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Old 3rd Jul 2022, 10:27 pm   #1
vintage_8bit
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Default RAP Transatlantic & high H.T. (1937?)

History so far.

I changed the rectifier valve base from a non-original IO base to a UX4 base & fitted a new "80" valve. I then went through the usual recapping & resistor checks. The front end of this radio is mostly hidden under the wave change switch. Several wires have to be removed to lift this assembly out to gain access to the components, it’s quite a pain.

After refitting a rewound output transformer, I was pleased to find on power up a working radio. I have not checked alignment yet but it seems responsive & sounds good through its energised speaker.

I was concerned however at the high H.T. voltage at around 300V after the energising coil. Mains input was at 240V & input selector set to "240/250V" I did check for incorrectly wired mains transformer primary. Heater voltages are as expected. As the rectifier is directly heated the output, before the energised coil raises to 400V before the rest of the valves catch up. I know I can fit a dropper resistor to help the valves out, but would be interested in views from others. I have a nicely drawn circuit from another enthusiast, but there appears no other information available. Colin
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Old 3rd Jul 2022, 11:56 pm   #2
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Default Re: RAP Transatlantic & high H.T. (1937?)

The surge voltage is common to all sets with field coils and directly heated rectifiers.
Are the speaker and mains transformer original? I think they look as though they are.
If original then the most likely cause is insufficient current being drawn by the set causing too small a voltage drop across the field coil. This would typically be due to a weak (or wrong) output valve but it is also not uncommon for set designers to run valves VERY hot.

What is the 42 cathode voltage? Does the cathode resistor measure correctly at 500 ohms?
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Old 4th Jul 2022, 8:58 am   #3
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Default Re: RAP Transatlantic & high H.T. (1937?)

Thanks for the reply PJL.

The speaker construction seems of the period & origional as far as I can tell. The same goes for the mains transformer. There is virtully no Hum & the sound quality is very good. The output valve looks good on a VCM as well.

The cathode resistor was low & the cathode current was 40mA. I have increased the cathode resistor value to a bit over 500 ohms to bring the current down to around 35mA. I can get exact values when in the workshop latter but the cathode voltage is now around 17V (AVO 8)

What would you say the H.T. should be 220 / 250V perhaps?

I have measure the voltages with a calibrated AVO 8. All in all & apart from the H. T. volts concern, the set performs well. Colin
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Old 4th Jul 2022, 9:03 am   #4
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Default Re: RAP Transatlantic & high H.T. (1937?)

I like your original 'British Insulated' reservoir capacitor. Did you re-stuff the innards or is it original?
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Old 4th Jul 2022, 11:03 am   #5
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Default Re: RAP Transatlantic & high H.T. (1937?)

I re-stuffed the 'British Insulated' cap box & all the cardboard tube electrolytics. The rubber insulation around the mains transformer & capacitor box couldn't be saved. With the paper caps I used modern replacements but they are predomantly black so it looks fairly origional.
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Old 5th Jul 2022, 12:36 pm   #6
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Default Re: RAP Transatlantic & high H.T. (1937?)

So I can find nothing obviously wrong so I think I will put a dropper resistor in & try to get the H.T. down to around 240V. As there is a separate feed from the rectifier to the local osc side, I was thinking of putting the dropper in the centre tap line of the H.T. winding. This will keep the differences between the L/O feed & the main H.T. line the same. Any thoughts welcome. Colin
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Old 5th Jul 2022, 3:51 pm   #7
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Default Re: RAP Transatlantic & high H.T. (1937?)

I restored one of these sets recently and the HT voltage of around 300V is about what I was getting, I was surprised too! Check what voltage you have at the end of the dropper resistor chain 37k + 40K to the oscillator. If you look at the characteristics for the 42 it gives a design centre reference of 375V anode volts. With an anode voltage of 285 it states a cathode resistance for bias of 440 ohms, I'm guessing they designed it for 300V HT as the cathode resistance is 500 ohms
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Old 5th Jul 2022, 5:20 pm   #8
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Default Re: RAP Transatlantic & high H.T. (1937?)

Thankyou for that information Peter. I ran the radio with no modifications & at the typical mains voltage for my workshop, around 245V. I had an H.T. voltage of 290V.
The cathode current of V4 was 38mA. Looking at the 42 characteristics & design centres, (Now you have pointed me to them thanks). The total design current is 40.5mA if I'm reading it right.

The voltage after the R12/R14 dropper resistors is 176V.

So perhaps I was worrying unnecessarily? I have restored a few radios now & not seen more than about 260V, until now. I fitted a N.O.S. rectifier which of course helps keep up the volts. Colin
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Old 5th Jul 2022, 8:19 pm   #9
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Default Re: RAP Transatlantic & high H.T. (1937?)

The specification to watch for is maximum anode dissipation as too much can result in thermal runaway.
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Old 7th Jul 2022, 7:00 am   #10
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Default Re: RAP Transatlantic & high H.T. (1937?)

If total valve current is 35ma and screen current is about 7ma according to spec sheet, anode (plate) dissipation is only 8w (maximum is 11/12w), based upon a plate current is 28ma and plate voltage (anode-cathode) of 283v. You've worried about the valve unnecessarily I would think. I would restore the cathode resistor to the original value which should also lower global HT a bit in case this makes a difference to the other valves
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Old 7th Jul 2022, 9:01 am   #11
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Default Re: RAP Transatlantic & high H.T. (1937?)

Thanks for the replies. If I have done this right....

I measured the actual anode to cathode voltages. I then worked out the separate anode & screen currents from the volts drop across the output transformer primary & cathode resistor.

I concluded with my measurements that the anode dissipation is 7.4W. Design centre is 11W. (Not to far away from Gabe001's calculation)

The screen dissipation is 2.6W. Design centre is 3.75W.

The cathode resistor is now fixed at 500 Ohms.

With the price & rarity of some output valves I was initially concerned it may of been over stressed. With the forums help, I can sleep easy now.

Colin
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Old 15th Jul 2022, 3:16 pm   #12
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Default Re: RAP Transatlantic & high H.T. (1937?)

I can report that the RAP chassis is back in its cabinet & working well, currently tuned to Radio Caroline.

The following list details some of the work.

Re-fitted correct rectifier & its associated valve base. (Previously modified some years ago with an I,O base & 5Z46 this valve was compleatly dead on one side.)

One open circuit resistor.

Cathode resistor on V4 was low. I hid another resistor in series to correct the value.

One open circuit output transformer. Ed sorted that one out, thanks.

All waxes changed.

All electrolytics re-stuffed.

1 modern looking & temp fitted 50K lin pot for tone. Still looking for a suitable vintage looking replacement.

Full alignment. I have not done that without a procedure before, but it seemed to go well.

The radio was origionaly purchased from Bennett & Brown in Gravesend. They are still in business, but sell furniture now.

Thanks to all that contributed to this thread


Colin
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