First time biasing and I have some questions

[pompeiisneaks]

It's really hard to tell on those bias pots. It seems like the second one is wired differently from the first. The pictures blurry though. Can you tell us which pins are 1,2,3 on those?

~Phil

[professormudd]

One is flipped upside down so the resistors to ground had a straight shot. Sorry for the blurry quality. The lead going to the board on each pot is pin 2. Pin 1 is the resistor to ground. Pin 3 is from the rectifier board.

The pins are actually numbered on these pots.

[pompeiisneaks]

Yeah, again looking at the photo's the new one you've posted seems to indicate pin 3 is near that small dimple.

initially I couldn't see a dimple at all on the right one, but I can see it clearly on the left potentiometer, but on the right one it's not super clear but it seems like it might be to the right of the green wire which means the pin 3 on the right connects to pin either 1 or two, I can't do the rotation in my head, just basing on the dent in the pot as the 'closest to pin3' but in the layout and logically, pins 3 should be the ones tied together on both pots.

Just double check that pin 1 connects to the 1.5k to ground, pin 2 off to the 220k on the board and then pin3 should be connected to the other pin 3 and the bias board.

~Phil

[professormudd]

Yeah, again looking at the photo's the new one you've posted seems to indicate pin 3 is near that small dimple.

initially I couldn't see a dimple at all on the right one, but I can see it clearly on the left potentiometer, but on the right one it's not super clear but it seems like it might be to the right of the green wire which means the pin 3 on the right connects to pin either 1 or two, I can't do the rotation in my head, just basing on the dent in the pot as the 'closest to pin3' but in the layout and logically, pins 3 should be the ones tied together on both pots.

Just double check that pin 1 connects to the 1.5k to ground, pin 2 off to the 220k on the board and then pin3 should be connected to the other pin 3 and the bias board.

~Phil

Okay, progress...

I went ahead and removed both pots. I put them each back in as carefully and thoughtfully as possible. You were right about the dent, I did not see it as a landmark for some reason. Lesson learned.

I also changed the transformer leads to the lower voltage as xtian indicated. Here are my results with no tubes attached and a 8ohm speaker cabinet attached:

• Voltage at pin 3 on every power tube socket is 405.
• Voltage at pin 5 on both sides range from -22.7 to -60.
• The interaction seems gone.

I have not added tubes yet, as I am curious if the voltage at pin 3 is too low. Please advise.

[xtian]

400v unloaded sounds great. Proceed with biasing!

[professormudd]

I take it back, Voltage at pin 3 is 437. I was running through a current limiter when I first checked.

[professormudd]

400v unloaded sounds great. Proceed with biasing!

I got it to bias at approximately 65% (47.3mV). So that is good, and I consider this a success. Now the fun begins, though, as there is no guitar signal getting to the speaker. I can hear that the speaker is powered and I can hear light static through the speaker when I touch points with my multimeter and it is powered on.

I double checked to make sure my cable tip does not have continuity to ground but the sleeve does.

All trim pots in side are at 12 o'clock.

[pompeiisneaks]

One quick test would be to check for a noise when probing voltages near the anodes of the tube, that almost always makes a noise at hte speaker, a kind of pop. If it's present at every anode of every tube, then it's likely a problem with the input itself, somewhere/somehow the signal isn't making it from the input jack to the grid of the first triode.

You can also inject some kind of signal and you should read a few mVAC of signal at the input of the first triode becoming a few volts on the anode past the coupling capacitor. If you have a scope it makes this a lot easier but often people dont' have one.

~Phil

[xtian]

I got it to bias at approximately 65% (47.3mV).

So what was your plate voltage AFTER you set the bias?

[professormudd]

I got it to bias at approximately 65% (47.3mV).

So what was your plate voltage AFTER you set the bias?

Bias is actually 49mV each side, plate voltage 405 each side.

[professormudd]

One quick test would be to check for a noise when probing voltages near the anodes of the tube, that almost always makes a noise at hte speaker, a kind of pop. If it's present at every anode of every tube, then it's likely a problem with the input itself, somewhere/somehow the signal isn't making it from the input jack to the grid of the first triode.

You can also inject some kind of signal and you should read a few mVAC of signal at the input of the first triode becoming a few volts on the anode past the coupling capacitor. If you have a scope it makes this a lot easier but often people dont' have one.

~Phil

I did get noise at every anode when testing DC voltage to ground. Some were more pronounced than others, but they were all audible.

I do not have a scope. My level of understanding is also not sufficient to translate the next test into probe locations on my layout, but I will put my nose down into some reading material and try to make sense of it.

[pompeiisneaks]

if you got noise at each anode, then it is very likely all the t ubes are working as expected. The lack of sound is possibly a wiring problem on the input jacks, I think most people tend to mess them up first time. Can you take a picture of the wires around the input jacks?

I think you might have had that in the other thread, (and why it's sometimes good to keep an entire build thread in one but not the end of the world) so if you can just link me to those, that works as well.

~Phil

[xtian]

The relays can also be tricky.

[professormudd]

if you got noise at each anode, then it is very likely all the t ubes are working as expected. The lack of sound is possibly a wiring problem on the input jacks, I think most people tend to mess them up first time. Can you take a picture of the wires around the input jacks?

I think you might have had that in the other thread, (and why it's sometimes good to keep an entire build thread in one but not the end of the world) so if you can just link me to those, that works as well.

~Phil

I don't think I'd posted a pic before. The input Jack was done by the person I bought the amp from, but I tested continuity to the top and sleeve of my cable and it seemed correct. Pics attached.

Also, please note that I also did not get guitar signal when I plugged straight into the fx return.

[professormudd]

The relays can also be tricky.

The relay boards were done before this got to me as well. I connected the boards to the other stuff and have quadruple checked continuity between points that are supposed to be connected per the layout.