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

Resistors are quite cheap if you're already ordering other parts and can share shipping costs.

The idea behind grid leak values is that the low value of 1 Meg. is pretty much guaranteed to work, but won't be optimum. Using higher values can increase the sensitivity, but eventually if you pick a value too high, it may stop working altogether.

Does anyone have some nice high impedance earphones they want to sell? I saw some other posts that talked about R-14 drivers being a good choice. That and they had an impedance of around 2K. I would only need a 5:1 Xformer or something like that. I think some phones would be nice for me and my son to use with his crystal radio as well.

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.

Always love to display real world solutions vs what the engineers postulate, lol.

High impedance headphones aren't cheap and you don't really need them since I gather you ordered the 7025/5. You can use any 'mp3', portable CD player, cellphone, etc., headphones with it or, put another way, it 'converts' them to high impedance.

Since no one has commented on the cost of this rig I am going to assume that 50-60 dollars is resonable for what I am building.

By happenstance, I was about to.
Mounts up rather quickly, doesn't it? Part of the 'trick' is figuring out who's best at which 'deals' and balancing that with combined shipping. http://www.esrcvacuumtubes.com/ is about the best price for tubes and they've got good socket prices too.
As you've discovered, variable caps aren't cheap these days but I just happen to see a 'deal' at radiodaze. They have a multi-section http://www.radiodaze.com/product/15438.aspx for $4.50. It's a tad bit bulky but you can combine sections to make up from between 20 pF to 438 pF. And it's geared, which would be nice for a regen. They've got others if you want to keep the bandspread but you might not need one with a gear drive.
While at radiodaze you might want to look at these turret sockets http://www.radiodaze.com/category/425.aspx They're kind of slick for mounting components. They've got regular sockets too and if you were getting the cap then might as well toss either of those in.
And then there are 'olden days' tricks like using a breadpan for a chassis. It still works
http://flipperhome.dyndns.org/Tin%20Man.htm

Oh, my, I hadn't looked before but just followed the link. No way I'd pop for 365pF variables and then rip plates out of them. I'd probably use the radiodaze 3 buck job and just let it tune a little wider but looks like the 75pF section is 6 plates for 12.5 pF each so if you pull 3 out and gang the two sections it would be 195 pF, which is dern close to his 194 pF. (I'd check it first to see if my 'guess' was right, though). Do the same thing on a second one except just use the one 25pF section for bandspread. Or, as I mentioned above, just use the 150 pf, 20 pF, and 28 pF sections on the geared job and let the finer tuning with the gear reduction substitute for the bandspread (plus the second 20 pF section if it needs the bandspread 'extra' to do the whole band).

I don't have a project in mind but may order some of those myself because it's hard to beat $4.50 and 100, 120, 150, comes to 370 pF, close enough to 365 for government work.

Following this thread has piqued my interest enough to build this (with minor modifications). On the subject of variable capacitors, only once have I ever ordered a new one from a catalog--they're just so expensive, and then there's the shipping cost. I've been buying them for years at swap meets at a fraction of the cost of new (very seldom more than $3) and now have two boxes of them. And almost never does one pulled out of an old set turn out to be unusable. A bit of cleaning, a bit of lubrication...good as new!

The one I bought from a catalog is a 365 pf unit with a reduction drive built into the shaft. I forget why I bought it, but this Bob's Data project would be the ideal place for it. And as Flipperhome mentioned, with a reduction drive there's probably no need for a bandspread. Time to heat up the soldering iron!

Tom

I hope I didn't give the wrong impression. I think using a wallwart is a great idea. My philosophy has always been to use the simplest cheapest component that will do the job.

However I stand behind my earlier statement that measuring winding resistance won't give a remotely accurate idea of impedance. Now, having said that, I must say that I've also found that impedance match can be off quite substantially before you start to notice much difference. So, I certainly wouldn't get too obsessed finding a transformer that matches the impedance exactly.

The simplest way to get a decent approximation of the turns ratio is to excite the primary and then measure the voltage on the primary and secondary. There's your ratio.

So if using a power transformer, such as a wall wart, where you know 120 goes in, and as an example, 12 volts comes out, that ratio would be 10 to 1.

No excitement necessary, lol.

Hehe. Well, that sounds logical but, you knew there would be a "but," right? It's not always so obvious what the 'AC in' is nor can you count on the 'rated' volts out. Reason being a lot, if not most, transformers are rated 115 VAC in and there will be sag under load, which is what the rated output is spec'd at. So, in reality, a 12 VAC output transformer will be (over) wound to do, oh say, maybe 14 VAC open load, which is the actual ratio at the nominal 115 VAC in, so that when it droops under load its 12 VAC.

Now, as Bob mentioned, the impedance isn't super critical but, by the same token, Murphy's Law says it rarely falls 'right on' an available voltage so it might make a difference whether, for example, you pick a 10 VAC or 12 VAC transformer because 20% is around the range it can be off, and that turns into almost 40% off in impedance.

As far as I know, unless it's a switchmode power supply(smps) the output rating on the xformer is based on the load applied. IE - 12v 500ma- the voltage at the secondary will only be 12 when you are actually drawing. 500ma. Hence the reason the voltage is always more when you measure it with a meter. You are aplying such a high impedance that you are not loading the xformer when using a meter.

I think the best idea is to turn up the primary until the secondary is 1v then measure the primary. This makes the most sense to me.