Bottlehead Forum
Bottlehead Kits => Legacy Kit Products => Stereomour => Topic started by: Tom-Huffman on June 03, 2013, 10:12:47 AM
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Resistance checks were right on except for 2 and 14 which were 13K and may be OK
Start - Finish AC voltage = 167 v
A1 to A4 = 2.5 v
C1 to C4 = 2.5 v
2A3s to not glow
Diodes oriented correctly
LEDs oriented correctly but no light
Transistors oriented correctly
I have confirmed to 12AT7 wiring to the PCB above it
HV+ = 449
Terminal voltages
1 = 440 v
2 = 175 v
3 = 0
4 = 440
5,6,7,8,9,10,11 = 0
12 = 440
14 = 167
15 = 440
16 = 0
17 = 440
18 = 0
19 = 440
20 = 0
A1 = 0
A2 = 445
A3 = 445
A4 = 0
C1 = 0
C2 = 445
C3 = 445
C4 = 0
1A = 445
1B = 445
OA = 174
OB = 166
Kreg both = 2.5
-reg both = 0
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Tom,
This is just a hunch. Did you check your 4 pin tube sockets to see that the larger holes are facing the back of the chassis (closer to the hum pot)? It appears that you are not getting any cathode current on the 2A3's. Reversing the tubes could cause this. This would also explain why no glow. Your filament pins are not connected to the filament windings.
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Knowledge plus a hunch = excellence
Everything is in line but two pair -
Terminal 2 and 14 = 167 v
OA, OB = 167
A few zeros are reading 15 to 18 mv
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Glad to hear it is working now. I don't think the 167V on the 12AT7 anodes is a problem. That number will be tube dependent, and will vary a lot. The design is tolerant to this and it will not affect the performance. I am sure someone can chime in with a range of acceptable values. Also, terminals 2 and 14 will not match exactly unless you are very lucky and get a tube where both triodes inside the 12AT7 are perfectly matched.
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167V is an unusually strong 12AT7, certainly no problem there.
-PB
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One last check before I check to see if it makes music.
Left hum pot .096 to .071 with .000 mv in between.
Right hum pot .110 to .067 with .000 mv in between.
Is this normal?
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Please check my previous post as well about the hum pot measurements.
I went ahead and hooked up the system with minimum components - Stereomour, DAC, CD transport and speakers. Result? No sound at all.
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Tom,
It appears that something is not right in the signal path. What voltage did you get on terminals 5 and 11? I am just checking to see if your output transformers are hooked up correctly. Also check to make sure the coupling cap between terminals 1 and 5 and between 11 and 15 are in place. The funny thing is that you seem to be getting hum on the speaker terminals as evidenced by your humpot measurements but no sound. I would expect much more hum than .1mV max though. I would expect the humpot measurements to get has high as 10 to 100 mV. The next thing to look at is to see that your input is wired correctly. Trace your inputs through the selector switch and the volume pot to make sure there is a connection from the RCA jacks to the output of the volume pot. If that is OK check the other coupling capacitor between terminals 2 and 4 and between 12 and 14. Keep us posted on your progress. You could also post a picture of the underside of your build. Perhaps something will pop out.
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5 = 58 v
11 = 60 v
I am starting the signal tracing now from input to volume pot
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I continuity checked each input to the selector switch and then to the volume pot. All are correct. I will remeasure all voltages and send to you.
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Tom,
This is a little baffling. Your cathode voltages are good and the output transformers are connected. I looked at your build and I did not see anything obvious. I noticed you are using a 16 ohm secondary wiring for your output transformers. I also assume you have the proper source selected when you tested for sound (switch position) and the volume was not at the lowest level. I drew up the schematic you had before you fixed your tube sockets and in your old configuration you would have had the full 445 volts between the grid and cathode of the 2A3's with 249K ohm current limiting resistor. This can damage the tubes but since you are getting the proper current now I assume the tubes are functional. I will wait to see all the DC voltages to see if that sheds any light on the situation.
Randy
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As previously posted, all input to selector switch and then to volume pot checked out
Here are the new voltages.
hv+ = 400
1 = 370
2 = 176
3 = 0
4 = 12 mv ??
5 = 58
6 = 0
7 = 0
8 = 0
9 = 0
10 = 0
11 = 60.8
12 = 13 mv ??
13 = 0
14 = 167
15 = 369
16 = 58
17 = 401
18 = 0
19 = 400
20 = 61
A1 = 58.8
A2 = 372
A3 = 18 MV
A4 = 58.8
C1 = 61.5
C2 = 369
C3 = 20 MV
C4 = 61.5
IA = 400
1B = 400
OA = 176.5
OB = 167.7
Kreg = 2.5
-reg right = 0
-reg left = 0
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Tom,
All the voltages look reasonable. The 400V is a little high but not too far out. Everything else is right in line with where it should be. I noticed that your volume pot was all the way down when you made these measurements. I re-looked at the way you had it hooked up before and there was only 1200 ohms limiting the grid current on your tubes. I am starting to wonder if you blew a grid connection inside the tubes. Do you have another pair of 2A3's to try? Perhaps PB or Paul or Doc can comment on the possibility of damaging the tubes.
Randy
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Are your speaker cables making good contact with the metal of the binding posts?
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No extra tubes. This is my first 2A3 project.
Speaker connections are via Kimber banana plugs. However, I did have trouble getting my interconnects to seat properly because they were so tight. I will try a cheap pair.
Question: should I be able to see the 2A3s glow? I have never seen that but they do get hot.
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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.
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And they are not real bright either like the 12AT7, kinda dull cherry red.
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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
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Interconnects fully seated, no 2A3 tube glow and no sound.
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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.
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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.
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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)
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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.
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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.
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"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.
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I switched to my Bottlehead interconnects.
No sound, no hum, no glowing 2A3s.
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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.
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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
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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.
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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!
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Good job, so far, sounds like you are on your way to sorting it out.
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Now we know you are getting signal to pin 5. Check your output transformer connections. I was looking at the picture you posted and can't tell for sure but you might have switched a couple of terminals. The resolution of the picture is not quite good enough. I think you are getting close!