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Old 11th Nov 2020, 9:12 pm   #1
Paul Barnes
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Default Fitting an SP61 valve to a Pye 18A radio.

I really like the radios Pye made in the 1940's-the short superhets.Sometimes with a 20ft aerial, a simple homemade FET booster and BFO I listen to amateur stations as far away as Germany and Spain on a Pye 15A.
Recently I bought a very rough Pye 18A of which I have restored the chassis , which works well and have tried to fit a valve RF stage to the front end.Needless to say I am doing something wrong and the adaption does not work.As I could not understand what I researched about RF amplifiers on the net, I decided to copy the Murphy A128 receiver which has an SP41 switched in on SW circuits.As the Pye heater voltage is 6.3 v, I used the SP61 (actually the CV118 equivalent). I enclose two attachments, the original front end circuit and the (in red) adaptations I have made.I am at the limit of my knowledge and would appreciate someone who has a real grasp of fundamentals to show me where I am going wrong. Finally, you will see there is a 7.5 ohm resistor feeding the anode.In the Murphy it is a 7.5 ohm coil that sits on its own in the chassis. I assume some kind of choke? Paul.
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Old 11th Nov 2020, 9:21 pm   #2
Restoration73
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Default Re: Fitting an SP61 valve to a Pye 18A radio.

Hi Paul and welcome.

Your modification and connection will have affected the sets agc. It would be better to
restore the set to the original and connect a single valve with tuned input circuits that
could deliver useful gain, EF39 would be ok, a suitable outboard amplifier Codar PR30
circuit on web can be copied for this application. 73
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Old 11th Nov 2020, 10:19 pm   #3
Mr 1936
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Default Re: Fitting an SP61 valve to a Pye 18A radio.

Hi

The Murphy A128 relies on an inductor in the anode circuit of the RF amplifier. Although its DC resistance is 7.5 ohms, its impedance in the SW range will be much higher, i.e. hundreds of ohms. If you simply use a resistor of such a low value, the gain will be very small indeed.

I don't know the exact value of the inductor, but would guess at something around 2 microhenries. I suggest you source a choke of about this value, or use a tuning coil (main winding) from a scrap shortwave set.

As a refinement, Murphy cunningly chose the inductor value so that in conjunction with its two coupling capacitors it gave some rejection at the unwanted image frequency, usually 910 kHz higher than the wanted frequency. I think you might struggle to achieve this nicety without accurate test gear.
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Old 12th Nov 2020, 2:44 pm   #4
Paul Barnes
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Default Re: Fitting an SP61 valve to a Pye 18A radio.

Thank you both ever so much for your quick and knowledgeable replies.

I have never heard of the Codar 30 before but from what I see I am better at this stage tacking something on the front of the receiver rather than trying to intrusively connecting and altering something that will have repercussions to the AGC.

I am assuming the idea of the choke on the anode of the SP41 was to ''force'' the shortwaves to the next valve stage.Having read what Mr 1936 has explained, I wonder why on earth I thought a resistor would do. Hind sight is a wonderful thing.

If its ok with you both I will let you know what happens once I have had a ''play''.Once again, Many thanks Paul.
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Old 12th Nov 2020, 6:18 pm   #5
Mr 1936
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Default Re: Fitting an SP61 valve to a Pye 18A radio.

Hi

I remember the Codar PR-30 amplifier/preselector which was sold in the late 1960's. It was mainly intended as a companion to the same firms CR-66 communications receiver. It was basically a single parallel tuned circuit using a variable capacitor and three switched high quality inductors for the different bands. This was followed (from memory) by an EF183 RF pentode amplifier, with its gain controlled by varying the voltage applied to the screen grid.

These are a useful add-on if you can find one. Some versions require you to "steal" a small amount of HT and LT from the accompanying receiver.
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Old 12th Nov 2020, 6:47 pm   #6
Silicon
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Default Re: Fitting an SP61 valve to a Pye 18A radio.

Does your modification have the grid of the additional SP61 RF valve connected directly to the aerial?

The Murphy A128 had an LC tuning circuit in front of the RF valve to improve selectivity.

You modification may result in a poor signal to noise ratio.
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Old 12th Nov 2020, 7:26 pm   #7
Paul Barnes
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Default Re: Fitting an SP61 valve to a Pye 18A radio.

Hi,
Yes the grid goes straight to the aerial(at the moment!) Regards Paul
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Old 12th Nov 2020, 10:19 pm   #8
kestrelmusic
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Default Re: Fitting an SP61 valve to a Pye 18A radio.

To be honest, I can't see the point of the modification. The Pye 18A has ample sensitivity with even a modest aerial. If you want to pull in weak foreign stations I would concentrate on the aerial.
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Old 2nd Dec 2020, 1:29 pm   #9
Paul Barnes
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Default Re: Fitting an SP61 valve to a Pye 18A radio.

Hi, I took Mr 1936's advice which makes good sense to me and removed the 7.5 Ohm resistor above the anode and replaced it with a 220uh choke (Mr 1936 advised 2uh and I bought a 220uh by accident which I will have to change).Actually get a crack noise now when I touch the control grid.Progress!

What I don't understand is the anode voltage on the CV118 valve is 116v. The Ht I took it off is 255v but when I plug the valve in the HT line drops to 141v.I actually bought 8 of these CV118 valves untested as a job lot and when I try the others the anode voltages vary from 80v to 124v.The 80v ones heaters seem to glow slightly brighter. Have I bought duff valves? What is exactly happening with these anode voltage drops? Can anyone help?

Regards Paul.
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