Express build 1st startup, B+ voltages too high?

[pompeiisneaks]

Am I seeing this right? Or did you do this until you can get it tested on the board itself? It looks to me like your white wire that connects to the bias point on the main board is not connected to anything? See arrows.

WhiteWire.png

~Phil

[shipwreck]

Yes, the bias wire was intentionally disconnected when I was troubleshooting. I will connect it and do a full layout/schematic check before doing a startup test.

If I have learned anything from this build, it is to be especially patient when getting close to the finish line. For my next build, I will definitely be more thorough and systematic, and check voltages before doing a tube startup. And I will probably ask fewer, but more precise questions

[shipwreck]

Today I checked my work against layout and schematic, did a startup without tubes, got B+ voltages from 410 to 404, and ... (Wait for it!) bias voltage of -33! Yay!

So I did a startup with tubes, but disappointingly got only low, distorted output. Anti-climax!

I then tested all the tubes in a working amp, and did not find faults with them.

So what next? I know the tubes are good, voltages are in the ballpark, wiring seems correct (although I will re-check), no smoking components... Not sure where to start troubleshooting.

[romberg]

Today I checked my work against layout and schematic, did a startup without tubes, got B+ voltages from 410 to 404, and ... (Wait for it!) bias voltage of -33! Yay!

Nice!

So I did a startup with tubes, but disappointingly got only low, distorted output. Anti-climax!

I then tested all the tubes in a working amp, and did not find faults with them.

So what next? I know the tubes are good, voltages are in the ballpark, wiring seems correct (although I will re-check), no smoking components... Not sure where to start troubleshooting.

• Did you check that those output jacks were wired up on the correct side of the jack's switch? I'm guessing they are since you do get some output.
• Did you bias the power tubes? I always set the bias voltage for max negative before changing/installing any new tubes. It is the safest setting. But it can bias the tubes so cold they are in cut-off (no current). This is safe for the tubes. But it produces no sound. More than once I have been alarmed when one of my new builds did not produce sound (or low distorted sound like you have) only to find I forgot to bias the tubes after the amp started and did not smoke .

Mike

[shipwreck]

I did check the speaker jack wiring. I have not biased the tubes yet. I just did a new startup and adjusted the bias knob whilst playing, but it still sounds like a lofi fuzz pedal. But at least I have sound!

I am attaching a new pic of my power tube, speaker and bias pot wiring. Too late tonight to continue working on the problem.

[romberg]

Good points, I have attached the schematic and the layout I have used.

On the layout, the presence pot is wired incorrectly. If you did your pot the same way then it will be too. Note that on the schematic the bottom of the pot and one side of the cap are grounded. The top has the negative feedback from the speaker out. And the other end of the cap is connected to the center wiper.

The layout incorrectly has the *center* wiper grounded. The ground connection needs to be moved to the bottom leg of the pot. The presence pot is part of the cathode resistance for the phase inverter. This mis-wire could be causing some/all of the trouble.

It might be a good idea to sit down with a copy of both the schematic and layout and go over everything using a highlighter. It is not uncommon for a layout to have an error or two. The schematic is probably right as it looks like the Kelly one from here.

Mike

[RJ Guitars]

Hey Mike,

Great work in finding that wiring error on the layout - Thank you! That is my fault by the way - I'll own that one.

After looking at it, if I am understanding things correctly, it's actually more wrong than just the wrong lug on the pot grounded. With that little ground tab going to the R-11 resistor the amp would work but the Presence control would not. That ground should actually come thru the Presence pot.

I've corrected the drawing and I'll send a copy to Shipwreck. I removed the "Do not publish" label and replaced it with a cautionary one. I sorta figure it's a little dangerous to publish an unreviewed layout document but simultaneously it's a little dangerous to build an amp with an unreviewed layout document. So it's gotta be O.K. to let folks have a look and get some fresh eyes on the amp and the document. I'll just have to find the thick skin that is required to have folks take a look at a new drawing... I'll ask your forgiveness and meanwhile fix the drawing.

Ultimately, I want to get a good layout out there for the Can Cap based Express build. This is IMHO the cleanest power supply layout for a Wreck build.

thanks,

rj

[romberg]

Oh! I did not see R11 grounded. Goes to show that it is difficult translating a working schematic into a working layout. I'm sure the work you are doing will be very much appreciated. Your layout does look much cleaner than the traditional pile of caps hot glued together.

What are you using to draw your layouts? I'm currently working up my own layout for an overdrive special and am always on the lookout for a better layout drawing application.

Mike

[shipwreck]

Thanks for the updated layout, rj.

I did find an error in my precence control wiring, so I rewired it according to the updated layout. Started up the amp again, and now I got something completely different than the low output distorted tone, namely some very loud feedback cacophony! I assumed I needed to reverse the OT primaries, and did so, but on next powerup it was still too much loud noise. So I probably need to do a yet another thorough check of all my wiring.

I wonder if one step in debugging could be to disconnect the NFB wire at the speaker out? Any other tips on how I can isolate this latest problem of high uncontrollable output?

[romberg]

Yes it is safe to (and might help you debug) disconnect the negative feedback wire from the speaker tap it is connected to. If it is out of phase, it can cause a run away oscillation. Make sure the disconnected end does not touch anything else such as the chassis.

The express is known for being an amp right on the edge of stability. So, lead dress of the signal wires is important.

A pretty descent way to debug which part of the circuit is causing trouble is to use a lead with alligator clips on each end. The idea is that you connect one end to ground and then the other you use to ground the input grid of each stage. For example you would ground R10 (side that connects to V3 pin 7. This nulls the input to the phase inverter. If the amp is still out of control the problem is in the power tube section.

Next ground R7 where it connects to pin2. If the problem is still there then it is in the phase inverter. Move on down the line (volume pot center tap and then finally the input).

Of course it is safest to connect up this clip lead with the amp off and unplugged (usual safety procedures). It should help you track down where the trouble is.

Often you see shielded cable used for the run from the volume pot to the second stage. This is a grid wire and is very sensitive to picking up things. Not sure if you used a shielded run here or not. But it might be worth doing.

EDIT: Also if switching the brown/blue wires from the OT is difficult, you can accomplish the same thing (swap the phase of the feedback signal) by swapping the blue/yellow wires coming out of your phase inverter. These two wires are usually much easier to move around if your feedback is positive instead of negative.

I'm assuming the uncontrolled feedback is occuring with no guitar plugged in and the volume set to zero?

[RJ Guitars]

You might consider pulling tubes one at a time, starting with V1.

This will help you locate the trouble stage.

If you still have the screech after pull V1 then leave it out and pull V2, rinse and repeat.

[joeboo88]

ROmberg and RJ's advice is perfect, divide and conquer.
I was looking at the picture on page 2 because it's clear.
Can you also check R4 -100k it doesn't look as though one side is soldered, but can just be the picture.

These guys will help you get to the bottom of your trouble.
Good luck

[shipwreck]

I will try the alligator clips method as soon as I have some free time to devote to this project.

Just wanted to say thank you to all of you who have given so much helpful advice.

[shipwreck]

Grounded R10: No sound

Removed alligator clip (regular startup): High pitched squeal with guitar connected. Without guitar, hiss and static noise, like FM radio between stations. Loud pop when turning standby switch back to standby.

[romberg]

Next try grounding R7 and then the wiper of V4.

Since it sounds like things change if you do or don't have a guitar plugged in, describe that some more. With no guitar plugged in does the volume control (or any other controls) have any effect on the static? When the guitar is plugged in, same thing.

Since there was no squeal with R10 grounded, your power amp and PI are probably ok. So, the trouble is probably earlier. My guess would be the tone stack.

Mike

EDIT: Also, is your feedback wire connected or disconnected?