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47'ing the Class A Joplin - Final Results

68.14.223.31

Posted by Lance on February 20, 2003 at 10:37:15

I finished mofifying the left channel for type 47 ST's. Results were similar to the right channel and the listening tests were equally impressive. I have sent photos to Brian so he can post them.

Here is the left channel info
(into 8 ohms)

@20 Hz

Po = 0.5 W, THD = 0.45%, Gain = 34.5 dB
Po = 1.0 W, THD = 0.58%, Gain = 34.5 dB
Po = 3.5 W, THD = 5.0%, Gain = 33.6 dB

@1 KHz

Po = 0.5 W, THD = 0.35%, Gain = 36.2 dB
Po = 1.0 W, THD = 0.54%, Gain = 36.1 dB
Po = 4.0 W, THD = 5.0%, Gain = 35.5 dB

@ 20 KHz

Po = 0.5 W, THD = 0.26%, Gain = 37.4 dB
Po = 1.0 W, THD = 0.36%, Gain = 37.5 dB
Po = 4.3 W, THD = 5.0%, Gain = 36.3 dB

Gain matching between the two sides is within about 1 dB.

I changed the plate to screen resistors to 100 ohms @ 1W from 82 ohms at 1/2 W because the 100 ohm resistors look better and it is then easier to calculate the screen current (no calculator needed!)

Let's talk about the 47 for a minute. The data sheet says that the maximum plate and screen voltages should be 250 VDC. A physical examination of the tube shows that it should handle more than that since the 46 has a maximum rating of 400 VDC. I called a friend of mine who is a retired tube designer at Eimac and he knows the 47 well from his early career (1940's). He said that it should be OK at the ~280 VDC that I am running here. The 47 is very similar to the 46 but, as I said in my earlier post, but it has a different grid structure and is a pentode instead of a tetrode. The filament and plate seem identical. The 47 has a bigger plate assembly then the 45 and the 45 is rated at Pd = 10W. I am guessing that the 47 is about 11-12W Pd as this specification is not on any data sheets. Finally, the current of 39.5 MADC is divided between the plate and the screen which are tied together for low mu triode operation (I'm guessing that the Mu ~ 6). In this case screen current is about 6 MADC which makes the plate current 33.5 MADC. This results in the screen dissipation being at 1.7W and the plate dissipation at 9.5W. These should well be within the tube design centers.

KYW mentoned that when he ran the 45's in his Joplin he had to use only one 2.5 VAC winding per pair of tubes since if he kept the normal configuration that would result in too high a filament voltage. I did not see that here in the U.S. as the 47 has a slightly larger size filament (2.5 VAC @ 1.75A) than the 45 (2.5 VAC @ 1.50A). My filament voltage at Vin = 123 VAC was exactly 2.50 VAC using the original four filament windings. There could also be a difference that is due to slightly different ratios caused by differences between the taps for 220 - 240 VAC operation in the UK and 123 VAC operation here.

The 47 is also a great looking tube and as soon as I can find four new globe 47's I'll put them in. One other neat little point about the globe 47. The gettering that was used in this tube can cause it to flouresce slightly under operation as stray electrons strike the glass. This does not mean that the tube is gassy (a gassy tube would have a blue glow within the tube elements). It looks very nice if you can get lucky and have four tubes that do it.

The sound, well I'm going through listening tests now, but initial results are certainly better than the 2A3's and very similar to that which could be obtained with 45's.

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