Sander deWaal wrote:
Patrick Turner said:
and I use the 4 ohms tap and connect 16 ohm speakers?
This way distortion will be 3 times less, but there is *no* risk
of damaging the tubes or the OPT.
There will be less maximum power.
With triodes the distortion would be less, with pentodes it would be
more.
--
Sander deWaal
Vacuum Audio Consultancy
The amp in question probaly has a lot of nfb applied, arranged so there
is
say 20 dB when 4 ohms is connected to the 4 ohm tap.
If the amp works in pentode mode, the Dn will be probably 0.4% at 1 dB
below clipping
with the above 4ohms to 4 ohm tap.
When 16 ohms is connected, the gain of the pentode output stage rises
by around 3 times, or 10 dB, so the applied nfb leaps to 30 dB, and any
increase in pentode distortion
is mainly prevented, depending on the amount of class A used.
With no load at all, there is a danger of excessive anode swings, and
pumping
a large input signal into the output grids will excite electro magnetic
phenomena resulting
in perhaps 2,000 vrms deveoped across the OPT primary.
This can cause an arc between the primary and anything earthy nearby.
But a 16 ohm load connected to the 4 ohm tap is an ample sized load to
absorb such
excessive voltages caused by back emfs in the OPT.
In practice, speakers have a very varied impedance, and its not uncommon
for]
a speaker to have an impedance peak of say 40 ohms at say 50 Hz.
Thus when 50 ohms is connected to the 4 ohms tap, the load seen by the
tubes
is way higher than designed for, and there isn't quite enough R to
prevent the
spurious over voltages caused by back emfs.
It can still happen if the 16 ohm tap is used with a speaker a peak
impedance of 50 ohms.
To prevent such phenomena, and the resulting strain on OPT insulation,
diodes should be connected from the anodes of each output tube to ground
to prevent the anodes from going negative,
or to a potential below 0V.
If one end of a an OPT primary goes so negative that current flows from
0V into the winding via the
very low on resistance of the diodes, then the other end which goes +ve
cannot rise
beyond the same volatge value in the side being shunted.
See the schematic with such diodes at
http://www.turneraudio.com.au/htmlwe...00ulabmono.htm
There are 3 x IN4007 in series, each with 1M resistors strapped across
them,
to form a clamping network which prevents the peak -ve voltage swing at
each primary
to never exceed the value of the B+ voltage.
Normally, the diodes sit there doing absolutely SFA, and have an utterly
negligible effect on the Dn.
But if some dude turns the volume way up but with no speaker connected,
the amp clips without straining the hell out of the insulation
between P&S windings.
Most tube amps don't have such clamping diodes, and so you get the
occasional OPT failure because of excessive voltages.
Its true that when triode operation is used, 16 ohms connected to the 4
ohm tap
would result in definately less Dn.
But the increase in gain in the trioded output stage is not great, and
there is no huge increase
in the effectively applied nfb when the RL the tubes see is a high ohmic
value,
so the amp tends to be more stable, but only if the quality of the OPT
permits this.
There are plenty of triode amps with FB that oscillate at HF just as
easily as a pentode amp
especially if the load is capacitive.
Before fiddling around with the mode of operation, one must be sure
about stability issues.
Most ppl prefer the sound of triodes.
The proviso is that there must still be an adequate ceiling for power.
Say you have an EL34 pentode AB1 output stage which makes 35 watts
with 4 ohms load connected to the 4 ohm outlet. The anode to anode load
might be 4k.
Suppose one only requires 1 average watt per channel.
Then one doesn't need 35 watts max power ceiling, and one could settle
for 15 watts which is all you'll get using AB1 triode connection with 2
x EL34.
If 16 ohms load is connected to the 4 ohm outlet, then the load value
seen by the
output tube leaps from 4k to 16k, and the power becomes
totally class A power, but you then will only get about 10 watts of
power.
But its a very nice 10 watts, and its probably enough if 1 watt average
is all that's used.
To find out if this is the case, a DVM with a peak and hold function
should be used
whilst playing the loudest music one wishes to ever hear with the chosen
speakers.
From this we can calculate the maximum instananeous power level being
used,
so if we read 12v peak on our meter, this is equal to 8.48 vrms, and if
the load
was 16 ohms, then the instaneous maximum power is 8.48 x 8.48 / 16 =
4.49 watts,
and even if there was a dip in the 16 ohm speaker impedance down
to 8 ohms, the power out max would be 8.98 watts, so triode operation
would be permissable.
With the OPT configured for 4k to 16 ohms, the voltage ratio is 15.8:1,
and if the B+ max swing was 400v peak at each end of the OPT pri, then
you have 565 vrms across the OPT pri, so the max output voltage
would be 565 / 15.8 = 35.7vrms = 50v peak.
With an impedance peak of 50 ohms being common in 16 ohm
speakers, one could expect to see
perhaps 45 peak volts of swing at the output of a triode amp.
All this may seem confusing, tedious, and boring for those without a
clue
or experience, but if one assumed that the load was indeed 16 ohms only,
and that triode power into 16 ohms was a max of 10 watts,
then the output voltage is 12.6 vrms, then the max peak
voltage permissable should be 17.7 peak volts.
So if you have a 16 ohm nomimal Z speaker, and you measure no more than
12 peak v at the output, triode operation should be fine, and give
pretty decent sound,
and possibly better sound than pentode operation.
I say possibly, because ppl have been arguing this one way or other for
at least as long
as the beginning of the use of multi grig power tubes in the early
1930s,
and the use of NFB before that.
I find most well designed ultralinear amps to be better than plain
pentode amps
or triode amps.
I also find the use of well implemented cathode feedback windings
to give about 13% of NFB to the output tube cathodes
is superior to any other configuration of multigrid output tubes.
The feedback from my customers tells me all this.
Patrick Turner.