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Author Topic: 6bq7 tube mixer  (Read 5947 times)
9A5BDP
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Posts: 110




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« on: December 15, 2012, 01:15:30 PM »

Hi..

I want to try during this weekend mixer with 6BQ7 double triode tube. I have on my breadboard some carrier oscillator on 10MHz and audio oscillator. My intention is to generate DSB signal. I need advice regarding biasing of this tube and what to expect.
Anode voltage is approx 300Vdc.
Thanx in advance..

73!
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WB6BYU
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Posts: 13152




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« Reply #1 on: December 16, 2012, 07:27:39 AM »

I looked through some circuits for dual-triode mixers, and the common cathode-
coupled designs used about 100K of grid leak and 100 to 2K ohms of cathode
resistance.

Pulling up the databook, for a Class A amplifier the recommend 150V and 220 ohms
cathode resistor, plate current about 9mA (maximum is 20mA).

At 300V you'll need -5V on the grid to keep the anode current under 20mA.

HOWEVER, the maximum anode voltage rating is 250V, except it may be 300V
"during cutoff conditions, or when the two stages are used in a series
cascode arrangement."
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9A5BDP
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Posts: 110




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« Reply #2 on: December 17, 2012, 12:05:32 PM »

Hi there...

I managed to find in one RSGB publications note, originating from ARRL Smiley, for following circuit with 6BQ7:

- both catodes share same resistor of 1k
- grid leak resistors are 100k
- IF and BFO is conected to grids via 100pF caps without gridstopers
- one plate is coupled directly to plus from PSU and another have potentional divider in plate circuit (upper resistor is 47k and lower is 10k)
- mixer output is taken in midpoint of these resistors
- PSU voltage is around 250Vdc

What do you think regarding this circuit?
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G3RZP
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« Reply #3 on: December 18, 2012, 04:25:31 AM »

This is the Pullen mixer. Are you looking for DSB as in AM, or DSB with suppressed carrier?
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9A5BDP
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Posts: 110




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« Reply #4 on: December 19, 2012, 12:06:53 PM »

I want to make some mixer for suppressed carier DSB and to feed this signal into crystal lader filter, tell me is this pullen mixer on right path for my design or do I use something else (pentode mixers)?
And one more question regarding quartz crystal disipation or on better words power handling...I read somewhere that new crystals (for digital era) is not suitable enough for using in ladders for tube equipment beacuse of low pover/voltage handling...its this true?
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WB6BYU
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Posts: 13152




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« Reply #5 on: December 19, 2012, 12:47:19 PM »

For a DSB signal you probably want some sort of balanced mixer that will attenuate
the carrier.  The common triode mixers don't do that.  Usually such a mixer will have
a balance adjustment to null out the carrier.

I will have to go back and look through my circuits for a suitable design, but I can't
think of seeing very many of them over the years.  The best approach might be
a tube variant of the FET mixer that Kenwood used in the front ends of some of
their transceivers (TS-450, etc.), with two push-pull triodes having the carrier
applied in phase to both cathodes:  in the absence of modulation the carrier signal
is then out of phase on each anode.

You don't need a balanced mixer to generate DSB, but by reducing the carrier
signal out of the mixer you don't need as much carrier suppression from your crystal
filter.


With newer (smaller) crystals the limitation is signal levels:  assuming that you have
capacitive coupling so there is no B+ on the crystals it comes down to the level
of the generated signal.  While solid state circuits often have lower signal levels
than GLOW-FETs, that need not be the case.

Where crystal size and signal handling is particularly important is in self-excited
transmitters where oscillators may be designed to several watts of power.  The
crystal currents required to do that may damage many modern crystals.  Again,
that isn't so much a matter of the device type (you could design such an
oscillator using power FETs that runs 20W output and cracks crystals) but the
signal level in the circuit:  since solid state devices are cheap and use less
power, it makes more sense to use more of them and keep the oscillator at a
lower level, which places less stress on the crystal.

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9A5BDP
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« Reply #6 on: December 19, 2012, 01:03:54 PM »

Do you mean something like on following page (middle section)?

http://www.super-phantom.ru/arh/111-eng/108/
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WB6BYU
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« Reply #7 on: December 19, 2012, 02:06:54 PM »

Yes - figure 10.

These mixers will be single-balanced.  To balance the carrier you would either run the audio input
and RF output single ended and apply the carrier in push-pull, or vice versa.  That way the
carrier leakage cancels itself in the output.  Figure 10B isn't quite the circuit that I'd use - I'd
probably put push-pull audio on the grids and RF to the common cathode resistor, but I haven't
tried building one to see how it works.

The adjustment in the B+ lead to the anodes sets the carrier balance by changing the
relative dynamic characteristics of each triode.
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G3RZP
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« Reply #8 on: December 20, 2012, 02:57:54 AM »

Dual triode balanced mixers have some problems, especially with stability over time. One basic one is to feed the two grids with push - pull AF, and the two cathodes go to each end of a 1 k potentiometer. The two cathodes are connected via 1000pF capacitors to a low impedance RF source. The anodes are connected in push-pull to a tuned circuit, with the anode supply on the centre tap. Adjusting the cathode potentiometer nulls the carrier.

You need the old fifth edition of the RSGB Radio Communication Handbook.

Generally speaking, the 4 diode balanced modulator is much better, even with germanium diodes.
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9A5BDP
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« Reply #9 on: December 20, 2012, 12:30:52 PM »

I will try design with push pull AF source for a start. My goal is to design all tube based transceiver so no silicone or germanium crystals are allowed.
What actualy mean phrase : low impedance source..it's that in magnitude of few ohms or hundrets ohms or thousands?

In general my push pull generating device will be 6C4 tube with equal resistors in plate and cathode (phase splitter).

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G3RZP
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Posts: 4479




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« Reply #10 on: December 20, 2012, 12:47:02 PM »

You should look at about 500 to 1 kohm source impedance at the most, preferably less. Remember that taking anti-phase signals from cathode and anode does NOT give equal source impedances.

Output impedance of a Class A cathode follower is 1/gm

73

Peter G3RZP
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9A5BDP
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Posts: 110




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« Reply #11 on: December 20, 2012, 01:15:42 PM »

If 6C4 is not suitable, I must check specs, one more tube laying on my workbench..5687WB
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WB6BYU
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Posts: 13152




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« Reply #12 on: December 20, 2012, 08:52:17 PM »

An audio transformer would give you push-pull drive.

I think G3RZP's comment about the phase splitter was that the simplistic design
might not have good enough balance to get good carrier suppression.  (This is because,
even with equal anode and cathode resistors, the effective load impedances
in the two sides will be different due to the negative feedback across the cathode
resistor.)  But you can certainly try it and see - it will depend on the adjustment
range of your balance control, and may be acceptable if your valve doesn't draw
grid current.

I suspect you'll need to do a bit of experimenting - even the old reference books
where I've found these circuits (as well as the reference you found) recommend
the use of copper oxide rectifiers instead as an easier method, though you might
be able to get by with some 6H6s instead.


You can use a center-tapped modulation transformer in the anode lead, such that
the B+ is applied to the center tap, each end of the winding goes to one of the
anodes, and the modulation is fed into a separate winding.  then take the RF off
of the anodes in push-pull and apply the carrier to the grids in parallel.

At least one of the old SSB transceivers of the late 1960's (?) included a carrier
balance control on the front panel, a likely indicator that the balance wasn't very
stable with temperature and/or voltage.

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G3RZP
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Posts: 4479




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« Reply #13 on: December 21, 2012, 05:25:01 AM »

Some of the late 50's SSB transmitters used phasing generation, and had two carrier balance controls on the front panel!

An AF transformer with a centre tapped secondary (typically a small driver transformer) would be a good approach for the push - pull drive. You certainly want to avoid grid current in the balanced modulator, too. Another method is to use two triodes for the phase inverter: use a common cathode resistor of about 1 or 2 kohm, equal resistors from each grid to ground, bypass one grid to ground with a 0.1 microfarad capacitor, and use equal anode resistors - say 10k ohm, and 12AT or AU or even AX7 for the twin triode.

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WB6BYU
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Posts: 13152




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« Reply #14 on: December 21, 2012, 08:28:43 AM »

Talking with another ham on the repeater on the morning commute, he said that
the Collins KWS-1 used a triode balanced mixer - you might look up the schematic
on line as an example.

He also remembered converting an ARC-5 transmitter to DSB.  They have  pair
of 1625 tubes in parallel in the output:  he screen modulated them in a similar
manner as would be done for AM, but fed the RF to the two grids in push-pull
rather than parallel.  He did, however, recommend using a buffer stage between
the VFO and the output:  the ARC-5 has the VFO driving the final directly and
he had some FM on his DSB.  But this approach would eliminate the need for
balanced audio drive.


[edited to add:] And here is a link to Chapter 2 of the Collins book on SSB techniques,
which discusses balanced modulators.

http://collinsradio.org/archives/ssb_fundamentals/Fundamentals_of_Single_Side_Band_Chap_02.pdf

The same web site (the Collins Collectors Association) should have a schematic
for the KWS-1.
« Last Edit: December 21, 2012, 09:21:34 AM by WB6BYU » Logged
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