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In Reply to: RE: Great reliability IME posted by casouza on March 18, 2008 at 19:39:01
this is a valid point ...pushing too far
in my application I have a pr of Primaluna 7's that produce 70w per channel using 4 power tubes ...this 'inefficiency ' compared to other amp configurations comes w/ the auto bias feature of the product ( allowing for the use of many different types of PT's : EL34 , 6L6 etc )
I wonder if this is kind or hard on the PT's ?
Follow Ups:
Regarding your Prima Luna monoblocks, the power tubes at 35 push-pull Watts per pair are operated very conservatively and should last long.
Re your concern with the power transformers, my suggestion is to measure or feel by hand how hot are the power transformers after about one hour of normal operation (music playing).
If you can touch the transformers for 10 seconds (and not feel any pain), they are in the "safe zone". If you can not stand the heat for more than 2 seconds, the transformers are stressed (this is rough layman's approach).
If the PTs are near tubes, touch them on the opposite side. Some heat radiation from the tubes is normal and will not usually harm the PTs.
I use an inexpensive Fluke infra-red thermometer to measure parts/tubes/transformer temperatures.
When I get a new piece of gear I write down a "thermal map" of the gear. If there is a tube failure or any doubt about the performance, I measure again and compare with the "normal" temperatures. Using the IR thermometer, one can easily identify power tubes that are close to end-of-life (current-hungry). Of course this is in addition to the usual bias / current measurements with a good DMM.
I have also found a few op-amps and resistors working under extremely high temperatures (as compared to the data sheet) in a couple of high-end products. I attach a heat-sink and give the part plenty of breathing space, it usually drops temperature by 10 degrees celsius (~20 degrees farenheit??).
I hope this helps
thanks for your informative post ...
I heard from Kevin D regarding my amps and he reported the same thing ..
it wasn't me that asked about the temp of the PT's but it is good to know this !
re your Fluke infra-red thermometer is this a point and shoot type ( like a laser ) ?
seems like I should have one ...very helpful
do you have Primaluna gear ?
Greetings!
I have the Fluke 61 point-and-shoot IR thermometer. It does look like a "laser gun". I bought it from tequipment.net.
There are also good offers on Ebay.
If you decide to buy one, make sure it reads up to 300 degrees centigrade (roughly 580 Farenheit) or more, because some transmitting tubes may reach 220 degrees centigrade (450 F) at the plate, under normal operation.
Your output tubes will probably measure 300 F or less, pointing the meter at the plate.
I do not have Prima Luna amps. The hint about power transformer temperature came from a friend and applies to most amps, because power transformers are wound with enameled copper wire, which has a maximum temperature spec.
Enjoy your amps!
I have checked out the 61 and goes to 275C only ...the 65 goes to 500C
maybe it is a better bet ?
they are not shaped like a pistol but rather a remote control ..is this your model ?
cheers
Yes, Fluke 61 is the one I have.
I did not realize it measures "only" up to 275 C.
My advice and description were based on the cheaper IR thermometers I saw on Ebay.
On second thought 220 C is the highest tube glass envelope temperature I have seen on tube data sheets.
I suggested a wider range to make the tool future-proof, because a transmitting tube with red-hot plates will measure more than 300C. Anyway if you are not into "electric chair" DIY high-power SET amps, the suggestion is moot.
I have measured at the most 150 C on class A KT-88s, pointing the laser straight at the plate structure.
I hope this clarifies.
so perhaps the 61 would be fine ....
thanks for your input !
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