Okay. Hello, all.
You've heard the story a thousand times. Found an old amp in Grandpa's attic, wondered if it still works. At a glance, it looked pretty dang good - considering its age and that it's been in a hot/cold un-insulated environment for decades. So, after staring at it lustfully for 20-30 minutes, I could no longer resist.
I plugged her in. And lo and behold, the power indication light sprang to life....the tubes warmed up....and I got a constant low hum out of the speaker. Pleasantly surprised, I plugged a guitar into it (...after allowing a few minutes to ensure it wasn't going to catch fire on me). No guitar signal was making it through to the speaker, so I switched off and unplugged and took it out to my shop.
A few hours later, I did some additional recon work. Plugged the amp back in (no guitar this time). All tubes glowing? Check. All of the sudden, I started to hear short, sporadic "squeaks" in the otherwise constant output hum......one.......two........three..........then another.....then another. Uh oh. Any rehab efforts I was willing to do, were going to likely be more involved than I might have hoped, right? So....I turned her off, and......
...immediately after I unplugged it, the amp circuitry began to smoke. Not an obscene amount of smoke - I never reached for the fire extinguisher, but about the amount of smoke coming off an idle cigarette. I waited for the smoke to dissipate, then let the old gal cool off.
I resumed my inspection efforts later that day (...while the amp was unplugged, of course). I opened her up to find a couple of issues:
1. A fried and cracked carbon comp resistor running from the 6v6gt to the 5y3gt tubes.
2. Some sticky, sweaty residue on a good portion of the wiring. My initial thought on this was that the overload lightly melted some of the wiring insulation. However, this residue could quite possibly have been there before I ever found the amp - perhaps from a blowout in the 70s....or from exposure to extreme attic heat for years.
3. On the Mallory FP can filter capacitor, there was some obviously melted black resin residue on the exposed open chassis mount. Not much, but enough for me to conjure up another audible sigh.
Note: I have not plugged it in since, but the 5 watt fuse never blew...and visual inspection confirms this.
I'm not inclined to take it to a tech just yet, but will if it comes down to it - I want to educate myself first, if nothing else. I need to identify the wattage of the comp resistor that is toasted. It's discolored, but schematic indicates 6.8K......I just need the wattage (not indicated on the schematic). Schematic and close-up of the resistor included below.
Also would love anyone's opinions on a possible diagnosis of what the smoke culprit was/is.....and any other insight. I'm not a total noob. I have some basic wiring/mod skills. However, I have no intention of electrocuting myself. I just want some insight as to what I might be dealing with here.
Thanks for your time.
Model: Gretch 6150T (aka 6151) (Valco) 5 watt
Schematic: http://www.oldfrets.com/Valco/Schematics/gretsch6151.pdf
You've heard the story a thousand times. Found an old amp in Grandpa's attic, wondered if it still works. At a glance, it looked pretty dang good - considering its age and that it's been in a hot/cold un-insulated environment for decades. So, after staring at it lustfully for 20-30 minutes, I could no longer resist.
I plugged her in. And lo and behold, the power indication light sprang to life....the tubes warmed up....and I got a constant low hum out of the speaker. Pleasantly surprised, I plugged a guitar into it (...after allowing a few minutes to ensure it wasn't going to catch fire on me). No guitar signal was making it through to the speaker, so I switched off and unplugged and took it out to my shop.
A few hours later, I did some additional recon work. Plugged the amp back in (no guitar this time). All tubes glowing? Check. All of the sudden, I started to hear short, sporadic "squeaks" in the otherwise constant output hum......one.......two........three..........then another.....then another. Uh oh. Any rehab efforts I was willing to do, were going to likely be more involved than I might have hoped, right? So....I turned her off, and......
...immediately after I unplugged it, the amp circuitry began to smoke. Not an obscene amount of smoke - I never reached for the fire extinguisher, but about the amount of smoke coming off an idle cigarette. I waited for the smoke to dissipate, then let the old gal cool off.
I resumed my inspection efforts later that day (...while the amp was unplugged, of course). I opened her up to find a couple of issues:
1. A fried and cracked carbon comp resistor running from the 6v6gt to the 5y3gt tubes.
2. Some sticky, sweaty residue on a good portion of the wiring. My initial thought on this was that the overload lightly melted some of the wiring insulation. However, this residue could quite possibly have been there before I ever found the amp - perhaps from a blowout in the 70s....or from exposure to extreme attic heat for years.
3. On the Mallory FP can filter capacitor, there was some obviously melted black resin residue on the exposed open chassis mount. Not much, but enough for me to conjure up another audible sigh.
Note: I have not plugged it in since, but the 5 watt fuse never blew...and visual inspection confirms this.
I'm not inclined to take it to a tech just yet, but will if it comes down to it - I want to educate myself first, if nothing else. I need to identify the wattage of the comp resistor that is toasted. It's discolored, but schematic indicates 6.8K......I just need the wattage (not indicated on the schematic). Schematic and close-up of the resistor included below.
Also would love anyone's opinions on a possible diagnosis of what the smoke culprit was/is.....and any other insight. I'm not a total noob. I have some basic wiring/mod skills. However, I have no intention of electrocuting myself. I just want some insight as to what I might be dealing with here.
Thanks for your time.
Model: Gretch 6150T (aka 6151) (Valco) 5 watt
Schematic: http://www.oldfrets.com/Valco/Schematics/gretsch6151.pdf
