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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 22nd Aug 2017, 12:18 pm   #1
vosperd
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Default R1155 psu

Hi,
In the R1155 psu circuit here:

http://www.vintageradio.me.uk/milita...2/R1155psu.gif

What is the function of the 5K 25W variable resistor? Is it really necessary?

Don m5aky
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Old 22nd Aug 2017, 12:25 pm   #2
G6Tanuki
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Default Re: R1155 psu

The 1155 circuit needs a negative bias voltage for certain valves - this is developed by way of a resistor in series between the -ve output of the PSU and the radio's earth (the WWII-vintage AR88 receiver does the same thing).

In some 1155s this resistor is fitted internally, in some it's in the power-supply.

The value of it (irrespective of where actually fitted) can need changing if, for example, your 1155 has had something like a 6V6 audio-power stage fitted in place of one of the DF valves (a very common, and worthwhile 'mod' that was popular in the 1950s/1960s).
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Old 22nd Aug 2017, 12:52 pm   #3
vosperd
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Default Re: R1155 psu

Hi,
Thanks for the reply.
I think in my case the bias resistor is in the receiver as the existing, rather large, psu which I am replacing does not have a resistor. The DF valves are still in the 1155 and I will be using the 6V6 amplifier as shown.
5K 25 Watt pots are not that common these days.
Don m5aky
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Old 22nd Aug 2017, 4:01 pm   #4
vosperd
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Default Re: R1155 psu

Just found a previous thread of a few years ago that I'd completely forgotten about.
Looks like in this case it's not essential.
Don

Last edited by vosperd; 22nd Aug 2017 at 4:20 pm.
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Old 22nd Aug 2017, 4:41 pm   #5
M0FYA Andy
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Default Re: R1155 psu

Quote:
Originally Posted by G6Tanuki View Post

In some 1155s this resistor is fitted internally, in some it's in the power-supply.
I'm puzzled by this comment - unless you are referring specifically to R1155s which have been subjected to amateur 'improvements'?

Andy
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Old 22nd Aug 2017, 4:54 pm   #6
G6Tanuki
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Default Re: R1155 psu

One of the Naval versions I recall seeing inside while at a joint Army/Navy summer-camp near Folkestone in the mid-70s had the bias resistor in the power-supply rather than in the receiver itself. It also used circular metal-shell plugs/sockets for the interconnecting cables - like the ones used on the TCS-series of naval radios - rather than the traditional Jones plugs.
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Old 22nd Aug 2017, 6:22 pm   #7
M0FYA Andy
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Default Re: R1155 psu

Very odd! Presumably 'locally modified' for some reason, so arguably not really an R1155 at all any more!
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