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?

Also, the fix to WP was a hack so I have to change up some URL. If you have problems with pictures or sound, lemme know.

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.

Feel free to join in!

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/ 0 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.

I have purchased the best amp I have ever heard.    This is a 2006 Gibson GA 20 RVT.   It’s not perfect, I might have to mod it, but I am definitely in love.   My strategies about building the Songbird and other things are all up in the air now.

Let see why..

Once again.. this is the best amp I have ever heard.  I suspect that other people get better tone (T-Bone Burnett playing for Plant/Krause comes to mind) but I have never heard anything personally that even comes close.   In the first 20 minutes after starting this amp for the first time, I got a bit concerned.   It is voiced very dark and I’ve backed my Strat pickups off thestrings already to moderate Fender shrillness.  However, I very quickly came upon settings that were just magnificent.

Specs

Here are the specs from Gibson (with comments):

15W Class A   -   12AX7 pre-amps,  6V6 x 2 Output

The circuit really looks like it’s Class A.   No phase splitter.    12AX7 to 6V6, just like the old 50’s Champs and Skylarks.   The two output tubes are in parallel, but not push-pull (very rare .. last seen in the BR-9 I beleive), so I wonder if you couldn’t disable a tube for even more low volume fun.   I haven’t really pushed it very hard yet, so I don’t know how clean it would stay in a club situation yet.    Rumor is they use Sovtek or Chinese tubes in production.   I’ll replace with quality when the time comes.  It can only help.

Do I still need the Songbird?   Hmm..

Hand Built, Point to Point

I’m really not a purist when it comes to these things.    The Fender Hot Rod / Blue Deluxes are circuit board and they sound great.   My concern was that I would not be able to repair them myself.    Any fool can beleive he can repair a PTP amp.

2 channels plus mix input

Looking forward to trying the other channels someday :-)

Tube Reverb,  Tube Tremolo

The reverb is the most un-Fenderish reverb you can imagine.   It’s got a deep reasonant hall sound to it.. not bright and springy, but very very filling.   Hard to describe.. I’ll work on sound samples this weekend.  It’s a great sound and it’s nice to not be making the same sound everyone else does.

There is a tremolo but it could not really suck more.    This, to me, is amazing because the 50’s and 60’s  Gibson Tremolos sound fantastic.    They had an oscillator varying the power tube bias and were great.

Because of the fact that this has both reverb and tremolo, I’ll bet a lot of people bought it for either surf or very bayou swampy sounds and they were probably very disappointed in that.  Too bad for all concerned.   Gibson loses some luster with such a crappy tremolo (taking up two knobs and a full tube).

Again, the reverb is different but outstanding.

Pentode/Triode Switch

This is nice.   Cuts power and lets you reach a higher level of preamp saturation before you wake the kids.    Some tone change, but nothing untoward.

12??? Eminence Legend Speaker, Asymmetrically Placed

I am very interested in how this responds to other speakers.   It has various outputs so I’m wondering how it will sound against a classic Jensen Ultrasonic or the new Weber Alnico.    I’ve heard that this is a good first mod as these are probably not the made in Britain speakers.

Metal Film Resistors

Note :  How is that a feature?   Metal film is widely believed to cause sterility in amps.  This is balanced against the fact that the amp is silent even when dimed if you don’t play something.

DC Powered Filaments

No hum.. it must be working but it seems to be AC to the power tube filaments.    I wonder why.

Hand Wired Turret Board

Again, I’m not a purist, but this should make repairs/mods easier.

Two-Tone Vintage Brown and Tan Tolex

It does look good, but you can’t use right angle cables because the case is too thick and too close.

Comments

This thing streeted for 1k in 2006 when it was made.   Honestly, I wasn’t even shopping for an amp, but I saw that come up on ebay for 300 to start, I bid 300 and until the last few hours, I was the only bidder.   I got it for $406.01.  For a hand built PTP amp, that is a phenomenal deal.

So far I have only barely touched the “crunch” and mix channels and really haven’t taken it out of Triode mode much either.  I literally stop the backing tracks all the time now and just listen to the amp play the notes.   I’ve never enjoyed hearing a guitar more!

As I said, it is voiced very dark.   You have to shape your tone a fair bit (which I am not really used to doing) but it’s glorious once you get there.

Drawbacks

• Tremolo is completely useless to me.
• Very loud pop when switching to standby.
• No master volume.
• Only one control for reverb.
• The channels share pre-amp tubes, so you cannot swap tubes in a channel

Nothing end of the world here but unsurprisingly, I am starting to think about some …

Mods

• Yank the Tremolo and add a master volume.   You could get one per channel since the Trem takes two knobs.
• See if there is a place in the reverb circuit to adjust the dwell.   Pop a pot in there and bring to front if I only use one master volume.
• revamp the pre-amp topology so that I can use a 12A?7 to fiddle with the sound on channels

Truth be told.. I might never do any of those mods.     Maybe I’ll just leave this pristine and wrap up the McTube and the Songbird.

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.