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10th Nov 2008, 12:11 am | #1 |
Nonode
Join Date: Jul 2005
Location: Hull, East Yorkshire, UK.
Posts: 2,083
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Weyrad TRF Circuit
This circuit is looking very like that we have found in the constructional kits of the late forties & early fifties. There's no o/p stage, but I have got loads of 6V6's I can put to good use.
In their time these radios were known to use some very strange valves. Here Weyrad specifically reccomend a 6K7 as RF amp and 6J7 as anode-bend detector. I suppose you could also use A EF39 and EF37A would draw less HT current, and the rectifier would be less stressed. I was of a mind to construct the circuit. I have a chassis & case with tuning capacitors and a bagful of military surplus capacitors. There are two problems. I don't have any guide as to setting up the RF and detector tuning so theyr'e exactly in step with each other. How were these TRF circuits of the period aligned? The last touch for a period feel to the project would be a metal rectifier. I am going to be awkward and see if I can get a new one. Don't expect this to be a quick build. |
10th Nov 2008, 9:10 am | #2 |
Dekatron
Join Date: Feb 2005
Location: St.Ippolyts, Hitchin, Hertfordshire QRA IO91UW
Posts: 3,517
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Re: Weyrad TRF Circuit
Neil,
Look at the circuit - you will see three trimmers on each coil down to deck - these are for peaking the coils in the ant and det stages. Cheers Sean
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10th Nov 2008, 9:54 am | #3 |
Dekatron
Join Date: Oct 2003
Location: Croydon, Surrey, UK.
Posts: 7,550
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Re: Weyrad TRF Circuit
Neil.
There is no 'tracking' as such in a TRF set. Usually you just peak the coils for maximum signal starting at the detector then peaking the aerial coils. Some were more elaborate than others where the reaction winding was peaked at a certain point in tuning but that doesn't look like the case here. Rich.
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10th Nov 2008, 11:25 am | #4 |
Nonode
Join Date: Jul 2005
Location: Hull, East Yorkshire, UK.
Posts: 2,083
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Re: Weyrad TRF Circuit
First off I would check the dial pointer was in the right place when the tuning capacitor vanes were fully open (horizontal).
I was wondering at what point in the medium wave band one should peak the trimmers. I wondered would you do the detector first, or the aerial trimmer? It seems like one would do the detector on a station near the low end of the band, so the station you chose was tuned n the right place on the scale. The difficulty I see with this is to bypass the RF stage temporarily or it is a see-saw business between the two trimmers. |
10th Nov 2008, 8:18 pm | #5 |
Retired Dormant Member
Join Date: May 2008
Location: Preston, Lancashire, UK.
Posts: 428
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Re: Weyrad TRF Circuit
I have never found it all that critical. With two valves, trimming would probably increase the volume a bit, but still ample without. If you don't want to bother trying to find the "genuine" Weyard cails, just use RF chokes, they work just as well. For MW, I use 200uH (two 100uH in series if you can't find a 200uH choke). Saves a lot of messing about & they are very inexpensive.
Bob |
11th Nov 2008, 2:18 pm | #6 | |
Retired Dormant Member
Join Date: Aug 2005
Location: Keyworth nr. Nottingham, UK
Posts: 140
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Re: Weyrad TRF Circuit
Quote:
Cheers, Neil |
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11th Nov 2008, 8:32 pm | #7 |
Nonode
Join Date: Jul 2005
Location: Hull, East Yorkshire, UK.
Posts: 2,083
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Re: Weyrad TRF Circuit
This circuit is about as simple as they come.
Old issues of PW show that people like Premier used some odd valves at times live a CV1510 (B9G base) and a ATP4 (Mazda Octal). They did revert to using a normal 6V6 in their AC only TRF's. It seems that the obscure output valves pulled more anode & screen current, so i'll keep with a EL32 or a 6V6. I dunno what the average o/p power would be for a EL32 & 6V6 in single ended use. |
11th Nov 2008, 8:52 pm | #8 | |
Dekatron
Join Date: Oct 2003
Location: Croydon, Surrey, UK.
Posts: 7,550
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Re: Weyrad TRF Circuit
Quote:
I had one of these TRF's in the 60's. Unfortunately it didn't survive my curiosity when a capacitor failed! Rich.
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12th Nov 2008, 10:48 pm | #9 |
Nonode
Join Date: Jul 2005
Location: Hull, East Yorkshire, UK.
Posts: 2,083
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Re: Weyrad TRF Circuit
I used the National Valve Museum data to work out the anode & screen current for the valves that the 3 valve TRF's might use and the necessary rating for a rectifier stack.
Those transmitter valves are obviously would do a reasonable job. I am curious about them but not enough to install one because I have a lot of 6V6's and a pair of VT52 (EL32) for the audio stage. I may sell the EL32's. The 6V6 is more powerful, but is biased very conservatively. The Repanco diagram left it to the builder to choose a AF o/p stage of their own. I am taking this kit radio right down to the bare chassis. It is going to be a single band 3 valve TRF, hopefully looking as much like a amateur project radio of yesteryear as is possible. This would mean a metal rectifier stack of course. It will happen if I ever hear from Cougar Electronics again. I have got a bunch of old Federal 1004A's. I will try one pair and see if they are fit to use. I would not leave the radio unattended, but I don't use it heavily. |
17th Nov 2008, 11:00 pm | #10 |
Nonode
Join Date: Jul 2005
Location: Hull, East Yorkshire, UK.
Posts: 2,083
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Re: Weyrad TRF Circuit
I have seen a listing of the Federal 1004A in what I assume is a advert of the product range. Trouble is, I don't know what'd be the sort of fwd & reverse resistances to expect in a good one and how much drift puts the thing in danger of getting all hot, & stinky.
The requirement is to rectify 250V, so that means a pair of the Federal 1004A or Brimar RM3. Hmm..... Can I tell if a NOS rectifier has aged badly from a check with my AVO 8? |
19th Dec 2008, 9:10 pm | #11 |
Nonode
Join Date: Jul 2005
Location: Hull, East Yorkshire, UK.
Posts: 2,083
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Re: Weyrad TRF Circuit
This TRF did not have a 'good sound' before the rebuild and it didn't after, which I attributed to the anode bend detector. I put a 0.047uf @ 600V capacitor between the 6V6 anode pin and chassis. It gives a much more mellow result without sounding muffled.
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