hum with VTA70

Hi all:

I have a slight problem here with hum. I got a new VTA70 last week, and it was extremely quiet when I started it up. It started to hum a little (ears near the speaker) but then it started to get a little worse.

So -- I turned the bias pots all the way counterclockwise, and put a new rectifier in to test that (the rectifier is new to me, anyway -- it's a Sovtek 5AR4 with a date code of 06 04).

At this point, I had not turned the amp on at all today. So I turned it on and the fuse blew. Unfortunately I don't have any more fuses, and it's too early here to go get some.

What I was wondering...has anyone else had this experience? I suppose the new rectifier could be bad, but I have no way to test it.

I would normally write Bob about this, but his email is bouncing and I'm going to leave town in a few hours.

Thanks.

dougmon wrote:Hi all:

I have a slight problem here with hum. I got a new VTA70 last week, and it was extremely quiet when I started it up. It started to hum a little (ears near the speaker) but then it started to get a little worse.

So -- I turned the bias pots all the way counterclockwise, and put a new rectifier in to test that (the rectifier is new to me, anyway -- it's a Sovtek 5AR4 with a date code of 06 04).

At this point, I had not turned the amp on at all today. So I turned it on and the fuse blew. Unfortunately I don't have any more fuses, and it's too early here to go get some.

What I was wondering...has anyone else had this experience? I suppose the new rectifier could be bad, but I have no way to test it.

I would normally write Bob about this, but his email is bouncing and I'm going to leave town in a few hours.

Thanks.

As you did "several things at once" you may have spoiled the chance to figure out the source of hum.
One possibe source could be powertubes that begin to be unbalanced, this could be
examined by measuring the cathode voltages. And if the were unbalanced an adjustment of bias would have silenced it.
I suggest you reinstall the previous rectifier, replace the fuse and restart
with an voltmeter ready for bias adjustment.
I hope the bias pots were turned fully left "counterclockwise" as you speak in us.

If the fuse still blows, repeat with all tubes out.
Then install rectifier only and verify high voltage etc. It's all documented in the building instructions.

peterh wrote:

dougmon wrote:Hi all:

I have a slight problem here with hum. I got a new VTA70 last week, and it was extremely quiet when I started it up. It started to hum a little (ears near the speaker) but then it started to get a little worse.

So -- I turned the bias pots all the way counterclockwise, and put a new rectifier in to test that (the rectifier is new to me, anyway -- it's a Sovtek 5AR4 with a date code of 06 04).

At this point, I had not turned the amp on at all today. So I turned it on and the fuse blew. Unfortunately I don't have any more fuses, and it's too early here to go get some.

What I was wondering...has anyone else had this experience? I suppose the new rectifier could be bad, but I have no way to test it.

I would normally write Bob about this, but his email is bouncing and I'm going to leave town in a few hours.

Thanks.

As you did "several things at once" you may have spoiled the chance to figure out the source of hum.  
One possibe source could be powertubes that begin to be unbalanced, this could be
examined by measuring the cathode voltages. And if the were unbalanced an adjustment of bias would have silenced it.
I suggest you reinstall the previous rectifier, replace the fuse and restart
with an voltmeter ready for bias adjustment.
I hope the bias pots were turned fully left "counterclockwise" as you speak in us.

If the fuse still blows, repeat with all tubes out.
Then install rectifier only and verify high voltage etc. It's all documented in the building instructions.

I actually didn't build this one -- I bought it wired. So the tubes are brand new. The bias pots were turned fully to the left.

I will be putting the old rectifier back in to try this again. Then with the new rectifier. And if the hum persists, I'll try different power tubes.

Thanks.

Bob Latino wrote:Hi Doug,

The most common cause of hum in a VTA ST-70 or ST-120 is the center driver tube. Try swapping the center (voltage amplifier) tube with either of the SIDE driver tubes. In the CENTER position a tube which is slightly noisy will most of the time show no noise in one of the side positions. This is due to their different functions in the amp.

Bob

Thanks, Bob. This started with the tubes that came with the amp, and I did try new driver tubes (Mullard CV4003), but the hum persisted. I will move the tubes around, though, to see what the effect is.

Got a new fuse, put it in (using the old rectifier tube), biased it up, and...there is still hum. However, I can't hear it from where I sit, so unless a fuse pops again, I'm going to ignore it. I did move the driver tubes around. I would like to figure out why there was none before, then a lot, now just a little.

But at least I'm not listening through a solid state amp.

Thanks for the input, everyone.

dougmon wrote:Got a new fuse, put it in (using the old rectifier tube), biased it up, and...there is still hum. However, I can't hear it from where I sit, so unless a fuse pops again, I'm going to ignore it. I did move the driver tubes around. I would like to figure out why there was none before, then a lot, now just a little.

But at least I'm not listening through a solid state amp.

Thanks for the input, everyone.

now it's time to adjust bias. Just do it and see if hum is reduced.

( best done with shorted inputs )

peterh wrote:
now it's time to adjust bias. Just do it and see if hum is reduced.

( best done with shorted inputs )

Bias is best done with shorted inputs?
Pray ... tell me more. This is new to me. I can see where it would eliminate the rest of the audio gear as the culprit if you're trying to track down hum ...

PS ... could just need a common ground with any other gear in the chain.

If your power amp was built with a three-prong AC plug, you probably have a ground loop.

sKiZo wrote:

peterh wrote:
now it's time to adjust bias. Just do it and see if hum is reduced.

( best done with shorted inputs )

Bias is best done with shorted inputs?
Pray ... tell me more. This is new to me. I can see where it would eliminate the rest of the audio gear as the culprit if you're trying to track down hum ...

PS ... could just need a common ground with any other gear in the chain.

Adjust bias should be done and might reduce or eliminate hum. Shorting inputs will prevent external hum.

The shorted input eliminates ALL audio input from outside the Unit Under Test. Bias should be set with no audio signal at all. Only the idle current is being adjusted and measured. If there is audio in the circuit, it will be reflected in the reading on the meter when setting bias.

peterh wrote:

dougmon wrote:Got a new fuse, put it in (using the old rectifier tube), biased it up, and...there is still hum. However, I can't hear it from where I sit, so unless a fuse pops again, I'm going to ignore it. I did move the driver tubes around. I would like to figure out why there was none before, then a lot, now just a little.

But at least I'm not listening through a solid state amp.

Thanks for the input, everyone.

now it's time to adjust bias. Just do it and see if hum is reduced.

( best done with shorted inputs )

Just to clarify - when I wrote 'biased it up', I probably should have, in the interest of accuracy, said 'adjusted bias on each tube to ~.40 v'. Is that what you meant by 'adjust bias'? (Yes, this is a serious question -- I want to make sure that I'm not bypassing a step that could solve my problem.) The thing about using shorted inputs, though, is new to me. I'll make some as soon as my ambition overrides my 'hey, this record is great!' feeling.   

I once knew just enough about tubes to be dangerous. I'm trying to work past the danger zone now.

dougmon wrote:

peterh wrote:

dougmon wrote:Got a new fuse, put it in (using the old rectifier tube), biased it up, and...there is still hum. However, I can't hear it from where I sit, so unless a fuse pops again, I'm going to ignore it. I did move the driver tubes around. I would like to figure out why there was none before, then a lot, now just a little.

But at least I'm not listening through a solid state amp.

Thanks for the input, everyone.

now it's time to adjust bias. Just do it and see if hum is reduced.

( best done with shorted inputs )

Just to clarify - when I wrote 'biased it up', I probably should have, in the interest of accuracy, said 'adjusted bias on each tube to ~.40 v'. Is that what you meant by 'adjust bias'? (Yes, this is a serious question -- I want to make sure that I'm not bypassing a step that could solve my problem.) The thing about using shorted inputs, though, is new to me. I'll make some as soon as my ambition overrides my 'hey, this record is great!' feeling.   

I once knew just enough about tubes to be dangerous. I'm trying to work past the danger zone now.

You have a problem here "adjusted bias on each tube to ~.40 v'". This is wrong.
You should adjust each tube to a current of 40-50mA . The voltage seen on the grid ( the one you adjusted to 40V) is the effect and not the cause. Some tubes need more "voltage" some less.

The usual method is the measure the voltage across the cathode resistor. In the VTA-70 it's 10 ohm and you have 4 measuring points and 4 adjustment pots. Each measuring
point ( reachable through the octal plugs on the front) should measure 0.4V referenced to chassies ground. Best done with no signal but speakers connected.

peterh wrote:

You have a problem here "adjusted bias on each tube to ~.40 v'". This is wrong.
You should adjust each tube to a current of 40-50mA . The voltage seen on the grid ( the one you adjusted to 40V) is the effect and not the cause. Some tubes need more "voltage" some less.

The usual method is the measure the voltage across the cathode resistor. In the VTA-70 it's 10 ohm and you have 4 measuring points and 4 adjustment pots. Each measuring
point ( reachable through the octal plugs on the front) should measure 0.4V referenced to chassies ground. Best done with no signal but speakers connected.

Sorry, typo in my post. I've biased the tubes so that the reading is ~.400 vdc - this is using the inputs in the front of the with a probe from a multimeter. The docs seem to say this is correct.

Sorry about the typo -- very misleading.

dougmon wrote:

peterh wrote:

You have a problem here "adjusted bias on each tube to ~.40 v'". This is wrong.
You should adjust each tube to a current of 40-50mA . The voltage seen on the grid ( the one you adjusted to 40V) is the effect and not the cause. Some tubes need more "voltage" some less.

The usual method is the measure the voltage across the cathode resistor. In the VTA-70 it's 10 ohm and you have 4 measuring points and 4 adjustment pots. Each measuring
point ( reachable through the octal plugs on the front) should measure 0.4V referenced to chassies ground. Best done with no signal but speakers connected.

Sorry, typo in my post. I've biased the tubes so that the reading is ~.400 vdc  - this is using the inputs in the front of the with a probe from a multimeter. The docs seem to say this is correct.

Sorry about the typo -- very misleading.

My misunderstanding.

what happened to the hum ?

peterh wrote:

dougmon wrote:

peterh wrote:

You have a problem here "adjusted bias on each tube to ~.40 v'". This is wrong.
You should adjust each tube to a current of 40-50mA . The voltage seen on the grid ( the one you adjusted to 40V) is the effect and not the cause. Some tubes need more "voltage" some less.

The usual method is the measure the voltage across the cathode resistor. In the VTA-70 it's 10 ohm and you have 4 measuring points and 4 adjustment pots. Each measuring
point ( reachable through the octal plugs on the front) should measure 0.4V referenced to chassies ground. Best done with no signal but speakers connected.

Sorry, typo in my post. I've biased the tubes so that the reading is ~.400 vdc  - this is using the inputs in the front of the with a probe from a multimeter. The docs seem to say this is correct.

Sorry about the typo -- very misleading.

My misunderstanding.

what happened to the hum ?

A couple of things happened. The hum lessened, but I don't know why. As was written earlier in this thread, I did too many things at once to figure out what the problem was exactly.

I did move the driver tubes around, and that could have helped.

I'm going to try with the new rectifier tube again, and see what happens.

In any case, thanks for trying to help. When I actually build one of these things, much of the input I've received from you and others will be very helpful.

When I first plugged in my VTA upgraded ST-70, I had a hum that was haunting me. Bob made the same suggestion to me with regard to the driver tube in the center. I had an extra CV4024 so I just swapped it out, but with no improvement. I chased this thing around for a couple of days and then on a whim swapped the outside 12AT7s with each other and the hum moved to the other channel.

It turns out that I had a wonky JAN Phillips tube that was causing the hum...even in the outside position. Crisis averted.

Cheers,
Ed

edgobb wrote:When I first plugged in my VTA upgraded ST-70, I had a hum that was haunting me.  Bob made the same suggestion to me with regard to the driver tube in the center.  I had an extra CV4024 so I just swapped it out, but with no improvement. I chased this thing around for a couple of days and then on a whim swapped the outside 12AT7s with each other and the hum moved to the other channel.

It turns out that I had a wonky JAN Phillips tube that was causing the hum...even in the outside position.  Crisis averted.

I did try this, and it didn't make any difference. Great idea, though.

I also thought it might be the rectifier, so I put in a new rectifier. The power tubes heated up, but wouldn't bias up. So obviously I have seen a few bad rectifiers, but if those are the cause of the hum, I can keep trying new ones. The original rectifier blew and took two fuses with it. And then another fuse and then I learned my lesson...