19-07-2020, 05:18 PM
(This post was last modified: 19-07-2020, 05:23 PM by Murphyv310.)
Thanks Rob.
OK so now lets gets this fixed tonight.
Question time.
1.... is the hum heard at three feet or more or is it ear to the speaker?
If its ear to the speaker then I'd say that is normal. On a Push -Pull output stage like your amp the output stage effectively cancels out hum if it is working OK which if it is very slight I'd say it is working as it should.
2...... Not so much a question but the next step to take if the hum can be heard at a distance from the speaker. Remove all the ECC83's leaving just the two EL84 & EZ81 in place. Does doing that remove the hum you have?
If not then you will need to measure the the Anode voltages on both the EL84's (pin7) screens (pin9) the control grids (pin2) and the cathodes on pin3. There will be a slight difference between the voltages on the anodes due to the way the transformer is wound, there is normally a difference between the windings.
3...... Again if there is significant hum I want you to scope the ripple on C14 (reservoir) cap and the smoother C4. Take a picture if you can and tell me the p-p value of the ripple on both the caps.
4........ If the hum is quiet does it get louder with all the valves in place? if so between V1 & V2 which one is worst (V3 needs to be in place for this test)
5......... Back to significant hum with just the EL84's in place........ One further thing does pulling the EZ81 stop it altogether? Not such a stupid question as if the transformer is not well screened it could be inducing hum to the output transformer.
OK so now lets gets this fixed tonight.
Question time.
1.... is the hum heard at three feet or more or is it ear to the speaker?
If its ear to the speaker then I'd say that is normal. On a Push -Pull output stage like your amp the output stage effectively cancels out hum if it is working OK which if it is very slight I'd say it is working as it should.
2...... Not so much a question but the next step to take if the hum can be heard at a distance from the speaker. Remove all the ECC83's leaving just the two EL84 & EZ81 in place. Does doing that remove the hum you have?
If not then you will need to measure the the Anode voltages on both the EL84's (pin7) screens (pin9) the control grids (pin2) and the cathodes on pin3. There will be a slight difference between the voltages on the anodes due to the way the transformer is wound, there is normally a difference between the windings.
3...... Again if there is significant hum I want you to scope the ripple on C14 (reservoir) cap and the smoother C4. Take a picture if you can and tell me the p-p value of the ripple on both the caps.
4........ If the hum is quiet does it get louder with all the valves in place? if so between V1 & V2 which one is worst (V3 needs to be in place for this test)
5......... Back to significant hum with just the EL84's in place........ One further thing does pulling the EZ81 stop it altogether? Not such a stupid question as if the transformer is not well screened it could be inducing hum to the output transformer.
