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Old 6th April 2011, 05:18 AM   #1
BRSHiFi is offline BRSHiFi  United States
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Default Quad KT88 driver

What is the best driver stage for a quad of KT88 tubes? I have been doing research and have come up with some ideas but largely drew a blank. Some of you probably have more experience in this matter. First off I have seen a circuit published in the 50's that uses a standard MKIII driver board with the quad.

www.the-planet.org/dynaco/Misc/Transformers.pdfwww.the-planet.org/dynaco/Misc/Transformers.pdf


The original Dynaco driver board does not look ideal but the Poseidon upgrade board looks more promising, as it sounds pretty good in my MKIII. My only question is it can it drive the capacitance and 50k ohms without incident?
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Old 6th April 2011, 11:27 AM   #2
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hey-Hey!!!,
The general topology of the dynaco circuit is fine...but the implementation is not good. Consider a better pentode; 6EJ7 perhaps, followed by a 6AH4 for the split-load phase splitter. Run the pentode with 15-20k plate load and the 6AH4 with 10k loads. Set the pentode plate voltage at 20-22% of B+ available to the split-load.
cheers,
Douglas
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Old 6th April 2011, 11:35 AM   #3
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Morning Douglas,
Would you have a schematic you can share???

Thanks,
Ray
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Old 10th April 2011, 08:10 PM   #4
BRSHiFi is offline BRSHiFi  United States
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Here is an amplifier I found on this site made by gingertube.

http://evatco.com.au/project_files/kt88.htm

http://evatco.com.au/project_files/dud120.jpg

http://evatco.com.au/project_files/dud120-2.jpg

Here are some changes I will make.

- Add a beefier B+ power supply by replacing the resistor with a regulator circuit.

- Replace the 6SN7s with E88CC's in the cathode follower. Is it better to use then the 6SN7s there?

- What about the Zobel network? This was designed to dampen an erratic ultrasonic ringing in the Hammond transformer (see The Set To Work description). Is it really important to correct that sort of anomaly? I am not equipped with a frequency sweep generator, so I how do I detect that erratic response if it is necessary?
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Old 11th April 2011, 02:27 PM   #5
BRSHiFi is offline BRSHiFi  United States
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The reason I am wondering about the Zobel network is because I am using a Heybor transformer instead of the Hammond.
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Old 11th April 2011, 05:54 PM   #6
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by kingneb View Post
The reason I am wondering about the Zobel network is because I am using a Heybor transformer instead of the Hammond.
Those networks are specific to the Hammond transformer used, that said IMHO you really will need a scope and generator to get this thing stable and properly compensated.
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Old 11th April 2011, 06:20 PM   #7
BRSHiFi is offline BRSHiFi  United States
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I have a function generator. Do I need a sweep function generator, one that can make a bode plot? I have at my disposal a Tektronix scope as well.
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