In article , Keith G
wrote:
OK, I've never seen/heard a decent explanation of what goes on with a
valve amp with 4 and 8 ohm taps when it is hooked up to speakers of 6
ohm nominal impedance.

Instinctively I use the 8 ohm tap but, quite recently, one of the yoofs
in HFW wrote 'simply use which one sounds the loudest' or words to that
effect. I know it doesn't appear to make a lot of odds, but summat must
happen differently in the amp on each of these tappings. I've seen
references to the valves 'working harder' one way and the trannies 'not
liking it' another and so it goes on.

Anybody really know? - I would love to hear the correct explanation....

Two basic effects from one cause.

The cause is that the choice of transformer tap/ratio changes the load
'seen' by the plates of the o/p valves. This means that the amount of
current the valves are asked to produce for a given swing in plate voltage
will be changed.

The effects a

1) Using a tap that isn't optimum for your speaker may mean that either the
amp voltage limits, or current limits, at a lower power level at the
speaker. Thus the result is either quieter or more distorted.

2) Change in the level of distortion.

A third effect is possible, This is that an unwise tapping choice may lead
to instability or valve damage.

Can't be specific without knowing a fair bit about both the amp and the
speaker.

The problem for valve amps of 'traditional' designs is that speaker
impedance is very variable. So in general no tap is 'perfect', and you have
to choose one that is the 'best available'.

Slainte,

Jim

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