Preselector - UK Vintage Radio Repair and Restoration Discussion Forum

darren67 · 7th Jan 2015, 11:42 pm

Hi all,
Whilst on-line ages ago I came across detailed plans for a self powered preselector, which I intend to make, as I am interested to see if performance would be improved on the Trio 9R-59DS.

After much searching through various old parts, I have massed together most of the parts required.
I am nearly ready to proceed, with the exception of the stated valve, an EF183, which I don't have

I was considering the use of another RF pentode which I do have to hand, an EF89. Now I understand that various valves have different pin outs, and have taken this into consideration and compared the two valves' data in my book.

I am fairly confident it will work, but would I have to alter the circuit in any way to accommodate the EF89?
I would be grateful for any advice. Hopefully I can upload the circuit.

kind regards Darren.

G8HQP Dave · 8th Jan 2015, 1:16 pm

You may need a slightly larger value for R4, the cathode bias resistor.

G6Tanuki · 8th Jan 2015, 3:13 pm

An EF89 should work OK.

What sort of performance improvement are you seeking? Preselectors can be used to improve front-end gain, give better overall noise-figure (particularly on the upper HF bands where the likes of 1940s/1950s valves have high noise-figures), or add extra RF selectivity to help reduce image-interference (particularly a problem on the upper HF bands where you have a low IF).

Or a combination of all these.

I'm suspecting that in the case of a 9R59, which has relatively modern front-end valves, the greatest gain (pun intended) will be improved image rejection due to the additional signal-frequency tuned-circuit the preselector will provide. Extra front-end gain isn't always a benefit - it can increase the likelihood of cross-modulation and blocking.

Andrew2 · 8th Jan 2015, 3:21 pm

I had a 9R59DS from about 1972 to 1985 and the higher bands suffered from drift, pulling and monumental image problems - I always had a choice of two frequencies on which to listen! Obviously the preselector isn't going to stop the drift or the pulling, but it may help a little with the images if the selectivity can be made tight enough.
When I finally gave it away to a young SWL and replaced it with a Trio R1000, things got a lot better....

Sideband · 8th Jan 2015, 5:18 pm

I recognise that circuit? Radio Constructor?? I built that very unit when it was published to go with my homebrew shortwave receiver (from the same magazine). It worked extremely well and in fact I used it with several sets built later.

EF183 is a frame-grid valve with a high gain. EF89 will work but gain may be lower (and more noise)

G8HQP Dave · 8th Jan 2015, 5:59 pm

In a typical HF general coverage receiver much of the RF stage noise comes from the tuned circuit, not the valve. An EF89 should be fine, and will reduce the risk of overloading the receiver mixer.

TrevorG3VLF · 8th Jan 2015, 6:24 pm

I would use a bridge rectifier to make it easier on the transformer.

HamishBoxer · 8th Jan 2015, 7:11 pm

I should have an EF183 spare,PM me if stuck and I will find you one this weekend.

Bazz4CQJ · 8th Jan 2015, 7:26 pm

Quote:

Originally Posted by TrevorG3VLF

I would use a bridge rectifier to make it easier on the transformer.

I'd be inclined to see if you could get away with stealing power from the 9R-59?

B

G6Tanuki · 8th Jan 2015, 9:12 pm

Agreed. Even if you end up using a separate heater transformer, the HT requirement's only going to be a few milliamps so should be well within the wiggle-room of the 9R-59's supply.

timewave · 8th Jan 2015, 9:40 pm

I made that preselector and used it with a 9R 59D. I sold the RX but the preselector is on the bench at the moment waiting to be restored (made safe). I was but a lad when I made it and the wiring needs some attention.

I seem to recall that HT and heater volts are available on the octal socket at the rear of the RX.

John G4IJD

Biggles · 9th Jan 2015, 5:28 pm

Looking at the circuit I can see that careful consideration will have to be given to layout to prevent instability due to the bypass switching arrangements, otherwise you will have an oscillator on the input to your set. Goes without saying I suppose...
Alan.

darren67 · 13th Jan 2015, 11:30 am

Crikey I didn't expect this much response but it's all welcome.

Cheers Dave, any idea of the value for the cathode bias resistor?

Thank you G6tanuki, this radio has one RF stage. I was interested to see if any performance would be achieved with a RF amp pre selector and at my current location there is a fair bit of interference so selectivity might help matters too.

Andrew, I have been thinking about upgrading the Trio R1000. I have been thinking of trying to get for a while but they are few and far between.

Sideband, you are right I found the circuit ect on vintage radio and electronics site radio constructor section.

Trevor, I was thinking of a bridge rectifier so I will try it thanks.

Hamish, that would be fab if you have a correct valve I will pm you later.

Bazz, I was thinking of this but I have concerns that the receiver's transformer handle an extra valve? g6tanuki says it can hmmm food for thought.

Timewave, that's an interesting point if HT and heater voltage's are available through the octal socket this would simplify things a lot I shall study the circuit diagram and open up the receiver to have a look.

Biggles, I understand your point about the layout but would screening the the stages help to prevent oscillation?

many thanks to you all for your sound advice some will say I'm mad but, I am new to homebrew equipment and want to be successful in the spirit of home construction. I will post as I make progress it may take some time with family commitments but I will get there.

kind regards Darren.

G8HQP Dave · 13th Jan 2015, 11:40 am

I would try roughly doubling the value of R4 if you use an EF89.

Biggles · 15th Jan 2015, 8:35 pm

If you are new to home construction the main trick is to prevent any output signals getting back to the input which usually means attention to layout (keeping input circuits as far away as possible from output circuits, or to include screening between the two) and to keep connection leads as short as possible. Have a look at some of the RF stages in a commercial set and you will pick up on the methods used. RF engineering is a complex subject and I couldn't even scratch the surface here. Try and build it and see how you get on. It may be perfectly stable and well behaved!
Alan.

7th Jan 2015, 11:42 pm	#1
darren67 Tetrode Join Date: Jun 2014 Location: Plymouth, Devonshire, UK. Posts: 70	Preselector Hi all, Whilst on-line ages ago I came across detailed plans for a self powered preselector, which I intend to make, as I am interested to see if performance would be improved on the Trio 9R-59DS. After much searching through various old parts, I have massed together most of the parts required. I am nearly ready to proceed, with the exception of the stated valve, an EF183, which I don't have I was considering the use of another RF pentode which I do have to hand, an EF89. Now I understand that various valves have different pin outs, and have taken this into consideration and compared the two valves' data in my book. I am fairly confident it will work, but would I have to alter the circuit in any way to accommodate the EF89? I would be grateful for any advice. Hopefully I can upload the circuit. kind regards Darren. Attached Thumbnails

8th Jan 2015, 1:16 pm	#2
G8HQP Dave Rest in Peace Join Date: Sep 2008 Location: Solihull, West Midlands, UK. Posts: 4,872	Re: Preselector You may need a slightly larger value for R4, the cathode bias resistor.

8th Jan 2015, 3:13 pm	#3
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 14,007	Re: Preselector An EF89 should work OK. What sort of performance improvement are you seeking? Preselectors can be used to improve front-end gain, give better overall noise-figure (particularly on the upper HF bands where the likes of 1940s/1950s valves have high noise-figures), or add extra RF selectivity to help reduce image-interference (particularly a problem on the upper HF bands where you have a low IF). Or a combination of all these. I'm suspecting that in the case of a 9R59, which has relatively modern front-end valves, the greatest gain (pun intended) will be improved image rejection due to the additional signal-frequency tuned-circuit the preselector will provide. Extra front-end gain isn't always a benefit - it can increase the likelihood of cross-modulation and blocking.

8th Jan 2015, 3:21 pm	#4
Andrew2 Nonode Join Date: Mar 2010 Location: Dukinfield, Cheshire, UK. Posts: 2,038	Re: Preselector I had a 9R59DS from about 1972 to 1985 and the higher bands suffered from drift, pulling and monumental image problems - I always had a choice of two frequencies on which to listen! Obviously the preselector isn't going to stop the drift or the pulling, but it may help a little with the images if the selectivity can be made tight enough. When I finally gave it away to a young SWL and replaced it with a Trio R1000, things got a lot better.... __________________ Andy G1HBE.

8th Jan 2015, 5:18 pm	#5
Sideband Dekatron Join Date: Oct 2003 Location: Croydon, Surrey, UK. Posts: 7,578	Re: Preselector I recognise that circuit? Radio Constructor?? I built that very unit when it was published to go with my homebrew shortwave receiver (from the same magazine). It worked extremely well and in fact I used it with several sets built later. EF183 is a frame-grid valve with a high gain. EF89 will work but gain may be lower (and more noise) __________________ There are lots of brilliant keyboard players and then there is Rick Wakeman.....

8th Jan 2015, 5:59 pm	#6
G8HQP Dave Rest in Peace Join Date: Sep 2008 Location: Solihull, West Midlands, UK. Posts: 4,872	Re: Preselector In a typical HF general coverage receiver much of the RF stage noise comes from the tuned circuit, not the valve. An EF89 should be fine, and will reduce the risk of overloading the receiver mixer.

8th Jan 2015, 6:24 pm	#7
TrevorG3VLF Rest in Peace Join Date: Apr 2011 Location: Matlock, Derbyshire, UK. Posts: 1,378	Re: Preselector I would use a bridge rectifier to make it easier on the transformer.

8th Jan 2015, 7:11 pm	#8
HamishBoxer Dekatron Join Date: Aug 2007 Location: W.Butterwick, near Doncaster UK. Posts: 8,935	Re: Preselector I should have an EF183 spare,PM me if stuck and I will find you one this weekend. __________________ G8JET BVWS Archivist and Member V.M.A.R.S

8th Jan 2015, 9:12 pm	#10
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 14,007	Re: Preselector Agreed. Even if you end up using a separate heater transformer, the HT requirement's only going to be a few milliamps so should be well within the wiggle-room of the 9R-59's supply.

8th Jan 2015, 9:40 pm	#11
timewave Pentode Join Date: Apr 2006 Location: Clitheroe, Lancashire, UK. Posts: 173	Re: Preselector I made that preselector and used it with a 9R 59D. I sold the RX but the preselector is on the bench at the moment waiting to be restored (made safe). I was but a lad when I made it and the wiring needs some attention. I seem to recall that HT and heater volts are available on the octal socket at the rear of the RX. John G4IJD __________________ Old radio engineers never die, they just go intermit...mit...mit...mit...

9th Jan 2015, 5:28 pm	#12
Biggles Rest in Peace Join Date: Feb 2004 Location: Hexham, Northumberland, UK. Posts: 2,234	Re: Preselector Looking at the circuit I can see that careful consideration will have to be given to layout to prevent instability due to the bypass switching arrangements, otherwise you will have an oscillator on the input to your set. Goes without saying I suppose... Alan.

13th Jan 2015, 11:30 am	#13
darren67 Tetrode Join Date: Jun 2014 Location: Plymouth, Devonshire, UK. Posts: 70	Re: Preselector Crikey I didn't expect this much response but it's all welcome. Cheers Dave, any idea of the value for the cathode bias resistor? Thank you G6tanuki, this radio has one RF stage. I was interested to see if any performance would be achieved with a RF amp pre selector and at my current location there is a fair bit of interference so selectivity might help matters too. Andrew, I have been thinking about upgrading the Trio R1000. I have been thinking of trying to get for a while but they are few and far between. Sideband, you are right I found the circuit ect on vintage radio and electronics site radio constructor section. Trevor, I was thinking of a bridge rectifier so I will try it thanks. Hamish, that would be fab if you have a correct valve I will pm you later. Bazz, I was thinking of this but I have concerns that the receiver's transformer handle an extra valve? g6tanuki says it can hmmm food for thought. Timewave, that's an interesting point if HT and heater voltage's are available through the octal socket this would simplify things a lot I shall study the circuit diagram and open up the receiver to have a look. Biggles, I understand your point about the layout but would screening the the stages help to prevent oscillation? many thanks to you all for your sound advice some will say I'm mad but, I am new to homebrew equipment and want to be successful in the spirit of home construction. I will post as I make progress it may take some time with family commitments but I will get there. kind regards Darren.

13th Jan 2015, 11:40 am	#14
G8HQP Dave Rest in Peace Join Date: Sep 2008 Location: Solihull, West Midlands, UK. Posts: 4,872	Re: Preselector I would try roughly doubling the value of R4 if you use an EF89.

15th Jan 2015, 8:35 pm	#15
Biggles Rest in Peace Join Date: Feb 2004 Location: Hexham, Northumberland, UK. Posts: 2,234	Re: Preselector If you are new to home construction the main trick is to prevent any output signals getting back to the input which usually means attention to layout (keeping input circuits as far away as possible from output circuits, or to include screening between the two) and to keep connection leads as short as possible. Have a look at some of the RF stages in a commercial set and you will pick up on the methods used. RF engineering is a complex subject and I couldn't even scratch the surface here. Try and build it and see how you get on. It may be perfectly stable and well behaved! Alan.