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 Post subject: One Tube Preamp
PostPosted: Sep Tue 24, 2019 11:57 pm 
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Joined: Jan Mon 17, 2011 2:10 am
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Location: Revere,Massachusetts U.S.A.
Hi Folks, I'm interested in building a one tube preamp (Simple Circuit) I would like to use an audio tube from the 30's or 40's. "The Coke Bottle Shape" ex. 6C5-G

Any ideas would be appreciated...Tony


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 Post subject: Re: One Tube Preamp
PostPosted: Sep Wed 25, 2019 1:38 am 
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Location: Tucson, Arizona U.S.A.
The first question is - what do you want to do with it? There are all sorts of preamps for all sorts of purposes; without more information we can't help.

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 Post subject: Re: One Tube Preamp
PostPosted: Sep Wed 25, 2019 5:51 am 
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Location: Revere,Massachusetts U.S.A.
Hi Jim, I've built a couple of projects from "The Boys First Book Of Radio & Electronics", one is a tube radio and two amps.
I need more gain to power the amps so I'm asking for a preamp using similar parts & tubes.
They are very simple circuits using just a few parts so it's a nice breadboard project.
Thanks, Tony


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 Post subject: Re: One Tube Preamp
PostPosted: Sep Wed 25, 2019 3:49 pm 
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Location: S. Dartmouth MA 02748-1225 USA
In the back of most tube manuals are sample circuits, an older manual from the 30's should have an amp circuit.

Later amp designs will include filters to change the curve of the amp to make it more suited for phono record reproduction.

Choose a low gain amp tube so less likely to have a feedback problem.

Will the power source be with the amp or will the power be "stolen" from some other project like the power amp or the radio?

Have any idea what the input impedance required? High impedance for things like guitars pickups and phonos, low impedance for some radio outputs and "modern" devices.

Some building precautions will be needed for high impedance, very likely to pick-up hum.

No really a breadboard project, some sort of chassis to keep prying fingers off the plate voltage and hum at bay.

Regards, good learning experience,

GL

Chas

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 Post subject: Re: One Tube Preamp
PostPosted: Sep Wed 25, 2019 5:17 pm 
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Joined: Nov Mon 06, 2017 2:35 pm
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Location: Texas, U.S.A.
....
Referring to the circuit diagrams from the book, shown following:

I'd be inclined to use a high mu tube for the preamp, like 70 to 100. Would need to look for a suitable 1.4 v filament tube.
Transformer coupling to the preamp tube grid can be used as shown on the existing power amp. I would try increasing the resistance on the secondary of the audio transformers to 220 k, this should increase the amplification of the detector and preamp.
I'd look into cathode bias for the preamp tube, if the tube data indicates that additional bias is needed.
Shielded twisted pair from the detector output to the preamp, and from preamp to power amp.
The plate voltage for the preamp would need to pass through the primary of the audio transformer shown on the existing 1G4 amp. (this conductor is one of the wires in the shielded twisted pair from preamp to amp)

Back to the detector, if a regeneration control that does not reduce the detector plate voltage can be implemented, it would give more audio output from the receiver. Don't have time right now to post a diagram.


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Eric LaGess
WB5HDF


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1tubeRegen_Boys_1st.jpg
1tubeRegen_Boys_1st.jpg [ 89.54 KiB | Viewed 463 times ]
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Audio_amp_boys.jpg [ 72.14 KiB | Viewed 463 times ]
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 Post subject: Re: One Tube Preamp
PostPosted: Sep Wed 25, 2019 10:59 pm 
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Location: Tucson, Arizona U.S.A.
Are these the circuits you are using? If so, I would try changing the 500K pot in the receiver to something like 50K and connecting pin 3 of the tube to pin C of the coil as well as the arm of the pot. This should substantially increase the audio output. You may have to adjust the number of turns on the tickler coil.

Are you hoping for "room filling volume" from the amplifier? A 1G4 tube isn't capable of that although it should be audible. Be sure you are using a high impedance speaker or a low impedance speaker with an appropriate output transformer.

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 Post subject: Re: One Tube Preamp
PostPosted: Sep Thu 26, 2019 7:11 pm 
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Location: Texas, U.S.A.
...
Regeneration control hookup as suggested by Mr. Mueller for the A. Morgan detector. This hookup preserves the full B+ voltage and audio frequency current at the plate while providing a variable resistance across the tickler that can be varied from much greater resistance than the radio frequency inductive reactance of the tickler to much lower resistance than the RF inductive reactance of the tickler, thereby providing operator control of the RF current through the tickler.
Attachment:
Mueller_regen_control.jpg
Mueller_regen_control.jpg [ 34.63 KiB | Viewed 386 times ]


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Eric LaGess
WB5HDF


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 Post subject: Re: One Tube Preamp
PostPosted: Sep Fri 27, 2019 6:38 pm 
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Location: Texas, U.S.A.
....
It would probably be desirable, as a starting point in building the regeneration control circuit shown above, for the maximum regeneration control resistance to be around ten times the inductive reactance of the tickler at the highest operating frequency. The idea being to make most of the RF go through the tickler at the maximum resistance setting yet permit gradual reduction of current through the tickler with control rotation.

For example:

Typical medium wave (MW) broadcast band tickler inductance: 30 microhenrys (uH).

Reactance of 30 uH at 1700 kHz: 320 ohms

320 x 10 = 3200 ohms. So probably a 5 k ohm pot would be the closest commonly available.

Also the effect of the capacitance of the regeneration control variable resistor becomes more significant as the operating frequency is increased. I don't think it will be significant in the MW band.

Once the resistance setting of the control for the desired regeneration in the receiver is known, a portion of that total resistance could be made a fixed resistor and a smaller resistance used for the control; the idea being to give less critical control adjustment. Allowance would need to be made for resistance variations needed by different frequency range coils, if used.

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Eric LaGess
WB5HDF


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 Post subject: Re: One Tube Preamp
PostPosted: Oct Fri 04, 2019 11:33 pm 
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Joined: Nov Mon 06, 2017 2:35 pm
Posts: 90
Location: Texas, U.S.A.
....

It would be helpful if you could post the schematic diagram of your 6BJ6 - 6C5 amplifier hookup as you have it now, showing the present component values and B+ voltage and how it is interconnected to other equipment. Or write up the description of the circuit.

Thank you,
WB5HDF


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 Post subject: Re: One Tube Preamp
PostPosted: Oct Thu 10, 2019 3:48 am 
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Joined: Nov Mon 06, 2017 2:35 pm
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Location: Texas, U.S.A.
The following may be helpful in optimizing your 6BJ6 - 6C5 amplifier.

The overall voltage amplification or gain of two amplifiers cascaded (output signal of first amplifier is input signal to second amplifier) is the product of the two amplifiers voltage amplifications. For example if the first amplifier voltage gain is 10 and the second is 8, the total gain would be 10 x 8 = 80.

For a resistance coupled two stage amplifier such as described on pages 194 - 199 of The Boy's First Book of Radio and Electronics, the voltage gain of the first stage is found by dividing the signal voltage at the grid of the second tube by the input signal voltage at the input terminals to the amplifier. The second stage voltage gain is found by dividing the signal voltage at the output terminals of the amplifier by the signal voltage at the grid of the second tube. Then the overall voltage gain can be calculated by multiplying the two stage gains. Assuming your 6BJ6 - 6C5 circuit is similar, the amplification can be found the same way.

To determine the overall voltage amplification that the audio amplifier stages following the regenerative detector should provide, the following must be known:
1. Desired maximum output voltage from amplifier.
2. Desired radio frequency (RF) signal voltage input to detector for maximum amplifier output voltage.
3. Ratio of detector audio output voltage to RF input voltage.

Example:
28 V peak to peak (p-p) is chosen as the desired maximum output from the 6C5, and this output is to be attained with 50 microvolts p-p RF into the detector. 135 V B+ supply is chosen. The ratio of detector audio output voltage to RF input voltage (detection gain) of a typical Armstrong circuit triode regenerative detector is given by H. A. Robinson as 1,650 (H. A. Robinson, "Regenerative Detectors", QST, Feb. 1933).

50 uV x 1,650 = 82 millivolts p-p audio output from detector.

28 Vpp / 0.082 Vpp = 341, the desired overall voltage gain of the two-stage amplifier.

The maximum amplification factor of the 6C5 is 20. The actual amplification obtainable with the chosen 135 V B+ and typical plate load resistance will be 10 to 12, so 10 is chosen here.

341 / 10 = 34, the amplification that the 6BJ6 stage needs to provide.

---------------
Eric LaGess
WB5HDF


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 Post subject: Re: One Tube Preamp
PostPosted: Oct Tue 15, 2019 7:46 pm 
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Location: Vancouver Island, Canada
Right, that is a good explanation of the calculations, and if the example is close, makes sense Best to find a High Mu Triode for your preamp tube. Then you can reduce the gain with a voltage divider circuit, bleeding off some of the gain. A volume control pot works well as part of the divider.

But overall my approach would be to install a dual triode tube in your existing amp, or add a socket, and find a circuit already created for those tubes. But it is fun to be inventive, and so whatever floats your ground. :lol:

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