"Arfa Daily"
"Phil Allison"
Anyone got any opinions about just strapping the screens to the anodes on
each pair via a switch, to implement a triode mode ?

** It better be a damn good switch, insulation and voltage rating wise -
most makers who have such triode/pentode switching use a very hefty slide
switch.

Yes indeed, my thoughts as well. Anode supply is quoted at 330v nominal,
so a switch rated to 250v ac, as most are, should be good for that.


** Nope.

Switches ( and relays) rated for AC operation are not suitable for DC
voltages above about 24 to 30 volts at the same current. DC arcs are tough
SOBs to break.



At the end of the day, many tube amps use the same switch for the HT
standby switching,

** All *sensible* designs switch the AC secondary of the transformer.
Those that switch the DC rail directly often arc and burn - even though
the current is generally only 100mA or so.

Fender ( for example ) did this in many older models, but chose a particular
switch that had a very large opening clearance to survive the inevitable
arcing - ESPECIALLY if the standby switch was ever operated under full
power.

Some Boogie models made the massive blunder of having the standby switch
parallel two electros ( one charged, one not) when it was closed. Splat.


Also, the power output will of course drop in triode mode and each EL84
will dissipate more heat under overdrive. Combined with their use of a
"concertina" phase splitter, I suspect that overdrive clipping will not
be symmetrical.


I hadn't considered that there might be any changes to the phase splitter
output as a result of such a change.

** Please try reading what is there.

The PS dos not itself change ( ??) but the job it has to do does - cos
the required grid drive to the EL84s is much higher in triode mode.
Concertina ( single triode ) PS circuits have less output and clip
asymmetrically compared to twin triode PS circuits.

Not recommended.

Well, not something that I would normally recommend either, and the owner
is happy to live without the mod. He just thought that he would like to
give it a try if it was practical, whilst I was in there doing the other
work. Apparently, he would like to use it in 'triode' mode when he is at
home practicing.

** Wot a ****wit.

It only drops the power by half or 3 dB.


..... Phil