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 →]
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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.
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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
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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.
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Analog computers are way way old school. Essentially you would wire up circuits to do math functions and when you put a signal in, the voltage out was the answer. It could do multiplication, addition, integration, differentiation and other functions. The answer was visible on an oscilloscope screen (often included).
This is a Polish Model (from wikipedia, I think). Could it possibly look any cooler? That’s gotta be an old school desk for a base.
Look at that bad boy!
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2nd Stage
Not sure here. In fact, I have to admit to a bit of confusion. See the output of a guitar pickup is roughly 0.5V peak to peak. Maybe as much as a volt on a high output pickup. The gain of the first stage is roughly 40 or more. Now this will be cut down by the volume pot but even so, at full volume would I be seeing 40 V or more going to the grid of the second triode in the 12AX7? No. Of course not. I need to read a bit more, but I think that its current that is actually amplified and that the load will determine the actual voltage change.
What I really need to do is get it on the scope, but clearly, there is a disconnect here. I have another idea.. stay tuned.
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Ok, so clean power and rectified heaters and some other stuff were covered in the previous requirements post.
Tube Biasing – 1st Stage
As I understand it from my readings, the standard way of biasing, self biasing, is a complex reaction. Essentially as the power drawn from a tube increases, the bias goes more negative, somewhat thwarting the increase in power. This could be a very good thing. To my mind, this sounds like a plausible explanation for why compression happens. Now, as you all know, my understanding of what’s going on in these amps is mild, at best. My thoughts for biasing are actually three fold. Here is the first stage of the GA-5 :
The Fender Champs (5E1 and 5F1) and original Skylarks biased at 1.5V. Interestingly, the schematics from Crestline Skylarks (GA-5 and GA-5T) are annotated with voltages and despite the fact that these are exactly the same circuits up through the second stage, the ‘5 shows a bias voltage of 1V while the ‘5T shows 1.5V
I want to use a potentiometer here, to play with various values. Note that setting the bias is not really an issue for 12AX7 / 6EU7s in this case. The circuit is self biasing. I just want to see if fiddling here would change the tone.
Additionally, I want to be able to switch the bypass cap on and off. The 5E1 Champ had it but the 5F1 champ did not. Finally.. and here’s the kicker, I am considering a straight bias option. Sending a fixed voltage to the cathode. This is how tubes used to be biased, with a battery or other source directly supplying the right voltage. So this would be a complex little bit of business, just so I could play around with biasing options. Knowing me I’ll find a setting I like and never change it. Dave Hunter’s Two Stroke amp has a three way switch with two different caps and a no cap selection.
But still, that’s not complex enough! Next up an LED/diode combo to tell me when the grid voltage is higher than the bias voltage (Actually when Vg + Vb >= 0). No point in buying a lot of extra tubes if I can give myself a warning before I cathode strip them.
More Coming…
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Wordpress is up to 2.7 but my favorite theme is not ready. Bear with me, it shouldn’t be long.
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I guess before I dived into the requirements, I should have talked a bit more about the amps upon which I am basing the Songbird.
Unsurprisingly, I am starting with a Gibson Skylark. To be specific, I am starting with the pre-crestline GA-5. This is a lovely little class A, single ended amp, rated at about 5 watts. She uses a 12AX7 as a pre-amp, 6V6 power tube and a 5Y3 rectifier.
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