Skylark Faceplate with plenty of rust (click for fullsize image)

Now it’s not going to matter one bit if I don’t do anything about the looks, but it would be nice to get her cleaned up. There is no way I am going to be able to clean around the silk-screened lettering or lines. So the whole thing is going to have to come off. I don’t even know how you clean these things.

Now, the chassis is actually just a single piece of metal, bent and punched. I’m not sure if it’s steel or aluminum but either way, it’s easily bendable.
Here is a picture of one out of the cabinet :

5T Chassis out of cabinet and looking from behind

So I suppose I could just have one remade. That would surely cost more than the amp, but I’d come out of it with the specs for new metal, which would be a good thing for anyone else trying to do this. Then I would need to get the art redone or redesign something. Again.. favors or money and no improvement in the sound.

I’m leaning towards leaving it, for obvious reasons, but very early in the process I’ll have to make the decision.. anyone have any thoughts?

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.

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

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.

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…

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.

This was a student model amp, and a direct competitor with the Fender “Champ” amps.  Fender started making the champs in ’48 (I’d kill for a 5B1 schematic, but I’ll bet it’s damn close the ’52 Les Paul amp).    By the early to mid 50′s these student models were all pretty damn similar.

To the left is a  classic Fender Champ 5E1 schematic.  12AX7, 6V6, 5Y3 and almost the exact same thing as the  …

… Champ 5F1 schematic.   In fact, the choke has been replaced with a resistor (much cheaper) and the bypass cap has been removed from the 1st stage preamp cathode.  If you look carefully, you can see that the cap was simply erased, as the dots are still there.

And here is the GA-5 Skylark schematic that I will be using for a baseline.  There is really almost no difference.   Fender has a 1MΩ resistor at the input, the bias resistors are slightly different on the preamp…    just little stuff mostly.    However, don’t think you can just swap the OTs on these things.    Fender uses 3.2 or 4 Ω speakers and Gibson almost always uses 8 Ω speakers.    The GA-5 also uses 10 and 20 μf capacitors for power conditioning compared to 8s and 16s in the Fenders.   I’ve read that this is expected to mellow out the highs.   I don’t disagree, I just doubt that I would notice.   Who knows though?   There is never any trouble getting ampophiles to argue over which part is most important.

A little caution is in order when discussing the Gibson Schematic.  They are prone to small errors.   Note that V3 is listed as a 6Y3  or arguably a 6V3 if you want to be kind, but either way, it’s actually a 6V6 of course.

Why the Gibson?

To my ear, the Gibson sounds just a little warmer, and the Fender is a bit harsher.   Some ears will prefer the earlier transition to overdrive/distortion on the Fender.   I prefer that ‘barely holding it together’ feeling I get from the Skylarks.   When you back off with the pick it’s plenty smooth, but hit it hard and it will bite back.

Since these are all 5 watt amps, they can reasonably be dimed in the studio (or your garage).   Try bringing your Marshall stack into a little studio and see what happens.   The Skylarks I play at home right now are 8-9 watts and Push-Pull instead of SE.   This Skylark will give me more overdrive at even lower volumes.

Later on, when I discuss power tubes in more detail I’ll drag in the Gibsonette schematic, which just uses two 6V6′s in a class A, SE configuration.   This could give more power .. although I’m not really sure that is a goal.

So.. to wrap it up.. these are classic Studio/Bedroom amps.   Great tone and the ability to push it hard even with the kids in bed.   This is what I want to build.

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.

So thats the key motivator, but secondary considerations abound.  This is a simple amp,  5Y3 rectifier, 12AX7 pre-amp and 6v6GT power amp.  Maybe 30 parts in all.   If this was just a straight copy of the old amps, I’d be done already.   I have some ideas that I’d really like to try.  Most of the things I want to do different were design decisions in the original.

I point out that the original GA-5 was the lowest priced amp in the Gibson line.   It’s purpose was to get an amp that could be played at home or in the dorm room into the hands of as many kids as possible.  When you are building an amp the two basic ways to cut costs are to put fewer parts into it and pay less for the parts.   Gibson and Fender both did as much of the first as possible and then started trading off quality parts to keep the costs down.   Nothing wrong with that, but I am trying to capture the sound, not save money.   I am going to use high quality parts (although I won’t be buying 80 dollar capacitors, no matter how freaking good they are).

I want to improve the circuit that Gibson made by doing the things I think they would have done if cost were not an object (as can be seen in the more expensive amps).  I have a few ideas of my own too.   Some new options, and every damn thing I can think of to keep it noise free.

Power 

This amp was only designed to push about 5 watts.   You can still piss off your housemates with a 5 watt amp, but the neighbors might just resent you rather than develop outright hatred.  This became the genius of the amp when that overdriven sound got popular and people started realizing that big amps do not work well in the studio.

I want to keep the max power the same, but I am considering a Triode/Pentode switch.  In theory this would allow me to push the thing into overdrive at even lower volumes.   Probably with some changes to the tone, but who knows it might sound better.

Also, I am strongly considering separating the PT and rectifier circuit completely from the rest of the chassis.   Essentially, I would have two chassis’ :  one for audio and one for power.   I expect to run the power and heater lines to the bottom of the chassis and punch them in exactly where they are needed.

Heaters

I am almost certainly going to rectify / condition the heater voltages.   This will not be tube rectification, of course.

There is way more..  stay tuned.

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.

So, I bought a NOS Mallory T8 Attenuator on ebay not too long ago. The attenuator acts like potentiometer, but it shows a constant impedance (8 Ω) to the ‘source’ side. The thing is, just sticking a resistor in the output circuit is dangerous and even when done right, it changes the tone.

It’s dangerous because if you go too far from the expected impedance at the output transformer, you can blow the OT. It changes the tone because the relationship between the OT and the speaker is now changed. The ‘right’ way to do it is get an active circuit in there. The simple, cheap way to do it is get the right resistance in there.

My Lil’ Lanilei amp uses a combination of a power soak and a hybrid power section to give you the same sound at any volume. Less speaker interaction, I think it really does it, or at least damn close. You can pay THD a couple hundred or more for a ‘hot plate’ that seems to do the same thing. The THD may well be worth it to bring a ’59 Bassman into the studio or something like that, but I paid $15 for the T8 so lets see if it can work. [Note: this wasn't perfectly clear before, but the whole point of the T8 is maintain that constant impedance. It may color the sound, but it will not damage my OT]. There are lots of people who have built special resistor boxes for various amounts of dB cut and were perfectly happy with the results.

So here we go…

In order to work, attenuators need to get in between the output transformer and the speaker so on a simple old amp like the GA-5, that means I am going to have rewire the speaker connections. That’s fine, I’ve been meaning to put in/out jacks in the back anyway. The T8 is pretty much like your basic potentiometer, as you’ll see. Here is the usage :

So it’s a big wirewound potentiometer with 3 lugs. It dissipates 15 watts of audio, so for a Crestline Skylark, which is lucky to push 12 watts, we are in good shape.

The original plan was a shorting jack so that I could send to a cabinet with a speaker out and have the attenuator on board. Here is the diagram :

So that’s pretty simple, OT to attenuator to shorting jack. If there is no plug in the jack, the internal speaker gets the signal. If there is a plug, the internal speaker is cut off.

I wired it up with nice thick speaker wire, drilled out my back panel for the plug and then knob and we are good to go. Give yourself extra wire between the OT and the T8 and the speaker and the jack. When you open this thing up you’ll need the room.

How does it sound? Great! I guess I can hear some of the coloration because on the neck pickups the bass is very muddy at high gain. However, this amp ain’t that great with overdriven bass response on the neck pickup anyway.. so hardly a big loss.

I am really excited by this mod, it was pretty easy and it sounds great. I’ll get some samples up soon. However, I made a huge blunder and I’ll be rewiring this soon, more on that later.