Thanks for taking the time. I’m mindful of going off topic on the original post and nervous about starting a thread as I already did this weeks ago on a different site. So I apologize for any unintentional wrongdoings. Sharing a few details to answer your questions here and if it’s better I can start a new thread.
The amp is a set of monoblock 300Bs (3.5k to 8ohms) based on the popular Radiotron circuit for 2A3. Most of the circuit is the same as drawn by JELABS with the exception being in the PSU: PT (with higher secondary voltage), reservoir cap and choke. I have tried 3.3uF, 10uF and (currently) 20uF size reservoir cap. Original 5H choke replaced with 10H. The latest changes (20uF / 10H) rendered a measurable, but only minor, improvement.
IIRC shorting output tube to ground as mentioned did not influence the hum.
Voltages seem within range and ok.
There is a hum pot and it’s adjustments change 60hZ.
8mV is audible on my > 100db efficient speakers and sounded high based on others reports. Not egregious but investigating helps me learn so I kept experimenting as I could without a scope and proper education.
I tried to DM you 6A3sUMMER but unable, my account is still under moderation.
The amp is a set of monoblock 300Bs (3.5k to 8ohms) based on the popular Radiotron circuit for 2A3. Most of the circuit is the same as drawn by JELABS with the exception being in the PSU: PT (with higher secondary voltage), reservoir cap and choke. I have tried 3.3uF, 10uF and (currently) 20uF size reservoir cap. Original 5H choke replaced with 10H. The latest changes (20uF / 10H) rendered a measurable, but only minor, improvement.
IIRC shorting output tube to ground as mentioned did not influence the hum.
Voltages seem within range and ok.
There is a hum pot and it’s adjustments change 60hZ.
8mV is audible on my > 100db efficient speakers and sounded high based on others reports. Not egregious but investigating helps me learn so I kept experimenting as I could without a scope and proper education.
I tried to DM you 6A3sUMMER but unable, my account is still under moderation.
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If my estimates are close, the ripple at the junction of 20uF and 10H is about 8Vpeak to peak (the current draw is about 127ma for 2 output tubes, 2 driver tubes, and the B+ bleeder resistor; Xc of 20uF @ 120 Hz (full wave rectification).
Then, at the junction of the 100uF and the end of 10H, the ripple is about 14mVp-p (10H is 7500 Ohms and 100uF is 13 Ohms; divides 8Vpp by 13/7500). 14mVp-p /2.828 is about 5mVrms. After the voltage division the
And the root of the impedance ratio: (3500/8) = 21; the B+ ripple at the output should be about 250uVrms.
If your DMM is good enough, you should be able to measure the ripple at the junction of the 20uF and 10H choke (caution, very carefully).
It appears that your 8mV ripple at the amplifier output transformer secondary is caused by:
Either 8Vp-p across the choke magnetically couples to the single ended output transformer,
Or your AC powered filaments are the problem.
Remove the choke off the chassis (do not touch it), and see if the ripple reduces significantly. If it does, you need non-magnetic washers and non-magnetic screws to mount the output transformer off of the magnetic steel chassis.
Or use an aluminum chassis.
If the hum is still there, it is time for two DC power supplies for the 2A3 / 300B filaments.
Experts: did I miss anything?
If I missed something, then fixing the above items will not fix the problem.
Then, at the junction of the 100uF and the end of 10H, the ripple is about 14mVp-p (10H is 7500 Ohms and 100uF is 13 Ohms; divides 8Vpp by 13/7500). 14mVp-p /2.828 is about 5mVrms. After the voltage division the
And the root of the impedance ratio: (3500/8) = 21; the B+ ripple at the output should be about 250uVrms.
If your DMM is good enough, you should be able to measure the ripple at the junction of the 20uF and 10H choke (caution, very carefully).
It appears that your 8mV ripple at the amplifier output transformer secondary is caused by:
Either 8Vp-p across the choke magnetically couples to the single ended output transformer,
Or your AC powered filaments are the problem.
Remove the choke off the chassis (do not touch it), and see if the ripple reduces significantly. If it does, you need non-magnetic washers and non-magnetic screws to mount the output transformer off of the magnetic steel chassis.
Or use an aluminum chassis.
If the hum is still there, it is time for two DC power supplies for the 2A3 / 300B filaments.
Experts: did I miss anything?
If I missed something, then fixing the above items will not fix the problem.