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Post by Richard on Sept 29, 2022 14:19:01 GMT 12
Of course the heaters on the 71 tubes are also the cathode, thats why they have separate windings on the power transformer and R25 is the bias resistor. Does the Audio output perform oK at present? if so, dont fix it IMO.
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Lance Neame
Tinkerer
Ham - Amateur Radio Operator, keen on valve / tube based technology
Posts: 51
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Post by Lance Neame on Sept 29, 2022 15:18:06 GMT 12
Hi Richard,
Excellent points, unfortunately the audio is very low, just okay in a quiet room.
And the supplied voltage to the 42 is under even 5 volts as measured.
Thanks very much for your valuable time to reply. Warm regards, Lance Neame.
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Post by Richard on Oct 1, 2022 8:13:00 GMT 12
good luck with the fault finding, a signal tracer would help you and drawing the circuit out "as built" to see if there are mistakes in the design. The low heater voltage may not be all that is wrong?
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Lance Neame
Tinkerer
Ham - Amateur Radio Operator, keen on valve / tube based technology
Posts: 51
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Post by Lance Neame on Nov 7, 2022 12:17:16 GMT 12
Hi again 😁 After a break on the Philco model 220 radio, I am back at it. With good fortune I acquired some more valves in Christchurch, and amongst those was a type 41 valve. A type 41 is a smaller output tube than the type 42 that was in place when I first looked at the radio. The 41 and 42 have the same pin out connections and number of pins. But being smaller, in turn the heater required less current, 0.4 vs 0.7 Amps at 6.3 volts. So the type 42 that was in there was sucking at least 0.7A and probably dropped the voltage from the original type 71A separate windings. Long story short, the 'new' valve gives far more satisfactory volume and I must measure the heater voltage, I'm sure it will be a lot better, or closer to 6.3v!!! I still need to draw the circuit out, which I will start soon. But I wonder where to next? It is still dodgy with the extra 27 triode, that shouldn't really be necessary since the other type 27 should drive the 41 by itself? Best I draw it out, and come back to you all. Thanks kindly for your time to help! Very best regards, Lance.
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Post by Philconut on Nov 7, 2022 14:51:59 GMT 12
The filament winding for the 71As is rated at 0.5A (0.25A for each tube). At 0.5A the voltage will be close to 5V, unloaded it will be around 0.5V higher. Even with the 41 heater, the voltage will be just above 5V so the 41s emission will be degraded. The maths say that the 71As require 2.5 watts for the pair and as the 41 requires 2.52 watts, the existing 5V winding will adequately cope with the requirements. I have had a look through my transformer box and can easily magic up an auto transformer that will give you 6.3V for the 41. Let me know if you want to proceed.
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Lance Neame
Tinkerer
Ham - Amateur Radio Operator, keen on valve / tube based technology
Posts: 51
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Post by Lance Neame on Nov 8, 2022 8:22:40 GMT 12
www.jaycar.co.nz/12-6v-ct-7va-500ma-centre-tapped-type-2853-transformer/p/MM2013That was a good point, the valve will not be getting sufficient voltage for the heater still. I should have measured it first, but was in a fit of enthusiasm after hearing it work so much better!!! I could always use a Jaycar type transformer to supply that tube? No great difference, except I have one of those transformers new on the bench here. I am still not 100% sure about the current wiring, and are tracing the circuit out. Many thanks for your good advice! Very best regards, Lance.
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Post by Peter Walsham on Nov 8, 2022 8:40:45 GMT 12
Good morning Lance
You may already know this. You will need to dismantle the Jaycar transformer to modify it (not at all difficult to do, as I have used those transformers to feed 6.3 Volts to replacement valves I have fitted in to sets that originally had 2.5 Volt valves fitted) to provide a single 6.3 Volt winding by paralleling the centre tapped 12.6 Volt winding. The secondary coil bobbin is separate to the primary coil bobbin & can be very easily modified to provide 6.3 Volts. You will need to remove the outer layer of tape & disconnect the three 'tails' from the winding. Then wire the two 6.3 Volt windings in parallel (making sure you get the phasing correct) & refit two tails to the ends of the paralleled windings, then reassemble the transformer & test it.
Peter
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Lance Neame
Tinkerer
Ham - Amateur Radio Operator, keen on valve / tube based technology
Posts: 51
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Post by Lance Neame on Nov 8, 2022 19:17:20 GMT 12
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Post by Peter Walsham on Nov 8, 2022 19:54:03 GMT 12
It might be OK. One of the 6.3 Volt windings will be wound over the other and if you use the inner winding, it may get a bit hotter than if you use the outer winding.
However, my comment about paralleling the windings was to reduce voltage drop more than anything, and using as much of the copper on the secondary as you can, and improving the voltage regulation (by reducing the 'I R' losses), which also allows the transformer to run cooler. Using half of the secondary winding is certainly within the transformers ratings if you'd rather not dismantle the transformer. There's certainly no harm in trying the transformer out unmodified to begin with, measure the 41's heater voltage, and monitor the transformer to see how warm it gets. The toroid transformer would certainly be an option for sure, but the Jaycar one is quite suitable. Try it & see what happens. I'm no transformer engineering expert.
However, with more current 'available' from the heater transformer, you could substitute a 42 output valve for the 41 and get more power output from the radio. The type 42 valve does have a slightly higher transconductance (effectively( meaning that it will produce more output for a given input - although the difference between a 42 & the 41 is not all that great. Any gain is better than no gain though.
Peter
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Lance Neame
Tinkerer
Ham - Amateur Radio Operator, keen on valve / tube based technology
Posts: 51
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Post by Lance Neame on Nov 11, 2022 10:13:23 GMT 12
Hi Peter, Your are definitely correct of course, I am always testing my luck! 😀 I have done a dodgy set-up on the bench as shown, and note that the multimeter readings are actually the same, just they need to be calibrated (or binned and a decent pair bought!). The transformer is only sitting at above ambient temperature. The two Russian 6V6 tubes are running quite happily at just over 6.3v . I would love recommendations for a good true RMS multimeter by the way from anyone? I would actually like to get rid of the extra 27 tube and 41, and wire in a 6SJ7 and 6V6 in place. Anyway, I might be still delirious in recovery with that idea🤣🤣🤣 Thanks once again! Best regards, Lance.
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Post by Peter Walsham on Nov 13, 2022 9:50:47 GMT 12
That combination (6SJ7 & 6V6) might give you too much audio gain. However, you could possibly connect the grid of the 6SJ7 to the anode of the 24 detector valve through capacitor 'C20', & 'bypass' the original type 27 1st audio valve & see how much audio gain you then have.
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