100 WATT UL AB1 MONO BLOC  AMPLIFIERS
Content of this page.......
Picture of  two 100W UL-AB1 mono amps, general description,
Schematics for :- 1 power amp schematic, 2 power supply schematic,
3 bias and protection circuits.
graphs for :- 4 power vs load graphs, 5 THD graphs.
Full explanations of all included.



I sold these two amplifiers in early 2004 to a delighted customer living near
Cairns with a tropical climate.
He wished to use them with Quad ESL63 speakers used mainly during the
dry season, because high humidity can be a problem with ESL.

These amps gave rock solid bass, clear and natural mids, and well detailed
treble with an overall ease and dynamic precision from a high proportion
of class A power within the total AB power.

They have aluminium chassis, 470 mm long x 260 mm wide x 220 high,
and have fairly open protective steel grilles over the tubes.
Weight is approx 22 Kg, ( 49 Lb ), for each mono amp.
The largest transformer is the output transformer with a 75 mm stack of
44 mm tongue E&I silicon steel lams, seen just behind the output tubes.
The power transformer is 100 mm stack of 38 mm iron, seen furthest
from view.
Silicon rectifiers are used, with C-L-C filtering using a choke of 2H and
large value electrolytics, to maintain an impeccably low noise level.
Output tubes are 6 x EL34 or 6CA7.
Input triode is 1 x paralleled 12AU7 or 12AT7.
Driver triodes are 2 x 6SN7.
The amp was sold in 2004 with matched 6CA7EH which I feel are more
rugged than EL34, and excellent sounding.

 
SCHEMATICS AND TECHNICAL NOTES :-
Sheet 1, 100w UL AB1 power amp schematic.
Sheet 2, 100w UL AB1 power supply schematic.
Sheet 3, 100w UL AB1 bias and protection circuits.
Sheet 4, 100w UL AB1  power vs load graphs.
Sheet 5, 100w UL AB1 THD graphs.
These schematics are of the 100w mono amps as supplied to my
customer in 2004. 
 
Sheet1 100w UL AB1 power amp.

 
Fixed bias is used, and there are six adjust pots and volt meter test points
accessible through 6 access holes in the side of the chassis, so bias can be
adjusted easily with a cheap dc voltmeter and screw driver without moving
the amps to a work bench or removing any covers.

Driver tubes are 2 x NOS 6SN7 with both triodes in each paralleled.
This pair of tubes are set up in a differential amp aka long tail pair driver/phase
inverter with their commoned cathodes connected to a "long tail" using one
MJE340, which has such a high collector dynamic impedance in the circuit
that it can be regarded as a constant current sink.
This makes the two output opposite phased outputs from the 6SN7 tubes
exactly equal in amplitude which depends solely on the equality of their
resistor loads.

The MJE340, has no sonic signature, and does no active amplification and
acts merely as a high ac impedance source of cathode current and it replaces
what would be pentode tube which would work no better.

Input tube is 12AU7 with both triodes paralleled.
The output of the input triode feeds one of the grids of the LTP via a LF gain
shelving network to tailor the open loop gain and phase response to ensure
unconditional stablity when global NFB is applied.
The 47nF + 1M in series with the LTP grid cause an open loop loss of gain
below 20 Hz, but extend the final pole of LF determined by the 0.47 uF and
1M + 220k grid bias resistor.
The effect of this measure is superb LF stability.

Series R + C networks are also used across each half primary on the OPT,
1.5k + 0.0022uF, and at the secondary of the OPT, 0.22uF + 15 ohms to
ensure the amp is loaded by a resistive load at HF when typically inductive
speaker loads are used.
The traditional global negative voltage FB resistor of 2 x 2.7k ( 1.35k ) is
bypassed with 1,000pF to advance the phase of the global NFB voltage
signal applied from the active speaker terminal to the V1 cathode circuit.
The 1,000pF compensates for the phase lag in the open loop phase response.
There is also some small amount of global negative current feedback applied
from the negative speaker terminal and taken to the V1 cathode circuit point
above the 2uH and 0.1 ohm LR network.
Above 20 kHz, some negative current FB applies itself to prevent ringing
in square wave responses with capacitor loads.

These measures ensure complete freedom from any oscillation with any
pure capacitance load or with no load at all. The seriesed silicon diodes
from the output anode to 0V are to limit the peak output voltage swing to the
430 volt supply voltage, since these diodes conduct when either side of the
primary tries to swing to a negative value, caused by back emf phenomena
in the OPT when no load is present, and a high output voltage is used.
This measure safeguards stressing the OPT insulation by excessive voltages.
Power output can be seen plotted in the the sheet 4, for selectable triode or
UL connected amps.
Bandwidth at 72 watts, 8 ohms, is 10 Hz to 65 kHz.
Output impedance < 0.5 ohms,
Distortion < 0.3% at 100 watts, 4 ohms, and less than 0.05% at 3 watts.
Noise is very low.
The amp is fitted with the usual active protection circuits, shown on sheet 3.
An led on the front of the chassis indicates clipping, or any fault condition.
 
Sheet 2, 100W UL AB1 amp power supply


The power supply has all solid state rectifiers. A CLC filter ensures low B+ rail noise
and a 50 ohm resistor limits peak charge currents into the two 220uF series input
caps to the CLC for B+.
The 50 ohms also keeps the B+ at about +410V to enable a high idle bias current
thus ensuring a large amount of class A power.
DC is applied to V1 heaters and fixed grid bias and B+ supplied to V1 is shunt
regulated. There is a relay in the power transformer HT secondary circuit which
is operated by the protection circuit
shown in sheet 3.
 
Sheet 3,  100W UL AB1 bias and protection schematic.


The fixed bias adjustment pots are arranged as shown to give each tube a
range of applied grid bias between -33V and -48V.
Each pot for each output tube grid bias is adjusted so 0.5Vdc appears across
the same tube's 10 ohm cathode resistor.
This indicates 50mA of idle current.
The process of setting the bias for the amp is repeated 3 times after the amp
has warmed up because bias adjustments are interactive.
The circuit is actively protected against bias failure in one or more tubes,
or against excessive and continued use with loads which are too low in value.
Each cathode of each output tube is grounded through 10 ohms and points
K1 to K6 are all fed to a common cathode monitoring signal path which has
its voltage reduced by the R divider of 4k7 and 2k7 with a 220uF cap to
remove unwanted ac signals during operation.
Should one of more cathode dc currents rise to dangerous levels for more than
a couple of seconds, the SCR will trip and  the 16V relay supply will be pulled
to 0V since the 50 ohm 10W resistor is grounded at the relay end.
If the SCR is tripped the fault LED will turn on. The same LED will flash
if the amp clips since the error signal from the output of V1 is fed through
a high resistance path to a darlington pair of bjt driving a second gain bjt to
turn on an LED.
All the protection parts will fit on a small board under the chassis and all
can be easily sourced at any electronic parts shops.
There is a 6 second delay for turn on of the B+.
 
Sheet 4, power vs load graph.


The graphs show maximum power levels at thd <2% just before clipping
with a range of load values.
Curves A and B are for Ultralinear with either 2k : 6 ohm load match
or 4.5k : 6 ohms.
Curves C and D are for Triode with either 2K : 6 ohms or 4k5 : 6 ohms.
Any value of load along the RL bottom line can be chosen and the
output power max can be seen on the curves.

Using the higher Z ratio on the OPT results in less maximum  output
power for most common loads between 4 and 8 ohms but the class A
portion of total power increases and although the total AB power reduces
the THD and output impedance are both much lower.

For example, if the OPT is set for a load match for 4k5 : 6 and an 8 ohm
speaker is used, then the maximum UL power output only 52 watts,
but it is virtually all class A and the THD will be about 1/2 that of 6 ohms.

Load matching is all very confusing to most people. A common
misunderstanding is that the more ohms a speaker has, the more difficult
it is to drive. Common sense tells us carrying 10 bricks is more difficult
than carrying 1 brick.

But more ohms means a less difficult load. But like so many things in
electronics basic commonsense seems reversed to a novice. If anyone
cannot understand what ohms are, they need to study Ohm's Law,
which might lead to more questions about many other basics of tube
operation and load matching effects and distortions. Books such as the
1955 4th Ed of Radiotron Designer's Handbook is an excellent read,
or they might read my other pages on tube basics.
EDUCATIONAL AND DIY.

Sheet 5, 100W UL AB1 THD figures for two amps.

 
The graphs of  total harmonic distortion, thd, are for one pair of
100 Watt tube amps with identical schematics. There are two curves for
amps A and B and there is up to 7dB difference between THD at below
1Watt.
The graphs were made during completion work on the two amps.
The voltage scale is linear and the THD scale is *logarithmic* to display
small quantities of THD more easily.
The amp A had the highest THD which is listed below with power levels
and what would be SPLs using  average modern speakers of 6 ohms and
90dB/W/M :-

1 watt , 2.45vrms, 0.02%, 90dB SPL,
2 watts, 3.46Vrms, 0.03%, 93dB SPL,
4 watts, 4.89Vrms, 0.047%, 96dB SPL,
8 watts, 6.93Vrms, 0.09%, 99dB SPL,
16 watts, 9.8Vrms, 0.15%, 102dB SPL,
32 watts, 13.8Vrms, 0.20%, 105dB SPL,
64 watts, 19.6Vrms, 0.37%, 108dB SPL,
88 watts, 23.0Vrms, 1.0% 109dB SPL, and this is
so loud, it wakes the dead.

Like nearly all amps the fidelity increases with a higher load value but with
a reduction of maximum output power.
Most people will find that with two amplifiers average levels of 1/2 a Watt
from each will produce SPLs of 88 dB approximately although peaks in
drumbeats and transients will go a lot higher but these will be very easily
dealt with by these amps. The average level for most loud music in a
domestic situation is 88db for men, and 84dB for women and that includes
both channels, so indeed these amps have quite enough power even for
teenagers who like bass boosting if possible in the preamp.
( OK, you have a killer teenager?... I don't want to know..)

Much is said about tube amp distortions spoiling performances but let's
get this into perspective. If the speaker voltage is 5Vrms at 4 Watts of level,
then the THD = 0.05%.
Therefore the actual distortion voltage within the signal is 0.0025Vrms,
and would be very difficult to hear from across a room with speakers rated
for 87dB/W/M, because the distortion voltage alone gives 1 micro Watt
of power, which gives an SPL that is 60dB below the 4 watt level of 93dB,
so the distortion produces an SPL at 33 dB which would be below the
sound level of heartbeats, breathing, and natural background sound levels.

The THD spectral voltages in tube amps such as these at low levels up to
4 Watts is usually a mixture of predominantly 2H and 3H with other
4H, 5H, 6H, 7H, 8H, 9H etc all at least 12dB below the combined levels
of 2H and 3H indicated on the above graphs. Either 2H or 3H harmonic
may be greater at low levels because the 2H at low levels varies because of
unavoidable slight unbalance in the 2H currents in each half of the PP output
circuit because the 2H currents in each 1/2 of the PP circuit are slightly
different in level, thus never completely cancelling as a result of push pull action.
There are slight differences in the "matched" output tubes used in the output
stage and driver stages. Plus the input stage produces some 2H since it is a
single ended triode stage. The 2H content is the main reason for the
differences between the two amps' THD.
But at high output levels the THD of the pair of amps becomes nearly
equal and mainly 3H. The 2H at low levels can be minimized by placing
tubes in either side of the PP circuit so that ac signal currents measured
across the 10 ohm cathode resistors for each side have equal totals,
or as close as one can get, and this is a very tedious thing to do and to
monitor with a distortion meter.
Unfortunately, this needs some technical expertise to achieve, and
usually does not lead to any betterment of the music.

Triode operation was tested but very little differences with THD were
recorded at the same low levels. Changing from UL to triode operation
means the total circuit gain is reduced about 4 db. Therefore the amount
of applied global NFB is also reduced by the same amount. With less
global FB one would expect output resistance and distortion to rise
but the triode connection itself gives a compensating reduction in THD
and output resistance so the change from UL to triode does not require
any change to the global NFB network resistances and the output
impedance and THD will remain very similar in either UL or triode.

These amps were prepared for someone with QUAD ESL63 and the
amount of NFB is not high and could have been increased to levels
used more commonly by other makers who might use say 20dB
which would reduce all the above THD figures by about 9dB, or to
1/3 of the figures mentioned. I felt there was no need since the
output impedance and THD was low enough.
 
There is nobody I know who can tell the difference between triode
and UL connected output stages if the power levels are well away
from clipping and if the general total levels of NFB are similar as
I suggested above.

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