Red Star Radio; some general technical info.

[Ricardo]

Dear all,

I would appreciate any general information about this set plus I have some more detailed questions for anyone either with experience of this set or perhaps old battery valve sets in general.

I have obtained a circuit diagram for this model that was drawn by someone in February 2004 that gives the valve line-up as PM1LF detector, PM1LF audio amplifier and PM2A output stage. The same set of valves is listed for this set on a website that I came across recently.

My set however has a PM2HL fitted in the detector stage and unfortunately I can't see any markings on the other two valves (any hints for making them visible or identifying these valves?).

Whilst the PM2HL doesn't seem to be listed as a direct equivalent for the PM1LF it is described as a detector valve according to the National Valve Museum data so it might be a suitable alternative. Any comments?

Also if I am adventurous enough to try and get this set working I will need to make myself a battery eliminator and I am slightly uncertain about the supplies needed.

The heater supply for these valves is 2 volts (maximum?) at about 400mA - I was thinking of running them at about 1v5.
My circuit shows separate grid bias supplies for the audio amplifier and the output stage, but the actual set only appears to have one connection. I am uncertain as to a suitable voltage for this as the data I've seen for the valves and their equivalents either doesn't quote a value or it varies quite considerably. Would -6V be suitable and is their any way of determining this either theorectically or by testing?
The HT supplies are also a puzzle to me. The circuit shows three seperate HT+ supplies one for each stage and sure enough my set has three leads. In practice would these all have been connected to a common HT voltage or would they have been connected to different taps on an HT battery. A high impedance loudspeaker needs to be connected in series with the anode of the output valve so perhaps this HT supply was higher. Any suggestions as to suitable HT voltages would also be welcomed [90V or perhaps 120V?].

Apologies for all the questions, but this set is a real challenge for me. The previous valve sets that I have repaired have all been covered by Trader Service Sheets.

Once again any information or advice would be welcomed.

Regards,

Richard

[Paul_RK]

Hello Richard,

Immediate pangs of guilt here as just such a Red Star was among the few sets I destroyed at about the age of ten some 36 years ago when first intrigued by old radios. It had come to me without valves, and I rapidly despaired of discovering what they should be. I think the front bronze escutcheon is still around somewhere. Anyway...

There do seem to be a few around still, and I'm fairly sure I've seen one or two sold with their original instruction leaflets - if anyone here happens to have that it would answer several of your questions. My own experience with battery sets of the period remains very limited, but as regards the LT supply the nominally 2 volt accumulator would have delivered probably 2.2-2.3 volts when fully charged, and would properly have been regarded as discharged once the voltage dropped below 1.8 or so... I'd say just aim for a two volt supply. Prolonged under-running of the filaments would do more harm than good.

The single grid bias connection sounds a little unusual in my experience, but, yes, the PM2A's bias is quoted in my data as -6 volts, and I don't think harm would arise from providing that for the whole set: wiser counsels may follow from other hands For the HT supply, normally the output valve would have received something near the full 120 volts of a typical HT battery, with the others getting something probably in the 50 to 90 volt range. Such things often were a little experimental - a 5 valve Pye portable here has its lower HT supply voltage (57 volts) written in hand on the instruction leaflet during final testing on the 12.8.1927

And, yes, I'd guess myself that the PM2HL would substitute for the PM1LF without problems, though finding a PM1LF shouldn't be difficult if you wanted to - they were staples of three- and four-valve home constructor sets.

Paul

[Ricardo]

Andi,

Thanks for the tip. Sadly it didn't work with my valves.
Since it doesn't look as though the set has been powered up in ages (the battery leads have kind of welded themselves together in places and need completely replacing) maybe I'll just have to hope the valves fitted are the correct ones.

Paul,

Thanks for all your information. Most useful, especially regarding the likely heater and HT supplies for the set. I had seen some valve data stating 2 Volts as the maximum heater voltage, but from what you have said regarding the accumulator type supplies that would have been used in the past I shall now go for 2 Volts nominal as my heater supply.

Anybody,

Paul seems to think he has seen an instruction leaflet for these sets. I'd like to obtain a copy if possible.
Also I forgot to ask in my original post; my set is missing a knob for the wavechange switch on the front - does anyone know what the original would have been like? A picture would be great.


Thanks again for your help.

Richard

[boiss]

Hi, I have played with a few of old TRFs and have found that under 1.6V on the filaments means no go at all, 2V is fine. As for the HT look at the data for the valves themselves, typically the RF stage uses 90-120v, the detector stage around 50-70V and the output stage around 90V. The grid bias -V being around -7.5V and is often just on the output stage, however without any data these are reasonable starting points. I have found that tying the HT supplies together at say 90V works OK on some sets but on others there are some weird feedback effects and performance suffers drastically. The best bet is try it and see. The GB -volts were often quite high in order to reduce the current in the O/P stage and prolong the battery (and Valve?) life, again try it and see what works best.
There's nothing like trying to coax a signal from these old sets especially ones with 5 or more variable caps.
Cheers,
Boiss.

[Ricardo]

Boiss,

I really appreciate the information which in the absence of any specific data on the Red Star should give me a good starting point when I get around to powering it up. I have just ordered a suitable transformer for my battery eliminator.

Perhaps it should be part of a separate thread, but I wonder how the HT voltage for the various stages would have been determined theoretically? Most of the basic valve data I have seen quotes absolute maximum values for the anode voltage, current, load resistance, etc.

I read with interest an article on Reaction tuning in the Summer 2006 issue of the BVWS bulletin and I am looking forward to playing with one of these sets myself. Fortunately the Red Star only has 2 variable capacitors to worry about!


Regards,

Richard

[boiss]

Hi Richard,
I found some data somewhere regarding the valves that are in one of my radios and found recomended working voltages for them when utilized in a particular function, eg.RF Amp. Basically It's a which came first the radio or the batteries, the batteries made for radios had either taps at the various voltages 22.5V, 45V, 67.5V,90V and 120V or a particular Voltage output, So there wasn't a great deal of choice, Grid Bias batteries had taps at 1.5V steps.The circuits used are generally very low tech from the 20s and early 30s and with the wiring that was used, it surprises me that they work at all. I have a home constructed set from the early 30s which is a Scott-Taggart design, With only 10 ft of wire for an antenna (Aerial) it is far more sensitive than a typical 6 transistor radio from the early 90s but it's a real pig to tune in, 3 of the 5 variable caps must be kept constantly in step otherwise it's completely deaf to all but the strongest signals.
Anyway get your power supply built and have some fun, Yesterday evening I could get at least 50 stations from as far afield as Russia, N Africa and N Europe on the speaker not headphones, by the the way I am in N Italy.
Cheers,
Boiss

[Paul_RK]

Originally Posted by boiss

...the batteries made for radios had either taps at the various voltages 22.5V, 45V, 67.5V,90V and 120V or a particular Voltage output, So there wasn't a great deal of choice...

I think there were often a few more voltage taps on offer. The only "conventional" HT battery I'm young enough to have bought new, an Ever Ready Winner 120 which was still available circa 1970, had I think tappings every twelve volts at least toward the upper end: and the "57 volts" written in for my old Pye gives away the presence of a 57 volt tap on its intended battery (Ever Ready W.812).

Paul

[squier]

I've got one of these sets and they really do work quite well. The "Red Star" brand was produced by Telsen (Birmingham) in 1929/30. I wouldn't worry too much about the type of valve to use, virtually any B4 based triode will do for the detector and 1st audio stage.

The output valve can be a PM2, LP220 etc., the grid bias voltage will depend on the valve type and how much HT you have. Don't try using 1.5v on the filaments as it won't work, probably 1.8 is the minimum you should use (yes 300mV does make a difference).

You can use an old 6.3v heater tranny + rectifier/smoothing circuit and regulate output voltage with a cheapo LM317 regulator , make sure that you are supplying the filaments with DC as they are very intolerant of even a small amount of ripple).


I use something like 45 volts on the detector valve, 75 volts on the 1st audio and 120v on the output stage, I use a 4.5v cycle lamp type battery for grid bias (maplins still sell them). Don't be tempted to run the detector valve at anything higher than 45v as you might have problems with microphonics and stability/reaction.

[Ricardo]

Boiss,

Thanks for the information regarding recommended working voltages for valves used for particular functions.

Your 1930s home constructed set certainly seems to perform well.


Paul,

I really appreciate your contribution giving details of HT batteries and their likely tappings.


Squier,

It was especially good to hear from someone else with one of these Red Star battery sets. Does your set have a knob for its wavechange switch - if so any chance of a picture? Also what do you use as a speaker/headphones? - I believe the power output is quite low (into a high impedance load), but I am thinking of finding a suitable audio matching transformer to drive a modern speaker.


Just to update you all on my progress. I have almost finished the construction of my battery eliminator. I am just waiting for some 5W zener diodes to regulate my 120V HT supply and also to provide some suitable 'tappings' for the various stages. My LT supply, as suggested, uses an LM317 regulator to provide up to about 500mA at 2V DC.


Thanks again for your input everyone.

Richard

[squier]

Richard,

My set does have a knob on the wavechange switch. It looks to be made of hardended rubber rather than bakelite or ebonite. From memory, the "switch" contacts are just a couple of bolts and the rotor a small piece of brass strip.

Make sure you use a heatsink on that voltage regulator, even when drawing 0.5 amps it'll get quite hot. I use the "huskier" version of the 317 which is rated at 1.5 amps so I can use it with sets using more then 3 valves.

The output impedance will obviously depend on the type of output valve you are using, generally speaking, 2 - 3K is the norm for these early battery sets. Output is about 250mW, but this is sufficient for normal use.

If you want to get more output you could substitute a screened grid output valve but make sure you use one where the screen connection comes out on a terminal on the side otherwise you'll have to change the socket.

[boiss]

Richard,
The advice on the 317 regarding heater current is VERY important, One of my 3 valvers does draw 500mA (200mA+200mA+100mA), check the data for your valves before connecting the psu to the radio, better still try a dummy load simulating the valves first.
Cheers,
Mike

[Ricardo]

Squier,

Thanks for the information on the wavechange switch.


Squier / Boiss,

Thanks for the warning about the LM317 regulator and the likely heater current.


My transformer for the LT supply has a 9V secondary winding rated at 12VA. The LM317 regulator is the type with a maximum rating of 1.5A. I had fitted it with a heatsink because as you say it gets very hot without one. I have tested the supply with a 2R2 dummy load and it is OK supplying about 900 mA. (There is a series resistor which drops my unregulated supply to about 4.6V at the input to the regulator on full load. More heat!)

The battery eliminator is now fully built and tested. The remaining tasks before the big switch on are to replace the old battery leads and obtain a suitable output transformer/speaker for the Red Star Radio.


Once again I appreciate the help and advice.


Regards,

Richard

[Ricardo]

Thanks to the help and advice received on the forum my set now works quite well.

My experiences with its operation and performance to date are as follows:

It needs a decent earth connection. I initially used a short length of copper pipe in the garden, but that was not as good as a connection to the central heating pipework or the mains earth. I know that the mains earth is not normally recommended as a RF earth, but it seems to work quite well for me.

I first used what I thought would be a reasonably good aerial, about 30 ft of copper wire tacked along the top of a wooden fence in the garden. With this aerial the set picks up four stations on its SHORT (medium) waveband at quite loud volumes. The tuning with this aerial ranges between Radio 5 Live on 909khz and Three Counties Radio on 630khz. The stations are all either very local (within a few miles) or come from powerful transmitters.
Little selectivity was evident and the VOLUME (reaction) control had no effect.

The grid bias resistor was missing on the detector stage. I have tried fitting a 5 Megohm resistor as per the circuit diagram that I have as well as various values down to about 200K without any obvious effect upon the sets performance, beneficial or otherwise.

I disconnected the reaction coil completely and I could still receive the same four stations on medium wave. I checked all the components around the detector valve and their connections and couldn't find anything wrong, but even with an oscilloscope I couldn't convince myself that the reaction circuit was working.

On Long Wave however, where I could pick up quite weakly Europe 1 on 183khz, Radio 4 on 198khz and RTL on 234khz, the reaction control did have a positive effect [excuse the pun!]. At the high frequency end of the long wave band I was suffering some breakthrough from the strong medium wave stations mentioned before.

I have continued to experiment today and achieved much better results on Medium wave with shorter aerials indoors (between about 5 and 10 feet long).
With these aerials the set covers a higher range of frequencies with apparently better sensitivity and selectivity. The reaction control now has a positive effect and with care I can tune in nine stations on frequencies between 828khz and 1359khz.

With these shorter aerials on Long Wave I can only detect RTL on 234khz.

On both wavebands the tuning has shifted towards higher frequencies with these shorter aerials.

It seems that I need a short wire for good performance on medium wave, but my outdoor aerial is needed to pick-up Radio 4 on Long Wave.

Some of my issues seem to relate to the presence of several very strong local stations on medium wave. As mentioned already I get some breakthrough from these stations when tuning on Long Wave. Perhaps an external low pass filter circuit for the long wave band might cure this?

Does anyone have any comments or suggestions regaring suitable aerials for these sets? Particularly for long wave reception which isn't great. Would some sort of loop aerial be any better than my 30ft outdoor aerial?


Richard