A question about KT120s

Does it stand to reason that a set of KT120s should run cooler in a ST-120?

I picked up a matched quad of low hour KT120s from a guy local to me. I had planned to change to KT120s when it came time to replace the KT88s. Besides I feel better knowing I have a spare set on hand.

For grins and giggles this morning I swapped them in, set the bias and they've been playing for about 3 hours now. I swear that the amp is not putting off nearly as much heat with the KT120s in place.

I guess I'll need to dig out the little laser thermometer thing and take some measurements.

jhoak wrote:Does it stand to reason that a set of KT120s should run cooler in a ST-120?

I picked up a matched quad of low hour KT120s from a guy local to me. I had planned to change to KT120s when it came time to replace the KT88s. Besides I feel better knowing I have a spare set on hand.

For grins and giggles this morning I swapped them in, set the bias and they've been playing for about 3 hours now. I swear that the amp is not putting off nearly as much heat with the KT120s in place.

I guess I'll need to dig out the little laser thermometer thing and take some measurements.

The tubes or the PT?


Steve

The tubes.

The PT seems about the same. Like I said I haven't measured it yet. I'm going to do that as soon as I can figure out where I last used my thermometer.

It would be an interesting experiment.

1. That the amp has been on the same amount of time in both cases ..

Easy enough. I light it up every day at the same time.

2. Both sets of tubes are biased the same

.550 volts or so. I've yet to get them all exactly .550V. More like .549 to .551 but I don't think .002V one way or the other makes a lot of difference.

3. That you use the thermometer on exactly the same spot on the amp or tubes...

That'll be a little tougher. The plate designs are a bit different. Would measuring the very top and maybe the bases be "consistent" enough? Which raises another question... Is the thermometer measuring the temperature of the plate in side the tube or the glass side?

4. That the ambient temperature in your room is about the same ..

Pretty consistent here. 76 to 77 degrees all day.

5. The location of the amp in the room is the same.

It never moves.

6. Your playing music at about the same volume level in both cases.

Pretty consistent during the day. Typically around 75 to 80 dB. My speakers are VERY efficient (101dB/1w/1m) so the amp is NOT working hard at all. Well under 1 watt. Evenings after a beer or two I tend to crank things up a bit but rarely over 90dB. I doubt the amp ever sees more than 1 watt of output. I probably could have saved myself some $$ by buying a ST-70 kit.

As it is right now my "perception" is completely unscientific and based on nothing more than me holding my hand over the amp. On the other hand I am pretty sure that the amp is giving off less heat with the KT120s installed.

Now I just have to prove it.

I'm going to guess that your transformer will run hotter under the additional load.
Just a guess with nothing to back it up.


Steve

I am in the process of developing some manner of "empirical" testing methodology to prove my theory. By trade I am an engineer and well aware of precision testing methodology. Of course my area of expertise is NOT thermodynamics but still.

On the flip side I'm one of those guys who uses the "hand over the coals" method to determine when my grill is ready to cook on. Do I have a "calibrated" hand? Of course not. But as far as the grill goes that method has served me well for many years.

There's a fair chance that I'm completely wrong but I'm here to tell you that the KT120s are running significantly cooler than the KT88s were. My "un-calibrated and unscientific" right hand just don't lie to me

If you are using an IR thermometer you will not know which surface temperature you are measuring--envelope or plate. To measure the envelope surface temperature you will want to use thermocouples--$10 at Fry's--plugged into your DMM. Of course the thermocouple measures a local temperature so multiple measurments would need to be made and averaged. Slow going.

Hi:
I have been alternating 2 sets of KT-120's and a set of KT-88's in my ST-120 for almost two years after starting out with KT-88's. I bought a spare quad of the KT-120's (in case of disaster) as I prefer their sound because over that time I have noticed that my amp just sounds a bit better in subtle ways with KT-120's and the bias at .60 rather than at .55
There seems to be no effect on the amp temp whichever set of tubes or types I'm using but my question is would this higher bias point make it run any hotter?
I have also rebuilt my WZ-68 "Coppercap" with 3 amp 1000v diodes and that seems to have solved a lot of my problems with power surges and outages blowing them. I didn't notice any obvious changes in the apparent temp of my amp when using the educated hand test either but that is very a subjective test.
Even after a few hours of operation the temp seems to every stable and I found the sound becomes more complete if that describes it with a slightly higher bias after the amps are warmed up so getting warm is a good thing in some ways. The PT is quite warm to the touch but the OT's are not hot at all only warm to the touch.
I'm thinking that it makes very little difference which tubes are in there as far as temp goes. A careful test with the digital thermometer will be of interest to me as I don't have a digital thermometer.
P.S. given the cost of the output tubes don't forget to invest in a $20 GFI plug adapter to make sure you don't fry either the rectifier or the tubes when the power does a quick off and on which eventually it will no matter where you are. Its cheap insurance.

Could you drop me an email, or describe in detail for those interested, how you went about the mods in the GZ68's. I have wanted to "beef things up" since hearing of the possibility of blowing a GZ68 in one of the threads. I'd love to get both of mine a little stiffer, in case of a problem. We have had lots of storms here lately with the power going on and off, but I usually don't have the amps, or anything else in the house, running when the thunderstorms approach.

Is their modification a tough job? Where did you find the parts? How do you get inside the little buggers? Thanks (My email is k s r i g g at g m a i l dot c o m .) I appreciate this very much.

Sutton

Hi:
I posted a description of the rectifier modifications on this forum some months ago on Nov. 28, 2010. It is titled "Fried rectifier solution". I checked and it is still easy to find on this forum.
This pretty well lays out what I did.
I might add that I had to sweat out the leads that were connected to the pins and you have to use the new long ends of the diodes I would not reccommend trying to solder the new diodes to old leads that are cut off the old diodes.
There is also site that sells the blank octal bases identical to the base Weber uses but I don't have it at my fingertips right now I will look for it in my bookmarks and get back to you on that in the forum as soon as I find it. You might also find it in one of my old postings. They were dirt cheap.
I used IN 5408 diodes that are 3 watts 1000 volts and they have been very reliable.
With plenty of power weirdness and a lot of outages down here in Costa Rica where I live they have performed flawlessly.
If this posting is missing any details just ask and I will try to answer you questions.

BTW There is a current inrush capacitor in there that I have not had to replace and it has not had any problems with the diodes it seems to be fine with these diodes.
Baddog1946