TheRadioBoard

I've been using my bogen t725 off and on during the contest and I am
a bit disappointed. At first I got nothing out of it at all, and I think that
may have been the switches I used to build a unimatch type of circuit
with a '104' cap and 100k pot benny. I took it all apart and started playing
directly with the xfrmr and moving to various taps and while I do get
audio now, the signal is significantly less than when I connect the
phones directly to the tank circuit. The best performance so far has been
using the recently discovered unmarked scavenged diodes (one or two
red bands on the cathode side) directly to my clevite brush crystal phones.
For taps I've been using the white as the top of the input, the black as
the common, and the green or blue to the phones. I've also tried several
1n34a's fo-215's and some older diodes (scavenged from old philco car
radio tuners with a black, grey, and red stripe and a C1 or G1 marked in
green). Note this is all with the benny removed.

So, would the more experienced/knowlegable guys here have any suggestions?
Is it just that I need even higher impedance on the
input? I would have thought I'd have gotten SOME improvement by
moving from the straight phones to the bogen. If it comes down to testing
things for impedance etc, I should say my only test equipment is a
$15 DVM. I don't even have an oscillator of any kind working at
this point.

Hi, FT,
What you should be getting with the Bogen xfrmr is not necessarily more volume (although this could happen, too) but more selectivity. The Bogen will allow you to more lightly load the coil/capacitor detector circuit, increasing its Q, and giving better selectivity.

Have you found this to be the case?

Best regards,
Jack

Hi Jack, I know I should expect better selectivity, but that is very hard
for me to determine. I have not heard any new stations using the bogen,
and have not been able to separate the signals much better, but I
attributed that to the volume being SO much lower that I can't hear
the signals well enough to determine if I am hearing a different signal
or not. Maybe it will just take a little more in the way of patience to
determine if it is doing what is expected. I knew I was not expecting a
much louder signal, I just didn't expect it to be this much lower...I was
expecting something much closer to the level I get with the straight
phones, maybe a bit lower, but with the increased selectivity.

The black/grey/red striped diode (and probably the season) has allowed
me to increase my station count (so far) by about 20% over what I had
during the August contest sponsored by this board.

Hi FT ,

Your brush headphones are higher impeadance than the bogen transformer , so the is no befenifent by using it . The benny being only 100k about the same impeadance as the brush headphones is way to low ,the bennys resisitance in this case should be 1 to 5 meg ohms .

Aside from cost the bogen isn't a very good matching transformer for a high end crystal set. Auto transformers do not have any current gain when matching impeadance so are not as efficient as a step down matching transformer .

Mark

uhm the bogen is NOT an autotransformer, it can be rigged as one but...
more here.
http://schmarder.com/radios/misc-stuff/t-725.htm

but to be hones the 40k is bit on low side, a 350k would be good for many high end sets, and if we add a tapped secondary with say 16, 150, 300, 600, 1k, 1k5, 2k. we would have a great transformer.
sadly we would also have a expensive one.

Hi OEjan .

I should have said used as a auto transformer ,like it is being used 99% of the time with crystal sets .

Mark

Not to go off topic, but I have a question for you guys. I know there is no such thing as a stupid question and all that, but I have one that makes me feel stupid, but maybe I just have been left out in the dark for too long.

But what the heck is a "benny?"
Curt

_________________
Curt, N7AH
CW forever
Connoisseur of the cold 807

Thanks for the info bat. I was under the impression my brush phones
were about 2Kohms, but I'm perfectly happy to find out I'm wrong.
I've looked on the net and I've not found any information. Does anyone
out there know, or know where to find the impedance of these phone?

Guess I'll play with the bogen on some other phones...

Hi Curt ,

It s a high value pot (100 k to 5 meg) with a .1ufd cap in parallel placed in series with the detector diode and the matching transformer to equilize AC and DC impeadace's to provide maximum volume and clarity of audio in the headphones of a crystal radio. Named benny for Ben Tongue who devised and popularised its use .

OK. Thanks for the explanation.

FtForger- crystal headphones usually have impedances rangeing in the 100K ohm or more region. That is why you can not use them in the plate circuit of a tube without a resistor shunting them. Actually, their impedance is more of a capacitive reactance than what most consider impedance, but impedance can be resistive, capacitive, or inductive.
Curt

_________________
Curt, N7AH
CW forever
Connoisseur of the cold 807

Thing is, Curt, crystal phones are generally insensitive mechanically - or defective

An exception would be those piezo-gizmos that were being discussed a few months ago which are apparently quite effective.

Even still, in a top notch DX rig some impedance transformation might be advantageous.

My experience with a Bennie has been mostly to eliminate distortion on stronger signals which occurs on a truly hot radio. I can't say with any authority that it helps with weak-signal sensitivity on my radio but as you know, just having a tweak-knob to turn on a weak combination of signals can often give an edge over having no adjustment at all. On a really hi-q rig I can see where an additional edge can be gained in selectivity.

Did you listen to that mp3 clip that I posted a few days ago? Nobody commented if that was typical or not but my bennie experience sez that that sort of range is to be expected, at least in the context of distortion.

Exray ...


can you give your YT link please ...

_________________
regards,

George ..
__________________________________________
I thought I had amnesia, I just forgot I didn't.

http://www.sparkbench.com/20040401wxew.mp3

The first part of the clip has the Bennie being adjusted in and out.