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Old 11th December 2006, 02:50 PM   #1
Sherman is offline Sherman  United States
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Default Citation II OPT?

I've been reading the thread on a scratch-built Citation II oscillating. One of the issues seems to be the selection of a Hammond OPT. It seems the bandwidth is limited (Hammond states 30Hz to 30K Hz).

I've been thinking of building a Citation II from scratch as a winter project and was wondering, short of original Citation II OPTs what would you recommend?

I've been looking at the James 6273HF. It has a primary Z of 3.4K, 200mA capability, 70 watt rating and a stated frequency response of 5Hz to 110K Hz -1 dB. I understand the original HK Citation II OPTs varied from 3.2K to 3.5K so it seems a good fit there. At 15 pounds each they would make for a hefty amp!

Any Citation II owners or builders with suggestions?
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Old 11th December 2006, 04:56 PM   #2
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Default Re: Citation II OPT?

Quote:
Originally posted by Sherman

I've been looking at the James 6273HF. It has a primary Z of 3.4K, 200mA capability, 70 watt rating and a stated frequency response of 5Hz to 110K Hz -1 dB.

Any Citation II owners or builders with suggestions?

This James tranny offers such a wide tempting b/w that it must be a toroid in disquise .. my E&I 20 pounders from other manu's are quoted 20-45K or sim. Instinctly, when using wide b/w transformers, the shunt cap used in global feedback loop has to be optimised for ideal squarewave shape and the interstage feedback network caps increased in value to reduce output stage ringing. This is quite a challenge and I've heard this before with Citation 2 mods. There is a web "mods work page" around.

...Has anyone had a consistent full non distorted LF push pull output power from a toroid with misbalanced output tubes in fixed bias ? (I doubt it)

It pays to look at the 12BY7 datasheet. It has darned good voltage gain at modest current levels (in pentode and triode config) and requires a respectful tight layout to avoid this RF tube oscillating on the quiet. A coupling wire from a cap hanked on to the anode will do just that !


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Old 11th December 2006, 08:43 PM   #3
kff322 is offline kff322  United States
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I remember reading in another audio forum that the best OPTs for a "modern" Citation II amp would be Plitron's PAT4004 series OPT. The impedance and bandwidth are near the specs of the originals iron.

The only reason I didn't get them for my Cit II clone is because I did not know Plitron even existed. Oh well. (And they are $311CAN each which makes it even harder being a high school student working at a gas station)

good luck
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Old 11th December 2006, 11:57 PM   #4
Sherman is offline Sherman  United States
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Quote:
Originally posted by kff322
I remember reading in another audio forum that the best OPTs for a "modern" Citation II amp would be Plitron's PAT4004 series OPT...

... they are $311CAN each which makes it even harder being a high school student working at a gas station...


$311CAN is a lot even for someone a 'bit' older than a high-school student! I will take a look at the Plitron OPTs though. I haven't completely decided on the Citation II but have been leaning that way. After following your thread I am having second thoughts.

I hope you get the ocillation issue sorted out. I'm looking forward to hearing the results of your tweaking.
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Old 12th December 2006, 12:04 AM   #5
Sherman is offline Sherman  United States
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Default Re: Re: Citation II OPT?

Quote:
Originally posted by richwalters



This James tranny offers such a wide tempting b/w that it must be a toroid in disquise ...

It pays to look at the 12BY7 datasheet. It has darned good voltage gain at modest current levels (in pentode and triode config) and requires a respectful tight layout to avoid this RF tube oscillating on the quiet. A coupling wire from a cap hanked on to the anode will do just that !


richj
Rich,

Not that it means much but the diagram of the James OPT seems to show that it is not a toroid. Check the diagram on this page-

http://euphoniaaudio.netfirms.com/ea...1780b344ea3d80

If that doesn't work just navigate to the correct page on the Euphonia website. (I wish they were selling again, I got my SE James from them at a good price and they were delivered extremely quickly.)

I understand the 12BY7 is one of several reasons the Citation II is well regarded. Like any other complex bit of electronics there is more to it than one element and I'm interested to see that the interaction between OPT, input and driver tubes and the complex feedback loops seems quite critical. A bit different than the usual tube amp "hey, get within 10% of it and you're good" way of thinking.
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Old 12th December 2006, 01:03 AM   #6
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Quote:
I understand the 12BY7 is one of several reasons the Citation II is well regarded.

Uh, huh. The 12BY7 is a video output device, with huge bandwidth. Notice the pattern. Everything in a Cit 2. flows from bandwidth. The KT88 is a very respectable RF tube. The same "guts" are in the TT21. TT = transmitting tetrode.

The large bandwidth allows NFB to perform "flawlessly". The amp does not run out of gain at the frequency extremes. Combining over 30 dB. of NFB and unconditional stability was no mean feat.

Every good amp has a good PSU. The Cit. 2 is no exeception. The voltage doubler is well executed and has plenty of current plus a low impedance.

Stu Hegeman was an engineer's engineer.
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Old 12th December 2006, 06:43 AM   #7
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Sherman, Yes this James tranny is obviously made from exotic core materials and the throughput power is roughly halve compared with same sized core using standard M6 silicon Fe...the cost obviously reflected. The o/p tranny in the orig Citation 2 was not particulary optimised regards leakage inductance, as the secondary windings were "tapped" so the design must have had a workable stabilty margin and response probably made to roll off quite early. A pity I sold mine as I should have examined it more carefullly.

Daresay the Citation 2 was around before the Electrostatic LS made headway. Anyone using a Citation 2 with an electrostatic LS ? That's the acid test.

On my Citation 2 schem' there is no "roll off cap" in the 1st stage. It's obvious that a small value pF cap in any feedback network will have a profound effect on phase behaviour. Asking for trouble.......with an extra wide b/w o/p tranny...-> yes. The EF184 is sim.

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Old 12th December 2006, 01:19 PM   #8
jlsem is offline jlsem  United States
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Quote:
Sherman, Yes this James tranny is obviously made from exotic core materials and the throughput power is roughly halve compared with same sized core using standard M6 silicon Fe...the cost obviously reflected.
I believe the James is made with M4 equivalent core material. The Citation transformers were made by Freed, and those guys sure knew what they doing! I have military surplus transformers (input and line out) made by them that have very wide bandwidth.

John
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Old 12th December 2006, 05:45 PM   #9
Sherman is offline Sherman  United States
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Thanks to everyone for their input.

Quite frankly, given the comments here and in the thread re: Citation II oscillation I'm getting more and more leary of trying to scratch build one, though it does seem the James OPT might be up to the task.

If I do decide to go ahead the next issue might be sourcing the James OPTs since Euphonia isn't currently selling. Perhaps direct from THLAudio.

Then there are always plans B and C.

Plan B- build a Dynaco MK III, lots of support out there for that. Maybe it doesn't have the Citation 'magic' but at least it seems possible to scratch build one

Plan C- the PP KT88 schem on the Ghat site using an interstage phase splitter looks interesting though no idea yet of cost for those trafos.

Oh, and there is a Plan D, find a Citation II at a flea market or estate sale for cheap.
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Old 12th December 2006, 07:40 PM   #10
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Hey-Hey!!!,
There is a pair of originals on ebay now... or get a really ratty amp. I got a pretty rough looking one, and at some point I will have one of its outputs unwound and copied. I suspect it will be a fairly complex wind, but not quite as bad as the Peerless 20-20 Plus S-271-S I just had done...

Or somebody else can wreck one of their Deuce outputs...
cheers,
Douglas
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