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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 22nd Feb 2021, 5:49 pm   #21
Skywave
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Arrow Re: Output Valve bias meter? how to wire in correctly?

David: Thanks for your post 19.

As for a metal zero-adjusting screw, hazard arising as a consequence, that is something I've never considered. As I sit here and write this, that is a consideration that I do not know to be applicable to my stock of meters: something I will have to check-out and bear in mind from this moment on.

Al.
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Old 23rd Feb 2021, 10:52 am   #22
Robert Gribnau
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Default Re: Output Valve bias meter? how to wire in correctly?

The earthing of the filament supplies puzzles me a bit. Both the supply for the 71-A filaments and the 6F5 filaments are earthed. Probably this does not create big problems anyway because of the relative high value of the two 50 Ohm resistors. But still I wonder what will be going on current-wise.

The schematic from post #17: https://www.vintage-radio.net/forum/...7&d=1613918787

Addition: If this is too much off-topic, just ignore this post.
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Last edited by Robert Gribnau; 23rd Feb 2021 at 11:13 am. Reason: Addition
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Old 24th Feb 2021, 6:09 pm   #23
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Default Re: Output Valve bias meter? how to wire in correctly?

Quote:
Originally Posted by Whaam68 View Post
Hi All
Thanks for the input tbh I wasn’t expecting much interest!

Usradcoll1 I’ve attached the power supply schematic. The rest is in the firs post. From memory I made a couple of mods to suit what I had on the guidance of Philip at Bluebell audio and to further separate the power supply feeding each channel.

Andy yes I’m old enough to remember Joe 90 and quatermass but the John Mills 80’s version...not the black and white original!

I have read up on how to measure the anode current in fixed bias manually which seems to be measuring the dcr of the output tx primary then the voltage drop from b+ to anode using I=V/R....what I’m still not 100% on is how to meter it in circuit (simply).

So to recap just double checking all I have to do ....accepting the sensible advice re the meters (they are all plastic and I’d only ever use an insulated screwdriver to adjust) is to wire a meter in series with the anode of each of the 71A output valve? So not measuring the voltage drop across a 1r sensing resistor? That almost seems too easy!

Out of interest a pic of the original chassis is attached.

Mike
I was looking at the original chassis!
I know that the valves on the left are voltage regulators but the center black coated one is? The large one on the right is maybe a 807?
Dave, USradcoll1 interested!
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Old 25th Feb 2021, 10:58 am   #24
Whaam68
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Default Re: Output Valve bias meter? how to wire in correctly?

To be honest Dave I can't recall...I asked at the time on the forum (several years ago) and it was deemed to most probably be a power supply from perhaps an old telephone exchange. That was as far as we got...there were a couple of variable pots and lots of tappings....one was an 807 and as you say voltage regulators x 2 ....I still have all the spare parts in the loft ...one day the mammoth 400-0-400v power TX will power maybe a 300B amp or something similar as it has 6.3/5 and 4 volt heater taps. It was a v cheap ebay purchase.
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Old 25th Feb 2021, 1:24 pm   #25
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Default Re: Output Valve bias meter? how to wire in correctly?

Quote:
Originally Posted by Skywave View Post
As I sit here and write this, that is a consideration that I do not know to be applicable to my stock of meters: something I will have to check-out and bear in mind from this moment on.
Definitely one of those things best learned from the experiences of others. It can take a long time for your eyebrows to come back round to the front.

I found out about it as a schoolkid when I asked about the perspex plates mounted on extensions of the screws holding classic round meters in the panels of a walloping big broadcast transmitter. Our guide knew the reason and explained it well. I think he was rather pleased to get asked.

David
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Old 25th Feb 2021, 1:28 pm   #26
ms660
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Default Re: Output Valve bias meter? how to wire in correctly?

The shaft of the meter set zero pot in the HRO receiver can give a kick, amongst other adjustments.

Lawrence.
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Old 25th Feb 2021, 6:26 pm   #27
usradcoll1
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Default Re: Output Valve bias meter? how to wire in correctly?

Quote:
Originally Posted by Whaam68 View Post
To be honest Dave I can't recall...I asked at the time on the forum (several years ago) and it was deemed to most probably be a power supply from perhaps an old telephone exchange. That was as far as we got...there were a couple of variable pots and lots of tappings....one was an 807 and as you say voltage regulators x 2 ....I still have all the spare parts in the loft ...one day the mammoth 400-0-400v power TX will power maybe a 300B amp or something similar as it has 6.3/5 and 4 volt heater taps. It was a v cheap ebay purchase.
Best
Mike
I have a Heathkit W5M amplifier with the matching pre-amp. I was impressed by the elegant approach to setting the output valve bias. Of course, this is a push-pull circuit. When I restored it, it was handy to check it out for proper operation. It was originally furnished with KT66's.
Fully knowing this is slightly OOT, but it's worth looking into.
I'd be lost without the excellent Heath schematic website.
Dave, USradcoll1, retired but, seldom bored.
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