Low volume problem, Airline GIM 9011A

[Realist]

Greetings, This amp is a basic 3- 12AX7, and 2- 6L6, cathode bias amp head, with a low volume problem. My understanding of circuit logistics is my weakness here. I fortunately have two Airline amps built from this schematic. One loud and great sounding the other like my deluxe on 2. I also have an oscilloscope and all the necessary tools. I have determined that the problem is in the preamp, just cant figure out how or why? Thanks

[Jazz P Bass]

I would start with no signal basic Vdc tests.
Plate & cathode voltages will tell if a tube is conducting or not.
If all is well, send a signal through & see what is the maximum Vac signal you get at the 6L6 grids.

[Realist]

V1 Pin1=209, Pin3=1.91, Pin6=285, Pin8=2.86
V2 Pin1=202, Pin3=1.86, Pin6=285, Pin8=2.86
V3 Pin1=271, Pin3=20.4, Pin6=248, Pin8=20.4
VDC no signal. Not sure what I'm looking at here. These voltages are similar to the good amp btw.
Both of these amps have a Master Volume installed at the point where the preamp s exit into the phase inverter. In an attempt to isolate the problem I applied a sign wave to both amps. With the MV at 0 and all preamp controls to 10, the good amp showed apx ten times the signal strength than the amp in question. Thanks

[Jazz P Bass]

" the good amp showed apx ten times the signal strength than the amp in question"
Where was the test point?
At the MV control?
If you use your scope to probe each grid, you should be able to narrow down which circuit is at fault.

[Realist]

Yes that probe was at the MV feed.
I'll do that again. At the time wasn't sure where to look, or how to interpret what I was seeing.

OK Pin 2 has input signal, pin 7 amplified apx three times.

[Realist]

Oh well back to illogical testing. The phase inverter and output tubes are apparently good, I connected the preamp of my working Airline to the quiet Airlines PI input with equally loud results from both power amps. Just for grins I changed the tubes again with the amp running a sign wave. I was hoping for a burst of volume while I moved the tubes in their sockets. V1 was very quiet with the volume up and no signal. V2 had static and I was able to introduce a 60 cycle hum at that socket. I'll have a better look at it tomorrow. I also repaired two cold solder joints that I found while probing with a chop stick. I changed 6 resistors that were further than 5% towards open from spec, The voltages are much closer to spec now compared to my first run. I jumped across The caps in the preamp with a good Cap. "no results yet"

[Realist]

Greetings,
I can"t figure this out.
My amps low volume problem appears to be isolated in the preamp, but common to both channels. The mechanics are ok, tube sockets volume/tone pots etc. I ground and soldered all the riveted ground tabs to the chassis with big Bertha,etc..The tubes are functioning normally as far as I can tell (by comparison to my other amp) but at reduced amplification.
There is a problem with the supply voltage to the preamp. The voltages at each filter cap are about +10v over the schematic, with the exception of the last one which is approximately -10v under. Most of the resistors in that circuit had drifted open past 10% value. So, I changed the 22K feeding the voltage from the previous cap, and both 270k's feeding the tubes plates, and the 100k's in the channel mixer, as well as that filter cap. None of this had any effect on the low volume problem. The voltage remained -10 or so (under schematic) at the filter cap but approximately +5v (over schematic) at the plates on the other side of the 270k's.
Any ideas? Thanks

[ReadyTeddy]

You're in a good position to solve this problem because you have two identical amps. I would do this. Turn the MV pot all the way down so you won't kill your ears or speakers. Set the other pots associated with the preamp to some intermediate value. Inject a sine wave into the input of the good amp, and then look at each grid and plate in turn. Check with the scope on AC so you can focus in on the sinewave, and DC so you can see the whole signal. Write down the voltages you see on each grid and plate in the good amp, and then do the same experiments on the bad amp. Somewhere you will see a difference that will allow you to focus your attention to one spot. Once you do that, you should be able to resolve the problem fairly quickly.

[Realist]

Thanks, The editor wouldn't allow me to attach a photo of the schematic with my post , but you can see it here; http://music-electronics-forum.com/t5707/ Scroll to the bottom thread.

[Jazz P Bass]

Here are the files.