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Gibson G 30

Started by gbono, December 27, 2012, 01:41:35 AM

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J M Fahey

#15
Quotefor safety reasons this arrangement would not get type approval in Australia,
FWIW, neither in Argentina.
Our rules and tests are quite strict .... and bribing does not carry very far, if any.
QuoteDoes the UL really accept this kind of crud I wonder?
Looks like UL is neither that strict nor mandatory  :o
Quotemy inclination would be to get the mains right off the board altogether.  If it were on my bench I would cut the two connections from the back of the pot to the PCB and turn them up into tags, lift the mains and tranny wires from the near side, add a mains rated in-line fuse holder
:tu: :dbtu:
Now, where's the "clap clap clap" applause icon?

Those Gibson guys are *STUPID CRAZY* :grr
WHO designed that PCB? :trouble

**EDIT**: just out of curiosity, I Googled "2N2148"
Ouch!! It's a *germanium* trabnsistor !!!
And a very poor one at that.
Looks strong in its metallic TO3 case, almost like its "partner", the workhorse 2N3055 .... NOT!
Puny 12W dissipation, 2A max current, Vs 115W and 15A. Oh well.
Plus its base diode drop is much smaller than a silicon one.
2 (Si) diodes are waaay too much.
In fact, I would not mess with this amp, using it as-is (obviously correcting the 120V power wiring as Roly suggested) .
And the crossover distortion ?
Short answer, leave it as-is.
And if *absolutely* unbearable, consider placing an LM3886 there, fed by the present PSU.

gbono

Well the amp has a UL sticker on the side of the cabinet - remember this was designed and built in the late 70's. I have had some experience with type approval for consumer electronics and in the US you can dodge radiated and susceptibility requirements while other agencies (VDE) are not so nice. I doubt this design would pass UL approval in 2012.

I took my chances and added another diode in series - no thermal issues after extensive testing and monitoring with IR.

On to the next one ;)

THX

Roly

Oh noz!!!  Germanium - I blew it!  :-[  My profound apologies (nobody expects the Spanish Inquisition).

@gbono - JM is right, this is a problem - two silicon diodes biasing a pair of germaniums is way overcooked, and I'm surprised you didn't encounter problems.  Did you actually measure the idle current (via R16 voltage drop)?  What did it come up as?

I've never found the original circuit for the Gibson G-70 that is the subject of my article, but perhaps the output stage had been rebuilt to silicon for this sort of reason - it certainly wasn't original by the time it came to me, and up to this moment never even suspected that it might have originally been germanium.

There is a saying about getting a car that was started on a Friday and finished on a Monday, but in this case it could have been the work experience lad during the engineers lunch break.  I've seen some pretty stupid things done in my time, but running mains across the preamp board is right up there with the worse.   :duh
If you say theory and practice don't agree you haven't applied enough theory.

gbono

Well PNP power transistors were hard to come by in the 70's so I assume that Gibson reverted to Ge devices. :duh

I rebuilt the output section using two "modern" BJTs - Idle current and thermal stability are fine. The amp originally presented with damaged output transistors.

There are several versions of the G30 - this apperas to be an early version.

With the original speaker and even a "MDF" type enclosure this little combo is very light (portable). I have a friend who is using this G30 in a blues band playing a harmonica through it. He loves the amp. What can you say?

Roly

Quote from: gbono
I rebuilt the output section using two "modern" BJTs

Well I can say phew!   8)

Quote from: gbono
I have a friend who is using this G30 in a blues band playing a harmonica through it. He loves the amp.

...and very pleasing.   :dbtu:
If you say theory and practice don't agree you haven't applied enough theory.