Power Transformers. Decisions

[kff322]

Hello World,

Does anyone know the specs or a power transformer that would be a good match in a Harman Kardon Citation II?

Thanks Guys

[danFrank]

My SAMS fotofact says:
180v @ 1.7 amps,
6.3v @ 6 amps,
70v @ 10ma (?) for bias,
and
6.3v @ 6 amps

[kff322]

Thanks a lot
Now does anyone know of a suitable replacement?
hmmm

[kevinkr]

I used to sell these on eBay, you might try there from time to time..
Otherwise you are probably talking about a custom made replacement.

Are you repairing an existing Citation II or building one from scratch?

I used to repair and restore lots of these in the 1990's as part of my day to day business, and am quite familiar with the design and its limitations. (Yes it's not perfect )

If you are building one from scratch check out my site and find the old OTS/PF articles there as there are a number of mods mentioned in them that you might find useful.

If scratch building I do not recommend using the stock doubler circuit, this will give you a range of choices from Hammond.
Note that the idle current in the Citation II was 100mA per tube, and this is a huge burdon on any extant power transformer, not to mention the fact that the only modern tube I have seen that can handle the dissipation at 450V is a KT99.. Otherwise I would run them around 60 - 70mA.

You could use 750VCT - 800VCT @ 500mA transformer to run a clone Citation II. Or as mono blocks 750VCT - 800VCT@ 250mA should work fine, in both cases assuming tube rectification.. Something in the Hammond 37X series ought to work. Using solid state fullwave (not bridge) rectification something right around 700VCT@ >500mA would be ok. If you elect to use a bridge 360V @ >500mA would be fine for a stereo amplifier.

One of my more popular mods was to boost the B+ to about 540Vdc, this made a surprising difference to the sound quality. I also had an entire line of amplifier designs (and a few commercial amplifiers) built around Citation II iron. MFA built a line of 120W monoblocks using a single Citation II output transformer, and I also built some commercially. This output transformer is very conservatively rated for a 60W transformer, and in fact with 540V B+ and a quartet will easily get over 100W..

Hope some of this is helpful and not too far off topic.

[kff322]

I am building a clone,
Because large power transformers are very expensive, I am planning to build a 4 stage voltage doubler, bring 120 Volts to around 480 Volts.Would there be any problems in doing this?

Btw - Your site was very useful!

[Tweeker]

Your not trying to omit the transformer I hope? Dont.

Look to industrial transformers for cheap power iron. If bridge rectification is ok it might be tenable use a 120V winding in series with a 240V winding or 2 120V windings for 360VAC.

[kff322]

I am still using an isolation transformer of course! But whats wrong with a voltage doubler?

[EC8010]

They have very high output impedance. Also, your isolation transformer may not be up to the job of isolating those voltages. Go for kevinkr's suggestion and use a proper transformer, 360V with a bridge would do nicely. Money saved on transformers is time wasted on construction. You're building it to produce something good so don't skimp on the materials.

[ray_moth]

I agree, voltage doublers are OK for low current. I use one myself, for the negative supply in my amp, to feed voltage amp stages and to bias the OP tubes. The current involved is less than 30mA and it's all class A (doesn't vary with the signal).

However, for the main B+, which has to supply all stages, including the OP tubes whose current demand depends on the signal amplitude (unless they're in class A), you need a low impedance supply. A voltage doubler is not a very good solution for this. (I know that H-K used one but I think it must have been for economy reasons).