Troubleshooting request for 15 watt LP PCB build

[Blue Parrot]

I completed the LP 15 watt PCB build from RJ Guitars last night and am having some problems. After an initial consultation with RJ, he suggested I post here to troubleshoot this build, both for ease for him to access as well as to solicit input from forum members.

I went through the initial startup using a current limiter first without tubes and then with tubes; things appeared to me to be OK. I then proceeded without the limiter and obtained voltage measurements that are listed below. Before realizing some of the voltages were off, I played for a few min (maybe 10 min) through the amp to make sure I was getting sound (which I was). I then started to get popping through the speaker and noticed V5 was red plating. From comparison to info posted for the regular LP, it seems some of the voltages are off. I highlighted some measurements in red for V3 that seem almost twice the voltage they should be and voltages on pins 2 and 3 on both V4 and V5 are too high. Also it seems some of the high voltages may not be as high as expected? I have posted several photos of the build below. I could certainly take additional photos and post if it would be helpful. I am hoping the error is something that may pop out and be obvious to someone that has completed this build, or to other forum members experienced with trouble shooting. Any insight into the problem would be greatly appreciated. Thanks.

Scott

[RJ Guitars]

Scott,

I am going to add some documentation that should help us trouble shoot. I made voltage entries onto a gutshot image that you can compare with your measurements.

It's not impossible that we are just pushing the El84's too hard, but I would expect both of them to act the same... look at the EL-84's closely and verify that they are the same tube number. If everything seems ok you might swap them from socket to socket and see if the problem follows the tube.

Let us know what happens after that.

thanks,

rj

[Blue Parrot]

Thanks RJ. It is more accurate to say I was in a hurry to turn the amp off and noticed V5 red plating. V4 probably was also redplating, just not to the same extent. I did check the V4 and V5 tubes and they are both the same type (6n14n). I’ll verify that V4 is actually redplating with these tubes and do the suggested tube swap later today and report back.

I did an additional study where I put a balanced 12AX7 in V3 and put 2 new matched EL84s in V4 and V5. Both the EL84s redplated immediately when the amp was taken out of standby. So, it looks like V4 and V5 actually do act the same.

[Tillydog]

Hopefully not butting in:

I wouldn't worry about your V3 (PI) voltages - trying to measure the grids directly with a normal meter doesn't work. (FWIW, the PI grid voltage will be the same as the voltage where the 4 resistors R10..R13 join.)

The concern is the non-zero voltages on the EL84 grids -

Check that you have solid continuity to ground from the junction of R17 & R18 - I'm *guessing* that the board relies on the stand-off connecting to the chassis where the solder tag from ground wire from the presence control is fixed for this (I'm sure RJ will correct if necessary).

[RJ Guitars]

Hopefully not butting in:

I wouldn't worry about your V3 (PI) voltages - trying to measure the grids directly with a normal meter doesn't work. (FWIW, the PI grid voltage will be the same as the voltage where the 4 resistors R10..R13 join.)

The concern is the non-zero voltages on the EL84 grids -

Check that you have solid continuity to ground from the junction of R17 & R18 - I'm *guessing* that the board relies on the stand-off connecting to the chassis where the solder tag from ground wire from the presence control is fixed for this (I'm sure RJ will correct if necessary).

Not butting in at all... you are quite helpful. I asked Scott to post this for trouble shooting so we could have additional eyes on it.

Yes the ground path goes through the standoff from that terminal. Verify that it does indeed ground... the voltages are trying to tell us something there. I printed out both yours and mine so I can sit and take a look at later this morning... I'll let you know if i find something.

[martin manning]

I seem to recall a similar issue for another builder... was the solder mask preventing the mounting hardware from contacting the trace that is meant to be grounded?

[RJ Guitars]

I am working from my mobile phone so I am hamstrung visually... but you might try just running a temporary wire direct to ground and measure again.
When I get home I can look at mine, I have it open on the bench now.

[RJ Guitars]

I am working from my mobile phone so I am hamstrung visually... but you might try just running a temporary wire direct to ground and measure again.
When I get home I can look at mine, I have it open on the bench now.

[Blue Parrot]

Thanks Tillydog – I appreciate your input! Actually you may have gotten right to the root of the problem. I checked the ground lug and although this was firmly attached to the PCB and standoff, the screw that holds the standoff to the chassis had worked loose. I didn’t notice this loose screw as the amp was upside down during testing. I have firmly attached the standoff to the chassis and verified the ground is now good.

There are a few things I want to check before trying the amp again but I will try later today and report back on whether this fixes the issue.

Thanks Martin for your input. I recall this being a problem presented in a thread a while back too. I think that one solution that was suggested was to use an internal toothed lock washer to facilitate ground connection to the PCB. I went to the hardware store to get some of these washers and will install these at PCB ground points if necessary.

Thanks RJ, I can try the temp wire as well if the problem persists. If the amp is behaving correctly after fixing the ground, I’ll measure the voltages again and post.

Scott

[martin manning]

Yes! https://tubeamparchive.com/viewtopic.ph ... 531#280531

[Zippy]

I know I am a slow builder but one of the reasons is that I don't assume anything. I always check ground connections to make sure they read less than one ohm between the circuit and an arbitrary location on the chassis.

All grounds need to be solid and reading less than one ohm to ground.

[Blue Parrot]

Thanks Zippy, good advice. I did check resistance from ground points to chassis and they seemed to be good (< 1 ohm) when I checked. I should have verified that all the external standoff screws were tightened before the initial startup. I certainly will do this in the future!

I did some initial tests with the current limiter and proceeded from there. The V1 tube went bad for some reason, but after replacing this 12AX7 I think the amp is getting there. It is no longer red plating the V4 and V5 tubes and there is no odd popping or extraneous noise through the speaker.

I attached an updated voltage table below. Although my voltages are a bit lower than what RJ posted, the measurements are much closer to the targets than before.

RJ & others - do you think the voltages are now close enough to the target values to call good, or does this amp require further work before trying to play through? Thanks.

Scott

[martin manning]

Voltages look pretty good (if a bit high), except for the PI where the cathode is ~2x what it should be. Check that the component values are correct and that the ground path through the presence pot is good. You should measure ~44.5k ohms to ground from the cathodes (pin 3 and 8) of the PI.

[Blue Parrot]

Thanks Martin – this brings up a point of confusion I had on the PCB version. In the normal LP schematic, the path you are talking about goes from pins 3&8 through R13 (1.0K) then R11 (39K) and finally through R14 (4.7K) to ground to give a total resistance of about 44.7K. R14 is the confusion for me since this is not shown in any of the photographs of the 15 watt PCB version I have seen. Because of this, I did not include R14 in my build so my measured resistance from the PI pins 3&8 to ground is much higher at 59.3K. I asked RJ about R14 in my original consult e-mail but don’t recall that we really resolved this particular issue. If I crudely hand place a 4.7K resistor from R11 to ground I get the expected measured resistance of 44.5K from pins 3&8 to ground, as you suggest.

So the question is, should R14 be placed for the 15 watt LP PCB version?

[RJ Guitars]

Thanks Martin – this brings up a point of confusion I had on the PCB version. In the normal LP schematic, the path you are talking about goes from pins 3&8 through R13 (1.0K) then R11 (39K) and finally through R14 (4.7K) to ground to give a total resistance of about 44.7K. R14 is the confusion for me since this is not shown in any of the photographs of the 15 watt PCB version I have seen. Because of this, I did not include R14 in my build so my measured resistance from the PI pins 3&8 to ground is much higher at 59.3K. I asked RJ about R14 in my original consult e-mail but don’t recall that we really resolved this particular issue. If I crudely hand place a 4.7K resistor from R11 to ground I get the expected measured resistance of 44.5K from pins 3&8 to ground, as you suggest.

So the question is, should R14 be placed for the 15 watt LP PCB version?

Hmmm.... this might be interesting. I have mine open on the bench right now and I might need to try this. I admit to doing little with my build since I completed it because it worked so well... maybe there is still more?

Also I meant to mention - this is a 270-0-270 power transformer so the voltages will run a bit hot for a Liverpool. In addition to that, I generally get 123-125 volts AC out of the wall socket here so my voltages will usually be a bit higher than some places.