Seeking Valves

[Refugee]

What resistor are you measuring?
It sounds to me like the cathode resistor (connected to pin 8 of the KT61) and it should be left at the marked value.
The most likely cause of low anode volts is bad biasing on G1 of the KT61 (pin 5).
Measure the voltage from the chassis to Cathode (pin 8) and post it here. It should be about 4.5 volts.
Do the same for G1 (pin 5). It should be less than 0.5 volts.
If pin 5 is above 0.5 volts replace the capacitor that is connected to pin 5 with the other end connected to the other audio valve.
That should reduce the anode currant and allow the anode voltage to rise.

[Nowhere-Man]

What resistor are you measuring?
It sounds to me like the cathode resistor (connected to pin 8 of the KT61) and it should be left at the marked value.
The most likely cause of low anode volts is bad biasing on G1 of the KT61 (pin 5).
Measure the voltage from the chassis to Cathode (pin 8) and post it here. It should be about 4.5 volts.
Do the same for G1 (pin 5). It should be less than 0.5 volts.
If pin 5 is above 0.5 volts replace the capacitor that is connected to pin 5 with the other end connected to the other audio valve.
That should reduce the anode currant and allow the anode voltage to rise.

I've been swamped overall with work as I'm trying to fix up 2 seventies canal boats before the weather turns nasty towards Winter. Plus, I seem to have a growing workload of more valve radios all waiting their turn. I tend to work on my radios on the rainy days and, as it was raining yesterday, I packed up my boat work and went to Maplins. Then today, it was sunny weather again but I decided to carry on and do some radio related work. Why not?

The output valve you refer to is, in this case, an N78 although the manual cites the KT61. Funnily enough, I do have a KT61 on a stored HMV and it's bulkier than the N78. Anyway the GB resistor was measuring 160 Ohms and the manual specifies it rates at 100 Ohms. Also, the GB resistor itself seemed to have leaked and bubbled in its past so I'd decided to replace it for a modern 100 Ohm from Maplins. I was expecting this would also lift the voltage a touch on the output anode of the N78. Anyway I kept with the specified 100 Ohms resistor for now as that's what the manual lists although I know when you fit a silicon rectifier, you get into murkier waters.
Soldering that resistor was tricky as the modern one is larger in bulk and I found no way to safely squeeze it in. I winded up using a short run of DC wire to one end of the resistor and the valve pin as the only option and I seemed to get a really strong joint in the end. I then tested for Ohms and all was good.

On testing, it seemed to me output anode voltage was a bit lower if anything. I tested the rectifier ripple voltage and that was fine (around 275). I tested my new 100 Ohms resistor and it was merely 2.4 volts between pin and chassis. However, I ought to point out it's a 7 watt resistor and not a 5 watt!

So, now we can consider the 1 K series resistor between rectifier cathodes and the HT capacitor. That, as I'm aware, tends to be a big bone of contention since a lot of engineers feel strongly you don't need the series resistor, just the diodes and capacitors. Or others may recommend a token 200 Ohm resistor to smooth the current rush a bit on start-up. As I'm not a designer, it would be out of my scope to argue any case for or against. I do have a 500 Ohm resistor from Maplins and I'd be interested to see how that might alter the voltage readings. Surely it will go up a bit as I already measured a 30 volt drop across the 1 K that I have.

Lastly, I find myself wondering if at this point I may be better off getting a rectifier valve now I know it works. It would be a bit safer perhaps than silicon diodes soldered to the valve base pins. Of course, that would add at least another tenner to my costs (I think I spent close to 35 pounds to get to this point).

To Refugee: thanks for the tips and I'll follow up on it tomorrow.

[Refugee]

There was some confusion about the valve line up.
I have looked at the data sheet for the N78.
http://www.r-type.org/exhib/aaa0341.htm
I have worked out the value for the resistor and come up with a figure of 150 to 180 ohms. the voltage here should be between 5 and 6 volts.
This resistor should be connected to p1n 2.
Pin 1 is G1 and should be close to 0 volts and no more than 0.5 volts.
The capacitor on pin 1 must be replaced if the voltage is more than 0.5 volts as this will make the anode volts low and cause the valve to get too hot.
The old resistor has indeed been hot.

[Nowhere-Man]

There was some confusion about the valve line up.
I have looked at the data sheet for the N78.
http://www.r-type.org/exhib/aaa0341.htm
I have worked out the value for the resistor and come up with a figure of 150 to 180 ohms. the voltage here should be between 5 and 6 volts.
This resistor should be connected to p1n 2.
Pin 1 is G1 and should be close to 0 volts and no more than 0.5 volts.
The capacitor on pin 1 must be replaced if the voltage is more than 0.5 volts as this will make the anode volts low and cause the valve to get too hot.
The old resistor has indeed been hot.

The resistor I changed specifically is R13. You can see it if you go to Radio Workshop and click on the GEC BC5432 schematic. It was the only resistor that visually looked a bit off. On removal it had blisters and gunge. It measured 160 Ohms. That may have been the original value and may have been used in cases where the N78 replaced the KT61?

I think I can check some of the voltages you outline and make sure the rest of the resistors are rated correctly. I seem to recall I did check the grid stopper and that was fine. I should check R10 again which should be 330 K.

The capacitors ought to be find as they're all brand new. I tend to use the red Wima radials but I extend the short leads with sleeved, thin, copper wire.

Another problem I keep finding is dates. Last night I found the date was 1950 for this radio but originally I thought it was forties.

Anyway, I'll follow up on your suggestions and try to let you know what the voltages are relating to the grid. I'm also now thinking I ought to consider dramatically reducing the series resistance I put in the HT line. I was going to remove the 1 K and insert a 500 Ohm but now I think maybe I'll just parallel the 500 Ohm across the 1 K. That will give around the 300 and something range.As I said, I uprated my capacitors and the Wimas are all at 1200 volts max. Plus I have more high voltage caps across the silicon diodes.

Today my soldering iron packed in. It simply wouldn't heat. I just bought a new 30 Watt unit. I need to tidy up my soldering around the rectifier as at this experimental stage I have HT wire just 2 inches from chassis!!

P.S. Last night I was reading other people have had issues with the HT smoothing cans on this set. They are often replaced or bypassed with a new one. Mine seems a bit suspect to me as it doesn't seem to hold any charge once switched off.

[Nowhere-Man]

As it happens one of the companies sent me an e-mail and have also located the valves for this set. That made me fee la bit awkward as they'd taken time to get the valves and test them but by that time I had already bought what I needed from another company. I think perhaps I ought to make a polite gesture by buying a rectifier valve off them. They had actually traced an X61M which turned out should have been an X79 Mixer. I suppose a plug in rectifier tube will always be a bit safer and tidier than diodes soldered to base pins and hot series resistors.
Should anyone want an X61M let me know as they offered it me for a tenner.

[Refugee]

Are those red capacitors the ones from the 1970s?
It may be worth lifting the end off pin 1 and measuring the voltages again.
I have no idea how well those type of capacitors last over time.

[Nowhere-Man]

Are those red capacitors the ones from the 1970s?
It may be worth lifting the end off pin 1 and measuring the voltages again.
I have no idea how well those type of capacitors last over time.

They're red and not expensive. I've used them before on an Ultra and the result were really good. I even used them as rated mains lines filters at 1200 volt rating. As I said I tend to solder copper wire to the pins and sleeve the copper wire in insulation so I can then use the radials like axials. The downside is there is more soldering involved and the joints have to be solid and safe.

I was held up today by the soldering iron packing in. I may resume work in half an hour test the new iron.

[Nowhere-Man]

This project is now pretty much finished. The output valve anode is now measuring around 275 volts and the series resistor is now 220 Ohms. The GB bias R13 (I think) showed a bit more voltage. The sound quality is surprisingly good. I mean, very impressive. People don't realise how good they had gotten the audio sound even by 1950.

Does anyone have an idea about safety regarding the HT wire to the Smoothing Can? What I finally did was to insulate the 220 series resistor in heat shrink. I then tied it to a thick cable so the whole thing is wrapped in a red heat shrink tube. What I wouldn't like to see happen is the HT wire ever coming off the HT smoothing can and contacting any part of the chassis. So far as I can see, if it did happen the wire would pop upwards and away from the chassis but even so..... I wonder if this ever happened to anyone? I could try taping the wire in such a way as if it did ever come off it wouldn't budge too far??

I was also thinking of a quick blow fuse. Maybe 1 amp? Possibly even modified to fit a plug?

Apart from that it seems O.K. I find it sounds better if there's some earth connection to the earth wire.

[pwdrive]

It would look more professional if you mounted the rectifier diodes and the dropper resistor on a tag strip or board.

Putting the dropper into a heat shrink sleeve isn't usually the norm, if they were put into a sleeve it would be the fiber glass type of sleeve

Lawrence.

[Refugee]

I agree the heat shrink will continue to shrink and eventually crack. The resistor needs to be in open air so that it can keep cool. The heat shrink will act like a blanket and allow it to get hot. The wires on power resistors need to be looped around a tag at each end.
The only exception to this rule about power resistors is if they are mounted on a spring at the non rectifier end so that the joint melts and the resistor springs away from the rectifier.
Phillips used to use these ping off resistors on some 1950s TV sets.
I have an item of test equipment made by Phillips that has a very early version of this type of thermal fuse on the HT transformer.
The spring is attached to a solder tag that is folded around a copper slug that is slid between the transformer windings and soldered to it. If the transformer gets overloaded the solder melts and the spring pulls the solder tag off. The primary winding is connected to the copper slug and the power is connected to the spring.
It disconnects the power and just leaves a live spring hanging in the area that is insulated with that brown fiber board that is shown folded back for the photo to be taken.