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Hum in tube amp
Hum in tube amp
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Old 19th March 2013, 01:00 PM   #171
N4BBQ is offline N4BBQ  United States
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Old 11th May 2016, 01:49 AM   #172
Shef is offline Shef
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Asking for help from the forum members on same issue.
Well, the major difference to the OP's case is that if I shorten the direct inputs - the amp goes almost dead silent, otherwise. ..

The info:
- it is a Japan-made piece (ran through a 120->100V 500VA tranny), the ground pin on mains connector is floating, the ground topology seems a proper star;
- the grounds from fillament windings and bridges come to the 1st filter caps' negative terminal;
- it has switched inputs - direct and variable, if I switch to variable and turn the knob leftmost the hum goes to pretty much low level;
- all the input wires seem shielded cables, layed around the far side edge + in the corners;
- the 6.3 and 12.6 V filaments are twisted everywhere, neither CT on each so they are simply grounded on one end (that end seems same phased between two filament winding);
- the B+ filter is a proper CLCRCRCR;
- all transformers are EI and locatead as far from each other;
- there is no shield layer inside of mains transformer.

The inputs are isolated from the chassis, there are 100K + 100p in parallel to the input, and the 0.27uF soldered between each RCA's groung to the separate thick ground wire which is attached to chassis nearby the inputs. The jumpwire from star-center on 1st cap also goes here. The variable input has the 100K pot instead. Then the wires go to switch -> 2.2uF electrolytic -> the 1 MOhm in parallel -> 5" & 10" shielded wires to the fist stage's grid and the shield is soldered to the stage's local ground.

As I mentioned:
- if I shorten the direct input the amp goes most silent, the huge portion of audible hum is 60 goes away. At this point the residual 120 hum is very little, but if I then attach the additional jumpwire from RCA's ground to 1st (2nd) stage's ground - the 120 goes to almost inaudible;
- it does not make much of difference if the signal source is connected to the direct input or not. When it is - slightly higher level of 60 is present, but again, if I shorten the input - the hum goes to zero;
- the variable input works a bit better: if I have the pot at e.g. 12 the level of 60 is half as much;
- if I turn the power off -the hum stays on same level and decays as the filter capacitors drain.

Please advise where to look at.

I was thinking of the decoupling input transformer, like 600:600 so its secondary would short the input, but. .. would it make any difference? if almost same level of 60 is present with no signal source.

Last edited by Shef; 11th May 2016 at 01:56 AM.
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