Sunn 200s bias resistor

[pbman1953]

I'm biasing a a Sunn 200s and the bias pot is already down all the way. I need a recommendation on what value to use to help the adjustment be in a better range. Right now they are running too hot. Which resistor should be changed?

Thanks


http://www.prowessamplifiers.com/schematics/post/sunn_200s.pdf
 

[Latole]

Too hot, you need more negative voltage.

Replace the 18 K by a bigger value
or, if needed the 1K by a smaller
18 K send some voltage to ground, bigger value will send less voltage to ground.
1K feed some AC voltage to bias power supply. Smaller value will send more AC volt
Understand how circuit work ?

Or bias power supply is defective ? Or both tubes are short ?

Remove all power tubes ; Power amp On stnadby on Play : how much neg voltage do you read ?
Schematic show -55 volts.
You must read same voltage on pin 8 on each KT88 tubes
Fix negative voltage before putting tubes in amp.

[pbman1953]

I'm getting -75.7. Probing the middle leg of the pot . Which is connected to the 50/150v cap

[sluckey]

I'm getting -75.7. Probing the middle leg of the pot . Which is connected to the 50/150v cap

That should make the tubes run very cold. What is the minimum and maximum voltage you can get at pin 5 of each output tube?

[pbman1953]

with tubes in or out?

[sluckey]

out

[pbman1953]

-75.6  at pin 5

[sluckey]

I repeat... What is the minimum and maximum voltage you can get at pin 5 of each output tube?

You'll have to turn the bias pot. There will be two numbers.

[pbman1953]

-47.4 & -76

-47.4 &  -76

[sluckey]

You should be able to bias good tubes just fine. What will you monitor when you adjust the bias with tubes in?

[pbman1953]

I was using my Bias Rite. The voltage is at 514 and current is 60 , which puts me at 30 watts. I want around 22-23 watts

[sluckey]

I was using my Bias Rite. The voltage is at 514 and current is 60 , which puts me at 30 watts. I want around 22-23 watts

What is the voltage on pin 5 when your current is 60?

[pbman1953]

I was using my Bias Rite. The voltage is at 514 and current is 60 , which puts me at 30 watts. I want around 22-23 watts

What is the voltage on pin 5 when your current is 60?

-74.5, pot is maxed

[sluckey]

Measure the voltage at the junction of the bias diode and the 1K resistor. What have you?

[pbman1953]

Measure the voltage at the junction of the bias diode and the 1K resistor. What have you?

-78.2

[sluckey]

-78.2 is the absolute maximum voltage you can get. Put a jumper across the 1K resistor and recheck bias. I doubt you will get down to 22 watts.

You do realize that the KT88 is a 42 watt tube? Running at 30 watts is 71% which is about where most people would like to be. Why do you want to run at 50%?

My Sceptre (same power amp) has -62v and -64v on pin 5. Plate voltage is 495v and current is 60mA. That gives me about 29.7 watts at idle. I think your tubes are messed up or your bias rite is messed up. You should not need to run your bias voltage at -75v to get 30W.

[pbman1953]

I'm using a set of new Genelex Gold Lion kt-88 and the spec sheet shows that it's a 35W tube

[pbman1953]

https://shop.ehx.com/catalog/addimages/kt88-genalex.pdf

[pbman1953]

My friend bought the tubes from Upscale Audio and the sticker shows-


bias of 60
Transconductance- 3.82
G2 mA-4.87


Just wanted to give all the info

[shooter]

maybe a couple 1 ohms under the PA tubes cathode to ground, just to verify your equipment/setup?


If you really want to get to low 20W watts, maybe even do a temp bias change from fixed to selfbiased

[sluckey]

-78.2 is the absolute maximum voltage you can get. Put a jumper across the 1K resistor and recheck bias.

Do this temporarily. I'd just use a gator clip test lead. If you still can't get enough negative bias voltage then you are SOL, unless you are willing to use the HT winding as the AC source for the bias circuit. Easy to do. Just put a 100K to 220K resistor between the bias diode and the HT secondary.

Please note that even though your data sheet shows Pa max = 35W, it also shows Pg2 = 6W. That's a total of 41W and that is what your bias rite is actually measuring.

[sluckey]

OK, one more thing to try. Adjust the bias pot to put -60V on pin 5 of the KT88s. Now connect your bias rite and measure current and plate voltage. What have you? Do this quickly in case you see any red plating.

[pbman1953]

Replacing the 1k did too much so I decided to adjust the 18k. I ended up with a 41 k with these results-


Diode junction- -77


Pin 5- -75


41k resistor- on the pot side - 60


Both tubes avg- 27 watts

[pbman1953]

Is everything ok, with what I did?

[sluckey]

Yes.

I'm still puzzled as to why your KT88s require such a large negative voltage though. 

[pbman1953]

Thanks, and I have to say that they sound great. For me the kt120 still has the edge bt these sound very natural. Now  I see why they are so popular

[Latole]

Yes.

I'm still puzzled as to why your KT88s require such a large negative voltage though. 

That is why we should check bias and adjust each time we put new set of output power tubes

[pbman1953]

On another note, I wanted to compare in this amp how other Kt-88's, I have, sound. The only ones I had were some Valve Art's (forum member suggestion). I had to increase , turn up the pot, to get a proper bias to be around the same as the Gold Lion's. So would lowering the pot be a GL trait?  Sound wise the VA's were very good and actually they were more dynamic as I gone on the bass. But I did switch back to the GL's and they are special.

[Latole]

Or you may have issue with Output Transformer !

[sluckey]

Remove all power tubes ; Power amp On stnadby on Play : how much neg voltage do you read ?
Schematic show -55 volts.
You must read same voltage on pin 8 on each KT88 tubes

Pin 8 is connected to ground. Pin 5 is the correct pin to measure negative bias voltage.

[pbman1953]

pin 5 is -74 on both tubes

[pbman1953]

Question should pins 2 & 7 measure 3.3ac each or 6.3ac?

[sluckey]

Measure 6.3VAC ***BETWEEN*** pins 2 and 7. That means one probe on pin 2, the other probe on pin 7.

[pbman1953]

Ah,  I'm good on that, thanks.


What about t the - 74 on pin 5? Still have reservations?

[shooter]

Schematic show -55 volts.

+/- 10 I don't get interested, -20....
did you try disconnecting the NFB?  spaghetti on wall idea

[sluckey]

What about t the - 74 on pin 5? Still have reservations?

Yes I do. Would you mind putting your other pair of VA KTs in the amp, bias it as you like, and post your plate voltage and current from your bias rite. And also post the voltage on pin 5 of each KT.

Your GLs certainly bias up differently than your VAs or any tubes I've put in my Sceptre. That may be normal for GLs, or it may indicate a problem with your GLs. 

[pbman1953]

I'll send that next, but these GLs are Cryo'd. Does that change things?

[sluckey]

That doesn't mean anything to me.

[pbman1953]

That doesn't mean anything to me.
[/quo


Ok, with the VA set it's still - 74. One thing that i noticed if i max up the pot quickly the other way I get - 61 at pin 5

[sluckey]

Ok, with the VA set it's still - 74. One thing that i noticed if i max up the pot quickly the other way I get - 61 at pin 5

The voltage on pin 5 ain't gonna change 'til you crank the bias pot. -61v is about where my tubes are biased.

I would be searching the net to see if it's normal for GLs to require such a high bias voltage on pin 5.

[pbman1953]

I found somewhat of a similar situation


https://www.diyaudio.com/forums/tubes-valves/186700-tube-replacement-causes-bias.html

[sluckey]

Now you have a friend! 

[pbman1953]

Do you u agree with the poster that the lion's sre not s good match?

[sluckey]

Well, they certainly wasted a lot of your time. I won't be buying any of them.

[thetragichero]

they tend to be too pricey for my blood anyway. if I'm buying new from a supplier i tend to stick to jj because they seem to hit the sweet spot between cost and relative dependability (not relying on crazy marketing claims it a plus in my book as well)

[pbman1953]

Are you all saying that the GLs cant be used unless it measures - 55?

[sluckey]

I haven't seen anyone say that. Don't read between the lines in my posts because there's nothing there! WYSIWYG

[pbman1953]

I haven't seen anyone say that. Don't read between the lines in my posts because there's nothing there! WYSIWYG

Im not, just the vibe in getting. Im very appreciative of the help

[Latole]

Well, they certainly wasted a lot of your time. I won't be buying any of them.

With your skills Steve, it would take you 15 minutes to fix the problem if it happened to you.

[pbman1953]

I made a couple tests- no tubes in- testing at "D"  the -55 point


I subbed 2 different resistors for the 1K before the 10k pot

Here were the min/max ranges for each resistor-


1.5k-  -47 to -75


3.2k-  -44 to -71




After I did this I tested with the Bias Rite and the cathode current was over 100.So that didn't help.


After exchanging values for the 1k and 18K I ended up with removing the 1k and going direct. The 18K is now a 41k. This put me in range.


I'm getting -


-80 at the diode


The pot min/max is now -63 to -79. With the Bias Rite I'm getting it to settle around 28 watts