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Old 22nd Nov 2017, 9:39 pm   #41
Dave-N2DS
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Default Re: Most Minimal Superhet

Here is a one tube superhet I built a while back.
http://makearadio.com/tube/6ba7-superhet.php

73, Dave, N2DS
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Old 22nd Nov 2017, 10:25 pm   #42
Argus25
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Default Re: Most Minimal Superhet

Quote:
Originally Posted by G6Tanuki View Post

The "Conversion conductance" of the ECH81 is also better than the earlier Octal valves [think of 'conversion conductance' as essentially the gain of the mixer, though it's really rather difficult to quantify gain in a mixer]
Yes this is the important parameter for the converter, this is why an ECH35 is superior to a 6k8.
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Old 23rd Nov 2017, 9:54 am   #43
kalee20
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Default Re: Most Minimal Superhet

Quote:
Originally Posted by Neil Purling View Post
I have made several attempts at making a regenerative TRF. RF gain seems like the big problem & them how would you keep the RF & detector stage tracking perfectly?
You can't - but you can get pretty good.

At low frequencies, one of the circuits will be at a slightly higher frequency than the other, so put a core in it; add a turn; until inductance is raised to bring it in-line with the other. Or, separate the turns of the other, until its frequency is raised.

At high frequencies, one of the frequency will be slightly higher than the other. So, add a trimmer capacitor across it to bring the frequency down. This will mess up the low-frequency end, but repeating a few times will converge.

Don't adjust trimmers at the low-frequency end, because their effect is heavily diluted by the full capacitance of the variable tuning capacitor, so you'll have to make a big adjustment to have any effect, which'll screw things completely at the HF end!

It is certainly possible to be 'good enough', let's face it, if superhets can be made to track sufficiently well, when the two tuned circuits are at DIFFERENT frequencies, a TRF with them at the SAME frequency should be a doddle!
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Old 23rd Nov 2017, 9:59 am   #44
kalee20
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Default Re: Most Minimal Superhet

Quote:
Originally Posted by Dave-N2DS View Post
Here is a one tube superhet I built a while back.
http://makearadio.com/tube/6ba7-superhet.php
Wow. Just looked. What a thing of beauty!

Could you have reflexed the AF, back into the frequency changer, and got a bit of AF amplification to drive the headphone? Or did you try it and just found it's too prone to instability?
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Old 23rd Nov 2017, 10:41 am   #45
Neil Purling
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Default Re: Most Minimal Superhet

The ECH81 I have is a Russian clone.
I am still waiting to receive components & am like a cat on a hot tin roof.
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Old 23rd Nov 2017, 12:01 pm   #46
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Default Re: Most Minimal Superhet

Quote:
Originally Posted by Dave-N2DS View Post
Here is a one tube superhet I built a while back.
http://makearadio.com/tube/6ba7-superhet.php
That is a lovely project, and not just one valve but one electron stream! With that simplicity, perhaps a high-slope output pentode would be justified for driving a loudspeaker.. I can imagine a fixed-tuned 198kHz set with a good big frame aerial, 85 or 125kHz IFT and 6BA7/6CH6 line-up, or maybe a B7g version with 6BE6/6BJ5. (Some will object to the 6BE6 on the grounds of lower conversion conductance and higher noise, but it's only a strong signal set and a bit of fun...)
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Old 23rd Nov 2017, 4:11 pm   #47
Neil Purling
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Default Re: Most Minimal Superhet

Several packets of resistors & capacitors came just now through the letterbox.
I saw the postie still had his summer cargo pants on and on a day with gusts seemingly blowing from the open door of a fridge. Perhaps there's a contest at the depot over who is the hardest?
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Old 24th Nov 2017, 2:09 am   #48
Dave-N2DS
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Default Re: Most Minimal Superhet

Here is a real 1 valve superhet that a fellow in Canada built a few years ago.
http://electronbunker.ca/eb/OneTubeSuper.html
I don't think it gets better than that. Everything reflexed.
Dave
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Old 24th Nov 2017, 3:10 am   #49
kalee20
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Default Re: Most Minimal Superhet

Wow!!!!

RF Signal through valve... IF appears in push-pull at two anodes. Filtered and fed back to valve as a common-mode signal... Amplified common-mode output signal extracted, from letter and detected... AF fed back to valve, amplified a third time, drives phones or speaker!

So the valve amplifies at 3 different frequencies!
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Old 25th Nov 2017, 6:51 pm   #50
Neil Purling
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Default Re: Most Minimal Superhet

I have been carrying on with mine, robbing parts from the old kit radio. I removed the tuning capacitor & the drive pulleys.
These have now been fitted on the new chassis carefully aligned to use the reduction afforded by the spindle and pulley. I don't know the precise ratio.

Is the accepted colour for RF and oscillator wires usually red & blue? Those are the colours Denco used anyway.

I ought to complete the detector first and put 4**Khz into the primary of the only IFT. If I get a 1Khz tone from the speaker then I am on the right track.
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