Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: 82lespaul on December 29, 2017, 11:54:15 am
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Hello,
New to the forum; however I have been reading and learning from this great community for years. I have a question regarding a 1962 Gibson Falcon I am working on. Just finished recapping and installing a 3 prong cord. The amp is plugged into a DBT which is plugged into a Variac. No tubes installed, powered on (no standby switch). First off I notice the dim bulb never illuminates (at least to the naked eye). The jewel light lights up, I have 5 volts AC at rectifier heaters and 6.3V at light and tube heater pins. I have about 315V at rectifier inputs. The odd thing is I measure about 73 VAC at the fuse and power switch. I checked the output of the variac and it is solid at 120VAC. I took the Variac out and went straight to measured wall and still only measure about 73VAC at fuse or power switch. I do not want to continue. Have you ever seen this? Could I have a bad power transformer. Any help is greatly appreciated.
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If you have no tubes, then you have no power to light the bulb. If your amp is plugged into the bulb limiter you don't need the variac. Insert all the tubes back in their sockets, at least the rectifier. At turn on, the limiter will «flash» for a fraction of a second. If it stays bright you have a short. If it stays dim after initial flash, you are ok. Don't forget, the speaker must be connected if you plug the power tubes. You should be able to hear some hiss coming through the speaker.
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Greg, Thanks for the feedback. I was most concerned about only measuring 73VAC at the switch and fuse (red to either and black to chassis) instead of normal 120VAC. Not sure if transformer primary is bad, but somehow I am seeing a voltage drop of about 45-50 volts.
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Greg, Thanks for the feedback. I was most concerned about only measuring 73VAC at the switch and fuse (red to either and black to chassis) instead of normal 120VAC. Not sure if transformer primary is bad, but somehow I am seeing a voltage drop of about 45-50 volts.
Nothing wrong. That's the way it's supposed to be. You have a light bulb in series with your amp. In your case, 73 volts is dropped across your amp transformer primary and 45-50 volts is dropped across the light bulb. There's not enough current flowing to light up your bulb. If you use a smaller wattage bulb it will probably shine a bit, but it can never reach full brightness unless your amp has a problem that looks like a dead short.
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sluckey,
Thanks very much. I will move on to the next step and install the rectifier tube and bring up slowly with variac to form new electrolytics. Hopefully I am good to go from here. Appreciate the help from a great forum.
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You don't need to form new electrolytics. Just plug your amp into your lamp limiter, plug in all tubes and turn the amp on. If the lamp limiter does not glow really bright, then plug the amp straight into the wall. ♪♪♪ Check some volts... Strum a little guitar... Get down tonight ♫♫♫
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Well I installed the pre amp and reverb circuit tubes (3 6EU7s and 7199) and ran the amp for about 20 minutes. No issues, everything checked fine. Hooked up a speaker and installed the 2 6V6s. The dim bulb glows dim as expected; however after about 15 seconds (once the power tubes heat up) the amp gives off a high pitched squeal through the speaker. The only thing not plugged in is the 2 RCA plugs to the reverb tank. Any ideas?
Thanks
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Pull V1. Squeal go away?
Pull V2. Squeal go away?
Pull V3. Squeal go away?
Pull V4. Squeal go away?
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The reverb footswitch seems hardwired... If it is open, the recovery amplifier can pick-up sound via the hanging rca recovery cable and since the reverb control is in the drive circuit, the stage is left wide open with a cable hanging... Maybe close the footswitch or try the amp without the 7199 tube or maybe plug the reverb tank.
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Happy to report back that my amp is working and passed a bench test with a guitar plugged in. Tried the reverb footswitch first (no luck) and then removed tubes one at a time starting with V1. V3 (6EU7) was bad). My amp does not follow the schematic in that V2 is not used on the reverb switching side, actually half of V3 is used. I could never find a Gibson or Epiphone schematic that matched my amp and you can tell its factory wiring. Go figure. Again, thanks for all the help. I will check my voltages again and the bias and report back.
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Gibson was notorious about printing schematics that often vaguely resemble the actual amp, sometimes not at all. And it's not uncommon to find undocumented variations in amps that should be identical. That's the main reason I've never tried to copy any Gibson amps.
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Just finished checking the bias. I used the output transformer method.
V5 Pin 3 to Pin 8 of rectifier = 144 ohms, Voltage drop of 4.86V, calculated plate current of .0338A, measured plate voltage of 315V, plate dissipation of 10.6W
V6 Pin 3 to Pin 8 of rectifier = 160 ohms, Voltage drop of 5.56V, calculated plate current of .0348A, measured plate voltage of 315V, plate dissipation of 10.9W
So averaging about 10.75 W. I believe amp should normally put out about 12W. Pretty close. Could this be attributed to weak/aged 6V6s ?