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Old 4th February 2004, 07:51 PM   #1
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Question 6550, kt88's at 600+ Volts

I am still looking for a high power tube amp project. It seems that 3cx300a1, 845, and 211 in push pull at high voltages is not a good idea (too much money and complexity). I think my best bet would be to run 6550, kt88, kt90, or even kt100 at high plate voltages to get the kind of power I'm looking for. Has anyone tried this? The tube data sheets for the kt88 claim 100 watts for 600V B+ I believe, but I have never seen any schematics at these voltages.

My dilemma is that I want high power but refinement too. I would like to build the amp to sound good in the midrange frequencies and give that "tubey" sound. How would these tubes sound at such high voltages? I suspect (by experimenting with other tubes) that they will begin to sound "sterile" and ss like because the operating point shifts towards class B.

Any suggestions on other high power diss. tubes that dont require extremely high voltages (> 1000V) and primary impedances (> 6000 ohms).
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Old 5th February 2004, 04:17 AM   #2
angel is offline angel  Norway
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You can operate the 3CX300A1 at lower plate voltages, and it'll be happy with a primary impedance of anything from 450 ohms and up. You'd need a fan for high power in class A1, though. In class AB1, you could do without a fan, and peak 300W.. just set the bias right.

As to other valves, the 6550 has pretty okay drive requirements, and will be happy with 5Kohm primary impedance, though that's close to the 6K limit you mentioned.

6C33C-B should deliver 60W in class A1 push-pull, with about 200 ohm plate-to-plate impedance. You could sacrifice some power to go for a higher impedance, and gain damping..

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Old 5th February 2004, 08:02 AM   #3
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I would not recommend operating the 3CX300A1 without forced air cooling. You take the chance of melting the seals.
The tube is designed for forced air cooling under all conditions.
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Old 5th February 2004, 03:08 PM   #4
KlausB is offline KlausB  Germany
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Hi there,

with KT88, 6550A or even the EL34 you can get 100watts of power with sufficient anode voltage (600 to 800 V depending on the tube type).

The problem is that the maximum voltages of the screen grids in these tubes are limited to 400V. Hence you cannot (or should not) run your tubes in the commonly used ultralinear circuitry which uses feedback from the primary coils of the output transformer into the screen grid .

You are either limited to pure pentode operation (which I did with a 100W bass guitar amp with 2 x El34 at 800V and 11k Raa, Ug2 400V, not suitable for hifi ) or you inject your feedback into the power tube cathodes with separate coils.

To my knowledge the latter circuit was invented by Peter Walker (QUAD) in the fifties. It allows a free choice of the screen grid voltage and is seriously my first choice circuit.

I use an anode to cathode coupling in my DIY 50watts tube amp and it sounds beautiful.

Here the schematic


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Old 5th February 2004, 05:54 PM   #5
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Default Screen voltage

Just wanted to point out that, actually, the screen voltage max given is for tetrode operation. As the screen is from a fixed high stable source, and the plate swings below the screen voltage, the screen dissapation rises significantly. hence the voltage limitation.

As long as the screen dissapation isn't exceeded, UL and triode operation is perfectly fine with screen voltage peaks well above this rating. (Although I wouldn't run 600 or 800 in UL or triode mode)

Just be careful the screen dissapation isn't exceeded.
Radiotron Designers Handbook also discusses this subject.

Cheers to all.
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Old 5th February 2004, 06:46 PM   #6
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It sounds to me like you want something for nothing. Using the commmon hi-fi tubes which are designed for around 50W/pair, you'll have to parallel a good bunch of them and drive a low impedance. Power supply current will necessarily be high, so you may have to have a power transformer wound for you. The alternative is high voltage, which gains efficiency, but still isn't going to go above 60% for some nice class B operation. Efficiency almost intrinsically produces high distortion, requiring highamounts of NFB, masking the "tube sound". Could make a squeeky-clean class B amp with like 30-35dB NFB and use a low-volts preamp to add the distortion back in.

Or, you have the high power tubes, which tend to require high voltages anyway. This will get you to your goal, because it is essentially a doubly scaled-up version of the <100W amps using the more common tubes. That is to say, voltage and current are both increased with these tube types, with impedance staying relatively constant. With a good 100 to 500W plate dissipation to burn, you'll easily get the low-NFB smoothly distorted* output required.

And lastly, of course, there is the question of needing >200W "tube-quality" hi-fi in the first place. You must have some damned inefficient speakers (like...70dB range) to need so much.

*Distortion is in the eye of the beholder...since I like tubes I'll call it a good distortion

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