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Old 7th September 2004, 05:06 PM   #1
hacknet is offline hacknet  Singapore
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Default Kt88 Se

I`m gearing up for the end of the year project. i have semi-decided to build a KT88 single ended project.

i sat down and looked through a couple of proven designs and
this is what i have come up with. your inputs will be much appreciated.

the driver is a 5687 in SRPP and the Power tube is a KT88. its idling at about 100ma -35v triode and 85ma -30v pentode.

Click the image to open in full size.
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Old 8th September 2004, 02:57 AM   #2
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Hi,

The cap on the cathode of the SRPP seems a bit low, the impedance in this point is lower than the cathode resistor (Rk || (Ra || Ri)/u) this could typically be something like half of Rk. 6uF will then give a -3dB cutoff of ~37Hz, note! this is not an exact calculation.

Also the capacitor from screen to ground is very small, together with the 270ohm resistor it will give a cutoff of ~12Hz which will affect low end response, I wouldnt bother with this cap at all, just remove it.

BTW why not allow for UL coupling also?, (if the OT have a tap)

Regards Hans
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Old 8th September 2004, 05:31 AM   #3
arnoldc is offline arnoldc  Philippines
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stability of the screen supply...

i have always had problems with the screen supply for pentodes. i tried it the way it is on the schematic above (on an EL34) and there's no bass. does the KT88 *not* suffer from such an issue?

i also tried 807 and again no decent bass, even if i clamp the screen supply with zeners...

and for reasons i cannot comprehend, when i used 0C3 + 0D3 i get terrific bass.

i'd like to make a KT88 too so i'm interested in the results of this project... now i can use my 5687s
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Old 8th September 2004, 06:14 AM   #4
hacknet is offline hacknet  Singapore
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well, i kinda love the 5687. it sounds resonable at resonable prices...

anyway, what do you guys recommend i do to pump up the bass, i`m a bass fan and don`t mind alittle too much bass...

is the screen cap really redunant? would it help if i got a regulator instead?
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Old 8th September 2004, 06:45 AM   #5
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Quote:
is the screen cap really redunant? would it help if i got a regulator instead?
The reason you are using a series resistor and capacitor is to keep the screen grid voltage konstant but the voltage drop over the 270 ohm resistor is so low that decoupling in this case doesn't help much, the screen voltage will still follow the B+.

some alternatives are either to connect the screen to B+ which is common or to make an electronic regulator. I guess it depends on the stabilty of your B+. One other alternative is to increase the resistor to drop say 50V that would probably also work, that is the method that was used in many small radio sets with penthodes.

Regards Hans
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Old 9th September 2004, 12:04 PM   #6
hacknet is offline hacknet  Singapore
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would a voltage divider with a cap do the job?

how much current is expected to be drawn on the screen?

could someone recommend a powersupply to go along with this?
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Old 10th September 2004, 05:31 AM   #7
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QUOTE]would a voltage divider with a cap do the job?[/QUOTE]

Although I adviced that as a possible method earlier I found that Ig2 varies so much in this case that it would be an inefficient method, using G2 connected to B+ directly or an electronic regulator would be better.

Quote:
how much current is expected to be drawn on the screen?
It depends on the operating point, I found data of 6550 in SE class A, 6550 is similar to KT88. See here http://www.pmillett.addr.com/tubedat...Tubes/6550.PDF page 2

Ia idle is 87 mA, Ia max 105mA, IG2 idle is 4mA and Ig2 max 18mA, B+ 400V and VG2 225V.

2 OB2 connected in series regulates at ~210V that is probably close enough to 225V, I dont advice the use of Zeners as they have much higher dynamic resistance than gas tubes so regulation suffers. Othervise an electronic series regulator could also be used, I used that in my 6146 Push-Pull amp for the screens.

Quote:
could someone recommend a powersupply to go along with this?
A CLC supply could be used in this case, for calculations use PSUD2 see here http://www.duncanamps.com/psud2/index.html

Note! KT88/6550 can give 20W in SE penthode but distortion is then 13.5%! I would therefore advice you to use triode or UL connection instead.

Regards Hans
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Old 10th September 2004, 07:17 AM   #8
hacknet is offline hacknet  Singapore
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okay...

if i were to run it in UL. i will need a UL tap. how far down does this tap need to be? 43%?
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Old 10th September 2004, 08:23 AM   #9
arnoldc is offline arnoldc  Philippines
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hi, i just had one made with 40% UL taps single-ended. this is for my 6550 amp.
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Old 10th September 2004, 09:19 AM   #10
hacknet is offline hacknet  Singapore
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may you be so kind to share the schematic?
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