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

[afm_380]

OK ... I've scored a Plitron 654708 as a PT for this project. At 290VAC@.6A and 6.3VAC@8A, I shouldn't have any issues of overheating the PT. But, I'll never get into the transformer saturation phenomena either ... ;o). Short of just throwing what I could score that may be 'overkill' at this project, my head is spinning trying to decipher exactly what the tuba data sheets require. I'm using Duncan's TDSL and NJ7P Tube Database Search which verifies that running a pair of 6V6's in AB1-PP with a plate voltage of 285 the current draw at the plate (Ia) is 70mA and 4mA at the grid (Ig2) with a Zout of 8K, this combined with the 3 12a?7 preamp tubes (1.2mA per half) comes to 81.2mA of current draw at zero signal. Which validates that the Tweed PT with a rating between 90mA and 115mA is perfectly acceptable ... the smaller the more saturation ... the classic Tweed tone that makes it a Tweed.

With 6L6's running in AB2-PP at 360/270 volts Ia of 69 and Ig2 of 8 and a Zout of 6.6K, zero signal draws a total of 84.2mA. But if we run the plate voltage up into the AB1-PP range of 450/400 volts, Ia shoots to 116-210 and Ig2 to 5.6-22, which I'm assuming totals to 128.8 at zero signal and 239.2 max with a Zout of 5.6K. So a 150 to 200mA PT should work well with some nice saturation potential.

My head starts to hurt when I jump into the el34 specs, there are multiple Zout specifications for varying grid voltages ... which range from 450 volts with current draw at the plate (Ia) between 60-200mA and 5-50mA at the grid (Ig2) with a Zout of 8K, to 350 volts with current draw at the plate (Ia) between 70-240mA and 9.5-50mA at the grid (Ig2) with a Zout of 2.8K. Fortunately, I have 600mA with this Plitron so the current draw will not be an issue. So ... do I now select the OT based on the plate/grid voltage?

Oh ... the speakers that I currently have for this project are either a pair of Celestion Blues at 16ohms each or an 8ohm Ragin Cajun ... just to throw another variable into the mix. I do understand that a lot of the Gibson GA-77 magic came from driving a large speaker, I'm just not going there ... yet.

[txstrat]

Have you considered building a bass amp yet?
With this strong PT you could go for an Ampeg B-15 with a 4x6L6 power amp and a 120W OT. I believe that would leave every bass player slack-jawed.

[afm_380]

Well ... It's finally built
I scored an old Gibson GA-10 which prompted me to continue with this build. The cabinet and chassis are GREAT donor hardware (although the 10" speaker amy not last long ... ;o) I opted to eliminate the presence control and negative feedback, primarily because I ran out of front panel space, and I wanted the keep an un-modded appearance of the (sleeper)amp. I also used different iron than I had intentionally planned, again primarily to keep a close to stock appearance.
Being a scratch build from a platform that doesn't seem to be popular, I have a question about my power supply circuit. I've deviated from the original GA-77 design and built it with solid state rectification and some heavier filtering (I've attached my schematic). In Merlins new book (Designing Power Suppies for Valve Amplifiers) on page 74 he comments that 'voltage loss across each dropping resistor is easlit calculated, ... ' but he doesn't go into detail (or give any examples) on how to determine this. My B+0 is measuring 435V with a pair of EL34's plugged in, but my B+1 and B+2 are pushing 410V without any preamp tubes plugging in (I'm to chicken to plug them in until I do a little more research as they are rated at 300V max). The original schematic shows power tube voltages at 420V/425V, so I believe that the dropping resistor values should be REALLY close, but at a measured 410V with no load I'm concerned enough to ask first.

[jmaf]

Just a note: The attached Gibson GA77 schematic says EL34/5881 when the EL34 would require a zero or negative voltage on its suppressor grid, which is not connected on that schematic. Just a reminder in case you decide to use the EL's.

[afm_380]

I've standardized my builds to include the el34 conversion of tieing the suppressor grid to the cathode (I need to modify my power tube component in the schematic app to reference this correctly ...) Thanks

[afm_380]

With the preamp tubes installed my B+ voltages fall right in line (B+0 = 435, B+1 = 335, B+2 = 280) ... phew. Now I get to troubleshoot why it has has an horrendous hum (especially when the lower set of inputs (v1a) volume is increased). I've swapped the leads from the P/T to pin 3 on the power tubes, with no change. I think that I'll lift the cap (C6) off of the Fidelity control to see what change I get. Maybe I'll wire in the NFB as another test. Any Ideas ???

[afm_380]

No luck ... just a loud hum and now guitar amplification ... I've double/triple checked my schematic to my layout and chased all of the wiring from the board to the valves, pots, and other components. I've attached my layout diagram (it's a little hard to follow the wiring, but most of the connections are labeled). One interesting fact is the I have VERY little voltage at pin 5/6 of the power tubes. Another is that if I change out the 12ay7 and the 12au7 in v1 and v3 with 12ax7's the hum gets really annoying and will even oscillate at moderate levels. An extra set of eyes on my schematic and layout would be greatly appreciated.

[tubeswell]

So it works with a guitar but just hums like hell? Sounds like a grounding issue. How is it grounded?

[afm_380]

Nope ... it doesn't work with the guitar.
The grounds all measure out correctly and don't show up any place undesirable.
The hum can be varied with the volume controls, as is a little loud at zero the extremely loud as volume is increased. The tone controls also change the timbre of the hum.
I've also lifted the bass control to ground as another possible shunt to ground with no effect.

my voltages are
v1
p1 - 178, p2 - 0, p3 - 1.5, p6 - 182, p2 - 0, p3 -1.5

v2
p1 - 170, p2 - 0, p3 - 1.3, p6 - 280, p2 - 170, p3 -170

v3
p1 - 207, p2 - 0, p3 - 1.8, p6 - 270, p2 - 24, p3 - 68

v4 & 5
pin3 - 425, p4 - 432, p5 - 0, p8 - 38 w/.054mv across 1ohm resistor

Thanks ...

[tubeswell]

Try pulling out V1-V3, and put them back in one at a time (from V3 backwards) and listen for the hum.

[afm_380]

It looks like it was the input jacks. I stole some switchcrafts from another project, an now it produces music (still a little more hum that I'd like ... but not what it was). I still need to troubleshoot the channel with the fidelty control, anything other than dimed on this pot is muddy and scratchy. I'll post some pic of this sleeper when I get a chance ... Thanks

[afm_380]

Once I corrected whatever was wrong with the input wiring and switched the leads on the fidelity pot, this thing come to life. And I have to admit that I believe that this will be my favorite build so far. Here's a couple of crummy cell phone photos (the guts shot was taken during a trouble shooting session ... the wiring has undergone a major clean-up). I would REALLY like to encourage others to build some of those less popular designs, there are alot of great schematics available that need to be built more often ... ).

Thanks Again for all of the assistance and inspiration