Resistance cks OK voltage descrepancies

[rlyach]

The plate from the 2A3 hides most of the heater/cathode wires so you will only see a glow if you look at the top straight down inside the tube.

[Mike B]

And they are not real bright either like the 12AT7, kinda dull cherry red.

[Paul Birkeland]

The 2A3's are operating just fine, as are the 12AT7's.

Try turning the hum pots all the way clockwise, this should produce audible hum in your speakers.

If you have hum, let us know (we can work backwards through the circuit from the output).

Also, turn the volume control all the way up, then measure the resistance between each red wire on the volume control and ground.  They should all be about the same, but if there's a short from the foil, drain wires, rca jacks, etc. in the input wiring, it will give you a 0.

-PB

[Tom-Huffman]

Interconnects fully seated, no 2A3 tube glow and no sound.

[Mike B]

If the 2 and a half volts are on the filaments, and there is no glow, I suspect the filaments got blown by the 400 volts.

Check the heaters for continuity.

[Doc B.]

Recheck the DC voltage at A1 and B1. If you have 58-60 V at A1/A4 and B1/B4 the 2A3s are working. It is difficult to see the filament glow unless the room is very dark.

[Mike B]

Bedroom in daylight, curtains closed, no lights on.

(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Ffarm8.staticflickr.com%2F7398%2F8918023440_afca55b8de_o.jpg&hash=c5f809ff399434e6c2bbc5932d52fc770662ac26)

[rlyach]

Check the ac voltages on A1 to A4 and C1 to C4 as well. Lets just be thorough. Since your cathode voltages are right the tubes must be working but it is good to check.

[Doc B.]

OK, we can see that the tube filaments are all working. Try turning the amp off, let it cool. Then turn it on while connected to the speakers and see if you hear any kind of startup sound, like a slight hum that fades away.

[Paul Joppa]

"Left hum pot .096 to .071 with .000 mv in between.
Right hum pot .110 to .067 with .000 mv in between."

If this is AC volts (not millivolts) at the speaker connectors, at various positions of the hum pot, then it is normal and indicates that the 2A3 and output transformer are working correctly. In that case, you should hear hum from the speakers, especially if you set the hum pot to either end of its rotation. If you don't then there must be something wrong with the speaker wires, connectors, or the speakers themselves.

Here I am I'm going back to the signal tracing that PB described, working from the output back. The voltages tell us the tubes are working, and the hum measurements (assuming I understood the post correctly) indicate that at least the 2A3 and output circuit is working and making hum on the output connectors. You should be able to hear that.

[Tom-Huffman]

I switched to my Bottlehead interconnects.

No sound, no hum, no glowing 2A3s.

[Mike B]

Remove one of the 2A3's and check resistance between the 2 fat pins.  It should be very low.

The fat pins to either of the skinny pins should be open circuit.

[Paul Birkeland]

I switched to my Bottlehead interconnects.

No sound, no hum, no glowing 2A3s.

Ok, so with the hum pots twisted all the way clockwise, you get no hum through the speakers?

If so, flip the amp over and power it on.  Then measure the AC voltage between the chassis plate at terminal 5, then repeat for terminal 11.

I have a sneaking suspicion that you need to look over the wiring of your output transformers very, very carefully. 

-PB

[Tom-Huffman]

I thought I replied earlier but guess I did not.

Speaker cables are secure.  Switched to my Bottlehead interconnects to make sure that was not the problem.
#1 RCA pair; selector switch is in first position although I tried all three.

Volume pot was tried minimum to maximum position.

No sound, no hum, no glowing 2A3s.

[Tom-Huffman]

Here are the latest measurements.
Vol pot clockwise - all four red wires to ground = 94K
A1/A4 DC = 60 v
C1/C4 DC = 64 v
A1 to A4 AC = 2.5v
C1 to C4 AC = 2.5v
Hum pot ranges are in line.  I checked with a different meter an probably did not realize I was reading volts instead of mv
I checked one 2A3.  fat pin to fat pin = 2 ohms, fat to small = 0 ohms both pair
Terminal 5 = 60 v AC; 11 = 63 v AC

Next steps -
1. I will carefully check the output transformer wiring.
2. I will hook it up and measure the output signals
3. Check for glow from above in a dark room
4. I will attach different speakers.  Originals were fine before but random chance is a cruel master.

Thanks for all your help!