Version 1.32
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0.6R in series with the feed to V3/V4 (in series with X7, not X8, the side tied to 0V).
(2 x 1.2R metal film 2W in parallel). Depending on what combination of 6N1P (Ih =
0.55-0.6A) and E88CC/ECC88 (Ih = 0.3A) are fitted, this results in Vh of between
approximately 6.2 – 6.6V (6.4 with one of each type fitted)
0.39R in series with each of V1/V2 connected in series with the ac supply to pin 7 of
each; (0.39R wirewound 3W) results in a Vh of 6.34V
The modification is intended to reduce the heater voltages to their nominal 6.3V to
prolong the life of the valves, not for any acoustic impact; there was no discernible
difference.
Valve choices and bias.
The biggest impact on passive audible hum and ‘sound’ is the choice of input valve
V4.
I have tried the 6N1Ps that came with the amp (one of which was hummy if put in
either V4 position but seemed to have no impact if in a V3 slot - different cathode
voltages and currents?)
I have also tried HARMA 6N1P-EBs, Siemens ECC88s and Tesla E88CCs in the V4
slot, leaving the ‘best’ (least hummy in V4) pair 6N1Ps supplied with the kit in the
V3, driver position.
The best in terms of hum are the Siemens ECC88s, but they’re noisy (and sound
‘flat’); the 6N1Ps are low noise but depending on which valve is plugged in you get
changes in audible hum; all are in any event very low levels and totally ignoreable!
The best appear to be the Tesla E88CCs.
Whether this is due to the actual physical construction/characteristics of the valves or
whether it’s due to their different characteristics meaning they are not optimally
biased I don’t know!
Same applies to the output valves V1 and V2 EL34 vs 6CA7s. The 6CA7s seem to
have a more ‘boomy’ base response and a more defined top end than the EL34s but I
really can’t decide which I prefer! Again; would there be so much of a difference if
they were all set to pull the same quiescent bias current, or would it make the
differences more pronounced? Is the effect due to the changes in supply HT to
V3/V4? I don’t know! (See earlier comment about potential mod to allow me to set
them all the same).
I haven’t touched the bias of V3 or V4.. V3 is the output driver pair; and if I’m right,
V4B is the input pre-amplification and feedback/overall gain control, providing the
‘push’ via V3B and V1; V4A inverts the output of V4B with a gain of 1, providing
the ‘pull’ via V3A and V2.
V4B bias would seem to be the one to play with (R14 and R25), though shifting the
bias current in both V4A and V4B would seem to be a good idea – right now though,
I love the sound I get!!
