tube buzz/hum

I have the ST-120 that I built from a kit last year and I love it. Last year I purchased a 12BH7 tube for the driver position but got a buzz/hum through the speakers. Bob and Audiobill tried to diagnose the problem but the buzz was too loud for me to leave the tube in place (it wasn't real loud but it was noticeable more than just a few inches from the speaker).  I ended up using the 12AU7 Raytheon that came with the kit and that has worked fine.

I also bought a couple of older tubes from an E-Bay seller someone in this forum recommended - an RCA 12AU7 clear top and an RCA 5963. I tried both over the weekend. The clear top works very well and to my ears I prefer it over the Raytheon.  The 5963 gave me a buzz through the woofer, though. Not as loud as the 12BH7 but again, enough to bother me.

When I use the Raytheon or the RCA clear top, there is just a very faint hum through the woofer that is audible only with my ear up to the speaker grill and maybe an inch or two beyond that.

My question is do different tubes just not play well with the amp and I have to find the right combo, or is there something I can do to fix this?  It doesn't seem like a ground loop problem but if it is, then why would some tubes have a louder hum/buzz than others?

peterh wrote:

monkuboy wrote:I have the ST-120 that I built from a kit last year and I love it. Last year I purchased a 12BH7 tube for the driver position but got a buzz/hum through the speakers. Bob and Audiobill tried to diagnose the problem but the buzz was too loud for me to leave the tube in place (it wasn't real loud but it was noticeable more than just a few inches from the speaker).  I ended up using the 12AU7 Raytheon that came with the kit and that has worked fine.

I also bought a couple of older tubes from an E-Bay seller someone in this forum recommended - an RCA 12AU7 clear top and an RCA 5963. I tried both over the weekend. The clear top works very well and to my ears I prefer it over the Raytheon.  The 5963 gave me a buzz through the woofer, though. Not as loud as the 12BH7 but again, enough to bother me.

When I use the Raytheon or the RCA clear top, there is just a very faint hum through the woofer that is audible only with my ear up to the speaker grill and maybe an inch or two beyond that.

My question is do different tubes just not play well with the amp and I have to find the right combo, or is there something I can do to fix this?  It doesn't seem like a ground loop problem but if it is, then why would some tubes have a louder hum/buzz than others?

different tubes has varying types of filaments. Some has dual-twisted to avoid hum other
don't care.  Sometimes the filament can emit enough electrons at the edges of the cathode
to be detectable.
I'm afraid that 12bh7 is not made to be silent.  Try ECC99 instead.

Thanks, I guess the 12BH7 I bought is just not meant to be a good match. I do like the RCA clear top that is currently in there, though. And that one is quiet.

An audio tech that's helped me out a few times pointed out to me that a lot of the 'NOS' tubes being sold these days are probably around today because they were not used when new because they didn't meet standards of quality back in the day. Granted probably not all that way but I be cautious about paying a premium for something just because it's NOS.

That being said could the reason the tube buzzes because it's somehow defective old stock?

Though I've also read the 12BH7 has a tendency to be microphic. Could this be the cause of the noise?

Dogstar wrote:An audio tech that's helped me out a few times pointed out to me that a lot of the 'NOS' tubes being sold these days are probably around today because they were not used when new because they didn't meet standards of quality back in the day. Granted probably not all that way but I be cautious about paying a premium for something just because it's NOS.

That being said could the reason the tube buzzes because it's somehow defective old stock?

Though I've also read the 12BH7 has a tendency to be microphic. Could this be the cause of the noise?

I don't think the 12BH7 is microphonic as it just made this buzz through the speaker but not like when you tap on a tube. Well, who knows... at least the stock tube Bob sold me with the kit worked fine and the RCA I just tried over the weekend is good, too. I'm not really big on tube rolling (thankfully!). Most likely the 12BH7 just doesn't blend well with my amp.

Do you have an input connected when it's buzzing?

j4570 wrote:Do you have an input connected when it's buzzing?

Yes, it is connected to a Schiit Saga preamp. When I first turned on the ST-120, there was a faint buzz through the woofer using the 5963 tube, definitely louder than the Raytheon 12AU7 Bob had sold me when I bought the kit, and the 12AU7 RCA clear top. I have the delay timer installed on the amp so after the 17 or 18 seconds or so when the timer switched on, the buzz became louder. I could hear it through both speakers. It isn't audible from my listening chair but it was still bothersome so I put the clear top back in its place.

monkuboy wrote:

Yes, it is connected to a Schiit Saga preamp.  When I first turned on the ST-120, there was a faint buzz through the woofer using the 5963 tube, definitely louder than the Raytheon 12AU7 Bob had sold me when I bought the kit, and the 12AU7 RCA clear top. I have the delay timer installed on the amp so after the 17 or 18 seconds or so when the timer switched on, the buzz became louder. I could hear it through both speakers. It isn't audible from my listening chair but it was still bothersome so I put the clear top back in its place.

"the buzz became louder"... Interesting that you have slight hum before the HV relay delay activates. This happened to me once on a guitar amp build where I had hum immediately as the amp heaters were switched on - even with no tubes installed. Just happened to be the power transformer coupled 60Hz directly to the output tranny. I had to physically relocate the output transformer to eliminate the mutual coupling.

Not saying this is the same issue but do you hear the hum in both channels before the relay kicks in? My ST-120 is very quiet so surprised.

arledgsc wrote:

monkuboy wrote:

Yes, it is connected to a Schiit Saga preamp.  When I first turned on the ST-120, there was a faint buzz through the woofer using the 5963 tube, definitely louder than the Raytheon 12AU7 Bob had sold me when I bought the kit, and the 12AU7 RCA clear top. I have the delay timer installed on the amp so after the 17 or 18 seconds or so when the timer switched on, the buzz became louder. I could hear it through both speakers. It isn't audible from my listening chair but it was still bothersome so I put the clear top back in its place.

"the buzz became louder"... Interesting that you have slight hum before the HV relay delay activates.  This happened to me once on a guitar amp build where I had hum immediately as the amp heaters were switched on - even with no tubes installed.   Just happened to be the power transformer coupled 60Hz directly to the output tranny.  I had to physically relocate the output transformer to eliminate the mutual coupling.

Not saying this is the same issue but do you hear the hum in both channels before the relay kicks in?  My ST-120 is very quiet so surprised.

Yes, I hear the hum in both channels. With a quiet tube like the Raytheon or the RCA clear top, it is very faint and it is sort of a cross between a buzz and a hum. I can only hear it with my ear right up to the speaker grill. When the relay kicks in, the hum gets only slightly louder and it isn't audible more than several inches away. The 12BH7, however, was noticeably louder like a couple of feet away from the speaker. The 5963 wasn't that loud but it was loud enough that I decided not to use it. With the quiet tubes, I would say the amp is very quiet. The tweeters are silent. With the solid state amps I've used, I've heard some hiss through the tweeters and in a couple of cases, also a buzz through the woofer.

Do you have the stepped attenuator installed? If so disconnect the input and turn all the way down. If no attenuator short the inputs to ground.... Same issue?

Further you may try unplugging the center tube and turn it on.... If all quiet before and/or after the relay engages then start looking in the preamp circuit area. If not issue is further downstream.

I just don't see how the tubes could be amplifying noise before they have high voltage supply. What type rectifier are you using - tube or SS?

arledgsc wrote:Do you have the stepped attenuator installed?  If so disconnect the input and turn all the way down.  If no attenuator short the inputs to ground.... Same issue?

Further you may try unplugging the center tube and turn it on....  If all quiet before and/or after the relay engages then start looking in the preamp circuit area.  If not issue is further downstream.

I just don't see how the tubes could be amplifying noise before they have high voltage supply.  What type rectifier are you using - tube or SS?

The 12BH7 tubes were purchased early summer last year so I'm just going by foggy memory but the problem did seem to be the tube because with no center tube there was no buzz, and shorting the inputs made no difference. I have the stepped attenuator installed but can't remember if I did that before getting the tube or after. Anyway, Bob and Audiobill suggested some remedies and nothing worked so I have to conclude the amp just doesn't like the tube (or vice versa), especially since other tubes work just fine. Being that I know little about electronics, maybe it's best to leave it. But thank you for your help, though!

I'm with PeterH on this. Different tubes are made using different structures internally, and some are noisier than others.
Also some peoples ears are more sensitive to frequencys that sound like hum/buzz/noise. If you can't hear it under normal conditions (farther than 2 feet from your speakers)
than it's a non-issue for me.

"It isn't audible from my listening chair"

If so, I fail to see the problem ...

If you stand too close to a painting, all you see are patches of color. If you stand too far back, you can't see any of the detail. Right now this is your particular perspective and if you ask me ... you're a little too close.

Be a shame to pass up an entire group of tubes because of a little noise you can't even hear under normal use. I love the 12BH7's overall dynamics, and am willing to "suffer" any minor imperfections. Keep in mind that few of these tubes were even designed for audio ... they just happen to work.

PS ... got me thinking (ow ow ow! ... gotta do that more often ... it hertz!)

"Military Grade" tubes tend to be low noise, low microphonic. Think "Star", JAN, etc. Can't say as I've ever seen a 12BH7 with those designations ... no military application? I've also seen painted tips and other special markings to note tubes that test better for those characteristics, including super special "medical grade".

Maybe I'm just not looking hard enough.

I have some different brands of 12BH7's, ranging from GE, Westinghouse, CBS-Hyton, RCA and Matsushita. The Westinghouse and Matsushita pair are dead quiet in the front location of my M125s, the other brands you can hear a slight buzz within a foot of the speakers to a few feet from them. Buying old tubes is just pot luck it seems to me.

TM

I use a 12BH7 in the center position for my ST120. Rather surprisingly, I have an EH 12BH7 (non-gold pin) and it is dead quiet. I have tried 6SN7's with adapter but all of the 6SN7's I have have been somewhat noisy. I got a "low microphonics and noise" grade TungSol 6SN7GTB that was HORRIBLE in terms of noise. So my 6SN7 experiment was pretty much a bust. In general, I've found that the driver tubes can really affect the amp noise level; not so much for power tubes.