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 Post subject: TL431 to replace 6888 transformer?
PostPosted: Sep Thu 28, 2017 9:26 pm 
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Could the TL431 be used to replace the input transformer on the original 6888 transmitter?

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 Post subject: Re: TL431 to replace 6888 transformer?
PostPosted: Sep Thu 28, 2017 9:39 pm 
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I can't answer your question, but I have some information which may help others.

Inside the TL431 is a 2.5V reference and the input pin has a DC offset of 2.5V. The input impedance is very high (~1MOhm).

This means that the current through the 1M self-biasing resistor results in a voltage drop of approximately 1V which implies that the quiescent DC voltage on the output is approximately 3.5V, dependent on the value of the presently used 1M biasing resistor.

This quiescent output voltage may be altered (higher, but not lower than 2.5V) by changing the value of the 1M self-biasing resistor or using a voltage divider with an extra resistor from input to ground.

Looking at the schematic and thinking about it, a large value resistor to ground from the grid and a coupling capacitor from the TL431 output might be successful.

Another thought. Rather than using a separate 9V supply, it might be possible to connect the supply side of the 1K TL431 load resistor to the 5.1V Zener supply used for the oscillator.


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 Post subject: Re: TL431 to replace 6888 transformer?
PostPosted: Sep Fri 29, 2017 9:29 pm 
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If there is an interest in using a TL431 as the audio input on a 6888 transmitter, I wanted to point out that the TL431 can likely be used a transmitter itself.

I have built a number of regenerative-reflex AM receivers using only the TL431 and had it oscillating up to about 2 MHz.

Adding a 1 MHz crystal to turn it into a Pierce oscillator, leads to the circuit below, which I have not tried and may require some finessing to get it to work. Because I haven't built one, I also don't know what percentage modulation can be achieved.

I am presenting it only as a suggestion for any who might be interested in a simple, but reasonably high power AM transmitter and don't mind putting a bit of work into getting it going.


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 Post subject: Re: TL431 to replace 6888 transformer?
PostPosted: Sep Fri 29, 2017 10:43 pm 
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With a 9VDC supply a 5V crystal oscillator could be used?


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 Post subject: Re: TL431 to replace 6888 transformer?
PostPosted: Sep Fri 29, 2017 11:42 pm 
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Mike Toon wrote:
With a 9VDC supply a 5V crystal oscillator could be used?


Now you really have me thinking!

The left hand circuit below uses a 7805 regulator to supply the crystal oscillator.

The TL431 is driven by the oscillator in Class A mode because the TL431 has a quiescent current determined by the self biasing 1M resistor. The 10 mH choke permits modulating the TL431 but isolates the RF input from being loaded by the audio input source.

The right hand circuit is a similar configuration to the original tube circuit and will likely require a higher supply voltage.

I want to stress that these are only suggestions which will likely require some finessing.


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 Post subject: Re: TL431 to replace 6888 transformer?
PostPosted: Sep Sat 30, 2017 12:41 am 
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I was aware that there was a popular thread using the LM386 audio amplifier and a crystal oscillator in an AM transmitter.

I took a look at the circuit and see no reason why a TL431 could be used instead of the LM386.

When used as a shunt regulator, the reference voltage on a TL431 is 2.5V which can be modified by using a voltage divider on its input rather than the self-biasing 1M resistor. For 5V output, 2 equal values in the divider will create an output of 5V to supply the crystal oscillator. The output voltage can then be modulated by applying a capacitively coupled audio signal to its input.

The oscillator module requires 15 ma. at 5V which means that when using a 9V supply, the TL431 input resistor which has a 4V drop across it should be on the order of 270 Ohms or less.


viewtopic.php?f=12&t=293116&p=2452961&hilit=lm386+lm386+transmitter#p2452961


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 Post subject: Re: TL431 to replace 6888 transformer?
PostPosted: Oct Mon 02, 2017 7:47 pm 
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I breadboarded this amp and replaced the input transformer and am really happy with the results with 9V. No need for a volume control. It is 300% the volume! No need to max the volume of my little MP3 players. They run at 50% volume now.

I connected it to 5V and the volume dropped a little. I tried replacing the 1K resistor with down to 220Ω to recover the 9V sound and it got worse. Is there is a way to juggle the resistors to get that little bit more?

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 Post subject: Re: TL431 to replace 6888 transformer?
PostPosted: Oct Mon 02, 2017 10:45 pm 
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I'm not sure that a shunt voltage reference is the best choice for an audio amplifier, but if that's what you want to do...

As already suggested, a voltage divider on the Adj pin would work better that a single resistor. The current at the pin is not well controlled, so the the operating point will vary.

Similar references are offered with lower reference voltages. This would be useful at lower supply voltages. Or, there are versions with separate output pins that can pull down close to ground.

1.25V reference:
https://www.digikey.com/products/en/int ... ageSize=25

0.2V or 0.6V reference, separate output pin:
https://www.digikey.com/products/en/int ... ageSize=25

Ted


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 Post subject: Re: TL431 to replace 6888 transformer?
PostPosted: Oct Mon 02, 2017 10:49 pm 
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Mike what's the voltage on pin 5 of 6888???

If cathode is much lower than 5v the audio will be distorted... I generally bias my 6888 at closer to 6v...

Assuming voltage is a bit low, you can try a 68-100 ohm resistor in series with osc B+ and cathode, zener remains connected to osc B+... Feed TL431 directly from cathode... Also be sure the .1 cap is connected directly to cathode...


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 Post subject: Re: TL431 to replace 6888 transformer?
PostPosted: Oct Mon 02, 2017 11:28 pm 
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Mike Toon wrote:
I connected it to 5V and the volume dropped a little. I tried replacing the 1K resistor with down to 220Ω to recover the 9V sound and it got worse. Is there is a way to juggle the resistors to get that little bit more?

Image

One problem with running at 5V is that the threshold voltage of the TL431 is 2.5V and its output is likely to be close 5V which means that there isn't enough overhead. You can try lowering the value of the 1M resistor self-bias resistor which will lower its voltage drop and lower the quiescent output voltage giving more overhead.

I was also surprised that you did not capacitively couple to the grid and drop a large value resistor from the grid to ground. In the original 6888 circuit the grid is held at ground potential through the transformer winding. Directly coupled, the grid will be raised to somewhere about +5V.


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 Post subject: Re: TL431 to replace 6888 transformer?
PostPosted: Oct Tue 03, 2017 12:46 am 
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Pin 5 stays at 5.3VDC. My .1 cap is 2" away from pin. :cry: Audio isn't distorted at all.
I just thought since the 9V battery sounded a little louder, I would ask.Specs said 2 .5 to 36V.

Connected to pin 5 (5V) and after the dropping 1K resistor the TL sees 2.65V. Is it close enough to 2.5V?
Coupled with .1 and 1K to ground, grid now at zero. (Only breadboarded)

LM386, I had a bag of TL431s and I saw your schematic of this chip that used for his crystal radios. I tried it and loved the amplification. I dug out an old 6888 transmitter and thought this circuit might be better than the small Radio Shack audio transformer.
It is, by 300% and only 4 parts.


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 Post subject: Re: TL431 to replace 6888 transformer?
PostPosted: Oct Tue 03, 2017 1:11 am 
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Mike Toon wrote:
Connected to pin 5 (5V) and after the dropping 1K resistor the TL sees 2.65V. Is it close enough to 2.5V?
Coupled with .1 and 1K to ground, grid now at zero. (Only breadboarded)

If my memory serves, the spec sheet for the TL431 indicates that the input current for the device can vary from between 1 and 4 ua, which would imply a 1 to 4V drop across the 1M bias resistor.

Yours seems to be drawing less and 2.65V on the output should give you enough overhead.

Not being a tube guy, I don't know what effect raising the quiescent voltage on the suppressor grid would be. By directly connecting the output to the grid you are changing its bias from zero to + 2.65V. If there is an improvement when capacitively coupled, I would use a much larger resistor to ground than 1K so that it doesn't load the TL431.

Also a 0.1 and 1K would have a very poor low end frequency response. The lower 3dB cutoff frequency would be 1600 Hz.

If you keep the 0.1 cap I would use 100K for the resistor resulting in a 15 Hz lower 3db cutoff. If input 60 Hz hum ever becomes a problem, you can lower the value of the resistor to raise the lower 3dB cutoff frequency.


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 Post subject: Re: TL431 to replace 6888 transformer?
PostPosted: Oct Tue 03, 2017 2:30 pm 
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LM386 wrote:
I was also surprised that you did not capacitively couple to the grid and drop a large value resistor from the grid to ground. In the original 6888 circuit the grid is held at ground potential through the transformer winding. Directly coupled, the grid will be raised to somewhere about +5V.

Agreed...

I found early on without xfomer some 6888 were prone to current draw on G3 that cut off tube completely(maybe just leaky tubes?)... Still I'd go with a .05 coupling cap & resistor of around 68-100K... Also to improve high frequency response, the .001 can be reduced to 470-680pf...


Tom


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 Post subject: Re: TL431 to replace 6888 transformer?
PostPosted: Oct Wed 25, 2017 6:36 pm 
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I still have some 6888 tubes. If any member wants to build one just PM me a mailing address.

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