Paleo-Electronics Blog

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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The Songbird is the next project for me, after / along with the Real McTube.

The major goal of this project is to get me a Champ/GA-5 (Pre-Crestline) style amp for far less than the 500 or so for an original. I don’t like any of the re-issues, both the Gibson and Fender re-issues sound very thin to me. Also, I don’t think they use a tube rectifier, but I could be wrong there.

These are real Class A Single Ended amplifiers.   Three tubes, one knob and just fantastic sounding.   Although they are very similar, I am going to build off the Gibson GA-5 schematic.  The basic reason is that I like the Crestline GA-5 I have so I trust the Gibson.   Not that the Fender would sound bad, they are far too similar for that.   Thus the name “Songbird”.  I will be basing this on the original Gibson Skylark.

The primary motivation is to get that sound into my arsenal.

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If you aren’t familiar with the real McTube, it’s a one tube foot pedal type pre-amp.  It was designed by a guy named Fred Nachbaur and is by all accounts a very cool device.

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.

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You may recall that I ended this post : Mods to the working GA-5 with a problem.

The problem was that like a bonehead, I didn’t try to dry fit any of it together and once the T8 was in place, it was hitting the PT.

So I had to rip the damn thing out. Just as well. I went researching T8 prices and found that I had gotten a steal! I paid 15 bucks for an NOS Mallory T8 T-Pad Attenuator. I can’t find them for under $80. I didn’t know.. in fact, I had just lucked into the idea and bought the first one I found.

Since I wouldn’t be putting a it into every amp, I needed to build a separate box for it.

Here is the final (with the original box for show) :

The wiring is just the application guide drawing with jacks instead of Amp and Speaker :

And thats all she wrote.   Works great but as I play with it more I  definitely hear the loss of tone.

So yes, I have removed the hum, and to a large degree the amp appears to be working well, but I noticed that bass notes got kind of farty sometimes and some specific notes would warble all over.

Weird…  I’ll be putting the output section up on the scope again soon to see if I can figure out whats up..  I’m betting on a blown speaker, but I suppose the OT could be gone too.

One of the problems with an amp this old is that I don’t know what the primary impedance is  meant to be.  I can look for a short easy enough, but some shorts only appear at high power.

The speaker ought to be easy to test, I’ll plug in the good amp to that speaker.

Film at 11.

But I have done it! The amp sounds great. Some contact cleaner, a 3 prong mod and then I’ll button up the cab and give it a final run through, but I think I finally got it.

I’ll save the final bit for last, because I want to do a quick walk through all the steps it took.

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As you’ll have probably guessed by now, I don’t really know what I am doing. The GA-5T had a nasty buzzing that was independent of volume. So I re-did the power filter caps and now I have a hiss/buzz that is dependent on volume. I have run through with a DMM and every voltage is within 10% and the cathodes are all exact.

So, I was out of options essentially, other than replacing everything. I actually probably had a ton of options, but I just don’t know them. As luck would have it, I got a little bonus from a project I worked on two years ago and a sweet little Oscilloscope came up on Craigslist. 75 bucks for a decent HP 1725a! Excellent timing.

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This just went for $361 on ebay. I plan on building one from scratch and I’ll be lucky to do it for that.

What’s the difference between my amp and that one? Well, I’m not actually sure sonically. Mine (a Crestline Skylark) has push pull 6QA5’s. That one is a classic single ended 6V6 like the champ it took its basic design from.

Odds are.. you missed out. That one looked pretty good too.

Of course I’m gonna mod it, this ain’t a museum!

I’ve already done my 3 prong conversion, but now it’s time to make this thing more fun. Reminder for those reading along in sequence: I have two GA-5’s, one of them is the project that is not yet working and the other one is the one that worked after redoing the power caps. I use the working amp daily and that is the one I am modding.

The idea is to be able to play my working GA-5 at cranked levels in the conservatory (guest room) at night. This amp has great creamy cleans up to about 5 then it gets into a CCR crunchy sound followed by a very bluesy Claptony sound at 7 to 12 on the only dial it has. I want to be able to hear those sounds without waking the kids.

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So.. those of you following closely (both of you :-), will recall that I had last determined that an idiot had put in a couple of diodes in place of the 6X4 rectifier tube. Amazingly, this person may have owned a multimeter, because he seems to have recognized that the diodes increased the voltage out of the power supply circuit. He actually replaced the 1k dropping resistor with a 2k to bring the B+ voltage near spec (242V) while leaving the power to the center tap of the output transformer raw. This had something like 265V .. which is hot, but I think the iron can take alot of abuse on these things.

So I snipped off the offending diodes, and replaced the dropping resistor and voila! we are no longer blowing fuses and voltages are back in spec. We don’t know what damage may have been caused by those damn things… I stuck in a real 6X4 rectifier and I still get the loud hum.

To add to the fun, a buddy of mine, the guy who came up with ‘Paleo-Electronics’, found the Weber Copper Cap rectifiers. These are the RIGHT way to do solid state rectification. They are simply tubes with solid state components, but done right, they protect your other tubes in a way that diodes can’t.

In a tube rectifier, high voltage is not passed until the tube warms up. As the tube gets closer to temperature (various widely, but well under a minute) it produces more voltage. It’s not until the tube reaches operating temp that it puts out full power. This is a good thing because the other tubes are also warming up. If they get full voltage before they are warm, they can damage the tube immediately or at least have their lifetimes seriously curtailed.

The Copper Cap rectifiers mimic this warm up with what I assume is a capacitor network inside the bottle. They even mimic ’sag’ (the failure to produce full power when driven hard and quickly) although that is a conversation for another time.

You can get them here

I tried swapping the Weber out vs. the Original 6X4 I had in my other GA-5 and while I could hear a slight difference in the reaction to hard fast strumming, that was it.