Re: Norton amplifier from 1940's (pics)
- From: patrick jankowiak <eccm@xxxxxxxxxx>
- Date: Tue, 26 Aug 2008 22:30:33 -0500
Patrick Turner wrote:
patrick jankowiak wrote:This amp showed up. Gives an idea what it took to make 200 watts of
audio in 1940.
http://www.bunkerofdoom.com/Norton1/index.html
There is no model #, but if anyone knows where there's a manual with
specs and schematic, it would be useful. I have already drawn the shcem.
for the preamp and driver stage so you can see it.
All Kenyon iron too.
PJ
If you want to make 200W of power with tubes now,
and you wish to use triodes, then 4 x 845 is a nice way to do it
and you'd need a similar sized chassis for at least the power amp as in
1940 if you like to to things right.
I recently built a pair of class A 55W SET amps using 2 x 845 per amp,
and I used all the weight of iron they used in 1940 and perhaps more per
watt, being SE.
2 x 845 are also capable of giving 55W class A in PP,
with Pda at idle = 78W per 845, same as with SE use.
If one lowered the Pda to about 60W each tube, then you'd get about 40W
max of class A,
but maybe 100W in class AB1.
But today we'd use silicon rectifiers and voltage doublers to make the
B+
required for such amps, so the PSU chassis would be a lot smaller and
simpler than in 1940.
And the chokes for filters would now be smaller because we have reliable
electrolytic capacitors of enormous
capacitance values and small size, much better than the huge paper in
oil types they used.
Patrick Turner.
I was given a bunch of NOS 838's with the amp. I think the PP 6B4 driver can push a pair of the zero-bias class-B 838's, will be interesting to find out if it can push two pair, I have my doubts since a pair takes 7.5 watts and there's a matching or driver xfmr turns ratio issue due to peak g-g voltage requirement differences.
Right now I am ass-u-ming 1250V on the HV side, not tested the HVPS yet.
I am also ass-u-ming the amp takes 845's because it's what I was told by an old fart, and they usually know.
I will probably apply some bias with 838's as the 845 wants these conditions: 40mA resting per pair (4W/tube), -225V bias, 400V Pk. g-g- volts
The 838 at 1250V runs 148Ma per pair (93W/tube) at zero bias and 200V Pk. g-g volts.
It's funny the tube manual calls the 845 operation class AB and the 838 class B, even though it's obvious the resting current is supposed to be much lower with the 845's.
As if to recant this, the TT-3 RCA manual does give as an afterthought a set of "special" conditions for the 838 for "low distortion <4%" of 1250V plate, -15V bias, 50mA resting current(pair). 210V peak g-g volts. TT-3 has no curves for the 838's but refers the designer to the 805 curves.
From the curves, the 200mA max-signal plate current of the 845's might be obtained from 838's with the -15V bias conditions by driving only 150V pk g-g volts.
Well my suppositions above my all be hogwash. The proof if any will be in the test hopefully in a couple weeks. I have to replace a few caps in the driver and then check the other two chassis out.
;;
If building new I still might use 866's or the like with a FWCT transformer but I am certainly open to options. I like the way MV rectifiers look and they are also still alive after lightning etc. I prefer FWB or FWCT for my large RF stuff because it has better regulation when old iron is used than with doublers+old_iron. For me it depends on what's available and I don't mind old parts as long as they are good. I like the way it looks with lots of iron and oil caps. I don't mind doing it the old way if space and weight is not a critical issue. I would have no objection to exchanging the small-value oil cap for a big stack of electrolytics since the voltage is certainly less than 1500 in the Norton.
Another piece of gear uses a mercury FWCT with 8uF/4H/32uF oil filter for a pair of 3-500Z's at 3000VDC now (in a transmitter) and have only 6V p-p ripple at 600mA which is close to the full load condition. The bleeder draws about 120mA of this. The regulation is also better with the choke and a small input capacitor than with just a big C bank out there.
I am not disputing the value of modern components. Certainly if I had a modern PWD or equivalent 2500V 1A plate transformer with a secondary resistance of 30 ohms instead of the old 240 ohm 5KVCT pole pig core then it might be as good or better. But you know what they say, ham radio operators are the cheapest people alive and also like to use what is on hand or otherwise free/cheap.
Patrick Jankowiak
.
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