Gibson Guitar Board: GA-77 amp - Gibson Guitar Board

Jump to content

Page 1 of 1
  • You cannot start a new topic
  • You cannot reply to this topic

GA-77 amp

#1 User is offline   dkevin 

  • Member
  • Group: Members
  • Posts: 18
  • Joined: 01-February 12

Posted 03 July 2017 - 10:21 PM

I found a Gibson GA-77 amp chassis ...actually I got it on Ebay and also managed to get the cabinet it was paired with. The cabinet is not the tall, buffalo grained cab with a "77" on the front. It is more akin to the later version GA-77 called a Vanguard amp. However, this cabinet is a dead ringer for a Fender Pro amp circa 1959 or so. The side walls are thin and the top and bottom front panels are 2 1/2 to 3" wide. The tolex is a black/white sort of pattern that I have never seen on a Gibson amp before. The chassis is an early version (pre-Vanguard) with 7 tubes and 5 controls. I am wondering if any of you have ever seen a Gibson amp with a cabinet that looks like this. The cab is not constructed with finger-joints as the later amps are. This one more closely resembles the GA-25 or GA-20 with overlapping cuts. (Sorry..I forgot the technical term) I believe this version of the 77 was made from 1954-1957. In 1958 I think the amps were redesigned to use 6 tubes and 6 pots.
0

#2 User is offline   dkevin 

  • Member
  • Group: Members
  • Posts: 18
  • Joined: 01-February 12

Posted 18 July 2017 - 01:55 PM

The silence is resounding...I guess no one has ever seen a Gibson amp like this one. I peeled away the vinyl/tolex in order to replace a rotted lower panel and I confirmed that the joints are lapped and not finger joints. The cabinet is very similar to a Fender tweed Pro amp 59 or so. The covering is a black square pattern on a white textured background. The chassis is a stock GA-77 from 1954-1957 or so. It uses 2ea 12ax7's for the channel 1 and 2 inputs. Channel one is the hot channel. These are followed by a 6SL7. The next tube is a 6J5 that serves as the phase inverter. This tube feeds a pair of 6L6's in push-pull configuration. The amp is said to develop 25 watts or so. The 6L6's are cathode biased. The layout is a real mystery to unravel. There is a 22 tab terminal strip that runs from the PT to the 12ax7's. Most of the caps/resistors either fly to/or from this terminal strip to the tube pins or controls. There are 5 pots (as compared to the later tweed GA-77 Vanguard amp which sports 6 pots). Due to heavy rust on several of the chassis surfaces, I lost all of the exposed graphics on the chassis. That is a shame because they helped to explain the controls. Channel one has a volume, bass and treble pot. Channel 2 only has a volume pot and a tone pot. Also, this is the first Gibson amp I have owned that has a standby switch. I am waiting for parts to come but I'm gonna need the time to unravel this puzzle....
0

#3 User is offline   badbluesplayer 

  • ZIPPER CLUBBER
  • Group: Members
  • Posts: 5318
  • Joined: 14-April 08
  • LocationJonesborough, TN

Posted 20 July 2017 - 02:04 PM

[thumbup] [thumbup]

How about some pix?!
_________________________________

TENNESSEE AMPLIFIER COMPANY
0

#4 User is offline   dkevin 

  • Member
  • Group: Members
  • Posts: 18
  • Joined: 01-February 12

Posted 21 July 2017 - 07:46 AM

I have a lot to learn about the ins and outs of posting pictures. Every time I try to pull one of the many pictures of this amp the system notifies me the file size is too large.
0

#5 User is offline   dkevin 

  • Member
  • Group: Members
  • Posts: 18
  • Joined: 01-February 12

Posted 28 July 2017 - 07:58 PM

The 60 year old monster lives! The power transformer was good and the components have been renewed. I had a schematic (Thank you EL-34 schematics!) but it lacks voltages. Gibson amps schematics do not come with a layout as Fender amps do. That necessitates a hand-drawn layout and some good detective work. I rarely find an amp this old that has survived in "as-it-left-the-factory" condition. This one was no exception. There were several power supply doo-dads that needed to be removed. The power switch was completely messed up. I normally replace all the E-caps, but I left the 20/20 @500vdc cap can in hopes it was still good. Naturally it wasn't. I blew fuses until I spliced in a new 22/500 F&T caps in their place. I chose to trust this cap can because I could not find a 20/20@500 can.The amp sounded good but had a few oscillations. I checked the plate/cathode/screen voltages and most were ok. The exception was my last 6L6 plate voltage. It should have been 415vdc as the first 6L6 was but it read 0vdc. The screens and cathodes agreed but the plates would not. When I checked the continuity of the OT. only one side was functioning. I had hope it was a broken wire but I checked at the OT itself and the break must be inside the windings. That means I will have to buy a replacement OT...or have the original repaired. Hmmmmm.....Any suggestions?
0

#6 User is offline   badbluesplayer 

  • ZIPPER CLUBBER
  • Group: Members
  • Posts: 5318
  • Joined: 14-April 08
  • LocationJonesborough, TN

Posted 30 July 2017 - 11:05 AM

I have the Master Gibson Service Book from 1964. I can't find your amp in the book but if I could somehow identify it I could give you any info it has on that model. If it's in the book.
_________________________________

TENNESSEE AMPLIFIER COMPANY
0

#7 User is offline   dkevin 

  • Member
  • Group: Members
  • Posts: 18
  • Joined: 01-February 12

Posted 30 July 2017 - 03:30 PM

I looked at my copy of the Gibson Master Service manual and on page 220 there is a GA-77 amp. The schematic however has 6 tubes and uses no 6SL7 or 6J5 octals in the circuit. There is a handwritten note that say this version is a "1953-54" version. I do not believe this to be true. I think it is a later Vanguard version of the amp and was probably made in the late 50's-early 60's. I am more inclined to think that my version pre-dated this version because mine uses the older octal tubes in place of the more modern 12A_7 tubes. The schematic version that fits my amp is labelled "GA-77 1954 version" , and is found on the EL-34 website at the end of the Gibson amp section. On another topic, I have found several options for a replacement output transformer. Magnetic Components 40-18006 or 10-18010 or Allen Amps TO-35MT . They range in price from $60-$90 (including shipping).
I also have an interest in having the original OT repaired or rebuilt. Perhaps someone reading here has a suggestion for a person who repairs/rebuilds OT's.
0

#8 User is offline   dkevin 

  • Member
  • Group: Members
  • Posts: 18
  • Joined: 01-February 12

Posted 07 August 2017 - 12:05 PM

Well, I went for the Allen amps TO-35MT output transformer. It cost $75/$14 to ship. It has a primary of 4k and dual secondary of 4 and 8 ohms. I have purchased several OT's from Mr. Allen and they sound great. I am looking forward to putting this thing together and playing through it. I had 2ea 6L6's from my Bassman 100 amp (they tested in the 90's on my B&K 747b so I kept them) so I used them just in case I had a brain-fart and blew them up. I also had a 5V4GT from previous scavenging. I rebuilt the cab floor, reattached the tolex and hung a Magnavox P15N inside. The amp sounded good running on one 6L6. I can hardly wait to hear it with two output tubes.
0

#9 User is offline   dkevin 

  • Member
  • Group: Members
  • Posts: 18
  • Joined: 01-February 12

Posted 15 August 2017 - 11:47 AM

Got the Allen amps TO-35MT output transformer mounted and hooked up.The amp sounds really good!. This sucker is loud! I lost all the strange oscillating whistles and sounds when I went with the new OT. I re-checked the operating voltages and had to be schooled on how to interpret them. Most of my experience has been on smaller amps. Some have been push-pull but most have been single-ended. I usually use the Weber bias calculator. To be specific, my 6L6 plates were at 389vdc....screens were 260vdc....and the cathodes were running 21vdc with a cathode resistor that read 138 ohms. The schematic called for a 150 ohm cathode resistor. This amp is push/pull 6L6's and is cathode-biased. Before I just relax and play it for longer periods of time, I would like to be assured that it is operating in a safe manner. I posted on another forum and asked these same questions. I was recommended to Rob Robinette's bias calculator. According to this, the amp is biased somewhat cool as it stands. If I was to increase the size of the cathode resistor and the schematic call out, the amp would move even further into the cool/cold range. This would probably affect the overdriven tone in an unpleasant manner. As you can tell, I am not a professional tech. As I previously noted, this is one of the most powerful amps I have attempted to rebuild. I ended up replacing most of the carbon comp resistors and all of the wax-paper capacitors and electrolytic capacitors. The layout has to be seen to be believed. If I can be taught...and that still remains quesitonable...I will learn to post pictures and hand-drawn layouts to this forum so you can see what I am talking about. Next up is to mount the chassis (very heavy) in the cabinet and find a way to support it so it doesn't destroy the cab.
0

Share this topic:


Page 1 of 1
  • You cannot start a new topic
  • You cannot reply to this topic

1 User(s) are reading this topic
0 members, 1 guests, 0 anonymous users