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Old 19th December 2008, 03:55 PM   #1
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Default Single ended KT88 question

I am going to build my first amp, and I have some questions to ask.

I googled and found only Mikael Abdellahs and related ultralinear connected amps using KT88 in single ended, but i would need cca 25w of output so I was thinking about pentode operation. I found in svetlana datasheet that with anode at 400V, G2 225V, and anode I 87mA yields 20W output.

Now the question is how to get constant G2 voltage from PT?
Idle current of G2 is 4mA and 18mA at max signal. So if I lower the voltage from B+ cca 400V to 225V with resistor ther will be great differences in voltage between 0 and max signal. Should I ask my PT manufacturer to add extra taps for lower voltage?

Can I get make this amp working at 440V at anode with cathode bias cca -20 V so voltage across the tube is 420V and higher G2 Voltage so the anode current is 100mA. That would give 42W plate dissipation and probably 24W output?

Thank you!

Miha
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Old 19th December 2008, 04:06 PM   #2
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sounds a bit optimistic...25w and single ended.........thats more 211 or 845 or GM70 territory I think

just my two penneth, but I may have misunderstood your goal.

Ed
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Old 19th December 2008, 04:36 PM   #3
45 is offline 45  Italy
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Default Re: Single ended KT88 question

Quote:
Originally posted by overdrajv
I am going to build my first amp, and I have some questions to ask.

I googled and found only Mikael Abdellahs and related ultralinear connected amps using KT88 in single ended, but i would need cca 25w of output so I was thinking about pentode operation. I found in svetlana datasheet that with anode at 400V, G2 225V, and anode I 87mA yields 20W output.

Now the question is how to get constant G2 voltage from PT?
Idle current of G2 is 4mA and 18mA at max signal. So if I lower the voltage from B+ cca 400V to 225V with resistor ther will be great differences in voltage between 0 and max signal. Should I ask my PT manufacturer to add extra taps for lower voltage?

Can I get make this amp working at 440V at anode with cathode bias cca -20 V so voltage across the tube is 420V and higher G2 Voltage so the anode current is 100mA. That would give 42W plate dissipation and probably 24W output?

Thank you!

Miha

I don't think you will notice a difference between 20 and 24W in terms of SPL!

Instead running the KT88 at 35W rather than 42W could give an overall better sound because of less stress and the life of the tube will be considerably longer.

KT88 is not like 211...


The problem about screen grid voltage regulation is instead an interesting point.
Having a costant grid voltage, regardless of modulation, will result in lower distortion.
The simplest way to do it is using an series of zener diodes. It is not the best but it is cheap and works better than a simple RC coupling, IMO.

Cheers,
45
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Old 19th December 2008, 05:35 PM   #4
45 is offline 45  Italy
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Default Re: Re: Single ended KT88 question

PS
It would be better if you have an extra tap.

Then, together with the zener diodes, you will see the difference...
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Old 19th December 2008, 08:22 PM   #5
DougL is offline DougL  United States
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45's recommendations are Spot on.

To Make it more general, it is highly recommended to actively regulate your G2 supply. Because of the modest currents required, this can be done either with a series or shunt regulator.

Simplest shunt regulator: A resistor and a zener string.

Doug
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Old 20th December 2008, 11:17 AM   #6
bigwill is offline bigwill  United Kingdom
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Or VR tubes if you want to be pretty
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Old 20th December 2008, 04:51 PM   #7
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Quote:
Originally posted by DougL
45's recommendations are Spot on.

To Make it more general, it is highly recommended to actively regulate your G2 supply. Because of the modest currents required, this can be done either with a series or shunt regulator.

Simplest shunt regulator: A resistor and a zener string.

Doug

What type/brand of zener would be the choice to use?
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Old 20th December 2008, 07:47 PM   #8
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Default Re: Single ended KT88 question

Quote:
Originally posted by overdrajv
Now the question is how to get constant G2 voltage from PT?
Idle current of G2 is 4mA and 18mA at max signal. So if I lower the voltage from B+ cca 400V to 225V with resistor ther will be great differences in voltage between 0 and max signal. Should I ask my PT manufacturer to add extra taps for lower voltage?
Use an active regulator. I include this subsystem in my designs that use pentode finals. Nailing down that screen voltage and providing a Lo-Z impedance to AC ground greatly improves the sonics.

Active Screen Regulator

I used this one for my latest project. You could also do a solid state version as well.
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Old 22nd December 2008, 10:44 AM   #9
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Default Re: Re: Single ended KT88 question

Quote:
Originally posted by 45





Having a costant grid voltage, regardless of modulation, will result in lower distortion.
The simplest way to do it is using an series of zener diodes. It is not the best but it is cheap and works better than a simple RC coupling, IMO.

Cheers,
45
Thank you all for your replies!

But isnt it so that the modulation of the grid voltage would reduce the gain and distortion, acting more like UL or triode operation?

What about cathode resistor? Did I calculate right?

Thanks again,

Miha
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