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Espey AM/FM radio (pentagrid mixer, etc.)
Espey AM/FM radio (pentagrid mixer, etc.)
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Old 14th December 2010, 05:39 PM   #11
DF96 is offline DF96  England
diyAudio Member
Join Date: May 2007
Pentagrid mixers are not a good idea at VHF because they are too noisy. This means you need a high gain pentode as an RF amp, but you still end up with more noise and inferior signal handling. Better to use a triode or pentode mixer, with cascode or grounded grid RF stage.
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Old 14th December 2010, 06:41 PM   #12
Bobby Dipole is offline Bobby Dipole  United States
diyAudio Member
Join Date: Oct 2010
I have several Pilot T-601 FM tuners that use the 6BE6 as a complete converter (no separate LO tube). This tuner came out in 1946, one of the first to tune the modern 88-108MHz FM band. I find that only one out of four NOS 6BE6 tubes will oscillate properly at these frequencies. Most will either take off at UHF, or die out half way up the band. I suspect that Pilot either sourced specially spec'd 6BE6s, or cherry picked them in the plant. But, with the right tube, this little tuner is stable and sounds quit nice.

Bobby Dipole
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Old 15th December 2010, 12:59 AM   #13
7n7is is offline 7n7is
Join Date: Mar 2006
Location: VA
You can download information about the Pilot T-601 here: www.pilotuner.com
It has a 6BA6 RF amp and 6BE6 mixer/LO. The Espey 200 has a separate LO.

One's for sale on Ebay
Pilotuner FM radio pilot tuner vintage tube with manual - eBay (item 130465389808 end time Dec-19-10 19:07:34 PST)
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Old 6th March 2011, 07:46 PM   #14
7n7is is offline 7n7is
Join Date: Mar 2006
Location: VA
19 kHz limiter for stereo demodulator

I got the idea of putting a 19 kHz limiter in my Pilot 100 stereo demodulator from these three patents.

US 3235663 Fm stereo multiplex receiver having limiting means in the pilot channel

US 3287501 Multiplex detector circuit
clipper 35

US 3270138 F. M. stereophonic receiver having noise prevention means in the pilot circuit

The simplest modification seemed to be to use antiparallel diodes across the signal path to clip positive and negative peaks outside the +/- 0.6 volt range (for two silicon diodes). I put the limiter at point A in the schematic http://members.shaw.ca/pacifictv5/pilot100.jpg .
The limiter includes a 10k resistor between tank circuit A1 and the 470k resistor with the antiparallel diodes connected between ground and the connection point between the 10k and 470k resistors. I put a switch in the circuit to switch the diodes in and out of the circuit to compare the effect with and without the limiter. WMUC (88.1 MHz) broadcasts with 10 watts of power and when the Pilot 100 is set to stereo, usually a grumbling splatter sound can be heard during silent passages which seems to be from interference from nearby stations, like NPR on 88.5 MHz and probably other stations. Sometimes when WMUC is off the air I can hear a lot of noise at 88.1 MHz which sounds like other stations bleeding over into that frequency, as well as another distant station on 88.1 MHz. Switching the diodes into the circuit essentially causes the grumbling splatter sound to disappear, but some noise can still be heard at a lower level during silent passages. Its a distinct improvement. Before this, I put a 500k pot having a switch in the Pilot 100 between the slide switch stereo outputs as a blend control which can be switched on and off and adjusted between stereo and mono. Blending the left and right outputs tends to reduce noise since the stereo noise is out of phase in both channels.

Here's some other examples I found which use antiparallel diodes for limiting signals.

US 3250919 Amplitude limiter using tunnel diodes

antiparallel diodes 32 (fig. 2)

US 3102991 Sonar equipment for single-transducer operation

US 4357690 Switch circuit for exciting ultrasonic transducer elements

Popular Electronics Magazine
scroll down to and click link to the M/M/M Instrument Amplifier

Google Patents

Attached Images
File Type: jpg anti-parallel-diode-mod400.jpg (15.7 KB, 45 views)
File Type: jpg Pilot100ModOpen.jpg (121.1 KB, 45 views)
File Type: jpg Pilot100ModSide1.jpg (103.2 KB, 45 views)

Last edited by 7n7is; 6th March 2011 at 08:06 PM.
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Old 7th March 2011, 04:25 PM   #15
7n7is is offline 7n7is
Join Date: Mar 2006
Location: VA
If you scroll down on this page about the Dyna FM-3, you can find a link to the manual.
On the 5th page of the manual under "The Multiplex Inegrator" it says the triode half of the 6BL8 doubles the 19 kHz signal and also acts as a limiter, holding a constant 38 kHz level to reduce noise and preserve stereo separation. There doesn't seem to be a 38 kHz oscillator in the Dyna FM-3. It generates the 38 kHz signal from the 19 kHz signal after the FM signal is demodulated.
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Old 25th March 2011, 09:31 PM   #16
7n7is is offline 7n7is
Join Date: Mar 2006
Location: VA
I put a dynamic limiter in the Espey receiver with amazing results, ie. lower noise and better stereo separation. Details are at the bottom of this page.
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