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Old 1st September 2010, 03:03 PM   #41
jrenkin is offline jrenkin  United States
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I thought there was an update to the Bill amplifier using a CCS to replace R12.

In any case, that may be an easy modification that will solve most of the issues you may have in the ECC99 stage.

You might enjoy TubeCad software for modeling these circuits. Just remember that modeling software finds "by the book" operating points, not the "sweet spot".
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Old 1st September 2010, 03:24 PM   #42
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Quote:
Originally Posted by TheGimp View Post

You have 130V available in the cathode circuit. How about a current source instead of R12?
FWIW, Triode Dick's Mono Bill has a CCS on the cathodes in place of R12......

Triode Dick's Page (link is a little strange...click projects, then Mono Bill)

A 10M45S would be another CCS solution.

Last edited by boywonder; 1st September 2010 at 03:27 PM.
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Old 1st September 2010, 04:46 PM   #43
rongon is offline rongon  United States
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Quote:
Originally Posted by TheGimp View Post
13mA for the pair of ECC99 gives 6.5mA per tube.

top anode is at 440-(.0065*30000)=245V
A-K = 245-130 = 115V

Bottom anode is at 440-(0.0065*37170)=198.4
A-K = 198.4-130 = 68.39V

I believe the lower ECC99 will start saturating before you drive the KT88 fully and you will get high distortion.

I would lower the current through the pair by increasing R12, or change the anode resistor values to give less drop, or both. Depends on how the ECC99s are best operated.

I would have to look up the ECC99 and KT88 specs to give more detail.
JJ Electronic ECC99 data sheet

JJ Electronic KT88 data sheet

I know from playing around with 5687 and 7119 (USA types similar to ECC99) that these sound thin and weedy with only 6.5mA going through them. They begin to "turn on" at about 10 or 11 mA. I like them with at least that.

Looking at the JJ curves, it seems an ECC99 with 6.5mA through it is operating in the "knee" of the curves. That would indicate higher internal anode resistance, higher distortion, etc. That might be the source of "thin" sound right there. I think the ECC99 would perform better with more current going through it, not less. But that would require a total re-design of the circuit...

Why not try bypassing the cathode resistor of the 6SN7 as it is now, with a capacitor of about 330uF 16V. Any high quality electrolytic caps from 270uF to 470uF should work; whatever you have around (but not physically large, high voltage types). That would take away the negative feedback from cathode degeneration and maybe change the tone of that stage to your liking (higher gain, maybe more full-sounding). Easy enough to try, right?

--
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Old 1st September 2010, 05:21 PM   #44
tinitus is offline tinitus  Europe
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Quote:
Originally Posted by rongon View Post

Why not try bypassing the cathode resistor of the 6SN7 as it is now, with a capacitor of about 330uF 16V. Any high quality electrolytic caps from 270uF to 470uF should work; whatever you have around (but not physically large, high voltage types). That would take away the negative feedback from cathode degeneration and maybe change the tone of that stage to your liking (higher gain, maybe more full-sounding). Easy enough to try, right?

--
thats what it says here too..... What Is Biasing?

actually strange, because TriodeDick's first KT88 PP from 2001 had cathode bypass caps, on KT88
and the later 3 versions have none
is it cost saving ?
Attached Images
File Type: gif kt88_amp 2001.gif (9.8 KB, 344 views)
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Old 1st September 2010, 06:47 PM   #45
Green77 is offline Green77  Sweden
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Thanks all

So, my picture is right around the 6SN7? Then i will change my resistors..
And to get more mA in the ECC99 i change the anode- resistor to 20K and cathode resistor to 24,6K..right? That will bias the ECC99 to more like 10mA?

I have looked at a CCS.. Didn't understand **** And i have a seperate PSU and no free pins in my connector...

MANY thanks! //Daniel
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Old 1st September 2010, 07:53 PM   #46
rongon is offline rongon  United States
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Quote:
Originally Posted by Green77 View Post
So, my picture is right around the 6SN7? Then i will change my resistors..
Reducing the anode load resistor value to 27k will raise the harmonic distortion in that first stage. You generally want the anode load resistor value to be 4x or more the internal anode resistance of the tube. 6SN7 internal resistance is 8k ohms or so, so you'd normally want more like 33k or 36k as the anode load. But try the 27k, see what it sounds like. It might be good...

Quote:
Originally Posted by Green77 View Post
And to get more mA in the ECC99 i change the anode- resistor to 20K and cathode resistor to 24,6K..right? That will bias the ECC99 to more like 10mA?
20k resistor with ECC99 at 10mA would drop 200V.

24.6k resistor with 20mA across it would drop 492V!! That can't be right (and that won't work).

That ECC99 stage is trickier to figure.

We need 130V at the joined cathodes of the ECC99's, right? So 130/.02 = 6500. You want ECC99 cathode resistor to be 6500 ohms (two 13k in parallel would work).

In order to get 10mA through each ECC99 half, we want the anode voltage to be about 175V.

175V (anode) + 130V (cathode) = 300V.

That means the anode resistor needs to drop 140V from the Ebb of +440V. With 10mA going through it, the anode resistor will need to be 14k. That would work!

From the JJ data sheet, the bias for the ECC99 at that point should be -6V.

You will probably need to adjust the values of the 6SN7 anode resistor and the ECC99 cathode resistor to get the right relationship of voltages between the two. You need to get the anode voltage of the 6SN7 to be 6V lower than the 130V at the ECC99 cathodes. So the anode voltage of the 6SN7 needs to be +124V.

Now the question is, will -6V bias on the ECC99's allow enough voltage swing to drive the KT88's to full power?

--

Last edited by rongon; 1st September 2010 at 07:56 PM.
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Old 1st September 2010, 08:11 PM   #47
tinitus is offline tinitus  Europe
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that was a useful post, nice one

just a note
I noticed in TD's later BILL schemes that KT88 cathode bias resistor R13/14 are printed as just 1, and it should be 1K, right ?

I suppose to most here it goes without saying, but anyway, just in case
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Old 1st September 2010, 10:00 PM   #48
Green77 is offline Green77  Sweden
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ok, still don't get it Well, it's midnight here so i will read all again tomorrow and see if i understand then
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Old 2nd September 2010, 02:38 AM   #49
rongon is offline rongon  United States
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OK, maybe this editing of your schematic will explain better....

Click the image to open in full size.

Please note that I'm not suggesting that this is any kind of an optimal design idea. I'd change things around were it my amp. But I hope this helps get the idea across better.

--

Forgot to mention:
Notice how the +120V anode voltage of the 6SN7 goes directly to the ECC99 grids? The ECC99 grids are at +120V.

Now look at the ECC99 cathodes. They're at +126V. That puts the ECC99 grid 6 volts lower than the ECC99 cathodes. That creates the -6V grid bias for the ECC99 triodes.

The ECC99 triodes each have 10mA going through them.

The 6k3 resistor from the ECC99 cathodes to ground has 20mA going through it (10mA from each of the two ECC99 triodes). 6300 ohms * .02 ampere = 126V.

One more thing:
I'd definitely try bypassing the cathode resistor of the 6SN7 with a 330uF capacitor. Choose a high quality, low ESR electrolytic. You don't need anything more than a 6.3V rated part. A 16V part would work fine too. 63V would be complete overkill, and too physically large.

--

Last edited by rongon; 2nd September 2010 at 02:49 AM.
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Old 2nd September 2010, 02:46 AM   #50
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Quote:
Originally Posted by Green77 View Post
. Electro Harmonix sounded like ...
how long did you let this tube run in circuit?

and,

was it new?

Last edited by tomtt; 2nd September 2010 at 02:47 AM. Reason: 1666
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