Gibson GA-95 RVT

[Phil_S]

Well, there's good news and there's bad news. The good news is that lifting c25 doesn't do diddly. That's also the bad news. The readings are unchanged. Small flux is normal. Let's step back a minute asd see what we know now.

V1 plate and cathode voltages look good. Plate voltate is running about 30v high, which is consistent with the reading at #50. This suggests to me the problem is after V1.

V5 has some sort of problem I don't understand. Plate voltage is 38v high, consistent with the B+ rail. However, cathode voltage should be much closer to 8v. There are only a few parts here and we've bee over them. Bookmark this thought, as I think we need to come back to it. This is going to affect the signal to the finals. I think it's worth confirming the 470K bleed resistor is properly referenced to ground and is the correct value. Just trace that side of the connection for visual observation. Maybe reflow the solder.

V4 has some good news. On channel 1 (normal) cathode voltage is right on spec at 4.5v. Plate voltage is still way high at 257. I'd reconnect R14 and get new voltages here. I think the 330K load might bring down the voltage.

V4 on channel 2 has the same high plate voltage we've been seeing all along, and the low cathode voltage.

So, what's common to both halves of V4? The plate load resistors connected at #50. We are seeing appropriate voltage drop at V1 plates, so I think the B+ rail is functioning OK. Tubes were changed. I guess ( and this really is a guess) I don't like the 100K plate load resistors connected to #50. The strip code should be brown-black-yellow. Even if they look right, I think I would lift one end and meter them to be sure. BTW, if these turn out to be 100 ohm, 1k or 10k, that could easily explain it. (Don't be embarrassed if you did this -- we all do things like it; I know I have.)

I might have thought 94V is a misprint, except that in these positions on my GA20-RVT, the spec is 101 and 86, and, as best I recall, my amp is close to spec.

I'm quickly running out of ideas here. Let me sleep on it and see if I think of something else.

Meanwhile, we sure could use some help here. What would cause plate voltage above spec when the B+ is OK? There just isn't much in the neighborhood of the problem in the way of parts.

[Travst]

Ok, here's what I did... I got in and scrubbed the board some more. Then I checked every connection again and re-flowed several joints as well as trimming some leads that I thought might get bent to touch other components. New readings:

#13 162v
#14 3v
#31 175v
#32 0v -chopsticking resulted in some popping at this cup, perhaps I need to re-flow
#50 220v

V1 pin 6 59v
V1 pin 4 2.7v
V1 pin 7 58v
V1 pin 9 2.5v

V4 pin 1 178v
V4 pin 3 2v
V4 pin 6 210v
V4 pin 8 3.4v

V5 pin 1 263v
V5 pin 3 2.5v

I will continue tinkering with this tomorrow. I'm really inclined to pull the board and really clean it up and check the underside. I can see at least one spot where excess solder has accumulated beneath a cup.

[Firestorm]

Sorry I'm late. This thread is like reading War and Peace with all the editor's markups still in it!

Let me catch up a bit: we're using the EA 300 RVT schematic for consistency, right. That's fine, but we'll have some clarity issues if need to deal with components that only exist on the Apollo schematic (notch filter on the driver grid, for one).

What tubes are installed for testing? Apollo schem. shows V4 as 12AX7; EA 300 RVT has 12AU7. Driver tube before the phase inverter transformer is 12AU7, right? Can't see V5's cathode resistor (R42). It is 1K, right?

R14 and R30 are both reconnected at cup #13, right? They should probably should in order to match schematic voltages as much as possible. The 330K (390K per Apollo) and the 68K are there as a voltage divider to balance the outputs of the channels, but they also parallel the plate resistors.

As to V4: The voltage discrepancies are odd (especially the ever-changing voltage at cup #13). Both triodes are supplied from the same node and both have the same plate loads (and are equally affected by R14 and R30). In the EA300 schematic, they both have 3K3 cathode resistors. In the Apollo, V4B has 1K5 on the cathode. How is it set up now?

What in the world is going on at V1? We now have plate voltages down around 59V. Wasn't it closer to spec earlier?

[Travst]

Welcome to the party, Firestorm. I'll answer what I can, but I'm at work and not at the bench with the amp.

Let me catch up a bit: we're using the EA 300 RVT schematic for consistency, right. That's fine, but we'll have some clarity issues if need to deal with components that only exist on the Apollo schematic (notch filter on the driver grid, for one).

We are using the EA schematic for clarity, but I always check it against the GA schematic to note any differences, so I think we're consistent in that respect.

What tubes are installed for testing? Apollo schem. shows V4 as 12AX7; EA 300 RVT has 12AU7. Driver tube before the phase inverter transformer is 12AU7, right? Can't see V5's cathode resistor (R42). It is 1K, right?

I believe that the resistor value is correct, but I'll double-check tonight. I'll also check all the voltages again, as I was pretty worn out last night when I tested.

As to V4: The voltage discrepancies are odd (especially the ever-changing voltage at cup #13). Both triodes are supplied from the same node and both have the same plate loads (and are equally affected by R14 and R30). In the EA300 schematic, they both have 3K3 cathode resistors. In the Apollo, V4B has 1K5 on the cathode. How is it set up now?

I have a new set of Ruby 6L6GCs in the amp, along with known good preamp tubes. I double-checked every tube both on my Eico 625 and by trying other tubes in the position. I have the tubes in place per the chart on the GA schematic. I can't speak to the V5 cathode resistor since I don't have access to the amp.

R14 and R30 are both reconnected at cup #13, right? They should probably should in order to match schematic voltages as much as possible. The 330K (390K per Apollo) and the 68K are there as a voltage divider to balance the outputs of the channels, but they also parallel the plate resistors.

As to V4: The voltage discrepancies are odd (especially the ever-changing voltage at cup #13). Both triodes are supplied from the same node and both have the same plate loads (and are equally affected by R14 and R30). In the EA300 schematic, they both have 3K3 cathode resistors. In the Apollo, V4B has 1K5 on the cathode. How is it set up now?

I'll check this tonight as well.

[Phil_S]

Sorry I'm late. This thread is like reading War and Peace with all the editor's markups still in it!

Let me catch up a bit: we're using the EA 300 RVT schematic for consistency, right. That's fine, but we'll have some clarity issues if need to deal with components that only exist on the Apollo schematic (notch filter on the driver grid, for one).

Yes, I think I forced this into the thread because the Epi schematic is of better quality and easier to read. There are minor differences, as you observed.

What tubes are installed for testing? Apollo schem. shows V4 as 12AX7; EA 300 RVT has 12AU7. Driver tube before the phase inverter transformer is 12AU7, right? Can't see V5's cathode resistor (R42). It is 1K, right?

I missed the V4 tube difference. It's curious that both schematics call for same voltage with different tubes. I wonder, Gibson is famous for making the amp du jour -- whatever parts were on the shelf were used. Regardless of the schematic, what tube came with the amp, recognizing it's been passed around?

R14 and R30 are both reconnected at cup #13, right? They should probably should in order to match schematic voltages as much as possible. The 330K (390K per Apollo) and the 68K are there as a voltage divider to balance the outputs of the channels, but they also parallel the plate resistors.

I don't have with me the paper schematic with my notes, but IIRC, you are right about #13.

As to V4: The voltage discrepancies are odd (especially the ever-changing voltage at cup #13). Both triodes are supplied from the same node and both have the same plate loads (and are equally affected by R14 and R30). In the EA300 schematic, they both have 3K3 cathode resistors. In the Apollo, V4B has 1K5 on the cathode. How is it set up now?

Uh, huh!

What in the world is going on at V1? We now have plate voltages down around 59V. Wasn't it closer to spec earlier?

Wow, it was good before, can't figure what must have changed. I am guessing something is loose, weak solder, broken wire inside the insulation, unintended short/component contact -- something like that? Check both ends of plate supply wires for V1 and V4 and see where it breaks down.

Firestorm, thank you for joining the conversation. I will try to sit back for a while.

[Travst]

I missed the V4 tube difference. It's curious that both schematics call for same voltage with different tubes. I wonder, Gibson is famous for making the amp du jour -- whatever parts were on the shelf were used. Regardless of the schematic, what tube came with the amp, recognizing it's been passed around?

I don't remember the exact sequence that was in the amp when I got it. I do remember that one 12ax7 was swapped with a 12au7, but that shouldn't make a difference. I'm left wondering if someone tried to fire up the amp with 12ax7s in the 6eu7 positions. I know they are wired differently, but am unsure as to what the effects of such a mistake would be.

[Firestorm]

Firestorm, thank you for joining the conversation. I will try to sit back for a while.

Don't you dare! I hate these Gibson things. They built more models than anyone, changed them more often even than Leo, AND HARDLY EVER DOCUMENTED THE CHANGES I have that Gibson Master Service Manual thingy and it's more common NOT to find an amp listed than it is to find one.

[Firestorm]

I don't remember the exact sequence that was in the amp when I got it. I do remember that one 12ax7 was swapped with a 12au7, but that shouldn't make a difference. I'm left wondering if someone tried to fire up the amp with 12ax7s in the 6eu7 positions. I know they are wired differently, but am unsure as to what the effects of such a mistake would be.

I think putting a 12A*7 in a 6EU7 socket is pretty harmless, but putting a 6EU7 in a 12A*7 socket shorts the connections for plate2 to grid2 through the heater so that could be bad.

[Phil_S]

Firestorm, thank you for joining the conversation. I will try to sit back for a while.

Don't you dare! I hate these Gibson things. They built more models than anyone, changed them more often even than Leo, AND HARDLY EVER DOCUMENTED THE CHANGES I have that Gibson Master Service Manual thingy and it's more common NOT to find an amp listed than it is to find one.

Well, OK. This is a love hate thing. Love the challenge, hate the maker for the documentation failures. I'll pay close attention. I was thinking maybe a fresh voice and eyes would be beneficial.

[Travst]

Just don't leave me alone with this thing.

[Phil_S]

Just don't leave me alone with this thing.

Creepy feeling, is it?

[Travst]

Just don't leave me alone with this thing.

Creepy feeling, is it?

I just feel like it would become The Thing That Wouldn't Leave... if you remember the old Saturday Night Live skit.

[Firestorm]

Travst --

Get anywhere? Or has she beaten you into temporary submission?

[Travst]

Travst --

Get anywhere? Or has she beaten you into temporary submission?

I took the evening off. I've got to work up something for a Valentine's Day song for a gig, then I'm back to work on the Iron Lady.

[Travst]

Ok, I've lifted R14 again and taken CH1 out of the circuit. I'm back to the same voltages as before. I also replaced C13 on the GA schematic.