Some of you might have looked at the pictures I posted of a gassy 368A tube. Well I did a bit of experimenting with it's bigger brother the 388A. These are really odd UHF tubes and the 388 filament requires 1.5 volts at nearly 10 amps! To power it I used a variac and a normal filament transformer. With a 400 volt supply and a 6.5K Hammond 1629SE I was able to get close to a half watt just by driving it with with the 15 volt output from my distortion analyzer. However my observations don't follow the plate curves. At 400 volts on the plate and the grid at 0 volts it should be drawing something like 140mA. With those voltages I am seeing 65mA. I don't have any serious plans but sometimes it's fun to play with oddball tubes. It seems like a good candidate for running A2 as the grid is rated for 20mA and the plate is somehow rated for 50 watts.
Attachments
Presumably you have this data sheet. Curious whether connecting the filament ground to the other filament supply polarity has any effect.
Looks a lot like a type 845 with serious (battery operation?) limitation on space charge / filament power. Very interesting.
All good fortune,
Chris
All good fortune,
Chris
How many of them do you have? Could it be gassy? Have you experimented with more than one?
A while back I was thinking of building something single-ended with them because they looked so cool. I decided the filament current required was too much of a hassle.
It’s been 15+ years, but the last time I played with them I think I recall NOS examples matching the curves, at least at 0 bias.
I did end up building an amp with 316A’s although It never got past the bread board stage.
For what it’s worth, here’s a schematic I found on a Japanese DIY site 10-15 years ago:
althI’m
A while back I was thinking of building something single-ended with them because they looked so cool. I decided the filament current required was too much of a hassle.
It’s been 15+ years, but the last time I played with them I think I recall NOS examples matching the curves, at least at 0 bias.
I did end up building an amp with 316A’s although It never got past the bread board stage.
For what it’s worth, here’s a schematic I found on a Japanese DIY site 10-15 years ago:
Interesting schematic, thanks for sharing it. Both channels are shown but the output tubes share a bias resistor? I only have two 388A. One seems to have a manufacturing defect because one filament pin is slightly smaller in diameter than the other and my socket doesn't fit securely. I was hesitant to put almost 10 amps through it and didn't try the second tube. Maybe I will try it just to see if it behaves like the first tube.