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Bias Voltage ?
Posted:
Sun Apr 27, 2008 9:43 amby Gregt01
Hi
I am getting max of -48.3 vdc at H
Prior to th 4.7k resistor befor the 10k pot I am getting 75 vdc.
Once I try to bias the KT88's I get a minimum of 1.44 vdc and I can not
drop it down to 600 mV.
What am I doing wrong?
Also do I use a diode as shown on the diagram?
Greg
Posted:
Sun Apr 27, 2008 1:49 pmby TomMcNally
For KT-88's - some users have found that changing the 4.7K resistor to a 1K will make it work just fine. Yes, you need that diode off the bias tap of the transformer.
Let us know how you make out !
... tom
Posted:
Sun Apr 27, 2008 1:54 pmby erichayes
First, verify the values of the 4.7K and 18K resistors. Second, check to see if the 100 µF cap connected to the wiper of the bias pot isn't installed backwards.
Posted:
Sun Apr 27, 2008 11:13 pmby Gregt01
Hi Tom the 1K did the trick, I now can get the -55v with lots of adjustment +/-.
I am still getting a reading of 1.24v on the 10 Ohm resistor. Should I change the value of the 10 Ohm resistor?
Eric all is good. all values are correct. the caps are also correct, + to gnd.
Greg
P.S.
I changed the 10 Ohm to a 6 Ohm and it all looks good. I have been able to set the 700 mv for the KT88's at -55.2 v
Posted:
Mon Apr 28, 2008 8:07 amby TomMcNally
Hi Greg ....
Well, you can't just change the 10 ohm resistors to 6 ohms ... the 10 ohms is just a convenient value so you can easily calculate the current. Take a look at this link - it's a nice bias calculator:
http://www.webervst.com/tubes/calcbias.htm
You can fill in the blanks with your readings and see what your tubes are really doing.
Posted:
Mon Apr 28, 2008 9:14 amby Ty_Bower
What kind (brand) of KT88 do you have in there? It looks like you might almost need the full -75 volts before you can get it biased correctly. Where is the B+ sitting?
Here's another thread related to Poseidon biasing. The first half dozen posts might be helpful.
http://www.diytube.com/phpBB2/viewtopic.php?t=2296
Posted:
Tue Apr 29, 2008 11:10 amby Gregt01
Hi guys
The KT88's are the new Russian Genalex Golden Lion and the 4AR4 is a new Golden Dragon.
I have changed the 6 ohm resistor back to the 10 ohm value, I have taken readings after a 20 minute warm up.
H = -66.3
F = -62.5
G = -62.5
J = 408 (HOT)
K = 471 (Warm)
Point #1 = 546
Point #2 = 537
KT88 pin 3 = 529
KT88 pin 4 = 529
KT88 pins 1&8 = 700 mV
Changes made: 4.7K to a 1K
All resisrors are matched and are as close as 0.1% to there value.
I hope this will put some light on the matter. No hums No hisses No pops just nice and quiet
Greg