Gibson GA45-RVT low power

[1WoodHippie]

Well, when I inject an Ipod full volume into the input i get a nice loud output from the speakers. But when I put a guitar through it I hardly get ANY signal even at 5 on the vol knob. You have to turn it up to 10 to really get any kind of good audible signal with guitar, so that's why i did it that way.

So I guess I'm confused now. Where do I go from here?

thanks again for everyone's advice. This has just got me turned in circles! Driving me nuts!

[The Dude]

This may sound like a really stupid question, but where are your tone controls set?

[Enzo]

I was just looking for relative volume. If your ipod is ten times hotter than the guitar, then sure so will be the amp output. A couple resistors or a pot across that ipod signal will knock it down to the same level as a guitar signal. Or just keep having your friend play, nothing wrong with that, just not how I do it. I am just wondering how large is that signal at the output tube plates. Merely sounding strong on the tracer is not enough, I want numbers.

[The Dude]

This is a guitar amp, but any old music is good enough to troubleshoot. I use a feed from my cheap little shop stereo receiver tuned to a music station. No need to tie up yours or anyone else's hands playing guitar.

I do a similar thing. I use my soundcard out of the PC for testing. It's easy to adjust levels for different types of inputs and I can run internet radio, guitar samples, or change to test tones that I've downloaded at the click of a mouse.

[Chuck H]

replacing the PT and Filters are all that "I" did. That's not to say that the guy who had it before me that did all that horrible wiring and stuff didn't screw it up.

But you said:

I put in a new PT and changed all the filters. Now both channels have hardly any power.

Then Jazz asked:

Did the amp work before you changed out the PT & caps?

And you replied:

YEs It Did...

I'm not trying to be a dick. I just think we need to stay on track with the facts. It's most likely something you did that made the amp not work. To imply that we're chasing another persons mistake that made the amp not work is misleading and directs us to the entire amp. I think we should start by looking into anything you may have changed. Still not trying to be a dick, honest. Any grounds you lifted, tubes you removed or leads you may have lifted and put back to access stuff is more likely to be where the problem is than any part of the circuit you didn't work on. Even if the other guy did make mistakes they hadn't rendered the amp broken at the time it was put into your hands. So it's VERY unlikely that any of the other guys mistakes are causing the problem now.

A guy walks into the living room and see's his wife looking around and under things:
Husband - Did you lose something?
Wife - Yes. My keys.
Husband - Where did you lose them?
Wife - In the kitchen.
Husband - Then why are you looking for them in the living room?
Wife - Because the light is better.

[1WoodHippie]

how can i get ya the numbers? I've got a scope and and a function generator. How big of a signal should I put through the input. I can measure the outputon the tubes with the scope.

(bear with me. i'm learning here)

[Jazz P Bass]

Attach an 8 ohm dummy load to the output & hook up your scope to it.

Insert a generator signal with an amplitude of 100 mvs, at 1K frequency.

Turn all of the controls full on & measure the signal at the output tube grids.
Use a volt meter set to read volts ac & measure that signal.
It should be huge. (roughly 40 Vac)

The output should read 20 Vac into an 8 ohm load.

[nickb]

.....<snip>.. somebody had added extra filter caps. <snip>.... So I put in a new PT and changed all the filters....

Is there any chance that one or more of the removed caps was actually in the signal path, perhaps a mod and the removal is the reason for the volume and tone change? Did you keep these caps? Can you remember where they went?

Here's a suggestion: I always take a photo of anything on my bench before I start.

[1WoodHippie]

Ok, think i got this all set up right.... here's what I measured with a 1k freq with 100mvs peak to peak:

With all all controls up and 8ohm dummy load... Output tubes pin3: 170VAC Pin 5: 55VAC from chasis ground to tip of speaker hot connection 18VAC.

[g1]

100mv peak to peak is about 35mvAC at the input. 18VAC into 8 ohm load is about 40 watts. Not sure if that is far off for this model, doesn't seem to be too low power wise.
How does the output signal look on the scope, clean sine wave (similar shape to the input waveform) or is it clipped at the top or bottom?

[1WoodHippie]

now that you mentioned it should be 35mvAC on the input it made me go back and recheck again. I was getting 100mvAC with the multimeter on the input, not 35mvAC. I went back and rechecked my signal and it reads 100mv pk to pk on the scope and 35mv with the meter on the input now. And of course my voltages changed:

Pin3: 69vAC Pin5: 24vAC Output is 8vAC The sine wave looks great on the output. no clipping.

(sorry for the inaccurate reading before)

[Jazz P Bass]

I don't know.
If I measure my signal generator output with a meter (Fluke) set to read Vac & it reads 100Mvs, then that is what it is.
Pk to Pk is a scope term.

It sounds to me as if the amp is functioning properly.
100 mv in, 18 Vac out.
40 watts.

If in doubt, hook up a meter to the dummy load & bang a few chords with a guitar.(all controls full on.)
See if you get 18Vac.

If not, then measure the guitar signal.
Banging away should easily measure 100 mv.

[Chuck H]

Considering these new developments, why does/did the amp have almost no output? Have we been trying to fix a working amp?

[drewl]

Turn the "presence" switches up (off).
All they are is a .01uf cap on the input jack, cutting and muffling the guitar signal alot.
must be for mellow jazz mode