Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: 3choplex on July 29, 2013, 12:01:23 am
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So I continue to work on this gibson scout. The problems I am currently trying to tackle is that I am getting an ugly distortion sort of following notes when I dig in and the voltages are way off.
Tonight, though, I discovered something really strange: the coupling caps from the PI are not attached. Or rather, they are attached to pin 7 of both 6AQ5 power tubes, and not to pin 1. The 220k resistors from the cathode bias are the only connection to the grids of the power tubes. Despite this, the amp works. I am absolutely baffled. [bold]Basically, the PI is not connected to anything. I assume I must be missing something stupid, but how is this thing working at all?[/bold]
Quiescent voltages are off, mostly low, but one power tube is really screwy.
I don't have voltages for the reverb tube on hand. (V1)
V2A (First gain stage)
Plate 70v (50v below schem)
Grid 0v
Cathode 0.9v (0.3v below schem)
V2B (2nd gain stage)
Plate 70v (80v below schem)
Grid 0v
Cathode 0.9v (0.3v below schem)
V3A (PI)
Plate 140v (71v below schem)
Grid 0v
Cathode 0.9v (0.3v below schem)
V3B (trem)
Plate 96v (22v below schem)
Grid 0.45v (about 1v above schem and above cathode!)
Cathode 0v
V4 (6AQ5)
Plate 265v (35v below schem)
Screen 206v
Grid 0.38v
Cathode 3.76v (15v below schem)
V5 (6AQ5)
Plate 270v (30v below schem)
Screen 206v
Grid 4.39v (voltage above cathode!)
Cathode 3.76v (15v below schem)
Coming out of the PT is 252v per side, I'm told this is normal. On the rectifier cathode I am seeing 277v, about 30v below the schem value.
I assume connecting the coupling caps from the PI to the power tubes will change the power tube voltages quite a bit.
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Wired the caps in and it works, but started popping after a minute. Voltages didn't change.
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> pin 7 of both 6AQ5 power tubes, and not to pin 1
Did you look-up the 6AQ5 pin-out?
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The amp is how old? 50 years?
This plan? http://www.gibson.com/Files/schematics/GA-17RVT%20Amp%20Scout.pdf (http://www.gibson.com/Files/schematics/GA-17RVT%20Amp%20Scout.pdf)
And Gibson didn't make their amps as good as their mandolins.
I think you need to check ALL resistors and caps. I bet the Rs have drifted high/low by many-many percent, and half the caps leak bad.
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they are attached to pin 7 of both 6AQ5 power tubes, and not to pin 1
on 6AQ5 pin 1 & pin 7 are both connected internally to grid number 1.
http://tubedata.milbert.com/sheets/093/6/6AQ5A.pdf (http://tubedata.milbert.com/sheets/093/6/6AQ5A.pdf)
low B+ is likely that the power supply electrolytics need replacement but then i read your first post that implies that you replaced them? also, probably could benefit with a fresh recto and fresh power tubes. check resistor values in PS chain. distortion can be caused by a faulty cathode bypass cap or a poor ground or socket connection. make sure all the filaments are lit.
--pete
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:BangHead: misread the pinout somehow. Doh!
The PS caps are new, ill switch out the cathode caps. I've been going through the resistors and changing any that are off. Most have been pretty close.
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" And Gibson didn't make their amps as good as their mandolins. "
Truer words were never spoken. Gibson amps of this era were just ordinary record-player build quality...Danelectro-grade point-to-point builds, and not to imply that the wiring technique is a hard limit on how good the amp sounds or lasts. I believe that lots of times, we hear the nameplates on these older amps. Gibson amps were NEVER all that popular; not that numbers weren't sold because back then, you bot a LP Jr and an amp as a set. And they're cool in their way, some are much better amps than others. It's not anybody's fault that 50-60 year old parts stink from a performance standpoint.
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This is my 4th gibson amp. They all had mixed quality and sound, from dreadful (duo medalist) to great (skylark).
Except for the weird distortion, this one sounds pretty good, but very very bright.
There are some changed values inside (mostly to the reverb circuit), not sure if they were factory changes or not. It's a bit hard to work on, because there are two terminal strips and the longer of the two is kind of stuck behind the small one. Very hard to work cleanly.
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Changed some caps and resistors and the distortion is gone and most of the voltages are close to spec. Thanks!