• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

EAR834p clone with hum. Would appreciate some help

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Depanatoru

Member
Joined 2006
If what is showing the multimeter is real then the power supply is not good at all , maybe the transistor is damaged , wrongly connected , so on ...
All parts should be checked . What is the DC voltage before the regulator ? On AC is showing the same crap ?
 
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AweLoi

Member
Joined 2023
I don't know why but when I read AC on auto I get what you saw in the video, but if I read the AC from the manual ranging instead as Depanatoru suggested I get 0V AC on B+ and 0.2V AC on heaters. So maybe not so bad after all.

I changed all the diodes and the transistor (had extra). No changes regarding hum. Old diodes measured fine too. I measured all resistors still mounted on the board. All measured fine expect the two small ones R22 but that probably because they are still soldered to the board. I don't know how to test the shoke or caps.

I'm using the same PSU as everyone else and all is mounted as it should as far as I can see. Mods on the main board are well known and tested by others. Same goes for the grounding scheme.

This is all above my paygrade. I just thought that following Skunkies videos and Leocoheavens instructions would make everything world fine. Do not have the knowledge to fix this issue. It's not to bad so I can live with it but I am prepared to try out different suggestions.
 
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Depanatoru

Member
Joined 2006
I don't like how grounds are connected there in back with that screw . The star center should be at the filter caps on the board .
If you have mains ground connected there try without it .
Remember that for no audible hum everything must be perfect , every detail is important , RIAA is very sensitive :)
 
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AweLoi

Member
Joined 2023
The mains ground is mounted on it's own bolt next to where the star ground is connected to the chassis. I did have the star ground where you suggested at first (that was also the recommended place in the Lencoheaven guide) but I moved it since it was so crowded by the PSU and also inconvenient while moving around stuff searching for the cause of the hum. I also had hum then too. But of course, that was before I fixed the shielding problem.

I will move it back to that place. 🙂 May I ask why this is recommended? It is good to learn the proper ways.
 
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Depanatoru

Member
Joined 2006
Because a wire or trace on PCB is not a perfect short , there is a little minuscule voltage drop , If diffrent parts of the circuit share a single wire , like you have now from that screw to capacitors ground , can easily influence each other . The most sensitive part can amplify that voltage drop from other currents Or that voltage drop is like a positive feedback and would oscillate .
So wires must go like a star or tree branchesr from filter capacitors positive and negative , that are like shorts for AC , to diffrent separate parts of the circuit that perform diffrent functions and can influence each other
 
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stephe

stephe

Member
Joined 2020
skunkiedesigns.com/
The heaters are lifted.


EAR834 Clone Schematic.jpg
 
lcsaszar

lcsaszar

Member
Joined 2004
There are some common practices against hum:
  • Grid resistors, series 1k resistor directly on the socket, to prevent RF oscillation. Applied at each tube.
  • Ground loop breaker. This is a high current antiparallel diode between signal GND and metal chassis. 10R resistor parallel with the bridge. The earth symbol on the leftmost should be removed.
  • Input ground loop breaker. This is a 10R resistor between the signal GND of the 1st stage (included the RCA common) and the rest of other stages' signal GND.
  • The first 2200uF filter capacitor in the heating supply is too high, resulting in high peak charge current. I'd replace it with a 100uF.
  • Snubber across the output of the rectifier bridges.
  • 2M2 and 3M3 grid resistors seem extreme. I know the 2M2 is part of the RIAA ckt. I would divide the resistors by 10 and multiply the capacitors by 10.
 
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