The UTC S-15 is the output transformer .
http://www.bunkerofdoom.com/xfm/UTC_1955_WEB/031.jpg

Measure that wirewound resistor, blue-green-??? and report the value. The third paint dot could be missing or it could be black. It appears to be connected from pins 8 of the two output tube sockets to ground. Pin 8 is the cathode on most common output tubes. But......6N6's do not use cathode resistors since they are supposed to be internally self-biasing. This tells me one of two things. Either someone could have been playing with it in the past and added that resistor for some reason, or the unit is supposed to have different output tubes like 6F6's, and someone was either experimenting with the 6N6's or put them in there to fill empty sockets.

The difference in the connections for the two 250K resistors is probably to create a voltage divider on the one socket, which feeds the phase inverter with a reduced signal level. This was a fairly common way of making that connection. The one socket has a 20K from pin 6 to ground and the 250K from pin 5 to pin 6, which would give roughly a 10:1 ratio.

Also, measure the voltage on pins 3 and 4 of the output tube sockets so we have some idea of how much B+ this unit is providing. That may give another clue whether the 6N6's are supposed to be in there or not.

I can't see well on my phone but it looks like they have wires connected to common, 500 Ohms (headphones?) and 8 Ohms.
Agreed about 6N6s seemng a little unorthodox.

I'm also beginning to think that perhaps the 6N6s shouldn't be there. I would think that with 6N6s you wouldn't need the 6SN7.

-David

You would still need a phase inverter with the 6N6's, the first triode in those tubes is used strictly as a direct coupled driver to the output triode. To the rest of the circuitry in the amplifier they look like a single tube.

I also have questions after looking up the specs of the UTC S15 output transformer, which is only rated for 12 watts maximum. While a pair of 6N6's would probably not put out any more than that, it seems more suited to types like 6V6's or 6F6's. It would not be used with 6L6's.

I begin to wonder if someone has previously or recently highly modified the amplifier from the way it was originally built?

360 VDC on pin 3 and 4.0 VAC on pin 4.
Resistor with blue green dot reads 66 Ohms,
Know what, I would really love to see a schematic ! !
I tested on a couple of speakers that all play well on another small Amp I have with same result and just for fun I went and tried them on there Ohm Tappings with same results.
More later, Have to leave for that work thing now.
Peter.

If you have not done so make absolutely sure that every resistor is in tolerance. An out of tolerance resistor in the right spot in the circuit can cause the amp to not sound good.

Something's drastically wrong.

Max plate voltage for both sections of a 6N6 is 300 per the GE tube manual. That reading of 360 volts on pin 3 would cause premature tube failure at the very least. Pin 4 also needs to have a considerable positive DC voltage for it to work. Usually pin 4 on a 6N6 is connected to B+.

66Ω does not seem like a correct value for a cathode resistor and 6N6's don't usually have a cathode resistor.

NEW VOLT MEASUREMENTS.
OK, 6N6 closest to 5V4,
Pin
1, 0V
2, 0V
3, 346 DC
4, 352 DC
5, 5.3 AC
6, 4 AC
7, 6.3 AC
8, 6 DC
Next 6N6 to left
1, 0 V
2, 0V
3, 346 DC
4, 352 DC
5, 0 V
6, 0 V
7, 6.3 AC
8, 6 DC
I hope this helps,
All resistors check really on the money.
I double checked the left 6N6 pins 5 + 6 for a third time and both pins are at 0 V.
Where does this leave us?
Sorry I am slow at this,
I can usually get everything working well when I have a schematic.
Thanks for your patience.
EDIT, These readings are with a good Multi meter to chassis ground,
Peter

Pin 5 should be 0 volts, and there is no connection inside the tube on pin 6.

The one which has an AC voltage on those pins is the problem. Have to figure out why there is an AC voltage on the grid of the tube.

Other side of 6 is a .1 cap going to pin 5 of 6SN7.
So the resistor that is in this pic across the base going from pin 1+2 to pin 5 then off to pin 5 of the 6SN7 may be an issue,
Do I need to move or remove that resistor?
Both pins 1+2 on this 6N6 go to power transformer.

Pins 1 and 2 appear to be chassis ground.

I don't think the resistor is related to the problem, but something that is connected to pin 5 could be feeding AC to that point.

I would pull that 6N6 leaving the other one in place and measure pins 5 and 6 on the socket again. Since pin 6 appears to have nothing connected to it, this isn't making any sense at all why there should be a voltage reading there. If you still get the same or a higher AC voltage there without the tube in the socket then we are getting closer to the problem.

With no tube, Pin 5 reads 0, But pin 6 reads 4 AC, I went to the line in to pin 6 and found same reading no matter which line I hooked to being either - or +,,,
So it appears that I am getting stray voltage somewhere?
What can I try next ?
Peter

I took the liberty of adding numbers to a few of the tube pins so everyone can follow clearly. Let's make sure we are all on the same page.

Is this the socket which you are measuring the unwanted AC voltage at pins 5 and 6?

Yes, ANd just to be real clear, I went from line 1, and line 2, Being + and - from the wall outlet where it connects to switch and transformer to pin 6 and get 4 VAC no matter if I try - or + 110 VAC
I also just for fun, Closed my eyes and plugged in a set of good 6V6s and what do you know, Same grainy sound, Plugged the same speaker back into another Amp using the same 6V6s and same MP3 and nice smooth sound,
I keep thinking it's the 6N6s, But with the same distortion with a set of 6V6s it has to be something other than those 6N6 tubes.
Like I said before, I am not the best with this stuff yet, Most of my test stuff is 300 miles away and wont see it for a couple weeks and this Amp is about ready to visit the shelf of doom
But I think with your help it may avoid such a place,
I can work on drawing an entire diagram but it will take a bit,
Where-O-where can I find a diagram?
On the label it states "Licensed by Western Electric" Maybe that is a good clue,?,
I will keep looking for the obvious.
Peter

You cannot directly sub 6V6's in there without increasing the value of the cathode resistor, they will not be biased correctly. The B+ voltage is also too high for that tube.

what kind of speakers ? are they matching output impedance ? output transformer matching tube impedance ? troubleshooting over the air is a guessing game

If you are using a DMM you might get some voltage reading on a tube socket pin that is not connected to anything due to the high impedance of the DMM.

Speaker is in an old Ampro speaker which is 8 Ohm,
I know the 6V6 is NOT an acceptable sub, But it still worked for minute and still had the same grainy sound, I had to try something to try and rule out the simple thing, (Tube)
Peter