ANOTHER ST70 hum issue

What was a minor annoyance with my modern full range speakers has become unbearable with my highly efficient Altec VOTTs.

I’ve run through all the Curcio document as well as the sticky here. The 60hz hum does disappear with shorted RCA inputs. BUT, it is there with every other electrical device in the room unplugged, with or without inputs connected, lifted ground or not (irrelevant with nothing else attached). I even pulled the whole room circuit breaker and ran my bucking transformer off a UPS temporarily. Simply impossible for it to be generated externally.

Best I can tell all wiring and wire twisting rules have been applied (it is a factory-built Triode USA version). It’s on both channels equally so swapping driver tubes had no effect.

Any ideas where to look next?

Thanks,
Jeff

With the inputs shorted you don't hear hum? Then the source of the hum IS NOT the amp! The hum is being fed from the upstream components to the amp.

Plus the very efficient speakers make the hum more audible.

The amp is not the cause/issue.

Jim McShane wrote:With the inputs shorted you don't hear hum? Then the source of the hum IS NOT the amp! The hum is being fed from the upstream components to the amp.

Plus the very efficient speakers make the hum more audible.

The amp is not the cause/issue.

Thanks for responding, Jim.

If the hum exists with no other device connected besides the speakers, and all other electrical devices within 30 feet unplugged, what possible source could there be “upstream”? Also, the hum diminishes slightly by lifting the ground. With no other devices connected, how can this be a ground loop?

I am not trying to be argumentative. Just trying to find a way to enjoy an amp that otherwise appears to be a great match for my Altecs. But I am out of ideas. The amp is factory built Triode USA, stock with the exception of the “yellow sheet” diode mod, an inrush current limiter (my house current is 120-122 so I also use a bucking transformer at about 116) and 16 ohm output enabled for the Altecs. I am very pleased with the sound with the exception of the hum.

I don’t want to arbitrarily replace tubes without some hint that it might make a difference. I have checked and double-checked wiring, twisting is done where appropriate. The only assembly recommendation I have read about not implemented appears to be the “star ground” concept. Still trying to evaluate whether that is a practical mod in this amp.

Thanks again,
Jeff

Jeff,

You wrote "The 60hz hum does disappear with shorted RCA inputs". Shorting the amp inputs means you have zero signal coming in to the amp. If the hum goes away because you shorted the inputs then by process of elimination the hum has to be coming in through those inputs!

Nobody said it was a ground loop. Other things besides ground loops can cause hum.

You also wrote "what possible source could there be “upstream”?"

The interconnect cables could be picking it up, especially if you have unshielded cables. Also whatever electronics (preamp, disc player, etc.) you normally have plugged into the amp could be the cause.

And there is some possibility that the RCA input jacks aren't properly grounded and that when you short them you provide a needed ground path; or the internal wiring is picking up hum. But with shorted inputs that's unlikely although they are worth investigating.

With super high efficiency speakers like you have now getting rid of hum can be a very difficult process!

So to proceed further with Jim’s comments. The interconnect cables used to go back to the next piece of equipment must be checked. Try shorting them at the far end. If no hum then the next piece of equipment (preamp?) is plugged in. Test. Then the next. Somewhere the hum will remain. That will be where the problem originates from or is contributing to your issue.

Apparently I am not making the circumstances clear.

This amp is generating 60/120hz hum when NO interconnect cables are attached. The only things touching this amp are the mains power cord coming through a ground-lifted bucking transformer providing approximately 117vac, and speaker cables to a pair of unpowered Altec Lansing A5s.

No CD, no receiver, no preamp, no inputs in fact they are unplugged and unpowered. Every electrical device in the room has been unplugged from the walls including lamps, clocks, overhead lights, etc. Nothing powered by house current is within 30 feet of this amp. When the amp is turned on, it is silent for 15-20 seconds at which point the hum is very audible. If at that point shorted RCA plugs are inserted into the inputs, the amp is silent. The hum is louder if I reintroduce ground through the power plug.

The hum also disappears if the amp is powered on with no power tubes. The hum is unchanged with the changing of the 4 power tube positions. The amp functions very well and sounds great otherwise and in fact brief frequency response tests show excellent flatness throughout the audible range.

Besides looking for some advice as to where I might begin to troubleshoot without arbitrarily replacing tubes, I post this in the interest of advising that the conventional wisdom that if hum disappears with shorted inputs, it MUST be coming from outside the amp; this is simply not 100% true. THERE IS NO 100/120vac 60hz electrical device within 30 feet of this amp and it hums. The hum is not worse when my audio devices are connected and music is played, unless I remove the ground lift at which point it is much worse (ground loop).

Hope I have made the circumstances clearer.

Thanks for any input.
Jeff

The amp was quiet with shorting plug ? Then the issue is outside the amp.
Open inputs can pickup noise from the environent.

Well if there is truly nothing wrong then with this amp, it is truly unfortunate. I’ll have to be back in the market for a different tube amp, I guess because this level of hum is unbearable. Hard to believe that
ST70s are not suitable for driving high-efficiency speakers.

Neither of my SS receivers exhibit any hum with open inputs operating in the exact same environment as the ST70, but they struggle to drive these Altecs sufficiently.

Thanks all for the feedback.
Jeff

I think you are quitting too early.
The difference between you SS gear and ST70 is that the st70 has a much higher input impedance,
it will pick up electrical noise easier.
Connect your cables to the st70, short them at the far side. Does it hum ? Then your cables
might be your source of hum.

If it hums while connected to your preamp, then you have a ground loop that tries to use the
cables grounding.
Use a coarse wire and connect to the st870 chassies and the pre chassie. Does the hum
go away or reduce ? There is your problem.

jeffjmr wrote:Apparently I am not making the circumstances clear.

This amp is generating 60/120hz hum when NO interconnect cables are attached.

As Peter posted - having nothing connected to the amp inputs is not the same thing as shorting the amp inputs! This might seem counterintuitive at first, but trust me - it is 100% true and correct. His explanation about the higher input impedance being more likely to pick up noise/hum is also spot on.

I think you'd be wise to follow the advice you received here. It'll help you narrow down the cause of the hum.

I built a VTA ST-70 and its driving the horns on a pair of Klipsch Jubilee's. The system has some noise, but I have to stick my head inside one of the horns to hear it. So I'd say sensitivity is not the problem.

I think no one has explained the shorting issue quite right.

If your amp is on and connected to speakers, with nothing connected to the RCA inputs, it is completely normal for an amp to buzz. It may not but is certainly may. Yours is buzzing, but it may not be the same buzz you're hearing with cables and components connected to the inputs. I think that's what others were trying to tell you.

Now you short the inputs and the amp gets quiet. That tells you the amp doesn't have a grounding issue. MAYBE.

The ST-70 amp's input RCA's are CHASSIS GROUNDED. If they are not grounded properly the amp will act like nothing is connected to it and buzz even with cables and components connected to the inputs and music may even play.

Shorting the inputs might simply be providing a ground point to the input that is not grounded properly, thereby eliminating the buzz.

Now, not all ST-70's have chassis grounded inputs. You will have to determine this. If they are, remount them, making sure they have good contact with the chassis and that no one left insulators on the input connectors that are blocking them from making good chassis contact, also make sure the chassis is properly grounded to circuit boards and transformers.