R1155 psu - UK Vintage Radio Repair and Restoration Discussion Forum

vosperd · 22nd Aug 2017, 12:18 pm

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

G6Tanuki · 22nd Aug 2017, 12:25 pm

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).

vosperd · 22nd Aug 2017, 12:52 pm

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

vosperd · 22nd Aug 2017, 4:01 pm

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

M0FYA Andy · 22nd Aug 2017, 4:41 pm

Quote:

Originally Posted by G6Tanuki

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

G6Tanuki · 22nd Aug 2017, 4:54 pm

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.

M0FYA Andy · 22nd Aug 2017, 6:22 pm

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

22nd Aug 2017, 12:18 pm	#1
vosperd Hexode Join Date: Oct 2007 Location: Bristol, UK. Posts: 444	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

22nd Aug 2017, 12:25 pm	#2
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 13,998	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).

22nd Aug 2017, 12:52 pm	#3
vosperd Hexode Join Date: Oct 2007 Location: Bristol, UK. Posts: 444	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

22nd Aug 2017, 4:01 pm	#4
vosperd Hexode Join Date: Oct 2007 Location: Bristol, UK. Posts: 444	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.

22nd Aug 2017, 4:54 pm	#6
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 13,998	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.

22nd Aug 2017, 6:22 pm	#7
M0FYA Andy Nonode Join Date: May 2007 Location: Preston, Lancashire, UK. Posts: 2,511	Re: R1155 psu Very odd! Presumably 'locally modified' for some reason, so arguably not really an R1155 at all any more!