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Old 7th Nov 2019, 12:57 pm   #21
JohanBee
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Default Re: 6BE6/EK90 vs 6CS6/EH90 as frequency mixer in Trio 9R59DE ?

Edit: Ik is the cathode current Ic (katod in swedish) in the post above.

Last edited by JohanBee; 7th Nov 2019 at 1:09 pm.
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Old 7th Nov 2019, 8:29 pm   #22
turretslug
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Default Re: 6BE6/EK90 vs 6CS6/EH90 as frequency mixer in Trio 9R59DE ?

No problem, we're used to Ik for cathode current here, I suspect that using 'k' for cathode probably goes as far back as 19th century physics research or something like that. It also saves confusion with Ic for collector current!

I think that that extra coil associated with L112 is a booster coil, adding a bit of an extra kick to the circuit to maintain oscillation- it's not unusual to find that local oscillators struggle a bit on the highest frequency band as L/C ratio deteriorates and losses increase, especially when there is a wide tuning span as here. If a valve is tired, it'll often be on this band where it stops oscillating first as a result. Even when everything is in good order, the oscillation amplitude may be lowest here also. They may have left the extra coil in circuit just because they could get away with it and save a switch contact.
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Old 9th Nov 2019, 10:19 pm   #23
Synchrodyne
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Default Re: 6BE6/EK90 vs 6CS6/EH90 as frequency mixer in Trio 9R59DE ?

Quote:
Originally Posted by JohanBee View Post
The 3pcs 6BA6 can also be swapped to 6BJ6, to save another 450mA heater current.......(for a lamp to the S-meter).....?
Use of the 6BJ6 in place of the 6BA6 (with appropriate base wiring changes) will certainly reduce total heater current, but it might also reduce the large signal handling capacity of the receiver.

The 6BJ6 was intended both to reasonably replicate 6BA6 general performance at lower heater consumption, to suit mobile applications, and to provide improved VHF performance to better suit it for FM and VHF mobile applications.

The trade-off was a slightly lower transconductance, and, with a lower anode current, reduced signal handling capability. The latter was mentioned in RCA Application Note AN-127, available at: http://www.one-electron.com/Archives...-AppNotes.html.

The 6.3-volt, 150 mA heater made the 6BJ6 suitable for use in both AC and AC-DC receivers. In the latter case, its 6.3-volt heater (as compared with the 12.6 volts of the 12BA6) made it advantageous for use in American AC-DC FM-AM receivers where the total heater string voltage was limited to 117 volts, but the valve count was reasonably high.

The improved VHF performance was obtained largely by reducing the mutual inductance between the control and suppressor grid leads-out by changing the pinout arrangement. The mutual inductance provided a feedback path including the suppressor grid-to-anode capacitance. This change resulted in something like a halving of the input conductance at 100 MHz. This VHF capability was obtained whilst retaining its suitability for narrow-band IF applications. There was the same relationship between the sharp cutoff counterparts 6BH6 and 6AU6. (See “Service” magazine 1948 August p.34ff for background; available at: https://www.americanradiohistory.com...e_Magazine.htm. RCA AN-139 also provides some comparative numbers.)

Re pentagrids and the respective uses of g1 and g3, a further delve shows that the situation is quite complex, with for example the 6BE6 and 6BA7 sometime used the “wrong way round”. But that is getting off-topic…


Cheers,
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Old 9th Nov 2019, 11:25 pm   #24
G3PIJpeter
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Default Re: 6BE6/EK90 vs 6CS6/EH90 as frequency mixer in Trio 9R59DE ?

What about the Pullen mixer? Circuit at https://www.w7ekb.com/glowbugs/rx/Pu...rom%20RSGB.pdf and discussion at https://www.w7ekb.com/glowbugs/rx/Pu...%20Circuit.pdf The 12BZ7 is equivalent to the ECC84.

This double-triode circuit was the approved direct replacement for the 6BE6 in the Collins 75A-4 receiver. Same B9A base and a simple re-wire plus voltage check.

- Peter G3PIJ
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Old 10th Nov 2019, 2:54 pm   #25
G6Tanuki
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Default Re: 6BE6/EK90 vs 6CS6/EH90 as frequency mixer in Trio 9R59DE ?

That "Pullen" mixer circuit is familiar - I used it in my first 144MHz converter, downconverting to 14-16MHz. It worked well, and was a lot less-bothered by Band-II broadcast-breakthrough than a lot of other designs.

(Then I discovered hot-carrier-diode double-balanced-mixers which worked just fine without a RF-amp. But that's a story for another day)
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Old 10th Nov 2019, 3:09 pm   #26
G8HQP Dave
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Default Re: 6BE6/EK90 vs 6CS6/EH90 as frequency mixer in Trio 9R59DE ?

The Pullen mixer is a strange circuit. Take an LTP (which in itself would make an awful mixer); degrade it by using a short tail; unbalance it by starving one side of supply rail voltage, hence introducing second-order distortion (which is exactly what a mixer needs). Yet it seems to work OK, and may be better than some valves designed to be mixers!

I have tried to understand Pullen's own explanation of how it works but he has such a strange way of understanding circuits and writing about them that I gave up.
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Old 10th Nov 2019, 3:13 pm   #27
turretslug
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Default Re: 6BE6/EK90 vs 6CS6/EH90 as frequency mixer in Trio 9R59DE ?

Quote:
Originally Posted by Synchrodyne View Post
The improved VHF performance was obtained largely by reducing the mutual inductance between the control and suppressor grid leads-out by changing the pinout arrangement. The mutual inductance provided a feedback path including the suppressor grid-to-anode capacitance. This change resulted in something like a halving of the input conductance at 100 MHz.
That's an interesting re-visit of something apparently basic, and an excellent example of how some of the most important parts of a circuit can be the ones that don't appear on the circuit diagram! I gather that one of the reasons behind separating suppressor grid and cathode connection in many RF-specific pentodes was to prevent the combination of anode-suppressor grid capacitance and cathode connection inductance compromising control grid conductance.
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Old 11th Nov 2019, 4:59 pm   #28
JohanBee
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Default Re: 6BE6/EK90 vs 6CS6/EH90 as frequency mixer in Trio 9R59DE ?

Back to the Trio 9R59DE. I’ve been working on the oscillator to the mixer on the 2 upper bands C&D, as specially the D-band 10,5-30MHz was very veak, gave parasitics and stopped completely to oscillate at 15Mhz & up.
I took out the L112 coil, found and cleaned away a remaining wax chunk on the back of the coil, took off the main windings, as the 0,4mm wire was very brittle (was it copper?) and the green isolation came off.
Found the temperature compensating C104 inside the bottom of the coil and left it there.
Cleaned the coil former more from the rest of the Q-eating wax and rewound the 4 3/4 turns main winding with 0,5mm wire. Seen to the left on the first picture.

Took also out the L111 coil for inspection and to be able to clean more wax hidden on the back side. The wire was OK so I left it. It is on the right on the firt picture.

At the same time, with the coils out of the way, I demounted the L-shaped bracket, that holds the 2 tubular, flimsy polyethene trim-capacitors CT8 & CT9.
Seen on the second picture with the new ceramic trimmers in the background.
Lowest quality I’ve ever seen and I can’t believe the designer wanted these there. I think only the beancounters are responsible to this junk. They’re probably part of the instability problems at higher frequencies.
So I soldered a copper washer to make the new trim-caps to fit and screwed in a couple of 2-15pF ceramic tubular types, that I think the designer more opted for.
In the middle of the first picture.
Also seen on that picture is the low capacitance koax line to the antenna trimmer. Grounded only at the antenna coil side. Stops the oscillator to pull when trimming the antenna cap.
Also in the bottom of that pic is the lose wires (the red B+ from the OA2 and the shielded from RF-gain) close to the oscillator tubes secured and glued to the chassie.
The green systoflex covered wires around the band switch, in the middle of pic1 are the new 1mm solid core wires to the variable capacitors

After assembling back everything it is now oscillating good at all bands and even the D-band is rock stable and smooth after warm upp and only fluctuates a couple hundred Hz upp & down, due to mains variations (yes have the OA2 installed)
Had it spot on at 17780 +-200Hz (BBC at Ascension?) for an hour without drift in that test. Picture No3.

Yes I’ve mounted an (from eBay) outboard crystal controlled frequency counter with an IF 455KHz offset for monitoring and alignments. A half wave rectifier from the 6,3V heater voltage gives 8VDC, which is sufficient to power it.
Use the other half of the oscillator tube as a cathode follower for the signal to it.
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Old 11th Nov 2019, 9:10 pm   #29
turretslug
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Default Re: 6BE6/EK90 vs 6CS6/EH90 as frequency mixer in Trio 9R59DE ?

That's really encouraging- it goes to show that some time, effort and small expenditure really can make a difference. I don't think it's a bad thing to make competent improvements to old sets like this if it makes them more usable and effective- not least because it means that more are likely to be kept for longer and are less likely to be thrown away as no good, or stuck at the back of a shed to rot.

In the UK and USA at least, there were many excellent communications receivers from WW2 that appeared on the surplus market for a long time after the war for a fraction of what they cost the taxpayer and they still continued to be effective for decades- I'm sure that this must have made it difficult for makers of new receivers like this to make something that people would buy without cutting quite a few corners.
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Old 15th Nov 2019, 1:10 pm   #30
JohanBee
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Default Re: 6BE6/EK90 vs 6CS6/EH90 as frequency mixer in Trio 9R59DE ?

Thanks, my 9R59DE is now working very good when I now fixed the issues originating from the oscillator, that caused many problems.
I’ve now moved it from my workbench to my shack and use the following tubes:
RF=6BA6/EF93, OSC=12AT7/ECC81, MIXER=6CS6/EH90, 1st&2nd IF=6BA6/EF93, PRO.DET=6BE6, BFO+1st LF=12AT7/ECC81, LF=EL95.

Observe, the heater wires must be reorginized (also see below) when using the 12AT7/ECC81 instead of 6AQ8/ECC85, which fits better in vintage FM-receiver inputs in the use it was originally designed for!
There is a developing shortage of them, so stock up if you use them regulary in restorations. 6AQ8 is a semi-remote cut-off type and inferior to use in audio. Otherwise, if you really need the higher S&Mu of the 6AQ8/ECC85 in f.ex. a cathode follower, use the 12AT7/ECC81 (despite also semi-remote cut-off) instead, as they sounds better and still are plentyful out there.

To conclude, the real improvements of stability and functions to use the vintage 9R59DE(DS) as a short-wave receiver again, was to:
Install an OA2/C150 stabilizer tube, if not already there.
Move the B+ to the mixer tube resistor R7 & R8 to the stabilized OA2 voltage.
Change the wires from the S3f switch to the oscillator sections of the variable capacitors from the original stranded yellow wires to 1mm solid core in isolation tubes (systoflex)
Fix to the chassie with glue, the lose red B+ from the OA2 (I put it in a systoflex tube) and shielded BFO wires passing the oscillator
Clean throughfully all remains of all Q-eating wax specially on the C&D-band L111&L112 oscillator coils. Use carefully a small heat-gun and f.ex.cotton pins to soak up the molten wax. Don’t burn the coil wires!
Change the flimpsy polyethene isolated trimm-capacitors CT8&CT9 to ceramic types ~2-15pF
Change the oscillator tube socket to a ceramic type with shield base for J-slot shield (The original is from ”micanol” and changes residual capacitances when heating) and fit a shield.
Change the oscillator tube to 12AT7/ECC81 (lower total tube heating) With 12AT7, if you dont use the other half of the tube, you can connect only the heater wire to the oscillator section cathode to decrease the total heating of the tube even more (In original the 6AQ8 runs really hot in the thermos style mirror-shining shield)

There are still some issues with the BFO and product detector, with leaking, pulling and syncronization of the BFO to the IF frequency through the space charge from the virtual cathode to grid3, but I want to take it up in a later thread, or if someone knows more about it, you are wellcome to make a new thread about it.
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Old 16th Nov 2019, 9:05 pm   #31
JohanBee
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Default Re: 6BE6/EK90 vs 6CS6/EH90 as frequency mixer in Trio 9R59DE ?

I also want to show some photos of the now finnished Trio 9R69D, restored to better than new specifications. Only 1 electrolytic capacitor was bad, so all capacitors and components seen on top of the PC-boards are original.
The large grey electrolytic 3x40uF in the background was also found to be ok and left original.

The only difference you can see with the cover off, is the ventilated shielding cans on 2 tubes and cooling shielding cans on the oscillator and on the stabilizer tube OA2 behind the main tuning capacitor.
That tube was also moved to the empty hole for the calibrator tube socket, further away from the main tuning capacitor, not to heat it up.

Also one knob on the BFO capacitor was missing, but I found a similar to the antenna trimmer knob and it fits quite good and looks even better than the bigger original knob.

On the last picture is on the restored and calibrated military signal generator, built in Sweden by CEMEK in1958, which was used for all alignments of the Trio.
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