Crawls Backward (When Alarmed)

IconProjects, musings about guitar builds, guitar repairs, vintage tube amplifiers, old radios, travel, home renovation, and other stuff.

Say you have some tubes...


Say you have like, some tubes.  And you have, like, a bunch of parts and stuff like maybe an output transformer and like an aluminum chassis and some switches and some other old junk.

And one day you say to yourself, "Hey, self, you got all this neat old junk!  Maybe you, self, could actually make something out of it!  Like, maybe even an actual vacuum tube amplifier!"

Then, before you know it, you are starting to plan an amplifier from scratch!

That, dear reader, is where we find ourselves.  Or, more accurately, I find myself.

Never at a loss for some new plan or project (ask my friends), I had way in the back of my mind for about a year, a vague plan to build a "Basic 3 watt power amp" as someone posted on the Antique Radio Forums a while back. This was a great thread and really got me going.

Update 15 March 2017: Someone commented (below) asking about the schematic. Seems it's no longer on ARF. Fortunately, I had a print copy and scanned it. It's below. It should print to 8.5 x 11 paper. I adjusted it as best I could for improved readability.

You will notice that quite a few of my posts have a phrase such as "I've been planning this for a while..." or "I wanted to do this for some time..." and this is one of those cases.  It's actually Phase One, or Proof of Concept of a really really neato cool amp I want to build - a stereo 2A3-based amp.

But before I lay out the cash for some semi-pricey, really good quality, really good sounding output transformers (which, of course, I have planned for already), I thought it might be a good idea to build a simple amp to further hone my dull skills and get an idea of What I Might Be In For if I build a full-blown 2A3 amp.  Plus, if this one turns out ok, I can actually use it!

So with these thoughts rattling around in my head, I gathered some bits and started in.

A little about the amp:  it's about as straightforward as you can get - it only has 3 tubes: a rectifier, an input tube and an output tube.

The rectifier is a 6Z4/84.  Most rectifiers I'm familiar with need a 5 volt filament voltage, but the 84 uses a 6 volt (actually 6.3 volts) filament.  Which makes your power transformer cheaper since it's doesn't have to have a 5 volt winding in addition to a 6.3 volt winding for your other filaments.  So, all of your tubes run the same filament voltage.

The input is a 6J5, a medium-mu triode, with an amplification factor of 20.  And the output tube is a 42, a pentode used in the late 30s to 1940s.    A single 42 is good for 3 to 4.5 watts, depending on the plate voltage.

As luck would have it, I happen to have all three of these tubes in my tube  I actually find I have at least 8 42s alone.  That's it...three tubes, Class A single ended.  Whoo hoo!

I should add that I love, love, love the look of "ST" tubes - with the shoulders.  Joseph Esmilla referred to these as "Art Deco" tubes and I agree.  Very very cool looking.

I really wanted to be able to build this amp with only parts I had on hand, but I had to spring for a power transformer and a couple of small bits.

I'm determined to plan as much as possible before I start drilling, soldering, or anything.  So as a very first foray, I take my three lovely tubes and my two transformers and lay them out on the chassis box I'm going to use.

I want to take good assembly practices into factor, these being, in order of importance:

- Keep the power supply as far away from the input as possible.

- Keep the circuit from "crossing over" itself.

- Mount the power and output transformer cores at 90 degree angles to each other to minimize hum.

- Use a star ground for the power supply, and a separate star ground for the input section grounds.

The layout you see above is close to the final one I chose.  From the left we have the 6J5 input tube, the 42 output, and the 84 rectifier.  Behind the 42 is the output transformer, and behind the 84 is the power transformer.  Looks good, eh?

I liked the look of the 6J5 being centered on the chassis, but in the end, I lined it up with the other two tubes.  You'll see what happens as we move along.

I have this "old" Hammond 125E output transformer I took off an old single-ended 6V6 guitar amp build.  The inexpensive Hammond transformers are a good deal for the money, but they won't win any beauty contests.

I decided to paint mine in the hopes it will look a bit more attractive on the final amp.  So, I take some 150 grit sandpaper to it, and mask it off.

I have no idea how this thing will sound.  The frequency response is 150 hz to 15Khz I think.  You can read that as Not Much Bass.  What-ever.  If it works, I can always upgrade to another output transformer.  I actually have some pulls from old radios which might be heftier, but I figure I should get this thing going and then fiddle with it.

Here we have our little Hammond primed and ready for a final coat of paint.  I didn't get a shot (shot?  get it?) of the final color, but it's a dark grey hammertone.  You'll see it in some upcoming posts.


Post a Comment 2 comments:

  • Anonymous said...
    March 13, 2017 at 12:31 AM
    Can you please post the schematic you used? The link to it is dead.
  • Yr Fthfl Blggr said...
    March 17, 2017 at 7:17 PM
    I've added the schematic above.

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