NE602 radio in sim

In the sim the RF is 8 uv pk-pk (output from the transformer) and the LO input is .3 volt. Q1 is AGC. At .6 volt it reduces the output. At 1 volt it kills the output.
I'm looking at the parts bin and see a core and a NE602. Why not?

DBM and Dual Gate Mixer

Refer to the 40/80

We built the 40/80 from the ground up with a little discussion about the components as we went. The dual gate mixer and DBM are both used in the 40/80.

We used bought transformers and wound our own. The binocular was the best. It is something about the signal isolation with the binocular that made them the winner. (this was our perception.)

 The center tap on the transformer connected to the LO is grounded. If you take the signal from it you will get feed thru of the LO. The binocular cores helped with the signal isolation too.

 The first mixer uses the dual gate. So you have an example of the DBM and dual gate mixer in the 40/80. If you follow the link at the top of the page it will take you back to the time the project was on going. From that point you can back up thru the development.

One "TV" tube radio

Look at pages 113-122. The one compactron radio using what could be called a "TV" tube.

projects book

Miller effect and gain band width

The voltage multiplier will be used to illustrate the circuit reaction to component changes.

I added a 1p capacitor to see the frequency response. You can see it falls off on the high end.

I changed it to 10p and you can see it falls off faster.

The 20p falls off even sooner. From this we can see the capacitance is our enemy when trying to get high frequency response. Our circuit will have base collector capacitance and it will limit out high frequency. What can we do about it?

A circuit such as this can be resolved to a single vector at a specific angle. In simple terms Xc and Xl cancel each other. One will dominate. So What if we put a coil in our circuit?

 Look at the effect the coil had.

As you can see the coil helped. What else can we do?

I changed the frequency to show it is a frequency response in play.

I reduced the input Z to show the effect.

I varied the gain to show the effect is at a certain frequency. At low frequencies the gain effected the output but a higher frequencies the gain fell.
If you followed this you should see the gain effects the roll off frequency and the input Z effects the roll off  frequency. To get higher gain at high frequencies we need less capacitance. Placing a coil in the circuit can help with this. Using a lower Z will also improve the high frequency response.

Remember the capacitance will be there. You may find a better transistor for high frequencies. Look at the Ft.

A lower Z circuit will have better frequency response. Some RF designers use 50 Ohms for their I/O for this reason. The low Z circuit will have a higher current draw. For this reason a 5 volt amp may have lower power consumption than a 12 volt amp and produce the same gain. You can use this calculator to see the cut off frequency for different R C values.

1/(2*pi*r*c) calculator

Current mirror - observations and final design- ?

I put the current mirror on a board and it does sing. (to well actually)

The test board worked so well I added the jacks, switch and a volume pot.

The problem is it is to sensitive. I put a pair of back to back diodes across the input to limit the drive. With a wire clipped to the input it sounds like a radio between stations. With a tank on the input I could tune it and select a station from the mix. We have a local low power day time station about 12 miles from us. This mourning I turned the amp on and could hear it. Low in the background but  there. With a test lead clipped to the input it is an easy listening level. So it's back to the drawing board. The simplest way to reduce gain is to reduce stages.

This is the amp as built. With the 2.2K resistor in series with the earbud it is still to high gain.

Here is the final version. I bypassed the middle stage. I have two resistor representing the earbuds. The red graph is one and the yellow is across both. Much better but it could still use a gain control.

With a 10K pot adjusted from 100 Ohm to 10K Ohm in 100 Ohm steps, this is what the output looks like.
So I replace the gate resistor for the output fet with a 10k pot and bypass the second stage.

I think a new build is in order. I can use the one I have as a signal tracer. I need to make a probe for it. Another project to add to the list ;).

*** I did the new build with two stages and I'm using 8 Ohm headsets. I connected a DBM and picked up AM1160. It is a low power (1 KW) in Dallas Texas. That's about 600 miles.

Tramsmitting Tube Data from RCA

RCA transmitting tubes

A couple of modifications to the current mirror

I put the amp on a board last night and it does sing. It needs an volume control and jacks for the I/O. It was a little unstable, when I clipped a jumper across the input it oscillated. So a closer look at the circuit.

There is current in the battery. This is causing inter-stage modulation.

A little more filtering and the battery current is steady. If you want a high Z output this is a good time to address that.

When adding the filter on J3's drain you can use it to feed a high Z headset. I'm showing a dual output. In this sim we have a headphone on the source and drain of J3. So you can connect it to your favorite crystal set and share the listening experience with a friend.
If you don't have the 2SK2539 you can build the circuit with any Jfet. The biasing may require some adjustment. I substituted J112s and it still function but at a lower gain.
Look at the first set of curves on the last post. You would need a 3.3K Ohm or 4.7K Ohm resistor with that Jfet to get 1 ma. It would function if built using the 1K Ohm but would have a very high current drain on the battery.