47 Penthode in triode mode.

[Diy martin]

Hello Diy Members.

I have a pair of Philco 47 globe tubes and want to built a Se amp with these tubes .
Now I am looking for the triode data .

Thanks, Martin.

[Bill*B]

He runs them as pentodes, but if you haven't already seen it, Gary Pimm has an interesting article about a push-pull 47 amp on his web page.

[PRR]

> 47 globe tubes and want to built a Se amp with these tubes. Now I am looking for the triode data.

Back when they were writing datasheets for the '47, you could buy a perfectly good power triode and socket for less than the cost of a '47 and its odd/new 5-pin socket, so they didn't bother to measure the triode curves. Maybe someone has measured a '47 in triode; good luck finding that data.

We can do some guess-work from the pentode curves. We know the triode curves intersect the pentode curves where plate voltage equals the G2 voltage. In the published curves, G2 is held at 250V, so we know the triode grid current and plate current at that voltage.

There are two simple approximations for triode plate curves. We can assume they all point to the origin, or we can assume the zero-bias curve is slightly curved to intersect the origin and the others are parallel. For most real triodes, the truth is in between these two simplifications.

We should also plot the limiting ratings. Both plate and G2 are 250V max. There is no plate dissipation rating, but the "typical" condition is usually about as much as the tube can stand, and it is 7 or 8 watts plate dissipation.

In attachment: Published pentode curves black, my approximated triode curves in red, 7.5W and 250V in yellow.

The blue line is a possible load-line. Using straight-line approximate triode curves, the power is about 3/4 watt and the THD is high. Using bent triode curves, THD is lower, but power before clipping will not be a lot higher. Maybe a Watt at 10% 2nd harmonic.

As pentode: 2.7 Watts; as triode, estimated 0.5 to 1.0 watts. Is this reasonable? A quarter the power by going triode? Look at 6L6: 5 or 10 watts pentode, 1.4 watts triode. 6L6GC can do a lot better than 1.4 watts triode because it is allowed a high plate+G2 voltage. But we should not run more than 250V-275V on irreplaceable globe '47s.

[Brett]

Originally posted by Bill*B
He runs them as pentodes, but if you haven't already seen it, Gary Pimm has an interesting article about a push-pull 47 amp on his web page.

It's a great amp and worked really well with my horns.

[Diy martin]

I found this link with triode data of the 47 Penthode


http://homepage2.nifty.com/Kame/47syu.htm

[amperex]

Some of the triode curves good nice per link.

[rdf]

What a great trick PRR, thx for that demonstration. Now off to look at some 828 pentode curves....