UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Specific Vintage Equipment > Vintage Radio (domestic)

Notices

Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only.

Reply
 
Thread Tools
Old 22nd Apr 2021, 5:53 pm   #1
Skywave
Dekatron
 
Skywave's Avatar
 
Join Date: Jun 2006
Location: Chard, South Somerset, UK.
Posts: 7,112
Question Where is the RF amplifier?

I've seen a number of British designed valve radios of 1940 / 1950 vintage that use a 3-gang tuning capacitor yet do not have a valve RF amp. associated with it - typically a 6K7 or an EF39. So, why the 3-gang capacitor?

Al.
__________________
Chard, Somerset - The birthplace of powered flight and prosthetic limbs.
Skywave is offline   Reply With Quote
Old 22nd Apr 2021, 6:00 pm   #2
ms660
Dekatron
 
ms660's Avatar
 
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 10,677
Default Re: Where is the RF amplifier?

Band pass tuning for low IF receivers.

Lawrence.
ms660 is offline   Reply With Quote
Old 22nd Apr 2021, 8:28 pm   #3
Skywave
Dekatron
 
Skywave's Avatar
 
Join Date: Jun 2006
Location: Chard, South Somerset, UK.
Posts: 7,112
Question Re: Where is the RF amplifier?

Lawrence - thank you.
When you say "Low IF", is that the common 455 - 470 kHz or freqs. much lower, such as 110 kHz?
I know what bandpass tuning is and the advantages it can bring, but of those advantages they're not really a substitute for an active RF amplifier, are they?

Al.
__________________
Chard, Somerset - The birthplace of powered flight and prosthetic limbs.
Skywave is offline   Reply With Quote
Old 22nd Apr 2021, 8:51 pm   #4
ms660
Dekatron
 
ms660's Avatar
 
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 10,677
Default Re: Where is the RF amplifier?

Yes, much lower IF, eg: Murphy A4 (117kHz)

Lawrence.
ms660 is offline   Reply With Quote
Old 22nd Apr 2021, 9:06 pm   #5
kalee20
Dekatron
 
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 5,693
Default Re: Where is the RF amplifier?

Ever Ready 5033 is such a radio - 3-gang tuning capacitor which uses two sections for RF bandpass filtering (at least, on LW and MW. SW takes the aerial input to the second section of the filter).

The IF of this radio is 455kHz.

I don't know how good the tracking is, I'm not in a position to assess it right now - but certainly a single tuned circuit can cause tracking compromises. The adjacent channel selectivity all comes from the IF amplifier, whereas immunity to spurious responses, image frequency etc all has to come from the front-end signal tuned circuit. And if it is sufficiently selective to do this, it may well result in sideband cutting, or mistracking, whereas a bandpass filter with a response 20kHz wide will be easy to track, yet give superior attenuation beyond its passband.
kalee20 is offline   Reply With Quote
Old 22nd Apr 2021, 11:08 pm   #6
Skywave
Dekatron
 
Skywave's Avatar
 
Join Date: Jun 2006
Location: Chard, South Somerset, UK.
Posts: 7,112
Question Re: Where is the RF amplifier?

Kalee20 - thank you.

Your post causes me to ask you this: if such bandpass filtering was such a worthwhile feature with all the benefits it provided, why did some radios (of this vintage to which I have referred) use a valved RF amp. instead?

Al. / Apr. 23
__________________
Chard, Somerset - The birthplace of powered flight and prosthetic limbs.
Skywave is offline   Reply With Quote
Old 23rd Apr 2021, 6:32 am   #7
Sideband
Dekatron
 
Sideband's Avatar
 
Join Date: Oct 2003
Location: Croydon, Surrey, UK.
Posts: 6,108
Default Re: Where is the RF amplifier?

The Philips 470A is another set with a three-gang and bandpass tuning. The IF is 110Khz
__________________
On Spike Milligan's headstone......I told you I was ill...
Sideband is offline   Reply With Quote
Old 23rd Apr 2021, 7:46 am   #8
jjl
Heptode
 
jjl's Avatar
 
Join Date: Jan 2003
Location: Ware, Herts. UK.
Posts: 954
Default Re: Where is the RF amplifier?

Quote:
Originally Posted by Skywave View Post
Kalee20 - thank you.

Your post causes me to ask you this: if such bandpass filtering was such a worthwhile feature with all the benefits it provided, why did some radios (of this vintage to which I have referred) use a valved RF amp. instead?

Al. / Apr. 23
The most likely answer is that an RF amp with its own tuned circuit gives even better performance but at extra cost. The fact that bandpass tuning was uncommon in later lower cost radios with 465 kHz etc. IF shows how cost conscious at least UK manufacturers were - they couldn't justify the extra cost of bandpass filtering and its required 3 gang tuner.

John
jjl is offline   Reply With Quote
Old 23rd Apr 2021, 8:45 am   #9
kalee20
Dekatron
 
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 5,693
Default Re: Where is the RF amplifier?

This would be surmise on my part, but here goes anyway.

A tuned RF amp as well as offering the benefit of an extra signal-frequency tuned circuit, (giving a better measure of selectivity than just the single tuned circuit, 2-gang system), will offer extra gain, too, giving better sensitivity. Whereas the bandpass input will just offer the selectivity without extra sensitivity. And by boosting the signal before the frequency changer, which is the major noise-producer, it should offer better signal:noise ratio.

Of course, adding a valve as well as an extra gang section with its associated coils, is costlier. There's also the fact that a critically-coupled, or slightly over coupled, bandpass filter can actually offer better filter characteristics than two simple LC circuits separated by a buffer.

So, use the RF amp in radios for long-distance reception, and the BP where there is a plethora of strong signals.
kalee20 is offline   Reply With Quote
Old 23rd Apr 2021, 9:46 am   #10
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 17,359
Default Re: Where is the RF amplifier?

You can have two resonant tanks, each tuned and couple them lightly together. This gives you narrower (= better) bandwidth than you'd get with a single tank. If you couple them too tightly together it degrades into just a single tuned circuit response and you've wasted a whole coil and one gang. If you couple them too weakly, the bandwidth narrows further but the insertion loss shoots up and your receiver becomes deaf.

Having an RF stage valve between the two tuned circuits acts as an isolator so it's easy to run them at high Q and good selectivity without light coupling and associated losses.

So the change in sensitivity of a set with a true RF stage is even better than the gain of the valve would imply, if the selectivity is kept the same.

This is one of those effects of a design compromise which isn't obvious on the circuit diagram.

Some comms receivers like the AR88 have TWO RF stages. Not because they need the gain, but because they need the isolation between RF tuned circuits in order to get more RF selectivity.

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is offline   Reply With Quote
Old 23rd Apr 2021, 3:24 pm   #11
kalee20
Dekatron
 
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 5,693
Default Re: Where is the RF amplifier?

Quote:
Originally Posted by Radio Wrangler View Post
You can have two resonant tanks, each tuned and couple them lightly together. This gives you narrower (= better) bandwidth than you'd get with a single tank. If you couple them too tightly together it degrades into just a single tuned circuit response and you've wasted a whole coil and one gang. If you couple them too weakly, the bandwidth narrows further but the insertion loss shoots up and your receiver becomes deaf.

Having an RF stage valve between the two tuned circuits acts as an isolator so it's easy to run them at high Q and good selectivity without light coupling and associated losses.
That's true, however, single tuned circuits with a buffer-isolator between them, don't offer the versatility in the response of two circuits partially coupled together.

If you couple them tightly, it certainly does degrade to a single tuned circuit response. Loosely, and you get the product of the two responses. Very loosely, and you get the same, with more loss. But there's a range of coupling where the response broadens without significantly increasing in amplitude.

It's impossible to get a Butterworth response (maximally flat) with two tuned circuits separated by an amplifier. But properly coupled with no amplifier between, you can.
kalee20 is offline   Reply With Quote
Old 23rd Apr 2021, 4:00 pm   #12
ms660
Dekatron
 
ms660's Avatar
 
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 10,677
Default Re: Where is the RF amplifier?

On the top three ranges, the RCA AR88 uses choke/capacitance coupling between the 1st and 2nd RF amplifier and between the 2nd RF amplifier and mixer:

http://www.vmarsmanuals.co.uk/archiv...8-D_Manual.pdf

Lawrence.
ms660 is offline   Reply With Quote
Old 23rd Apr 2021, 6:01 pm   #13
G6Tanuki
Dekatron
 
G6Tanuki's Avatar
 
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 9,887
Default Re: Where is the RF amplifier?

There were a number of cunning circuits used back in the days [1930s] of 'low' IFs [100-150KHz] to reduce the image-interference problem a low-IF presented: typically these had a series-tuned circuit at the image-frequency and so deliberately 'suck-out' image-frequency signals.

Designing front-ends to track properly across the entire MW band was a bit of a black art and often involved variable-capacitors with non-traditional vane profiles. I'm sure radio-designers were happy to see the move to 450-470KHz IFs for broadcast-radios which made the image-rejection issue much less of a problem.
G6Tanuki is offline   Reply With Quote
Old 23rd Apr 2021, 8:58 pm   #14
turretslug
Dekatron
 
Join Date: Nov 2011
Location: Surrey, UK.
Posts: 3,726
Default Re: Where is the RF amplifier?

Philips notably used the 2-ganged and lightly coupled input circuits (both top- and bottom-coupling combining to even up the effect over the frequency ratio) without RF stage set-up in a few post-war sets, even with 450-475kHz IFs- probably LW front-ends in particular could suffer sideband-cutting without measures to broaden nose response and in the days before broadcasters got paranoid about transmitted bandwidth limitation (was this as much about nudging folk towards VHF/FM as about observing nominal channel-width goal-posts?....) it may have been worth ensuring that even MW front-ends didn't constrain fidelity. There's also the question that Continental set designers in particular would have been aware of the multitude of powerful stations, particularly after dark, all piling in on the unfortunate mixer and necessitating a tight front-end even when ultimate sensitivity and RF amp wasn't on the spec sheet.

With comms receiver LF coverage, it's obvious from some circuits that keeping gain down with high L/C ratio RF stages became a concern and this would also have broadened nose bandwidth- e.g. 220k shunting resistors in the CR100's RF stages and 330 ohm series resistors (i.e. between L and C) with the AR88LF/CR91.
turretslug is offline   Reply With Quote
Old 23rd Apr 2021, 9:07 pm   #15
kalee20
Dekatron
 
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 5,693
Default Re: Where is the RF amplifier?

It's quite true that LW front-ends could cause sideband cutting, I'm right now building a superhet and calculated that I'd want a Q of not much more than 20 on LW.

Having wound myself a nice ferrite rod aerial with 30 strand 0.063mm wire, 3 pies, with a Q of nearly 300, I'm realising I'm going to have to add parallel damping resistors to kill it! At least it's under my control... With an extra gang that I haven't got, I could've made a bandpass front-end and probably got MORE signal into the frequency changer!
kalee20 is offline   Reply With Quote
Old 23rd Apr 2021, 10:01 pm   #16
Skywave
Dekatron
 
Skywave's Avatar
 
Join Date: Jun 2006
Location: Chard, South Somerset, UK.
Posts: 7,112
Thumbs up Re: Where is the RF amplifier?

Well thanks everyone for the many responses: I've learnt a lot from them and I can see that this topic is a good deal more involved than I would have expected. So, what I've learnt will do for now.

Al. / Apr. 23
__________________
Chard, Somerset - The birthplace of powered flight and prosthetic limbs.
Skywave is offline   Reply With Quote
Old 23rd Apr 2021, 10:44 pm   #17
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 17,359
Default Re: Where is the RF amplifier?

If you're trying to get the narrowest skirt selectivity for a maximum acceptable amount of insertion loss, then the conventional planned responses like Butterworth ('maximally flat' but really means monotonic accelerating slope) ot Chebyshev/Cauer shapes are off the menu anyway. The best paper on this for filters with a finite resonator Q is by Seymour B Cohn. I can only describe the shapes his studies resulted in as rather erratic looking with a main peak, and some moderately prominent ripples (almost sub-peaks) into the transition region.

My last receiver design was an airband job for actual aircraft use, so it had to meet full DO-185/ED-23 requirements. The specs for avoiding intermod and overload from FM broadcasters got toughened up not that many years ago. I opted for a coupled pair into an RF amplifier into another coupled pair into the mixer. Each resonator of each pair being varactor tuned. With a large-ish tuning range and fixed coupliing components, it was an interesting balancing act.

Fortunately I'd gone for a fairly high IF in order to wangle a nice trick with digitisation and higher order Nyquist zone folding later on (29.25MHz) and for which crystal filters were available. So the image was the best part of 60MHz away.

Designing a receiver is an interesting puzzle.

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is offline   Reply With Quote
Old 24th Apr 2021, 12:12 am   #18
Synchrodyne
Nonode
 
Join Date: Jan 2009
Location: Mt. Maunganui, New Zealand
Posts: 2,613
Default Re: Where is the RF amplifier?

Quote:
Originally Posted by Skywave View Post
So, what I've learnt will do for now.
Noted. However, as I had written the following before seeing that comment, I thought that I may as well post it.

For valved communications receivers with four-gang front ends, two single-tuned RF stages seems to have been the modal choice. Possibly circuit noise at the upper end of the HF band was a factor here. I think that we may “read back” from comments that Marconi made about its Hydrus solid-state compact ISB receiver in 1968, bearing in mind that this was before the paradigm shift in HF receiver front end design marked by the Racal RA1772.

“To reduce the effects of crossmodulation and intermodulation, highly selective circuits should be introduced between the antenna and the first active device in the receiver, consistent with a noise figure which may be usefully employed at the upper frequency limit.”

“The degree of pre-selection, and hence the number of tuned circuits which may be inserted between the antenna and the signal-frequency amplifier, is a compromise between sensitivity and selectivity. It is generally accepted that a noise figure of 6 dB to 8 dB may be usefully employed at the upper-frequency limit of the h.f band ( 30 MHz). Provided that the noise figure of the active devices used in the signal-frequency amplifier can be made sufficiently low, say 2 dB, then greater scope is offered to the designer in providing antenna pre-selection. During the past two years, field effect transistors (f.e.t's) have entered the scene and are now available at economic prices. These devices offer attractive advantages over the more conventional bipolar transistors, resulting in improved signal-handling capabilities at lower noise figures. Depending upon circuit configuration, improvements in crossmodulation performance of up to 20 dB have been measured. Coincident with better signal handling, device noise figures of 2 dB or less are realizable.

“In the Hydrus equipment it has been possible to introduce a band-pass coupled circuit in the antenna input, which produces 30 dB to 40 dB attenuation at 10% off tune. This circuit feeds the signal-frequency amplifier, which consists of two junction f.e.t's connected in cascode. Noise figures of between 4 dB and 7 dB have been achieved. The cascode arrangement is preferred as it gives good isolation between the input and output of the amplifier and it also provides a convenient terminal for the application of a.g.c.”


The implication is that the jfet cascode was quiet enough that it provided a noise margin against the desideratum, which then allowed the use of the apparently noisier (at upper HF) bandpass input circuit. In the valve era, with pentode RF amplifiers, and even with high-slope pentodes in the first RF amplifier, that margin was not available. At lower HF and MF, device noise was less of an issue.

Some confirmation of that position may be had by looking back at some of the Eddystone receivers. For the 680X, 730 and early 880 Eddystone had used the conventional 4-gang arrangement with two single-tuned pentode RF amplifiers. But for the 830, it used an ECC189 cascode as a single-stage RF amplifier preceded by a bandpass tuned circuit. Given that the double-triode cascode was likely quieter than a pentode in the 20 to 30 MHz range, it could have been that Eddystone was taking advantage of the noise margin that allowed the use of the bandpass input. Later, with the 940 and later editions of the 880, it used the cascode as 1st RF amplifier in a conventional 4-gang arrangement with single-tuned input, presumably to obtain better quieting curves at upper HF.

Eddystone had also used a 4-gang, single RF amplifier (12BA6) with bandpass input for its 909A marine receiver, whose tuning frequency range went only to 4.7 MHz. This choice seems to align with the notion that it was more at the upper end of the HF band that device and tuned circuit noise became an issue.

Returning to domestic-type receivers, Murphy was on record (post-WWII) as saying that a bandpass input was the best choice at LF and MF, this apparently independently of whether or not an RF stage was used. It did use a bandpass input, without an RF amplifier, on its A186 and A188C (both LW & MW only, no SW) receivers of the early 1950s. These had variable selectivity, with a wide IF bandwidth that I’d guess to be in the range 16 to 20 kHz or so. The use of a bandpass input might have avoided the already noted bandwidth restriction that could occur at the lower end of the MW band with single tuned circuits. In some wideband receivers/tuners (e.g. Quad AMII) the MW RF circuit Q was lowered (by switching in series resistance) in the wideband mode. Another technique, when there was an RF stage and so two single tuned circuits, was to slightly stagger tune them at the low end of the MW band.

Murphy’s TA160 export receiver was of the 3-gang with RF amplifier type. But on MW, two of the gangs were used for a bandpass input, with an aperiodic interstage. On the wide-range SW bands, a conventional approach with single-tuned input and single-tuned interstage was used. The single-tuned input would have been better from a noise viewpoint at upper HF (although not at MF and lower HF). The arrangement tends to confirm that Murphy saw the bandpass input as being superior on MW, even if it required an aperiodic interstage when used with an RF amplifier. But there was more to it. A high-slope, 6F1 RF amplifier was used in place of the customary remote cutoff type, such as a 6F15. On a standalone basis, that was not so unusual, as there were other domestic all-band receivers that also used high-slope pentode RF stages in order to obtain lower noise at the upper end of the HF band. Evidently Murphy saw this choice of RF valve as increasing the risk of cross-modulation with strong signals on MF, and opted for the bandpass input as a way of minimizing this. Murphy also had another reason for using the 6F1. On the bandspread SW bands, it used an image-rejection circuit in the interstage, this apparently reducing the RF stage gain somewhat, and requiring the high-slope valve to provide enough gain overall at upper HF. So the input tuned circuit and valve choices were somewhat interdependent.

Dynatron also used a four-gang, single RF stage front end on some of its receivers, namely the later T69 iterations (T69C and T69D, I think) and the T99. The earlier T69A and T69B had been three-gang. As best I can work out, the additional tuning gang supported a bandpass circuit placed in the interstage, not at the input. If so, this placement was logical, given that the SW coverage went up to 30 MHz and the RF amplifier was a pentode. (I suppose though that there is the possibility that the band switching might have been arranged to put the bandpass at the input on MF, but at the interstage on HF.) The later T139 reverted to three-gang, but I suspect that may have been something of a forced choice, as I think it used the standard Weyrad bandspread front end. Even so, Dynatron added an aperiodic-input grounded grid RF pre-stage on the bandspread and upper HF bands. This would have provided constant input conditions for the following tuned circuits. Thus not having to cater for the variable source impedance of random connected aerials, and with less concern about noise introduction, they might have been “tightened up” somewhat – speculation on my part, though.


Cheers,
Synchrodyne is offline   Reply With Quote
Old 24th Apr 2021, 1:36 am   #19
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 17,359
Default Re: Where is the RF amplifier?

Thanks, there's a lot of information there!

Noise concerns say that you want a low noise amplifier as early as you can get it in your structure, and you want to minimise losses before it.

Big signal handling concerns say you want good narrow selectivity as early as you can get it in your structure and you don't want much gain until you've got through your channel-defining filter.

Clearly these concerns are at loggerheads. And dynamic range concerns say you want as much as you can get of both.

So we wind up with a nasty compromise and no real optimum solution. Consequently people have played many, many games with these structures and parameters.

The devil really IS in the details. Details of how good an amplifier/mixer/filter you can make can cause dramatic shifts in the structural choices... and you're still left wondering if there was a better solution.

We can't be surprised that there are so many variants.

Al was asking about broadcast radios of the 40s and 50s. Did radios compete then on detailed performance? Did buyers and advertisers understand such things... I don't think the radio world ever has its 0-60 time equivalent. I'm sure most manufacturers of broadcast sets just did what seemed like a good idea at the time, working to get sensitivity for their more exotic/expensive sets, and just living with any consequences.

So we see a wide profusion of approaches.

It's a valid issue and a class of problem which we still have no general solution to. If we do too good a job in one respect, we can usually twiddle things to trade some of the benefit off into a more needy area.

It still gets frightfully empirical.

I designed receiver structures for HP on elaborate spreadsheets modelling thir performance. In many respects these were models, not exact provable calculations. The spreadsheets told me what I'd got, but not how to do better. To an extent a lot of design software does analysis, but you're paid to do synthesis. So you make a guess, analyse it, vary your guess to analyse it and see if you're onto an improvement and so on and so on. It seems unscientific. Yes there are sub-areas which are analytic and provable, but any real system outgrows them rapidly.

If there were analytic solutions, we'd need computers alone and no designers....

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is offline   Reply With Quote
Old 24th Apr 2021, 9:03 am   #20
ms660
Dekatron
 
ms660's Avatar
 
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 10,677
Default Re: Where is the RF amplifier?

Murphy used R/C/L coupling between the 2nd RF amplifier and the mixer in the B40 for most of the ranges and used R/C coupling for the highest range, receiver schematic(s) in a link in the link below:

http://www.portabletubes.co.uk/boats/murphy3.htm

Lawrence.
ms660 is offline   Reply With Quote
Reply

Thread Tools



All times are GMT. The time now is 5:35 am.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2021, vBulletin Solutions, Inc.
Copyright ©2002 - 2021, Paul Stenning.