Voltage drop from B+2 to B+3 question

[markedwardsims]

Hey all! I am attempting to complete my first build, a Trainwreck Express, and have been following the great guide that Ron Worley assembled. I tried to find the answer to this in existing posts, but don't see anything so I'm starting a new thread with the hope of getting some solid guidance. Here's where things are at.

- I've got the wiring and assembly completed and carefully reviewed and highlighted the schematic to verify
- Followed the startup advice and used both a variac to form the filter caps slowly, and a current limiter (incandescent light bulb box)
- These steps all passed muster and there was no red plating or shorts
- I put the tubes in, brought it up and started checking voltages

The issue I'm seeing is that the B+3 voltage does not drop after the 9k1 resistors (R26a & R26b). Here is the complete list of B+ voltage readings:

- Wall = 120VAC (i used the variac to ensure i was starting right at 120)
- B+1 = 395V
- B+2 = 387V
- B+3 = 387V (should be more like 298V)
- B+4 = 363V
- B+5 = 343V

The relative drop after B+3 looks to be correct. I tested out R26a and R26b, and they are testing properly and within tolerance.

Any advice on how to continue trouble shooting this? I've attached an annotated picture of where i'm measuring voltage, as well as the schematic I'm following.

Thanks in advance!

Screenshot 2023-04-08 at 5.11.36 PM.png

Express Build Guide V2.0 (NXPowerLite) - Schematic.pdf

[xtian]

Are you getting sound?

Possible wiring or component error. Post detailed photos.

[markedwardsims]

After a bit more generic searching, this sounds like maybe an Ohm's Law thing and there could be no load on B+3. Per the circuit, it looks like B+3 powers the phase inverter, so that could be a good place for me to look. I honestly haven't plugged a guitar up yet. I was concerned about the high voltages so I powered back down pretty quickly, but I suppose in the time I can take measurements, I could also do a sound test. I'll do that later and post back. As requested, attached are some images of the build. Let me know if anything looks suspect.

Thanks!

IMG_9056.jpg

IMG_9058.jpg

IMG_9057.jpg

(as I'm looking at these images, the first-time builder vibe is really showing. the black + and - were markings for drilling that weren't cleaned up after a relocation.)

[gktamps]

First time builder? Don't get too discouraged yet. Just take your time and triple-check everything. First thing is to make sure all of your B+ nodes at their entrance to the preamp board have continuity throughout the circuit. Unplug the amp and discharge the capacitors, then set your DMM to Ohms and do a continuity check where each B+ node wire connects to each stage on the board. Cross-check all connections to the layout and schematic. If all nodes are wired correctly, check for any breaks in the connections or incorrectly wired/soldered components on the board, including your tube sockets.

Everything after B+3 is what you are trying to assess; if voltages don't drop at those points forward, the circuit is not drawing power. You first noted you had tubes in the sockets, which is essential. Let's also make sure your tubes are good.

After checking every solder joint and wire connection, and ensuring you have everything wired correctly, go ahead and plug in the amp when you have made the work area safe, and start probing voltages at the board. It's easy to have your probe slip on a tube socket pin, so except for checking heater ACV, I suggest using the board for probing.

If you still have no current draw from B+3 forward, disconnect AC, discharge caps, and check all caps and resistors on the board. Let us know what you find after doing these things.

[markedwardsims]

Thank you for the guidance! I'll work on this and report back. One of the things that I'm discovering is that the V2 and V3 aren't seating as deeply as V1, V4 and V5 in the socket. I think I may have over-soldered and got some solder in the socket. Any tips on how to clean that out? If I understand correctly, the tube is what will pull current, so I could see where that PI tube not seating in there all the way would cause an issue.

BTW, I did enough reading and research before starting to know that assembly is on a portion of the journey, and especially for a first timer the real fun begins in this stage. I suspect this is where the real learning begins.

[xtian]

Thank you for the guidance! I'll work on this and report back. One of the things that I'm discovering is that the V2 and V3 aren't seating as deeply as V1, V4 and V5 in the socket. I think I may have over-soldered and got some solder in the socket. Any tips on how to clean that out? If I understand correctly, the tube is what will pull current, so I could see where that PI tube not seating in there all the way would cause an issue.

BTW, I did enough reading and research before starting to know that assembly is on a portion of the journey, and especially for a first timer the real fun begins in this stage. I suspect this is where the real learning begins.

It's easy to see if a preamp tube is pulling current--just measure voltage drop across the cathode resistor for each triode. If you see one or two volts DC, the triode is conducting and operational.

[markedwardsims]

I measured across the cathode resistors and got the following results:

R2 (V1b) = 1.7V
R5 (V1a) = 2.3V
R8 (V2a) = 3.7V
R13 (V3) = 1.4V

So this suggests all the preamp tubes are pulling current.

Also, these are all brand-new tubes and unfortunately, I don't have any spares. I traded them around between v1-v3, which was the best I could come up with to attempt to eliminate a faulty tube from the equation.

[Guy77]

One thing I like to do after every build is measure the resistance from the outside pins of the preamp tube sockets, not from the inside of the chassis. So I pull out all the preamp tubes and place one end of my multimeter on the pins 3,8, 1,6,4,5 and 9 and 2 and 7 and the other side of my meter touches my chassis (ground). With my meter set at Ohms I look for any resistance. If the resistance is infinite for the 4,5 and 9 pins then I know one of my heater solders is bad. Same for my signal pins 2 and 7. There should be some resistance there too.

Good luck

Cheers
Guy

[sluckey]

The PI circuit is the problem, not the preamp tubes. A very common error for a PI that's not drawing any current is the cathodes have no path to ground. In your case a missing ground connection to the presence pot is high on the suspect list. Check resistance between pins 3/8 and chassis. Should be about 10K with the presence pot turned to one end and about 15K with the presence pot turned the other way. If the resistance is over 100K then the presence pot is bad or not connected to ground. It's also possible that the 470Ω, 10K, or any wiring between pins 3/8 and chassis is open.

Another less common error is B+3 is not getting to the PI plates. Easy check to measure voltage on pins 1 and 6.

[gktamps]

^ What Steve said above - he posted just before I did...

Did you measure voltages at all pins of the the tubes, particularly the PI? What were the results? Suggest you download the voltage chart and make notes.

Also, I've used those same caps you did for your preamp tube cathodes, and I like them. However, their markings are confusing. If I recall correctly (and you could always double-check the manufacturer's datasheet), the black line denotes positive, not negative. In fact, you will find a little raised - symbol in the plastic housing, denoting negative. It is visible in the closeup picture below. There is also a bevel in the end of the housing. Please double-check the datasheet, as it's been years since I've used this type of cap.

Post up your voltage chart when you can.

EDIT: Looks like you've found datasheet evidence that the black stripe IS negative - good verification!

[markedwardsims]

Great call! It looks like everything checks out from a continuity standpoint on the tube sockets, so my amateur soldering seems ok in this instance.

I've gone back through and followed/highlighted the schematic again, and cannot find anything that doesn't match the circuit and layout I'm working from. I even spent extra time on the phase inverter portion, since B+3 powers that, but again nothing is popping out to me other than maybe I have a bad component? Focusing on the preamp board, all resistors checked out and the larger caps seem to check out, but my DMM won't measure the smaller caps.

Lastly, I checked for all the components that go to ground on the schematic, and they all seem to be proper.

Maybe I can get some guidance on the bigger picture and how to track down what seems to be a lack of current draw on B+3. Questions:

• Is it the PI circuit I should be looking at? OR, is the problem up or downstream on B+2 or B+4?

• Is there a technique to work backward and measure the current draw at each component?

The vibe here is super friendly toward first-timers, so thank you all for your patience!

[sluckey]

MEASURE THE VOLTAGE ON V3 PINS 1 AND 6. WHAT HAVE YOU? I WANT NUMBERS.

MEASURE RESISTANCE FROM PIN 8 TO CHASSIS. WHAT HAVE YOU? I WANT NUMBERS.

[markedwardsims]

As requested!

IMG_9060.jpg

not pictured:

• B+1 = 395v

• B+2 = 387v

• B+3 = 387v

• B+4 = 363v

• B+5 = 343v

sluckey, i'll get you numbers!

gktamps, great call on that capacitor polarity. I'll double-check, fix if needed and report back.

[markedwardsims]

MEASURE THE VOLTAGE ON V3 PINS 1 AND 6. WHAT HAVE YOU? I WANT NUMBERS.

MEASURE RESISTANCE FROM PIN 8 TO CHASSIS. WHAT HAVE YOU? I WANT NUMBERS.

check out that picture above. i measured all those voltages (except heaters) from pin to chassis ground.

[sluckey]

MEASURE THE VOLTAGE ON V3 PINS 1 AND 6. WHAT HAVE YOU? I WANT NUMBERS.

MEASURE RESISTANCE FROM PIN 8 TO CHASSIS. WHAT HAVE YOU? I WANT NUMBERS.

check out that picture above. i measured all those voltages (except heaters) from pin to chassis ground.

That's only half of what I asked for.