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Old 1st January 2004, 04:01 PM   #1
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Default Harman Kardon Citation II : love/hate relationships

I have a Harman Kardon Citation II. It is a very problematic amplifier. It is my suspicion it was constructed incorrectly. When I first obtained the amplifier, it sounded horrible. I had to trace the circuit with a schematic and make corrections. There were many mistakes.

I believe this amplifier never sounded good, because of mistakes made during construction. Therefore, it has been stored unkempt in garages and attics, and has probably never been used.

The tube sockets need to be replaced, many components need repair, and it needs re-tubing. Some of the 12BH7s have blue ionization clouds when running.

The bias on the 6550s is way too hot. I suspect it is because the amp was built for 115VAC and my power is usually 121VAC. Last week, the 6550 output tubes went into runaway and blew a fuse. I need to do some component changes to modify the bias circuit to modern voltages.

So hereís the question: is it worth the money to rebuild the amp to original condition, or should the whole circuit be redesigned?

Hereís another way of asking the question: is spending money to rebuild the original driver circuit using six 12BH7 pentodes better or worse than designing and building a new driver circuit?

There's people who say they hate the Citation II, and there's people who say they love the Citation II. Which one are you, and why?
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Old 1st January 2004, 04:52 PM   #2
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Yes.........a new year surgical case........I did the same for my amp, found the chassis in a skip and recovered it. Are you sure about the front end and phase splitter tubes, on my sch it's a 12BY7A all round. Big difference......
The citation 2 has DC multiple feedback loops between O/Pstage tube anodes to phase splitter. There are also alot of 0.47uF 600V caps in it.. (pricy).... I'm not against this but there are many simpler UL push-pull circuits around which operate equally well and some better. In most high power fixed bias circuits, there is no failure interlock, i.e if for nay reason the neg volts goes down, the output stage can operate with reduced power or shut down, as it's easy to write-off the pricy output tubes. In addition the Citation 2 has a HT doubler power supply and that I don't like. I don't have values for the mains tranny not HT details but probably 450-470V B+
In your case, the list of faults can be high and is high. The UL stage is attractive and can stay intact, similiar output powers obtainable to KT88. If you are staying with the original and opting for a refit, and hoping for the best then all original components, pots, (used for neg bias) and caps should be replaced with new as also tubes

If it was my amp I would change the circuit. The output trannies are gold dust. For absolute simplicity, you could do as I did some time ago, butcher an orig GEC KT88-50 circuit and fit a higher power output stage and mains tranny in lieu.

The good common mode performance of the ECC82 triode drive as used in the GEC 88-50 circuit has enough guts to drive pair 6550's at 500V and is dirt simpler, when compared to the complex arrangement used Citation 2.

If it was my amp......the surgury to a simpler and more reliable version would be inevitable.

Only my opin!

rich
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Old 1st January 2004, 04:58 PM   #3
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I'd rebuild the original circuit. It's not for nothing that this amp became a classic.

If after that you're not satisfied for one reason or another, then you can hack the input stages. But the original was stable, reliable, well-designed... in all, a classic amp.
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Old 1st January 2004, 07:52 PM   #4
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Iím looking at the original schematic Ė I made a typo Ė it is the 12BY7A pentode.

I see six different feedback mechanisms:

-big global feedback from output transformer to input stage

-local feedback from grid 2 of input stage to cathode of input stage (capacitor C1 and C18)

-local feedback from plate of positive phase splitter tube to grid of the positive phase splitter tube

-local feedback from plate of negative phase splitter tube to grid of the negative phase splitter tube

-semi-global feedback from positive 6550 plate to negative phase splitter grid

-semi-global feedback from negative 6550 plate to positive phase splitter grid

Yowza! I havenít calculated all the gain from the 12BY7A stages, but it appears this amp employs heavy feedback. Based on this information, Iím guessing when it functions properly, it will sound like a solid-state amplifier.
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Old 1st January 2004, 09:35 PM   #5
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The message getting across seems to be the same........It did forget to mention that it's worth studying other circuits before deciding on a complete re-design. Okay most 50 watt amp driver circuits have enough umph to provide the drive for 100Watts.

Looking at the HC2 circuit there seems an awful lot of electrolytics in the power supply stages. If they have been mouldering for years in the back of a garage then they are well past their useful life. I can't make out the SS diodes, make etc, but this an area for improvement.
To design philos'y, alot of US amp designs did make alot of inverse feedback and sharp cut off pentodes, unlike in europe where more straightforward designs prevailed.

I don't know the noise figure for the HC2 but with all that internal feedback it could be quiet perhaps around -85dB or better .........most other P- amps are around -70dB; in other words, if you want to detect the background noise, it is just about disconcernable. perhaps someone with a spec of the HC2 could come forward.

My experience using pentodes ((as longtailed phasesplitter drivers.....is .. I don't like them) ) is that ageing characteristics are more likely to effect the operating conditions when compared to using triode stage configured as a common mode driver. So what happens in practice? Most of these longtailed phasesplitters require an AC balancing pot for symmetrically balancing the output stage drive (as unsymmetrical drive ends up as higher THD.. )
Who knows how to set it up ? I do; but one shouldn't really be expected to **** about with it. The simpler and common-mode approach as used in Williamson configuration and others does solve this problem. The drawback.....overall lower gain = mid 20dB, however if you use a concertina phasesplitter before this with a 30dBgain stage then things look reasonable. There is an advantage in not having too much stage gain in the middle driver stages ..so to speak of..as this can bring the noise of the previous stage up. That's why most amps using the concertina and paraphase phase splitters have noise figures around the 70dB @below full o/p.. The paraphase splitter (ECC83) as used in the GEC 88-50 configuration is also worth examining for simplicity.

At the end of the day anyone can look at all this and compare like for like, I'd put it forward; that the THD +IMD of the HC2 is probably no different to other tube amps which have far less internal gubbins which probably sound better. < Simplest is always the best and less likely to play up >...>any more ??
I'd abandoned many designs which offered darned good bench results, also on paper; but the amp sounded subjectively different when finished. In the end I went back to the simplest configurations and I'm more than happy with them....

rich
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Old 2nd January 2004, 01:57 PM   #6
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Quote:

I'd rebuild the original circuit. It's not for nothing that this amp became a classic.

SY is 100% correct here. Unfortunately you ended up with a bast**d unit that wasn't wired correctly. Very simple solution, undo it all and rewire it paying close attention to what the hell your doing. When you do this you will be VERY pleased with the result. Its easy to point the old finger and question why a circuit was designed a certain way. Have some faith as our fore fathers that designed tube circuits and paved the way for us knew what the heck they were doing. The amp is a classic! Restore it and enjoy it or sell it to me and I will give it a good home.

J
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Old 2nd January 2004, 03:36 PM   #7
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A total refit with same concept ....pricy.....and if it doesn't come up to scratch ? good luck... (the concept isn't for me). I agree with Kash'..
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Old 4th January 2004, 04:06 AM   #8
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The place to go for Citation information is http://pages.prodigy.net/jimmcshane/ - schematics, specs, etc.

Jim sells kits of parts to rebuild these, and he rebuilds them too.

In good working condition, it's probably worth $800 and up, mostly depending on external cosmetics. Unlike the Macs, modifications won't hurt the value that much - but you'll find it hard to beat the original.
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Old 4th May 2014, 07:29 PM   #9
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Quote:
Originally Posted by richwalters View Post
The citation 2 has DC multiple feedback loops between O/Pstage tube anodes to phase splitter. There are also alot of 0.47uF 600V caps in it.. (pricy).... I'm not against this but there are many simpler UL push-pull circuits around which operate equally well and some better. T

The UL stage is attractive and can stay intact, similiar output powers obtainable to KT88.

If it was my amp I would change the circuit. The output trannies are gold dust. For absolute simplicity, you could do as I did some time ago, butcher an orig GEC KT88-50 circuit and fit a higher power output stage and mains tranny in lieu.

The good common mode performance of the ECC82 triode drive as used in the GEC 88-50 circuit has enough guts to drive pair 6550's at 500V and is dirt simpler, when compared to the complex arrangement used Citation 2.

If it was my amp......the surgury to a simpler and more reliable version would be inevitable.
Rich,

Found this old post of yours. If I can ask you to scratch your brain,
or at least show us what mods you did. It might be worth considering
for me.

Got your notebook handy? or pics? or Schematics?

I'm sure I"m not the only one who would appreciate it.
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Old 5th May 2014, 08:16 AM   #10
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Sync,

Just refurb your "Deuce" ala Jim McShane. The only vintage amps in the same league as a Cit. 2 are by McIntosh. Frank McIntosh and Stu Hegeman were anything but stupid. Modern parts allow us to jack up the performance of the vintage wonders, but the original circuit designs should be respected.
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