Phono 2 3 3S - ECC83 5751 - Cathode Bypass Caps - 3180uS 318uS mod

[Richard]

A popular valve swap in Phono is to use 5751 instead of ECC83 and this can be done without problem. They're two very similar valves and 5751 has a great reputation for good sound. Whilst they're similar, 5751 has lower gain and lower output resistance than ECC83 which lowers the gain of the amp and changes its RIAA corrections slightly when used in Phono. Whilst trying 5751 I looked into what changes the valves make and whether the already-good accuracy of the RIAA could be improved at all. Here are 2 ideas;

The first is to add cathode bypass capacitors to the first stage cathode resistors. This lowers the output resistance of the valve and improves the first, 3180uS, time constant (the second, third and forth are unchanged). It works very well with both ECC83 and 5751 resulting in more tuneful bass and better dynamics imho. Particularly good with 5751.

Gain with ECC83>standard amp. Gain with 5751=standard amp.



The second is to adjust the first and second time constants to more accurately follow the RIAA curve without using cathode bypass capacitors so there's no increase in gain. The exact component values are made up from readily available ones shown. This improves the accuracy of both the first 3180uS and second 318uS time constants whilst the third and forth are unchanged. It's aimed at ECC83 users who do not want more gain from their amp but would like to try improving fidelity, and also works well with 5751.

Gain with ECC83=standard amp. Gain with 5751<standard amp.