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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only. |
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Thread Tools |
15th Jan 2021, 1:18 am | #1 |
Pentode
Join Date: Jul 2020
Location: Borehamwood, Hertfordshire, UK.
Posts: 216
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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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15th Jan 2021, 10:30 am | #2 |
Nonode
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 2,386
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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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15th Jan 2021, 10:50 am | #3 |
Nonode
Join Date: Jul 2005
Location: Hull, East Yorkshire, UK.
Posts: 2,087
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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. |
16th Jan 2021, 12:06 am | #4 |
Pentode
Join Date: Jul 2020
Location: Borehamwood, Hertfordshire, UK.
Posts: 216
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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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16th Jan 2021, 9:55 am | #5 | |
Dekatron
Join Date: Mar 2008
Location: Worthing, West Sussex, UK
Posts: 5,185
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Re: Champion Model 784 with Microphonic Valve
Quote:
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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16th Jan 2021, 10:43 am | #6 |
Octode
Join Date: Nov 2010
Location: Cullompton, Devon, UK.
Posts: 1,435
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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 |
16th Jan 2021, 11:10 am | #7 |
Moderator
Join Date: Dec 2010
Location: Exeter, Devon and Poole, Dorset UK.
Posts: 6,875
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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 Mike T
__________________
Invisible airwaves crackle with life or at least they used to Mike T BVWS member. www.cossor.co.uk |
16th Jan 2021, 4:12 pm | #8 |
Pentode
Join Date: Jul 2020
Location: Borehamwood, Hertfordshire, UK.
Posts: 216
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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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16th Jan 2021, 10:37 pm | #9 | |
Nonode
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 2,386
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Re: Champion Model 784 with Microphonic Valve
Quote:
Last edited by cathoderay57; 16th Jan 2021 at 10:53 pm. |
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17th Jan 2021, 1:08 am | #10 |
Pentode
Join Date: Jul 2020
Location: Borehamwood, Hertfordshire, UK.
Posts: 216
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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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