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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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In Reply to: What would make one output tube go bright cherry red!! too much drama posted by henrybasstardo on November 30, 2006 at 22:09:27:
There are a number of possibilities:1. Was the bias properly adjusted?
2. Were the tubes properly matched? (it would seem not)
3. Was one tube defective or damaged in transit?Since the old tubes are undoubtedly worn, they likely would not show the same symptoms. But if you set the bias to the schematic voltages you have only 40 minus 28.5 or 11.5 volts of negative bias. If the new tubes you had are toward the "hot" end of the normal range then they could easily be overstressed.
Finally, any of the vintage pieces that use 330K resistors for the output tube grids are at some risk when using either the JJ or EH tubes. In the RCA tube manual the maximum grid resistance was spec'd at 300K. Plus those 330Ks typically have drifted up about 10% over the years to the 360K range. You can get away with it with the old stock tubes, but the new production isn't as tolerant.
I recommend you change the grid resistors to 180K or so an chnage the coupling caps to .1 uf. Then you can use the old and the current tubes safely - as long as the tubes are well matched and not defective/damaged.
I know Mr. sgmlaw doesn't agree, he's a purist and doesn't want to make the changes. He advocates tracking down NOS or good UOS tubes instead. That's well and good, but the 7591 old stock supply is finite - and it's quite thin already. So it seems that at some point even the purists will have to use current tubes.
The current tubes are nice sounding, and very reliable with one exception - they are not tolerant of the real high grid resistor values in many vintage amps. It was the exceptionally low grid leakage in the old tubes that allowed the makers back in the day to exceed the tube grid resistor spec by 10% right out of the box!
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Follow Ups
- Re: What would make one output tube go bright cherry red!! too much drama - Jim McShane 05:20:09 12/01/06 (3)
- Re: What would make one output tube go bright cherry red!! too much drama - el_37 07:04:11 12/02/06 (2)
- Re: What would make one output tube go bright cherry red!! too much drama - Jim McShane 10:17:48 12/02/06 (1)
- Re: What would make one output tube go bright cherry red!! too much drama - radiance 22:12:13 12/04/06 (0)
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