Hum from my Tube amp (300B) - Please help!

[tomchr]

Does the amp have a built-in volume control? If yes, does the hum go away if you turn the volume to minimum? How about if you turn it to maximum?

If the amp does not have a built-in volume control, does the hum go away if you short the input? I.e. connect the RCA centre conductor to the RCA shell.

If you have no hum with the inputs shorted (or volume at minimum), the amp is not likely to blame. The only exception would be if you have zero hum when the volume is at minimum or maximum, but you have hum at intermediate settings of the pot. In that case you'll probably find that it's worse with the volume around 2 o'clock (-6 dB). In that specific case, you can probably get rid of the hum by grounding any metal of the volume potentiometer.

If you have hum when the volume is at minimum (or the amp input is shorted), the hum is either getting in through the heaters, the bias, or the B+. If you're running AC on the 300B filament, change it to a 5.0 V DC filament regulator. If you still have hum, I'd probably go after the B+. A high-voltage regulator such as the 21st Century Maida Regulator can be helpful here.

Tom

 

[Saffy]

Thank you Tom for your response, and sorry for the delay.
300b amp does has a passive volume control, and the low hum remains the same whether at 5%, 95%, or in-between volumes (this with a cheater plug on either on the DAC or amp).

Without a cheater plug: a high pitched hum is heard.
Interesting that high pitched hum is eliminated (equally) with the cheater plug either on DAC or amp. Therefore, given a choice, I guess I'm better off placing it upstream (DAC) rather than on amp?

Still have the low hum problem.
Thanks

 

[tomchr]

It's probably hum on the B+ then. A regulator will address that, though it does need a bit of headroom, so you'll have to live with a lower B+ voltage or bump it up a bit going into the rectifier.

Tom

 

[Jim Matthews]

I gather you're driving something efficient with these?

 

[Saffy]

Yes full range 98db speakers, I have plenty of headroom at 2:00pm volume

 

[Saffy]

It's probably hum on the B+ then. A regulator will address that, though it does need a bit of headroom, so you'll have to live with a lower B+ voltage or bump it up a bit going into the rectifier.

Tom

Please excuse my ignorance, though is there another solution that doesn't require changing amp components?

 

[pma]

To change the whole amp for SS. You have very efficient speakers and these SE triode power amp circuits are perfect hum/buzz and heat generators, with sound as a byproduct.

 

[tomchr]

Please excuse my ignorance, though is there another solution that doesn't require changing amp components?

If you're dealing with hum induced via the B+, your options are:

• Regulate the B+
• Add more reservoir capacitance (or other filtering)
• Turn the amp off

Sorry. But most tube amps are not the best performers in the hum department. This is especially true of SETs like the 300B. The turns ratio on the output transformer is maybe 20:1 so if you have a few hundred mV of hum on the 360-400 V B+, you will have tens of mV of hum on the amp output. That's definitely audible.

I developed the 21st Century Maida Regulator to deal with B+ hum in my DG300B. The amp was completely hum-free after I added the regulator.

Tom

 

[Saffy]

Thanks Tom, where does your regulator install?

 

[Saffy]

If you're dealing with hum induced via the B+, your options are:

• Regulate the B+
• Add more reservoir capacitance (or other filtering)
• Turn the amp off

Sorry. But most tube amps are not the best performers in the hum department. This is especially true of SETs like the 300B. The turns ratio on the output transformer is maybe 20:1 so if you have a few hundred mV of hum on the 360-400 V B+, you will have tens of mV of hum on the amp output. That's definitely audible.

I developed the 21st Century Maida Regulator to deal with B+ hum in my DG300B. The amp was completely hum-free after I added the regulator.

Tom

Thanks Tom, where does your regulator install?

 

[Saffy]

To change the whole amp for SS. You have very efficient speakers and these SE triode power amp circuits are perfect hum/buzz and heat generators, with sound as a byproduct.

Understood, though the amp. didn't come with the hum. How can i tell if what the culprit is?

 

[tomchr]

Thanks Tom, where does your regulator install?

It goes after the first smoothing cap. So basically:

Transformer -> Rectifier -> Smoothing Cap -> Regulator -> Amp

The biggest challenge when fitting the regulator in an existing amp is that it does require a heat sink, so you need to fit that in.

Tom

 

[Saffy]

To change the whole amp for SS. You have very efficient speakers and these SE triode power amp circuits are perfect hum/buzz and heat generators, with sound as a byproduct.

I'm, contemplating moving to SS - looking at Eval-1 from VTV. Your opinion? Thanks

 

[tomchr]

I suggest reading reviews of VTV's amps.

I would steer you in the direction of my Purifi/Hypex Buffer if you want to build your own amp, but I'm obviously biased. You can probably find more inspiration in this thread: https://www.audiosciencereview.com/forum/index.php?threads/diy-purifi-amp-builds.10478

Tom

 

[Fernando]

I managed to stop the noise by changing the tube, it was faulty.

 

[tomchr]

Oh, cool! Was it a bad driver tube or was the 300B defective?

Tom

 

[Saffy]

I managed to stop the noise by changing the tube, it was faulty.

Thank you good idea. I've tried every form of isolation: physically disconnecting all inputs into my amp including their power cords from my power bar, then bypassing power bar directly to outlet - no difference in hum. I have since used an extension cord from my amp to various outlets / circuits also to no avail. Before I start swapping out tubes one by one, is there any other test I can perform? BTW which tube was the culprit in your rig?

 

[tomchr]

Do you have hum in one channel or in both? How many driver tubes does it have? Are the tubes shared between channels?

Tom

 

[MakeMineVinyl]

As mentioned previously, single ended triode amplifiers are just prone to exhibiting low level hum/buzz since any ripple on the B+ supply just finds its way straight to the output of the amplifier. The only real cure is to regulate the B+ in some way, but this is probably not practical in most cases unless you're comfortable with tearing into the circuitry (not to mention that the high voltages which can remain on capacitors even with the power disconnected can cause some very unpleasant experiences). On my own 300B amplifier, I inserted a choke before the smoothing capacitors, and doubled the amount of smoothing capacitance. This cut down the buzz to an acceptable level.

Lacking the ability to electronically modify your amplifier yourself, you're probably better off with a solid state amplifier with your very efficient speakers.

 

[Saffy]

Do you have hum in one channel or in both? How many driver tubes does it have? Are the tubes shared between channels?

Tom

Both channels hum equally. 2-input tubes: 6sl7-gt & 6sn7-gtb. Rectifier is 5ar4.