In recent posts on 300B PP power,
I mentioned the formula to find the load
for maximum class A1 PO for a SE triode,
For SE, RL = ( Ea / Ia ) - ( 2 x Ra ).
For PP, RL = ( 2 x Ea / Ia ) - ( 4 x Ra ),
Where Ra is anode resistance in ohms of ONE triode, and Ia is idle dc in
ONE triode.
Now for PO in class A1, Ia increase approx = Ia decrease in any tube in
an SE amp or PP amp.
Note I have not included the derivation of these formulas, a long story,
including some geometry and trigonometry, which will be obvious to
those smart enough to understand a load line drawing on a set of Ea/Ia
curves for any
negatively biased triode.
PO max for given Ia in SE = RL x Iadc squared / 2.
Example, SE amp with RL 2.8k, and 60ma, PO max = 2,800 x 0.06 x 0.06 / 2
= 5.04W.
PO max A1 for given Ia in PP = RL a-a x Ia dc squared / 2.
Example, PP amp with RLa-a 5.6ka-a and 60mA PER EACH TUBE,
PO max A1 = 5,600 x 0.06 x 0.06 / 2 = 10.08W.
The first thing to know when setting up any outpus tubes is the anode
voltage
which the tubes will be happy with.
So for 300B, from 350V to 420V is OK, and Pda should not exceed
32W, with 30W being OK for class A1 SE or PP.
About the same can be said for trioded KT88 and 6550,
although 33W for KT90 would be fine.
SO, we can avoid ALL the load line analysis if we are good at math.
Load line design by graphic means should be used to verify
the math and the test results of the amp when we have built it.
Let us say we have a pair of 300B set up for class AB.
Let us suppose we have Ea = 400V, and Ia in each tube of 60mA.
Each tube then had Pda at idle = 24 watts, and the tube life will be
long.
For maximum class A1, the RL for PP = ( 2 x 400 / 0.06 ) - ( 4 x 800 )
= 10,133 ohms.
PP Class A1 max = 10,133 x 0.06 x 0.06 / 2 = 18.24 W.
We know that with a lower value RLa=a we would get more PO,
and that it would become class AB1 power.
Expect that the maximum AB1 PO with triodes could be a lot more than
the pure class A1 if the Pda is low to begin with.
However, where Pda is 2/3 maximum Pda rating of the tube or slightly
more,
we can expect perhaps up to 50% more AB1 PO.
There is a way to calculate this because we know the slope of the Ra
line at Eg = 0.0Vdc bias.
So we can calculate the Vswing with any load class B load value.
But for hi-fi, it isn't necessary because what we are interested in is
the first 10 watts.
Suppose RL = 6k a-a for the two 300B, and we have the same 60mA dc in
each tube at idle and Ea = 400V.
PP Class A1 PO = 6,000 x 0.06 x 0.06 / 2 = 10.8W.
If the load was a mismatch at 3k a-a ,
then you can see max PP class A1 = 5.4W, because
A1 PO is pro****tional to RLa-a.
a dismally loaded amp struggling with 1k5 a-a produces only
2.7W of class A1.
Notice that as the idle current is increased, the amount of class A PO
increases pro****tionately to Ia squared,
so that if one did have a 6k a-a load, and Ia was raised to 75mA from
60mA,
then PP A1 PO = 10.8W x ( 75/60 squared ) = 13.5W.
Some ppl do load their amps dismally.
A good load for 300B in PP is 10k a-a.
Great fidelity, good natural DF and low THD/IMD without any GNFB.
But suppose the amp is an ancient one, and has an old fa****oned
oulet labelled 16 ohms.
Assuming the anode load is 10k when 16 ohm speaker loading is used,
then where some idiot connects 4 ohms, the anode load drops to only
2.5k,
and THD/IMD rise by 4 times watt for watt or more compared to when 16
ohms is used,
and DF is poor, and class A power ceiling is low, and no amount of GNF
will make it sound or behave
very well.
In modern amps, the load for AB1 is usually too low.
The loads commonly chosen for AB1 amps today was usual for only PA amps
where a large amount of not very clean class AB was prefered.
High maximum power output watts are good for sales, not fidelity.
So wherever you see the 4 and 8 ohm labelled outputs on any modern amp,
always assume the makers probably have a load on the tubes that is too
low,
ie, with more than 75% of total PO = AB1, and less than 25% A1.
So only try the 4 ohm outlet for any speakers rated above 4 ohms.
Output PO max will be lower than the maker boasts that it can be.
So what, hi-fi usage will NEVER EVER want to run any power amp
right up to clipping, and if a PP amp is rated for 60W max,
a hi-fi user may only ever take it up to 40W even on peaks,
so connecting an 8 ohm speaker to an outlet marked 4 ohms
will be fine. The THD/IMD will be lower, DF doubled,
and if there is any GNB, it is made more effective
because tube gain rises with load value.
There are limits though.
I know a guy who tried to use a Leak 2020 to drive Duntech Sovereign
speakers
to levels he was used to with 100W amp.
The 2 x EL84s per channel fried after awhile.
Patrick Turner.
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