Tube Amps


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4/3/17:  Just completed chassis layout V1.1.  Found an even better GBU805 diode bridge in a salvaged washing machine DC motor controller along with two cherry 680-mF electrolytics. This diode features 600 PIV (had no specs for the previous diode) so I also added a 6H choke.  The block is bedded to the chassis side, using it as a heat sink. Calculated resonance came out to ~2.5Hz based on formula f=1/2PI * sqrt (LxC).   Adding the choke knocked down the ripple from ~ 350 mV to a few microV with the 30K ohm load resistor.   The best I could get with the half-wave diode was between 3 and 5 volts of ripple.  I was concerned about flyback voltage hurting the electrolytic cap on power down but neither the DMM nor the ‘scope showed any spike.  The cap is rated at 200 VDC and the load voltage measured at 142 VDC.

I have a bunch of dual diode tubes including 6 and 12 volt types so I’m working out which to use for phase inverter and which for PP power stage.  Also added 3A fuse to AC mains and 1A fuse to AC secondary winding, then re-arranged power supply parts to one end of the chassis for isolation.  Preamp and PA tubes will be at the opposite end.

3/13/17: Decided to do a build-along while we’re on the audio amp analysis, using junk-box tubes and parts.  In the slide show above,  the tube right in front of the transformer is a 6SN7GTB dual triode from a HP 200AB sine generator I got at a hamfest.  That gives me the phase splitter/preamp.  Undecided on PP tube but have a few octals that could work.  My 6J6 reads weak on the B&K tester.  Next to it is a 5Y3GT I might use for B+.  The repainted junk box transformer puts out 105 VAC nominal and 6.3 for filaments.  Underside shows half-wave rectifier pack from an appliance  tear-down. I’ll try to filter out the bumps and use either the 5Y3GTB or the 35Z5 that came with the Philco AC/DC radio if I can’t.  Maybe.  Have to see if I can get away with only 100VDC B+;  theoretically should be possible but I’ll have to redo the component values.

I’ll look for a few 12AX7s and 6360s at the Maine State Hamfest this weekend just in case. FWIW I am amazed that some on-line vendors want over $100 apiece for so-called “gold-pin” tubes for each type,  as if I’d be foolish enough to cough up that kind of cash for run-of-the mill tubes.

I’m always amazed how much time it takes to chase down parts and build stuff.    My hat’s off to Paul for not only putting together the projects but also shooting and editing the videos. That’s all very time consuming.

I intend this as a ghetto-class, bread-board effort, simply a test fixture  to learn on using  expendable parts if I goof up.  I’ll be saving the higher voltages and high-power tubes for follow-on projects.  BTW, the line cord is not fused b/c I’m connecting to mains current via a fused VARIAC.   I hope we get to reading tube data sheets and how to plan a design.   Where to put the stake in the ground and start is always the hard thing for me.



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