Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: ronco05 on May 06, 2012, 08:24:41 am
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I just finished rebuilding a 1976 OR80. The amp sounds great. My question is when i turn the amp on the tubes look fine. After 20 minutes of hard playing the plates start to turn orange. When i stop playing they stop glowing. Is this normal? The tubes are JJ 6CA7's. The plate voltage is 560 and the bias is 35Ma. Thanks.
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After 20 minutes of hard playing the plates start to turn orange. When i stop playing they stop glowing. Is this normal? The tubes are JJ 6CA7's. The plate voltage is 560 and the bias is 35Ma.
NO! :w2:
The 6CA7's are red plating, ie, running too hot. You'll burn them up. They are biased wrong.
They are passing too much current and can not dissipate the heat. Your plate voltage seems high also, but I'm not familiar with this amp.
You have to change the bias so the 6CA7's draw less current. Don't play the amp anymore or leave it running until you do this.
Is this the right schemo for the amp you built?
http://www.el34world.com/charts/Schematics/files/orange/orange_otr_80_120.pdf (http://www.el34world.com/charts/Schematics/files/orange/orange_otr_80_120.pdf)
Did you install/wire up an adjustable bias when you built this amp? It's easy to do and is very easy to adjust your bias for different output tubes then. All power tubes even from the same manufacturer will draw different amounts of current.
Brad
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The plate voltage is too high, and it's because the PT is wound for 115VAC. First thing is try to set the bias lower, but it may be no use. You can try running it on a variac, or try to lower the B+ by another method. Regarless, first thing I would do is install 2.2K 5W screen resistors to replace the 1K's (Even modern Matamps use 2k's. And check out this related thread about Marshalls.
http://ampgarage.com/forum/viewtopic.php?t=10502&postdays=0&postorder=asc&start=0&sid=24da36afad9c6b24a5e73178355e498d (http://ampgarage.com/forum/viewtopic.php?t=10502&postdays=0&postorder=asc&start=0&sid=24da36afad9c6b24a5e73178355e498d)
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why would the plate voltage be too high? the 6CA7 can handle 800V on the plates.
the grid voltage can be max 425V.
lower the bias and you'll be fine.
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Thanks for the info.
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It's not the bias. It idles fine.
It's not just the high voltage.
Re-check the load connections. A too-low load impedance (especially combined with higher voltage) can cause overheating at high output power.
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It's not the bias. It idles fine.
It's not just the high voltage.
Re-check the load connections. A too-low load impedance (especially combined with higher voltage) can cause overheating at high output power.
Right, that's what I meant by "too high a B+", that it's too high in combination with low primary impedance. And yeah, they can handle 800vdc on the plates or whatever, but that's in combo with the right primary Z, and low screens. The Orange is definitely NOT set up to operate like that. So yeah, it's too high for that amp, and plus like I said, it was not designed to run that high with the orginal PT primary being 115, not 120.
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It's not the bias. It idles fine.
It's not just the high voltage.
Re-check the load connections. A too-low load impedance (especially combined with higher voltage) can cause overheating at high output power.
now i would like to know how you know this is the case without knowing what schematic and what transformers ronco05 used.
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I used these two schematics.
http://www.orangefieldguide.com/OFG_SCHEM/OR120schem_74.gif (http://www.orangefieldguide.com/OFG_SCHEM/OR120schem_74.gif)
http://www.orangefieldguide.com/OFG_SCHEM/OR120schem_post74.jpg (http://www.orangefieldguide.com/OFG_SCHEM/OR120schem_post74.jpg)
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Interesting.
Ronco: what is the bias voltage you have now? I'm talking about the negative voltage at pin 5 of the output tubes.
The original schematic you used shows a factory setting of -37v. Do you have less than -37v? If so, you'll want to bias cooler, perhaps even a hair more negative, like -40v, due to the higher-than-schematic voltages you have.
Recheck load as PRR said, but if all else fails, you may have to bias cooler to keep the tubes within dissipation when they're cranking out power.
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-45v.
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Is that what you had when you arrived at 35mA at idle?
If you don't have any issues with the speaker jack (it isn't loose is it?) or with an impedance select switch, then you'll have to bias cooler to keep dissipation in check at full tilt.
That, or... find a set of output tubes that naturally idle cooler. That might be tubes with lower transconductance, or a lower idle current measurement (different vendors do their matching differently, and might spec either of these). The old example is Groove Tubes, where you'd want (I'm guessing) a lower number (3 instead of 9).
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saw your post on dpi pacemaker ea 50t.
i have posted today about amp with same model no., but it's got only 3 tubes.
know anything more about these old amps?
thanks
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Bias is about idle.
Overheating at FULL output (but OK at idle) is load-related.