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Is this a grounding issue or too much gain?
Is this a grounding issue or too much gain?
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Old 18th September 2020, 03:07 AM   #1
slave2music is offline slave2music
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Join Date: Aug 2020
Default Is this a grounding issue or too much gain?

I recently converted a vintage console amp for standalone use with the helpful guidance of some fine members on this forum.

Unfortunately it has an issue, it has a 120hz (I'm pretty sure it's 120 vs 60) hum and and is picking up RF.

I'll do my best to explain the hum and noise scenario(s):

- When no signal source attached (no rca cable attached) - no 120hz hum - just the normal low tube amp buzz with ears up close to efficient speakers
- When try to connect RCAs, at first contact it picks up a radio stations; with the wires connected if I touch the other end of the RCA lead I pick up a radio signal (and normal grounding noise)
- When attaching my tube buffer (SRPP) to it there is a little more noise due to the additional tubes but not enough to really be heard at normal listening distance
- When I add my DAC to the chain there’s a loud thud when connecting the RCAs and the hum gets a little louder– its when I send a signal through the DAC from my PC when the hum gets significantly louder – when playing a signal it seems to be present in between the notes / sound (if that makes sense?) – when I stop playing the hum is louder and constant (the DAC is diy with no issues playing with my other gear)
- Turning off or removing the RCAs of the DAC results in a loud thump but the hum subsides
- When I use other sources – different USB dac or my phone directly into the amp it doesn’t exhibit the same behavior

I redid all the grounding (for the 2nd time) with 3 stars - one for the input and driver tube section; one for the output tubes and the 3rd for the power section - all tie back to the first start near the inputs - so there's really only one main ground point to the chassis.

RCA connectors are grounded to the chassis - I also ran a common ground wire from the connectors to the 1st star. I tried a different set of RCAs wired outside of the chassis with a common ground wire going to the star #1 - to see if my doubling on the grounding was causing the hum - no luck.

Sorry for the long winded explanation but trying to “help you help me”. So is this a grounding issue / loop? Or too much gain (gear matching)? I've checked all my solder joints many times but I'm certainly not a pro, can a bad solder joint cause such an issue? Should I be using shielded RCAs? Should I add a resistor to the input jacks to shield the RF?

Please note I'm working with limited knowledge and limited tools - the extent of my testing equipment at the moment is my DMM.

Appreciate any guidance.
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Old 18th September 2020, 01:20 PM   #2
duncan2 is online now duncan2  United Kingdom
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Join Date: Mar 2013
Location: Scotland.
If you amp starts picking up broadcast signals when an "aerial/antenna " acting aa a phono/RCA lead is applied you need to reduce the input bandwidth by applying a small value ceramic capacitor ( class 1 ) from input to ground .
"Present between notes " sounds like harmonics / HF instability.


Of course if the amp is very sensitive and you apply a large signal you will get problems and yes if the input has a very high impedance especially if directly connected with no impedance reducing network.


I got that a lot with tube/valve amps before I reduced the input impedance/sensitivity --way in the background you could hear radio programs.

OR---- you have insipid HF oscillation only coming on when a signal is applied .
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Old 18th September 2020, 04:47 PM   #3
slave2music is offline slave2music
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Join Date: Aug 2020
My inputs are configured as follows: RCA jack (connected and grounded to chassis) > .068uf film cap > 100k resistor > grid of 12ax7.

Will try adding a .002uf ceramic (I believe they are class 1?) from input to ground, thanks.
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Old 18th September 2020, 05:21 PM   #4
duncan2 is online now duncan2  United Kingdom
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Join Date: Mar 2013
Location: Scotland.
If the 100K resistor is going from the input grid to earth try reducing that but first fit a GRID STOPPER resistor which will be in series with the input capacitor and attached to the grid .
Try a 10K value first ( it can always be reduced ) make sure the 10K resistor is connected DIRECTLY at one end to the grid ( no long runs ) must be close to the INPUT grid which could be -pin 2 or pin 7.


Let me know if that helps or not.


No use a much smaller value ceramic no more than 300pf .
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Old 18th September 2020, 06:13 PM   #5
slave2music is offline slave2music
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To clarify the existing 100k resistor is connected to the input capacitor lead and tied to ground just before the grid.
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Old 18th September 2020, 07:02 PM   #6
duncan2 is online now duncan2  United Kingdom
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Join Date: Mar 2013
Location: Scotland.
If its directly tied to the grid AFTER the input capacitor just leave it there just now but connect that grid stopper resistor onto the same connection as the 100K resistor -- cut the wire leg from the input capacitor that goes to the grid and insert the 10K resistor in SERIES with the capacitor so that the 10K resistor is soldered to the grid along with the 100K resistor .
Recap= 0.068uf capacitor THEN 10K resistor then grid.
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