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Old 15th Jan 2021, 1:18 am   #1
clockman
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Smile Champion Model 784 with Microphonic Valve

Hi i have finished reviving this Champion 784 Radio it works ok but valve 2 the 6AM6 /EF91 is very microphonic so i could do with a replacement one.i wonder if a member might dig one out for me at a fair price. This seems to be common with these little B7G valves am also in the market for a 25Z4 as a spare
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Old 15th Jan 2021, 10:30 am   #2
cathoderay57
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Default Re: Champion Model 784 with Microphonic Valve

Hi, I've got a Mullard EF91 tests 100% and a 25Z4G that tests 75%. You have a PM. Cheers, Jerry.
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Old 15th Jan 2021, 10:50 am   #3
Neil Purling
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Default Re: Champion Model 784 with Microphonic Valve

I have a bunch of EF91.They are also the 'long life' version with the blue band.
How many variants of the 784 are there I think there's three.
The mid fifties model 832 is just a variant of the same theme.
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Old 16th Jan 2021, 12:06 am   #4
clockman
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Smile Re: Champion Model 784 with Microphonic Valve

Hi i have been soak testing this set and i have found with this model it gets very hot you could fry eggs on the top so i am thinking of changing the valve line up as my other Champion with the B8A valves runs really cool the heat problem with this one is because the large mains dropper and valves 25L6 and 25Z4 get very hot so i am thinking of obtaining valves 6BJ6 and 6BH6 on ebay i have the 35L6 and 35Z4 and small mains dropper and as the valve base pins are the same it should be a straight forward swap this set i think should have had a 600 ohm line cord not the mains dropper .So this is my plan ?
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Old 16th Jan 2021, 9:55 am   #5
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Default Re: Champion Model 784 with Microphonic Valve

Quote:
Hi i have been soak testing this set and i have found with this model it gets very hot you could fry eggs on the top
I have two of these sets, they certainly do run hot, both of mine have stress cracks on the top of the cabinet above the dropper/rectifier/OP valve.
There should be a piece of hardboard that slots in above this area, although it does not reduce the heat that much, the set will certainly run hotter without it!


Mark
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Old 16th Jan 2021, 10:43 am   #6
jonnybear
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Default Re: Champion Model 784 with Microphonic Valve

You could used a capacitive heater dropper or a auto transformer to reduce the heat problem or even a diode.food for thought https://www.vintage-radio.net/forum/...d.php?t=168453.
John
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Old 16th Jan 2021, 11:10 am   #7
Cobaltblue
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Default Re: Champion Model 784 with Microphonic Valve

As JB has pointed out an alternative dropper method may reduce the heat there is a great deal of information on the alternatives and pros and cons here :
https://www.vintage-radio.com/repair...per-calcs.html
Cheers

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Old 16th Jan 2021, 4:12 pm   #8
clockman
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Default Re: Champion Model 784 with Microphonic Valve

HI They both have the hardboard in the top i have painted it with silver hammerite paint allso have put PTFE oven sheet in the bottom as i think this helps
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Old 16th Jan 2021, 10:37 pm   #9
cathoderay57
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Default Re: Champion Model 784 with Microphonic Valve

Quote:
I am thinking of obtaining valves 6BJ6 and 6BH6 on ebay I have the 35L6 and 35Z4 and small mains dropper and as the valve base pins are the same it should be a straight forward swap
Unfortunately it would not simply be a straightforward swap of valves. There are 2 different circuits published in the Radio & TV Servicing data sheet: Original Model using line-up 6BA6/6AM6/25L6/25Z4 and Type B using 6BJ6/6BH6/35L6/35Z4. Firstly, V2 6AM6/6BH6 are not pin compatible: pins 6 and 7 must be reversed. Also the biasing conditions differ for V1 and V2 and so a small number of resistors would have to be changed, as well as the mains dropper to account for the difference in heater voltages. Not too big a job, but would need to be done with care. I don't think that changing the valves is going to alter the heat dissipation much although you might still want to do it for the fun of it! As detailed above a capacitive dropper or a step-down transformer are the best ways to get the set running cooler. I don't think anybody supplies replacement line cord droppers any longer. Hope that helps. Jerry

Last edited by cathoderay57; 16th Jan 2021 at 10:53 pm.
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Old 17th Jan 2021, 1:08 am   #10
clockman
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Default Re: Champion Model 784 with Microphonic Valve

Thanks i have all the published circuit diagrams and have done a modified one it has all the notes that you make including the valve pin and dropper changes it should match my little maestro this does not get very hot with the same valves i do not think this will be a big problem for me as i have replaced all the resistors and capacitors and this set also has a perfect condition speaker but the transformer gets very hot
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