Maximum Plate/Screen Voltage 6550 in UL - diyAudio
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Old 28th April 2013, 02:21 AM   #1
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Default Maximum Plate/Screen Voltage 6550 in UL

I have two of John Atwood's One Electron BFT-1B PTs that I want to use in a pair of 6550 monoblocs. This PT has two HT windings, one is 770V CT, the other is 520V CT. The 770V tap translates to about 530V plate quiescent.

The 6550 datasheet seems to limit the applied voltage to 450V. Can I cheat on this up to 530V, and if so then I guess the OT has to be able to take it too. I'm looking at a 5K Edcor OT.

If not, then I'll probably use the 520V CT secondary winding as the screen supply, but this will degrade the performance some I think.

I've built several 50 to 100W guitar amps, lately mostly Dumbles, so I'm used to playing fast and loose with design maximum numbers. Do I need some re-education in this application?
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Old 28th April 2013, 02:35 PM   #2
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kt88's are better suited for UL operation. might aswell use them then instead.
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Old 28th April 2013, 02:48 PM   #3
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Default KT88

Thank you Ruben. I know that the KT88 is generally considered to be tougher than the 6550, however, is a NOS Tung-Sol 6550 perhaps tougher than a current production KT88?

In a pinch I can rise to the cost of four NOS 6550, but certainly not to four NOS KT88, even if I could find them.
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Old 1st May 2013, 07:53 PM   #4
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Quote:
Originally Posted by fleghorn View Post
Thank you Ruben. I know that the KT88 is generally considered to be tougher than the 6550, however, is a NOS Tung-Sol 6550 perhaps tougher than a current production KT88?

In a pinch I can rise to the cost of four NOS 6550, but certainly not to four NOS KT88, even if I could find them.
i have been running two pairs of kt88-98's for about 6000 houres at 450 volt ultra linear with an output impedance of around 3kohm. i might have gotten an really good quad or they are strong.

but if the new production kt88's arent strong enough might aswell bump it up to an kt90.
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Old 1st May 2013, 07:56 PM   #5
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You may give a try to KT120, too. Very good tubes and very affordable.
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Old 1st May 2013, 08:29 PM   #6
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I'm running EH KT-88s at around 430ish SEUL no red plating sounding great at 5K OPTs down to 1-2 ohm load speakers. If you're using cathode bias maybe just upping the cathode resistor some could help??

Cheers,
Bob
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Old 1st May 2013, 09:19 PM   #7
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Quote:
Originally Posted by fleghorn View Post
...however, is a NOS Tung-Sol 6550 perhaps tougher than a current production KT88?
Quite possibily so. True NOS 6550s were reliabily manufactured. And they were used in tube regulated power supplies with higher plate voltages then that. I personally use GE Jan 6550As in ULPP with almost 600 volts on them with no problems. Yes, there may be an occasional NOS failure by fireworks, but these would probably have failed anyway even under lower voltage. I certainly would not feel comfortable using new production 6550/KT88s at this level. Can't speak for KT120s since I've never used them. I have a lot of NOS to use up first.
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Old 1st May 2013, 09:20 PM   #8
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Thanx for your responses. I've been able to find some instances of +500V UL operation of KT88s so I'm going to proceed on that basis. Right now Golden Lion seems to be a well regarded choice in KT88s.

For 70W per channel I'm thinking 5K is about right for RLaa onthe OT. I'm also looking at using Patrick Turner's 50+50 power amp design as a basis, which uses 6K6 OTs. See pic attached.
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Old 1st May 2013, 10:37 PM   #9
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I have used Tungsol Reissues 6550's at 560 volts as per the GEC circuit for multiple KT88's, they lasted for at least 10 years, but that is with 39K as recommended in the GEc circuit on the Grid using Fixed bias, not 100K. Current production tubes such as the KT120 recommend 50k not 100K. Don't try 560 volts with Psvane KT88's or Chinese tubes.
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Old 1st May 2013, 11:00 PM   #10
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That's interesting. I saw somewhere that "Tung-Sol reissue 6550s don't like anything above 500V". Grid bias not specified. Shows you how misleading incomplete info can be!

Turner's design uses 68K grid bias at 520V and I'll be at 520V too, not 560.

I would think your comments on those current KT88s would be cause for concern even at 520V.
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