Gibson GA-95 RVT

[Phil_S]

It's my belief that a 12AU7 belongs in V5. My basis for this conclusion is the two known iterations of the GA20-RVT of the same era. The GA20-RVT and the GA95 differ in only minor and insignificant ways all the way from the input jacks to the interstage transformer. The only meaningful difference is the power amp section. For the GA20-RVT, both the published schematic and the one inside my amp, both use a 12AU7 for V5.

The main difference between the two GA20-RVT's is V4. The published schematic uses a 6EU7 (functional equivalent of the 12AX7); mine uses a 12AU7 like the GA95.

I wasn't paying attention until just now. One big difference between the GA95 and the EA300RVT is V4. It is the same difference as the two GA20-RVT's -- nearly an identical difference. Where the GA95 schematic seems to fall apart is that the given V4 voltages are what we'd expect for a 12AU7, but the cathode resistors (which set bias for the tube) are identical the the GA20-RVT version that uses the 6EU7 in V4.

I am sorry for this long-winded post and the late observation. After looking at all 4 schematics on the table at once, my gut reaction is to put a 12AX7 in V4 like the GA95 schematic says and ignore the voltages. Its voltages and cathode resistors are begging for the 12AX7. This will step up the signal hitting V5. Let's just do it and see if that is the bottleneck.

BTW, the voltages for V4 (6EU7) given on the GA20-RVT:
p4, 1.7
p6, 175

p9, 1.2
p7, 150

This really does resemble what is being reported here and I think it supports my conclusion that the voltages printed on the schematic are simply wrong. With the 12AX7 nominally expected to give us 4-5x the gain of a 12AU7, I think that we NEED the 12AX7 in V4. Firestorm managed to cover this in about 10 words about an hour ago.

I"m attaching a very good clear copy of the GA20-RVT schematic that is the basis for saying V4 needs to be a 12AX7.

[Travst]

I changed V2 to one of the new 12au7s. After that, I measured the voltages in the chart below. I'll pick up in a bit with checking the board as you described.

Phil, it's definitely not as loud and full with the 12au7 in V4.

[Phil_S]

John,
I know this may be testing your patience. We will do some things over and over. That is the nature of such a tough problem and limited test equipment.

I recommend putting the 12AX7 in V4 and reporting a new set of voltages.

A few errors are noted in your voltage chart for the schematic values:
V4, p 6 should be 94 not 34
V4 p 8 is 4.5; you show blank
V5, p3 should be 8.2 not 1.2

I'm thinking your tube readings all look remarkably close to the schematic. I think with a 12Ax7 in V4, we can expect plate voltage to rise and cathode voltage to fall. Given the installed cathode resistors, I think we will see a marked improvement, as you reported you heard.

Assuming V4 really needs the 12AX7, and voltages now being within expectations, the amp appears to be operating well. The mystery is, where is the signal getting lost?

I see no downside in making a tester to probe the preamp as Firestorm suggests. You may get something useful out of it.

I am wondering about a bad ground or wiring on the output transformer/speaker jack/speaker. The Apollo schematic shows stuff there and I suggest that you simplify it to eliminate some of the possible output diversion. I'd disconnect the external speaker jack and C17 (Apollo schematic). Make sure the sleeve ground is in place -- this may be done at the OT or at the jack -- don't take the schematic literally.

Test the speakers for DCR. A 4 ohm load should read about 3 ohms tip to sleeve on the plug. This will be far from exact.

[Travst]

New voltage chart with 12ax7 in V4.

Disconnecting the external jack had no effect.

[Phil_S]

Those voltages all look reasonable to me. This is a real stumper. Time to trace the signal as Firestorm suggested.

[Firestorm]

Considering the wacky voltages we were seeing along the way in V4, I was curious to see what a proper 12AU7 would do there. Indeed very close to schematic values (given the higher overall B+). With the existing schematic values for Rk and Rp, the difference between using a 12AX7 and a 12AU7 in V4 will only be about 4dB. Noticeable, but not enough to account for the low output. Signal is being lost (or cancelled) somewhere. Just need to find out where.

[Travst]

I'll work on finding the spot. So far, all the out-of-spec components and other issues that we've identified have improved the amp considerably. It's much louder (10-15 watts loud now) and sounds much better. I also took the liberty of removing the second notch filter.

[Travst]

I rigged up the .047 cap and hooked it up to a speaker while using a CD player for input. I can hear the CD when I touch V1-6 and very slightly at V5-1. The signal is also heard at R19, which is connected to V1-6, so that makes sense. After probing all around with this rig, I'm simply not getting a good connection with the prob wire, or the signal's not getting through. The volume, even when heard, is very low.

[Firestorm]

The volume will be low using a speaker. Some people use a little amp (like those pocket-size battery powered ones). Volume all the way up? Tone controls up too? You said you can hear it at V1-6. The tone controls aren't especially lossy (by the looks of the circuit), so I'd guess at V4 the signal should be significantly stronger. 15 or 20dB maybe. So you should be able to tell even through the small speaker. Is the cathode resistor at V4 bypassed?

A couple of things to try: the signal should sound as loud on both sides of C11 as it does at V1-6. With the volume pot all the way up, it should as loud on the wiper as it does on the pot input. At C25, it should be much louder.

Maybe try the same thing on the other channel.

[martin manning]

For the normal channel you want to probe V1 7, and V4 6. For the Reverb channel you should probe V1 6 and V4 1. The two channels are mixed through R15 and R46 into C11, and you should see signal from either channel at V5 1. I would think V5 1 should be pretty loud, since this is essentially the driving voltage for the power stage. Don't forget that your probe is loading the circuit quite heavily unless you feed it into a high impedance like the input of another amp.

If you can feed a tone into the input of the Gibson, then you can test points along the signal path with your DMM and get a quantitative measurement, without loading the circuit.

[Firestorm]

Martin --

There isn't a published schematic for this exact amp. The "Apollo" schematic posted at the beginning of this thread is close, but even closer seems to be the Epiphone EA-300RVT which is linked on page 4 of the thread. We've been using the the 300 for component references here.

John --

Martin's reference to R15 and R46 (I think he actually means R44 -- the schematic is awfully hard to read) would be R14 and R30 on the Epiphone schematic. C11 on the Apollo schematic is C25 on the Epi.

Martin noted some computer-based tone generators. If it's difficult to connect your PC to the amp, maybe you could find a way to burn a CD with 15 minutes of straight 1KHz tone to use as the test signal. Then, instead of probing with a speaker, you could use your meter since the input signal would have consistent amplitude. You could still use the speaker method to locate test points since the circuit board looks like -- in Phil's terminology -- a rat's nest.

[martin manning]

OK, yes it's R44 on the Apollo scheme. Not trying to confuse things here, hope that's not the case... Rat's nest indeed!

Wow, I see that 6EU7's are getting pretty pricey. It might be tempting to rewire those two sockets for 12AX7's at some point...

[Travst]

Tom (Structo) sent me a program that should allow me to burn the signal to disc and use the CD player I have set up to provide input to the amp. Thanks for the good instructions. I have been getting a signal as you say... at V1-6 for the effects channel and V1-7 for the normal channel. I'll try again once I have the tone generator set up, but I haven't been picking up sound from V4 at all.

My new Honeytone amp for the bench lasted only a few seconds with the probe before going toes up. Any idea what's wrong there?

[Phil_S]

Tom (Structo) sent me a program that should allow me to burn the signal to disc and use the CD player I have set up to provide input to the amp. Thanks for the good instructions. I have been getting a signal as you say... at V1-6 for the effects channel and V1-7 for the normal channel. I'll try again once I have the tone generator set up, but I haven't been picking up sound from V4 at all.

My new Honeytone amp for the bench lasted only a few seconds with the probe before going toes up. Any idea what's wrong there?

Can't help on the Honeytone

If you aren't having luck with the homemade tracer, you might try employing a jumper. Find the coupling cap after V1 (Epi C4/C11). Attach the jumper to the V4 side of that cap. Use the jumper to bypass the tone and volume controls for the channel. I think I'd work backwards. Run the jumper to the grid of V4. This bypasses all the control knobs and essentially makes the amp wide open. If you get 100W type volume from doing this, well, then, you know the problem lies between the two jumper points. Start working backwards until you experience the low volume. On channel 1, Epi schematic, some suggestions on where to attach the jumper:
Where R8 meets the outer leg of the volume pot.
The other side or R8.
The wiper of R7 (Bass pot)

[martin manning]

My new Honeytone amp for the bench lasted only a few seconds with the probe before going toes up. Any idea what's wrong there?

Too much voltage for a solid state amp! You might have another project now... a quick look shows a TL072 Op-Amp at its input. The good news is they are less than a buck.