Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: bigsbybender on August 14, 2014, 12:22:59 am
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Hi again,
I've now got on my bench an early 1965 Gibson Falcon (GA-19RVT). This one is an oddball since it follows this "Maestro" schematic and not the typical one that is listed on this site. http://www.classictubeamps.com/schematics/Gibson/Gibson%20Maestro%20M-216%20schematic.pdf (http://www.classictubeamps.com/schematics/Gibson/Gibson%20Maestro%20M-216%20schematic.pdf)
Once I got the right schematic this amp is making more sense, however I'm stumped on the reverb. It doesn't work, but it squeals when the knob is over halfway up.
Here's what I've done to troubleshoot:
1. Swapped cables on the reverb tank. No change (Cables are soldered into the circuits as opposed to jacks as on a Fender)
2. Traced cables back to their sources and connected them to the proper jacks on the tank. No Change
3. Substituted in a known good tank. No Change.
4. Tested the cables with DMM for shorts and continuity. Tests show no shorts, No breaks in continuity. (Since these are soldered into the circuit board, which are holy hell to work on in these Gibsons, I cannot easily sub a cable.)
5. Subbed Reverb Driver and Recovery tubes, No Change.
6. Hooked a speaker to the driver side reverb cable, Got signal through the speaker, however got oscillation-like squeal when I turned the knob past 5.
So I'm stuck at the moment.... Any insight and help would be greatly appreciated!
Thanks,
J.
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Do you get any reverb at all before the squeal starts?
Sounds like the squeal is in the driver circuit. I wonder if the reverb pot is OK. Check the resistance of the pot by connecting your ohm meter between the pot wiper and ground. Slowly turn the pot from one end to the other and watch for any drop outs. Maybe temporarily tack a 1M resistor from V1-8 to ground.
Use a scope or listening amp to determine if the squeal is present on the plate of V1 (pin 7).
Bridge a 10-20µF cap across that filter C10?. That cap feeds 4 stages.
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fwiw I think Sluckey covered everything, I fixed a ga-5t awile back and the 6eu7(your V1) was squealing bad. I re-wired socket for 12au7 and the owner was happy.
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Do you get any reverb at all before the squeal starts?
No reverb at all.
Sounds like the squeal is in the driver circuit. I wonder if the reverb pot is OK. Check the resistance of the pot by connecting your ohm meter between the pot wiper and ground. Slowly turn the pot from one end to the other and watch for any drop outs. Maybe temporarily tack a 1M resistor from V1-8 to ground.
Use a scope or listening amp to determine if the squeal is present on the plate of V1 (pin 7).
I don't have a scope but I have a listening amp, I'll give that a try. Gibson amps from this era are just awful to work on compared to their counterparts.
Bridge a 10-20µF cap across that filter C10?. That cap feeds 4 stages.
All the filters are bulging so I'm in the process of replacing them too. I did notice how lightly filtered these are.
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I fixed a ga-5t awile back and the 6eu7(your V1) was squealing bad. I re-wired socket for 12au7 and the owner was happy.
So perhaps the 6EU7 has too much gain for this application and not enough current? Possibly why the more common version of the GA-19RVT uses the pentode side of a 7199. This one is unfortunately the oddball.
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It could be C10. that 10uf electrolytic. If it is non-functioning, the slightest noise or oscillation can feed back via plate resistors (R25 and R23) to itself on the 2nd section of V1, AND to the PI stage via the 470K plate of V2..... that 10uf C10 is supposed to be a path to ground for all AC including instrument signal, stray noise, etc...
Some of those Gibsons put the electrolytics under the board. You could tack in any electrolytic (8uf to say 20uf) with alligator clips to see if its the issue or not.
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It could be C10. that 10uf electrolytic. If it is non-functioning, the slightest noise or oscillation can feed back via plate resistors (R25 and R23) to itself on the 2nd section of V1, AND to the PI stage via the 470K plate of V2..... that 10uf C10 is supposed to be a path to ground for all AC including instrument signal, stray noise, etc...
This was our culprit! Problem Solved. The 6EU7 was also a bit microphonic so I replaced it. The prices keep climbing on that tube....
The two 10uF filters were in one cap can (cardboard tube type with wires coming out) I siliconed a couple 10uF radial caps together and stuffed them in the old cap can. The original caps were a pain because they aren't easy to spot a failure by looking at them.
Thanks for all your guidance everyone!
J.
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I left it stock with the three 6EU7 tubes. Pretty much for that reason. I've seen a few modded for all the tubes to be 12A_7 but I opted for stock and so did the customer even though the tubes are pricier.
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One thing about a 6EU7 that makes it tough to find a (plug-in) substitute is the heater connections being pins 1 & 2. That's a characteristic I myself have never seen on a 9-pin tube, though obviously that doesn't mean it doesn't exist. But it's pretty unusual.
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FWIW when I fixed the GA-5T, I couldn't find anything, so I talked it over with the owner, price for NOS, future repair, resale etc n he opted for the re-wire, I tried an AX7, AT7, and AU7. The owner liked the AU7 best. I did update the chassis n left a modified schematic for some future tech.
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One thing about a 6EU7 that makes it tough to find a (plug-in) substitute is the heater connections being pins 1 & 2. That's a characteristic I myself have never seen on a 9-pin tube, though obviously that doesn't mean it doesn't exist. But it's pretty unusual.
GE essential characteristics (14th Ed.) indicate that it is built on a 9LS base. 6EU7 is the only tube listed with the 9LS base.
--pete