I want to screen my GM70s to reduce their light output. I like to listen in the dark, and these light up the ceiling and walls! So a bronze mesh "arbour"or arch is favourite at the moment. But what is the maximum permissible temperature for the GFM70? I have an infra red thermometer, which gives a reading of 262 degrees at the moment. I will be adding a low speed fan to increase airflow, but I can't find any temperature ratings for the GM70.
It's not rated for temperature but 20 volts @ 3 amps on the heaters. So each one is a 60 watt light bulb. Best you can do is block the light.
Neither of the ones I have do, but one of them lists 55°C as the maximum ambient temperature.
I just measured 145°C (293°F) with 100W plate dissipation in a room with an ambient of 21°C. (with a direct contact thermal probe on the envelope) This is consistent with past measurements.
I would not allow them to go over 200°C.
Something else to think about is overheating the sockets..
I guess I have been running my amps for about 9 years now, they really cook over the 1000 - 1500 hours I am generally able to run them before they are completely clapped out.
The light they make doesn't really bother me, but they are the brightest output tubes in the system, the rest are 300B and are pretty dim.. LOL
I just measured 145°C (293°F) with 100W plate dissipation in a room with an ambient of 21°C. (with a direct contact thermal probe on the envelope) This is consistent with past measurements.
I would not allow them to go over 200°C.
Something else to think about is overheating the sockets..
I guess I have been running my amps for about 9 years now, they really cook over the 1000 - 1500 hours I am generally able to run them before they are completely clapped out.
The light they make doesn't really bother me, but they are the brightest output tubes in the system, the rest are 300B and are pretty dim.. LOL
Your question is somewhat awkward ...
If you (your amplifier) follows datasheet specifications (heater voltage and current) and maximum anode disipation, you cannot 'reduce light output' ... you can reduce/change operating point, but that affects sound, power output etc ...
I'm running mine at 1200VDC @ 80mA. Heater voltage is 20V 3A, with modified SMPS.
Regards
Thanks for the replies.
egellings; the datasheets were the first things I looked for, but no help.
kevin; these GM70s are running at 262 degrees C (using an IR thermometer), but the only way I could reduce that is with fan cooling. I didn't make these amps.
Flikoman; I am not trying to reduce the light output, but rather make a shade without overheating them. This shape, with diffusing mesh over the top half, as the light is emitted in a line.
The 845s I used to have were under a shelf with a bead curtain in front. The new ones are on the top.
egellings; the datasheets were the first things I looked for, but no help.
kevin; these GM70s are running at 262 degrees C (using an IR thermometer), but the only way I could reduce that is with fan cooling. I didn't make these amps.
Flikoman; I am not trying to reduce the light output, but rather make a shade without overheating them. This shape, with diffusing mesh over the top half, as the light is emitted in a line.
The 845s I used to have were under a shelf with a bead curtain in front. The new ones are on the top.
An externally hosted image should be here but it was not working when we last tested it.
Hi Awkward,
I would suspect that what you are measuring is in fact the temperature of the plate and not the envelope.
I don't currently have access to an IR thermometer but my recollection is similar temperatures at the plate to what you measured.
Envelope temperature is what is going to matter most here. 😀
I would suspect that what you are measuring is in fact the temperature of the plate and not the envelope.
I don't currently have access to an IR thermometer but my recollection is similar temperatures at the plate to what you measured.
Envelope temperature is what is going to matter most here. 😀
I thought that, but on the other thread about EL34 max temp, someone suggested it was the glass temp, glass being opaque to IR. right or max bulbe temperature
Ah, but GM70 does not use standard glass, it could never withstand normal operating temperatures. I no longer remember exactly what that glass is, but I recall it is either pyrex or quartz glass. I am almost sure the glass temperature is not 500°F.. 😀
I suspect you are right about what I am measuring, but glass melts at a higher temperature than that. Anything between 900 F and 3000 F. I used to work with stained glass, and the artist was setting the kiln to around 1300F with the glass we used. Still, it gets damned hot!
BTW, how do you get the degree symbol on your keyboard?
BTW, how do you get the degree symbol on your keyboard?
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Perhaps I can answer this. The degree symbol is part of the alternate character set, of which there are many. If you have a PC, press and hold the ALT key, type 248, release the ALT key and ° will appear. Got a MAC, can't help you.
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Alt 0176 also works, and I've found in some cases 248 doesn't for some reason. (Old android device IIRC)
One thing to note is max temperature may really be determined by the glass seal at the pins rather than max glass temperature.
I agree that is very likely going to be the point of failure, and I wouldn't recommend allowing the glass to get over 200 - 210°C for this and other reasons. (My recollection is vague, but a Russian EE friend years ago may have pulled those numbers out of a long forgotten recess in his brain.. 😀 )
Edit: Such a failure would be spectacularly unpleasant to witness..
Edit: Such a failure would be spectacularly unpleasant to witness..