Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: uki on January 13, 2026, 08:50:18 pm
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Hey guys !!
I built this amp about 12 years ago, and with the help of the good folks around here got the amp working properly!
This is the amp I use for giging (https://youtu.be/s0mNYvYM9kw)!
But now the amp is humming what looks to be 60hz, I may be wrong thou
PA tubes apparently are working hotter, but it is summer and its very hot
Hum increases as volume goes up either channel
I did pull the PI tube to listen if the hum would stop or stay, it does keep happening w/o PI, although with very low volume
Amp does not have master volume
Here some voltage readings, it did not chance from years back
Bias -43
Node A 405 vdc
B 404 vdc
C 380 vdc
D 242 vdc
E 294 vdc
F 275 vdc
Some guidance to fix this hum down would be appreciated, thanks in advance!
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Also i did a bulb(100w) test
At power on it did flash full light, then faded down to a dimmed light, then with standby on the light increased.
no input signal
follow pictures of the bulb
thanks
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Maybe one of you're power tubes is not as balanced as it once was. If you pull the PI and there's still hum it points towards the power section.
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Maybe one of you're power tubes is not as balanced as it once was. If you pull the PI and there's still hum it points towards the power section.
I did found the old notes, it is from 1ohm cathode resistor, to compare with today.
old: 25.8mv, 26.2mv, 24.6mv, 24.8mv
today: V7 20.8mv, V8 20.5mv, V9 20.9mv, 21.5mv
also I've compared the grid resistors:
old: V7 1607ohms, V8 1632ohms, V9 1636ohms, V10 1569ohms
today: v7 1666ohms, V8 1840ohms, V9 1844ohms, V10 1622ohms
Screen resisitors:
I have no notes from before, those read today:
V7 477ohms, V8 498ohms, V9 568ohms, V10 477ohms
All resistors in the amp are carbon composition.
Any help is appreciated, Thanks
picture from 2015
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I agree with Al, tubes first. Got spares? You must if you gig with it. If swapping tubes does not reveal anything, my next step would be to check all the grounds. Easy after 12 years for something to come loose or build up some corrosion. Clean the pots and all plug/jack connections? Filter caps should last 12 years, but :dontknow: Anyways, that's some easy stuff.
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Quite a bit less current now than before. What are your plate voltages at the power tube plates now? The grid voltage hasn't changed?
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I agree with Al, tubes first. Got spares? You must if you gig with it. If swapping tubes does not reveal anything, my next step would be to check all the grounds. Easy after 12 years for something to come loose or build up some corrosion. Clean the pots and all plug/jack connections? Filter caps should last 12 years, but :dontknow: Anyways, that's some easy stuff.
No spare tubes, yet. Indeed i found some loose stuff, but nothing related to the hum thou, grounds are good
Quite a bit less current now than before. What are your plate voltages at the power tube plates now? The grid voltage hasn't changed?
It does read the same as years back
v7-10 408 vdc
Also there is a reading from pin 5 to ground in ohms, its different now
old: v7 294k, v8 294k, v9 283k, v10 283k
now: v7 275k, v8 275k, v9 258k, v10 258k
the power tubes does feel hotter, i may be wrong thou
thanks
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Are you measuring with the light bulb still in circuit?
If so that would reduce voltages from operating conditions.
Otherwise, it seems strange there would be less current but similar voltage at the plate. Perhaps it's a leaky filter cap. Also, what's your DC voltage at pin 5 of the power tubes?
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Are you measuring with the light bulb still in circuit?
If so that would reduce voltages from operating conditions.
Otherwise, it seems strange there would be less current but similar voltage at the plate. Perhaps it's a leaky filter cap. Also, what's your DC voltage at pin 5 of the power tubes?
No light bulb while reading voltages
Pin 5 of PA tubes -42 vdc
Thanks
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Probably want to check resistances across your filter caps with the amp off and drained. Try with the stand by switch open, as that will isolate the reservoir caps from the other caps in the chain.
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Probably want to check resistances across your filter caps with the amp off and drained. Try with the stand by switch open, as that will isolate the reservoir caps from the other caps in the chain.
I've disconnected the + side of the reservoir caps from the circuit and measured uF and resistances
cap 1 - 21.98uF , there is ohms and raising, did read it until 4-5m
cap 2 - 21.58uF , there is ohms and raising, did read it until 4-5m
DMM can read 200m ohms
there was a moment when cap 1 was reading like 160m ohms and raising(confusing), then after reading again it gave the reading above.
DMM battery is new
also i did this test (https://www.youtube.com/watch?v=T2dvMu__JEk) and result seen to be the good case
Also i did a esr test with scope(hope i did this right) follow on pics
Thanks
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What about the filter caps on the pre amp side? You shouldn't have to lift one end unless you get low readings, there should be no other paths to ground.
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What about the filter caps on the pre amp side? You shouldn't have to lift one end unless you get low readings, there should be no other paths to ground.
Ok got new readings w/o disconnecting the caps.
6m ohms for the power section
16m ohms for the preamp section
Did play a bit with amp to hear what is going on,
amp start with low noise level, the noise increase as the amp warm up,
did play for about 20 minutes.
Thanks
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Maybe the power amp caps are weak? Maybe it's time to focus again on a tube change. The fact that one of the screen resistors is off spec is interesting. You could pull 2 tubes, one on each side of the output transformer, check for noise, and then check the other 2 tubes the same way, and see if you can isolate where the noise is coming from.
The screen resistor should probably be replaced either way.