"Normal" temperature for power tubes?

Electrolytic capacitors don't like heat and those above chassis capacitors
are close to the tubes, a good reason for fan cooling.
Also a good reason for heat shielding those elyticaps if it seems necessary, roll paper core sections wrapped in alu foil, shiny side out, is very effective.
 
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Some of my best burns have been from Dynaco, ARC amps and some guitar amp that used 7591s. Ouch. Never measured the temperature though.
 
Conrad - Yup - a reminder that tubes like it hot. I have accidentally bumped up against a 7591A in my recent days. Gotta keep those bottles in their jobs ! I'm going to try out the original Sylvania 7591's that came with the 500-C. I'm curious now to see the difference between them and the new Tung-Sol's to which I have become accustomed.
 
I tried the old Sylvania tubes which are considerably smaller and probably have considerably less GM. The mA dropped from 34 to 21 ! I put the Tung-Sol's back in though the Sylvanias did sound pretty good even at the way out of range specs.
 
Once its hot enough to remove fingerprints, thats about the proper temp.

Tubes bias at different points, thats why the adjustments are good to have. If you really want to know if its tired, bias it to normal current and do a power output test. If the tubes are weak, it will not make anywhere near normal output power.
 
Electrolytic capacitors don't like heat and those above chassis capacitors
are close to the tubes, a good reason for fan cooling.

This is another perennial semi-wives' tale that constantly gets retold about these Fisher tube receivers every decade or so, to justify blowing fans on them.

Yes, the big filters in these do have a temperature rating, just like any other electrolytic design.

But these large can filter electrolytics are among the hardiest I've encountered. Extremely well made General Instrument models in most cases. They are effectively mounted in a way to minimize evaporative electrolytic losses for many, many years, marching into decades of service.

For many years of working on these, they nearly always reform up just fine, unless they have been abused in the past. And they do not need any special cooling provision made for them other than the passive ventilation manufacturer recommendations.

I remained an advocate of leaving the original big cans in up until just a year or so ago, as they are now reaching 60 years of age. Some are now starting to more heavily fade in capacitance. In the 800B mentioned above, we left the originals in until only a few months ago, and we have had and played this unit for many decades, fanless, in an open-back cabinet arrangement. And the 800B has the biggest tube complement of them all.

It is the smaller under chassis electrolytic capacitors that dry out on these, as the seals are less enduring. Those are only good for about 25-30 years maximum. But heat is not what kills them. Seal failure does.

Again, there is no need to provide active cooling on these tubed receivers, unless you are mounting one vertically or in a wall or other space contrary to the passive clearances. On top of the electrical noise these fans impose, all I've found from the ones running under fans is a lot more dust and crud to clean up (and it even gets into the tuner front end mechanicals and the dial).

These are extremely sturdy tubed instruments as far as consumer grade equipment goes. After performing any necessary internal refreshes, adjustments and recaps, just plug them in and enjoy them as the original designers intended. They don't run all that hot as tube gear goes. They are not transmitters. And even the old-time high-power ham operators that I cut my teeth with never put these kind of fans on their outputs. These home receivers will run for another 50 years just fine without fans.
 
Conrad - Yup - a reminder that tubes like it hot. I have accidentally bumped up against a 7591A in my recent days. Gotta keep those bottles in their jobs ! I'm going to try out the original Sylvania 7591's that came with the 500-C. I'm curious now to see the difference between them and the new Tung-Sol's to which I have become accustomed.

The newest Tung-Sols are actually a pretty nice sounding output. A little sweeter than the older EH types. And much more reliable than the JJs. As to durability and dependability, so far so good for the ones I am aware of in the field.

Still not as nice as the old Sylvania and Westinghouses. But you use what you can get nowadays with this output family. And it's nice to have any new production. I remember when people were butchering these up to accommodate other outputs.
 
Once its hot enough to remove fingerprints, that's about the proper temp.

Sage advice ... right up there with "go ahead and tighten that bolt till just before it snaps" ... <G>

And ya ... some truth to both sides here. Yes, cooler tubes can last longer and do less damage to other internals IF they're still operating within the recommended range. Regular convection can and should provide proper cooling, and a fan SHOULD be overkill if the equipment is up to snuff. Kind of a crapshoot with some of the older stuff, so use your best judgement.

I built a Dynaco based (Latino) kit and didn't much care for how tight the old design was, so I went with a custom case that spread things out a lot more. My temps run significantly lower than the stock setup, even with the big KT120's. Again, not a necessity, but I'm happier with it, and the tubes are doing fine with a LOT of miles on them since the build. Here's the average temps after the amp's been running a while.

tube-temps.jpg


PS ... you'll notice a few empty holes in the chassis - early construction there - I just used that pic as it was handy. Also shows the holes I drilled around the tubes so I get good convection cooling from under the chassis plate, which helps a LOT. If your case doesn't have a good sized ventilation screen on top and a way for air to get in from the bottom, I'd go that route instead of a fan.
 
The case for the 500-C (including the after market case that came with my 500-C) has about 1 inch between the bottom of the case and the bottom of the 500-C chassis. That should be fine for underneath circulation. My 500-C sits on top of a Marantz 2265B. There's just no other good place for these gems. But there's plenty of open air around them so I'm good with removing the fan based on information in this thread. The metal baffle on top of the 500-C is wide open. To tell you the truth, I like going fanless if for no other reason than it's one less thing for my cat to knock over. HaHa. Thanks for all the discussion. It's been interesting and educational - about electronics and people.

I would guess the predominant sentiment is that the original Sylvania 7591As is preferred than the new Tung-Sol's if for no other reason than that's what was available when the 500-C was designed. It would be interesting to try the Sylvanias's again, bias them, and see how it sounds long-term. For the few minutes I listened to them with the extremely low bias, they sounded pretty good especially in the bass register.

Anyway, it's Christmas Day and we're off to the movies. Merry Christmas to those who celebrate. Thorne
 
Depends. If the Tung Sols are fresh and the Sylvanias near the end, the former may sing with a little better ‘pop’ on a heavier load.

But all else equal, the original US production, and the Westinghouses in particular, are still the standard in this output family.

It sounds like you have plenty of airspace around yours (provided the Marantz under it is not running and outputting heat). Just enjoy it in the natural air and watch out for the cat. Cats love to jump on warm things to nap, and grilles get bent up or the animal can get hurt.

Here’s a ‘cat off’ tip: When you are leaving for that movie, or perhaps going to bed tonight, place some clear packing tape, sticky side up, on what it is you don’t want the cat on. The cat will only jump on it once more, then never again. You will find the packing tape, in a ball covered in cat hair somewhere later. But the cat will never jump there again.
 
I measured the Gm of my 4 Sylvania 7591A tubes on my Hickok 600A. Average Gm is 6300. The 4 tubes measured 7500, 7500, 7600 and 8000. That makes these tubes above average.

Then I measured one of the Tung-Sols. Only 6300 making it average. Considering that the Tung-Sols have been in use only about 9 months, does this make sense that the Sylvanias measure that much higher? Baffles me.

UPDATE: I had to measure another of the Tung-Sols. Maybe the 6300 was just a weak tube. The 2nd one measured 6500. What the ??? By the way, the Sylvanias state "7591A-P" on them. I wonder what "-P" means.
 
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The modern tubes tend to want more negative grid voltage than the old ones. Plugging an old tube into something set up for a new tube will usually have it running at an overly low idle current. It doesn't necessarily mean the tube is weak, just that it needs the adjustments re-set.
 
Tube tester should work with new or old production. I'm thinking Gadget is referring to the bias setting on the equipment the tubes are used in. Not uncommon even for different brand NOS tubes to require that to be reset when swapped in. Main reason I added a meter circuit and easily accessed bias adjusters to my custom build here. I can swap in a set of tubes, and if the meters show the same levels after warm up, can safely assume I'm good to go. I'll still do the old VOM thing at the test points eventually, and my setup allows me to set each tubes bias individually, so no real issues with matching if I break a set.

meter-vom-jacks.jpg


PS - some amps make you work way to hard to bias them ... kinda sorta sucks, but ya does what ya gotta does ...
 
Oh sure, different tubes are very likely to require re-biasing in the receiver. But in the tube tester I would think that everything else being equal that any 7591A tube would be able to be tested and could be compared to one another. So when I saw the Tung-Sol's measure significantly below the Sylvania's, my conclusion was that the Sylvania's should have more "pop" to them. Or maybe there's such a thing as biasing the tube tester for different tubes? I have the bias controls easily accessed on my 500-C on purpose.
 
Question - I'd like to try the Sylvania's for a while. Will I have to measure the pin 3 voltage and do the P=I*V math to get the mA to the 70% of max wattage from the tubes? Or since both tubes have the same max wattage of 19 watts, is it enough just to re-bias the tubes to get to the 34mA I have with the Tung-Sol? As a reminder, when I tried the Sylvania's earlier today for a minute, pin 5 read .21 as opposed to the .34 for the Tung Sols.

And then when I put the Tung-Sol back in, is there any danger of red-lining them because I re-biased much higher the circuitry to accommodate the Sylvania's? I would guess no because I didn't have the IBAM circuit installed when I first installed the Tung-Sol's way back when.

UPDATE Dec 27: I realized there is a simple solution. All I have to do is note the pin 3 voltage when I install the Sylvania's before re-biasing them. Then, when I'm ready to reinstall the Tung-Sols, before I remove the Sylvania's, I simply adjust the bias of the Sylvania's to what I noted. Easy peasy !
 
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Tube tester should work with new or old production. I'm thinking Gadget is referring to the bias setting on the equipment the tubes are used in. Not uncommon even for different brand NOS tubes to require that to be reset when swapped in. Main reason I added a meter circuit and easily accessed bias adjusters to my custom build here. I can swap in a set of tubes, and if the meters show the same levels after warm up, can safely assume I'm good to go. I'll still do the old VOM thing at the test points eventually, and my setup allows me to set each tubes bias individually, so no real issues with matching if I break a set.

meter-vom-jacks.jpg


PS - some amps make you work way to hard to bias them ... kinda sorta sucks, but ya does what ya gotta does ...

What did you use for the connectors on the top of the chassis there? I want to put some test points on top of my 500c so I don't have to keep flipping and unscrewing every time I want to check my bias.
 
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