built a Gibson GA-9 (6V6 parallel SE) clone...

[billc]

a few days ago and love the sound, the best of several amps I've constructed over the years....this one is based on the 6SJ7 plus parallel 6V6 version.
FWIW built it on an old DIY chassis I put together a few years back and it has some wrinkles that might be interesting to others...I used two PT's, a Triad VPS230-190 (about $20 IIRC) connected to a diode bridge rectifier and a 12V filament xfmr from Radio Shack (about $10)...went with 12V since NOS 12V tubes are easy to find and cheap (especially 12V6's like I used, they were used in car radios back in the day) The CT of the filament xfmr is wired to the cathodes of the power tubes to float the heater string on about 17 volts DC and the amp is very quiet.
Used a Hammond 125E for the OT and it sounds good but like the amp so much I'll probably spring for a real SE OT eventually...and FWIW mounted the OT on the underside of the chassis, this was common on hifi tube amps. Only one pot is used for volume, did not attempt to add a tone control as I've read that 6SJ7's don't work well with them and it worked out OK, I like the tone as is...



[img:640:480]https://lh5.googleusercontent.com/-KOm7 ... 144424.jpg[/img]

[xtian]

Very creative and cool. Would love to see gut shots.

[M Fowler]

This is my kind of project since I have so many old chassis, stash of old odd tubes, and transformers I want to build some odd ball amps from.

Looks good to me, more photos

Mark

[billc]

Thanks! the inside is messy, this amp has had at least 9 lives and is starting to fall apart from being reworked too many times...also there are a bunch of extra parts from previous incarnations. FWIW the chassis was made from a cut down steel computer case with end pieces welded in and used three sets of parallel terminal strips to wire the components...anyway here is a current photo of the inside and schematic of a Gibson GA-9 (I made some minor changes to eliminate the field coil, wire a SS power supply, etc, but stuck with the stock component values for the most part)


[img:640:480]https://lh4.googleusercontent.com/-22hI ... 144923.jpg[/img]

[img:771:543]https://lh4.googleusercontent.com/-KSOp ... 71/ga9.gif[/img]

[M Fowler]

That's organized compared to some I've seen. I looked at using computer case for chassis but they seem too thin on the CPU's I have. Salvaged IEC, power cord and power supplies though.

[rp]

Anyone want to explain this schematic to me, Ive heard of SE paralleled output tubes but I thought each would have it's own OT.

- So you just hook the two plates together and off you go?
- OT Pz is the same as for one 6V6, I guess, 5-7K?
- Is this like paralleling an input dual triode? You get a little kick in gain and some fatness? Or does it actually double the output power like a Push Pull?
- What's the point? Not for the OP to build it for for Gibson to have bothered?

[M Fowler]

You can even go with three 6V6's if you wanted to.

AX84 site has a bunch of SE designs.

[billc]

Anyone want to explain this schematic to me, Ive heard of SE paralleled output tubes but I thought each would have it's own OT.

- So you just hook the two plates together and off you go?
- OT Pz is the same as for one 6V6, I guess, 5-7K?
- Is this like paralleling an input dual triode? You get a little kick in gain and some fatness? Or does it actually double the output power like a Push Pull?
- What's the point? Not for the OP to build it for for Gibson to have bothered?

One grid gets a resistor, otherwise the tubes are wired in parallel. OT Z is half what a single tube gets, I used 3K per recommendations I found (like the design for the Angela Super SE amp) Power is roughly doubled and is about 9W, probably what Gibson used for the model number...and I imagine they built them to fill out their amp lineup, the GA-9 would fit nicely in terms of price, power, and size between a Les Paul Jr. (basically a 5C1 Champ copy, one 6SJ7 and one 6V6) and a PP 6V6 amp like the GA-20, GA-30, etc
This one definitely has a sound of its own, I much prefer it to the 5C1 I built and it is a good bit louder and I also prefer the sound to my BR-6F and a GA-30 I built (they both use a 6SJ7 for the preamp and PP 6V6's for power)

[pops]

Nice, i have been thinking of building a parallel output amp. A few years ago i build a double champ in a princeton cab. Two complete amps, one power supply, two output transformers. I can use both at once each with a volume and tone or flip a switch and they are two separate amps and a friend can play or i use it for harp and mandolin. When used together it is not twice as loud but really much more intense. Great sound.

[rp]

One grid gets a resistor, otherwise the tubes are wired in parallel.

What's the resistor for?

6SJ7's are great. i was looking for a way to make a louder 5C1, I looked through the Gibsons but missed this one.

[billc]

One grid gets a resistor, otherwise the tubes are wired in parallel.

What's the resistor for?

6SJ7's are great. i was looking for a way to make a louder 5C1, I looked through the Gibsons but missed this one.

AFAIK it is a grid stopper, there is some info on them at
http://www.valvewizard.co.uk/gridstopper.html

[rp]

AFAIK it is a grid stopper, there is some info on them at
http://www.valvewizard.co.uk/gridstopper.html

I was wondering why just on one side or one stopping both grids?

[billc]

AFAIK it is a grid stopper, there is some info on them at
http://www.valvewizard.co.uk/gridstopper.html

I was wondering why just on one side or one stopping both grids?

I dunno, way beyond my knowledge....though FWIW the Angela Super SE uses two

also found a reference to using a few grid stoppers on a triple 6V6 SE build
https://lonewolfblues.com/forum/viewtop ... gif[/img]