Express fires up but no sound (not even hum)..help!

[fzfwyv]

A little frustrated because I have my Express built but no sound whatsoever.

Any help to assist me in finding the trouble I would be grateful.

Heaters are OK (voltages).

Not even sure where else I should start measuring voltages.

Should I be able to get a reading from my OT primaries? I get .422 when I measure using the DC voltage (red probe to the blue wire, black to ground)

.508 DC reading from power tube pins 4 & 6 to ground

Please help me troubleshoot this. I so want to hear a sound from it today!

Thanks,

Rich

[LeftyStrat]

Start from the PT and work forward. You can set your meter to read AC and measure the voltage where the PT connects to the diodes. Measure between these points and ground. You should see something like 300 volts. Next set your meter to DC and measure the DC at the output of the diodes (referenced to ground). You should see something like 425.

If you've got this far and everything is in the ballpark, grab a voltage chart from here:

http://www.maximoaudio.com/forum/viewto ... tage+chart

V1A PL 151.3 151.8
V1A CA 2.22 2.33

V1A is the first tube first triode, PL is plate. For a 12ax7 plates are pins 1 and 6.

CA stands for cathode, pins 3 and 8. All mesaurements made with respect to ground.

Some required reading to help you out:

http://www.drifteramps.com/safety.html

http://www.paulrubyamplification.com/in ... rstPowerUp

[fzfwyv]

Start from the PT and work forward. You can set your meter to read AC and measure the voltage where the PT connects to the diodes. Measure between these points and ground. You should see something like 300 volts.

Yes, this is correct on my build = 300 Volts


Next set your meter to DC and measure the DC at the output of the diodes (referenced to ground). You should see something like 425.

At the diode output it reads 408

If you've got this far and everything is in the ballpark, grab a voltage chart from here:

http://www.maximoaudio.com/forum/viewto ... tage+chart

V1A PL 151.3 151.8
V1A CA 2.22 2.33

May have a problem here> My readings:

V1A PL -.501 -.498
V1A CA .007 .193


V1A is the first tube first triode, PL is plate. For a 12ax7 plates are pins 1 and 6.

CA stands for cathode, pins 3 and 8. All mesaurements made with respect to ground.

Some required reading to help you out:

http://www.drifteramps.com/safety.html

http://www.paulrubyamplification.com/in ... rstPowerUp

Thanks for the reading. I will be reading that stuff now.

I'm still open to any suggestions.

Thanks,

Rich

[slajeune]

Hi Rich,

ok so the readings are off on the plates (and cathodes). Time to back track. Can you take the reading off of the filter caps? This will give you the various b+ (b+1, b+2, etc). We will able to know where the drop in voltage comes from.

Thanks,
Stephane.

[LeftyStrat]

Also give us the readings for all of the tubes. For the preamp tubes, 1,6, 3, 8. For the power tubes, 3, 4, and 5.

Also, with the amp off check the resistance from each cathode 3 and 8 on each preamp tube to chassis ground. If you have a bad ground it will be open, otherwise it should be the value of the cathode resistors (1k5, 2k7, 10k).

Typically when debugging, you either want to "follow the power" or "follow the signal"

"Follow the power" means working from the power supply out identifying voltages until you find a bad voltage, find the error, fix it, and work your way on out.

"Follow the signal" means using a signal tracer, which is nothing more than a small amp with a cable adapter (1/4 phone plug on one end, alligator clips on the other). You start at the input: is signal present at the input jack? is signal present at pin 7 v1? etc. Just make sure you clip on points where there is no DC component, meaning after each coupling cap (guaranteed if clipping to pins 2 and 7),

[Legin]

Did you make the board yourself? If so did you remember to do the underboard wiring? B+4 and B+5 etc
Nigel

[fzfwyv]

Hi everyone. Thanks so much so far.

Most of the amp was built by Nik of Ceriatone.

Here's some voltages...more to come:

B+1 = 408
B+2 = 403
B+3 = 308
B+4 = 286
B+5 = 267

Power Tubes (both readings are about the same for both tubes)
p1 = 0
p2 = 3.0 (AC to ground)
p3 = 410
p4 = 404
p5 = -158
p6 = 404
p7 = 2.9
p8 = 0

Preamp tubes are still not right...getting voltages now...

V3 (closest to the power tube)

p1 = 181.4
p2 = 12.7
p3 = 36.3
p4 = 2.9
p5 = 2.9
p6 = 193.6
p7 = 11.3
p8 = 36.3
p9 = 2.9

-Rich

[LeftyStrat]

Keep them coming....

[rhinson]

hello there, if you really do read -158 on pin 5 of your power tubes then your problem is in the bias circuit. at -158v, the tube is drawing zero current and is turned off so you'll get no sound. double check your wiring and resistor values for the bias circuit. i don't know what style bias circuit you are using but make sure you have either your last resistor or pot connected to ground in some fashion---sounds like maybe you forgot a ground connection or (for instance) put in a 220 ohm resistor instead of a 220k resistor as the first resistor in the bias circuit. anyway, double check that bias circuit. rh

[Allynmey]

fzfwyv, take a picture of your bias board. Therein probably lies the problem. If you can get good detail so we can e the color bands.

Allynmey

[slajeune]

As a recap, I think that Rich has 2 problems:

1- first preamp tube has wrong voltages (first two preamp stages)
2- power tubes aren't biased properly

For 1, Rich can you check that there is a wire going from b+5 to the 100k plate resistors as Legin suggested? If you do have a wire from b+5 to the first 2 preamp stages, verify that you have the right values for plate / cathode resistors for these 2 stages.

For 2, I will let other people help you out has I haven't built a push-pull amp yet. Did you adjust the pot/trimpot to obtain -33vdc as per the schematic?

Thanks,
Stephane.

[fzfwyv]

Hi guys,

Well, I blew through all my fuses so I have to run out and get some more.

Before that, had a bad standby switch. That corrected the B+ problems.

So still absolutely no sound, not even hum or anything so I double checked my OT connections. Seem to be fine.

I'm heading to the Shack to get some fuses and I'll come back with some detailed pictures as well.

I almost want to say this is "fun" having to troubleshoot, but I'm a little stressed.

But then again, I am gaining knowledge thanks to all of you. I think you guys have been very, very helpful to me.

Ok...to the store...pics soon...

-Rich

[slajeune]

Hi Rich,

don't worry, you'll find the problem. It took me about 15 hours to find the source of hum in my amp. It's a long process but you learn alot in the process. Makes the second build easier!

I'm worried that you are blowing alot of fuses. Something is not right with that. When you said that corrected the b+, you mean that the preamps had the proper voltages now?

I guess the next step is the bias on the power tubes. This is probably why you aren't hearing anything.

Thanks,
Stephane.

[Allynmey]

F, I don''t remember you saying your blowing fuses!
Pull your power tubes and turn on the power with the standby off. If the fuse blows check your mains input for shorts, PT hookups and rectier (diode connections for polarity. If not, turn on the standby, if it blows, check your standby switch for shorts and the proper connection. If it doesn't blow, shut it off put in the tubes and repeat. If it blows, Replace the tubes and repeat. Shorted tubes is the first thing I'd check.

Allynmey

[fzfwyv]

Sorry forgot to mention...I was blowing fuses because I keep brushing the MM probe against other pins when I measure at the power tubes. Otherwise the amp powers up fine and doesn't blow any fuses.

I did find that there was no ground wire going from the bias circuit.

After taking readings the the voltages look more in line but zero sound.

I'm going to check the wire from my input jack to v1p7 to make sure nothing funny there.

reporting back shortly..

-Rich