Gibson Gibsonette GA 8

[Domocaster]

Hi everyone,
Does anyone know what the correct impedance of the speaker, on the Gibson Gibsonette, GA 8 [1960 model],is?
I've looked at several schematics, but it's not on any.
Also, does anyone know what the voltages are on the Power tubes [6V6's]?
As I want to set the bias correctly.
Many thanks for your help.
Cheers, Dom.[UK].

[Scotty72]

Check here:
http://www.schematicx.com/view-schem...ier-schematic/

[Domocaster]

Cheers Scotty72, nearly right!
Unfortunatley wrong era.It's the Gibsonette GA 8. [1960] 1x12AX7, 2x6V6's 1x5Y3.
The one listed is an earlier model, or later!
Good schmatics though, good detail.
Thanks.

[Scotty72]

Ok, voltages here:

http://www.schematicheaven.com/gibso...gibsonette.pdf

[Domocaster]

Hi there again, you must know more about schematics than me!
I can see the one you sent me to,but,which figure am I looking at?
Is it the one thats on a dot on the top line?ie; 320?
I appreciate your help with this,
Regards, Dom.

[JohnnyCrash]

Both of those schematics (EL84 and 6V6 versions) show a cathode biased circuit.

You do not set the bias in cathode biased circuits, folks often call them "plug-and-ply or "self biasing," etc.

That 200 ohm resistor coming off of the 6V6's cathodes is your bias resistor and the 20uF (or 25uF) capacitor in parallel with the resistor is your cathode bypass capacitor.

All you've gotta do is plug in the new tubes and play!

In an amp that old though, I'd replace the old filter caps (a 20uF and three 10uF's with the same values and at least a 400+ volt rating) and electrolytic cathode bypass caps (the 12AX7's 20uF and the 6V6 pair's 20uF with the same 20uF values and anything rated for 25 or 50 volts is fine).

[Scotty72]

Yeah, those are the voltages.

Around 320 volts at both plates of the 6V6.
16 volts at the cathodes (both together going to the resistor)
160volts/1.5volts for both 12AX7s anodes/cathodes
etc, etc

You'll be okay 10 or so volts more or less at the anodes.
The cathodes should be closer to those values.

ampage.org has a vintage amps forum (http://music-electronics-forum.com/f...play.php?f=21). You should ask there for the speaker impedance, it's a great forum (like this one) where I learned a lot about amps.

[Domocaster]

Thanks to both Scotty72 and Johnny Cash!
Great stuff! At last, found somewhere to get questions answered about old tube amps!
Thanks for pointing me in the right direction on the schematics Scotty.
Cheers to Johnny for that info, I've done the Filter caps, but as it is an old amp I needed to know if the bias had drifted out of speck after48yrs.
And do you know? it hasn't! 320 volts with 6V6 oin class A should be 35.6[re; D.Torres, Inside Tube Amps, Bias Tables].
Mine reads; 35.7. Isn't that amazing?pretty good components, to last that long!
Cheers Again. Dom.

[Scott S]

Parallelled output tubes, interesting...

[jhundt]

Parallelled output tubes, interesting...

yeah, about 20 years ago I started to get an interest in tube amps, so a friend gave me one of these Gibsons w/ 2 6V6s in parallel. Back then it was almost impossible to find any wiring diagrams or schematics (this is before the internet!) and I couldn't figure out what I was looking at. I knew about Champs, and tweed Princetons, and tweed Deluxes, but I just couldn't figure out this Gibson amp. It didn't help any that it was wired TRUE point-to-point, with no circuit boards at all. It was only good luck that I saw a parallel-tube diagram in an old RCA Tube Manual and that got me thinking "outside the box".