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Old 4th Aug 2022, 1:19 pm   #1
Thyratron Valve
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Default Utra T401 UU6 Rectifier

Some advise please.
I have a Ultra T401 that I am renovating, looking at the actual circuit around the UU6 Rectifier, the heater and cathode wiring is connected to pin 8 of the base, not on pin 1?

This will mean the HT will be taken from the other side of the heater, this is certainly as from factory as has the blue paint on the the solder.
Could this a manufacturing error, if left could this be detrimental to the valve?

I am going to rewire parts of this set as some of the original wiring has gone brittle, so changing that would not be a problem.

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JOHN.
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Old 4th Aug 2022, 2:01 pm   #2
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Default Re: Utra T401 UU6 Rectifier

According to the National Valve Museum, it should be pin 1. Maybe it doesn't matter? It's obviously been OK from new and survived all these years......http://www.r-type.org/exhib/aaa0706.htm

Being directly heated and AC fed I can't see that it would make much, if any, difference.
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Old 4th Aug 2022, 2:14 pm   #3
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Default Re: Utra T401 UU6 Rectifier

The UU6 is indirectly heated.
Perhaps the strange wiring was done in an effort to use the rectifier heater as a HT fuse - a HT short would pass excessive rectifier heater current causing the heater to open. I'm not convinced that this would work though.

John
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Old 4th Aug 2022, 2:16 pm   #4
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Default Re: Utra T401 UU6 Rectifier

The UU6 is an indirectly heated rectifier, the valve museum and various receiver schematics shows pin 1 as h+k, in an Ultra T401 schematic I'm looking at it shows pin 8 as h+k.

EDIT: Post crossed.

Lawrence.
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Old 4th Aug 2022, 5:55 pm   #5
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Default Re: Utra T401 UU6 Rectifier

Quote:
Originally Posted by ms660 View Post
The UU6 is an indirectly heated rectifier, the valve museum and various receiver schematics shows pin 1 as h+k, in an Ultra T401 schematic I'm looking at it shows pin 8 as h+k.

EDIT: Post crossed.

Lawrence.
Thank you, Yes I did noticed that, but it seems strange it is going against the valve manufactures data.

Yes it will work, but do I stick the valve data or the ultra schematic?
i have just tested the transformer out to make sure this is okay, otherwise i could be doing a lot of work only to find the TX defective.

Regards

john
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Old 4th Aug 2022, 5:58 pm   #6
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Default Re: Utra T401 UU6 Rectifier

Quote:
Originally Posted by jjl View Post
The UU6 is indirectly heated.
Perhaps the strange wiring was done in an effort to use the rectifier heater as a HT fuse - a HT short would pass excessive rectifier heater current causing the heater to open. I'm not convinced that this would work though.

John
That is a interesting theory, i think a HT fuse would be cheaper than a new UU6
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Old 4th Aug 2022, 6:00 pm   #7
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Default Re: Utra T401 UU6 Rectifier

Quote:
Originally Posted by Sideband View Post
According to the National Valve Museum, it should be pin 1. Maybe it doesn't matter? It's obviously been OK from new and survived all these years......http://www.r-type.org/exhib/aaa0706.htm

Being directly heated and AC fed I can't see that it would make much, if any, difference.
Thank you it just seems strange

John
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Old 4th Aug 2022, 6:28 pm   #8
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Default Re: Utra T401 UU6 Rectifier

Maybe the designer found it reduced hum on the other side?
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Old 4th Aug 2022, 8:52 pm   #9
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Default Re: Utra T401 UU6 Rectifier

Kevin is onto something here. I have come across this wiring for a UU6 on another set. I rewired it 'correctly' using pin 1 for the cathode, and found that the hum did increase. Put it back to how it was on pin 8, and the hum reduced again.

Not sure about the physical construction of the valve, but I would leave it on pin 8 if that is how the set is wired.
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Old 5th Aug 2022, 12:16 pm   #10
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Default Re: Utra T401 UU6 Rectifier

It's odd that the hum reduces when wired "incorrectly", my simplistic thinking is that the AC heater supply would be imposed upon the pulsed DC at the valve's cathode giving an extra 50Hz ripple component. What actually happens is probably more complex than that though.

John
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Old 5th Aug 2022, 4:11 pm   #11
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Default Re: Utra T401 UU6 Rectifier

Yes, I didn't play around to find out what's going on. Maybe as simple as the heater's resistance having a filtering effect?
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Old 5th Aug 2022, 7:21 pm   #12
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Default Re: Utra T401 UU6 Rectifier

Quote:
Originally Posted by agardiner View Post
Kevin is onto something here. I have come across this wiring for a UU6 on another set. I rewired it 'correctly' using pin 1 for the cathode, and found that the hum did increase. Put it back to how it was on pin 8, and the hum reduced again.

Not sure about the physical construction of the valve, but I would leave it on pin 8 if that is how the set is wired.

Thanks for that
When I am nearer the time, Its going to be easy to swap around, will see if it has a detrimental effects on the hum.

John
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Old 6th Aug 2022, 4:55 pm   #13
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Default Re: Utra T401 UU6 Rectifier

Quote:
Originally Posted by jjl View Post
The UU6 is indirectly heated.
Thank you....I stand corrected!
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Old 7th Aug 2022, 12:51 am   #14
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Default Re: Utra T401 UU6 Rectifier

Wouldn't the effect on hump simply depend on the phasing of the 4V rectifier heater supply vs the HT secondary supply to the UU6 anodes?
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Old 8th Aug 2022, 1:45 pm   #15
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Default Re: Utra T401 UU6 Rectifier

Can you explain this to me?
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Old 8th Aug 2022, 1:55 pm   #16
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Default Re: Utra T401 UU6 Rectifier

Quote:
Originally Posted by agardiner View Post
Yes, I didn't play around to find out what's going on. Maybe as simple as the heater's resistance having a filtering effect?
Now that's a thought. The UU6 may be indirectly heated but it's cathode connection is commoned to the heater rather than having a separate cathode connection. If you connect the valve so that the HT is taken from the 'wrong' heater connection, then there is the resistance of the heater in series with the cathode. Maybe this does produce a filtering effect?
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Old 8th Aug 2022, 1:57 pm   #17
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Default Re: Utra T401 UU6 Rectifier

The UU6 like most other Mazda rectifiers can only be described as terrible and is one of the only rectifiers that suffer repeatedly from low emission. It might be worth checking if BOTH halves of the rectifier are serviceable. It maybe operating on half wave. Just a thought, John.
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Old 8th Aug 2022, 2:17 pm   #18
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Default Re: Utra T401 UU6 Rectifier

The heater resistance is negligible because it is shunted by a very low resistance transformer winding. If the non cathode pin is connected to the reservoir capacitor, a 4V RMS potential at 50Hz is added to the output which is DC with a 100Hz peaky ripple superimposed upon it. So, two peaks will be reinforced, and the following two diminished. Given the usual magnitude of ripple at this point, and considering the action of the reservoir capacitor, I would estimate the difference to be very slight.

UU6 rectifiers were not Mazda's finest hour - flashovers can also be added to their shortcomings. I think their final triumph must be the U801 which looks like two of these in one envelope with an 80V heater....

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Old 11th Aug 2022, 9:30 am   #19
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Default Re: Utra T401 UU6 Rectifier

Thank you all for your help
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