Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: JustMike on September 04, 2021, 07:57:22 am

Title: 1955 Gibson GA-20 How Would You Approach This?
Post by: JustMike on September 04, 2021, 07:57:22 am

 A friend brought over this old gem and just said "Make it work". I tested the tubes and they're OK although the output tubes are mismatched. One's a Tung Sol and the other is a Sylvania. The power cord is rotted to the point that is I move it, the rubber jacket crumbles away. So I can't even plug it in yet.


 Step 1- new grounded power cord.


  Step 2- I assume the filter caps should be replaced after 65 years. And what about the signal caps?


 Step 3- fire it up and listen for popcorn (cc resistors)?

Title: Re: 1955 Gibson GA-20 How Would You Approach This?
Post by: shooter on September 04, 2021, 09:23:05 am

me, I'd fix the power cord
put all the tubes in, make sure I had a GOOD, known speaker HARDWIRED to the OT secondary NO jacks
put my meter across the 200 ohm cathode R for PA tubes
and power up, NO signal in


IF you get a VDC across Rk, then measure the B+ tap for the power tubes, do the math.  Anything close is good


Now add input signal and evaluate

Title: Re: 1955 Gibson GA-20 How Would You Approach This?
Post by: pdf64 on September 04, 2021, 09:34:28 am

After a properly grounded new power cable is fitted, power up via a light bulb limiter for sure.
Just the rectifier valve in place and a low wattage bulb, initially, to see what the ecaps do.
At least the coupling caps aren’t bumblebees, but they still need checking for DC leakage.