Emergency Amp

[spacelabstudio]

I acquired some time ago some unknown piece of old tube radio equipment because it was in a cool looking chassis.  A little thing.  It had a label on it: "For emergency use only."  So I've been calling this my emergency amp.  Been trying to figure what to build in it.  It actually used a pair of 6JG6A tubes, which have an unusual large 9 pin base, about the size of a typical octal socket.  I find no reference to anyone using those to drive a speaker, although it's not obvious to me from the datasheet why you couldn't.  But I have a pair of unused Sovtek 7591XYZs in my cabinet that otherwise have no use, so I think I might throw octal sockets in there and try those, with an Edcor 5k:8 PP output transformer.  They're supposedly just 6L6s rebased to have a 7591 pin out. 

Trying to keep things small inside the chassis.  It's a small box.  And I went on a subminiature buying spree earlier this year and haven't done anything about it yet.  So I'm thinking a 6948 http://tubedata.milbert.com/sheets/084/6/6948.pdf preamp, with an 18 watt single knob tone control, because I've never tried one of those.  And a 5703 http://tubedata.milbert.com/sheets/127/5/5703.pdf split load phase inverter. 

Rough schematic attached in PDF and Jschem format.  Any thoughts?

[spacelabstudio]

Oops.  And here's the PDF.

And forgot to mention, Hammond 269AX power transformer, shooting for about 330V on the output tubes, about 200V on the little tubes.

[DummyLoad]

E.M.T.

emergency musical tubes.

[PRR]

> a label on it: "For emergency use only." 

What IS it?? Pictures!

> 6JG6 ...I find no reference to anyone using those to drive a speaker

TV H-sweep tube. No great difference among them. They are tuned to use 130V DC (off the 117V wall), pass HUGE current (700mA) at low plate voltage. CRT scan coils can be wound to such impedances. Speakers aren't, so we need a transformer. TV input and output is radio light and sound, no naked metal in sweaty user hand, so we audio guys (especially stage-amps) need a PT anyway so 130V operation is no benefit.

You do find arrays of H-sweep tubes in transformerless-output tube amps, a bastard style which has tube flavor but no OT iron flavor. Some hi-fi geeks like them despite an awkward lot of vacuum needed. Also the last of the big tube amps (in the dying days of tube TV) tried sweep tubes at high voltage with OT (mostly for cost).

Speakin' of which: NOS 6JG6 go for $55 each. And they are mildly popular, and they aint making new ones. That's a good reason not to get hooked: if one fails you will be grubbing for loot for new tube.

[spacelabstudio]

> a label on it: "For emergency use only." 

What IS it?? Pictures!

I don't have a picture handy at the moment, but it's actually exactly like the yellow and black box this young lady is using:



I suspect that kind of box was common as a general purpose project enclosure way back before I was born.  I liked the lattice work cage.

Chris

[HotBluePlates]

You can still get cane metal like that at Lowe's... you just have to do your own metalwork to make the cage.

[spacelabstudio]

You can still get cane metal like that at Lowe's... you just have to do your own metalwork to make the cage.

Interesting.  At some point I'll take the intro metal shop class here:

http://techshop.ws/durham.html

I already took a powder coating class that has given me a lot of ideas.

[HotBluePlates]

By "metalwork" I just mean you need to take the large sheets they have available, cut them to size, fold the metal to form a box-type cage, and use some method to crimp/fold the corners of the box to have neat edges.

The biggest challenge is only to figure out the good flat template to cut the metal sheet so it will become a good box when folded. Most of Lowe's metal is already finished, but you could scuff-sand it prior to folding to prep for final paint in whatever color you like.

[spacelabstudio]

Well, that's what I'm going to class to learn how to do!

[spacelabstudio]

Just realized that my design needs 2.35A of heater current and that PT only has 2.  Back to the drawing board.

[spacelabstudio]

I'm learning towards just using 6V6s--knocks 0.9A off the heater current and brings it well back under 2A.  Really trying to stick to octal just because the holes that are already in the chassis are too big for regular noval.  Thought about separate heater transformer, but I really don't have much space to work with.

[spacelabstudio]

If I push more than 10W through that Edcor am I going to fry it or just lose freq response?  Guessing actual output might wind up more towards 15W but not sure.

[spacelabstudio]

Spent some time fleshing out schematic.  Am now at least guessing a value for each component and taking some wild stabs at what voltages might look like.  Corrected some errors, and changed the PI to be DC coupled with a protection diode ala Merlin.  New schem attached.

[spacelabstudio]

I know it's been a while, but I finally built this thing.  There were a few minor tweaks, and a major overhaul of DC coupling to the phase inverter, since what I have above won't work.  (Luckily I figured this out *before* I built it, and could just tweak my design.  Hooray for long showers.)

At any rate, I've published my findings here, complete with final schematic, real measured voltages, sound samples and photographs:

http://ampnerds.spacelabstudio.com/projects/emergency/index.html

Let me know what you think!

Chris

[DummyLoad]

nice! thanks for sharing. 

it's a cool sounding amp - doug's stout TS and 6948/5703 missile tubes - nice touch.

--pete

[DummyLoad]

have you experimented with 5718/5719?

seems like a decent match for a 3 gain stage CF/concertina pre-amp.

--pete

[spacelabstudio]

No.  I went on a submini tube shopping spree on Ebay a while back and amassed a little stash, but don't have either of those.

[DummyLoad]

they're definitely candidates for a build. RC coupled amp charts included in the sylvania datasheets.

http://tubedata.tubes.se/sheets/137/5/5718.pdf

http://tubedata.tubes.se/sheets/137/5/5719.pdf

pm if you'd like to give a pair a spin on a breadboard. i don't have sockets to spare but 8 pin DIP IC sockets work if you form the leads carefully.

--pete

[spacelabstudio]

Just for the curious, someone is selling on Ebay right now the exact thing I started with:

http://www.ebay.com/itm/Antique-Shortwave-Tube-Type-Linear-Amp-Amplifier-HAM-Radio-2-6JG6-6AQ5-/151031689680?pt=US_Ham_Radio_Amplifiers&hash=item232a30b1d0

In my estimation, retrofitting this chassis was more trouble than it was worth.  But here's the before shot, at least.