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Old 6th October 2006, 02:59 PM   #1
Ardee is offline Ardee  England
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Default KT88/6550 as Triode

The data sheets show two recommended operating points for using these as triodes, one of which is 350v, 76mA and 4k p-p load. This is supposed to give about 17W output, but I get less than 10W both in real life and using computer emulation. It looks as if grid current starts to flow at this point, but surely that cannot be right? AB2 for 17W?
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Old 6th October 2006, 06:13 PM   #2
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Double check your measurements and use known good valves for the test. The g2 resistor strapped to anode is usually between 100 to 470 ohms although I've seen 6550 circuits without g2 resistor. However it's a quick way to ruin a good valve by overdrawing screen current. Never operate without these in place.
The GEC note actually mentions 400V.
more later


richj
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Old 6th October 2006, 08:31 PM   #3
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I'm using 100R on g2, and I think the 400v is Vb which allows for self bias. It's the fact that the TJC calculator results match what I'm getting that concerns me. Not the usual situation at all.
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Old 7th October 2006, 06:25 AM   #4
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Default Re: KT88/6550 as Triode

Hi Ardee,

per chance I have a fixed bias, trioded KT88 PP power stage on the breadboard right now, and at 450V plate-cathode, 82mA Ik per tube, Raa=6.6k, it reliably exceeds slightly over 20W Po before visible onset of clipping at the scope.

Also, I had the chance to measure (trace) through quite some old stock and new production samples, and I wholeheartly can confirm what Patrick Turner found out: When wanting the most power at the least distortion, donīt use old/historic KT88/6550 spec sheets for new production tubes, because they are considerably different.

I suggest to have a close look at Turners article on load matching to KT88/6550

Excellent reading.

Tom
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Old 7th October 2006, 10:04 AM   #5
Ardee is offline Ardee  England
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Tom,

Thanks for this. Looks like I need more volts and higher load impedance, then!
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Old 8th October 2006, 07:30 AM   #6
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Quote:
Originally posted by Ardee
Tom,

Looks like I need more volts and higher load impedance, then!
This implies vacuum not so deep as it should be.The only way to get to the bottom of this is to check the depth of vacuum. The reverse g1/cathode leakage current. This will be around 0.1uA upwards beyond atandard test equip.
Another way is to check distortion.This test is more tricky but this will indicate optimum grid placement for a specific output configuration.

I find the JJ 88 version requires some 15% less - grid bias voltage for the same anode current when compared to Svetlana 88's. The g1 in the JJ is somewhat closer to the cathode. IMO the JJ 88 isn't quite a drop-in substitute for a "standard 88" although it can take more anode punishment than a standard 88. In AB1 p-p it also a bit more THD than the standard 88.

Some years ago I had some replica 88's made by the old Tesla company. These were Chelmer custom made electrical replicas to the original GEC 88's spec and performance was identical. My Svetlana 88 are pretty close.

Alot more examination is required, especially examining the more popular 6550C,5881 series and espec the raft of EL34's produced.

later I shall compile some comparison graphs.

richj
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Old 8th October 2006, 09:42 AM   #7
Ardee is offline Ardee  England
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I've tried KT88-98 and 6550C with results too poor be explained by manufacturing variations. This could explain why I've never seen the 17W triode configuration used in an amplifier design - it just doesn't work, and never has!
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Old 8th October 2006, 08:16 PM   #8
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Okay Ardee keep hair on......

Done some checks with refudged old KT88 fixed bias amp in triode output configuration..
general conditions; Ra a-a =4.5K; Iq where stated; NFB 18dB;
B+ where stated; thd at 1KHz. Anode-> 100R to g2.

NOS GEC KT88
B+=550V;Iq=75mA;Pout=36W;thd=0.5%.
B+=380V; Iq=90mA;Pout=18W;thd=0.35%.
B+=300V;Iq=100mA; Pout=9W; thg ignore.

SVET 6550B newish last 5 yrs manu
B+=380V;Iq=90mA; Pout=19W; thd=0.3%.

NOS Mull EL34
sim results as 6550B but lower output Z.

Ei KT90 recent.
B+=400V; Iq= 100mA; Pout 25W; thd=0.6%.

The slight variation in o/P tranny input Z doesn't have a great influence on output powers. 4-5-6K a-a seems to perform fairly sim on most beam tetrode amplifiers re configured as triodes.
The GEC KT88 at 550V illustrates how triode configuration adores high B+, compared to the miserable performance at 300V B+.

The SVET 6550B often has been slammed as an slack unexciting valve. I take the assumption that if the B version is fine then the C version (fabricated with better materials) must be a better performer. Both are limited at 470 B+.

The NOS Mullard EL34.....no comment..a brilliant valve with expected performance.

The KT90. This valve I'd normally give the thumbs down as a lousy bottle suitable for clay pigeon work. At high voltages this valve doesn't give the performance as claimed by the Ei data sheet....however it eschews the fact it in triode mode it gives the highest power output v.s B+ at a modest thd compared to others. A case of overengineering, the benefits of lower internal R suggest a better match for the 4.5K a-a Z.
Despite it's wacker size anode, I've got a poor set as they glow in the middle even at 60mA Iq at 500V. To be frank, an EL34 performs better. Only my opin!

The most recenly made valve (last 5 yrs) applying in your case is prob the 6550B.
Double check your o/p tranny configuration. Check the heater voltage is correct. The 6550 series I find notoriously slow warm-up to settle to consistent emission compared to other classes.
Hope this helps.

Richj
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Old 8th October 2006, 08:19 PM   #9
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omission..... Iq = per tube
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Old 9th October 2006, 08:32 AM   #10
Ardee is offline Ardee  England
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Thanks for this richj. Your results do seem to confirm that you need to put in a lot more power than the data sheets say. 380v/90mA is rather different from the 335v/76mA GEC suggest for the same sort of output power.

The EL34 result is a real suprise.

P.S. There is a link from the Lundahl site to someone showing a design for a 20W triode KT88 amp which uses 320v HT!
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