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 Post subject: using 24A as a regen detector
PostPosted: Oct Sun 12, 2008 11:19 pm 
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Location: Zip : 80751
A few questions.
1. Should the screen bee grounded or kept around 20 volts + for use as a regen detector. I'm planning on using a ticler coil and using a throttle cap. and using a 1:3 trans and a 250K resistor in series. for the plate and regen.

2 Is there enough drive to run a #47? Or should I add another 24 as an audio amp and then the 47?
I'm tring to keep the tubes from a certain time era to fill an empty cabinent, and build a simple regen with older tubes. All the regens i've built have used triodes

Thanks for any input


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 Post subject:
PostPosted: Oct Mon 13, 2008 12:06 am 
Silent Key

Joined: Jan Thu 01, 1970 1:00 am
Posts: 34326
Location: Sandpoint, IDAHO 83864
The 24A makes a wonderful regenerative detector. Just be aware that some can cause some hum in the output from AC being on the filament. Tubes were not tightly controlled in their production back then, like they later were.

The one I had used a fixed throttle condenser and had a pot in the screen circuit that varied the screen voltage between zero and about 50 volts and I think I had 180 volts on the plate, if I remember correctly. It seemed to work best with around 18 to 25 volts on the screen grid. Smooth regenerator, but the wirewound pot I had was quite noisy, so adjusting it would make a lot of noise in the phones.

It was R/C coupled into a 27 triode for the audio amp stage. I can not see any reason why it would not feed a 47 for speaker volume.

Actually, I never built that set. It was built by a young ham back in 1931 when he got his ham license. Built on a wood chassis with a bakelite, or whatever it was, front panel The variable capacitor had a nice DeJur reduction drive on it. The AC power supply was on a separate board and used one of the Thordarson Power Compacts, which was the power transformer, filament transformer for the rectifier, and the filament transformer for the receiver and a choke in the same case. The rectifier tube was an 80 (globe style of course), and the filter capacitors were a pair of Paravolt oil filled capacitors in metal cans that stood upright just like an oil filled transmitting capacitor would.

Neat little homebrew set and it worked on 40 meters quite well, actually. Just don't ask me how many times I got bit from the headphone pin jacks on the front panel! The phones were simply in series with the plate to the 27. No coupling capacitor or anything, so my phones had 180 volts of B+ on them!

I wish I still had it, as it was a fun set to tune around the band with. And it looked neat also, with globe tubes in it, and their filaments glowing.
Curt

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 Post subject:
PostPosted: Oct Mon 13, 2008 2:42 am 
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Location: Circleville, OH, USA
With tetrodes, screen control of regeneration works best. It is smooth and easy to control. RC coupling would be best to couple into a following amplifier stage. The high plate resistance of a tetrode is too much of a mismatch for transformer coupling.

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 Post subject:
PostPosted: Oct Mon 13, 2008 3:47 am 
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What value should I start out with for the fixed throttle cap? Its going to be used on BCB band first to work out any bugs. I'm going to revise my schematic and use screen control. and R/C coupling to the #47. THanks everyone


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 Post subject:
PostPosted: Oct Mon 13, 2008 4:43 pm 
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Location: Circleville, OH, USA
With screen control, you shouldn't need a throttle cap. Just bypass the B+ end of the tickler coil to ground with a .001 cap to remove RF. Be sure to bypass the screen to ground with a .05 cap.

You will be surprised how little voltage you need on the screen to get good regeneration.

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 Post subject:
PostPosted: Oct Tue 21, 2008 2:25 am 
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Image

This is what i've come up with so far. Transformer is an old one that i salvaged, seems to put out about 300 VAC, and about 150 Ma. The fillament voltage for the tubes will come from a seperate x-former. Any suggestions.


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 Post subject:
PostPosted: Oct Tue 21, 2008 10:44 am 
Silent Key

Joined: Jan Thu 01, 1970 1:00 am
Posts: 34326
Location: Sandpoint, IDAHO 83864
The only suggestions that come to my mind is that you should hang a bypass capacitor off the point off the 45k resistor that drops the voltage for the first stage. May want another 10uF there. Regenerative sets are very succeptable to hum, especially the detector stage, and if it can be decoupled from the audio stage, so much the better.

Also the screen voltage on the detector can swing from zero to ninety volts in your schematic. That may make controlling the regeneration a bit touchy. I would jocky the value of the resistor and pot that sets the screen voltage so you only have a swing of about 0 to 45 volts on the screen. If you have good coils, it should regenerate smoothely at around 18 to 22 volts, which would put you mid point in the pots rotation.
Curt

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 Post subject:
PostPosted: Oct Wed 22, 2008 2:49 am 
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OK will give it a try maybe this weekend if i have time to build it.


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 Post subject:
PostPosted: Oct Wed 22, 2008 6:32 pm 
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Location: Circleville, OH, USA
For tuning the BC band, 50 pF is a bit low across the grid leak. I would suggest 250 pF. Also at the detector, I would use .001 mF to bypass RF. 100 pF is a bit light.

Curt is right about the screen control voltage. I have a set using a pentode detector. It only needs 8-10 V on the screen for good regeneration. Your circuit has too much voltage swing which will make control very touchy.

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 Post subject:
PostPosted: Nov Sun 30, 2008 2:14 am 
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Heres an idea for a radio circuit. All you have to do is adjust the filament voltage and voltage dropping resistor values for the 24A and 47 tubes. The audio choke helps to get maximum output from the regenerative detector. An interstage primary can be used in place of it.

Image

Norman


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 Post subject:
PostPosted: Nov Sun 30, 2008 9:44 pm 
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Here is another great idea to use:

Image

Very similar to the other one I posted.

Norman


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 Post subject:
PostPosted: Nov Sun 30, 2008 10:45 pm 
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Location: Norman Ok 73026
Those are a couple of fine schematics. That 32 and 33 are a great combo. You can add a 34 variable mu pentode tube for rf too so you don't radiate and to boost weak signals. A lot of old circuits used a 50 or 100k resistor in series with the 50k regen control. screen-grid tubes are easy to make regenerate without detuning the stations by the use of adjusting screen voltage. 22 1/2 volts was about typical for screen voltage, and as I think someone mentioned you need the series resistance to make control less critical. High output impedance of tetrodes and pentodes means a high input impedance into the next circuit. A grid circuit from a following stage satifies this requirement. In impedance coupling (like in the circuits above) the higher the henries, the lower the bass that will pass. I've read that some people used ignition coils. If you use an audio interstage, tie the primary and secondary together. The total inductance works out to be greater than the sum of the two separate.
You might try a space charge connection; use the screen for control and put from about 4 1/2 to 22 1/2 volts on the grid.


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 Post subject:
PostPosted: Dec Wed 03, 2008 9:39 am 
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I found a 640 H choke on ebay. The price is kind of high, but these are difficult to find these days:

http://cgi.ebay.com/640H-grid-choke-6dj ... 7C294%3A50

Norman


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 Post subject:
PostPosted: Dec Wed 03, 2008 7:28 pm 
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I had a look at that choke. It's a pair for fifty bucks. New made in China. Catering to the audiophile crowd I guess. You can get a lot of Heneries from an audio interstage transformer. Highr the ratio, the better. I just checked one. primary was 12H, secondayr was 320H. Tied together it was 495. The primary will always have a reasonable amount of inductance to keep the low frequencies from shorting to ground, so a high-ratio secondary will have a whole lot more and a whole lot of inductance. This transformer was unmarked. Another one I tested is Thordson 6-1 ratio 9.4H pri. 260 sec, 375 total. Total inductance always greater that sum of individual windings.
Normally (from what I read) the smallest drawn wire they had pre-war was 40 awg, which is just slightly more than a ohm per foot. So generally the higher resistance secondary the more windings, assuming that it's wound with 40awg.


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 Post subject:
PostPosted: Dec Wed 03, 2008 8:06 pm 
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Location: Baltimore Md
Neat schematics. I have several 22's that Ive always been tempted to use for a regen. Higher filament but they were the very first tetrodes Im told.

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 Post subject:
PostPosted: Dec Wed 03, 2008 9:30 pm 
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The nice thing about 22 tubes is that they have a 3 volt filament. You don't need a voltage dropping resistor for them! :)

Norman


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 Post subject:
PostPosted: Dec Wed 03, 2008 9:54 pm 
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Something else Ive decided to go ahead and ask even if a bit on the wierd side. What does anyone think about using a TV flyback transformer as an inductor for an audio choke?

Its core is designed for much higher frequency than a 60Hz transformer (which Ive seen used in audio service elsewhere on the forum). What is it, 20some KHZ? Would that be too high??

Ive never had a flyback trans to measure so dont know what its inductanc/resistance might be but I would expect pretty hi L and fair R.

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 Post subject:
PostPosted: Dec Thu 04, 2008 12:12 am 
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Here's another similar question from Rune. Does anyone have an answer for these?

Rune wrote:
I have a core from a flyback transformer on hand here. It's AL value is approx 450 with a litle gap between the two halves. 15000 turn would give me about 100H. What do you experts think of the performance at audio frequencies of a core like this?


Barney O, your question is not as weird as you thought!

Norman


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 Post subject:
PostPosted: Dec Thu 04, 2008 10:48 pm 
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tack wrote:
High output impedance of tetrodes and pentodes means a high input impedance into the next circuit. A grid circuit from a following stage satifies this requirement. In impedance coupling (like in the circuits above) the higher the henries, the lower the bass that will pass. I've read that some people used ignition coils. If you use an audio interstage, tie the primary and secondary together. The total inductance works out to be greater than the sum of the two separate.


I found an example of using an ignition coil at this site:

http://www.qsl.net/kl7h/12v.htm

Also, if you use an interstage transformer, do you tie the B and G together then connect the P to the plate and F to the B+? Is this the correct way to tie the primary and secondary together?

Norman


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 Post subject:
PostPosted: Dec Fri 05, 2008 9:45 pm 
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I found a good summary of RC vs. LC coupling on The Radio Board forum:

Rob Lenski wrote:
To sum up then, I'd say R/C coupling would be the way to go in a broadcast band set, or if you've got a robust power supply. When building a shortwave set, or you are limited in what you can provide B+ wise and fidelity isn't a prime consideration, then a choke coupling scheme has it's merits.


That explains why you need such a high B+ voltage when using RC coupling as in Toad's schematic near the top of this thread. Basically, LC coupling allows more B+ voltage to get to the plate and permits you to use a lower B+ voltage supply. RC coupling is less efficient, but provides better fidelity.

Norman


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