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Old 11th November 2009, 10:48 PM
Richard Richard is offline
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Join Date: Dec 2005
Location: Notts
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Default Re: Phono 3 with 5751 valves

Hi John,

Yep, that looks right. Checked carefully twice now with 2 amps and 4 sets of valves, comparisons in the same amp in the same sessions, today with GerryB. I used the 600ohm input and -40dB output of the Hagerman iriaa rather than include any step-up transformers and reffed at 2v p-p 1KHz on the scope at 0.5v/div showing a span of 20 graduations.

The drop in gain from ECC83 to 5751 is 14/20 on the scope = -3dB. That relates well with the difference in mu of the 2 valve types.

After a straight swap from ECC83 to 5751 there is a slight treble lift. After re-setting the trimmer caps there is a just-perceptible lift (still a net roll-off) which results in a -6dB point at 65KHz instead of 50KHz. I believe the slightly lower plate resistance of 5751 would account for this but we'd have to try a few more different makes to be more sure.

Usage-wise, we are back to that intangible quality - the sound - and to my first post where I did say these may not be better than anything else but are viable alternatives . I've taken so long to try them as I knew they were different electrically and wanted to know what effect they may have rather than just plonk them in.

I recd some 6072s today which would have been interesting to try, if the gain is enough, but they're not what they should have been so have gone back...

Rich

Last edited by Richard; 16th March 2010 at 08:19 AM. Reason: I believe the...
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