'59-'60 6BM8 Gibson GA-5T Schematic Needed

[Natedawg63]

I am in need of a schematic for the ECL82 (6BM8) SE version of the early blonde-tolex Skylark series. I can find the 5Y3GT, 12AX7, 6V6 SE version schematic... but this is a 5Y3GT, 12AX7, 6BM8 (ECL82) SE version with tremolo.

Any help in locating a schematic would be soooo helpful.

Kind Regards...
Nate

[Enzo]

Got the Gibson book? Page 63.

The Maestro-Gibson GA1-RT-1 looks like it, just ignore the reverb pan at the input.

In the Gibson section of Schematic Heaven it is listed under Maestro, but the link is broken. If you download the whole Gibcon manual from there it ought to be included.

I posted it on www.ampix.org in the Enzo gallery.

[Natedawg63]

Thank you Enzo... when I awoke this AM I also had a reply from Jon King, who has a web page dedicated to the GA-5 amp... He provided me a scan of an original schematic the kind that they paste inside the cabs... kinda hard to read, but cool to know it was published. He also provided a hand drawn layout that is easy to read and has all the correct values. Gonna attach those for future reference also.

Thanks for your speedy assistance... the 2 (Maestro and GA-5T) schematics look almost identical to my untrained eye - only the voltage charts are a bit diff readings.

[Natedawg63]

Hand drawn schematic...

[Enzo]

If you work on old Gibson amps much you will quickly find they SELDOM match the schematics, and often as not your GA something amp will match an EA something Epiphone schematic and vice versa. I have in my files here for a GA-15, GA-19? I forget what, some Gibson in that range, and I have three published schematics for them plug two hand drawn ones for other variants.

The Gibson master service book is a great help, since you can scan the pages looking for tube complement to help identify some amp chassis when teh model numbers don't work. That is on Schematic Heaven, a 20MB download. A real paper copy is a lot easier to leaf through though.

[Alex_SF]

GA-5T - additional schematic, and questions

I have one of these amps, and scanned the schematic that I got with it. Attached for your convenience.

I have a couple of questions about this amp.

First, the tremolo is not very deep at all, and I'd like to mod it. I know that on a Princeton Reverb, you can get more tremolo depth by roughly halving the value of the resistor connected to the plate of the tremolo half of the trem/PI tube (e.g., in a Princeton Reverb, changing the 1M resistor tied to pin 1 of V4 to a 470K). But I'm not sure which resistor in the GA-5T is the equivalent one to swap to change the tremolo depth: it looks like it might be either the 510K or the 1.5M resistors connected to the switch. Does anyone know?

Second, since playing a GA-8 Gibsonette not long ago (parallel single-ended 6v6 putting out ~ 8-9 watts) and being very impressed with it, I've gotten the idea to build an amp along those lines, and am brainstorming the project right now.

I've got a line on a few possible donor chassis that are set up for 6V6 output tubes, but I've also seen a number of 6BQ5/EL84 based amps out there that might also work for a conversion. Given that 6BM8s fit in an EL84 socket (with different wiring), I got the idea that instead of following the GA-8 circuit (which is very close to a 5F2 Princeton but with parallel output tubes), I could use an EL84 chassis, base the circuit on the GA-5T, and do the necessary mods for parallel output tubes (e.g., halve the value of the cathode biasing resistor, reduce resistor between first two filter caps, change output transformer), giving me an 8-9 watt parallel single-ended with tremolo. However, if paralleling the output tubes were the only circuit change, that would leave me with an unused triode half in one of them ... would there be any detrimental effect from this, such as uneven tube wear? Is there something else I could do with the extra triode half of the other output tube, such as add a gain stage (switchable, or tappable by a second channel)? Ideas?

[domocaster]

Gibson GA 5T schematc ect..

at last!!
I've been searching for this schematic for ages,tried everywhere, even Gibson themselves, they sent me the wrong one!
I'd given up thinking that any existed, and then, finally, I found this website and you guys.
Fantastic! I know what I'm doing now[ almost].
It's nice to find some people who've actually got one of these, aren't they fab?
I'm from England, by the way, had to import mine from the US.
The one problem I'm having is that she is running a bit on the hot side,any one else had this problem?
Now I've got the voltages I can check..
Alex.. I am lucky enough to own a Gibsonette GA 8, your right, they are lovely amps, superb overdrive and dynamics.
There are two of blonde tolex models; GA 8 and GA 8T, the tremolo model runs two ECL 82's [like the GA 5T], the non tremolo, as you state is two 6V6's.
They don't have much of a clean sound, very bass light, no "body", I've uped the coupling caps,which helped, however, to get a descent clean sound it needs two .1uf's,sounds great at the lower end of the volume, but turn it up and it starts to phut, phut, [motorboat].
The output transformers were small, [that's partly why they overdrive so nicely],putting a modern one in[ mercurcy do one] apparently does improve things alot.
Go here for lots of good Gibson circuit info;www.rru.com/~meo/Guitar/Amps/Gibson
Thanks again every one, take care..Dom.

[capnjuan]

However, if paralleling the output tubes were the only circuit change, that would leave me with an unused triode half in one of them ... would there be any detrimental effect from this, such as uneven tube wear?

Hi Alex; Not on the pentode sides - each would still be doing the same amount of work. The triode halves would 'wear' unevenly but then in most push/pull 6BM8 designs, the triodes aren't doing the same thing so their lack of match would hardly matter. CJ