Variac if it already hums

[mashaffer]

Just a quick question. If an amp hums when turned on in the normal fashion is there any point to bringing it up on a variac to try to reform the caps enough to determine what else works or doesn't or are they likely too far gone? Just thinking it would be nice to get an idea of where one was at before pulling a chassis and starting to sling lead.

mike

 

[Jon_Logan]

It wouldn't hoit.

I think your gonna want a scope though, or measure AC voltage on your caps.

 

[Chazb11]

I'd be interested in hearing what the tube gurus have to say on this too.

I've read that bringing a tube amp up on a variac is never a good idea as the resulting slow warm up just causes soot to form in your tubes. I have no idea how much truth there is to that.

I personally use a variac with tube amps, but only to step my wall voltage down to 110, not to bring it up slowly. (my wall voltage runs 123 ~ 127 vac)

 

[sloober]

I was under the understanding that the variac should be used for old gear to slowly ramp up power and let the caps reform. If you already went and plugged it in directly, you skipped all this. While you still may be able to reform them some, ask yourself how old is this amp. If it is 30-40 years old, I would suspect they are bad anyway. If you plan on using this amp, be safe and just replace them. You will sleep better. John

 

[mashaffer]

Yep, plan on recapping anyway just thought it might be nice to know a bit more about how the rest is working before digging in. I am watching the bay for variacs in any case.

mike

 

[Jim McShane]

Here's a bunch of answers for all the prior posts.

1. As has been mentioned, if the amp has already been plugged into the AC line then there's no gain to be had by using a variac to bring it up slowly now. Whatever happened when full voltage was applied has already occurred.

You should know that to a certain extent capacitors are self-healing. If there is a short at some spot inside the cap it will often arc at that spot and burn open, fixing the short. So if any of that was going to occur in the original poster's amp, it has done so by now.

2. Bringing up the voltage slowly does no harm to your tubes at all. No "soot" forms.

Be aware that bringing up the voltage slowly is ineffective if there is a tube rectifier in the gear you're working on. This is because no current passes through the rectifier until it gets enough voltage to function. Once you get the voltage high enough and the rectifier starts passing current you are already not too far from the wall voltage. There are ways around this, but they involve temporary subbing SS rectification.

3. As was said previously, don't leave 30-40 year old caps in there. Is not a question of IF they will fail, it's WHEN! :yes:

Those caps were designed to go 10-15 years, so they are WAY past their lifetime. And even if they don't blow up, often the capacitance value is just a fraction of what it should be.

Bite the bullet and replace them (along with the cathode bypass and coupling caps)! You'll be happy you did.

 

[mashaffer]

Thanks for the replies. BTW is this THE Jim McShane? Cool. The console in question is SS rectified so the best step seems to be replacing the Se recto and recap before any further diagnostics. I still plan to pick up a variac though as I think it would be handy.

While on the topic when you replace a selenium recto with diode and resistor is there any real reason to physically remove the old recto? Was thinking of leaving them in place in a couple of builds and just bypass them.

mike

 

[Jim McShane]

Thanks for the replies. BTW is this THE Jim McShane? Cool. The console in question is SS rectified so the best step seems to be replacing the Se recto and recap before any further diagnostics. I still plan to pick up a variac though as I think it would be handy.

While on the topic when you replace a selenium recto with diode and resistor is there any real reason to physically remove the old recto? Was thinking of leaving them in place in a couple of builds and just bypass them.

mike

Mike - unless I owe you money, it's me!

Do get a variac, they are enormously handy!

There's no need to remove the old selenium as long as it's disconnected and safely isolated from the live circuitry. Replace the caps, put good diodes in there and you are on your way!

Have fun with your project! :thmbsp:

 

[dshoaf]

The console in question is SS rectified so the best step seems to be replacing the Se recto and recap before any further diagnostics. I still plan to pick up a variac though as I think it would be handy.

While on the topic when you replace a selenium recto with diode and resistor is there any real reason to physically remove the old recto? Was thinking of leaving them in place in a couple of builds and just bypass them.

mike

Hi Mike,

It isn't a bad idea to see if the unit works before you start tearing into it. This helps once you've finished your recap and you run into problems of some sort. Knowing what might not be working would help there. The tradeoff is that you risk failure due to buildup and you're prudent to be prudent about that.

I'd recommend you have a variac on hand. I always use one when I'm bringing up an amp after I've done work to it. It is a safety thing and I'll monitor bias and plate voltages as it comes up, too. Think of the variac not only for reforming but as a diagnostic and safety tool.

Selenium rectifiers have more voltage drop across them than silicon diodes. This implies that, for bias voltage circuits, you will need to have some way of adjusting the bias voltage after rebuild to bring it back into specs.

The way this is done will vary from amp to amp. Post a schematic of your amp for some folks here to help with that. For instance, my Mac MC-225s needed to have their bias voltages adjusted by setting the output tube plate voltage to spec then adjusting a resistor in the bias circuit to get the tubes back to spec. There's a discussion of this technique in the Hints and Kinks stickey over at the Mac forum here at AK.

As for removal, I have left the old seleniums in when there was room but they are completely removed electrically from the circuits. In tight quarters, however, I always pull them out.

Cheers,

David

 

[mashaffer]

Mike - unless I owe you money, it's me!

ROTFLMAO

Have note been able to find a schematic it is the GE console shown in this thread.

http://www.audiokarma.org/forums/showthread.php?t=224132

That orange cap don't look too healthy.

mike

 

[gadget73]

I'm not in the automatic selenium removal camp. I'd re-cap it first, and measure the voltages. If B+ is where it belongs, I wouldn't be in a big hurry to replace the rectifier. At a minimum, i would suggest bringing it up with the original setup to get an idea of how much current its drawing so you can more accurately calculate the drop resistor.

 

[Kegger]

Here's a bunch of answers for all the prior posts.

1. As has been mentioned, if the amp has already been plugged into the AC line then there's no gain to be had by using a variac to bring it up slowly now. Whatever happened when full voltage was applied has already occurred.

You should know that to a certain extent capacitors are self-healing. If there is a short at some spot inside the cap it will often arc at that spot and burn open, fixing the short. So if any of that was going to occur in the original poster's amp, it has done so by now.

2. Bringing up the voltage slowly does no harm to your tubes at all. No "soot" forms.

Be aware that bringing up the voltage slowly is ineffective if there is a tube rectifier in the gear you're working on. This is because no current passes through the rectifier until it gets enough voltage to function. Once you get the voltage high enough and the rectifier starts passing current you are already not too far from the wall voltage. There are ways around this, but they involve temporary subbing SS rectification.

3. As was said previously, don't leave 30-40 year old caps in there. Is not a question of IF they will fail, it's WHEN! :yes:

Those caps were designed to go 10-15 years, so they are WAY past their lifetime. And even if they don't blow up, often the capacitance value is just a fraction of what it should be.

Bite the bullet and replace them (along with the cathode bypass and coupling caps)! You'll be happy you did.

What he said, except with a tube rectifier you can still do the low voltage thing
with the variac, it just takes longer for it to heat with the low voltages present.

I use variacs pretty much everyday, new builds, tests, getting certain ac voltages,
I'd suggest anyone into building or rebuilding have a variac or TWO..

 

[seta1]

Just a quick question. If an amp hums when turned on in the normal fashion is there any point to bringing it up on a variac to try to reform the caps enough to determine what else works or doesn't or are they likely too far gone? Just thinking it would be nice to get an idea of where one was at before pulling a chassis and starting to sling lead.

mike

You need to change those power supply capacitors out. Fiddling around with a variac and trying to, as some say, 'reform' the capacitors, even if successful, is only prolonging the inevitable. The capacitors are going bad. Dump them. I've rebuilt a lot of tube gear, both guitar and hifi, and I will admit that some have perfectly good power supply capacitors. But, those that do have good power supply capacitors never hummed, even when powered up after sitting for years in a closet. Every amp I have ever worked on that had power supply hum from initial power up and did not have it's power supply capacitors replaced ended up humming worse and worse until one was forced to replace the capacitors. Save yourself some headaches and just replace those capacitors. But, make sure that this is power supply hum. You can check the AC ripple simply by setting your meter to measure AC and seeing what kinds of voltages you get at each filter stage. Right off the rectifier, it is not uncommon to see as much as 2 volts AC. But after the first dropping resistor, you should see the voltage drop to a couple hundres Mv. Once you get to the B+ source for the phase splitters, AC ripple should be under 20 Mv and at the preamp, it should be under 5 Mv. If you have a frequency counting multimeter (like a Fluke 87) see what frequency the AC hum is, at the speaker output terminals. If it reads 60hz, it is AC pickup via bad wiring, inductive pickup etc. If it is 120hz, it is the power supply. (given it is a full wave power supply which is what is normally found in a hifi amp). Just because a receiver or amp is made by a well known company does not mean it can include some bad wiring practices.

 

[DaveWM]

another good thing about a variac is if the set had diodes, and you replace them, you can check to make sure the B+ is +, I always bring up very slow after replacing just in case I had a brain fart and did something stupid.

One thing to watch out for, they do not isolate (at least mine does not) so be carefull on transformerless sets, you may still need an isolation transformer to be safe and if you are working on a radio and it does not tune across the dial (the LO quits) then double check that you have turned the variac up to the correct voltage. I chased that problem for an hour once before realizing I had simply forgot to turn up the voltage. The AM band worked and the low FM band worked but would stop mid way up. The LO would cut out at lower voltages (below 100vac).

 

[Jim McShane]

another good thing about a variac is if the set had diodes, and you replace them, you can check to make sure the B+ is +, I always bring up very slow after replacing just in case I had a brain fart and did something stupid.

Yes, the variac has saved me more than once! :yes:

One thing to watch out for, they do not isolate (at least mine does not) so be carefull on transformerless sets, you may still need an isolation transformer to be safe

It's not an option, it is a MUST! Not using an isolation trafo can be a FATAL mistake, and I'm not exaggerating. One slip and you are hooked directly to your fuse box, which will be the only thing limiting current through your body. PLEASE!! Always use an isolation trafo - or don't work on transformerless gear! Variacs DO NOT PROVIDE ANY ISOLATION from the AC line.

 

[Jim McShane]

What he said, except with a tube rectifier you can still do the low voltage thing with the variac, it just takes longer for it to heat with the low voltages present.

But... there is still a threshold voltage under which the recto will not pass any current. You can usually get something going somewhere around 75-80 volts or so, but under that general vicinity the tube is inactive. Set the variac at 35 volts and you can leave it on all day, you won't get any current flow.

 

[andy]

But... there is still a threshold voltage under which the recto will not pass any current. You can usually get something going somewhere around 75-80 volts or so, but under that general vicinity the tube is inactive. Set the variac at 35 volts and you can leave it on all day, you won't get any current flow.

It's true that tube rectifiers are very non-linear under low voltage conditions, but with care, you can still achieve any output voltage you want with a variac. You just have to go slowly, and monitor the DC voltage. I know, because I just did it on my recently acquired Magnavox 8802.

 

[Jim McShane]

It's true that tube rectifiers are very non-linear under low voltage conditions, but with care, you can still achieve any output voltage you want with a variac. You just have to go slowly, and monitor the DC voltage. I know, because I just did it on my recently acquired Magnavox 8802.

That's not what I said though. I said that below a certain variac setting there won't be enough heater voltage to "light off" the recto. Once it's lit off you can get a range of voltages, but if you are too low the heater simply isn't going to boil any electrons off the cathode. It has nothing to do with non-linearity, only the point where the tube has enough voltage to generate the heat needed for it to work.

Try it. Make sure the filter caps are discharged so they don't fool you into thinking there is some output from the rectifier, set the variac at 25 volts - you'll have zero output from the recto tube. :yes:

 

[mashaffer]

Just had a thought. When I get around to this project I will probably replace caps and measure the voltages in the PS. Then I am armed for twiddling a silicon rectified version. Since I will have to drop some additional voltage why not add a CL stage before what is now the first filter cap. The choke will drop some voltage give better filtering and the new input cap can be adjusted to get the desired B+.

What made me think of this approach is that I happen to have a PS choke from a Baldwin tube organ. I haven't the equipment to measure it but it would have to improve filtering to some degree. As to current capability I would have to see if it got too warm. It was not supplying plate current to the 6L6 outputs but it did seem to be providing the screen current for the 6L6s and the plate current for more than 60 preamp tubes so it may be able to handle the little pentodes in this amp. Seems worth a shot.

mike