Last year, I bought a 1960 Harman Kardon Citation II for a winter project. I keep a bunch of pictures of my progress here: https://photos.app.goo.gl/LSk2ugZ4eXJEJcaP9
Basically, I gutted out the box, re-using the choke, power and output xfmrs and most of the stuff on the back panel like those dual pots that I couldn't find anywhere and the bias meter and switch, and speaker terminals. From there, I used the original assembly manual for assembly. The power tubes are 6550.
After it was assembled it passed the resistance test and double checked every device and connection.
After presetting all the pots (balance to middle and the 4 Bias pots full clockwise), I inserted all 10 tubes and measured the 15 ohm resistor (voltage drop from power tubes cathode pin 8 to ground with 4-5 volts (should have been 1.5 volts in order for the bias meter to be set correctly). of course, the bias meter was pegged and the tubes began to get really hot after just 1 minutes, so pulled the plug. It's enough to say that any adjustment in the pots would have made the bias current even higher, so I had to sit and scratch my head.
I read a post by SY who says: Pull the output tubes. Turn the amp on. CAREFULLY check the voltage at the plates and screens of the KT88. It should be the full B+. Now, run the bias pot to one end and check the grid voltage. Run it to the other end and check the gid voltage. You should see something like -60V at one extreme and -25 at the other. If the voltage doesn't vary or you get wildly different numbers, you've found the fault.
I dont know if SY is right, but the voltage test in the back of the assembly manual says there should be -50 volts at the screens (tube terminal 5)
I I pulled the output tubes and while my B+ is 500 vdc (400vdc with power tubes inserted), with all bias pots turned fully clockwise I am reading -23v at the screens (tube terminal 5) and when I rotate the bias pot all the way to the other end, voltage goes to zero (same on other 3 tubes). This appears to be a problem I need help with. Some say I should increase the cathode bias resistor to reduce the cathode current, so I doubled it and all that happened was my voltage drop across that resistor doubled. Adding that resistance didn't change the cathode current at all and forces the bias meter with twice its rated current and that's not good. I put it back the way the book says and continued scratching my head. Another poster said to increase the resistor to tube terminal 5 to reduce the current, but that ended up increasing the cathode current (which did not make sense to me).
I'm hoping there is someone out there who knows the secret handshake and puts me back on track before I completely destroy this thing.
Thanks,
Walt
Basically, I gutted out the box, re-using the choke, power and output xfmrs and most of the stuff on the back panel like those dual pots that I couldn't find anywhere and the bias meter and switch, and speaker terminals. From there, I used the original assembly manual for assembly. The power tubes are 6550.
After it was assembled it passed the resistance test and double checked every device and connection.
After presetting all the pots (balance to middle and the 4 Bias pots full clockwise), I inserted all 10 tubes and measured the 15 ohm resistor (voltage drop from power tubes cathode pin 8 to ground with 4-5 volts (should have been 1.5 volts in order for the bias meter to be set correctly). of course, the bias meter was pegged and the tubes began to get really hot after just 1 minutes, so pulled the plug. It's enough to say that any adjustment in the pots would have made the bias current even higher, so I had to sit and scratch my head.
I read a post by SY who says: Pull the output tubes. Turn the amp on. CAREFULLY check the voltage at the plates and screens of the KT88. It should be the full B+. Now, run the bias pot to one end and check the grid voltage. Run it to the other end and check the gid voltage. You should see something like -60V at one extreme and -25 at the other. If the voltage doesn't vary or you get wildly different numbers, you've found the fault.
I dont know if SY is right, but the voltage test in the back of the assembly manual says there should be -50 volts at the screens (tube terminal 5)
I I pulled the output tubes and while my B+ is 500 vdc (400vdc with power tubes inserted), with all bias pots turned fully clockwise I am reading -23v at the screens (tube terminal 5) and when I rotate the bias pot all the way to the other end, voltage goes to zero (same on other 3 tubes). This appears to be a problem I need help with. Some say I should increase the cathode bias resistor to reduce the cathode current, so I doubled it and all that happened was my voltage drop across that resistor doubled. Adding that resistance didn't change the cathode current at all and forces the bias meter with twice its rated current and that's not good. I put it back the way the book says and continued scratching my head. Another poster said to increase the resistor to tube terminal 5 to reduce the current, but that ended up increasing the cathode current (which did not make sense to me).
I'm hoping there is someone out there who knows the secret handshake and puts me back on track before I completely destroy this thing.
Thanks,
Walt
Those are grids, not screens. Screen is pin 4.
First, with no output tubes installed, check the DC voltage at C17b.
Should be around -75 to -100 VDC to ground.
If that is ok, then with no output tubes installed, check the DC voltage at R26.
Should be around -30V or -40VDC to ground.
First, with no output tubes installed, check the DC voltage at C17b.
Should be around -75 to -100 VDC to ground.
If that is ok, then with no output tubes installed, check the DC voltage at R26.
Should be around -30V or -40VDC to ground.
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Assuming there are no other mistakes, if you get the OPT plate wires switched then you have a big oscillator.
According to his first post, the bias supply itself is bad, outputting much too low DCV (-23V to 0V).
Maybe a bad capacitor or resistor, especially R26 and R61, since the DCV goes down to 0VDC.
Maybe a bad capacitor or resistor, especially R26 and R61, since the DCV goes down to 0VDC.
Thank you, steveu, I will have a look at OPT plate wires. I did not know that.
Rayma, I am new to electronics and especially vacuum tubes. I got those grids and screens mixed up. I will measure again; this time the "screen" Thanks!
Rayma, I am new to electronics and especially vacuum tubes. I got those grids and screens mixed up. I will measure again; this time the "screen" Thanks!
You need to measure the grid circuit (going to the tube pin 5), which is where the bias is applied.
But measure only at the wipers of the bias pot, not directly at the tube grid pin.
The meter will load too much if you connect it directly at the tube grid (pin 5).
The screens (pin 4) won't be a problem, don't worry about them..
But measure only at the wipers of the bias pot, not directly at the tube grid pin.
The meter will load too much if you connect it directly at the tube grid (pin 5).
The screens (pin 4) won't be a problem, don't worry about them..
Typo, the grid is pin 5.
Your problem, according to the meter, is the bias supply.
Somewhere between the rectifier and the tube grid pin 5.
But since the bias DC voltage goes down to 0VDC, look at resistors R25 and R61.
This assumes your meter black lead goes to ground.
Your problem, according to the meter, is the bias supply.
Somewhere between the rectifier and the tube grid pin 5.
But since the bias DC voltage goes down to 0VDC, look at resistors R25 and R61.
This assumes your meter black lead goes to ground.
Yes, meter black lead to ground when I measure. In this case, would you expect R25 and R1 value too high? Can you suggest a value to get me started?
Ok, let's do this.
Turn off amplifier.
Black meter lead to ground.
Set meter to ohms.
Red meter lead to R26.
If zero, use the other end of the resistor.
What is the reading?
Repeat for R61.
The problem is that the negative DC bias voltage is not negative enough.
The resistor value could be wrong, or a filter capacitor could be bad, etc.
But since this is a new build, it also could be a wrong connection.
This is all the more likely since both channels are the same.
Turn off amplifier.
Black meter lead to ground.
Set meter to ohms.
Red meter lead to R26.
If zero, use the other end of the resistor.
What is the reading?
Repeat for R61.
The problem is that the negative DC bias voltage is not negative enough.
The resistor value could be wrong, or a filter capacitor could be bad, etc.
But since this is a new build, it also could be a wrong connection.
This is all the more likely since both channels are the same.
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I read on this page: http://www.mcshanedesign.net/adjust2.htm that 5.6k resistor can get dirty and open circuit, but mine is new and all grounds are good.
correction; they were 5.6k ohms when I installed them, but when I changed them, I dropped the "k" (huge mistake). Let me put them back to 5.6k and re-measure pin 5 grid volts. might be an hour or so. thanks!
Yes, that is your problem, and why the DCV goes to zero instead of about -25V.
But measure the DC bias voltages at the wipers, NOT directly at the grid pin 5.
This is important to get accurate measurements.
But measure the DC bias voltages at the wipers, NOT directly at the grid pin 5.
This is important to get accurate measurements.
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Rayma, I just finished changing the R26 and R61 back to original 5.6k ohms
Measured values without the big power tubes.
-25 to -14 from one end of the bias pot to the other on all 4 pots, measured from pin 5 to ground. looks like I'm getting closer, but not much change on the high end.
Measured values without the big power tubes.
-25 to -14 from one end of the bias pot to the other on all 4 pots, measured from pin 5 to ground. looks like I'm getting closer, but not much change on the high end.
Ok, one problem fixed.
First measure DC voltage at the pot wipers, instead of at pin 5.
Of course this reading will change with the pot setting.
First measure DC voltage at the pot wipers, instead of at pin 5.
Of course this reading will change with the pot setting.
There's your problem.
Either the rectifier, C17a/b, or resistor R35 is bad.
If any of these parts is not brand new, I would just replace them all.
The selenium rectifier can be replaced with a 1N4005 diode in series with a 1k resistor. Observe polarity.
The dual section C17 can be replaced with two separate capacitors.
Use a 2W or 3W resistor for R35.
Either the rectifier, C17a/b, or resistor R35 is bad.
If any of these parts is not brand new, I would just replace them all.
The selenium rectifier can be replaced with a 1N4005 diode in series with a 1k resistor. Observe polarity.
The dual section C17 can be replaced with two separate capacitors.
Use a 2W or 3W resistor for R35.
That makes sense, Rayma. the first time I plugged this thing in after assembly, I blew the little rupture diaphram on C17a. I didn't know why or even think about it and just replaced it, thinking I had its polarity backwards. Its not blown, now, but I didn't bother to look at it's resistor or the other capacitor. thank you and I will focus on that area. can you tell me what voltage I should see at C17b when it is right?