Preamp hum issues

[ryuji]

im using this circuit for my preamps. when i pull the preamp tubes all hum goes away, meaning the cause is indeed, the pre? stranger part is the 12at7 drivers for my 807's don't seem to hum at all even tho they have a much higher gain?

using a smps dc supply on my heaters.



its somewhat bearable if i dont pay attention to it but i would like to try and get it silent if i can.

[M Gregg]

Originally Posted by ryuji

im using this circuit for my preamps. when i pull the preamp tubes all hum goes away, meaning the cause is indeed, the pre? stranger part is the 12at7 drivers for my 807's don't seem to hum at all even tho they have a much higher gain?

using a smps dc supply on my heaters.



its somewhat bearable if i dont pay attention to it but i would like to try and get it silent if i can.

ryuji,

Just a thought!

It could be:

The cathode on tube 2 on the circuit is at 165V.
The heater supply is SMPS. So the SMPS output needs to be isolated from Gnd and lifted as in the Aikido circuit.

To prove this if you put a two pole switch in the connection from the heater supply then after its all working and up to temp switch the heaters off for a second and see if the hum stops.

This will show if its the heater supply problem.

Regards
M. Gregg

[ryuji]

My filament smps only shares a connection to my three prong outlet. I am using a pre-built product so its already pretty isolated. Ill try the heater supply trick later

[Vinylsavor]

Hi!

As the previous poster said, the cause could be the missing reference to ground of the heaters. The heaters should be elevated so that the heater-cathode voltage is maintained within limits. This is best done with a voltage divider from B+. Also out a cap from heater to ground, sa 47uF.

Another cause could be too much ripple on the B+ or a grounding issue

Best regards

Thomas

[M Gregg]

Originally Posted by ryuji

My filament smps only shares a connection to my three prong outlet. I am using a pre-built product so its already pretty isolated. Ill try the heater supply trick later

ryuji

Are you sure that none of the cables to the tube heaters are connected to the mains earth inside the SMPS?
The output from the SMPS is completely isolated?

Regards
M. Gregg

[ryuji]

Originally Posted by M Gregg

ryuji

Are you sure that none of the cables to the tube heaters are connected to the mains earth inside the SMPS?
The output from the SMPS is completely isolated?

Regards
M. Gregg

there is a mains earth connection on the smps. i could disconnect it. i could also use some insulating material on the metal on the smps chassis and the amp chassis for further isolation.

when i initially build my amp i continuity checked my heaters with b+ and ground, there was no connection at least when powered down.

Originally Posted by Vinylsavor

Another cause could be too much ripple on the B+ or a grounding issue

i would think that my higher gain 12at7 stage would hum too if there was b+ ripple. psud2 shows there being almost none.

[ryuji]

this is the rest of my tube amp, just im running one output tube instead of two.


due to my higher b+ the preamp stages run at 160-170v on the anode and ~211v on the cathode but its still well within datasheet limits

total consumption after warmup is 260 mA on b+

[M Gregg]

ryuji

Just try switching the heaters off on the pre amp for a second and see if the hum stops. It has to be both + & - of the SMPS.

You must not remove the safety Earth on the SMPS. Just check that the output is isolated from Gnd if you try to lift the heaters.

What is the Plot like on volts C1, C2?

Do you remember the plot on some PSUs I tried! C1 ripple is interesting. What is the largest cap you can use after the rectifier without damage? (Just to try it).

The charge pulses may be an issue.

Just a few thoughts!

The ony other thing is a Gnd loop!

Regards
M. Gregg

[ryuji]

Originally Posted by M Gregg

ryuji

Are you sure that none of the cables to the tube heaters are connected to the mains earth inside the SMPS?
The output from the SMPS is completely isolated?

Regards
M. Gregg

i just continuity tested with it unplugged again. both sides of the filament outputs have no continuity with the chassis, earth, or AC hot/neutral

[ryuji]

Originally Posted by M Gregg

ryuji

Just try switching the heaters off on the pre amp for a second and see if the hum stops. It has to be both + & - of the SMPS.

You must not remove the safety Earth on the SMPS. Just check that the output is isolated from Gnd if you try to lift the heaters.

What is the Plot like on volts C1, C2?

Do you remember the plot on some PSUs I tried! C1 ripple is interesting. What is the largest cap you can use after the rectifier without damage? (Just to try it).

The charge pulses may be an issue.

Just a few thoughts!

The ony other thing is a Gnd loop!

Regards
M. Gregg

testing disconnecting the pre's heaters will be difficult. my heater circuit would allow me to disconnect all of the 12a[x,t]7 tubes as a unit. much easier to pull up my whole amps 12v supply at once from the smps, but i imagine output tubes would react much more rapidly to loss of heater emissions?