New 45 build has weird voltages - need guidance please!

[page361]

OK so with both legs of replaced preamp filter can attached, preamp tubes OUT and GZ tube installed, I get the following

458vdc at 100k and 470vdc at standby

[toner]

OK so with both legs of replaced preamp filter can attached, preamp tubes OUT and GZ tube installed, I get the following

458vdc at 100k and 470vdc at standby

Even without preamp tubes installed, the V1 plate resistors supply (458vdc) seems high to me but it will come down with current draw...

The preamp socket wiring looks okay, assuming the under-board connections are right.

Anyway, regardless of the cause (faulty cap, meter problem, user error ), it looks like B+ is stable now so...

Unless Neil or others advise against it, I would next install V1-V3 tubes and check voltages again. V1-V3 plates (pins 1 and 6) should read higher than the voltage chart but should be close. I'd expect V1 plates to be around 225-260, without power tubes in (just a guess). If voltage at the standby or any other location drop way down again, stop and post what you have.

I noticed you changed the 250pF tone stack cap. I may have missed it, but was there a problem with the one in your original pictures (other than it being mis-wired)? Just curious.

[page361]

Nothing wrong with it - the pic I just posted was older and I actually replaced the Lemco with the RS. Just stuck the Lemco away for later use....

OK so I'm going to go ahead and put preamp tubes back in and test....fingers crossed!

[page361]

With preamp tubes installed V1-V3, I get the following measurements:

68vdc at 100K
82vdc at standby when ON

[page361]

OK so I have now swapped out all three filter caps with new replacements, have checked, re-checked, verified and re-verified that under board wiring is correct - the only thing that was incorrect were the two blue wires to V3 that were reversed and the 250pf that I had replaced but inadvertently soldered to the wrong turret.

So I re-tested with the new cans installed and w/ GZ tube in, and get approx 420vdc at STDBY when off. BUT when I turn STDBY switch to ON, this time I now measure approx 118vdc at STDBY switch. I feel like smashing my multimeter!

Although I tested the OT' primaries resistance against the center tap and it seemed to be OK ( I assume the numbers albeit lower than what Neil intimated, are OK for this particular RS repro OT) I have to wonder should I swap it out as well? With no tubes in V4 and V5 could it be the culprit? I do have a spare new Marstran OT for a 45.

....other than that last option I suppose I am at my wits end and at some point need to surrender and move on
all these fluctuations in voltages and swap out of components seems to have me no closer than when I started...bummer cuz I spent a pretty penny on all these NOS parts...not to mention a pretty nice custom head cab that will include the coffin logo.

[toner]

I don't think it's the OT, especially since you aren't drawing current through it with no power tubes installed, but you can do what Neil initially suggested and disconnect the OT center tap wire and check voltage at the standby switch.

Try a different meter.

Have you tried different preamp tubes? Although I've never seen this before, you could have a preamp tube with a short in it (like plate to cathode). I would install the 3 preamp tubes you've been using and then swap them out one at a time with a known good 12AX7 and check voltage at standby switch after each swap. You could also try just one good tube in V3, then V2, then V1 and check voltage each time to attempt to isolate it to a particular socket. There could be problem with one of the preamp sockets, like a loose solder blob or something.

The only other thing I can think of at the moment is what I mentioned before: An intermittent short in the choke that only happens when the preamp tubes start drawing current through it. You can disconnect the choke and run a wire between the two points it was connected to to eliminate it as the problem.

[page361]

IT LIVES!!! IT LIVES!!! WOO HOO!!!!!!!

[neikeel]

IT LIVES!!! IT LIVES!!! WOO HOO!!!!!!!

C'mon then - tell me more - this has been bugging me all week!

[page361]

Well I swapped out all of the filter caps, as well as preamp tubes (which I thought were OK) ...and started seeing voltages settle in like they should. Then WHAM...it freaked out again...was measuring funky voltages at the 100K's...so I did another under board inspection and saw that the yellow V1 wire was dangling like broken tree limb...must have broke after all the fiddling I was doing looking for broken wires...ironic in a way.
Anyway I replaced it...fired up the amp and voila! Only thing I may need to do is get a better GZ tube that will give me a bit more voltage. Currently getting 420vdc. In the process of all the troubleshooting though I ended up breaking one of the legs off of my faux 16uf RS preamp cap...bummer because it looked great.

I can't tell you how much I appreciate your support Neil (as well you Toner!)
Not only did you stick with me, but I learned more about logically troubleshooting a circuit then I ever thought I would. Positive experience all the way albeit a tad bit frustrating at times. This was my 5th build and it was a bear I must admit.

So if you or Toner are EVER find yourself in Texas, please let me know - will buy you a Shiner Bock ale!

Sincerely,
-Mike

[toner]

...so I did another under board inspection and saw that the yellow V1 wire was dangling like broken tree limb...

hmmmm, a disconnected cathode on V1 wouldn't cause the B+ drop you had. I think something else was going on.

Anyway, hopefully it's fixed! If not, Neil can take over again.

[page361]

I assume it was maybe the 32x32 filter

[page361]

Seems I have one slight problem....any idea what would cause the Bass pot to control the volume level of the High Treble channel? It’s not adding any bass to the mix in either channel. And when the bass pot is turned down all the way the High Treble works great – but when the bass post it is turned up it cuts the volume back in the High Treble big time.

[neikeel]

Show us your tone stack and tone pots wiring, you have a jumper misplaced

[page361]

All is good...this thing really sounds great...maybe the fifth time is a charmer! Now if I only had a head box......