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Old 3rd December 2008, 06:50 AM   #21
poynton is offline poynton  United Kingdom
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Location: A life on the ocean waves when I'm not at home in N. Wales (but I'm not Welsh so no sheep jokes!)
Quote:
Originally posted by revintage
To make this cicuit work you have to take out max currents through the 6080s which means their workingpoint should be Ia 125mA, Ua 100V.

You should also use a choke or a CCS from the cathodes to ground. A B+ of 130-170V will do.

Every triode has an theoretical (simplified 1/Gm)Rout of ca 140ohms, so adding a separate 10 ohm to each one will not make you loose so much. Without NFB Zout will be high anyway.

If going for a 8ohm load I would suggest using at least 5 but preferably 10 6080s/channel.

I suggested using a CCS some time ago.

OTL SE - Help Wanted with Idea

However, there was concern that oscillation would (had) occured.
Maybe a CCS per cathode?


Andy
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Old 3rd December 2008, 07:02 AM   #22
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Have anyone really tried a CCS as I know you suggested? Must be the most costeffective way to go as chokes cost a lot. Just using a resistor will make you lose to much power and worsen THD.

Think a CCS robbed from Pass would be fine......
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Old 3rd December 2008, 07:14 AM   #23
poynton is offline poynton  United Kingdom
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To be honest, I could not see why a CCS would cause oscillation and instability as it is well used, as you say in Pass designs.

And as you say, cost is minimal !!!

So far, I have not had chance to try anything due to work. ( I am now 200km off the east coast of India ! )


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Old 3rd December 2008, 08:50 AM   #24
Fuling is offline Fuling  Sweden
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If you can find transformer cores, bobbins and magnet wire there´s no need to pay 60 bucks each for cathode chokes. I made my own for my SE OTL with 4x PL504 (per channel):

600 turns 0,45mm wire on EI78 cores with ~0,5mm airgap.
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Old 3rd December 2008, 09:56 AM   #25
Klimon is offline Klimon  Belgium
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Quote:
60 bucks each
30$ each actually; appealing idea to diy however I'ld have to find a parts supplier here (maybe in Germany?)

Regards - Simon
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Old 3rd December 2008, 10:07 AM   #26
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Hey Fuling,
As I know you are keen to try new solutions, have you tried CCSs?

My (theoretical) idea is that one could lower B+ to consist of 100V over the 6080s, the negative swing at decided load and the voltage drop over the CCS.

So a std 115V secondary transformer should be sufficient.
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Old 3rd December 2008, 12:03 PM   #27
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This is a theoretical model of what I mean. Simmed it works well. Stole the CCS from Pass Zen and turned it upside down and changed P to N !

Almost like it is too simple as the grids are at ground level. 12,5W through each 6080. At 5% THD output is 12W/16ohm and 6W/8ohm. At 1% THD output is 2,5W/16ohm and 0,5W/8ohm
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Old 3rd December 2008, 01:25 PM   #28
poynton is offline poynton  United Kingdom
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That's pretty much what I had in my previous postigs although I cannot remember where my CCS came from!

What current have you through the CCS ?

How about -ve bias for the tubes -?

Andy
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Old 3rd December 2008, 01:36 PM   #29
Fuling is offline Fuling  Sweden
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Klimon: I bought my stuff from www.svebry.se
Look under "produkter", then "transformatorer" and then "transformatorbyggsatser".
I guess you can write to them in english and ask if they ship abroad.
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Old 3rd December 2008, 01:38 PM   #30
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As I said earlier 100V/125mA through the tubes. No need to bias the tubes otherwise than with the CCS. If you check UaIa-curves, Ug is in the ballpark of -30V. So there is enough room for negative swing even if we should loose 10V over the CCS.
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