Citation 12 Substitute Transistors

[aged one]

Hello All

I am planning to build the Citation 12 from the HK manual.

Could you suggest suitable replacements for the obsolete 40408, 40594 and 40595 transistors?

It seems to me that the specs for the 2n3055 closely matches those of the 40636.....what are your thoughts?

Appreciate any help you can give.

[PB2]

Originally posted by aged one
Hello All

I am planning to build the Citation 12 from the HK manual.

Could you suggest suitable replacements for the obsolete 40408, 40594 and 40595 transistors?

It seems to me that the specs for the 2n3055 closely matches those of the 40636.....what are your thoughts?

Appreciate any help you can give.

Hi,
I'm just wondering what your looking for with this? A copy that is as close as possible, low cost? It's probably one of the most common and copied designs out there, it is very close to the reference design from the very old RCA transistor manual. I built it from scratch over 30 years ago.
It would make sense to build it as a complementary design today given the availability, but either way is fine, do you prefer quasi? One side of the quasi is EF the other CFP so take your pick if you make it complementary.

[aged one]

I would like to build it as close to the original as possible.

Of course, because of the un-availability of the original set of transistors, I need to identify suitable alternatives. Ths is proving difficult as I am unable to locate the specs of those devices

[PB2]

Originally posted by aged one
I would like to build it as close to the original as possible.

Of course, because of the un-availability of the original set of transistors, I need to identify suitable alternatives. Ths is proving difficult as I am unable to locate the specs of those devices

It's odd that RCA had many parts that were similar if not seemingly exactly the same. The design is tolerant of transistor substitutions because it uses simple Cdom compensation. I don't like the 40636 or any of the slow (Ft .8 MHz) 15A, 115W parts. It is odd that the RCA reference design used the 40411 which is 20A, 30A peak, 200W and it had VI limiting which was removed in the Citation.
It would be nice to build it so that you could go up to +/-45 or 50V on the supplies, the 2N3773 is a very similar part and would allow this, it's actually a preferred part at On Semi. I'd go with a 2N3773 if you must have close to original. A more up to date TO3 part is the MJ15003 which has the best SOA.
If you don't mind plastic the modern choice is MJW21195 - 16A, 250V, 200W should be fine, I'd double up on them to be safe into 2 ohm loads and accidental shorts. If you plan to go surplus and want to use any of the 15A, 115W parts make sure their at least 90V and I'd double up on these also.
Again the HK drivers are odd, the RCA design used the 40409/40410, and I stated in another thread that the MJE253/243 (or MJE15034/35) plastic parts are a better substitute 4A/100V/40W, good beta, good Ft, this is a current part. Use a small heat sink with them.
Bias transistor in the HK is 2N5232 and a dual diode is used for thermal sensing. The RCA design uses a three diode string, 2 are 1N3754 which mount to the output transistors with a metal holder, I'd add a 100 ohm pot in series for bias adjust.
Predriver is a 40408, .7A, 90V, 1W, hFE 40-200@10mA, Ft 100 MHz. MJE243 is a good substitute, or 2N3440.
The diff pair is a PNP dual transistor which provides excellent matching. The HK manual states that there is no substitute. The RCA design uses 40406 transistors and suggests selecting them. You can hand match the old standard 2N5087, which Leach used and tells you how to match at his page:
http://users.ece.gatech.edu/~mleach/lowtim/part2.html
Get ideas on parts sources, and transistors Leach uses:
http://users.ece.gatech.edu/~mleach/lowtim/part3.html
You many notice that this is nearly a copy but made complementary CFP:
http://sound.westhost.com/project3a.htm
And this might be helpful also, see Fig 33:
http://www.dself.dsl.pipex.com/ampins/dipa/dipa.htm

[BrianL]

Later Citation 12s used matched pairs of 2N5087 for the
input diff pair. My experience (both from genuine HK C12'a
and from about 20 DIY clones of it) is that both the original
canned diff pair and the 2N5087s tended to get noisy and
then die, probably because of marginal voltage rating.

I honestly can't figure out why one would want to replicate
this design. There are any number of designs similar
in topology but more refined in execution. For example, the
various Doug Self "Blameless" designs take this simple
topology to much more refined heights (in terms of
"objective" measurements) and would likely "sound better"
as well.

The HK Citation 12 chassis and transformers do provide
a very good platform for DIY amp designs. I currently
have a modified Nelson Pass MOSFET-output version
in one chassis.

[Greg Erskine]

hi aged one,

Have a look in the Pass Labs forum. The Citation 12 gets talked about reasonably often.

[PB2]

Originally posted by BrianL
Later Citation 12s used matched pairs of 2N5087 for the
input diff pair. My experience (both from genuine HK C12'a
and from about 20 DIY clones of it) is that both the original
canned diff pair and the 2N5087s tended to get noisy and
then die, probably because of marginal voltage rating.

This is interesting I didn't see this problem in amplifiers using the compliment to the 2n5087 or ones of similar voltage rating. I'd guess that it was static discharge (ESD) that got those transistors.

Originally posted by BrianL
I honestly can't figure out why one would want to replicate
this design. There are any number of designs similar
in topology but more refined in execution. For example, the
various Doug Self "Blameless" designs take this simple
topology to much more refined heights (in terms of
"objective" measurements) and would likely "sound better"
as well.
[/B]

I don't see any reason not to build a Citation copy, it's based on sound engineering. The parts count in the Citation 12 is very low, it's impressive that it can perform so well being such a simple design. I also took another look at the specs, they're excellent, it's 20 kHz distortion is lower than the Leach. They went for more bandwidth than I would but they were holding to the wire with gain concept.

[PB2]

Originally posted by PB2

It would be nice to build it so that you could go up to +/-45 or 50V on the supplies, the 2N3773 is a very similar part and would allow this, it's actually a preferred part at On Semi. I'd go with a 2N3773 if you must have close to original. A more up to date TO3 part is the MJ15003 which has the best SOA.
If you don't mind plastic the modern choice is MJW21195 - 16A, 250V, 200W should be fine, I'd double up on them to be safe into 2 ohm loads and accidental shorts. If you plan to go surplus and want to use any of the 15A, 115W parts make sure their at least 90V and I'd double up on these also.

I simulated the Citation 12 and am having what I think are model problems. The 2N3055 included with LTSpics has very low beta, lower than worst case specs, around 5 IIRC and I think it is an error in the model.
I tried the OnSemi 2N3773 and it is on the edge of stability, it bursts into oscillation when driven into slewing with a square wave. Tried many other OnSemi models, MJ15003, MJW21195, MJ802 they're not as bad as the 2N3773 but also have very poor stability. They show very high beta when I use a test circuit, which is probably closer to the typical device that's seen in real life. I wonder if they've got the HF parameters right?

I've built the Citation with substitutes (2N3790, 2N3714 outputs) for all the transistors, and know of one built with 2N3773s they both worked fine - never any smoke. Had one uneventful transistor failure that was probably just a normal failure. I think there's a problem with the simulation.

[Mark A. Gulbrandsen]

"it's 20 kHz distortion is lower than the Leach."

Yea but the Citations 12's TIM levels are off the charts! I wouldn't waste my time with this as a project. I converted piles over to the Pass version back when I was a bench tech. All the customers that had it done fully agreed it was miles ahead of the original version. Overall the Pass conversion is a much better choice and still keeps the parts count very low.

Mark

[PB2]

Originally posted by Mark A. Gulbrandsen
"it's 20 kHz distortion is lower than the Leach."

Yea but the Citations 12's TIM levels are off the charts! I wouldn't waste my time with this as a project. I converted piles over to the Pass version back when I was a bench tech. All the customers that had it done fully agreed it was miles ahead of the original version. Overall the Pass conversion is a much better choice and still keeps the parts count very low.

Mark

I think it's an excellent first project either way that one wants to build it. The poster asked for as close to original and that's what we're talking about. He could always convert it later. Did you measure the TIM? I've talked a lot about TIM in another thread but I think it's rare that source material has fast enough rise time to send an amp into slewing/TIM.

I think the Citation had lower power into 4 ohms than 8 indicating an output current limitation, this is one thing I'd like to address, it was probably caused by beta droop.

Pete B.