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Old 3rd March 2012, 06:59 AM   #51
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Great. thanks!

actually there is no big switch but small switch since the main switch will just power a small transformer which will in turn close relay/timer contacts for all the LV and HV transformers.

One small suggestion I have for MV based amps is to have full set of xenon replacements. People don't think about it, but you cannot flip the amp on its side with MV in action. Every reading, oscilloscope probing ecc will have to be done withouth turning the amp. Scary thing to work under a 65kg chassis!

I have full 3b28 replacements but there is a surprise for the screen rectifiers. let us just say the rectifiers I have found are almost as large as gm100
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Old 3rd March 2012, 11:04 AM   #52
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Click the image to open in full size.

Click the image to open in full size.

Last edited by Alexontherocks; 3rd March 2012 at 11:32 AM.
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Old 3rd March 2012, 01:28 PM   #53
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sweet... only 3mA

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One thing which is important is for the flow to stabilize. Even adeqwuate preheating is not enough to get a steady stream between anode and cathode (steady ionization). Just an observation.
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Old 3rd March 2012, 01:31 PM   #54
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Also phase inversion between power transformer and hater transformer will "shift" the flow from one end of the cathode to another. Optimum setup requires full outofphase operation.

12mA

Click the image to open in full size.

Last edited by Alexontherocks; 3rd March 2012 at 01:38 PM.
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Old 3rd March 2012, 01:58 PM   #55
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I could stand for hours looking at the subtle variations in the electron flow... tubes are just wonderful devices. I should have (in a week or so) neon rectifiers. I will be able to do extensive testing between mercury, xenon and neon rectifier. I will find which is best to my ears and to my scope.
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Old 3rd March 2012, 05:03 PM   #56
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A question for you guys (tubelab especially since he has experience with big poweramps)

I must create or buy anode caps for the gu81m (preferably 8 of them to play around with g3 connections)

the top anode connections on the gu81m are 20mm in diameter. The largest cap size is (if I am not mistaken) 13mm (which I have in quantity).

Now to my question:

1. is there a commrcial solution ie: 20mm ID anode caps?
2. what is the temperature I might see at full load on the anode connections? Is teflon/ceramic the only way to go?
3. Is there another way? I'd rather not solder anything on the anode connections so a mechanical solution would be best.

Thanks!

Ah also....I plan on using 5w 100K resistors for balancing. These should provide both balance and bleeding. Is this okay?
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Old 3rd March 2012, 07:42 PM   #57
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flaking guinea pig...my tubes are NOS from 1962 (coincidentally produced 2nd of october which is the day of my birthday). I wanted to test them out so:

280v with 5H choke (53ohms) caps and a 33r 50w load.

according tu PSUD II I am pushing almost 300mA through the tube. 5 hours straight. I see no transformation from the plate. can't go further....my choke is supposedly limited to 200mA DC (and given this is not pure DC I decided to go 50% above)
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Old 4th March 2012, 08:24 AM   #58
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and here is the king of rectifiers... 100KW power huge ball of mercury vapors

Click the image to open in full size.

Unfortunately the plasma is shielded from the eye but inside you can clearly see the "show" a couple of these will provide perfect 600v to the gu81m.

This amplifier will be unique. A gem!
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Old 4th March 2012, 08:33 AM   #59
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Click the image to open in full size.

better photo
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Old 4th March 2012, 11:08 AM   #60
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regardless of the colour here are all 470uf 450v caps in series (very good italian brands I tested in the past). Tested and resistor balanced..

Click the image to open in full size.

5 in series x2

should provide adequate filtering and protection against HV and surges..

even four of these could easily provide 1800v and with an additional 10% headroom almost 2000v. With 5 I get a total max rating of 2475v. Total capacity: 188uf. Should be plenty for the anode voltage.

There was really no other choice in terms of weight, space, cost ecc. I can afford to have one blow per bank so pretty safe against failure. In this amp I implemented the "every cap has a backup" rule....so every cap stage has a symmetrical (or in one case smaller) stage behind it. This way failure will not result in open circuits and nice 2000eur explosion. I'd really not want my bias to fail during operation.

Also I have a spare -150v line onboard to provide a backup if the situation should arise.

Hopefully I am not straying in the wrong direction.

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