How do you tell when tubes are overheating?

[dougg]

I know very little about how tube amps actually work, so this may be a really dumb question. But how do you tell when tubes are overheating? I ask because I have a new PRRI, and it seems to get hot pretty quick - nothing looks particularly unusual, but the two 6V6s are glowing pretty bright with a blue haze, and the face of the amp (the Princeton Reverb logo) feels warm after about 20-30 minutes playing, and can get hot after an hour or so.

I owned a Blues Jr previously, and probably didn't pay as much attention to it, but it never seemed to warm up this much. Of course, it's a different amp. I kind of assumed that the PRRI would be biased pretty cool from the factory (I did read something about Fender doing that so the tubes would last though the warranty, but that could have just been internet trash talk) I think I noticed this only because I spent so much time farting around with the chassis rattle today, and it's not something I'm overly worried about but thought it worth asking. Are there specific things to watch out for with a tube amp, or will they pretty much just crap out on you when something goes wrong and you get them fixed?

Edited to add - may have posted this in the wrong area; meant to go into Amps & Cabs but accidently put it in here. Don't think it's entirely out of place but maybe a dopey question to ask the more technically minded...or maybe not?

[rob2001]

You'll know they are overheating when the amp sounds totally awesome!


Just kiddin....well, not really...... I don't worry much about heat. I'd think if you know you are within bias parameters, have at it......if one blows..replace it. I have yet to blow a tube and I use amps pretty hard.

[oldschoolguy]

I'm no expert but it probably would not hurt any to set up a small fan to blow air towards your amp providing it is open-backed. I've done this for my Siegmund compb that doesn't have an internal fan and I can play for hours and it doesn't heat up as much as without it but it didn't get overly hot to begin with. Heat is the enemy of amps, I've read but then too much is relative. Your tubes are "too hot" meaning they get too much power when the plates glow red. They won't last long like that--it means that they are biased way to hot---take your amp to a tech who can bias it properly if you don't know how. Anyway, the blue is normal and i think you should just see nice, glowing red at the bottoms and the very tops of your power tubes and not on the plates themselves. BTW, some amps just naturally play hotter than others--the nature of the beasts but like, like yourself, if I wasn't familiar with my amps I wouldn't know what is the norm.

[dougg]

You'll know they are overheating when the amp sounds totally awesome!

Yeah, I've actually noticed that the hotter than thing gets the longer I want to keep playing!

Good to know the blue is normal, too. Never noticed that on my Blues Jr.

[RussB]

Tubes generate a lot of heat. The PRRI has a tube rectifier, which also generate a lot of heat. The tubes hang down, and heat rises, so the chassis gets hot.

carry on, Gentlemen :)

[SatelliteAmps]

I usually do not recommend running a fan pointed at tubes (or installed into an amp). Most tube amps are designed to be able to handle the heat generated. By blowing cool air onto the tubes, the tubes have to work harder to stay within their operating temperature. Also, there can be a thermal reaction with the glass in a hot tube, when cold air is blown at it. It's called thermal shock, and can crack or break glass. That would be rare, but it has happened occasionally.

Tubes will usually run somewhere between about 150-250 degrees. Enough to hurt, but not enough to start a fire.

[rob2001]

Yeah, I've actually noticed that the hotter than thing gets the longer I want to keep playing!




For sure... on the weeknights my band practices, they are usually pretty short sessions. By the time everything is heated up ...we're done....HATE THAT!

[uberpict]

If they're not red-plating, the outer plates turn red from the heat, then you're okay. The blue glow is pretty normal, AFAIK.

[RussB]

I usually do not recommend running a fan pointed at tubes (or installed into an amp). Most tube amps are designed to be able to handle the heat generated. By blowing cool air onto the tubes, the tubes have to work harder to stay within their operating temperature. Also, there can be a thermal reaction with the glass in a hot tube, when cold air is blown at it. It's called thermal shock, and can crack or break glass. That would be rare, but it has happened occasionally.

Tubes will usually run somewhere between about 150-250 degrees. Enough to hurt, but not enough to start a fire.



The fan isn't there to keep the tubes cool, it's there to keep everything else from getting too hot. I'm a fan of cooling fans :)

[GearHeadFred]

I agree with Adam - no need for a fan. Fender's design of mounting the chassis "upside down" in combos causes the heat to build up in the top of the amp...Not the most elegant solution, but it has stood the test of time! Unlike fickle modern semiconductors, these components can take it! Play on!

[strat a various]

Unless someone cares to perform a study of tube amps with and with out a cooling fan, I'll remain unconvinced that a fan will negatively impact tubes. They are vacuum components ... the glass remaining slightly cooler (still far too hot to touch comfortably) is unlikely to have much effect on the inner workings. For the past 30 plus years, I've run a small fan behind my tube amps at 90% of gigs ... the only tube failures and solder or tube socket issues I've ever had were when a fan wasn't available for some reason.

[SatelliteAmps]

The fan isn't there to keep the tubes cool, it's there to keep everything else from getting too hot. I'm a fan of cooling fans :)

I guess I'm missing something here, to keep what else from getting too hot?

Unless someone cares to perform a study of tube amps with and with out a cooling fan, I'll remain unconvinced that a fan will negatively impact tubes. They are vacuum components ... the glass remaining slightly cooler (still far too hot to touch comfortably) is unlikely to have much effect on the inner workings. For the past 30 plus years, I've run a small fan behind my tube amps at 90% of gigs ... the only tube failures and solder or tube socket issues I've ever had were when a fan wasn't available for some reason.

I have never found a fan to help anything for tube or amplifier life. I have noticed over the years that people who installed fans in their Marshalls (was very common for a few years), all went through tubes much faster than people with no fans. Also, all the tube failures in those amps were all closer to the fan side than the far side. Its not quite a scientific test, and I guess it could be a huge, amazing coincidence, but I doubt it.

[RussB]

I have never found a fan to help anything for tube or amplifier life. I have noticed over the years that people who installed fans in their Marshalls (was very common for a few years), all went through tubes much faster than people with no fans. Also, all the tube failures in those amps were all closer to the fan side than the far side. Its not quite a scientific test, and I guess it could be a huge, amazing coincidence, but I doubt it.



experience trumps conjecture :)

[davemccarthy707]

I like to keep a fan on stage to keep a nice airflow to keep me cool. I call it my biggest fan!

[Slunderfungus]

I bought a little 5" fan to perch above the left hand vent of my Vintage Club, because those amps run hot by virtue of their design. My V50 which uses 6CA7s doesn't get nearly as hot as the 4 EL84 in my VC5212. At first I didn't think anything of it but when I had the amp apart trying to diagnose a problem I was having I noticed that the circuit board was MAJORLY warped. oddly it was the side that has the tubes that had the warp, so I imagine that if the tubes get hot enough to warp the board it gets hot enough to damage other components on that board so I have ran the fan on it ever since.:boxerBut arguning in favor of Satellite aMps It is the only amp I have ever used a fan on.

[SatelliteAmps]

I went and bought an infrared temperature checker at Home Depot. I'm going to do some tests and find out what kind of temperature differences I get. I'll keep you all posted.

[SatelliteAmps]

I only tested one amp tonight, but figured I would post the first batch of results. Amp is a 1968 Fender Bandmaster, complete beater, with a pair of 6550's running hot. Ran a signal into the amp, and let it go about 3/4 volume for about 20 minutes. Took multiple measurements to get hottest temp I could. Tubes maxed out at 218 degrees. PT was 99.5, choke about 95, and OT at 92.5. Hooked up a 5 inch, 125v fan, set to blow cool air directly on the power tubes. Checked immediately after installing fan, and tubes dropped to about 180 degrees (it crept back up pretty quickly). No change in PT, Choke, or OT. I let the fan run, same signal through amp, etc. for another 20 minutes. Tubes went up to 215 degrees, PT was at 98, choke at 94, and OT at 91.

Going to try this out on a few amps that are a bit more closed up, and see how others do. I'll post more when I know more. (probably have time to test more on Tuesday)

[mbratch]

For sure... on the weeknights my band practices, they are usually pretty short sessions. By the time everything is heated up ...we're done....HATE THAT!
Yep, playing with a tube amp is like barbecuing with charcoal. The coals are just right when you're all done cooking. Tasty just the same. ;)

[strat a various]

I only tested one amp tonight, but figured I would post the first batch of results. Amp is a 1968 Fender Bandmaster, complete beater, with a pair of 6550's running hot. Ran a signal into the amp, and let it go about 3/4 volume for about 20 minutes. Took multiple measurements to get hottest temp I could. Tubes maxed out at 218 degrees. PT was 99.5, choke about 95, and OT at 92.5. Hooked up a 5 inch, 125v fan, set to blow cool air directly on the power tubes. Checked immediately after installing fan, and tubes dropped to about 180 degrees (it crept back up pretty quickly). No change in PT, Choke, or OT. I let the fan run, same signal through amp, etc. for another 20 minutes. Tubes went up to 215 degrees, PT was at 98, choke at 94, and OT at 91.

Going to try this out on a few amps that are a bit more closed up, and see how others do. I'll post more when I know more. (probably have time to test more on Tuesday)

A few words about exactly how I've been using a fan with tube amps: I've never installed a fan "inside" an amp and I don't blow the fan directly on the tubes ... I've always mounted a small fan 12-18" behind the amp, blowing at about a 45 degree angle to the back of the chassis. This light breeze is equivalent to playing outdoors on a mildly windy day, at most.
I can't vouch for a fan blowing an inch or two from the tubes.
By the way, I start the fan before I fire up the amp, and turn it off with the amp.

[SatelliteAmps]

Didn't try it from that far away, but I will test that sometime tomorrow as well. My guess is there will be more of an effect of a fan close by (both in cooling factor, and potential temperature changes). My guess is that if I had started the fan, and then turned the amp on, it would have gotten to the same operating temperatures, just without the spike. Most people who use fans (and are the ones I've seen tube issues with) have had the fans installed in the amp, with the fan less than 6 inches away from the power tubes. Most usually blow, occasionally one will suck the hot air out, which I think would be better for the tubes.

[rockon1]

I guess I'm missing something here, to keep what else from getting too hot?



.

In my V32 the cathode resistor throws off a ton of heat and its smack in the middle of the board. That coupled with the tubes and other componants that get hot made the top of the amp uncomfortably hot. Installed a fan to blow cool air into the inside of the chassis and out a smal vent I made in the back. The amps guts and chassis stay much cooler which I figure is probably a good thing. The over all temperature inside the chassis stays about 30 degrees cooler with the fan running. Bob

[drbob1]

I've read somewhere that running a fan in an AC30 (which tend to run pretty hot, a LOT of tubes in an enclosed space without much air movement) does bad things to the tone. Can't vouch for it myself...

[SatelliteAmps]

In my V32 the cathode resistor throws off a ton of heat and its smack in the middle of the board. That coupled with the tubes and other componants that get hot made the top of the amp uncomfortably hot. Installed a fan to blow cool air into the inside of the chassis and out a smal vent I made in the back. The amps guts and chassis stay much cooler which I figure is probably a good thing. The over all temperature inside the chassis stays about 30 degrees cooler with the fan running. Bob

Is the fan only blowing inside the chassis (and out the vent), or is there some air circulation around the tubes too?

Cathode resistors do get pretty hot. (I think the V32 has a 10 watt resistor lifted off the board for circulation). Some resistors do change value with heat. More testing....

[strat a various]

Here's another thing: I often use my old Fender amps with tilt back legs, and I notice they run considerably hotter when tilted. That was actually the origin of fan use for me ... with a fan, the tilted amps ran about as hot as an upright amp with no fan. There is substantially less air circulation when tilted, since all of the hot air from the tubes and trannies gets trapped inside instead of drifting out and away. Fan required.

[rockon1]

Is the fan only blowing inside the chassis (and out the vent), or is there some air circulation around the tubes too?

Cathode resistors do get pretty hot. (I think the V32 has a 10 watt resistor lifted off the board for circulation). Some resistors do change value with heat. More testing....


Mostly into the inside of the chassis with about 1/3 of the fan area blowing a bit of cool aire into the tube area. I have measured a 30+ degree difference inside the chassis near the top with it running. As I installed a switch I can try it both ways for heat and sound. Bob

http://i42.tinypic.com/nzr8le.jpg

[donnyjaguar]

I believe its generally the stresses of going from cold to hot, or vice versa, that can shorted component life. I'm of the mind that if you have a fan installed in the amplifier you reduce these stresses and things will last longer. :)

[RobRoyMcCoy]

I only tested one amp tonight, but figured I would post the first batch of results. Amp is a 1968 Fender Bandmaster, complete beater, with a pair of 6550's running hot. Ran a signal into the amp, and let it go about 3/4 volume for about 20 minutes. Took multiple measurements to get hottest temp I could. Tubes maxed out at 218 degrees. PT was 99.5, choke about 95, and OT at 92.5. Hooked up a 5 inch, 125v fan, set to blow cool air directly on the power tubes. Checked immediately after installing fan, and tubes dropped to about 180 degrees (it crept back up pretty quickly). No change in PT, Choke, or OT. I let the fan run, same signal through amp, etc. for another 20 minutes. Tubes went up to 215 degrees, PT was at 98, choke at 94, and OT at 91.

Going to try this out on a few amps that are a bit more closed up, and see how others do. I'll post more when I know more. (probably have time to test more on Tuesday)

Good test results, thanks for posting these, that is very useful.
Rob