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Old 6th April 2003, 06:11 AM   #11
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No replies???

Hasn't anyone here ever worked with pentodes in UL? Should I get a lower DC-voltage at the 43%-tap than at the plate?
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Old 6th April 2003, 07:21 AM   #12
mig-ru is offline mig-ru  Russian Federation
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the plate and ul tap should have same dc voltage, the transformer doesnt provide much dc resistance, it provides an impedence load which only affect ac. if you thought of it as a dc load, you might have gotten the wrong quiscent point for your kt88, much higher than you expected, and the bias would be off too. thats my two cents. please correct me if im wrong.
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Old 6th April 2003, 07:27 AM   #13
Colt45 is offline Colt45  Serbia
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the UL tap should be the same DC voltage as the plate tap.

well technically, since there is some DC resistance in the winding, the UL tap maybe a couple volts higher than the plate tap...
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Old 6th April 2003, 07:58 AM   #14
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You guys are great! It all makes sense. When I measured 1 or 2 volts higher at the UL-tap I thought I had destroyed my new tranny. (Allthough I couldn't understand how that could have happened. I have had a DMM connected in series with the HT-winding of the transformer and never went over a few mA.)

But when I read the KT88 specs provided by Svetlana, there seems I should have 400V at the plate and 225 at the screen. That's why I yhought I should have roughly 225 (400V -43% = 225V) at the screen.

Also when I tried to bias the tube correctly I connected a DMM in series with the anode-winding and one DMM in series with the UL-winding. When I have no signal I don't seem to get any current running throught any of the windings. Should I instead measure the current at the cathode?

I would very grateful if someone could just take a quick look at page 2 of the Svetlana KT88-specs to see what I'm talking about.

When I look at the output signal in a scope it looks like the KT88 is working much too "low". I'll try to explain what I mean. The signal into the tube isn't perfect but the output seems too clip at the negative (lower) side of the sine-curve. It looks like the whole signal should be lifted a bit for the tube to be able to produce the entire sine-curve. It seems to get better if I adjust the bias. But it looks like i need well over 2kOhm as cathode resistor. Isn't that VERY high?
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Old 6th April 2003, 08:35 AM   #15
Colt45 is offline Colt45  Serbia
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if you look on the datasheet for svet kt88:
http://www.kolumbus.fi/flait/techzotic/SvetlanaKT88.pdf

that neat, its even got a UL graph.. never seen that before.

well lets see.. if we go to:

Va= 360
Vg= -40
Vg2= whatever.. ~360
Ia = 100
Pa = 36w

so 40v/.1A = 400ohm cathode resistor (use a 5w)

you could knock it down a few ohms if you want to run it a little hotter. or bump it up a few ohms if you want it cooler.

note that B+ is still 400v, its just that only 360v is burned off in the tube, 40v is in the cathode resistor.

The graph doesnt list screen current, but judging from your 87ma/4ma comment, screen current is rather unimportant (10ma won't really skew the operating point much at all anyways.)

bypass the cathode resistor with a cap, say 100 or 200uF, rated for say.. 50v or whatever (anything over 40v will work)

the 400/225v you mention is just a suggested class A point (non-ultralinear)

hopefully that made sense.. im a little tired
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Old 6th April 2003, 08:45 AM   #16
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There's one thing I don't get. Should I have a negative voltage applied to the grid? The -40V you mentioned...
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Old 6th April 2003, 08:52 AM   #17
Colt45 is offline Colt45  Serbia
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oh, i didnt realize you were using fixed bias.

you can either:

A) leave the grid at 0v, and use a 400 ohm cathode resistor

or

B) ground the cathode (no resistor) and apply ~ -45v to the grid.
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Old 6th April 2003, 09:23 AM   #18
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I applied a 470Ohm 15W resistor at the cathode. I get pretty much the same signal out as I put in the KT88 up to roughly 150VDC but after that something weird happens. I get at mirror-signal that exists both after the 6n1p and at the output of the transformer. I can't understan where it comes from. Can it be some sort of self-oscillation?

[edit]

I have 30mA over the cathode resistor at 150VDC
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Old 6th April 2003, 11:15 AM   #19
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Everything seems to work!!!

The mirrorsignal was apparently just my total lack of knowledge when it comes to oscilloscopes. Turns out there is a little knob that says "level". The amp distorts quite heavily when i put in a full 2VRMS at the input but of I go down a bit probably -12dB or something it seems to sound quite allright.

I now run the amp at 400VDC and have about 75-80mA over the cathoderesistor.

What would happen if I lower or higher the resistance at the cathode, soundwize?
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Old 6th April 2003, 12:20 PM   #20
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I put in my rheostat in stead of the cathoderesistor. At 1kOhm i have exactly 40mA over it. But I can't hear any difference when I turn it so I get 90mA.

Since my amp is limited by the input-stage... Can I run the KT88 a bit cooler? It seem to work beautifully at 1k, and I could probably go even higher. There's no point in running the KT88 harder than I need to or is it? What do I have to gain by running it at 91mA?
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