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I’m getting a hum in my integrated tube amp and it’s bugging me alot

Hi - first of all, here’s my system:

- Rogers High Fidelity EHF-200 mkII integrated tube amp

- Magico A5 speakers

- Merging NADAC + Power DAC

- Audience T2 power conditioner

- Purist cabling

I love the sound of my system. BUT…….I have been noticing a hum after about 10 minutes of use from my amp. There are two switches on the amp - POWER & OPERATE - that need to be turned on in order to listen to music. (The Power switch needs to be turned on about 30 seconds prior to switching on the Operate switch). With both KT150 and KT88 tubes (all matched), after about 10 minutes I hear an audible hum from the amp that is apparent from my listening position about 10 feet away. If I turn off the OPERATE switch, the hum disappears (but of course, then I can’t listen to music). I have contacted Roger (the company owner) who said it was noise from old tubes. But my KT88’s have less than 100 hours on them and they hum as well as the KT150’s (maybe a few db’s softer than the 150s’). 

 

Does anybody have any ideas about what causes this and how it might be fixed? Is this an issue that is endemic to tube amps? FYI - if this requires new tubes, then I need to switch back to SS as I’m not willing to invest approx $800/year in new tubes. 

 

I turn to the great wisdom of this forum to help.

 

Thanks - Matt

mwsl
perkri
907 posts
 

Since the hum appears after the tubes warm up, and not immediately, it sounds like a tube issue to me. Is the problem in both channels, or just one?

And just to confirm, the sound is coming from the speakers, and not from the amp. Is that right?

imhififan's avatar
imhififan
2,180 posts
 

Disconnect the speakers (and everything else), if you still hear the hum, it's obviously the amp, and likely transformer noise.

Please check with the manufacturer if it is safe to power ON this amplifier without speakers connected!

 

mwsl OP
52 posts
 

Again - sorry for the delayed reply, am currently traveling.

I powered on the amp with just the speakers connected and the hum was still present. I will try to power on the amp with nothing connected; my best guess is that the hum will still be present.

The manufacturer previously suggested that the hum was due to tube degradation. I replaced two KT150 tubes that were looking like they’d had more wear than the other two, but the hum was the same. As stated in my original post, I replaced all 4 KT150 tubes with a matched set of 4 KT88 tubes that all had limited hours on them (under 100)……and the hum was still present (although perhaps slightly softer?)

When I spoke to the manufacturer to arrange for sending the amp back, he said the only cause he could think of was the transformer. 

I will be sending the amp back to the manufacturer in early December; stay tuned for an update in mid-December (hopefully, there will not be excessive shipping delays!).

mwsl OP
52 posts
 

OK, I finally have an update: the amp has visited the manufacturer (Roger of Roger’s High Fidelity). Roger said that my hum was due to 2 (of 4) bad KT150 tubes. He had no explanation for why my new matched set of KT88 tubes were producing hum (at perhaps a slightly lower volume). I’m planning to purchase 4 new KT150’s from The Tube Store unless somebody out there has a better recommendation for sourcing reliable and matched tubes. 

I will post again after I receive the amp back and have the new tubes installed. Thanks for everybody’s help and interest.

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