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Gibson GA-95RVT
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Travst



Joined: 15 Jul 2008
Posts: 9535
Location: Birmingham, AL

PostPosted: Sat Jul 24, 2010 8:37 am    Post subject: Reply with quote

I wasn't able to test the 6EU7 tubes, but I swapped all the other preamp tubes out and it didn't change anything. There's still very little volume. I guess digging deeper is in order. The old caps are definitely causing a bit of crackle.

Swapped out one 12AU7 and things got louder and cleaner. The second 6eu7 is definitely microphonic.
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John
I need more practice, not more gear.
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Travst



Joined: 15 Jul 2008
Posts: 9535
Location: Birmingham, AL

PostPosted: Sat Jul 31, 2010 9:55 am    Post subject: Reply with quote

I've swapped out all the tubes in this amp and have done a cap job. I didn't replace the coupling capacitors. I'm guessing those are the two yellow caps in the pic that are marked 40u 150V DC. I've chopsticked and eyeballed everything. This has resulted in the previously very low output increasing by a factor of four. The tone is also nice and sweet. However, the amp isn't putting out near 100 watts... 10 at best guess.

I do have a bad midrange pot in the reverb channel and don't have a replacement on hand. However, the low output is present on both channels, so I ruled that out as the issue. The OA2 rectifier tube was NOS according to the seller. That's the one tube I didn't swap, since I don't have one laying around, but I have replacements on order.

The schematic is here:

http://www.mediafire.com/?lla9jfq9831sfxs

Any thoughts? I will start on the steps outlined here today:

http://geofex.com/ - go to tube amp debugging, low power section.
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Structo



Joined: 21 Nov 2004
Posts: 26415
Location: Salem, OR

PostPosted: Sun Aug 01, 2010 11:25 am    Post subject: Reply with quote

John your amp has solid state rectification.

The OA2 tube is a voltage regulator tube.
If it is glowing orange than it is most likely OK.

The two bright yellow electrolytics are most likely part of the power supply filtering and they look like they were added later.
They appear to be wired in parallel making the total 80uF if they are indeed both 40uF.

The coupling caps will be the film caps on the circuit board that join the stages together.
What they do is block the DC voltage from the signal path as it goes stage to stage.
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Tom



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Structo



Joined: 21 Nov 2004
Posts: 26415
Location: Salem, OR

PostPosted: Sun Aug 01, 2010 11:28 am    Post subject: Reply with quote

John, I keep meaning to ask, what is that thing that is either a jack or trimmer pot that is on the top of the chassis by the OA2 tube?
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Tom



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Travst



Joined: 15 Jul 2008
Posts: 9535
Location: Birmingham, AL

PostPosted: Sun Aug 01, 2010 5:02 pm    Post subject: Reply with quote

Tom, the jack by the OA2 tube is for the internal speaker. The footswitch (which I don't have) connection is the DIN type plug in the upper left of the chassis in that same photo. I do have the wiring diagram for the footswitch as you know. I'll have to make one later.

Thanks for clearing that up about the voltage regulator and those yellow caps. I have new filter caps on order to replace those two. I should have replaced them when I did the others.

I'll be sorting through the amp over time, learning how to read schematics better and testing all the components. I'm approaching this project as a learning experience. It does sound quite good now with the work that I've already done. However, the volume is an issues and there's some hum present.

The trick here is to fix the amp without spending more on it than it's worth. However, since I now have about $350 in it, there's some headroom to fix it up. Surprisingly, the goofy speakers that are in it sound good. Lots of bottom end and no icepick highs.
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