Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Lectroid on October 15, 2025, 04:34:27 pm
-
Last year I built an 18W (ish) amp and have played it several times since until this week when it turned on but makes no sound. See Voltages.png below for the current voltages. Schematic below shows the voltages a year or so ago. B+ voltages are up about 15% across the amp but the house voltage was almost identical, 122V yesterday and 121V last year. That strikes me as a little weird, no? :dontknow:
Main problem is the first stage is dead. It has barely 90V at the plates of V1a and V1b. It should have 145-150V on its plates, given the increased B+. I've tested plate resistors, cathode resistors, and the 4.7K C and D dropping resistors, all were nominal. I did find the cathode wire of V1b had snapped off and re-soldered it.
I've tried 4 different 12AU7s and all behaved the same--no output. Could the filter caps be bad?
Any ideas of how to proceed or where I might dig next? Thanks!
-
What HT voltages have you got (at each filter cap node)?
And what cathode voltage on the EL84s?
-
Broken wire found. Maybe a bad solder joint or other bad connection finally failed completely, or enough to cause the current problem.
Can you trace an input signal?
-
Sorry for the delay, chaps. Been a busy couple of days. Thanks for posting.
@tubeswell, B+ nodes:
2024: A=325, B=314, C=292, D=275 These numbers I recorded last year, see the schematic. Then last week, I measured these voltages:
2025: A=374, B=364, C=324, D=303
1. Why the sharp jump in voltages now?
2. And why would the V1 plate voltages have dropped ~40V below what I recorded a year ago(?). Both stages of the V1 tube have plate voltages. lower than they were when I recorded the amp's voltages last year.
@BrianS,
Broken wire found. Maybe a bad solder joint or other bad connection finally failed completely, or enough to cause the current problem.
Always a possibility with my soldering. :laugh:
I've been chop-sticking and continuity testing every trace I could but nothing found yet. Ongoing. I've tried 4 or 5 12AU7 tubes in that slot and all display the same behavior. IE, negative-function.
Can you trace an input signal?
No, that was actually my original problem, see above--no sound. With a scope I see a test signal arrive at the amp's Instrument jack. I see the test signal arrive at the grid of V1b. But from there, nothing comes out of that triode that I can detect.
Thanks! :icon_biggrin:
-
When I said trace a signal, I should have said do you have a scope and signal generator. Sounds like you do and are tracing the signal.
I think in the past I have bypassed a non-functional stage just to assure myself that the rest of the amp is working and maybe dial down the trouble spot a little closer. Not sure if that is necessary here or helpful at all really...
Good luck.
-
... B+ voltages are up about 15% across the amp but the house voltage was almost identical, 122V yesterday and 121V last year. That strikes me as a little weird, no? :dontknow:
Main problem is the first stage is dead. It has barely 90V at the plates of V1a and V1b. ...
V1 is not "dead." Plates have dropped from 128v to 90v, but cathode voltage is up from about 2v to 3.25v. This indicates higher current through V1.
2024:
2.18v / 1.5kΩ = 1.45mA
1.45mA x 100kΩ = 145v dropped
Supply Node D: 275v - 145v = 130v ---> exactly what you have in your schematic
Recent:
3.25v / 1.5kΩ = 2.17mA
2.17mA x 100kΩ = 217v dropped
Supply Node D: 303v - 217v = 86v ---> exactly what is expected if V1 draws more current
Recent V2 measurements show higher supply voltage, and higher tube-current, matching the trend shown in V1.
Recent V3 measurements show lower tube-current (different 12AX7 in this socket?).
Overall supply voltages are up. If not higher outlet voltage, then did you replace the 5Y3 rectifier? A Sovtek type 5Y3 is actually lower-voltage-drop (it's not a "real 5Y3"). And if there was a change of filter caps, higher-µFs leads to higher-B+.
Nothing in your voltage survey points to a reason for "no sound." In fact, everything looks like it should be working fine. I'd look for an open coupling cap, or an open/broken wire from OT to speaker-jack.
-
HBP:
Thank you for suggesting open caps, and for laying out the math so clearly. All of the cathode bypass and coupling caps have high resistances; I think they're good. So we come to the C and D node filter caps. They are under the board so I haven't tested their capacitance yet. But I've just measured their DCR (if that's the right term here) and both measure around 1500 Ω. I'll dig them out and unsolder them for a real test--just been avoiding the pain. At least it'll give me a chance to fix a bad layout.
Point taken about the 5U4 delivering higher B+. I was substituting it for a 5Y3 that I suspected bc I didn't have another 5Y3 on hand. But I think you're right. I'll go back to the 5Y3 for the rectifier. This amp really doesn't need super-high high voltage.
So if I think about such a failing filter cap in the abstract, wouldn't such a cap not filter out as much of the AC ripple at that B+ node? Would that 'unfiltered' AC appear on the B+ node's voltage now? Would it be measurable?
Thanks again
PS: I understand that 'dead' is not a technical term here. Just writing too fast. I will work on my posting etiqette. :icon_biggrin: