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Inducktion

Wed Nov 30 2011, 09:27PM

Registered Member #3637 Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068

I recently acquired 4 vacuum tubes from gren...

6AL11 (sharp cutoff pentode)

6EW7 (Dissimilar dual triode)

6GE5 (beam power tube, pentode, sweep tube)

6FQ7 (medium MU dual triode)

Could I possibly attempt to make a tube amplifier with these? I'm not sure what kind of design yet, single ended or push pull wise. I know I'll need an audio output transformer, however, and my teacher says he'll bring me one tomorrow... also, the 6GE5 and the 6AL11 don't have sockets...

radiotech

Wed Nov 30 2011, 09:58PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

They have Compactron sockets. Ask Gre... to hack some free from the old TVs.

klugesmith

Thu Dec 01 2011, 01:23AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

In a pinch, you can use individual pin receptacle contacts from DB-9 or DB-25 connectors.
(Are you too young to remember when every PC had both?)
The DB-connector pin diameter is practically the same as that of most vacuum tubes.

Solder a female connector insert onto some hookup wire, then insulate it with some heat-shrink tubing.
Plug it onto a vacuum tube pin, and connect the other end of the wire to your protoboard or plug-and-play breadboard. Repeat until done.

I once used 26 of those contacts to fabricate a socket for my first trochotron.

They were kept in alignment using a stack of phenolic sheets with the same 26-hole pattern.
The method is also handy for Nixie tubes with unusual footprints.

[edit] Here are 2 ways to keep, say, 12 individual receptacle contacts in alignment, if you can't wait to get a proper socket.

1) prepare enough individually-wired receptacle contacts as described above. Plug them onto one of your duodecal tubes, which will be your alignment jig (so its pins had better be good and straight). Then secure the pins to each other with lots of RTV silicone, and perhaps some solid reinforcing material. Epoxy might be too stiff (not enough lateral compliance for the socket to accept a different tube). I don't know about hot-melt glue.

2) drill small holes in a circle on a piece of copper-clad circuit board stock. Cut or etch the Cu foil to leave an isolated pad around each hole. Get round-pin-receptacle contacts with solder tails for through-hole connection to boards. Solder them all in place, using a good vacuum tube as alignment jig. Then, once again, reinforce them with silicone etc.

Inducktion

Thu Dec 01 2011, 03:24PM

Registered Member #3637 Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068

So, I made a soft start circuit for the tubes... (it's so that the filament has a time to warm up before the actual amplifier starts amplifying)

Has an LM393 and a voltage reference on it, with a 10 meg ohm resistor charging a .47 uF tantal cap. (it takes around 4.7 seconds) Once the cap reaches a certain voltage, the power mosfet (i'm not sure if I should use a relay instead...) I have hooked up to the output of the comparator will turn on, allowing the main voltage supply to turn on. It's pretty simple, and from my understanding really helps with the life of the tubes.

I also got a 25k to 3.5 ohm transformer. It's kind of small. (it says 5 watts max?)

But I wasn't really intending on the amplifier to be amazing anyway. Just something small. :)

Proud Mary

Thu Dec 01 2011, 08:04PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

The consumer televisions for which your valves were designed didn't use HT time delay switches - an unnecessary expense.

4.7 seconds is of no use at all as an HT delay for indirectly heated valves. Vacuum thermal HT delay switches of the period most often had delays in the range 30 - 90 seconds i.e. time sufficient for the cathode to be thoroughly heated.

Inducktion

Thu Dec 01 2011, 08:21PM

Registered Member #3637 Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068

Well, I could just change the capacitor to a higher value.

radiotech

Thu Dec 01 2011, 09:26PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

When you are considering the warmup timer, add a side circuit that only applies the
plate voltages after the tube has warmed up fully.

Inducktion

Thu Dec 01 2011, 10:33PM

Registered Member #3637 Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068

Right, Radiotech, that is what the circuit is for, preventing the high plate voltage from being on before the filaments are on. Though, technically I could make it a soft start (so the filaments warm up slowly) using a similar circuit. Just make it a voltage follower...

And I'm not sure where to begin with designing the tube amplifier...

Mark-H

Fri Dec 02 2011, 02:30PM

Registered Member #607 Joined: Tue Mar 27 2007, 10:39AM
Location:
Posts: 64

If you google EL509 (6ge5) amplifier, you'll get hits.
I started an amp about 8 years ago using a pair of either EL or PL 509 "sweep" tubes but never finished it.
Could be a start?
Cheers.

Inducktion

Fri Dec 02 2011, 03:49PM

Registered Member #3637 Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068

I think i'll go with something like this. Though, from my understanding it needs a decent preamplifier stage in order to have any decent output with Mp3 players and the like. It shouldn't be too hard to add, right?

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