WAY OT 1962 Gibson Skylark GA-5T

[javacody]

I need a schem for this amp. I just bought one in great working order, it looks like this:



It has a 10" speaker in it (blown) that I am going to replace with a Jensen 10" AlNiCo.

From what I've read, it is a Gibson version of the 6G2 Princeton. Can anyone suggest any modifications?

[petemoore]

Buy build or have make a cover for it.
 Write down the tube types.
 Scout for nice affordable replacements.
 Don't play with the caps yet till they leak, or replace all the filter caps at once. You'll have to note the farad value, use same or higher voltage for them...also their size and shape often present issues. Shop for the caps.
 The volume knobs could probably use looking at, cleaned, tested and replaced if scritchy, or exibit problems.
 Transformers...if you have a problem there it's another topic.
 Resistors...if it is working now you will probably never have a problem with them.
 I've heard old solders get 'weak?', I lend only small credence to this. however the fact remains 'it's been known to happen'.
 Jacks...now these can cause problems in amps [IMO there ARE better ways to make connections than 1/4'' jacks]...these should absolutely be inspected or just replaced with the best functiong items you can find...in my Exp. these can make or BREAK an otherwise wonderful unit...Ive become a stickler for excellent output continuity. A faulty speaker jack can ruin everything...or consistantly make connection...sneaky too [unlike inputs which WILL tell you if there's a problem].
  Minding the speaker ohmage load on the amp, add speakers to tastem and drive the input...I start low to med vol, alone and run it up fraction by fraction allowing any heat to develop and monitor the sound and heat [good to test sever running conditions under conditions where monitoring is intensive...listen for what levels and settings the speaker 'likes' and what you can get going that you like].
 Inspect your power supply cord...modern alternatives do have advantages...not to be overlooked.
 As you can see, I'm leaning more toward the maintanence...and drive it to it's limits subjects.
 There's usually a sound these amps were designed for, which Can be seriously altered [within parameters], I'ts in remarkably good condidion and I suggest considering Not trying to mutate it more than marginally, just use loading by way of output and input to find what it does well.
 Although It could be tweaked to do more of whatever it is you're looking for, there's most likely a lovely sweet tone to this one, and being tube will have many different characters to the sound when Stompboxxed.
 It' looks like there's actually room in the cab for reverb equipment...possibly you could shoehorn in another tube...with just a few screw holes [use depth guage, tape on the drill bit]...
  such a classic unit as it sits, non modified has it's values IMO.

[javacody]

pete, that isn't my amp, thats just a picture of what it looks like. Mine isn't nearly as clean.  I got it for $85. I want to mod the hell out of it!     I want to replace one of the jacks with a standby switch (is this hard to do?) and I want to replace the tremolo with reverb. I have one knob that could be used for reverb, and you are correct, there is room for a reverb tank in there. I was thinking maybe an 8" speaker as well as the 10" or maybe 2 8" speakers instead of reverb and just using a holy grail for reverb.

The tubes are all cheap and can be found in quantity on ebay.  I plan on buying at least a dozen or so of each type.

This amp is not noisy in the least, and the pots are not scratchy at all. It has good tone, but I would like to make it a little hotter. Its ten watts I believe, and stays almost completely clean even when the volume is maxxed.

I wouldn't mind adding a tone control to it, too.

[javacody]

Here is a rundown on the tubes:

2-6EU7's (pre amp/ phase inverter and tremolo)
2-6AQ5's (in a push-pull power amp design)
1-6X4 (rectifier)

Some more details:

With this design, the tubes were mounted horizontally on the back of the chassis (top of tubes pointed to rear).
Approx. size is 20.25" x 15" x  6.5".

The controls, now located on a sloping *front* panel, are; 2 inputs, volume, rotating on/off knob, jewel pilot light and fuse holder. The GA-5T includes a frequency control knob to control tremolo speed.

I wish there was a way to fit 2 10's in the this cab.

[Lonestarjohnny]

http://www.schematicheaven.com/
They have all 4 version's here,
JD

[javacody]

Unfortunately, I already looked at these. There is one that is close, but without the tremelo schem. I suppose I could trace it.

Also, I want to ask about the mods found on this http://www1.korksoft.com/~schem/mods/hotrodprinceton.htm page. Can I add the grid resistor to my two power tubes?

The GA-5 Crest is my amp minus the tremolo.

[javacody]

I also have questions about filter cap replacement. I believe the filter caps are 10, 10, and 20 uF. Can I substitute larger values for these? Smaller?

[Travis]

Same site: GA-5 Skylark.  Including trem:
http://www1.korksoft.com/~schem/gibsonamps/skylarkga5t.pdf

Tube type/count is all wrong, but the trem is probably the same.

[fernmeister]

the model numbering of gibson amps makes it hard to identify them at times.  some of the schems people are alluding to are quite different to the amp I think you have.

if you have access the latest version of Aspn Pittman's "the tube book amp" you'll find the schem for your model on page 95.  it has the same config as your one.

btw, i think 6AQ5 is a cool and underrated valve for guitar amps.

[Dai H.]

pete, that isn't my amp, thats just a picture of what it looks like. Mine isn't nearly as clean.  I got it for $85. I want to mod the hell out of it!     I want to replace one of the jacks with a standby switch (is this hard to do?) and I want to replace the tremolo with reverb. I have one knob that could be used for reverb, and you are correct, there is room for a reverb tank in there. I was thinking maybe an 8" speaker as well as the 10" or maybe 2 8" speakers instead of reverb and just using a holy grail for reverb.

seems kind of a shame to hack it up... If you make major alterations, you may regret it down the road.

[javacody]

Yeah Dai, I'm thinking of leaving it mostly stock, but I would like to be able to get a little distortion out of the thing and I'd really also like a standby switch.

[Jim Jones]

With a tube rectifier a standby switch isn't really that necessary -- I wouldn't worry about it anyway.  Besides, you can find 6AQ5's cheap everywhere.  

Jim

[javacody]

Good advice Jim.

How long can I expect the tube rectifier to last? I was thinking about replacing it with Weber's copper cap rectifier which is supposed to reproduce tube sag and never needs replacing.

I bought this amp as a learning project.

Also, any links for how to rebias?

[Jim Jones]

Hi java,

You don't really need to re-bias a cathode biased amp like yours -- the shared cathode resistor looks after that.  One thing to keep in mind is your wall voltage may be higher than the primary winding on your PT "expects" so you may have to raise the value of that cathode resistor to compensate.  Also, it's worthwhile replacing the bypass capacitor on that resistor as it may have dried up.

The copper caps are are great idea but you can find 6X4's really easily - I think I have a dozen or so if you need one.  I'd recommend building RG's Light Bulb Current Limiter for your initial power-up.  I built one and I use it all the time when firing up "unknown" amps for the first time.  If your amp ends up humming like a bugger it's likely the power suply filter caps will need replacing.

Back to biasing for a sec.  After you've done the intial power up with the current limiter and if everything is fine, plug the amp into the wall and fire it up.  Measure the voltage on the power tube plates and measure the voltage at the tube end of the power tube cathode resistor.  Subtract that second measurement from the first, and that will give you the "true" plate voltage.

Using Ohm's Law solve of current: I=V/R - so you're dividing the cathode voltage by the value of your resistor.  Now divide your result by 2 to account for the current for each tube.  So long as you're not exceding the maximum dissipation figure for that particular tube you're good to go.  Now keep in mind, many old amps ran the piss out of their tubes so I wouldn't be surprised if you're amp is running hot.  To me cathode biased amps sound better biased quite hot anyhow.  So long as the tubes aren't red-plating and it sounds good - go for it!  

Hope this helps some.

Jim

[javacody]

My only concern is that I've read that if your rectifier tube goes, then it could take your power tubes and transformers with it. Are there any warning signs of this?

Also Jim, do both power tubes share the same cathode resistor? Could I split them so they each have their own slightly different values to get better distortion?

Also, looking at the schem I see a 270 ohm resistor and a 20 uF capacitor running parallel and hooking into both power tubes. Is the 270 ohm restistor the cathode resistor? I also see a + 16 here, does that mean there are 16 volts going through this section? What wattage capacitor would I replace the 20uF with?

[Jim Jones]

Hi Java,

Yes, the 270ohm resistor is the cathode bias resistor.  You can use separate resistors for each tube, but having that resistor shared allows you to use less-than-ideally matched tubes without issue.    If you opt for single resistors you'll have to double the value.

As far as distortion goes, I bet that little thing will bark quite nicely as is!  If you want to make it a bit gainier the preamp is the place to make those changes.

Back to the bias resistor, that 20uF bypass cap should be no less than 25V.  It looks as if  the schematic calls for roughly 16V at that point - a little safety margin is always good.  You can also increase the capacitance there for more bottom end.  If you are replacing that cap, make sure there's a bit of space between the body of the cap and the big cathode resistor - those things put out a fair bit of heat!

I've never had a rectifier tube fail so I can't tell you which warning signs to look for.  As far as I know the tube would just stop rectifying and there would be no DC voltage powering the amp.  I think RG has a good writeup over at GEO on tube amps.

Jim