collison, IL USA
philwashere wrote:
hi everyone!

this is my first post. has anyone created a "Megadyne" receiver? Gernsback's design avoids a seperate AF amp stage. it looks like a Hiker's but the normal grid-leak has been replaced with...

Image

what glass tube (1.5 Volt or less filament) has the lowest filament current? metal tube? my 6418 only needs 10 mA (1.25V) but i prefer a real base; and i cannot find a source for a "sub-miniature" socket.

what kind of results are obtainable by using a Bogen transformer and low-impedance earbuds instead of the normal 2k ohm phones?

thanks,
phil


Hello Phil, and hello all, it's been a long while since I was here. Megadyne: I'd suggest a power audio tube to match the characteristics of most of the tubes Gernsback recommends for the circuit. The 38, which he says works best, is a low current output tube, just under a watt at lower voltages. Note that he rated his choices on the ability of Grid #1 to take higher voltage in the space charge circuit. As a rule I'd say power tubes would often have a sturdier grid than high gain RF types. For 1.5 volts, a 1A5, 1C5, 1T5, 1Q5 octal would probably be good. Those are all 50 to 100 mA filaments. Miniature 7 pin would be 3S4, 3V4 with 100 mA 1.5v paralleled filaments. These and any subminiatures may be marginal in this circuit. Be careful in applying voltage to grid #1, on these smaller tubes it may be hard to see the grid glow red when you push too hard.
For AC 6G6 octal, 6AK6 miniature are very close to a 38's characteristics (at lower voltages). The heater is 150 mA at 6 volts AC or DC. The seperate cathode is simpler if you want to play with biasing by inserting a cathode resistor.
The best Space Charge tube may be one that didn't exist when the Megadyne was designed. A 12K5 is designed to run as a space charge tetrode, for car radios with 12V on the plate, and on grid #1. The heater draws 400 mA and the plate and grid #1 draw a whopping 115-125 mA. The tradeoff for low voltage is higher current. It puts out 15-25 Mw into a 500-1K load, plenty to drive even inefficient ear buds through a transformer. 12V DC at 1 amp is generally available from a surplus wall wart in the junk box. It's also safer if you have any kids or animals who might get into the thing by accident. The 12K5 works well, I had a 12-in-One Knight kit when I was a kid that used a 12K5 for all the circuits. No spring clips, one had to solder the jumpers on. Lots of burned fingers before I learned which end of the soldering iron was hot!
To be brief, a tube universal output transformer (Hammond makes one currently) driving GOOD earbuds (Such as the form fitting foam Koss units that Radio Shack sells, or the better Sony units) beat a pair of Trimm Dependables hands down. And the fidelity is much better, the locals sound amazing when they play music.
It takes a while to find which taps work best, (my transformer had a center tapped primary and 8 secondary taps) but it appears to me the new exotic magnets more than make up for the loss in the transformer. Most earbuds that come along with average CD players are very deaf. Sony's players & radios have less audio power output, due to being much easier on one AA battery than cheaper units are on 2AAs. Thus they make more sensitive earbuds. In fact, the better Sonys and the Koss are sensitive enough that my DX-398 portable is still quite audible at minimum volume, and any hum at all shows up loud and clear!
There are some exotic earbuds and over the ear style phones that have even better sensitivity ratings than the units I tried, but they start at $100 and go way up from there! There are several good choices at $15-$30.
I tried many output transformers and earbuds to reach this result. There is a lot of room for experimentation using better tools than my ears and a crystal set, so I pass this on for what it's worth.


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 Post subject:
PostPosted: Nov Tue 07, 2006 5:03 pm 
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Joined: Nov Mon 06, 2006 8:36 am
Posts: 79
thanks to all. Neutrodyne i appreciate the long post and have more questions. any headphones stand out AFA comfort, sound, cost, and sensitivity?

Image

top: ebay one-off, 2000 ohm vintage, 600 ohm Collins, 1000 ohm Philmore, 2M ohm ceramic

bottom: 500 ohm magnetic, 600 ohm Telex, 4400 ohm Chinese, 1600 ohm Russian, 8-32 ohm headset/earbud

the headband types look uncomfortable. i prefer new/NOS to vintage. 2M's sound tinny. are those Philmore or 500 ohm magnetics any good? your thoughts welcome.

thanks,
phil


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 Post subject:
PostPosted: Nov Wed 08, 2006 5:23 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 459
Location: Collison, IL USA
I have had zero luck with the 500/1000 ohm dynamic earplugs (to use early transistor radio terminology). They are not at all efficient, though if you were loading a germanium transistor collector with one, I'm sure it would work OK.
I did my comparisonsa few years back with newer earbuds/plugs/headsets against a set of Trimm Dependables, a set of Trimm Featherweights I bought new/NOS in 1976 and an ancient pair of Baldwins that came in a Phanstihl 1924 battery set. At the time I had a 150 foot length of #17 aluminum electric fence wire about 15-20 feet in the air, end fed. The crystal set was a slightly modified MRL #2 circuit, the coil wound at 2 1/2 inches diameter with #14 enamel closewound, selected 1n34A diode.
I suspect the baldwins were about demagnetized, as they were quite deaf compared to anything else. The Featherweights were best of the old style phones, with the full size rubber cups on them, they were good for comfort rating and sealing.
I see Radio Shack and Koss no longer have the plug style buds I used, but I do find "The Sparkplug" to be the logical successor. $14.99 on the Koss site. 112db sound pressure level @1 mw sensitivity. The key to superiority is clearly the form-fit foam cusion which seals them in the ears, this is worth a whole lot of sensitivity. Comfort is excellent the way I usually used them, rolling from one side to the other in bed and having just one in the ear on top. They get a little warm (insulating your body heat, not heating the plugs!) after long periods with both plugged in, and can cause weird sensations if you yawn.
Next would be Sony's, I found the sports set (yellow) that had a band that goes around the back of the head to be one of best for long term comfort and close in sensitivity to the Koss. They make a half dozen styles of these now, just watch the rating, anything 109db/mW or higher is going to be good. The further they go into the ear and the better the seal, the better off this application is. They make some $100+ full size over-the-ear units with great ratings but I never tried them. Koss and others also make commercial communications units that may be better but also go pricey. I found no miracle units among the cheap, mediocre and factory included buds, a great example of "you get what you pay for" with $15 being the minimum with good performance.
I probably should mention that I have tinnitus and a hearing response curve tested at -20Db @400 cycles dropping quickly to -70~75 db anywhere over 10KC. I think this makes my observations worthwhile in that few will get worse results than me on a "barefoot" crystal set.
Now I understand the best sound powered phones with the right transformer are said to be the best, I don't doubt it but you won't find them in stock and ready to go like a Hammond transformer and a pair of current Koss plugs. I also understand Brush crystal phones are wonderful but it seems to be harder every day to find a pair that hasn't had the crystal element go bad or be ready to go once you buy it and let a UPS truck vibrate it a bit. I don't know if the crystal cartridge rebuilders fix those or not. For fidelity on a strong AM station, these new plugs are amazing, I understand now what people are trying to explain when they build a high fidelity broadband AM tuner. However if you use a tube circuit with an AC supply, you'll need to filter it well, as the old cans have little response at 60/120 cycles and thus covered many sins in power supply design.
For crystal set use, I suppose I could fudge and run an active equalizer to overcome my hearing loss, or break down and buy hearing aids but most wouldn't consider that entirely in the spirit of crystal set building. Similar with noise canceling phones and buds. But I'm sure they are better than the low cost things I tried.
I didn't keep good notes, nor did I use scope to read voltages. The long wire is long gone now and from my wheelchair I haven't many prosepcts of getting i reinstalled anytime soon, so that may be the end of my evaluations on crystal sets.


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 Post subject:
PostPosted: Nov Fri 10, 2006 12:01 am 
hi Neutrodyne,

thank you. those Koss Spark Plugs look perfect. for 18 volts B+ which transformer can i use for interstage and for low impedance headphone hookup: Bogen T725 (have two), XICON TM-117, Hammond HX124A? sorry to hear you cannot put up an antenna. might some white #30 wire wrap stapled to a fence or taped across the wall (stealth) work? winter is here but soon the warm glow of these Raytheon 6418 tubes will be keeping me warm :lol:.

thanks,
phil


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 Post subject:
PostPosted: Nov Sat 18, 2006 10:58 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 459
Location: Collison, IL USA
Anonymous wrote:
hi Neutrodyne,

thank you. those Koss Spark Plugs look perfect. for 18 volts B+ which transformer can i use for interstage and for low impedance headphone hookup: Bogen T725 (have two), XICON TM-117, Hammond HX124A? sorry to hear you cannot put up an antenna. might some white #30 wire wrap stapled to a fence or taped across the wall (stealth) work? winter is here but soon the warm glow of these Raytheon 6418 tubes will be keeping me warm :lol:.

thanks,
phil



I am just back from a week in the hospital, which unfortunately doesnt have public wi-fi. For the rates they charge there should be a laptop with ever bed! Nothing more serious than what I'm used to, I'm simply a physical wreck and that's just the way it is.
The HX124A is a good choice for interstage. Triode Electronics has a few newly manufactured guitar amp transformers which can supply some different impedance ratios that might get the very last ounce out of each stage, but it would take a lot of experimenting. As long as you are sure the first tube doesn't draw more than 10 Ma or so through the primary winding, it's fine.
The Bogen T725 looks like something I should pick up while they last (the Bogen site doesn't have the spec sheet available anymore, a possible sign of future scarcity). Offhand, a speaker line transformer does offer a large number of possible combinations, but won't have as many primary turns, and hence can't go quite as high in impedance as a universal transformer. What you might want to do with a 38 or similar tube would be to measure the plate current and be sure it stays below 20Ma or so, while seeking the highest volume. Use the lowest power output tap to start.
What I used is an old Triad which is quite similar to the Hammond PT125C. The PT125A is the same thing but rated lower for power handling. ARS has them as does RadioDaze and others. These are the push pull transformers which gives a few more non-standards ratios via the center tap. It is spec'd to match something like 2000-25000 ohms into 1 to 16 ohms. This gives quite a range, and the primary will have substantially more turns than the Bogen. These will saturate the core (lose frequency response, distort) if you use them on a single tube with a heavy current draw, but Hammond does also make universal single ended transformers if you are producing speaker level power output.
If you use battery power or a power supply with a fairly low current output, it's hard to actually hurt a tube or transformer in a simple tube circuit if you keep an eye on the current draw or the B+ voltage for signs of distress. Tubes are very tolerant of bad matching as long as you aren't also trying to run them at high power levels.
One thing I got to do last week was read and I was poking around the Radiotron Design Manual (4th edition, I call it the Big Red Tube Bible) in the transformer chapters. If you want to know more than you ever thought was known about audio transformers for tubes, it is here. It almost gives enough information to make them from scratch, laminations and all! Anyway that book explains in great detail why and how some of these things work the way they do. Everyone who loves tubes and radio should have this on CD/DVD or grab an original to leave out on the coffee table and impress your friends. It's one of the thicker single volumes I've run across, almost 6".
I do have 30' of tv twinlead indoors that works fair on Shortwave when I can turn some things in the house off. One of these days when I can get back on my tractor, I'll get a fishing rod rigged with a weight so I can cast a leader ove a couple of trees and I'll be in business again.

John Smith


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 Post subject:
PostPosted: Nov Mon 20, 2006 1:59 am 
Member

Joined: Nov Mon 06, 2006 8:36 am
Posts: 79
Welcome back John!

Sorry to hear about your hospital stay. The tubes i am using only draw 10 mA of filament current :lol:. I would like a tiny (TM-117 sized) equivalent of the Bogen T725. I might end up doing something like Bill (exray) and make a 2-tube amplifier module. I like the idea of those Koss Sparkplugs. Also curious what the dB SPL/1mW rating of a crystal earphone might be.

Thanks,
Phil


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 Post subject:
PostPosted: Nov Mon 20, 2006 11:58 pm 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 459
Location: Collison, IL USA
It's good to be out of there, there's no doubt about that. Now if I'm a good boy maybe I can stay out for a while.
I just got the new Mouser catalog. I don't know why they keep sending it, I try to give them an order when I can but a couple of small ones a year hardly pays for that big book they send. It is quite useful, much easier than their website. Once you have the part number from the catalog you can then access the web listing and get whatever spec sheets are available. SO:

Mouser says the Ceramic earphone they sell is 20Meg ohm minimum impedance, 15000pf , and 57 db @1khz. Does that mean spl in db/1mw? Hard to say, far eastern companies often don't use standard ratings.
The dynamic phones on the same page are from 8 ohm to 1K ohm & are rated at 90db/mw. Same manufacturer. I read somewhere- can't remember where- that there's not much difference between crystal and ceramic earphones. I think all are ceramic now due to crystal's deterioration under humid conditions.
10mA heater would be a subminiature, probably an RF/AF voltage amp type? That's a tiny one all right. I didn't see one with filament current that low in the Sylvania tube manual I have at hand, but it's far from being a complete reference. I think I can safely guess there is not much current swing out of one of those. The ceramic/crystal earplug, despite the spec that appears to say it's pretty deaf, would go across a plate resistor of 100-200K most likely and there's no transformer loss that way.
I looked at the 1V5 & the 1AC5, with identical electrical specs, both are subminiature audio power tubes:
Fil.- 1.4v .040A With 30v on the plate it makes 5mW into 50K ohms
At 45v it makes 15mW into 40K ohms. At 67.5 it makes 50mW into 25K ohms. The Bogen transformer could probably come close to matching that tube with a 32 ohm set of earbuds on the right tap, probably better if you wire the two buds in series for 64 ohms. A very light impedance reflection into the primary that way. The plate current at 67.5V is just 2mA - not exactly earth shaking power.
I've seen a few voltage amp tubes used for audio power and they seem to make about 20-30% of the power you'd expect from an audio power tube with the same filament/heater current. If that holds true with subminiatures, you would be looking at 1mW or less out of one tube, if it matched well into the transformer.
You might use a transformer primary as a shunt feed choke (even a transistor type transformer would do with the low current and voltage), with the crystal phone across it. That should give more audio than a resistor. Of course should work and does work are two different things!
I haven't done much with subminiatures (other than replace them in Motorola "lunch box" portable VHF trancievers). I was a bit disappointed when I was a kid, by the feeble little filament in the '30. Had to turn the lights off to see it at a full 2.0 volts. I was hoping it would light up like the 26's in my Atwater Kent 47. I have a box of new 6J6's. Those glow nice and bright with a 450mA heater. I have about everything lined up and in the house here to see what one or two of those will do in a regen. I'll have a good hand warmer if nothing else.
Now I've gone and rambled all over the place, as usual. I hope there's something here that is helpful. Oh and I am curious as to what tube type you have there.

John Smith


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 Post subject:
PostPosted: Nov Tue 21, 2006 12:47 am 
Silent Key

Joined: Jan Thu 01, 1970 1:00 am
Posts: 34326
Location: Sandpoint, IDAHO 83864
John- good posting! You surely don't ramble on as much as I do and I enjoy reading posts with some information in them.

Yeah, 10mA for filament current is quite low. I think there may have been some subminature tubes made with that low of current rating, but I have no idea of what they might have been. In my Sylvania tube manual under subminiature receiving types, the 1.25 volt subminiatures seem to be drawing .04 amp., which is four times the 10mA. I wonder what these 10mA tubes are?

I know when I was a kid, I had a couple Raytheon CK512AX and CK522AX tubes I salvaged out of old tube type hearing aids. The 512 had a .625 volt filament and two were wired in series across a small 1.5 volt A battery about the size of an "N" cell. The 522 was the output stage and had a 1.25 volt filament. B+ was from a 30 volt battery slightly less tall than a standard 9 volt battery we all know of today.

My grandma had one of these hearing aids and she would always get the volume turned up too high and it would feedback and let out a howl that could be heard across the big house she lived in. So they can put out some audio power into an earphone.
Curt

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Curt, N7AH
(Connoisseur of the cold 807) CW forever!


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 Post subject:
PostPosted: Nov Tue 21, 2006 3:26 am 
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Joined: Nov Mon 06, 2006 8:36 am
Posts: 79
Hello friends,

Thanks John for your informative post! My "great" idea was usage of a mini pentode power amp, specifically Raytheon JAN6418. Alas Dave Schmarder (great guy) beat me to the punch. :shock:

Image

John i like your ideas, summarizing below.

1) 2M ohm crystal earphones (57 dB) across 100k load resistor (tube sees 100k).
2) two 1k ohm dynamics (90 dB) in series (2k) with 50k:1K transformer (tube sees 100k).
3) 8 ohm SparkPlugs (112 dB) with bogen 40k:8 (tube sees 40k).

I want to touch on the subject of ear protection. There is something to be said for weak audio. Does anyone use diode protection? Any thoughts?

Thanks,
Phil


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 Post subject:
PostPosted: Nov Tue 21, 2006 3:34 am 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 740
Location: U.S.
The 5886 and several other electrometer grade pentodes all had 10 mA filaments. I'm somewhat surprised to see a "power amplifier" :lol: pentode with such a low current draw.

Those 1K dynamic earphones are worthless. A crystal earplug, especially if selected fom a batch, can be incredibly sensitive, usually surpassing all but the best 1920s magnetic phones and of course the sound-powered elements.

73's


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 Post subject:
PostPosted: Nov Tue 21, 2006 6:32 am 
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Joined: Nov Mon 06, 2006 8:36 am
Posts: 79
Thanks DXer! And then there were four... SP phones, 600 ohm Collins, crystal earphones, and 1600 ohm Russian. Has anyone tried those Collins headsets or Russian "CW practice" phones?

Also does anyone have specs for the headpiece or microphone piece of those 1960's Bell telephones?

Image

Thanks,
Phil


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 Post subject: Re: Megadyne and other questions
PostPosted: Nov Wed 15, 2017 6:18 am 
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Joined: Oct Wed 15, 2008 3:39 am
Posts: 28
Location: Connecticut
philwashere wrote:
hi everyone!

this is my first post. has anyone created a "Megadyne" receiver? Gernsback's design avoids a seperate AF amp stage. it looks like a Hiker's but the normal grid-leak has been replaced with...

Image

what glass tube (1.5 Volt or less filament) has the lowest filament current? metal tube? my 6418 only needs 10 mA (1.25V) but i prefer a real base; and i cannot find a source for a "sub-miniature" socket.

what kind of results are obtainable by using a Bogen transformer and low-impedance earbuds instead of the normal 2k ohm phones?

thanks,
phil


Updated the Megadyne schematic link. The original location no longer exists.
Also please check out 'Regenerative Receiver with Crystal Detector' at:
http://www.theradioboard.com/rb/viewtopic.php?f=3&t=1140&p=10040#p10040

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Norman Bourassa
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