EAR509 100W monobloc amps, made 1979, re-engineered 2003.

This is the only very untidily drawn schematic I could find for
original EAR509 from 1979:-


There are many design problems with this amp.
I had two of these monoblocs brought to me by a customer who
probably paid little for them. One had nearly burned down a house,
and was badly damaged by smoke and flames under the chassis.
Someone else had tried to fix it, but amazingly they managed to lose
the burned out PT.
I wound a replacement with identical windings to those in the other
undamaged channel, and I used GOSS E&I, with same stack height,
and using a matching bell end cover. This ran much cooler than the
original other channel so EAR had not used GOSS in their PTs.

Notice the terrible bias pot arrangement which makes it very difficult
to set the bias properly. This is because the bias adjust network has
10k + 100k pot + 10k
to divide the -160V bias supply, so Vdc across
100k pot = 133Vdc. This means the range of Eg1 is far too big for
small rotation of pot, which will make many owners very angry while they
try in vain to set bias correctly, but find the tiniest pot rotation makes tubes
biased low or high, but rarely correct. This is compounded because for
PL509 with such high Ea and Eg2, the Eg1 grid bias then has to be
made very negative to keep Ia low and below say 30mAdc.  


This is my schematic with several changes to original.
But the tubes are the same, and my circuit works in the same
basic way as original.
The function is in fact very much like a McIntosh amp, with output
tubes working in what is wrongly called "unity gain" where OPT
primary cathode winding turns = anode winding turns, ie, there
is 50% CFB applied. Gain is in fact not 1.0, and is less than 2.0.
PSU is not shown but was same as original amp but with all electro
caps replaced.
An active protection circuit was added to counter the tendency of the
PL509 to overheat easily.
Bias condition was balanced and monitored with red-green LEDs
adjacent to each PL509, (not shown).
The PL509 has a 40Vac heater and noval base which is similar in size
to octal tubes like EL34, but with 9 slender pins, and not as good as octal.
More desirable and sensible tubes such as KT88, 6550, KT90, KT120
cannot be used without changing 2 noval tube sockets to octal, replacing PT,
OPT, and making huge change to PSU circuit, in other words, scrapping
all the dodgy efforts of EAR, and building something better.

The output transformer works similarly to the McIntosh amplifier.
The two equal turn primary cathode and anode each have CT, one at 0V,
one at +500Vdc.
These windings are not bifilar wound as in McIntosh, but don't need to be
because capacitors shunt ends of windings with same VAC, and this shunts
the leakage inductance between anode and cathode half primaries.

There are two windings with same turns as in anode and cathode and
with much lower thickness wire and each with CT, one at 0V and one at +260Vdc.
These are used for screen feeds biased at +260Vdc and for about 10dB of
balanced feedback to 6AQ8 driver stage which has bootstrapped anodes
to enable the production of about 170Vrms to each PL509 grid at clipping.
The OPT appeared to have ZR = 2k4 : 4r0 & 8r0.
I recall I measured max Po = 100Watts.
The PL509 work mainly in class B giving very little class A.
The BRLa each RL509 sees is only 600r, and peak Iac = 0.57A.

For the above Ea/Ia conditions, Pda can be calculated using formula,
based on Pda = P psu - Pout :-

To calculate AB1 Pda total, both tubes, for any value of PO,
Pda = Ea x [ ( 0.364 x Ia ) +  ( 1.8 x Va-a ) ( 0.364 x Ia x Ia )     ] - PO 
                                                    RLa-a           ( 2.83 x Va-a )  - Ia
Where Pda = Watts, total for both tubes,
0.364, 1.8, 2.83 are constants,
Ia = dc idle current for one tube,
PO = audio power out with THD < 2%,
RLa-a is anode to anode OPT primary load,
Va-a is signal Vrms across whole primary.

Applying this to EAR509,
Pda =
500 x [ ( 0.364 x 0.03 ) + ( 1.8 x 489 )  + ( 0.364 x 0.03 x 0.03 )   ] - 100.
                                              2,400             ( 2.83 x 489 )   - 0.03
= 189 - 100 = 89Watts.

NOTE. The old formula for a class B amp from the 1930s and from old books
may be written differently in terms of Va-a, Ea, RLa-a and without PO to give
Total Pda for all output tubes =
Va-a  x  ( [1.8 x Ea ] - Va-a )
Applying to nearly class B  EAR509,

Pda = 489 x ( [ 1.8 x 500 ] - 489 )  = 83Watts.

The class AB use with 30mA idle current gives slightly more Pda.

But both indicate Pda max for each PL509 = 44.5Watts, above the
allowable 35Watt rating.
You might say this is OK because music power does not equal sine
wave power, but if an idiot connects a 4 ohm load at output meant for 4r0,
EL509 could quickly overheat.
The PL509 are being worked too hard when 8r load is at Com-8r terminals,
or 4r load is at Com-4r terminals.
For home hi-fi, it is better to use 8r speakers at Com - 4r, and you get a better
maximum of 72Watts.
16r speakers may be connected at Com - 8r, for 72Watts, but its better to
still use the Com - 4r0 and get 42Watts.
4r speakers may be connected to 4r - 8r terminals. The ZR = 13k5 : 4r0,
and Po = 32Watts max.

The data sheets for PL509 show highest Eg2 at 190Vdc, and yet we see
them here with +260Vdc, and Ea at +520Vdc, both about 33% higher than
they should be for reliability.
FOUR PL509 tubes should have been used for 100W, NOT two!

With such high Eg2, the Eg1 bias must be a highly negative voltage and is
difficult to set correctly for PL509 and 2 x 6550 could make a nicer 75Watts
as done in McIntosh - which can be tested with a sine wave at 75W
without 6550 overheating. 4 x EL34 or KT66 or 6L6 would have been better.

I compared the sound of EAR509 to a normal UL 50W amp with 2 x KT88
and with fewer input tubes and only17dB GNFB. The UL amp sounded 
much better and with none of the hardness of EAR509.
The EAR509 amp is an example of how not to make a tube amp, IMHO.

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