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    Building, troubleshooting and testing of these amplifiers should only be
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    the safety precautions around high voltages.

OPT for SEKT88

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Next Tuesday (the 25th) is payday for me. I'm going to order my OPT's for my amp from Sowter. I just wanetd to know if the OPT I have specified is the "ultimate" for my amp.

Here are the specs:

> Type: SINGLE ENDED
>
> Package Style: "S" FULLY SHROUDED
>
> Secondary impedance: Single 8 ohms
>
> Valve type: Svetlana KT88
>
> Configuration: Other
>
> Primary impedance (ohms): 5000
>
> Internal rp (ohms): Seems to be 2k as pentode
>
> Screen taps: 43%
>
> DC mA: 91 mA ************UPDATED***********
>
> Power (Watts): 20
>
> Full power frequency (Hz): 45
>
> Size preference: SOWTER to optimise for bandwidth

Here is the schematics of the amp:

An externally hosted image should be here but it was not working when we last tested it.


The values of the components in the input stage is from Svetlana's tech. bulletin no. 43. I've had the inputstage running for about a week now connected to an oscilloscope, and it seems to be working beautifully. Assuming that the raster of the scope is correct I have almost 120V P-P from just a CD-player. That is measured with single sinus-curves at 0dB.

The current ratings for the transformer is from the pdf spec sheet of the KT88. On page 2 of the document it says that typical ratings at 400V is 87mA at the plate and 4mA at the screen at zero signal. Hence the 91mA.

I still haven't figured out the plate resistance of the KT88 in UL-mode. But from what I've read on the net it seems to be something near 2k. The reason I only specified the lowest freq. of the transformer at full power to 45Hz is quite simply that I don't need to go deeper.

The only thing I have to figure out now is the cathode resistor for the KT88. My 30W rheostat should arrive in the mail today or monday at the latest. I figured I could turn in the resistance with the rheostat and then measure it and put in equal fixed resistors.
 
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:bigeyes:
 
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Joined 2002
thanks frank,

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Thomas
 

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You might want to experiment with a little loop feedback on that amp, I found it greatly cleans up the sound on a SE pentode.

In my SE pentode amp, I split the preamp cathode resistor into two sections (in your amp it would be 900ohm and 100ohm, then I wired a 4.7K resistor in series with a 10K pot and wired it from the output + to the junction of the two resistors. Then you can vary the feedback by adjusting the pot.

Try it, you may be surprised.

Heres an interesting link

BTW I am jealous at your transformers, they look damn impressive. Please tell us what it sounds like when you are finished!
 
Just got them!!!

I just opened the package with my trannies...

OH MY GOD!!!!

People talk about The Niagra Falls, Grand Canyon etc. They should take a look at an OPT for a tubeamp. The feeling is almost religious. Well... not almost. It IS religous.

I have seen the light.

First of all, I just can't believe how big and heavy they are. My arms are numm after carrying the package a few hundred meter from the busstation.

Second, I never could imagine that something made out of metall could hold such beaty. I'ts breathtaking. The guys at the Sowter factory are not just ordinary people. They are artists, profets doing the work of the lord. I never considered myself a religious man untill now.

:cloud9:
 
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Hi everybody,

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I hope more people will love them & enjoy diy experience.

thanks

Thomas :idea:
 
It doesn't work

I've just got one channel up and running on my kitchen table. Something os horribly wrong. First of all, I seem to have the same voltage from both windings on the OPT, the anode AND the 43% UL-winding. Shouldn't I get a much lower DC-voltage at the UL-tap???

Also the signal I get at the output sounds and look like hell! Is it possible that one half och the 6n1p doesn't have the power to drive a KT88? :scratch:

My whole world has just fallen apart. I really hope you can give some answers. :bawling:
 
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.
 
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?
 
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
 
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
 
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?
 
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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