Next Build ... ???

[afm_380]

Hey guy ... I need your recommendations for my next build. I've been bitten by the amp building bug and having a blast here. So far I've built a 5e3, a Bassman, and just recently finished a scratch build from the schematic of a TopHat King Royale (with a few tweaks ... James Tone Stack and EL34/6L6's).

Now I'm back into the planning stage for my next build (I can't shake this obsession ... ;o). I'm leaning towards building a copy of the Gibson GA77 (although from what I can find on the web ... no two were ever built alike ... so, What's it going to sound like built to the common schematic available ???).

Any thoughts or suggestions ???

[afm_380]

Well, my initial post didn't bring out any objections ... so, I must be heading in the right direction ... right?
Here's my rendition of the original schematic ... I'm comfortable with cathode biasing the power section, so that's in the plan (although I'm sure that it wouldn't take much arm twisting for me to change it ... ;o). I like the fexibilty of being able to change out the power tubes from anything between the 5881's to El34's, and obviously that requires some tweeking of the resistor in the cathode bias section ... (there I go, twisting my arm again). I don't quite understand what the 'Fidelity' circuit is doing in the second channel, but it does add some uniqueness between the two channels. I've also stiffened up the power supply section. Next step is to build the layout on paper and turn this into my next project.

[jmaf]

Just a humble suggestion: it seems like you've had a few US builds, why not go for a British clone now? An AC30 is a complex build, the Orange Tiny Terror simpler. I'm a big fan of US amps, basically 70% of my builds are US tone. But you never know, perhaps an EL84 low wattage project to vary things a bit?

[afm_380]

Thank You for the suggestion jmaf. I seem to tend to building these projects with the traditional 'american tube amp' power tube section. THis is probably because I have a good selection of these tubes when I was going thru a phase of trying to 'tame' my Twin. I ended up with a quad set of yellow jacket adaptors that enables me to run el84's in the sockets. With a little tweeking of the preamp and PI tubes, my Twin Reverb RI singings like no other ... ;o). The TopHat King Royale is their take on building an AC30, although my version is voiced closer to an American build. It's clean channel in KILLER, and the dirty channel with it's pre-PI master volume gets quite 'voxy' (especially since I used a Vox Cambridge 30 as the donor cabinet, it just may be partially visual). Although I still need to get this amp out in public to get some 'feedback' from fellow (non-biased) musicians.

I'm REALLY enjoying the planning stage of these projects, almost as much as building, but nothing beats playing something built from scratch. Until of course a build goes 'Way Wrong', but with the help of this forum I doubt seriously that will ever happen. Thanks Guys

[jmaf]

Cool, afm_380, please share your results with us, once you decide on what to build next. Good luck.

[überfuzz]

So far I've always gone 6L6 builds. But now I'm sitting here trying to muster a EL84 schematic. More or less from scratch. (I've been looking at different amps copying things I like, here and there.) As of now I'm trying to match every stage to get a nice working amp...

Maybe you build something that'll beat the tone of 'my' amp. ;-)

[afm_380]

Any interest in building a GA-77 on a PCB ???

I've just completed the exersize of laying this out with a pcb layout utility. It's pretty compact (2.5"x6"), and it's my first attempt at laying out a pcb (so it might be better to change the layout to spread out the components a bit). If I can generate some interest with a few of you, we can get the pricing down to an extremely affordable level (say $25 a board if I can get 5 others involved). Again ... this is my first attempt, so if any of you can offer some suggestions that would help make this a successful build, I'd truly appreciate it. Here's the layout and schematic that I've genetated.

[tubeswell]

Hi afm_380
At 3A, that PTs 6.3V winding will barely have enough capacity to power filaments for 2 x 5881s and 3 x 12A_7s and a 300mA lamp. But unfortunately at only 115mA on the High Tension winding, it won't even be able to cope with 5881s or EL34s. It'd only be good for a pair of either EL84s or 6V6s and 3 x of those pre-amp tubes. So forget the 5881s or the EL34s or get a beefier PT. (Check out the relevant tube datasheets for heater filament current draw). In my estimation, a pair of 6L6s needs an HT winding of 150mA minimum - preferably 180 or 200mA

Also by "Deluxe' OT do you mean a tweed deluxe OT? - because that would only be good for a pair of either EL84s or 6V6s anyway. A pair of 6V6s or EL84s needs between 5k and 8k Primary load resistance, which a tweed deluxe OT typically provides. A pair of 6L6s or EL34s needs between 2k5 and 4k primary load resistance. 2CW

[afm_380]

Yes ... I do need to re-assess my transformer selection. My first build was a 5E3 clone which has been morph'd a few times into different builds. I've added a small 12.6v/2a PT for the heaters of the pre-amp tubes ... I like the lead dress of powering these tubes on 12 volts.

The selection of OT's still eludes me ... all of the manufactures seem to have slightly different recomendations for tube types supported. Merlin's book doesn't get into this ... Is his next book going there?

[tubeswell]

yes the heaters will work okay with that extra PT, but their still isn't enough juice on the High Tension winding for EL34s or 5881s

(Merlin's website does discuss power supply and output stage design. Don't know what he's got in mind for book 2.)

[loudthud]

A little mistake on the schematic is you have the feedback and presence control going to ground instead of the cathode of the driver tube. The PCB seems to follow this mistake but it's a little hard to tell. Also, you need to ground one end of the speaker winding on the OT. Don't forget to connect pin 1 to pin 8 on the power tube sockets for EL34's.

[mooreamps]

It all looks like a very good project. On the schematic, I'm not seeing any difference between the high gain and low gain inputs. But that is a reflection from the orignal layout. I'm not sure if going to change that or not. Otherwise, it should be a good sounding amplifier. Good luck.

-g

[raiken]

On the schematic, I'm not seeing any difference between the high gain and low gain inputs.
-g

There is a 6dB voltage gain difference between the high and low gain inputs as shown on his schematic.

RA

[afm_380]

Thanks Guys ...

I stopped by Torres Engineering this weekend, where I was able to play thru a couple of Gibson circuited amps, and talk to them about this project. This will indeed be a fun build. On to spec'ing the proper iron and deciding what platform to build it on (... pcb, turret board, terminal strips, classic point-to-point) .... decisions decisions

[mooreamps]

There is a 6dB voltage gain difference between the high and low gain inputs as shown on his schematic.

RA

I see it now.

-g