This is a Gibson Skylark in the “Crestline” series.  It is one of the many amps released with the GA-5T model number.   This was made sometime in the early 60′s as the later model “Whiteface” amps would be released in ’65.   This is not as popular as the true Class A SE Skylarks from the 50′s so it cost me $202.50 with shipping a couple of years back.  It’s not working right now and I’ve decided to refurb it.

When it worked (which it actually did at some point) it had the best tremolo I had ever heard.   It is so weird that Gibson tremolo is the shame of high end amp world these days.  They used to do it brilliantly.   However, I don’t like trem, so it’s going out..  in it’s place is a Master Volume.   I’ve read all the gasbags saying “It’s not the same!  You won’t get .. “  but the truth is I don’t care.  It makes a good sound.   If I get a different sound from 11 on both dials, great!  That’s just more versatility to me.

Here is the plan in general :

• Use the original  Iron, Speaker, Cabinet and Chassis as much as possible
• Mod back to original GA-5 specs, removing tremolo circuit.   except as noted below.
• Replace all discretes using original bone stock values
• Add Standby switch (hopefully I can do this with a rotary switch like they have now)
• Use multiple parallel caps for filtering ( a la TUT5)
• Add Master Volume before the Phase Splitter
• Add Pentode/Triode/Other switch
  • Pentode – Standard operation except provide 1k flameproof resistor for each screen
  • Triode – Put the screen in the plate loop (keeping the 1k resistors)
  • Other – Lower the voltage supplied to the screen (should be something between the other two in terms of breakup)
• “Galaxy” grounding, again, a la TUT5
• Careful layout and use of shielded wire on the signal path.

Should really be doable in a weekend if I have the parts, but I really want to nail this build, so I’ll take my time.

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.

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

1. Received Amp, turned it on, loud hum with volume at 0 and nothing in the jack.
2. Opened back and found missing 6X4 rectifier with two shitty diodes instead.
3. Ripped out diodes, compensating resistor and death cap, plugged in 6X4. Same hum.
4. Replace all of the filter caps with 500v rated caps. Lost hum at zero volume. Hum now follows volume knob with or without input.
5. Scope traced a reference signal and isolated signal issues to output pin of power tubes.
6. Tried different power tubes. All is well.

The very first step I took was to swap out tubes but that didn’t help. It didn’t occur to me to try the tubes again after step 4 because I had already tried that. It didn’t occur to me to start from the beginning … but that was the right call. I could have fixed this without the scope, but that’s ok, because it was definitely fun to trace the signal.

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.

Then I started shopping for a signal generator on CL and Ebay… well it turns out that this is the 21st century! Who knew? I found a mac software product called Signal Suite 2 and it is a perfectly good signal generator. Sine, square, triangle and sawtooth.. bunch of other options I haven’t tried yet. It’s control interface is shown here (click any image for the big picture)

Here is a shot of the amp on the bench. Notice that the chassis has been cleverly designed so that it can rest on the cabinet while you are working.

Now the scope. Yes that is a glass of scotch.. amp debugging is thirsty work.

[Image coming back as soon as I explain to my mom about the drinking.]

Very cool! Scope and generator, done. So I’m off to trace the signal. First up.. the reference signal

Nice and clean, standard 1khz signal 0.5 V p-p. Out of the 1st preamp stage we have

Still plenty clean, now at about 18V p-p. Second stage is after the volume knob (which I have at about 1)

Now this is back down to about 0.12V p-p and has picked up some noise (not easily seen in the thumbnail, but pronounced on the screen) and a ‘ghost’ signal or something. I don’t mean the very faint much larger sinusiod, which I assume to be a scope artifact, I mean the double lines in the signal. Not what I expected to see. That volume pot is noisy as hell so maybe it is the culprit.

Next up is the output from the splitter (half a 6EU7 on the GA-5T and just a 6C4 on the GA-5)

The noise is still there and it looks like the whole signal ‘fattened’ but the ‘ghost’ is still there. Finally, we get really weird :

That is the signal from the output tubes (Pin 5 of the 6AQ5). I’ll be ordering / swapping tubes asap to see if this is just a tube problem. Also, some contact cleaner in the pots will be happening soon.

Furthermore the scope will hide DC ‘lift’ from me in AC mode and it’s not made for the DC I see. (it can’t do more than 50 VDC)

So I have some more clues now.. I guess. If anyone knows what output signal that means, I’d love to hear it.

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.

You certainly need to get an earth ground hookup in these old two wire amps, but you don’t have to ruin the resale value of the amp to do it.

Here is how I do it, I take a small plastic project box and stick a 2 prong receptacle on one end, a three prong (IEC) receptacle on the other end and I pull out the earth ground wire and solder it to the PT chassis ground.

Here is a dumb diagram :

Doing this means you can leave the old plug in place, assuming it is not frayed and dangerous. You still get the benefits of 3 Prong safety, assuming you do a good job of soldering the earth ground (the half round part of the 3 prong plug) to the chassis. I did this at the ground wire of the PT. Don’t go alligator clipping this thing on or anything stupid like that. If it gets loose, you have a good chance of blowing something.

Now you can use a standard 3 prong computer power cable, and just plug the two prong plug into the other end.

This and removing the death cap will make your amp much safer, but it does have a drawback. You will now discover how well your amp is grounded. My GA-5 Crestline is pretty well star grounded but I still get some popping when I touch the strings.

Let’s have a look shall we? If you’ll recall from my previous post, the rectifying circuit in the GA-5 looks like this :

The death cap, a .022 mF is there to help suppress hum. Have that removed!! It is actually very dangerous, and is no longer allowed in circuit designs (in fact, I think it was outlawed in the mid sixties). I found my first reference to the death cap here and a subsequent conversation with somebody with a lot more experience than I do made the problem clear.

When that cap goes, and it will, it will do one of two things. It will blow open, or it will short. Open is no problem, but shorted means that there is now a direct path from power to ground. Just touching your metal cord shield should be enough to put you in the circuit (I hate metal plug assemblies on cords). If you are touching your strings or metal on the guitar, your odds of being electrocuted are much better.

Remove the death cap and ground your amp with a three prong plug first. More details on the way.

So I took off the back to have a first look. And sure enough.. there was something I had missed. There was a piece of silver tape over one of the tube sockets. WTF? The missing tube is the 6X4, which is a rectifier (converts AC into DC, sort of). There are two diodes soldered onto the tube socket. So I go googlin’ for a reason and find out that some people replace the tube rectifier with a diode network.

I email the seller, explain what I found and ask if the amp’s been modded. He says he doesn’t know anything about amps, but it was blowing fuses and he took it into the shop and now it sounds great, honest!

Modern rectification has come a long way from the old tube style rectification. With a little thought, you can replace the old tube with a solid state circuit. There are good ways to do it, but this wasn’t one of them. Lets look at the circuit :

V5 is the rectifier tube. When you power up the amp, two things happen : 1. About 260 VAC starts hitting the plates of the rectifier (on the left and right sides) and 2. 5 VAC starts hitting the heaters of the 6X4 tube (the ‘V’ with x’s above at the top of the tube).

Once the tube has warmed up, current will start flowing up the cathode (shown leaving straight up from the tube). However, until it warms up little if any current will flow. This gives the other tubes in the circuit that are being powered a chance to warm up before they get hit with 240 V on their plates.

Diodes, of course, don’t warm up. For reasons that I don’t want to go into, you really want the tubes to warm up before you hit them with power. Mind you, I’m not talking about playing the guitar, just powering up.

So now all my tubes are suspect. Yay!

The first step, of course, is symptoms. When I unpacked the amp, I cleaned it out a bit, just to be sure no packing material would cause shorts or anything like that. I turned all knobs down all the way, and plugged it in. It wasn’t turned on, just connected to the power. Again, just checking for shorts. I listened for anything at all and sniffed around. No obvious problems.

The next step is power. Still nothing plugged in to the inputs. Powered up and after about twenty seconds (tube warm up) I got a loud hum. So much for the “Sounds great!” description from the asshole on eBay. Oh well, I had not really expected a whole lot of truth there.

Turned up the volume, and the hum got just a bit louder, but not really as much as you’d expect from being turned from 0-11. So.. bummer but at that point I knew I was going to have to go in.

Finally, I crawled out of the shell and started looking for an amp I could play at home at night and also in a studio/practice situation. I settle on the “Lil’ Lanilei” by Tris at Songworks. However, that took some eight weeks to build and I was getting antsy …

Then I read about Clapton using the Gibson Skylarks in the studio. A little research turned up some interesting points: They sound great, are damn cheap and have very simple circuits.

Turns out the real ‘Class A’ single ended Skylarks are not so cheap. This one went for $255 + 40 shipping and that is as cheap as I’ve seen one go in a long time. 400+ is the usual. They look like this :

After a couple of weeks on ebay, I found two 60′s Crestline GA-5′s (well, one GA-5T (Tremolo) and one GA-5). These are both Gibson ‘Skylark’ amps, but not likely the kind used by Clapton They each cost 200 with shipping and both arrived with significant hum. New, they looked like this :

As the GA-5 arrived first, I worked on it first. I removed the old speaker wire and added a longer run of real speaker wire, and I replaced the power filter capacitors. That amp rocks. Simply fantastic clean at low volumes and great 60′s crunch as you turn the (only) dial. The other ‘dial’ is the on off switch.

The other one still has a nasty hum.. I’ll start telling that story soon.