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#501 |
diyAudio Member
Join Date: Feb 2012
Location: Dublin
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I think i answered my own question, i have been testing the amplifier with RMAA and seeing identical gain and measurements on both channels with the gyrators set to 3ma, and the bias voltages the same. Plates were reading 123v & 116v.
Ale if your reading this i need help, i think i have damaged one of my Gyrator boards. I had both set to 3ma (measured across the 10ohm resistor R5), i went back to tweak it and one channel read 8ma. Didn't understand why so i quickly wound the adjustment down to 3ma again, and now i am seeing 75v on the plate which makes no sense. Double checked the bias & filament voltages are correct, and tested the tube which is fine. Any idea what is going on? I suspect i may have damaged the BF862 in J2? |
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#502 | |
diyAudio Member
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Quote:
Do you prioritize matching gain over similarity in THD spectra or try to get similar THD spectra and then adjust the gain elsewhere, for example, at the voltage divider just before the SF? Thanks . nash
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F5TV3, AVC, BA3b pre, Salas DCG3b, OBL-15, SonyVfet2pair monos, Millett Nutube Bal pre |
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#503 |
садовник
diyAudio Member
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With preamps, where the output swing is only a few volts, the DC Anode voltage needs only to be in the right range to yield a suitable bias level. So this is least important for tight control. The triode gm varies with anode current, and this is more relevant for good performance. So I would say aim for a matched anode current - and you can adjust the anode voltage and/or the bias voltage to get there, so long as they don't end up being wildly different.
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#504 | |
diyAudio Member
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Quote:
The traders/vendors measuring "pairs" with tube testers. The average -good- tube tester measures tube at given operating point where gm near -or not so near- to datasheet's value in this point. I shows, that in this point (changing from one tester to another, no "fix" point of this tube) the tube condition near to required value or not ("good"-"not good", "%" etc.). It says nothing from distortions, which is a function of given curves (Ua-Ia at given -Vg) to relative datasheet curves. If you a lucky, and own a curve tracer (uTracer, eTracer etc.) which shows curves, you can compare curves from tube to tube, relative to datasheet's curves. It' a "real" pairing, but slow, labour-intensive, costly (do you have few bunch of hundred years old tubes?). Instead of this, the gm measuring at given operating point is accepted. After this you can do THD spectrum analysis, but I say in advance that you will not two "identical" spectra, even measuring tight "pairs". BTW some years ago I measured a few bunch of 801/801a tubes. The difference of THD spectra was larger (better-worse), than measuring -good, but not excellent- 801a/VT-62 tube versus 25/VT-25! |
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#505 |
diyAudio Member
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Thanks very much for this detailed response. Coming from Mosfets it shows how little I know about tubes. Will be fun!
nash
__________________
F5TV3, AVC, BA3b pre, Salas DCG3b, OBL-15, SonyVfet2pair monos, Millett Nutube Bal pre |
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