HK Citation II Mods
James Mcshane
It is overly complex, but not because it has 3 12BY7A's/channel. After
all, that's only three gain elements, versus 4 in two dual triodes. What
drove me crazy was the congested sound I heard. This amp has multiple
local feedback loops as well as global feedback. (I'm sure that's why all
the pentodes got used-most of the gain was used up by >30db of NFB).
Your balance problem is not unusual. The pots are good quality, and
aren't usually at fault, but check 'em anyway to be safe. You may have
some problems with the driver tubes or the output tubes themselves. You
also may have difficulty with the bias adjustment. I recommend you
(carefully!!!!!) measure the actual bias voltage with a DVM. The original
bias meter reads correct bias when 1.5 VDC is developed across the 15 ohm
cathode resistors. This biases the KT88s @ 100ma.!! Wow that's hard on
Chinese tubes. I recommend a bias of 60 ma. (.9 volts across the
resistors). Also be sure to replace the selenium rectifier in the bias
supply TODAY. A 1N4007 is a good choice. Be sure to reset the bias when
finished, then adjust AC balance.
The power supply voltage doubler will be fine if left stock, but it can
be improved with better diodes and more capacitance. A film cap bypass at
C15 is a good idea also.
Finally, I have a couple of extensively modified Cit IIs. Both use triode
voltage amp and phase splitters; 12AY7/6072 voltage amps and 6201 long
tailed pair phase splitters. I use VTL KT90 output tubes (I think the
Svetlana 6550s would be good too, but I haven't tried them), and about
14db of global feedback. I have circuit details, schematics etc., if you
are interested. The sound quality is awesome, and you can convert to
triode output easily.
Let me know if I can help you more.
-
JAMES MCSHANE MLJ...@prodigy.com
V. Keith Warden
I bought a Cit II about three months ago and have been working on it off
and on ever since. When I got it, it hummed and motorboated. I replaced
most of the PS caps and all the coupling caps. As for the sound I think
it beats my heavily modded ST70. I read several times about it being more
complex than necessary and by the looks inside I would tend to agree. Six
12BY7A (1 input/2 drivers per channel) and four push-pull KT88's or 6550's.
The balance looks best when the controls are turned to there extremes
(all the way left for the left channel and right for the right channel).
Plus the bias/balance meter fluctuates on the left channel. Something's
not quite right.
It has solid state rectifiers and (to me) a hokie voltage doubler in
the power supply. The OPT are very highly regarded. It runs very hot.
Those 12BY7A's run a lot of current through them. It has a phase-splitter
circuit I've never seen before and feedback/traps all over the place (30dB
of FB). If I knew more what I was doing, I'd be tempted to yank the innards
and throw in an all triode input/driver circuit.
Which brings me to my question. Does anyone know of mods for the HK
Citation II? Aren't the KT88/6550's easy to drive, like EL34's? If I
had a scope I'd be more tempted to lash out on my own, but alas. If no
one knows of any mods, how about schematics from other amps using
KT88/6550's? Is triode mode as easy to do as with the ST70? Or will all
the feedback circuits need to be changed.
Thanks for any assistance,
Keith - kwa...@tyrell.net
* Q-Blue 1.9 [NR] *
James Mcshane
James Mcshane
V. Keith Warden
> From: MLJ...@prodigy.com (James Mcshane)
> Newsgroups: rec.audio.tubes
> Subject: Re: HK Citation II Mods
> Date: 9 Sep 1995 17:30:51 GMT
>
> Comments on your Cit II situation:
>
> It is overly complex, but not because it has 3 12BY7A's/channel. After
> all, that's only three gain elements, versus 4 in two dual triodes. What
> drove me crazy was the congested sound I heard. This amp has multiple
> local feedback loops as well as global feedback. (I'm sure that's why all
> the pentodes got used-most of the gain was used up by >30db of NFB).
It;s that hokie looking phase splitter that I'm referring to. I just
can't get over it.
> Your balance problem is not unusual. The pots are good quality, and
> aren't usually at fault, but check 'em anyway to be safe. You may have
> some problems with the driver tubes or the output tubes themselves. You
> also may have difficulty with the bias adjustment. I recommend you
> (carefully!!!!!) measure the actual bias voltage with a DVM. The original
> bias meter reads correct bias when 1.5 VDC is developed across the 15 ohm
> cathode resistors. This biases the KT88s @ 100ma.!! Wow that's hard on
> Chinese tubes. I recommend a bias of 60 ma. (.9 volts across the
> resistors). Also be sure to replace the selenium rectifier in the bias
> supply TODAY. A 1N4007 is a good choice. Be sure to reset the bias when
> finished, then adjust AC balance.
I changed the balance pots, because the originals were shot. The 12BY7A's
are a set of tested/numbered Tektronics from GE/RCA. Two RCA inputs, four
GE Drivers/Splitters. I only have an Emission tester, but the tubes still
had great emission at 5vac on the heaters. I tested this because of the
reduced heater voltage on the inputs. The outputs test and bias up just
fine and are set to 1vdc. I changed the Selenium rectifier and added more
capacitance to the bias circuit. I disconnected the bias/balance meter and
brought the wires out to two external binding posts so I could use a DVM.
I still use the meter to set the balance, because I seem to get erroneous
readings with my DVM (not true AC I suppose). I do plan to get a set of
the Svetlana 6550's when they become available.
> The power supply voltage doubler will be fine if left stock, but it can
> be improved with better diodes and more capacitance. A film cap bypass at
> C15 is a good idea also.
I put two 1N5408 1000 volt 3 amp diodes in the B+ supply and increased
the doubler caps to ~270uF (the caps are actually unbalanced. A new multi
section cap rated at 200/40/40uf @ 385vdc (all tied together) and a old
metal can rated at 250uF @ 300vdc. With the way Electrolytics are rated
(+50%, -10%) I don't think that makes much difference. I tied all the
terminals together on an old quad cap (30/20/20/20uF @ 500vdc) that I
took out of my ST-70. I measured the B+ at 450vdc. I replaced the 50/50uf
and the bias can. The only one I didn't replace was the doubler 250uF @
300vdc. I will eventually though. The film cap goes in next. What size do
you recommend? I noticed two things when I was testing it the other day.
The first is the pulsing on the meter when trying to balance it and maybe
when setting the bias as well. The second is I measured the voltages as
listed in the copy of the manual I have, and found that on pin 7 for V2/V3
the voltages are 230vdc and 350vdc and changed very little when the balance
was rotated. V5/V6 was 317 and 265vdc. I'm going to look and see if it was
wired right. Maybe those big power resistors are shot.
> Finally, I have a couple of extensively modified Cit IIs. Both use triode
> voltage amp and phase splitters; 12AY7/6072 voltage amps and 6201 long
> tailed pair phase splitters. I use VTL KT90 output tubes (I think the
> Svetlana 6550s would be good too, but I haven't tried them), and about
> 14db of global feedback. I have circuit details, schematics etc., if you
> are interested. The sound quality is awesome, and you can convert to
> triode output easily.
Yes, YES!. definitely interested. The amp sounds quite good the way it is,
but I feel it has much more potential (no pun...). I'm ready to mod. Do you
have a schematic of your mods.
> Let me know if I can help you more.
Thanks.
> JAMES MCSHANE MLJ...@prodigy.com
James Mcshane
you now.
1. Do you have access to a scope? If so look at the B+. How much ripple
at idle? Are you certain the old caps you used are good? Any ripple on
the bias supply?
2. The pulsating balance meter worries me. What load are you using to set
the balance? H-K recommends a 16 ohm resistor, > 20 watt rated from the
16 ohm terminal to common. Be sure you are close on this. Also what kind
of shape is the 50/50 mfd. 450 v. dual section cap used to decouple V1
and V4? Are you using the test signal developed internally by the amp? If
so, you will be driving the amp to a reasonable output. poor decoupling
could definitely cause you trouble with the test signal applied.
3. I should have mentioned this before- what kind of shape are the tube
sockets in?
4. Save your self some money on output tubes- set the bias @ 60 ma. It
also eases the total load on the power supply at idle by 160 ma. (4 x 40
ma).
5. Unless you are prepared to do a lot of detective work, you will want
to solve the operating problem you have now before any big mods. I'm not
trying to preach to you, I learned the hard way myself.
Let me know what else you need.
-
JAMES MCSHANE MLJ...@prodigy.com
V. Keith Warden
> From: MLJ...@prodigy.com (James Mcshane)
> Newsgroups: rec.audio.tubes
> Subject: Re: HK Citation II Mods
>
> Yes I do have schematics, etc., but first, a couple of questions to help
> you now.
>
> 1. Do you have access to a scope? If so look at the B+. How much ripple
> at idle? Are you certain the old caps you used are good? Any ripple on
> the bias supply?
No scope darnit. I moderately sure that all the PS caps are ok with the
exception of the top cap of the doubler (250uF @ 300vdc). It's an old can.
I will try to measure any ripple with my DVM and see what I get. I thought
about using a 50-60 volt zener on the bias. I did increase the bias cap
from 20/20uF to 50/30uF.
As I mentioned in my first message, I measured the voltages as listed in
the copy of the manual I have, and found that on pin 7 for V2/V3 the
voltages are 230vdc and 350vdc and changed very little when the balance
was rotated. V5/V6 was 317 and 265vdc. I'm going to look and see if it was
wired right. Maybe those big power resistors are shot.
> 2. The pulsating balance meter worries me. What load are you using to set
> the balance? H-K recommends a 16 ohm resistor, > 20 watt rated from the
> 16 ohm terminal to common. Be sure you are close on this. Also what kind
> of shape is the 50/50 mfd. 450 v. dual section cap used to decouple V1
> and V4? Are you using the test signal developed internally by the amp? If
> so, you will be driving the amp to a reasonable output. poor decoupling
> could definitely cause you trouble with the test signal applied.
The pulsing meter worries me too. I bought a 15ohm 25watt sand cast power
resistor for balancing. The 50/50uF @ 500vdc cap is NOS from AES. The cap
after the choke is the one from my ST70 and has what appears to be a date
of 1992 on it. I use the test signal on the amp. I suspect the problem is
in some way related, at least partially, to the voltage unbalance referred
to in the above paragraph. The only major thing I did was to put two 10uF
@ 450vdc caps on the screens of the drivers, between pins 8 and the 22k
3watt resistor. One cap for each set of drivers. I'm sure it pulsed before
that though.
> 3. I should have mentioned this before- what kind of shape are the tube
> sockets in?
The sockets are ok, as far as I can tell. I don't think the Chinese KT88's
could be the problem, But I'll buy new regardless. I want to try the new
Svetlana 6550B's and the Sovtek KT88's.
> 4. Save your self some money on output tubes- set the bias @ 60 ma. It
> also eases the total load on the power supply at idle by 160 ma. (4 x 40
> ma).
I set them to a max of 73 milliamps (1.1volts)
> 5. Unless you are prepared to do a lot of detective work, you will want
> to solve the operating problem you have now before any big mods. I'm not
> trying to preach to you, I learned the hard way myself.
Agreed, but discarding the old boards would most likely eliminate any
problems, or at least narrow the cause down to the PS.
> Let me know what else you need.
I need a scope;) I would like to see the circuit you used. I've seen a
triode circuit I've been thinking about using. It consists of a 6CG7 one
half is the input, the other is a direct coupled phase splitter (ala ST70
type) with another 6CG7 as drivers with each half cap coupled to PP EL34's.
A ST70 mod. It's from Andy Moss aka tub...@cml.com. I thought I read that
KT88/6550's were easy to drive, so I think this circuit would work fine.
I would like to take a look at what you're using, if I may.
> JAMES MCSHANE MLJ...@prodigy.com
John Chervinsky
I also have a Citation II that I am upgrading. Although I have been
running this amp for 15 years, I have noticed that the power transformer
is starting to leek a little. It has always ran hot to the touch, that
is, I can hold my hand on it indefinitely, but is starts to feel
uncomforable. Also, there is a faint buzz coming from it.
Words of wisdom would be appreciated.
Thanks,
John
James Mcshane
Audio Note, etc. became widely available. They are useful to restorers,
but to be honest, I've never heard of a bad Cit II output. Some folks may
need to replace a missing unit though.
-
JAMES MCSHANE MLJ...@prodigy.com