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?
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:
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
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
Last edited:
Erik,
All other voltages are under the specs.
All other voltages are under the specs.
An externally hosted image should be here but it was not working when we last tested it.
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
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
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
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
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
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!
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.