Stepdown Transformer for K-12G or New Power transformer?

[phaze]

Hi All, a bit of a newbie but anyway I'll try and make sense.
I have bought and mostly built the K-12G amplifier from s5electronics.

As I live in Australia I am running off 240VAC. I uderstand that I either need to replace my transformers or run the amp of a stepdown transformer. I have opted for the stepdown transformer path just because it seems alot easier. I have found some stepdown transformers locally at www.altronics.com.au they come in 50watt 120watt and 250watt models.

Does anyone know what the minium I require will be?

Optionally if anyone knows where I can buy a suitable power transformer I would also be forever greatful as I have been able to source only the output transformers.

[erichayes]

Get the largest stepdown transformer you can afford. Amateur high fidelity is the aural equivalent to amateur astronomy: the amplifiers/telescopes just get bigger and bigger.

Just a thought . . . try rattling Dick Smith's cage. Maybe if enough of you upsidedown folks make serious noise regarding your transformer needs, they'll consider accommodating you.

[EWBrown]

g'day,

The newer 5670 & 6005-based S5 amps now have dual primaries on their power trannies, so they can run either 120 or 240VAC 50 - 60Hz anywhere in the world.

(I presently have the K16 PC board built, and the trannies currently "hanging" off the board, for a qquick power-on "smoke" test, but it is not yet chassis mounted. I'm going to go with better OPTs, either Hammond 1615s or 1620s)

I'd go with a 240/120 step-down for the K-12, if you plan on future power upgrades with 120VAC power, or plugging in additional equipment, then go for the higher VA capability. Otherwise, at least 120VA should be more than enough, for this amp... 50VA would be too close to the safety "edge", and 240VA would also be fine, and allow for future expansion.

/ed B in NH

[TerrySmith]

Hey Ed,

Are the outputs on your new '16 the same as the K-502?

I've got a pile of 6005 / 6AQ5's to burn up, maybe I'll cobble up a no pcb L-16, as I have two extra OPT's.

[EWBrown]

Exactly the same "UTK" trannies, as on the K-12 and K-502s. Check the large format photo, the model numbers should match up exactly.

The OPTs are OK for 8 to 10 watts, but really stretching it to handle 16 watts. The PPP 6005s like to see around 5K P-P load, they achieve this effect by specifying the output impedance to be 4 ohms. Since the OPT has a 35:1 turns ratio, this effectively looks like 4900 ohms CT to the PPP cirucit.

It's ll work, but the low freq and hi-freq response will suffer by a few dB.

I'm probably going with some Hammond 1620s in the final build. I had thought of the 1615s, they are 5K, 15W, bu thte 1620s, even though ithey are 6600 ohms p-p, it is a better OPT for the price, and besides, I already have two of 'em leftover from a re-habbed DIY35 project. .

The basic circuitry is nearly the same as the K-12 or K-502, and the actual resistor and cap values can be gleaned from the Big Picture Yellow_Light_Colorz_PDT_01 All resistors are 1/2 watt, 5% unless otherwise specified.

VA Rp is 220K, and Rk is 1500 ohms. The phase splitter is 68K 1/2 watt Rp and Rk. Coupling caps are the low-cost mylar 0.22 uF / 250V, I went with some WIMA 0.15 uF 40V caps that I had on hand - they are a perfect fit to the PC board.

The NFB loop is the same as the original, 68K bypassed with 33 pF. The groups of 4 6005s each have their cathodes tied together to a single 150 ohm, 3W resistor, it may be better to use 560-620 ohms, 2W for each tube, and bypass with 100-330 uF / 35V if you prefer. The G2 screen grids all have their own individual 220 ohm, 1/2 watt resistors, and the other end of those resistors are all connected together, then to the LV side of the 470 ohm PSU resistor.

The OPT B+ is approximately 225 VDC, then dropped thru the 470 ohm 1W resisor to about 215VDC, then thru the 2.2K 1/2 W resistor to feed the VA and PI 5670s at 210VDC. The VA and PI cinsume less than 1.5 mA combined (each channel), and the 6005s limp along around 25-27 mA each, so they should last a LONG time. It's simple enough it could be wired P2P very easily.


/ed B in NH