model 20 hum (detector circuit?) - Printable Version
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RE: model 20 hum (detector circuit?) - OZ4 - 10-25-2015
I thinking the hum is coming from the power supply. Try hanging a about 20uf cap on the bias line coming from the power supply to ground.
RE: model 20 hum (detector circuit?) - OZ4 - 10-25-2015
And as for the bias on the 27 since both 27 and the 71s bias come from the same line and with the bias voltages on the 71s are correct it must be either the bias resistor or the meter you are using. I would try an old analog Simpson meter or hanging a 200k resistor across a digital one.
( I was thinking a 5meg resistor would be a correct load. But using the a Simson on the 10v scale at 20k/volt would be 200k loading)
RE: model 20 hum (detector circuit?) - Tubesforme - 10-25-2015
(10-25-2015, 10:57 AM)OZ4 Wrote: I thinking the hum is coming from the power supply. Try hanging a about 20uf cap on the bias line coming from the power supply to ground.
It isn't the filters as they have now been replaced three times, each time with a different brand, but of the correct value. I put a 10uf across the output capacitor and then tried the input with no change. Extra filtering isn't the answer.
RE: model 20 hum (detector circuit?) - Kenneth F. Besso Jr. - 10-25-2015
Had the same problem once,,,,I put a 20 ohm 25 watt power resistor on the AC,, coming in,,dropped the voltage down to 115,,,was 129,,,to much voltage ,,,,Plays H**l with that old stuff,,transformer and speaker,,
RE: model 20 hum (detector circuit?) - morzh - 10-25-2015
Not a bad thought there, take Variac, lower the voltage, see if that cures it.
Another thing, the hum might be your xfmr vibrating. Not too much can be done but the hum also gets louder with increased voltage.
RE: model 20 hum (detector circuit?) - Steve Davis - 10-26-2015
I don't think it is the transformer vibrating because the hum stops when he pulls the 27 tube.
Steve
RE: model 20 hum (detector circuit?) - OZ4 - 10-26-2015
I wasn't thinking extra filtering was the answer.
If the tube and the socket are good then it must coming in on the coupling cap or the bias line.
You did say that a cap did lower the hum when applied at the grid.
I am thinking maybe there is some weird feed back from the final section or some other source of the hum coming in on the bias supply.
With the bias supply line common for both the 27 tube and the coupling transformer center tap which is the bias to the 71s, the transformer is a good suspect.
RE: model 20 hum (detector circuit?) - OZ4 - 10-26-2015
Another way to verify the bias supply would be to lift the bias resistor and use a different bias supply. A different power supply. Maybe a 9 volt battery and pot to adjust the bias. This would isolate it from everything else.
RE: model 20 hum (detector circuit?) - OZ4 - 10-26-2015
You haven't mentioned having a scope. It would be helpful here.
RE: model 20 hum (detector circuit?) - Tubesforme - 10-27-2015
(10-26-2015, 11:24 AM)OZ4 Wrote: You haven't mentioned having a scope. It would be helpful here.
Yes it would, but I sold most of my equipment and parts inventory. I am retired now
RE: model 20 hum (detector circuit?) - Tubesforme - 10-27-2015
(10-26-2015, 10:18 AM)OZ4 Wrote: I wasn't thinking extra filtering was the answer.
If the tube and the socket are good then it must coming in on the coupling cap or the bias line.
You did say that a cap did lower the hum when applied at the grid.
I am thinking maybe there is some weird feed back from the final section or some other source of the hum coming in on the bias supply.
With the bias supply line common for both the 27 tube and the coupling transformer center tap which is the bias to the 71s, the transformer is a good suspect.
I would agree, but the transformer was replaced with a NOS Stancor listed for that set as the original had i/2 of the sec open.
RE: model 20 hum (detector circuit?) - Tubesforme - 10-27-2015
(10-24-2015, 07:04 PM)mikethedruid Wrote: Does the hum go away if you pull the type 24 tube ?NO
RE: model 20 hum (detector circuit?) - OZ4 - 10-27-2015
So that's new? Earlier you said that pulling any of the 24 tubes had no effect. Still humming.
RE: model 20 hum (detector circuit?) - Tubesforme - 10-27-2015
(10-27-2015, 08:43 PM)OZ4 Wrote: So that's new? Earlier you said that pulling any of the 24 tubes had no effect. Still humming.
I apologize I was quickly reading and am upset with the chassis and thought you wrote 27 tube, not 24. The hum stays with all of the 24's out of socket or any one of then out. It naturally leaves when the 27 is pulled and is gone when the 27 grid is grounded. Filtering the source of the grid bias for the 27 did not affect the hum.
RE: model 20 hum (detector circuit?) - Steve Davis - 10-28-2015
In an earlier post mikethedruid asked if you have checked the ground on condenser 14. Have you done that? Also, check the value of that condenser and resistor 18.
Steve