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Old 4th June 2012, 08:08 PM   #181
7N7 is offline 7N7  United Kingdom
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Well I just know that valve rectifiers do not like large capacitances (the largest I know of is 60uF for GZ34) and those enormous ones would look like prolonged short circuits. As an example the famous U19 HV rectifier had a limit of just 4uF. And as I say there is no need for those huge capacitors. You can I am sure quite easily calculate what your output ripple voltage will be; as I recall calculating ripple current is less easy. By the way be sure to fit snubbers to your choke input supplies; something like 0.22uF at 2kV I guess.

I presume the current capacity of the 10H chokes is sufficient for the job? remember that in a choke input supply the choke has to have a rating much greater than the load current and the higher the voltage the higher the additional requirement.
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Old 4th June 2012, 08:13 PM   #182
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yep...excessive capacitance will stress the tube but the choke should have allowed for such large value of capacitance.

the choke is 10H rated at 1A continous and tested at 2500v. I suspect it will do the job nicely (predicted screen current is max 500mA at full load).

This is just screen voltage so max 650v.
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Old 4th June 2012, 08:17 PM   #183
7N7 is offline 7N7  United Kingdom
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That's a BIG choke!

But why the huge capacitors? Really we need someone else to join this - I am a bit "rusty" to say the least.
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Old 4th June 2012, 08:20 PM   #184
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as I said i had them handy (scavenged for a dime)....were new...and looked nice (very technical reason) also all simulations said they would have worked...

Maybe a load is required to see some voltage?

yes I hope either tubelab.com or pmillet or any other expert will join the conversation. This is stretching tube psu design so it might just help people avoid stupid mistakes (like the ones I made).

the other boards, on the other hand, work perfectly
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Old 4th June 2012, 09:01 PM   #185
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update... the voltage is there, just in the tiniest amount...39mV more or less.
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Old 4th June 2012, 09:20 PM   #186
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Without a load, a choke-input power supply's output will soar to the voltage of an unloaded cap input. Is it possible that the 700 volts worth of caps have been destroyed, and failed to a short?

All good fortune,
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Old 4th June 2012, 09:28 PM   #187
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I never powered the hv xformer above 50% mains so i doubt the caps are busted. There no bulges smoke or leaks... the real test will be bypassing them and using a smaller bank. That will solve it hopefully.

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Old 5th June 2012, 12:44 AM   #188
Sheldon is offline Sheldon  United States
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Quote:
Originally Posted by Alexontherocks View Post
assembly of top plate completed. Remaining slot is for an additional gain stage.

Looking good!

Click the image to open in full size.
You asked earlier, and I don't know if it was answered. But yes, I'd say those large electrolytic caps will get way too much radiated heat. I would put a reflective shield between the caps and tubes, and probably a small fan to clear the air inside the shield. Maybe a u-shaped shield. Or perhaps a cylinder around each cap with a cm or so of clearance (again add something to actively move cool air around the cap - small fan should do).

The tubes themselves can tolerate the radiated heat better than the caps, as long as you de-rate them by 25% or so and move the air.

Sheldon

Last edited by Sheldon; 5th June 2012 at 12:48 AM.
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Old 5th June 2012, 07:40 AM   #189
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Hi Sheldon, thanks for posting! The truth is maybe the caps are useless anyways so just for aestethics I'll leave them on the top panel but somehow filterthe screen voltage within the amp. The temperature is actually much lower thann expected. With 866A the closest section of the capacitors barely gets warm!

the filaments are unfortunately undervolted by 16% (too much drop in xformer voltage output) but can that account for NOT having a hot surface?
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Old 5th June 2012, 11:47 AM   #190
djn is offline djn  United States
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If you are not going to use the caps, it might benefit the amp to take them out so you get better air flow. Anyway, just a thought.
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