Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: ululufut on August 28, 2019, 04:38:40 pm
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I have a vexing issue made more so by that I solved the same issue earlier and now it's back, this time without a findable solution as of this time.
A 5E3-type circuit I'm working on is producing a low level 60 cycle hum on standby. Live the hum is louder, although in each instance it is not obtrusive. I would say that, on standby, the hum is what I would expect from the circuit when live, and the live hum is a little more than that.
I have checked the voltages on the V1-V2-V3-V4 plates on standby and they all seem normal - a trivial number of millivolts. Live, the values are all pretty high - I have bigger transformers in there. But they seem reasonable - in the low 200's on the preamp tubes and low 400's on the EL34's.
For the filter caps - on standby they're all running about 445mV. Is this normal?
The transformer/rectifier connections all seem legit - moreover the amp was quiet on standby earlier so this seems like an unlikely source.
One thing I'm unsure of, however, is that the rectifier (I'm using a plugin diode) pins 2 and 8 are at 327V on standby, and 423 when powered up. I frankly don't know what the values should be here.
Other ground connections:
Cathode bias 250Ω resistor - grounded
Input jacks -> potentiometers - grounded
PT high voltage center tap - grounded
PT shield wire (Allen TP40D) - grounded
PT 50V bias tap - grounded (should this be grounded or heat shrinked? It didn't cause an issue earlier)
Here's my standby switch wiring:
1. Rectifier pin 8 to Standby
2. Standby to positive end of first filter cap.
3. OT center tap to positive end of first filter cap.
One thing I just noticed is that the turret lug at the base of the + end of the first filter cap is a little loose in its footing. I don't imagine that makes a difference but I don't know exactly.
I seem to be running dry on solutions. What else can I check?
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low level 60 cycle hum
that indicates filaments, possibly jostled wires picking up hum, ground connections loose
did you roll in different tubes for testing?
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that indicates filaments, possibly jostled wires picking up hum, ground connections loose
did you roll in different tubes for testing?
Yep - swapped preamp tubes as well as a pair of output tubes.
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PT 50V bias tap - grounded (should this be grounded or heat shrinked? It didn't cause an issue earlier)
BINGO! PT is probably damaged at this point.
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It’s a bad idea to connect a 50V power source to 0V.
Try disconnecting it and you may be lucky.
What value mains fuse has the amp been fitted with?
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Sluckey & pdf - I was just thinking "how could I have been such an idiot?", but it turns out I didn't have the 50V tap grounded after all. "So I have that going for me... which is nice". On the flip side, I still have a humming amp.
It was after multiple hours - unveiling several weak spots, nonetheless - that I came up with the 50V tap theory. I was just tired. Anyway, looking for the source of the whiff helped me clean up the wiring - still have a few days until I have to take it on vacation with me.
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What grounding scheme did you use? Star; Buss?
silverfox.
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On the flip side, I still have a humming amp.
Teach it some words. Start with something simple and repetitive...
Baby shark, doo doo doo doo doo doo
Baby shark, doo doo doo doo doo doo
Baby shark, doo doo doo doo doo doo
Baby shark!
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No, no not baby sharks, my grandson wants baby sharks every time he visits. I can’t handle any more “Baby Sharks” can’t stop humming that tune, going crazy. I might have to learn that riff, what key is it in...?
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Try the key of A shark...
A#
Grandpa shark, doo doo doo doo doo doo
D#
Grandpa shark, doo doo doo doo doo doo
A#
Grandpa shark, doo doo doo doo doo doo
F
Grandpa shark! :icon_biggrin:
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I'm going with "Baby Snakes"
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What grounding scheme did you use? Star; Buss?
Buss.
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I'm going with "Baby Snakes"
Might be too ambitious for a 5E3. :wink:
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Just turned noon, into the Icehouse already?
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:laugh:
it's sunny n 75 here.........
retirement you only have 2 times sleep n whatever the hell else you want :icon_biggrin:
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You need to allocate some of your time to planning ahead for the early bird special at the Cracker Barrel. Then there's doctors appointments....
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Cracker Barrel.
:laugh:
I did that when I worked, stop #1
but now they're closed permanent Ecoli they can't kill :icon_biggrin:
n I worked for Doc's for 33yrs, it's over-rated
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You need to allocate some of your time to planning ahead for the early bird special at the Cracker Barrel. Then there's doctors appointments....
Hey! You been reading my calendar? :l2: