So where are we ?   I built out my McTube clone and it actually worked.   Here are a couple of pictures :

So yeah, she’s nice looking ..  I’m a fan of the clean look.   The back has the instructions / names of the controls.   The three banana plugs on the top are for heater and plate voltages (recall that my power supply is separate).

Here are the guts.. quite a bit less nice looking :

So what happened here is I didn’t carefully lay out the components before hand.   I ended up with a lot of bare wire crossing a lot of other bare wire and naturually got some interference.  In fact.  I got an AM radio station and had to tie some ground points together before I got a good sound.

How did it sound?  Well, I plugged it into a crappy Frontman 15B (that I got for 13 bucks) and it sounded like a tube amp.   Not a great tube amp, but a tube amp nonetheless.  It would be very fun to A/B with this setup and a good solid state amp to really get a chance to play with the ‘sound’ of tubes.

For a distortion box, I would tweak it I suppose, because I don’t have anything that can give you that barely hanging on sort of distortion and this doesn’t get there, although it gets pretty close.

If I were to use this more often, it would need a complete rewire.   I will probably consider it though because as always.. I have a plan

]]> http://www.paleoelectronics.com/blog/2009/10/the-real-mctube-wrap-up/feed/ 2 http://www.paleoelectronics.com/blog/2009/05/real-mctube-we-have-power-again/ http://www.paleoelectronics.com/blog/2009/05/real-mctube-we-have-power-again/#comments Sun, 10 May 2009 00:38:49 +0000 Dilapidus http://www.paleoelectronics.com/blog/?p=144 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

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.

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.

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.

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.  

12AX7

The tube is 12AX7, a very popular pre-amp tube, so knowing how it sounds and how it used in a circuit will be very handy.   This is a dual triode.   It houses two triodes in one glass bottle.

Power

Each triode is rated for 1 watt of power.    Power is    P = VI, so we can draw that curve on the standard datasheet just to keep ourselves safe.    The datasheet shows voltages from 0 to 500 and currents (in mA) from 0 to 4.0.    All we need to do here is pick a number, do the math and plot the point.

I’ll start with 250 V for no reason at all and find the current that creates 1 watt of power.  Please see the datasheet below

1 W = 250 V  * I  ->  I = 1/250 A = .004 A  = 4.0 mA.   Isn’t that handy?  Right at the top of the graph.  Wow, what luck!

Lets do some more

So just draw those points on the graph and connect them with lines to get a good idea of the safe ranges for this tube :

12ax7 data sheet with max power curve

Click on the sheet for a better picture, but there is no way we are gonna get close to the max power rating, it’s not even in charted territory.    Recall from the real McTube article that the max voltage is 140 V.    That would be over 7 mA of current and this is a stomp box meant to go in front of your amp.   I don’t think we need to worry.

Next up.. where on this chart are we?

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 :

Primary: 117 volt, 50/60 Hz
Secondary: 125 V, 15 mA
Filament winding: 12.6 volt, 0.3 amp
Mounting centers: 2″, U-bracket
Weight: 0.7 lbs.

Way more secondary current than we need (the doc says 500μA) and since we will have DC for the 12A*7 heaters, this thing can push twice the heater current required.   (Hmm..  perhaps a second tube .. two different circuits side by side .. switchable or stereo…  hmm..)

Where were we?  Oh yes.   The transformer is on the way, and cheep.   Time to start crunching numbers and see if I really am getting any of this.   More later.

It uses a 12AX7 as a two stage pre-amp in much the same way that many other guitar amps do. The ouput of the first stage drives the output of the second stage with pot in between.

You can find the original article here and I recommend printing it or better yet storing a PDF locally.
As designed, it should be fairly cheap to build. That was a large part of the intention. Transformers are usually the most expensive part of any amp (outside of the cabinet) and this idea uses two standard ‘wall wart’ adapters (7-10 bucks) to power the heaters in a pretty clever two stage deal.

(Click on image for a better look) As you can see, the first transformer steps down from 120 VAC to 12 VAC. The heater power is tapped off and a second transformer steps back up from 12 to 120. A couple of bridges and some filtering and you get clean power. This ends up with about 140V or so at the tube plates. A pretty good value when you consider that the old GA-5 Only ran 165 to the plates of it’s 12AX7. Hotter is not more distorted, btw, in fact, the reverse may be true under some conditions.

So I went for the power section first. Cramming some components into a tag board and then dry testing. I had just about the voltages expected without load so I started soldering it all together. Unfortunately, I left the power on while I soldered the damn connection. Luckily for me, all I did was blow both of the cheap transformers. DOH! That’s a major fuckup though. I probably could have been hurt.

Forget the price. I’m gonna buy a hammond, maybe a 269BX (150 – 0 – 150) or EX (190 – 0 – 190) so that I can play around a little with higher plate values. Throw in a solid potentiometer and I could probably learn a lot about plate voltages and sound.

More as soon as I get the part.