Citation II... can't get enough bias! - diyAudio
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Old 26th August 2015, 05:47 AM   #1
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Default Citation II... can't get enough bias!

Hi folks, recently I did an upgrade of my citation II amp and installed new sockets, caps and electrolytics and I also changed the resistors of the bias adjustment from 7.5K to 10k.

Putted to test.

The bias indicator in the rear panel is not working, so I followed Jim Mcshane tip for adjusting using the pin 8 of the kt88 to 1.5V DC.


The problem is that the maximun I can get is 0.45V DC with the bias pot at full clockwise.

The other voltages are correct B+ 465V, -53.

There is also a strange and constant bump, 2 for second in the 2 channels when I put the bias pot at the full anti-clockwise position.

As Mcshane says the full clockwise is the minimum bias position, in my case is the maximum, sure something is really wrong...

Can someone help me with this?

Last edited by murillollirum; 26th August 2015 at 06:11 AM.
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Old 26th August 2015, 06:24 AM   #2
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Does it apply to all 4 KT88s?

What is the voltage at G1 when you have the pot in full clockwise position?

What resistor value do you have between pin 8 (cathode) and GND?

pictures can also help!

Erik
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Old 26th August 2015, 07:15 AM   #3
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Quote:
Originally Posted by ErikdeBest View Post
Does it apply to all 4 KT88s?

What is the voltage at G1 when you have the pot in full clockwise position?

What resistor value do you have between pin 8 (cathode) and GND?

pictures can also help!

Erik
Hi Erick,

The voltage full clockwise is 0.45V full anti-clock is 0.00 to negative. Yes all 4 kt88.
The resistor is a 15ohms
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Last edited by murillollirum; 26th August 2015 at 07:18 AM.
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Old 26th August 2015, 07:19 AM   #4
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Murillo,

can you measure the voltage netween G1 (pin 5) and Gnd?
Erik
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Old 26th August 2015, 07:27 AM   #5
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-51DC full CW -85DC anti CW all 4.
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Old 26th August 2015, 07:33 AM   #6
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Erik,

All other voltages are under the specs.

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Old 26th August 2015, 07:45 AM   #7
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Values do look good indeed.

And the tubes are new, or measured well before the modification?

The 15R resistors are indeed 15R? besides, I see you use quite big (10W) ones - is that recommended practice for these amps? I do ask as sometimes these resistors are also used as "fusistor" (fuse + resistor): selecting a small Wattage one will burn the resistor in case of a tube runaway, opening the circuit, hopefully saving the expensive OPTs.

Erik
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Old 26th August 2015, 07:54 AM   #8
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Yes Erik, you are right, I will have to change to 1W, but this maybe not end my problem. Any idea? I really don't now whats going on. It looks like MH370.
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Old 26th August 2015, 08:06 AM   #9
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Hi Murillo

tenho ideia não (I have no idea), besides that the output valves are worn out or that these resistors aren t actually 15R (but lower in value).

What you could try, at your own risk , is to turn the pots anti clock wise, parallel a 12k resistor to R26 and R61 to get the mininum bias value to about -40V, and try rebiasing again - but carefully!

Erik
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Old 26th August 2015, 06:11 PM   #10
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Quote:
Originally Posted by ErikdeBest View Post
Hi Murillo

tenho ideia não (I have no idea), besides that the output valves are worn out or that these resistors aren t actually 15R (but lower in value).

What you could try, at your own risk , is to turn the pots anti clock wise, parallel a 12k resistor to R26 and R61 to get the mininum bias value to about -40V, and try rebiasing again - but carefully!

Erik
Erik,

That's all really good advice. Whever we redo a Cit II we change R26 and R61 to 4.7K. That's with a 7.5K bias pot. Making the pots 10K and using the stock R26/R61 values will mean he can't get the bias to the point where the tubes pass enough current. R26 and R61 will likely need to be about 3.9 or 4.2K to bring the bias into a usable range - assuming the tubes aren't worn out.

Also, when he said the voltage at pin 5 was "correct" at -53 it is clear that he doesn't quite fully grasp what the negative voltage at pin 5 does. There is no "correct" voltage at pin 5 - the "correct" voltage is the voltage applied that sets the cathode current where it belongs.

Also, those giant cathode resistors (10 watts) will offer little to no protection in case of a runaway tube. I recommend 2 watts and a metal oxide film for good protection without unnecessary resistor popping.

Finally, I am afraid that people look at that voltage chart from my site and don't see the heading on it which says "Corrected for 124 Volt AC Lines", nor do they pay attention to the asterisks on some of the voltages - like the ones on pin 5 for instance. What it says about the asterisks on my page is:

"* These voltages can and should vary with the individual bias pot settings. They will not normally be equal."

But VERY few read that line for whatever reason!

I sent a detailed reply to the original poster, I believe he can sort it out now - especially with your good advice too!
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