New Express build with Low voltage ???'s

[Bruciep07]

I’m not clear on your statement “With a 25V-30V drop across the OT with nothing else installed, is it safe to say something isn't right.” Did you actually take a reading “across” the OT with no tubes installed? What do you mean by “across.” From one primary lead to the other? From the CT to one primary lead?

If you have no tubes installed, I think there should probably be no voltage difference between any of those 3 nodes since there is no current (that should be) flowing from the CT to either primary lead.

Sorry for the confusing statement, what I was getting at was since my B+ is a bit low to begin with without any tubes installed, if I were to disconnect the CT from B+1 and see a large increase in voltage, odds are something fishy is going on with the OT. That, however, is not the case. With the OT taken out of the circuit, my B+ stays the same, I checked it a little while ago. My voltages were a little lower but it was 4 o-clock in the afternoon and my wall voltage was around 112V. What I did find out was...
-With just the power tubes and PI installed, it doesn't hum at all...
-With the OT shunted with my DMM, the hum stops abruptly but starts again as soon as I remove one of the probes...
-With a plate voltage of 357V, screen Voltage of 336V, and current measured (shunted) at 49mA, (measurements taken with all tubes installed and amp on) you get about 3-4 seconds from the time you flip the stand by switch to on til the time the hum starts, once it does, the plate voltage drops 1V-2V but the screen voltage drops 15V or 16V...

[M Fowler]

For right now I would bench the DMM and start tracing all the wiring and make sure the amp is wired according to the layout and schematic checking for cold solder joints and wires touching other components when they should be, know what I mean. Once that is done then back to the measuring starting at the secondary of the PT and down the dropping string.

Hope you can find the problem.

Mark

[Bruciep07]

Thanks Mark,
I've actually done that already a couple of times, just this afternoon I completely pulled the power supply board out, cleaned and re-soldered everything (and I mean everything) with new resistors, caps, and diodes, and with 2 different sets of filter caps just to rule them out (and with just the caps connected to B+), checked the rectified voltage again, exactly the same as before I started. So I know for sure everything on the power supply board is the proper value, connected correctly, and working properly. I traced all the wires in the amp against a layout, used a marker and crossed out each wire as I went, all looks correct. I must have a cold solder joint somewhere that I am missing, or could just be completely obvious that I'm missing, this is only my second scratch build so I still have a lot to learn...

[M Fowler]

There is a solder joint loose in the power supply section tighten all ground bolts real tight. Make sure the center taps are grounded, follow the secondary wires to their points check them, follow the filter cap wiring and make sure there is no problems there.

A while back in a good working Liverpool amp I had a similiar problem that started about 2 months after I built it. I ended up finding nothing so figured it had to be the filter caps so I replaced the entire cap section. I check each cap and found nothing? Still not sure what the hell the problem was but I tore that amp apart and looked under both boards for some problem like you did. Now it is back to normal again? Not sure if I want to clean up those caps and reuse them in something or not.

Mark

[Bruciep07]

Well, I've all but completely rebuilt the amp, checked/re-checked/re-soldered just about every connection on the amp, fired it back up, checked voltages, on par with what they were before, biased it, worked PERFECT! Pots were still a little scratchy so I figured I'd tackle that next. I played it for about 15 min, then let it sit for another 15min. Picked up the guitar again, turned up the volume knob a bit, hit the E string, HHHHUUUUUUUUMMMMMMMMMMMMM!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! She just starts screaming (almost sounds like feedback, but it's a constant hum if that makes sense) at me, I can flip it back in stand by and then turn it back on and it will stop until I turn a pot a little or pluck a string or tap on the amp hard enough. May be someone can spot something obvious that I am missing...

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[M Fowler]

I am assuming the four 9k1 2w resistor on the power board are tied together under the board right?

Reverse the primary wires on the OT if it is squealing. If it is humming then we have PT, power section problems.

Mark

[Bruciep07]

I am assuming the four 9k1 2w resistor on the power board are tied together under the board right?

Reverse the primary wires on the OT if it is squealing. If it is humming then we have PT, power section problems.

Mark

Yea, the resistors are tied under the board so you cant see it from the top.
The sound it makes is definitely more of a squeal than a hum I think. The presence pot makes it much worse and sort of changes the frequency of the squeal. I'll try swapping wires on the OT to the tube sockets and see if that helps, though I'm sure I'll have to add some wire to do so...

[M Fowler]

Yeah just give it a try and see if that does the trick, its only wire right

Mark

[jjman]

Scratchy pots are an indication of possible DC on them. This could be from a coupling cap that is not properly blocking DC.

More urgent to verify if the primaries need to be flipped on the OPT.

[Bruciep07]

I'll hopefully have time to swap the OT primaries tonight and see if that cures the squeal, thinking back now it's in the Express Build Guide which color wire goes to which tube. I have a printed copy that shows Heyboer trannys that have Blue, Brown, and Red leads, but I'm using Pacifics that were Blue, Green, and Purple. I took an educated guess and it looks like I guessed wrong. I went back and looked at the build guide online and it was updated since I printed mine out, this one includes the Toneslut Vintage OT's which have the same color wires as my Pacific, if they are both wired the same then it looks like I swapped the wires, but I'll find out tonight when I swap them and see what happens. Thanks again for pointing that out Mark. Is there an easily explanation on why swapping primaries on the OT would cause the amp to oscillate like that?

Thanks JJ, Structo brought up the possibility of DC voltage on the pots earlier, hopefully I can get the squeal worked out tonight, then I can start troubleshooting the scratchy pots. Hopefully it's as simple as a leaky coupling cap... To be completely honest, this is a lot of fun for me, I'm sort of glad I'm running into problems with this build, there isn't much to learn when you just put together a bunch of parts off of a schematic (with instructions thanks to Ron and many others) but when you have to step back and figure out why something is doing what it's doing, it's a lot more rewarding when it's all done...

[paulster]

I'll hopefully have time to swap the OT primaries tonight and see if that cures the squeal, thinking back now it's in the Express Build Guide which color wire goes to which tube. I have a printed copy that shows Heyboer trannys that have Blue, Brown, and Red leads, but I'm using Pacifics that were Blue, Green, and Purple. I took an educated guess and it looks like I guessed wrong.

Heyboer -> Pacific
Blue -> Purple
Red -> Blue
Brown -> Green

Hopefully that'll tell you whether you've got your primaries round the wrong way (it sounds like it) without having to desolder them.

[Structo]

Looks like you have the 1R resistor going to ground from pin 8 and then you have pin 1 going to ground.

Is that intentional?

Typically you would jumper pins 1 & 8 together and then the 1R resistor to ground.

[Bruciep07]

I'll hopefully have time to swap the OT primaries tonight and see if that cures the squeal, thinking back now it's in the Express Build Guide which color wire goes to which tube. I have a printed copy that shows Heyboer trannys that have Blue, Brown, and Red leads, but I'm using Pacifics that were Blue, Green, and Purple. I took an educated guess and it looks like I guessed wrong.

Heyboer -> Pacific
Blue -> Purple
Red -> Blue
Brown -> Green

Hopefully that'll tell you whether you've got your primaries round the wrong way (it sounds like it) without having to desolder them.

Thanks, I know the center tap is correct, just think I swapped the purple and green wires between V4 and V5, we'll see when I get home...

Looks like you have the 1R resistor going to ground from pin 8 and then you have pin 1 going to ground.

Is that intentional?

Typically you would jumper pins 1 & 8 together and then the 1R resistor to ground.

I did that intentionally, I was told that was a "better way" to check cathode current, I tried it first with both pins 1 and 8 tied together, then through the 1Ohm but decided to switch it later, I plan on putting the jumper back (after I get the bugs worked out and bias set) so both just go to ground...

[M Fowler]

Pacific trannies are primary green and violet. The blue is the CT.

Mark

[M Fowler]

I see your using the 280v taps which are yellow to the diodes. I blew your pictures up and went over everything and I did not see anything wrong. I see that you ran your OT primary CT blue to the 1 K 25w resistor rather than to the Standby switch but is okay. I am assuming that there is a wire under the board from C9 the 0.1uf OD to R10 1m which goes to V3 pin 7.

Other than switching the OT primaries it looks good to me. If the amp howls on start up it means the OT primary is out of phase so by reversing the wires takes cares of the phase on primary side.

Mark