Paleo-Electronics Blog

This time, I have done most of the job (I’m missing a pilot light but I’ll add that later.)

As you may recall, I chose to break out the power supply because I thought it might be useful in other cases.   In fact, if I were to mock up the preamp of the Crestline Skylarks (130V on the plate) , this would be damn near spot on.   It’s a bit light for blonde Skylark/Champ (165V on the plate) but you can’t have everything.

Power On

Here we are with an unloaded 170V.   Very nice.  The switch in front is standby.

The Filament power is the bottom connector.   16.4V unloaded.

Plenty of room

That’s a 12 point turret board with the discretes on it.    I should probably dress those wires.

That is with power down.

WARNING : Those are good sized caps and there is still plenty of juice in them.   You’ll get a nasty shock if you short the leads with power down (and much worse with power up).    I should switch in a couple of resistors to ground when the power is off, but I’d need a DPDT for that and I don’t have one handy.

Buttoned up and resting on the mothership

Since I am failing to complete numerous tasks at once, I thought I would describe their status

Amp Computer

The problem here was banana cables.   At some point, the ‘Asshole Audiophile’ community got a hold of banana connections and now it’s damn near impossible to get banana cables at a reasonable rate.   I wanted to use banana cables instead of say, posts and alligator clips.    These guys :  Circuit Specialists solved that for me.   I paid a buck and a quarter per 20″ cable.  Woot!  Back on that soon, but the first two parts are in fab now!    I haven’t decided on the mounting system yet, Cheap boxes or panel/rackmount.   Tough call actually.

Real McTube III

This got some love yesterday.   The power circuit is done but not boxed yet.   I am going to box it separately from the foot pedal part because hey, it’s 140/12 power supply and it’s not like I’m gonna take it on the road.   I wired up the heater stuff, LED and footswitch yesterday and may take a crack at the signal today.   More on that heater circuit later, but these are the first two amp computer style components.

Real McTube Calculations

This is gonna take a while, I did the numbers and they worked out, but now I can’t find the book I used and describing this stuff is really painful.   I’m back into the Rider book but that is the most annoyingly pedagogical book I have ever read.   I’d love to find a good clean book on tubes, with our without the math but it must have full data sheet reading info

SongBird

I’ve started buying parts and will rip the old stuff out of the chassis soon.   There may be some original Gibson Transformers on EBay soon.  Maybe I will recreate the Crestline in the amp computer style.  Maybe the 102C84 will get up there.  That would be ridiculously hard to find anywhere else.

Gibson GA 20 RVT

I have purchased the best amp I have ever heard.    This is a 2006 Gibson GA 20 RVT.   It’s not perfect, I might have to mod it, but I am definitely in love.   My strategies about building the Songbird and other things are all up in the air now.

Let see why..

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The Real McTube II was designed with rectified filament voltages because Fred found that the original McTube could be noisy if not done impeccably well.    The original McTube (appearantly) had the 12 V AC from the transformers going directly to the heaters on the tube.

That is the way it’s always done on the old amps and I’m surprised in retrospect that he didn’t remember the simpler trick.   Better amps would place a 100 Ω potentiometer between the two sides of the 12 V tap from the PT.   Of course, I’ve just remembered that too now that my rectified filament is done.    Oh well.

So the way to determine quality with rectification is to ask how close you got to the desired voltages and how much ripple (AC in your DC) remains in the signal.   The only way you are gonna see this is on a scope or a dedicated tool for this.   You might get it measured with a DMM, but I never got that to work.  If the ripple is complex, the DMM will not settle on a value so you will be seeing snapshots that make no real sense.   Since the DMM is reading RMS,  the highest number you see will be about 2/3 rds the total ripple.

Unfortunately, I didn’t take a picture of the ripple in the Skylark when I had it open.   Suffice it to say that it was plainly visible.   With a scope, it’s easy to see.   You set the scope to AC (and you don’t have to worry about the DC voltage spec within reason) and you slowly dial down the volts/div.

For the McTube, on both the B+ and Filament voltages, there was practically no ripple.   At 0.01 volts/div at the slowest sweep, I saw about 0.005 swing that took a bit over a second.  My scope wouldn’t trigger on it but it was plain enough.   Normally you expect to see something at 60 Hz at least and I saw nothing.

So I guess it cost me some extra parts, but that is a clean power supply.

Next time I get an amp opened, I’ll shoot a pic of the ripple.

Ok.. finally got off my ass and wired up the transformer and finished the power section.    That little transformer was cheap and is dead on spec.    The rest of my components ..  eh.

So the B+ voltage is expected to be around 140V with load and 150V unloaded.  I got 173V unloaded so I might be running a bit hot, but that is ok, as this tube is rated to 250 typical and 300 max.  As we shall see when we do the calculations, this will actually make it just a bit easier to chart.

In fact, in the Champs, Fender ran this tube at 150V and Gibson ran it at 165V in the Skylarks, so we will be operating right smack in the late 50’s!

For the filament, I got 16.6V unloaded, which is close enough to the expected 15-16V that I am happy with the power supply.   I will button it up separately from the rest of the McTube stuff as this is stage one of the Tube Amp Computer!

More soon.

So.. with a transformer on the way, it’s time to start fooling with the calculations.   The article by Fred Nachbaur gives you a lot of info, but not a full analysis.

I’m not gonna do a full analysis either, but I want to cover some of the basics, and then measure once we are built.   I’ve done some back of the envelope already and I’m pretty sure I don’t know what I’m doing.   [Read more →]

At some point recently, I realized that my transformer for the Real McTube had not yet arrived.  Why I needed a new one is chronicled here.   A bit of looking around showed me that the reason I was not in possession of the transformer was because I never ordered it.

I was talking about the Hammond 269 before (not overly cheap, to be sure) but as luck would have it, I found a better choice for this application.   Antique Electronic Supply has a PT called the  P-T442 .  Which I got for about 16 bucks.   Here are the specs :

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Ah..  large sigh of relief.    I have found JSchem.   This is a free schematic capture application.    So far the only downside is that it doesn’t produce a netlist.    This will greatly simplify scratching out descriptions for the blog.

Put your search engine on it.

As I alluded to earlier, there are a thousand options that I want to try with the Songbird.   I only got through a third or so of them before I stopped writing and this is nearly the smallest simplest amp you can make (although I have a plan there too!)

Now many of the options that I was considering are simply to try them out, with all other things being equal.   I have no real idea what effect they would have…  I want to hear them for myself.

Here is a quick list of things I am considering :

Input

• Fender vs. Gibson style grid stoppers,  Old school cap and grid.

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I got the cutline theme to work again. Now I just gotta find out where the images ran off too.

It’s nice to be back.