Journey of my Bassman Micro build

[joulupukki]

Well, I’m now 2 for 2 … at having no guitar sound after all the power things test out fine. Guess I get to learn more about amp troubleshooting. I doubt it’s a pre-amp tube, but just in case I guess I’ll switch them out. The only sound I get is when I tap my multi-meter (checking DC voltages) on the terminals on the three caps closest to the tubes … from V3, the phase inverter. Oh, and when I switch the NFB switch. Everything else is silent. No amp hiss in the speaker or anything. I’m guessing maybe there’s either something not connected or I’ve got a short in the preamp circuits.

 

[Len058]

Is the pilot light wired to the rectifier tube? Try it without it. It should be in parallel to the power lines, if its a Marshall style light. It shouldn't be in the heater wire circuit.

 

[joulupukki]

Is the pilot light wired to the rectifier tube? Try it without it. It should be in parallel to the power lines, if its a Marshall style light. It shouldn't be in the heater wire circuit.

It’s a 6V pilot light. It lights up like it should.

 

[andrewRneumann]

awww no worky?

Is it this?

 

[joulupukki]

Found it (haven’t fixed it), but I have the shielded wire from the RECEIVE hooked up backwards. The shield is connected to the tip. Hold please…

Here’s the error. I rigged up a signal tracer with a 0.022uF coupling cap, a mono switchcraft jack, some alligator clips on wires, and my looper for a signal. I’d say that method worked really well at tracking down the exact location where the signal stopped.

 

[joulupukki]

It’s … alive! Just getting acquainted with it. So cool! Ok, I’ve gotta get this baby into the head cab and then take a couple glamour shots. I’ll work some courage up at some point and see if I can work out a good sound demo.

 

[joulupukki]

Ok, who’s the wise guy who left just barely 2” between the cab floor and the support boards!? Hmm. Time for dinner anyway … and time to break out the saw after that.

 

[wangdaning]

awww no worky?

Is it this?

He has a jumper under the board. On that, I would suggest in the future to bring under board wires all the way up through the turret. It minimizes the chance they you will have to remove the board to troubleshoot.

 

[joulupukki]

So it works ... mostly. I've been playing it for the last hour or so fiddling around. Sounds pretty good.

Except...am I already losing a tube? I'm playing along with a looper and suddenly I swear it drops in volume. Hmm. I took the back cover off and don't see any red plating going on. The two preamp tubes are fairly warm but not as warm as the phase inverter tube (v3). But, the sound is definitely coming and going in different volume levels and I'm getting the sound of a bad tube occasionally. Time to figure out my next steps. ...and this was all going so well.

Here's how it looks...

 

[joulupukki]

These are the current voltage readings I’m seeing:

Wall Voltage: 122 VAC
255 VAC on pins 2 and 6 of the rectifier (HV from the PT)
6.4 VAC on heater wires
B+1: 315 V
B+2: 313 V
B+3: 267 V
B+4: 180 V

12AU7 Power Tube
Pin 1
Plate voltage (to chassis): 314.9 V
Plate to cathode: 302.3 V

Pin 6:
Plate voltage (to chassis): 314.7 V
Plate to cathode: 301.1 V

Cathode resistor: 816 ohm
Cathode voltage drop: 12.78 V

 

[Hyakuya]

I don't know the specs for the 12au7 but 300v plate to cathode would make me worry if they were EF80's.

Maybe worth checking the dissipation % on those tubes...

 

[joulupukki]

I’ve really gotta learn to read and understand all these values better but this is the power tube I’m using:

https://www.jj-electronic.com/images/stories/product/preamplifying_tubes/pdf/ecc82.pdf

These are the preamp tubes I’m using:

https://www.jj-electronic.com/images/stories/product/preamplifying_tubes/pdf/ecc83s.pdf

 

[joulupukki]

These are the current voltage readings I’m seeing:

Wall Voltage: 122 VAC
255 VAC on pins 2 and 6 of the rectifier (HV from the PT)
6.4 VAC on heater wires
B+1: 315 V
B+2: 313 V
B+3: 267 V
B+4: 180 V

12AU7 Power Tube
Pin 1
Plate voltage (to chassis): 314.9 V
Plate to cathode: 302.3 V

Pin 6:
Plate voltage (to chassis): 314.7 V
Plate to cathode: 301.1 V

Cathode resistor: 816 ohm
Cathode voltage drop: 12.78 V

@robrob do these values look normal/acceptable? Should I take the same measurements on the preamp tubes? I‘ve got a couple extra 12AX7 tubes but I don’t want to replace them only to have them go out quickly again if I’ve got something wrong.

 

[Hyakuya]

You can plug in your measurements to this..

Tube Bias Calc

Tube Bias Online Calculator

robrobinette.com

General consensus seems to be that for cathode biased power tubes you're good up to 100% dissipation.

For your ecc82 you'll see in the spec sheet, under limiting values, Ua is 300v which I believe is the max cathode voltage.

I think you should be aiming for under 300v on the B+.

Your amp looks great by the way......

 

[King Fan]

Lots going on, I may have missed stuff. Were you losing a tube? Did you? Is your sound still wonky?

Yes, voltages across all sockets. I don’t know the 12au7 as an output tube, but IIRC Rob does say somewhere he aimed for B+ 315. Do I also recall this tube can handle somewhat higher voltages than the EF80?

 

[joulupukki]

For your ecc82 you'll see in the spec sheet, under limiting values, Ua is 300v which I believe is the max cathode voltage.

I think you should be aiming for under 300v on the B+.

I think the Ua is referring to the maximum plate voltage (the 'a' on the tube diagram is next to the plate ... but I could be interpreting that wrong). That's what I'm seeing too though is that the max is 300v on the datasheet. Not sure how I'd lower it to 300v (or if needed).

Your amp looks great by the way......

Thank you.

Lots going on, I may have missed stuff. Were you losing a tube? Did you? Is your sound still wonky?

Yes, voltages across all sockets. I don’t know the 12au7 as an output tube, but IIRC Rob does say somewhere he aimed for B+ 315. Do I also recall this tube can handle somewhat higher voltages than the EF80?

Yes, one of the tubes is on its way out (guessing a preamp tube). I'm trying to determine whether it's "safe" to start swapping them out for others that I have by making sure my voltages/etc are correct.

I'm not sure how the 12AX7 compares with the EF80.

Here are all the other measurements, which as far as I can tell seem fine ... but still learning so I could be wrong:

V1
Plate voltage: 180.5
Plate to cathode: 179.3
Cathode Resistor: 815
Cathode voltage drop: 1.53
Grid: 0V

V2A
Plate voltage: 146.2
Plate to cathode: 145.7
Cathode Resistor: 815
Cathode voltage drop: .993
Grid: -11.7mV

V2B
Plate voltage: 268
Plate to cathode: 120.4
Cathode Resistor: 99.8K
Cathode voltage drop: 147.7
Grid: 146.3V

V3A
Plate voltage: 212
Plate to cathode: 188
Cathode Resistor: 467
Cathode voltage drop: 24.68
Grid: 15.3V

V3B
Plate voltage: 187.5
Plate to cathode: 163.6
Cathode Resistor: 467
Cathode voltage drop: 24.68
Grid: 15.89

 

[joulupukki]

Also, am I using this correctly?

 

[andrewRneumann]

Also, am I using this correctly?
View attachment 988445

There is no screen current, so cathode current equals plate current. Use 15.7 in the plate current box. Looks fine.

 

[Snfoilhat]

Choose whichever calculator corresponds to the measurement you've taken. Cathode voltage* / resistance* = current** is often the easiest measurement to take, and the calculator estimates plate current and everything else based on that.

The lower calculator expects a direct measurement of plate current, like by measuring at the output transformer primary.

The reason they don't agree is because the top calculator can divide the cathode current up among multiple tubes' plates and screens that all are fed by the same cathode resistor.

*direct
**calculated

 

[Hyakuya]

I think the Ua is referring to the maximum plate voltage (the 'a' on the tube diagram is next to the plate ... but I could be interpreting that wrong).

I got that wrong... Obviously the plate.