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Author Topic: Boat anchor "preselector"  (Read 1017 times)
KD0ZGW
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« on: August 08, 2019, 07:54:55 PM »

what is the function of the "preselector" on the tube type rigs?

thx
KD0ZGW
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HAMHOCK75
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« Reply #1 on: August 08, 2019, 11:00:17 PM »

Image rejection. Those rigs often had first IF's in the 3-6 MHz range. For example, if the first IF = 4 MHz and you are receiving a signal at 14 MHz, then the first LO could be at 10 or 18 MHz. Assuming 18 MHz, then 18-14 MHz = 4 MHz but the problem is that 22 -18 MHz also = 4 MHz so a 14 and 22 MHz signal would both fall in the 4 MHz IF. The preselector is usually an adjustable bandpass filter to eliminate the image at 22 MHz.

Modern rigs do it differently. Many have a first IF up around 50 MHz. Assuming 50 MHz as a first If, the LO frequency usually chosen to receive 14 MHz would be 64 MHz ( 64 -14 = 50 MHz ). The image would be at 114 MHz ( 114 - 64 = 50 MHz ), so they low pass filter below the first IF ( let's say at 30 MHz ) which eliminates the image and also prevents leakage of the 64 MHz LO out the front end.
« Last Edit: August 08, 2019, 11:11:16 PM by HAMHOCK75 » Logged
VK6HP
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Posts: 552




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« Reply #2 on: August 09, 2019, 01:17:55 AM »

To add to HH75's note, if the radio is a transceiver the same control, and the same set of resonators (inductors + capacitors), peaks the grid drive to the tube PA.  The control could be labelled "pre-selector", "drive", "exciter" or similar.
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W1BR
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« Reply #3 on: August 09, 2019, 05:46:06 AM »

It also reduces the likelihood of spurious receiver responses from strong signals that fall outside of the preselector's peak.  IMD, overload, etc.

The preselector can also provide impedance matching between a low impedance antenna system to the high impedance of a vacuum tube grid.  That is how most solid state rigs with low impedance signal paths avoid needing one, they use 50 ohm band pass filters.

Pete
« Last Edit: August 09, 2019, 05:49:06 AM by W1BR » Logged
KM1H
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Posts: 5541




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« Reply #4 on: August 09, 2019, 07:31:15 AM »

Are you talking about the old single conversion 455 kHz IF  RECEIVERS?  If so the main function is to overcome the converter/mixer noise and some added image rejection also resulted but it required two stages to become tolerable on the higher bands above ~ 15 mc

In a tube transceiver which always had either a high first IF followed by one at 455 kHz for  selective filters or just that single high IF when quality crystal filers arrived it tuned the receivers input as well as the TX final stage. Image rejection was no longer an issue as it was so far removed from the operating frequency. With only one RF RX amp using more modern tubes than prior the sensitivity was better and the overload problems reduced.

Carl
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G3RZP
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« Reply #5 on: August 09, 2019, 09:35:48 AM »

Very old preselectors, such as the pre WW2 RME one also had useful RF gain and lower noise. Another commercial one post war was from Millen, which was pretty much a commercialised version of the 'R9er' in GE Ham News  Vol 1 Nr 4 of November/December 1946 and Vol 2 Nr 1 of Jan/Feb 1947 which are at http://n4trb.com/AmateurRadio/GE_HamNews/ge_ham_news.htm. The 'R9er'design used a 6AK5 low noise tube.
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KM1H
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« Reply #6 on: August 09, 2019, 11:08:38 AM »

The prewar RME DB-20 with a pair of 6K7 noise generators for tubes was a laugh and was built to enable their POS RME 69 with a single 6D6 RF stage to hear more noise and not signal! It did reduce images.

The postwar RME DB-22A was much better with 6BA6's but still did more to make the RX S Meter bounce than improve SNR where needed on the higher bands since the full open gain was ~30 dB with 0 dB minimum. They also put the gain pot in the first stage. Many owners technical (plus RME's engineering) ability was not high even then but those small details were not on the FCC written exams Roll Eyes

I modified my DB-22A to a single 6GM6 and continued with all 3 tuned circuits. With gain now adjustable up to ~ 15dB at a very low noise figure it now became a real nice addition to my vintage station for 10-15 and sometimes 20M and all in between. It even made a SX-28 useable on 10M  Grin

Carl
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G3RZP
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« Reply #7 on: August 10, 2019, 08:15:08 AM »

There were never very many SX28s over here - a few came with US military in WW2. There were much greater numbers of HROs and AR88s. But in 1947/8, I'm told that SX28s  were more than adequate for 10 metres, and were probably so in 1957/8 when 10 metres often didn't close at night at all...Even the HRO with 6D6s in the RF stages  had stuff rolling in all night then.

The great days when we had sunspots - where are the Palos Verdes sundancers now?
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KM1H
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« Reply #8 on: August 10, 2019, 12:35:07 PM »

During Cycle 19 my first factory radio was a used SX-25 that was a complete POS on 15, actually the the whole band 4.
I had my dad return that since he insisted it had to be good if it was a Hallicrapers, and used MY money with no further parental interference bought a late version used HQ-129X. With a lot of listening to my HS Science/Math teacher, Brother Patrick Dowd, W2GK (SK in 2014 and famous among vacuum tube historians) I started with some tube swaps and it became excellent on 15 and 10 M and I wound up with over 90 DXCC as a Novice. The school club station, W2ZLK, had a HQ-129X so I was already hooked before the SX-25.

Dad could barely wire a socket, he was a textile salesman in NYC, but went ahead soon after and bought his first TV, a Halli POS of course. After several (many) service calls he had the store take it back and splurged on a Dumont floor console model. It never burped and was replaced by a Zenith SS color set in the 70's which was still working after he died and mom brought it with her when she moved in with us in 1998 and passed in 2002 at 93. It still works using a converter box for digital and is in my BR as a reminder !!

With a pair of good 58's and 57 mixer or later 6C6/6D6's in the front end the early HRO is still passable on the A coil, mine is from the second run in 1935 with the 2.5V tubes. Without all the losses of a typical haywired band switch they sure enjoyed a long life ending in 1968 with the final HRO-60 run. I have one of those also Shocked

Carl
« Last Edit: August 10, 2019, 12:44:47 PM by KM1H » Logged
G3RZP
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Posts: 1321




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« Reply #9 on: August 10, 2019, 02:14:37 PM »

My HRO is a bit unusual. Dad bought in pre-owned in 1947: it had been in private hands throughout WW2. But when he was a Voluntary Interceptor for MI8, his last HRO was, most unusually. fitted with a vernier scale so stations could be logged to better than a tenth of one division. (no, I don't know how to scan and load photographs). When at the end of the war, that HRO wen back to government, it went without the vernier scale and without the SG Brown (extremely uncomfortable!) headphones, which I still have. the vernier scale went on the HRO he bought, and is there today, although the insides are very different to its original 1938 build. 6SG7 - 6SK7 Rfs, 6AC7 mixer, 6J5 lo, pair of 6K7 IF, 6SQ7 detector/AF/AGC 6SJ7 bfo/product detector combined, 6V6 output and VR150 stabiliser.

I am proud of the BFO/product detector, though. I think it's pretty novel.
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KM1H
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« Reply #10 on: August 10, 2019, 06:07:19 PM »

That sounds like a great HRO conversion Peter, you should write up the PD-BFO.  I built one for the -60 in a NBFM adaptor carcass and the IF has a couple of fairly sharp KIWA ceramic filters that do well on AM and SSB as all those IF tuned circuits are already pretty sharp on the top and the skirts need the sharpening up.  It is sometimes used as a battle conditions radio on 75 Grin

My only other tube HRO is a stock 50 with just the 2 IF's so that is used only on low QRM AM bands. That and a pretty stock NC-183 take care of most of the "hi-fi" audio needs with their PP 6V6's.

Does your camera have a mini USB output? If so the adapter cable to the regular USB port on the PC is a common item on Fleabay and in stores.

Carl
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G3RZP
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« Reply #11 on: August 11, 2019, 02:42:00 AM »

Carl,

My camera uses ASA200 35mm film! When you are taking pictures of steam locomotives running at 70mph, you need a camera that reacts immediately you push the button and doesn't stop to think about it! 70mph is about 100 feet per second. and that's what the steam specials are going at on the line 1/4 mile away.

I believe there are now electronic cameras that will do the job, but they are expensive here.
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G3RZP
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« Reply #12 on: August 11, 2019, 03:10:37 AM »

Carl,

The BFO/Product detector using a pentode. One has to realise that for the usual diode detector to function well, it needs at least 10volts pk input and perhaps even more to produce the AGC.  So in the early days of SSB, people used the BFO to get the diode detector into a reasonably linear state and reduced RF gain  prevent overloading. But it also paralysed the AGC.

If you look at the mu of the suppressor grid of a pentode such as a 6SJ7, it's low enough that it can stand a fairly large signal without overload. (This doesn't apply to a 6AS6 or similar) So you can make the BFO a Hartley  oscillator with RF grounded screen and plate. Apply a small amount of signal to the suppressor grid though a small - few pF - capacitor with a resistor to ground the suppressor grid to DC and in the plate circuit, provide an RF Choke and bypass cap to get rid of the RF component and then a plate load resistor and coupling capacitor to the AF stage. Some people have told me that one should use a 6SA7, but fail to realise that has too much gain from the signal grid. Because the coupling of the electron stream to the suppressor grid is small, the amount of oscillator signal appearing there is small and with the high impedance of the small coupling capacitor, the AGC still works correctly, not being swamped by the strong BFO signal.

It was published many years ago in 'Technical Topics' in the RSGB RadCom magazine and again, more recently, in 'Signal' the journal of the Vintage and Military Amateur Radio Society, issue 33, November 2014.
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KM1H
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Posts: 5541




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« Reply #13 on: August 11, 2019, 07:25:25 AM »

Quote
Carl,

My camera uses ASA200 35mm film! When you are taking pictures of steam locomotives running at 70mph, you need a camera that reacts immediately you push the button and doesn't stop to think about it! 70mph is about 100 feet per second. and that's what the steam specials are going at on the line 1/4 mile away.

I believe there are now electronic cameras that will do the job, but they are expensive here.

Ive been a steam buff since I was a kid and there was still one running on the LIRR between Jamaica NY and Montauk Point called the Fishermans Special. Go out in the wee hours Saturday and return Sunday evening; my grandparents had a summer home in Hampton Bays which was one of the stops.

In recent years Ive watched several of the larger steam restorations while at speed as well as all I could find on the Internet including your Flying Scotsman and 60163 Tornado.


Quote
It was published many years ago in 'Technical Topics' in the RSGB RadCom magazine and again, more recently, in 'Signal' the journal of the Vintage and Military Amateur Radio Society, issue 33, November 2014.

Any links to where I can make a copy without being a member Roll Eyes

Carl
« Last Edit: August 11, 2019, 07:36:45 AM by KM1H » Logged
G3RZP
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Posts: 1321




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« Reply #14 on: August 11, 2019, 07:47:48 AM »

Quote
Any links to where I can make a copy without being a member

Send me your email address to  peter.chadwick@ieee.org
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