There is a website, "BOB`S DATA, that shows a nice 2-tube regen radio, which uses a 1T4 and a 3V4 radio tube. It is a regen radio, and the article specifies using a 10K ohm to 8 ohm audio output transformer. I cannot find a way to reach this Bob. I guess he does not want to be bothered! Anyhow. Does anyone have a parts n umber for a Hammond or Triad Magnetics transformer, which would work nicely with the 3V4 output tube. I have looked at the Digi Key and Mouser sites, and the different types of transformers seems limitless. ***? I am not an electrical engineer! Can anyone suggest a parts number for the above type transformer? I read some numbers off of the photos of the audio transformer in the article. They are 1005836 and ??47hf. Any help appreciated.......William

A hamond HX-125X-se will work .Also a bogen T725 Can be made to work
http://www.radiodaze.com/index-alt.htm
http://makearadio.com/misc-stuff/t-725.php
http://peeblesoriginals.com/catalog/72.php

There must be plenty of people (including myself) who have a surplus of old 1950's and later 3-way portable radio parts chassis lying around. If you put a want-ad in the Classifieds, you might get one for just the cost of shipping. 3V4 was the most common output tube used in these sets.

Dave

I found a SKU300-039 line voltage matching transformer, 70volts, 15watt , frequency responce, 40 to 20,000 khz, and secondary impediance of 4 or 8 ohms. The transformer is at" Parts Express" Will that work? Will someone in the know check this out? It`s only $5.95, plus shipping! William

A transformer capable of handling 15 watts of audio power is vast overkill for a 3V4. I can picture the transformer being the biggest part in the set.
Curt

What would be a safe wattage for a transformer, 10k to 8 ohm transformer, which would be hooked to the 3v4 output, which I just noticed has an output of .27 watts. I think a small PCB mount transformer would work! William

Yes, a small output transformer like that would be fine. However, using a PC mount transformer in a hard wired circuit can be fun due to having to solder leads to it.
Curt

Being the wild and crazy guy that I am, I am trying to make a circuit board version of that particular 2 tube regen radio. It is kinda fun! William

I'm trying to revive this thread. I stumbled upon this post and i have the same issue with "bob's" part list. Someone suggested a hammond as one alternative. I'm looking for a low cost solution. The turns ratio on this particular Xformer is about 35.5:1. If the 3V4 was so common, i've had a hell of a time finding ANY 10k to 8 or 32 ohm Xformers. I had another thought about adding a unity gain cathode follower to the output just to drop the output impedance to a decent level. I have built a few tube amps including a fender champ based design and a 12ax7/6dj8 headphone amp. The headphone amp does not use an output matching transformer. Although its impedance is supposedly around 100 ohms as far as i can remember. That is my inspiration behind the follower circuit. I think 35 to 40 bucks is a little steep for an Xformer just to play with my first regen radio. Im looking for an idea like use a 10:1 in series with a 3:1 to get me down to 35 ohms. I thought about winding my own but i don't want to go that route if i don't have to. I figured there would be someone with some sage advice like, you could use a 100v line transformer or i know XYZ transformer will get you close. I like the design of this radio, but if i have to modify it or look elsewhere for another circuit idea i will.

I found a great site that helps you calculate turns ratio and impedance. http://www.radioremembered.org/outimp.html

I have a pair of radio shack pro35a headphones which are approx 50 ohms. times that by 1.25 gives you approx 62 ohms. work that into their calculations and i really only need a 12:1 transformer. i might have an easier time finding that or a 10:1 just to experiment. And of course impedance changes with frequency so 10:1 might not be too bad.

Use a Tamura 3FD-312. They cost about $5 at Digikey or Mouser. It has the correct turns ratio, but it's actually a small power transformer. It will work just fine at audio frequencies for a homebrew receiver, as long as you're not expecting hifi. I've used them several times for audio.

Wasn't the general configuration for the 3V4 10K to 4 ohms vs 8 ohms?

Tom

Wow, i wasnt expecting such a quick reply. Thanks for the info i will check it out. i'm sure i could also find a pair of 8 ohm headphones if necessary.

I looked at the specs hoping to see some impedance data, but i didnt see any. I would buy some of these and at least do some measurements in accordance with the website in my previous post. Thanks again.

Get some 2K earphones, or even better some sound powered ones. You will want them if you are going to play with regens. You can then couple the phones via a o.o1 cap. 1T4?? You are running batteries?? If you are running a lower voltage for your B+, then you can run the B+ right through the phones. Up to 45V or so I would probably do it that way.

I just finished repairing an Emerson 559 portable that uses a 3Q4, electrically equivalent to the 3V4, that had an open winding on the audio out transformer, which also had the 10K to 3.2 ratio.

I got out my box of extra wall warts and began testing them, connected between the tube's output and the speaker.

I came up with three that worked quite well.

The 1000/6 ohm ratio was the best, but the 65/1 and 88/1.5 also worked quite well.

No need to spend money yet, if you have some of these laying around...

The walwort idea is genius. The first transformer you tried you stated as 1000 ohms to 6 ohms? Are these actual measured values or ratios? The other numbers looked like ratios to me. Also, do you have a good voltage to start with like 6v, or 3v? I really appreciate all the replys I am getting. I can post the schematic that I am trying to duplicate later. At that point if someone has a better design I would/could go with that. Also I'm not opposed to making a mains powered set. This one does happen to be battery powered, my thought was to make it portable to show off without mains voltage necessary.

My son is taking an interest in radios and i thought it would be cool to take it to school and show the other kids what you can make from stuff lying around. We just finished a crystal radio from scraps. I started a "foxhole", with a oxidized razor blade and a pencil. With my homemade capacitor and about 120 turns of #18 wire on a 2 inch form, we could get only one station, AM 1480 Geneva,IL. I live in Batavia. I told him we would experiment and change the design to try and improve it. We got some IN34 diodes in the mail last night, that sounded WAY better. No more messing with the sweet spot on the razor blade. I was then able to discern about 3 radio stations, but 1480 took up about 90 percent of my coil tuning range. I think i need a better capacitor. My cap is 2 toilet paper tubes with aluminum foil. HAHA. It works but it must be a low value. I want to test it later this week.

Here is the website for the set i'm contemplating building. http://www.bobsdata.com/regenerative_receiver/

see the attached schematic. His values for the components are on the site. I am in the process of updating the schematic with inductor values and other pertinent data including the values on the schematic.

The "ratio" numbers are simply DMM measured resistances on each winding. each of them were 120/12 volt supplies to begin with, although I checked all the units that I had (5/6/12/13/18 volt).

For your crystal set, you might try a Schottky (hot carrier) Diode.

Resistance readings won't be much help because primary and secondary will have radically different wire sizes.

To find the impedance ratio, divide the input voltage by the output voltage. That gives the turns ratio. Then square that value to get the impedance ratio. Then multiply that number by the speaker impedance to get the primary impedance.

Example:
Walwart with 120V input and 6V output. Turns ratio is 120/6=20
Now square that number to get impedance ratio: 20*20=400
Multiply impedance ratio by speaker impedance (8 ohms) to get primary impedance: 400 x 8 = 3200 ohms

That's a lot lower than the desired 10k primary, but if you can find a wallwart with a 3 volt output then you're in the right range. Or find a wall wart with a 240 volt input.

You're unlikely to get an accurate turns ratio from winding ohms because the wire sizes are usually different.

For audio purposes the limiting factor is core saturation, the point where the iron can no longer respond to increasing flux, and, so, impedance crashes. For audio that results in clipping. For a power transformer it causes excessive current draw and transformer overheating. The lower the frequency the more iron is needed to avoid it and the worst possible case is DC, but that is precisely what a single ended audio tube puts through one. (The 3V4 normally idles at about 10mA DC but I don't know what the referenced circuit does)

To ameliorate that problem single ended audio output transformers incorporate an air gap to reduce core flux at DC.

The point to that brief summary is that the power and line transformers being discussed do not have an air gap so comparing their (AC only) 'rated power' to the audio power you're trying to use them for does not equate due to the comparatively large DC component, which is there irrespective of the audio bandwidth or, indeed, of any audio at all.

I'd suggest a line transformer because they have multiple taps you can 'fine tune' with and the two I use are the SPECO T-7025/5, a 5 Watt job with a gaggle of taps starting at a 20,000 ohm maximum (8 ohm output), and the SPECO T-7010, a 10 Watt job starting at 8,000 ohm max with both 8 and 4 ohm outputs. Both are sub 5 bucks cost.

While not of paramount importance to a little regen it is worth noting that if you increase primary impedance by using a high than normal load on the secondary, e.g. 16 Ohms on the 8 ohm output, you reduce base extension and the converse is true.

I used a 7025/5 in my 12AL8 regen but did have a bit of asymmetrical clipping due to the onset of core saturation. The T-7010 might have tolerated it better but I'm reserving the two I've got for a future push pull amp using the 'dollar days' 6HB6.

I just ordered a 7025 off ebay. I also calculated that i could use a 9v transformer using your guys calculations. I have a pair of Radio Shack PRO 35A headphones which ohm out to 60 ohms. So, i guess i have some options. Also i could take the high imp. output into my 6DJ8 headphone amp and see how that works. I appreciate all the time and effort you guys have put in. I'm sure anyone else following this thread will have a pretty good idea of how to proceed.

On a side note, I looked at the Bob's Data site under discussion here and happened to notice that while the parts list calls for a 1 meg gridleak resistor, the underneath view shows that he actually used a 2.2 meg instead. Something to experiment with if you build this rcvr, I guess.

Tom