Next Build ... ???

[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

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]

woohoo ... It Works

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