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Old 20th November 2011, 10:04 PM   #1
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Default el34, kt88, kt90, 6550 to 8417 conversion

Yep, this is a strange one. I have 16 matched pairs of 8417's bought new over the last 5 years. I matched them within 5%. I sold the amps that I squirled away the tubes for. 4 dynaco Mark VI amps. Now I have 2 Manley studio 250's which use 10 el34's each. These tubes seem to be in very good shape. I also have 3 manley MB350's which use 8 KT90's, or 8 6550's each. Two of the amps have mix matched KT90's. One amp has mix matched 6550's. All the tubes had a layer of dust on them. They all test pass, but not like a new tube would. I want to replace all the tubes in the MB350's which would take 24 of either kt90's or 6550's. So my initial thought was to sell the 8417's and buy KT90's. I also have 20 Penta Labs kt-88sc's new and unused. Same thought there, sell the kt88sc's and buy what I need. In light of how the economy is going and looking at past ebay sales for 8417's , now wondering if it would just be better to rebias the MB350's to use the 8417's. Rebias as in either changing the bias circutry to support 8417's or adding a seperate internal transformer along with the bias circutry changes to support 8417's. Any other considerations?
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Old 21st November 2011, 03:42 AM   #2
dgta is offline dgta  United States
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Yes. Bias is a minor affair, what you need to figure out is the operating point of the existing tube vs. new (proposed replacement). I.e., how does your OPT impedance work with the new tube at your existing B+ .

I don't see the logic of going through a lot of trouble to replace tubes that are in current production with tubes that are not.
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Old 21st November 2011, 04:00 PM   #3
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Quote:
Originally Posted by dgta View Post
Yes. Bias is a minor affair, what you need to figure out is the operating point of the existing tube vs. new (proposed replacement). I.e., how does your OPT impedance work with the new tube at your existing B+ .

I don't see the logic of going through a lot of trouble to replace tubes that are in current production with tubes that are not.

you're right....most folks won't get it.....you have 1600 bucks worth of new tubes which you can't use unless you change about 16, 5 cent resistors. Then later, when you retube and/if you decide to put back in there what it had, just change the resistors back.

Or go out and buy 1600 bucks of the tubes you need today, and let the 1600 bucks worth of tubes you can't use sit in a closet.
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Old 22nd November 2011, 01:42 AM   #4
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Or sell the tubes,get the $1600,and buy some tubes that you can use.

If you plan to use the 8417's,just be aware that they can be kind of picky/touchy. Be on the look out for oscillation/runaway,etc.
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Old 22nd November 2011, 03:01 AM   #5
dgta is offline dgta  United States
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Fritz, I think you missed my point.

The Sylvania data sheet for 8417 gives an operating point of 560V plate, 300V screen and 4200 ohm P-P per pair, for 100W out. If your amp doesn't match these, you have to modify the PS, get a new PT, get a new OPT or all of the above. That may not be cheaper than buying tubes.

Alternately, you can calculate a different operating point that matches what you have available, and hope to find such an operating point that puts the tube still in a favorable part of the curve.
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Old 22nd November 2011, 10:06 PM   #6
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OK, thanks for the feedback. I guess I'll just sell the tubes and buy what my amps call for.
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