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 Post subject: Untuned RF & Hartley Regen using 6418 tubes
PostPosted: Dec Wed 10, 2008 3:36 am 
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I've have a bunch of NOS 6418's I'd like to use in a LF band battery powered regen.

Criteria is:
1. Untuned RF stage
2. Hartley Detector

Similar to this design.
Image

After doing some reading, the RF stage shouldn't be a problem but I haven't seen any pentode hartley regens, just triode hartleys. So a little help here is needed. Thanks.

Also, why is the 6418 called a pentode when it has beam forming plates that are passive and not a second screen?
Image

John
KB7NRN


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 Post subject:
PostPosted: Dec Wed 10, 2008 4:01 am 
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John

Usually beam forming plates are considered a third grid giving the name pentode. I've also seen these tubes called tetrode with beam forming plates.

Input to first stage should connect to grid. That's a lot of voltage for 22 1/2 volt tubes. Circuit will work but don't expect a lot from tubes with 1.25 volt @ 10 ma filaments. Transconductance of 300.

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 Post subject:
PostPosted: Dec Wed 10, 2008 1:00 pm 
Silent Key

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Location: Sandpoint, IDAHO 83864
And you are showing connections to a cathode element of a tube. These tubes are directly heated and have no cathodes. Back to the drawing board, I am afraid.
Curt

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 Post subject:
PostPosted: Dec Wed 10, 2008 5:03 pm 
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Location: NJ, USA
With gm that low you'd probably want to use reflexing to get decent performance. I would reflex the first tube for both RF and AF amplification.

As Curt pointed out these are directly heated tubes and your choice of grounded grid front end and hartley detector are not optimal. It will be easier to use grounded cathode topologies for both tubes. Also since the tube are pentodes there is no reason to use grounded grid RF amp - screen grid provides the desired i/o isolation.

However If you really want to stay with that cofiguration you can do bifillar winding for both RF stage cathode choke and hartley cathode feedback winding and supply heater via that bifillar wire pair.


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 Post subject:
PostPosted: Dec Wed 10, 2008 7:53 pm 
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OK,
So what you all are saying is that this would be more better;
Image
Thanks to Dave Schmarder.

I was trying to only wind one coil. 250 turns for LF band is alot of winding!

Yeah, I figured the directly heated cathode would throw a curve ball in my plans.

Instead of a variable antenna coupling cap and fixed resistor what about a fixed cap and varible resistor for RF attenuation?

I'm not going to reflex AF or RF since I'll have an additional audo section capable of driving a speaker. The set will use only 6418 tubes, four in the audio section alone.

John
KB7NRN


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 Post subject:
PostPosted: Dec Wed 10, 2008 8:16 pm 
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That is fine. Two observations:

1. High impedance input is not a good idea without tuning tank - I would replace that 1M resistor with a 10k pot for RF gain/volume control and use a fixed cap small enough to attenuate AF. Otherwise you will pick a lot of hum from the front end.

2. You can control regen via screen grid voltage on the second stage pentode.


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 Post subject:
PostPosted: Dec Wed 10, 2008 8:51 pm 
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Yep, a 3pf cap or two wires twisted. I've also used a 1k pot for RF attenuation with some solid state regens so anything between 1k - 10k should work.

Screen voltage, wonder where it goes into oscillation?

John
KB7NRN


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 Post subject:
PostPosted: Dec Wed 10, 2008 8:56 pm 
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Greetings;
The old-timers often used an input like yours; low pf variable capacitor and a high value resistor between antenna and ground. The resistor was used to save money; an inductor was preferred, 85mh or better. Sincew being you are building an LF receiver, you should use a much larger choke if you want to go that route. The resistor equally passes a portion of all signals to ground, good and bad. The choke passes off more desireable signals, while shorting out bad stuff like 60hz hum to ground.
The two favorite methods of regeneration were a throttle capacitor on the feedback loop in the case of a triode, and if the tube had a screen then a 50k pot used to vary the screen-grid voltage. The pot method works very well. You can use both! Use one to get close, and the other to fine tune.
The best method to couple antenna input to the first tube is with arf transformer. You may want to try this if you ever want to try to tune the RF input for greater selectivity.


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 Post subject:
PostPosted: Dec Sat 13, 2008 6:38 am 
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Here is a schematic I came up with. Borrowed ideas from Dave Schmarder and Tony Wells.

Image

No, it's not a grounded grid RF amp or Hartley detector. A very conventional regen. Your comments are welcome.

I'll draw out the audio section next.

John
KB7NRN


Last edited by War Bird Radio on Dec Sat 13, 2008 6:55 pm, edited 1 time in total.

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 Post subject:
PostPosted: Dec Sat 13, 2008 8:50 am 
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War Bird Radio wrote:
Here is a schematic I came up with. Borrowed ideas from Dave Schmarder and Tony Wells.

Image

No, it's not a grounded grid RF amp or Hartley detector. A very conventional regen. Your comments are welcome.

I'll draw out the audio section next.

John
KB7NRN


Hi John,
That circuit looks reasonable to me. I think you forgot to draw in
the small capacitor from the plate of the rf amp to the detector
and maybe the tank coil or rfc, depending how you are doing it.
You may have to fiddle with the value of the resistor on the plate
of the detector. It depends on what you are driving.

The 6418 makes a really nice tube for these little radios. I selected
it for the low voltages and currents needed to make the radio
operate.

I just noticed your input capacitor. That is good for SW but way
too small for LW. I would make that as high as a .001uF. The
capacitor for between stages might be as high as 100pF, but
could work better with a lower value.

Let us know how this turns out. The design looks good.

73, Dave
N2DS


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 Post subject:
PostPosted: Dec Sat 13, 2008 1:53 pm 
Silent Key

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Location: Sandpoint, IDAHO 83864
I know it is only 4AM here, but how are you getting B+ to the RF stage? Looks like an OOPS to me.
Curt

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(Connoisseur of the cold 807) CW forever!


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 Post subject:
PostPosted: Dec Sat 13, 2008 6:59 pm 
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Ok, I updated the schematic.
Image


Dave,
Rearranged the grid leak to block DC from the detector grid.
How about 500pf for the antenna coupling cap?

Curt,
Now we have B+ on the plate of th RF tube. Will probably work better this way! ;)

John
KB7NRN


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 Post subject:
PostPosted: Dec Sat 13, 2008 9:03 pm 
Silent Key

Joined: Jan Thu 01, 1970 1:00 am
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Location: Sandpoint, IDAHO 83864
No, not quite. Now you have pure B+ on the plate. You need to move the connection to the plate to the bottom of the coil so the coil develops some signal voltage across it. And bypass it with a capacitor to keep the RF where it should be.
Curt

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(Connoisseur of the cold 807) CW forever!


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 Post subject: 6418 LF regen Ver.3
PostPosted: Dec Sun 14, 2008 12:15 am 
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Thanks Curt, got it now.

Anything else you see needs to be fixed?


Image

John
KB7NRN


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 Post subject:
PostPosted: Dec Sun 14, 2008 1:34 am 
Silent Key

Joined: Jan Thu 01, 1970 1:00 am
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Location: Sandpoint, IDAHO 83864
Try it out and see. We can sit here in front of our computers all day long to come up with "do this" and "do that", but the proof is in the pudding.
Curt

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(Connoisseur of the cold 807) CW forever!


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 Post subject:
PostPosted: Dec Sun 14, 2008 2:42 am 
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Location: Okie Land
Could I ask what the tuning range you're shooting for is?

Yeah, build it and see what happens. Then try some modifications to see how performance is affected:

Use a high-value rf choke at the input between antenna and ground (at least 100mh)
Try a larger value pot at the input between antenna and ground, from 100k to several megs. As it now is, a lot of RF is going to leak away.
Try connecting the lower end of the tuning inductance to ground, and connecting the RF tube's screen to a variable resistance (to vary gain) as this can sometimes help with selectivity. Since this tube is a "pentode", with a means of secondary emission supression, and thus no tetrode kink, it's ok to have the anode and screen at the same potential as you do now, but it may not be the best way.


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 Post subject:
PostPosted: Dec Sun 14, 2008 4:57 am 
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Could I ask what the tuning range you're shooting for is?

Yeah, build it and see what happens. Then try some modifications to see how performance is affected:

Use a high-value rf choke at the input between antenna and ground (at least 100mh)
Try a larger value pot at the input between antenna and ground, from 100k to several megs. As it now is, a lot of RF is going to leak away.
Try connecting the lower end of the tuning inductance to ground, and connecting the RF tube's screen to a variable resistance (to vary gain) as this can sometimes help with selectivity. Since this tube is a "pentode", with a means of secondary emission supression, and thus no tetrode kink, it's ok to have the anode and screen at the same potential as you do now, but it may not be the best way.


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 Post subject:
PostPosted: Dec Sun 14, 2008 6:12 am 
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I've started building the case and getting parts in order.

Aiming for 100 - 500 kc tuning range.


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 Post subject: That set using the 6418 tube
PostPosted: Mar Sun 08, 2009 4:35 pm 
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Since you using a pair of 6418 tubes...Just go and connect them in pararrell and use a single 10 ohm ( might have to be adjusted in value possibbly a 10 or 15 ohm single resister to match the total current draw of those two tubes in parellel...) In the hot lead of that 1.5volt dry cell type of battery...
That resistor could be eliminated intirely and replaced with Nicads or any other rechargeable battery 1.2 volt C, D, aa or aaa...
Also make that " RF Pie network " into a full netweork input and output cap say 100pf input and that 470 pf output...
Also across the Screen Grid Potentometer a cap of at least that 470 pf...


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 Post subject:
PostPosted: Jun Mon 28, 2010 6:00 am 
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Hey, isn't the suspense killing everyone? WarBird did you ever finish this one? I searched "2-tube regen" and this old link came up.

Curious how it went.

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