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Old 31st March 2008, 03:15 PM   #1
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Default Directly heated Cathode/insulation question

I propose* to insert a 5R4GBW diode in series after a bridge rectifier.
The line in which the diode is to be inserted is at a potential of +1200V approx (rectified from an 800Vac secondary winding). As you may know the cathode is directly heated by 5 volts ac/dc.

I will be using another 5V winding to supply the filament. This winding will then float at 1200Vdc. My concern is this; will the internal insulation of the transformer be able to cope? Does anyone have experience on the subject??

*FYI I want to use the warm-up time of the 5R4.. to act as a delay mechanism before HT is applied to the amplifying triode plates.

JB
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Old 31st March 2008, 03:36 PM   #2
nhuwar is offline nhuwar  United States
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You need a filament transformer made to have that voltage across is. Not just any transformers will do.

I have mine custom made with a saftey factor of atleast 2 time the voltage stand off.


Nick
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Old 31st March 2008, 03:38 PM   #3
kmaier is offline kmaier  United States
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JB,

The winding voltage insulation is a transformer spec... look at what you're buying and see what the rating is. Note that the 5R4Gxx being directly heated is not going to have much delay at all in voltage output. If you want to delay the voltage using a rectifier tube, look for an indirectly-heated cathode type.

Regards, KM
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Old 31st March 2008, 05:41 PM   #4
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Thanks both Nick and KM.
I'll look for an indirectly heated cathode.
The diode will need to operate at high voltage, although the PIV will not be an issue, as it will be installed after the choke, in fact between the choke and one end of the OPT. The other end of the OPT is connected to the anode of the output triode (845).

Any idea on what I might use instead of the 5R4... given that I'm looking for a long warm-up delay?

Cheers
JB
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Old 31st March 2008, 05:47 PM   #5
nhuwar is offline nhuwar  United States
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You got me on that one I use mercs on my supply. But I don't know if you will have any luck finding one that can handle that high a voltage.

Plus you still have to have the filament supply elevated above ground as to not stress out the cathode insulation.



Nick
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Old 31st March 2008, 05:55 PM   #6
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Yep! forgot filament to cathode consideration. I guess it's back to the drawing board....or buy a new filament Trans.
Thanks again
JB
Hold on the filament will not be grounded, it will float. So it should not bother the cathode insulation. Right?
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Old 31st March 2008, 07:29 PM   #7
nhuwar is offline nhuwar  United States
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If there is 1kv on the cathode and you ground the filament then you would probably have a failure.

Your filament supply would be about the same for both indirect and directly heated tubes.

You would still need a good filament transformer that could stand off a couple kv in my mind.


Nick
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Old 31st March 2008, 08:10 PM   #8
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You are right, better safe than sorry
JB
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Old 31st March 2008, 08:22 PM   #9
kmaier is offline kmaier  United States
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Most 5V indirectly heated rectifiers have the cathode tied to one of the filament lines in, like the 5AR4, 5V4G, etc. so you can't have that situation. Also, most filament transformers are rated at 2000 VRMS insulation... even the Hammonds meet this spec. What current range are you requiring for the B+?

Regards, KM
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Old 31st March 2008, 08:47 PM   #10
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I was just looking at the 5AR4 data-sheet online, and that is shown as strapped internally.

So it would appear that this is common practice.

But it does seem to violate safety, by bringing high dc voltages back into the transformer? Which was the basis of my concern when I started to wire things up to include the diode .

PS
I expect the 845s to draw about 120 ma each so total 240ma, max fused at 200ma per channel just in case I lose bias
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