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Old 24th Sep 2014, 9:03 pm   #1
RobinBirch
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Default 1155 psu

Hi Everyone,
I am in the middle of rebuilding a PSU that a friend's father built many years ago that was loosely based on the attached circuit. What I don't understand is the role of the variable resistor in the -HT line. I thought that the actual voltage on this line was determined by the receiver depending on the mode set.

Ideas anyone.


Kind regards

Robin
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Old 24th Sep 2014, 9:34 pm   #2
G6Tanuki
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Default Re: 1155 psu

The resistor is a way of providing a negative-voltage to bias the valves.

It's not an unusual arrangement - the RCA AR88 receivers use the same approach. It has the advantage that the valve cathodes can be connected directly to earth/chassis which provides a better/lower-impedance cathode-to-chassis path than the more-usual approach with a R/C bias-network between each valve cathode and chassis.

The other benefit is that the bias-voltage across the resistor is generated by the sum of *all* the HT-currents drawn by the valves - so is less disrupted by changes in voltage when AGC action reduces the current through the RF/IF valves.
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Old 24th Sep 2014, 9:42 pm   #3
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Default Re: 1155 psu

Hi Robin.
I have looked at that circuit with the idea of rebuilding an 1155 psu and wondered the same.
The psu I have has a floating HT- and a separate bias supply for the Audio amp.
This circuit could have been designed to give a negative bias for the audio amp and still a floating 250V for the 1155.
When I rebuild mine I think I will use a different arrangement for the audio valve and leave out that resistor.
Don m5aky
Just looked at the filament and cathode connection, is it me or does that look a bit odd?

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Old 24th Sep 2014, 9:51 pm   #4
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Default Re: 1155 psu

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Originally Posted by vosperd View Post
This circuit could have been designed to give a negative bias for the audio amp and still a floating 250V for the 1155.
When I rebuild mine I think I will use a different arrangement for the audio valve and leave out that resistor.
You still need a source of negative-bias for the receiver stages, irrespective of what you do regarding non-standard audio-stages. The resistor-in-the-negative-lead approach works well even if you're adding something like a 6V6 as an audio-amp. OK, you need to use a somewhat lower-value resistor, but the additional current from the 6V6 actually improves matters because the AGC/RF-gain-control-induced current-changes through the bias resistor are actually a smaller *percentage* of the overall current so the bias-change with RF-gain/AGC-action is lower.
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Old 24th Sep 2014, 10:05 pm   #5
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Default Re: 1155 psu

I agree, looking at the circuit again I think my theory is wrong anyway.
However as Robin says I think there are resistors in the 1155 that do this. Mine works fine with the floating HT- and no resistor. I've not had my 1155 that long so not fully familiar with the details as it's been working fine.
Cheers
Don
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Old 24th Sep 2014, 10:20 pm   #6
RobinBirch
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Default Re: 1155 psu

Yes, the problem I had with this circuit was exactly what Don suggested.mthe 1155 has got a set of components that manage the floating -HT to provide the bias that's required for what ever you are doing and it has always been a point that's made about 1155 PSUs that the -HT is left floating as the radio sorts it. If this wasn't to drive an 1155 I'd completely agree with G6Tanuki but.........
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Old 25th Sep 2014, 8:17 am   #7
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Default Re: 1155 psu

There's one thing about this design of PSU which originated in the 1950s as a Wireless World publication that worries me and that's the unloaded secondary of the fragile audio output transformer. It's intended to feed a headphone load of about 500 ohms or less I believe and in this design it feeds a 500K resistor via a 1 uf capacitor which has a reactance at 1kHz of about 160 ohms. So effectively no load at all.

I wonder whether it might be better to lift the connections to the transformer and fit an anode load resistor of about 47k to 68k and an 0.1 coupling capacitor or some other idea that will protect that fragile and rare transformer. Everything could be left in place so as to make minimum disturbance to authenticity.

Jim
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Old 25th Sep 2014, 9:04 am   #8
RobinBirch
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Default Re: 1155 psu

Jim,
That's an interesting mod and one I may well include. The circuit hasn't been followed slavishly and has a few add ons that I'm still trying to get my head round. Most of the components are dead through old age so its going to be a cosmetic rebuild with the original chokes and so on.

Still trying to get to the bottom of the variable bias resistor as I'm sure it isn't needed and will defeat part of the 1155s bias circuitry.

Robin

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Old 25th Sep 2014, 9:32 am   #9
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Default Re: 1155 psu

Quote:
Originally Posted by RobinBirch View Post
Still trying to get to the bottom of the variable bias resistor as I'm sure it isn't needed and will defeat part of the 1155s bias circuitry.
It looks to me to be someone's add on to fix an R1155 with a bias fault rather than find the proper reason. Providing all the decoupling capacitors are good, preferably replaced, the R1155 works quite well and need no additions. In that circuit it's probably the big capacitor across the variable resistor that contributes to "fixing" whatever problems the original set had.

Don, the filament connection looks odd because it's drawn so that it looks to be connected to the cathode. With a better quality diagram there might be a little clearance evident.

Jim
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Old 25th Sep 2014, 9:55 am   #10
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Default Re: 1155 psu

R1 (2K) in the receiver itself develops the required bias. The variable resistor in this old PSU circuit is unnecessary and would, if adjusted to zero, short out the bias thus defeating the designer's intention. I've been operating R1155s for a number of years without this resistor without any problems (and, frankly, where would one find a 5K 25W component these days?).
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Old 25th Sep 2014, 2:26 pm   #11
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Default Re: 1155 psu

Keith,
That's what I'd thought. It hadn't occurred to me that there might have been a fault in the original receiver.

Thank you

Robin
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Old 25th Sep 2014, 2:58 pm   #12
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Default Re: 1155 psu

Perhaps apply Occam's Razor here: a simple design error? It would be instructive to know the original source of that cct. diagram. Considering that (a) the R1155 has been around for many years, (b) the style of that drawing and (c) that 25-watt pot. , I reckon that that original source was from many, many years ago.

Al.
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Old 25th Sep 2014, 5:25 pm   #13
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Default Re: 1155 psu

That schematic probably originates from the days when hard info about military surplus equipment was very hard to come by. These days with a wealth of information now "liberated", its hard to appreciate just how little was known about much of the wartime equipment. Back in the early 70s, when I was playing with WS31s, WS88s, WS19, WS62, R107, etc, we had no schematics for any of this kit. We had user handbooks, which told you the valve lineup, but that was about it. And when you tried to get schematics, nothing was forthcoming. I pestered and pestered GWM Radio in Worthing and Arthur Sallis in Brighton for data, but they couldn't find anything.

And that problem partly explains why so much of the stuff was smashed up for components. Most of it didn't even have a user manual, nor could it be exactly identified. Its hard to value something when its little more than a collection of bits.....


Richard
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Old 25th Sep 2014, 5:33 pm   #14
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Default Re: 1155 psu

This circuit, I believe, originated from Wireless World back in the 50s. The PSU that I am refurbishing was derived, I think, from it and I've been struggling to identify what all of the circuit actually is. As a result I was trying to map it back to the WW diagram.

I think I've worked it out and the resistor in question isn't in the cct that I'm working on.

Now I've got to make the original work which given that nearly all of the Rs and all of the Cs are defunct will probably result in a slight redesign to use the surviving components - Ls and Ts with new Rs and Cs.

This, shouldn't be a problem. The fun part will be the "modified" 1155 that goes with it. It was converted by an amateur in the 50s for his use at sea when he was an engineer office in the merchant marine. Fortunately he documented what he did to that and so I think I'm in for an easier ride

Kind regards

Robin
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Old 25th Sep 2014, 6:19 pm   #15
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Default Re: 1155 psu

Thought-experiment: could the adjustable resistor be there to compensate for extra current (by shunting the existing bias-resistors) when the standard 1950s "remove the DF-components and fit a 6V6 audio-power-stage in the freed-up space" mods were done to an 1155?

I'm thinking that a typical 6V6 would pass around 45mA extra - and the variable resistor would be tweaked to set the 'negative HT-lead-generated' bias voltage back to the originally intended value?
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Old 25th Sep 2014, 8:14 pm   #16
RobinBirch
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Default Re: 1155 psu

A good thought but I suspect unlikely as one of the standard configs of the 1155 was "no DF" as they didn't put the valves in if it was going in a non DF position. As a result the bias circuits had to deal with both conditions.
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Old 26th Sep 2014, 9:02 am   #17
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Default Re: 1155 psu

Yes but would the bias circuit deal with the extra 45mA HT current from a 6V6 audio amp?
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Old 26th Sep 2014, 9:43 am   #18
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Default Re: 1155 psu

Quote:
Originally Posted by RobinBirch View Post
A good thought but I suspect unlikely as one of the standard configs of the 1155 was "no DF" as they didn't put the valves in if it was going in a non DF position. As a result the bias circuits had to deal with both conditions.
Robin,

which R1155 variant was that then? I don't recall seeing a reference to this in either AP.

Richard
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Old 26th Sep 2014, 3:10 pm   #19
RobinBirch
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Default Re: 1155 psu

Para 86 AP 2548A vol 1 chapter 2
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Old 26th Sep 2014, 11:56 pm   #20
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Default Re: 1155 psu

Apologies for poor scans, using the old one in the Workshop.
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