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dkevin

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About dkevin

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  1. I realize this thread is very old but I ran across it just this morning...I have replaced OT's in Gibson amps that used 2 6v6's in parallel output/single-ended configuration with an OT from Allen Amps. From the Allen Amps website: "TO11S Heyboer output transformer rated 4,000 ohms primary to 4 or 8 ohms secondary. It is the same as the above TO11C but mounted on its side the short way to prevent transformer coupled hum. It will now be the standard OT for the 10W Chihuahua. Excellent choice for a 6L6 or EL34 single-ended amp using either a 4 or an 8 ohm speaker load. 3-1/8" mounting centers. It is the same physical size as my TO22 transformer"
  2. It appears to be an early GA-5 without the "Skylark" designation that has 2 inputs added by a modifier. I can't think of why you would need 2 more inputs for a 5 watt amp...The cabinet is the earlier variant with larger edges at the top and bottom of the face. These amps are great! Congrats!
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  4. It sounds as if this amp uses a Field-coil speaker. This type of speaker does not have a permanent magnet but is energized by the power of the amp circuit. The extra wires carry the circuit's power to the field coil. If they are not hooked into the circuit correctly, they cannot allow the speaker to reproduce sound. If they are inadvertently grounded against the amp chassis, they could cause personal injury or permanent damage to the amp. To advise you further, It would be best if you posted detailed pictures of the amp, speaker and circuit. Be very careful as there are lethal voltages stored in the circuit and you can be injured or killed.
  5. It sounds as if this amp uses a Field-coil speaker. This type of speaker does not have a permanent magnet but is energized by the power of the amp circuit. The extra wires carry the circuit's power to the field coil. If they are not hooked into the circuit correctly, they cannot allow the speaker to reproduce sound. If they are inadvertently grounded against the amp chassis, they could cause personal injury or permanent damage to the amp. To advise you further, It would be best if you posted detailed pictures of the amp, speaker and circuit. Be very careful as there are lethal voltages stored in the circuit and you can be injured or killed.
  6. dkevin

    GA-25.5

    "GA-30 Gibson’s midline amp was changed for the third time in three years, as the short-lived GA-25 (which replaced the short-lived BR-4) was superseded by the identically priced GA-30. Cosmetically, the porthole speaker openings were replaced with a large, rectangular picture frame-style with a Gibson logo across the middle. The 1949 Electric Guitars catalog showed the GA-30 with a two-knobbed control panel, as on the 25 (this was a new photo, not a reprint of the 25 from the ’47-’48 catalog), although most GA-30s had separate volume controls for the mic and three instrument inputs. An extra 6SJ7 preamp tube was assigned to the mic input to accommodate the extra volume control, in turn requiring the twin 6J5 phase inverter of the 25 to be replaced with a single 6SC7 to maintain a six-tube chassis. The two 6V6 power tubes and 5Y3 rectifier were basically unchanged. A bass “Tone Expander” switch inserted/bypassed a low-frequency blocking capacitor in the negative feedback loop of the Instrument channel circuit; on the two-knob version, the switch was located on the control panel between the volume and tone controls, on the three-knob version, the mic input was moved into the row of instrument inputs and the Tone Expander was installed directly below, in its place. The deeply textured “dark brown leatherette” covering of the 25 and early GA-30s was short-lived, replaced after about a year by a smooth, light brown material. This model would run until the change to the more powerful top-mounted chassis two-tone model in 1954. From Vintage Guitar Magazine Gibson Post WWII Amps
  7. dkevin

    GA-25.5

    Long time no post...recently I found several of these Gibson amps for sale. I will post a link to them.... https://www.ebay.com...T4AAOSwG-xb5I83 https://reverb.com/item/14326895-1950-gibson-ga-30
  8. I have just started on a GA-20 1st version amp. I bought it from a private seller and it was as-is. The PT and OT were solid and it came with a Jensen P12R too. Check out the pictures... https://imgur.com/a/SN8CqFR
  9. I am a big fan of tweed Gibson amps, so I am not the most impartial advisor. I love the larger speaker and extra tremolo control. I guess it would depend on the price of the 8T.
  10. The PT and OT worked out great! The amp sounds very good! I've got the chassis reinstalled into the cab with the warehouse G12Q and it sounds really nice! I had to tighten up the octal socket pins. The third time I played through the amp, I turned it on and ....no sound. I checked my rail voltages and they were all high and not dropping into normal operating range. I chopsticked the output tubes and when I touched the 6V6 plates (3) I got a audible pop. I checked the solder joints and they were all clean. So I removed the tubes and looked at the pins inside the sockets. They were spread out so I closed them up a but with a small machinist's scribe. The amp started and played from then on with no more symptoms.I am making a back cover for the chassis innards...the seller said he lost it somewhere. I happen to have a GA-20T that has the back cover so I'll just copy that one. The two amps are of the same vintage but the circuits and layouts are vastly different. The 20T is a horror story to work on with components mounted on both the upper and lower surface of the component board. It is the sort of Gibson amp that leaves techs shaking their heads and swearing never to work on another Gibson amp. Compared to the 20T, this rebuild of a GA-20 is a "walk in the park".<div><br></div><div><br></div><div>https://imgur.com<div><br></div><div><br></div></div>
  11. The beast lives. Voltages very close and sounds very good! More testing and then I'll wrap this up...meanwhile, look at some more pictures... https://imgur.com/a/xcEyRck
  12. Update https://imgur.com/a/KKTolZg
  13. I am looking for a power transformer and output transformer for a Gibson GA-20 with nine pin tubes in the preamp. I think the amp is a 1956 version and the PT currently in the amp is not only incorrect but it is also fried. The OT is an off-the-shelf replacement and I would prefer to have original transformers if possible. The PT is stamped 'GA-20-P" . The OT markings are not readily evident from the pictures I have. The OT is often "lost" when the original P12R speaker is fried and replaced. It was riveted to the speaker frame. Thanks for your help! https://imgur.com/a/T1ZZnVB
  14. I was in a local (Seattle) guitar shop and was pleased to find several older Gibson guitar amps for sale. One in particular caught my eye. It was a BR-6 amp from 1952-53. I have one of these and it is a very sound amp. This one however had a curious addition. The BR-6 and 6F have 5 octal tubes laid out in a straight line. The chassis is located at the bottom, rear of the cabinet. This amp has 5 octals all right , but also has a 9 pin (noval?) socket tucked in behind the preamp octal. It appeared to be a 12AX7 tube. My first instinct was to suspect an owner modification to increase the preamp gain. But the socket appeared to be of the same vintage as the others. I looked for another schematic but the only one I could find was the 5 tube variety. It may well be a home-made mod but imagine the surprise of the new owner when his tech tells him that his amp is not stock. The asking price is not low. I did not mention the issue to the shop salesperson. Let me know if you have encountered older Gibson amps like this one.....Gibson_br_6.pdf
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