HK Citation II Bias "issues" - diyAudio
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Old 3rd March 2006, 06:37 AM   #1
kff322 is offline kff322  United States
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Default HK Citation II Bias "issues"

Hello guys,
I just finished my attempt at building a HK citation 2 from scratch and being a tube n00b there’s a slew of problems following my build. The sound is REALLY quiet like 1/12 of a watt, its supposed to be around 30 watts a channel Min.

I buzzed out the entire circuit and I know this is a bias problem, ether being caused by my power supply or faulty Electro-Harmonix KT-88EH power tubes. I have tried both sets of tubes and no difference, hopefully ruling out dead tubes.

A Heathkit 400 volt variable PS is powering the mains on this thing.
Bias is taken care of by a smaller PS that is completely separate from the Heathkit. All the voltages on all the tubes are good including pin 5 on the 2 KT88's BUT Pin 8 where the BIAS measurements are taken is -0.001V (idealy 1.5V) on my DVM overall current draw is only 19ma per channel, supposedly 70ma bias per tube for KT88EH.

Other Things of note
Main power will not rise above 25ma, if you raise the voltage above 200V The sound looses depth and acquires hash distortions.
Output Xformers are Hammond 1650N's 4300 Ohm Pri Impedance
Any thing else you guys want to know please ask. I have asked every guy who knows a little about tubes and I NEED YOUR HELEP!!!1!
oh yeah
Complete Schematic
Other Citation II resources
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Old 3rd March 2006, 11:17 AM   #2
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You say that in normal operation there should be 1.5V at pin 8 of the KT88, 1.5V over 15 ohm gives a current of 100mA so it differs from what you say is the requirement 70mA.

However this is not the main issue for now but instead that the KT88s doesn't draw any current at all as you have all but no voltage drop over the cathode resistors.

My advice is to check that all voltages are correct anode supply and bias supply and if that is correct please check the resistors around the bias network, R25a, R25b and R26. R25a together with R26 form a divider which will allow the bias voltage on the grid to be varied by adjusting R25a, (the same goes for R25b for the other tube) if for instance R26 is open circuit or not connected to ground the voltage on the grid will be very negative and current will be low.

Regards Hans
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Old 3rd March 2006, 11:44 AM   #3
SY is offline SY  United States
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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.

edit: which Heathkit supply are you using?
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Old 3rd March 2006, 02:21 PM   #4
kevinkr is offline kevinkr  United States
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Citation II is a 60W per channel design. Supply voltage should be in the range of 480V. I found big wins sonically in increasing the supply voltage to as much as 550V, watch the dissipation in the output tubes though.

Bias voltage to the KT88 should be in the range of -55V to -70V depending on the output tube transconductance, and chosen plate voltage. (dissipation)

Nominal idle current per tube is 100mA or 1.5V across 15 ohms, you might want to reduce this to 60 - 70mA with modern tubes, incidentally the Citation II was pretty hard on tubes. As I recall the output tubes delivered from were HK branded MO KT88 and they are the only type rugged enough to run at the rated dissipation long term other than an EI KT90.

That first 12BY7A is critical to the sound you will need to select one for low microphonics and that is a serious problem with this tube type. (Apparently microphony in video pentodes used in video is not an issue??)

Your output transformers are very different than the oem Citation II opts and you may need to change the compensation to make it stable. There are multiple nested internal feedback loops in the driver stage and overall feedback in this design including the global loop to the output transformer is over 30dB.

IMHO most of the magic in this amplifier was due to the output transformers, although the design by Stewart Hegeman was quite clever and unusual compared to standard design practice.

I have overhauled many original Citation II over the years, it was an early specialty of my (now defunct) business.

I have used the output transformers in several of my own designs as well.
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Old 3rd March 2006, 02:36 PM   #5
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Default Re: HK Citation II Bias "issues"

Quote:
Originally posted by kff322


A Heathkit 400 volt variable PS is powering the mains on this thing.
If you have a IP 2717 A Heathkit 400 volts variable power supply, like mine .

This power supply is internally protected to supply a maximal output current of 100 mA and in any way it can feed the four KT88 with 70 mA each...
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Old 3rd March 2006, 08:31 PM   #6
kff322 is offline kff322  United States
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Quote:
you have a IP 2717 A Heathkit 400 volts variable power supply, like mine .
Yes its a IP 2717, I am only powering 1 channel and the amp wont take more than 25 ma, turning it past 200volts and it runs away with no sound.

Without any tubes in the sockets the screen Bias is -74.7 Volts on pin 5, adjusting the pots will vary the Voltage +/- 10 volts, The screens and plate voltages are all B+ except pin 8 which is -.001

If the Feed Back loop from from V1 to the OPT is not connected, could that cause this volume/bias problem?

Thanks Guys
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Old 3rd March 2006, 09:20 PM   #7
SY is offline SY  United States
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Whoa, you're saying that you can only adust your grids from -65 to -85 volts? If so, I think we've found a problem.
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Old 3rd March 2006, 09:44 PM   #8
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Quote:
Originally posted by kff322


Without any tubes in the sockets the screen Bias is -74.7 Volts on pin 5, adjusting the pots will vary the Voltage +/- 10 volts, The screens and plate voltages are all B+ except pin 8 which is -.001

If the Feed Back loop from from V1 to the OPT is not connected, could that cause this volume/bias problem?

The fact that the feedback is not connected has nothing to do with the problem.
As Sy have told you , the problem is in the range of negative bias voltage...
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Old 3rd March 2006, 11:42 PM   #9
kevinkr is offline kevinkr  United States
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It's just a simple resistor value change in the divider string for the output tubes on each channel or an adjustment to the bias supply.

You aren't still using the 400V supply are you?
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Old 4th March 2006, 08:45 PM   #10
kff322 is offline kff322  United States
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Yes I am still using the 400 Volt power Supply! I will eventually get a ~475 V PXformer as well as a variac, but for now......

I fixed the variable Bias voltage problem, the Bias now varies from -35 V to around -75 Volts. on the screen pin 5. Raising the bias voltage causes the current and sound to cutoff and lowering the bias causes the tubes to run away as well as the sound. Ok somethings working correctly..

BUT - sound is still a blow-me-away 1/12 of a watt

There is still no voltage or current of any kind on the cathode pin 8. I even lowered R26 from 5.6K to 4.75, no dice.
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