You are here

Log in or register to post comments
Reptiles00
Reptiles00's picture
Offline
Last seen: Never ago
Joined: Mar 16 2006 - 6:28pm
Hum from Tube Amplifier

Hi all,
I'm trying out a Vellman (Dynaco) Stereo 70 tube amp, with integrated volume control from a local shop and I have a hum from my speakers which does attinuate when you raise the volume control with ot without a source connected.
I have the (I believe Integrated Amp since I have to use my pre-amps tapeout loop for connection to the amp) powered through a power line conditioner (tried without too) and sitting in my rack under the other equipment (3rd shelf down).

I have tried to connect a seperate ground from chassis to ground and no change.

3 of the EL34 tubes look the same and 1 is silghtly taller so I'm guessing its different.

I tried to connect the amp using my pre-amp out but this leads to the hum being heard in the next room and the volume control can barely be used.

If I connect with the tape out loop I can use the volume on the amp and it sounds wonderful and 1/4 volume is rockin' the room, but I know that hum is there very low in the background.

Suggestions?

Thanks,
Don

Jeff Wong
Jeff Wong's picture
Offline
Last seen: 3 years 4 months ago
Joined: Sep 6 2005 - 3:28am
Re: Hum from Tube Amplifier

Hopefully someone with some experience with this classic design can offer some ideas. But, I have my own questions and general thoughts (which you should take with a grain of salt because your problem may be specific to this design, which I've only read about in passing.)

The hum attentuates when you raise the volume? If the hum is actually decreasing in level when you do this, it seems like it could be a grounding issue. Does the level change when you just touch the knob?

Is it in both channels? I suppose it could be a transformer, or a tube, or tubes. Is that one tube that is taller properly seated? If the hum is only in one channel, I wonder if you might be able to swap tubes to see if there is a change or shift to a particular side.

Reptiles00
Reptiles00's picture
Offline
Last seen: Never ago
Joined: Mar 16 2006 - 6:28pm
Re: Hum from Tube Amplifier

Hi Jeff,
Its in both channels. Audile from about 3 feet from speakers when volume is at 11 o'clock. No change happens with touching anything, and I checked seating of tubes and all seems ok.
When playing music you would have to work to hear the hum and be pretty close to the speakers.

Don

ampnut
ampnut's picture
Offline
Last seen: Never ago
Joined: Sep 4 2005 - 5:26am
Re: Hum from Tube Amplifier

Hi Don,

My suggestions :

1. Disconnect ALL the inputs to the Dynaco. Keep ONLY the power Cord and speakers connected. Turn on the Dynaco.

Do you hear a Hum ?

If not, your Dynaco, or its tubes are not the cause of the Hum.

( I suspect that there will be no Hum when the Dynaco is stand-alone )

2. If the Hum is present on the stand alone Dynaco, then its in the input tube(s) of the Dynaco, particularly if the Hum level increases if you turn up the Dynaco's volume level.

Your EL-34s are OK since the Hum is common to both channels.

3. If the Dynaco is Clean, the Hum is the result of a "Hum Loop" formed when the External Pre Amp is connected to the Dynaco, or the pre amp itself has hum ( I doubt the latter, though )

4. Check if the live and Neutral of the Pre Amp mains and the Dynaco mains are correct. I suspect 1 of these is connected the wrong way around.

5. If 4 does not get you results, Disconnect the Earth wire on the Dynaco's Mains lead. This will Ground the Dynaco, ONLY through the Pre amp.

I am not sure if this violates safety procedures, but I dont think it will, as long as the Dynaco is connected to the Pre....

This SHOULD CURE THE HUM ! ??

Do let us know.

CECE
CECE's picture
Offline
Last seen: Never ago
Joined: Sep 17 2005 - 8:16am
Re: Hum from Tube Amplifier

Yeah, ground a 15A ckt through the shielding of an RCA interconenct, how much current capaity does the shielding carry in the event of a short in the other device. the shielding of an audio interconenct is not for carrying fault current.

JoeE SP9
JoeE SP9's picture
Offline
Last seen: 1 month 3 weeks ago
Joined: Oct 31 2005 - 6:02pm
Re: Hum from Tube Amplifier

It may be that you have a ground loop problem. The easiest way to find out if it is a ground loop problem is to use a cheater plug (adaptor for grounded plug to two prong socket)on the Stereo 70. It may be necessary to do some filing on the cheater plug to allow reversing it. If reversing the cheater plug results in less or no hum. Get the service shop to reverse the hot and neutral inside the Velleman (stereo 70). I used to own an original Dynaco Stereo 70. I sold it. It was one of the dumber things I've done.

Reptiles00
Reptiles00's picture
Offline
Last seen: Never ago
Joined: Mar 16 2006 - 6:28pm
Re: Hum from Tube Amplifier


Quote:
1. Disconnect ALL the inputs to the Dynaco. Keep ONLY the power Cord and speakers connected. Turn on the Dynaco.

Do you hear a Hum ?

Yes


Quote:
This SHOULD CURE THE HUM ! ??

Do let us know.

1st off..... all of you are so awesome .... Thanks!

2nd..... I disconnected the pre-amp and the hum was still there. I then tried to connect AC from the wall vise the line conditioner.... hum was still present.
I then added a cheater plug to the amps AC power cord and reconnected it to the line conditioner....
Hum Gone!

Now my delima is..... do I use the pre-amp out connections or just leave it connected to the tape out as I have it now.
With the tape out it has tons of volume and at 9 o'clock its shakin' the walls. This is why I believe its really an integrated amp. It will run a source direct to the amps only inputs just like there was a pre-amp connected.

Or do I just get a good source route selector such as a DBX DAV-400 or DAV-600 and use that for source selection and use the amp's volume control.

If I connect to the pre-amp I will have to leave the amp volume at one position and use the preamps to control the volume.

Hmmmmmm.... decisions decisions.

Again, thank you so much for all the help. Jim T also was pumping me with suggestions so I definately need to acknowledge him along with everyone who answered here. You folks are great :-)

Don L

Monty
Monty's picture
Offline
Last seen: Never ago
Joined: Sep 16 2005 - 6:55pm
Re: Hum from Tube Amplifier

I think you are going to have to experiment to see which sounds best. I lean toward using the preamp with the source and amps wide open.

Many CD players that offer a variable ouput can squash dynamics when employed. The same situation could happen with the amps and leave the sound kinda lifeless. This doesn't have to be the case, but just as passive line stages can take the boogie out of the sound, so could be the case here.

At least with the hum gone, you can give it the ear test.

  • X
    Enter your Stereophile.com username.
    Enter the password that accompanies your username.
    Loading