Can tube amps overheat?

[nyindallas]

Unlike my vintage solid state amps which run quite cool, my older tube amps (both STromberg Calsons) run very hot. Is there a threshold in terms of how long a vintage tube amp should run? In the bedroom I have no fears around falling asleep with my Marantz 2230 or HK 930 (both solid state) on all night.

One thing that really helped was substituting new JJ 6L6GC's for the old 7027A's that were in my SC ASR-444.

Thanks

[toxcrusadr]

What's your line voltage? If it's running over 120V, that can cause power transformers to overheat, and in fact the filaments and everything runs at too high a voltage.

[nyindallas]

Good question. I am just running at household current. Do I need some type of conditioner??

[Celt]

Make sure the output tubes are biased correctly and balanced. Wouldn't hurt to make sure the amp's caps and such are still in spec too.

[toxcrusadr]

Many power companies are running higher voltages than they did back in the day. A tube amp made in 1960 is designed for 110-115V. If you run it at 125 (and there are documented cases of up to 128V these days), it's going to run hot.

Get a meter and check it at the socket.

More likely it's caps or bias as Celt said - have your older amps have had their electrolytics replaced?

[Steve O]

Unlike my vintage solid state amps which run quite cool, my older tube amps (both STromberg Calsons) run very hot. Is there a threshold in terms of how long a vintage tube amp should run? In the bedroom I have no fears around falling asleep with my Marantz 2230 or HK 930 (both solid state) on all night.

One thing that really helped was substituting new JJ 6L6GC's for the old 7027A's that were in my SC ASR-444.

Thanks

Assuming a properly designed amp with adequate ventilation and in good repair, it should be able to be operated 24/7 w/o overheating issues. If any assumption is incorrect, all bets are off. And there is some equipment out there where proper design practice was not followed for whatever reason, esp WRT ventilation.

I have a small collection of cheap bakelite tube radios. I once left for a long weekend with one of them on but volume down. It was A-OK in every respect when I got back 4 days later. However, IMO it's not wise to operate vintage tube equipment unattended unless you're absolutely sure of the state of repair. Too, energy consumption is usually a bigger factor with tube equipment so another reason to shut down if not directly using it.

[outlawmws]

A cheap voltage reducer could be a "Dim Bulb tester” which is simply a light bulb in series with your line input. An old fashioned ceramic bulb base and a Zip cord extension can create this quickly.

it looks something like this "Schematically"

>---------&------------=>

And with what results I have gotten, a 240 watt bulb comes in at 110 V. (YMMV)

[cademan]

If there are any issues related to heat, just get a computer fan, or a pair of computer fans to cool the power transformer. After about an hour, the power transformer on my Scott LK-48 would get somewhat hot to the touch, but using a couple of computer fans, I can leave it on all day and the transformer is only lukewarm.

[soundmotor]

Many power companies are running higher voltages than they did back in the day. A tube amp made in 1960 is designed for 110-115V. If you run it at 125 (and there are documented cases of up to 128V these days), it's going to run hot.

Don't I know that. At my old home in a somewhat rural area I left my Fluke 87 on 24H to monitor the peak & dip on the AC line. I don't recall exactly but the swing was something crazy like ~108 to ~122 throughout the interval! My Dynaco ST70 popped it's cork a couple times and I believe that is why.

[nyindallas]

Purchased a Volt meter. The current measures at 122V. Is there some device or conditioner I could put on the lines without spending a fortune or needing a inline light bulb to bring the volts down? Thanks

What's your line voltage? If it's running over 120V, that can cause power transformers to overheat, and in fact the filaments and everything runs at too high a voltage.

[mhardy6647]

Power transformers are designed and rated to run quite hot. EICO specified in their manuals that it is OK for the power transformer to be up to 190 degrees Fahrenheit and still be functioning normally. This is far too hot to hold your hand on.

BUT... A short circuit (e.g., shorted output or rectifier) in an amp with no fuse can overheat the power transformer, as can an internal short in the primary or secondary. The transfomer will smell bad, smoke and/or ooze tar, wax, or other unspeakable matter. This is a very bad thing.

The rated bulb temp for most output tubes is 250 degrees Celsius (this is oven temperature; 482 degrees F). It is OK for the output tubes to be that hot (well, almost that hot; at any rate, far too hot to touch). The tube rectifier will be similarly hot. The small signal tubes will be comparatively much cooler.

Output transformers should be cool to the touch (although in some amp designs they will soak up heat from nearby output tubes or other hot devices).

The main P/S filter caps should be cool/cold. In some amps, they will soak up some heat from nearby components and feel warm after the amp's been on for a while... but if you turn on a tube amp and in a few minutes feel the P/S can cap and it is quite warm, you've got trouble brewing.

HTH.

[outlawmws]

Purchased a Volt meter. The current measures at 122V. Is there some device or conditioner I could put on the lines without spending a fortune or needing a inline light bulb to bring the volts down? Thanks


Not sure what the issue is with the dim bulb tester line drop. The 240 watt bulb will be dim, and not particularly hot. (Hence the name "Dim Bulb tester"). It is a well established method to reduce AC line voltages on the test bench, and by using a series of increasing bulb wattages, (I start at 25w and go up from there) you can use it to reform caps in equipment that has been shut down for awhile.

It is pretty idiot proof, and the bulb will probably last forever, and probably outlast the amps it is protecting at that low a load.

If you wanted to get fancy, the bulb assy can be put inside a cage to “disguise” it

A Variac could also be used, but that will be more expensive especially if you get one with a built in voltmeter.

You might be able to find an isolation transformer that has 120 in and 110 out. Again some expense here.

[Brian]

it will depend on the design and condition. I have tube units that in 5 minutes will fry an egg and no fear and others if it gets even warm after several hours indicates an issue. Dyna stuff typically runs quite hot, which Mc quite cool. I'd not disregard any amp running how until I verified the unit is in specs as even if it is designed to run hot, it can still run hotter than it should.

As for the voltage, I used to use a variac. I also seem to remember EICO and a couple other companies had regulated power supplies to monitor and have fixed output voltages on the repair bench. The other problem seems to be a large amount of hash in the AC from sources such as motors that seems to need to be dealt with.

[toxcrusadr]

You can use a transformer set up in self-bucking mode to drop voltage. IIRC, you use a 120V pri/6V sec transformer and basically run one leg of the primary into one leg of the secondary or something like that, and it drops the voltage about 10%. (Do NOT follow these directions, my memory is a bit fuzzy, but I can look it up if you want to do it). This is quick and easy if you have spare transformers sitting around.

[nyindallas]

Thanks to everyone for all the replies. It would be nice to have something external to the amp. Maybe a power conditioning strip???

fyi - the power transformer gets very hot, the output transformers do not.

You can use a transformer set up in self-bucking mode to drop voltage. IIRC, you use a 120V pri/6V sec transformer and basically run one leg of the primary into one leg of the secondary or something like that, and it drops the voltage about 10%. (Do NOT follow these directions, my memory is a bit fuzzy, but I can look it up if you want to do it). This is quick and easy if you have spare transformers sitting around.

[toxcrusadr]

You could certainly mount either the light bulb or the self-bucking transformer on a DIY strip if you are handy and comfortable doing it. You'd want to make it very safe and well put together.

[nyindallas]

Thanks for your reply. This thread has become a bit confusing. Facts are my house current is about 122-123 volts. There was a loud pop when I shut off the SC ASR-444. When I did the test plugging a lamp with a 60 watt bulb into the switched power, the pop problem went away. I can see rigging the light bulb for testing, but I don't see anyone rigging a light bulb to their amp everytime I play it. If it is a matter of attaching a varister? I am confused.

fyi - I did try a $70 tripplittle strip with a voltage regulator today and it did nothing. Thanks

You could certainly mount either the light bulb or the self-bucking transformer on a DIY strip if you are handy and comfortable doing it. You'd want to make it very safe and well put together.

[dshoaf]

I think you may have 2 problems, here. Firstly, the popping on shutdown sounds like it could be from the power switch. See if it has a supressing capacitor across the power switch terminals.

Secondly, the heating issues look to be well-discussed. If you have any old filter caps in the power supply, they need to be attended to. If they're getting warm, there's a likely problem with higher-than-normal current flow through the power transformer due to this. They should be considered a safety issue until they're replaced.

As for the AC line voltage, a good method of reducing the voltage is to place 2 varistors inline with the high-voltage secondary winding of the power transformer. This is a technique that Jim McShane uses with the Citation II's to minimize inrush current, while reducing by a few volts the high voltage rails. I've also used this approach with McIntosh tube gear, too.

If the power supply is well restored, though, this is less of an issue, IMHO. As long as the bias circuit is set up correctly for the output tubes and no cap or tube ratings are exceeded, then it should be fine. I do find, though, that bias voltages tend to be a bit different if the amp employs a fixed-bias arrangement - as opposed to individual adjustments for output tubes.

Cheers,

David

[outlawmws]

Thanks for your reply. This thread has become a bit confusing. Facts are my house current is about 122-123 volts. There was a loud pop when I shut off the SC ASR-444. When I did the test plugging a lamp with a 60 watt bulb into the switched power, the pop problem went away. I can see rigging the light bulb for testing, but I don't see anyone rigging a light bulb to their amp everytime I play it. If it is a matter of attaching a varister? I am confused.

fyi - I did try a $70 tripplittle strip with a voltage regulator today and it did nothing. Thanks

Was this bulb wired in series or parallel to the amp?

To effectively drop the voltage to the amp it has to be is series. Plugging it into the same source is in parallel and won’t affect the voltage to any significant degree.

If it was in series, then you dropped the voltage to around 80VAC or so, and as you go lower in wattage, the bubs will shine brighter. A high wattage bulb (over 150 watts) is so dim it’s hard to tell it’s on at all in daytime.

[jpchleapas]

Purchased a Volt meter. The current measures at 122V. Is there some device or conditioner I could put on the lines without spending a fortune or needing a inline light bulb to bring the volts down? Thanks

A variac is a adjustable power supply that plugs in between the wall outlet and the tube amplifier. These are very nice to have when buying used tube amps to bring them up slowly to reform the capacitors. You can scrounge one local local to you at boat anchor (ham radio) swap meets, or look for one close to you on fleabay. These are pretty heavy so shipping can be $$. I paid $35 for mine that is about 40 years old. They allow you to manually set the voltage to what the amplifier was originally designed to be supplied with. You can find lots of info on how to swap in the SDS capacitor board to replace the can cap that was/is marginal to begin with. That will also tighten up the bass. I still have mine with the Kennedy mods that really made my amp sound much better than stock in storage. Good luck with your new amplifier. John