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Old 3rd December 2009, 11:38 AM
Richard Richard is offline
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Join Date: Dec 2005
Location: Notts
Posts: 5,203
Default Re: Phono 3 with 5751 valves

The 6072 replacements have now arrived so I looked at trying them in Phono.

They're considerably lower gain and lower plate resistance than ECC83 or 5751. They'll work but throw the third corner point a mile out without adjustment; the first 50Hz corner is better at 48.9Hz without a cathode bypass cap (51.8Hz with), the 500Hz corner is the same as before, but the 2122Hz corner is now at 2680Hz. This will put a step in the frequency balance, all lifted from 2122Hz upwards.

So, it will probably be a quite dynamic sounding change; the bass may be better and the treble will get a lift but we cannot just drop valves in willy nilly if we're concerned about fidelity. It can be corrected by changing the 220pF to 277pF but this is out of adjustment range for 3S trimmer so will wait for another day.

All told, I won't do it for a while at least as gain will be too low without a cathode bypass cap and the bass is then off. Perhaps it would be good to try in future with a full reworking to correct response and using bypass caps on both first and second stages to get the gain back.
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