~ LEAK Hi–Fi stuff ~

Picture: LEAK TL50 Valve Power Amplifier — H. J. LEAK & Co. ltd. — Brunel Road Westway Factory Estate London W3 —

LEAK ~ H. J. LEAK & Co. ltd. ~ Brunel Road Westway Factory Estate London W3 ~ Another name in British Vintage Hi–Fi History ~ Not as innovative as the Acoustic Manufacturing Co. in my opinion ~ In the late 1950s they had 3 basic valve monoblock power amplifiers and a low power stereo amp ~ All based on the same schematic

All the LEAK chassis have "acres" of free space so changing components like resistors and capacitors on the tag strips is very easy and encourages fitting oversized overpriced audiophoole components in an attempt to achieve some "Audio Nirvana" that is clearly not obtainable unless other circuit changes are made

Unfortunately most suitable "modern" replacement components for LEAK amps are smaller than the originals with short thin wire leads which look odd dangling between the tags ~ Fitting new 2W–3W metal film resistors and newer paper in oil capacitors like AmpOhm or NOS Russian K40Y–9 can look good and will work well

All LEAK power amplifiers are very sensitive ~ The matching LEAK pre–amps were very poor in design and performance ~ their signal to noise S/N was >20dB worse than the power amps and the output from a pair of EF86s for each channel was 125mV so the power amps needed to have enough gain to compensate

Like The QUAD 22 the LEAK pre–amplifiers were powered from the power amplifier via a removable umbilical cord with octal plugs and sockets ~ Unlike the QUAD which uses a series choke the LEAK HT supply from the power amp is via a high value resistor chain which also supplies HT to the driver stage valves

The voltage to the power amp first stage valve will drop greatly with a pre–amp connected and may be even lower than expected with aged capacitors in the pre–amp ~ When using mono–block power amps where only one supplies the pre–amp the two channels will be operating under different conditions

The LEAK PLUS range were smaller than their original designs and used an "Ultra Linear" output stage with grain orientated steel in the core of the transformers including the Mains Tr (unlike some modern designs) ~ The Stereo 20 and Stereo 60 were also Ultra Linear output with the better transformer core material

The table opposite shows the closed loop AV and open loop A0 gain and amount of feedback FB for the 1959 LEAK power amplifier line up calculated from the published specifications for sensitivity and FB for the 15–16Ω output tap ~ dBFB=20log(1+βA0)  β=FB ratio
Amplifier       AV            

TL 50 Plus    226x~47dB
TL 25 Plus    160x~44dB
TL 12 Plus    110x~41dB
Stereo 20      101x~40dB

FB                  A0

26dB~20x      4520x~73dB
26dB~20x      3200x~70dB
26dB~20x      2200x~67dB
24dB~15.8x   1600x~64dB

A "normal" power amp tends to have a sensitivity of about 0.5V to 1.5V for full output power so taking my preferred input of 1V this translates to a gain of about 23dB for 12W into 16Ω ~ Each doubling of power will require 3dB more gain for the same 1V input so my modified TL50 pictured above had a gain reduced to 30dB from 47dB

The "normal" valve power amp also tends to have a gain margin that is about 7-8dB and this is why I am working here in dBs ~ The manufacturer would like about 20dB or more overall feedback to lower the output impedance and keep the extremes of the frequency response "in line" with the response around 1kHz and of course lower the THD

The amount of feedback FB in dB is the reduction in gain when feedback is applied and the gain margin is the amount by which the open loop gain A0 or the feedback FB can be increased before the amplifier becomes unstable ~ The "normal" valve amplifier is class A or AB and A0 will be about 6dB higher without a load on the output transformer

The LEAK power amplifiers are about as normal as they come and we can assume that A0 and FB have been maximised for the valve line–up used and that increasing FB by 17dB as required for my 1V input TL50 is likely to cause instability ~ LEAK stated that they aimed for a 10dB ±3dB gain margin and 6dB of this is to allow for open circuit operation

The easiest way to lower the gain or sensitivity of any LEAK or similar power amp is to remove the cathode bypass capacitor of the first valve ~ Removing C2L and C2R will reduce the A0 of the Stereo 20 by about 10dB and removing C2 from the TL50 will reduce A0 by about 9dB but the closed loop gain will be reduced less than 1dB

Without the first valve cathode bypass capacitors you get an improvement in linearity (lower open loop distortion) and lower noise floor (due to lower gain of the first stage) ~ These changes alone may not make the amplifier sound any different but the damping factor will be worse and the reduced negative feedback (NFB) may change the sound

Having no bypass capacitor across the cathode bias resistor there is no question of capacitor quality affecting "the sound" at this point and this is my preferred option to reduce sensitivity even if the overall feedback still has to be increased to actually reduce the gain and to restore the low output impedance ~ Less really is More

The input valve in the LEAK configuration ~ whether a Triode or Pentode ~ will produce mainly 2nd harmonic distortion then a lower level of 3rd ~ With triodes the higher order harmonics tend to fall progressively but pentodes may produce higher odd harmonics greater than the adjacent even harmonics and detectable harmonics to higher orders

With an un–bypassed or partially bypassed cathode resistor the THD+N produced by a correctly biased single valve stage always appears to be lower than the fully bypassed or fixed bias option

Having applied the removal of the cathode resistor bypass capacitors to my own TL50s and several Stereo 20s and other valve amplifiers I was then introduced to "evidence" that an un–bypassed Rk may actually increase the 3rd harmonic and odd order distortion of a triode but I had never noticed this during the testing of many amps

The graph above is a reproduction of Fig. 4 from a 1950s? USA article 'Which Tube Shall I Use?' by George Fletcher Cooper and is used by him to demonstrate that varying the amount of un–bypassed cathode resistor can introduce 3rd and odd harmonic distortion and I think we all agree that odd harmonics are not what we want

The problem I had with George Fletcher Cooper's (GFC) comments were they jumped in and out of reality as he uses many glib (his own words) assumptions ~ The foreword to the article states "The author presents a lucid yet simplified description of the use of tube characteristic curves .  .  ." should it perhaps say a simplified report of a Lucid dream

To arrive at his Fig. 4 graph GFC calculates values for mutual conductance gm modified by Rk ~ which I will refer to as gmk ~ using the formula 1/gmk = 1/g+ Rk(µ+1)/µ where he assumes that "µ is always big enough for us to take (µ+1)/µ = 1" ~ This assumption is reasonable for an ECC81 and was used to produce the graph above

Another assumption GFC makes is that the curved  gm/Vg plot of a 12AT7 at Va=170 (or any valve) he later uses to graphically demonstrate distortion can be drawn as a straight line between gm=0 at the valve 'cut off' where V≈ –5V and gm=10mA/V at the point Vg=0V "Let us look at Fig. 4. To make the work easier I have drawn a straight line gm .  .  ."

Thomas Roddam (TR) in the UK published similar articles in the magazine Wireless World in June 1951 and October 1952 which I think explain better the use of mutual–conductance/grid–volts curves and the calculation of amplifier distortion due to the nature of these curves but he also makes some assumptions so read with care

Because the GFC article was the first I was aware of ~ and because it specifically referred to an ECC81 amplifier stage with un–bypassed cathode resistor ~ very similar to one I was working with at the time ~ it became the focus of an investigation into just how bad a problem using an un–bypassed or partially bypassed cathode resistor was

The curve manufacturers tend to publish is that of Ia/Vg which is known as the "mutual characteristic" ~ The differential of this curve is gm and when plotted on the same axis gives gm/Vg ~ If the Ia/Vg curve were originally a "perfect" straight line the gm/Vg curve would be a horizontal line indicating that the gain gm does not vary with the input Vg

Where the Ia/Vg curve is originally "curved" ~ as it always is to some extent ~ the gm/Vg curve may take on several forms depending on the degree and coefficients of the polynomial defining it

The graph above shows an actual curvature of gm/Vg at Rk=0Ω ~ The other curves Rk=100Ω Rk=200Ω and Rk=400Ω were calculated using 1/gmk = R+ 1/gm taking data from the Rk=0Ω curve rather than the GFC straight line ~ The coloured lines are trend lines but even without them it is clear that as Rk increases the curve straightens

I measured values of Ia/Vg at 0.2V steps using a good Mullard ECC81 (12AT7) and calculated values of gm=ΔIa/ΔVg at –0.2V steps for a fixed anode voltage of 170V which it would appear GFC also used for his Fig. 4. ~ Readings at other anode voltages also gave curved plots which become straighter at low Vg as Va increases

At low Vg and low Ia most triodes show a very curved Ia/Vg and while calculating the gm/Vg curve for the ECC81 at Va=100V I noticed that it was very similar to the Ia/Vg curve ~ The differential of the curve was the same as the curve which makes Rk=0Ω shown on the right an "exponential curve"

Deriving the distortion produced in un–bypassed cathode resistor amplifiers using GFC's gm/Vg "curves" does not correlate with real triode amplifier stages which tend to produce lower distortion than calculated his way

Looking at the GFC Fig. 4. graph above and comparing it with my ECC81 Va=170V graph I hope it is clear that the assumption to use a straight line gm/Vg is very misleading ~ The Rk=100Ω Rk=200Ω and Rk=400Ω curves derived from actual valve measurement (or even from published curves) become straightened by local feedback

It can be seen that the Rk=400Ω curve appears to be bending downward and that this should increase the 3rd harmonic ~ In practice an ECC81 amplifier operating at Va=170V and Va=100V always generated less 3rd than 2nd harmonics even when entering overload condition

In practice where a resistive anode load is used Va will vary as Ia changes ~ As I mention elsewhere it is this 3 dimensional surface defining the gain parameters that makes working with valves so interesting ~ If the change in gm with Va is taken into account we still see the 2nd harmonic much greater than the 3rd

With the above analysis applied to pentodes ~ as used for the first stage for the LEAK monoblock amplifiers ~ the curvature and hence 3rd harmonic distortion can be seen to be worse but again this is misleading because in practice a completely un–bypassed cathode resistor improves overall distortion and does not make the odd harmonics worse relative to the even

In summary the best way to reduce the gain of the LEAK series ~ and many other "normal" design valve power amplifiers using overall NFB is by removing any cathode bypass capacitor from the first stage and then increasing the amount of NFB ~ This way about 12dB (0.5V for full output) of stable gain reduction can be achieved or possibly more if you are prepared to adjust the pole capacitor and can test to confirm the stability

If you don't want to change too many things you can take the feedback from the 8Ω tap on the output transformer (OPT) where A0 is effectively 2.5dB less or from the 4Ω tap where A0 is 6dB less ~ This is similar to QUAD amplifiers where part of the OPT winding is outside the feedback loop which is more stable but not so low output impedance and with many speakers can give a relative increase in bass

The 17dB gain reduction required for my LEAK TL50s (c.1985) for a 1V sensitivity required a greater reduction of A0 than removing C2 alone would provide

I considered changing the EF86s for triodes or connecting them as triodes but I further reduced A0 by lowering the EF86 anode load resistors R5 to 100kΩ with an adjustment to R7 to lower the HT

Although the changes around the EF68 gave the required result I had also considered lowering the gain of the ECC83 cathode coupled phase splitter V2 by lowering the anode load resistors as I had done to my BBC LSM/10s ~ Due to the low value common cathode resistor the anode load resistors are different values to obtain a balanced push–pull output

I didn't make changes to the LEAK phase splitter but referring to similar schematics for the BBC LSM/10 and BBC LSM/8 highlighted a mistake on the LSM/8 schematic which was supplied to the BBC by LEAK because the amplifier was a TL12 point one ~ The problem is also on the LEAK TL12 schematic where R13 is marked as 57KΩ and on the TL10

Now that I have made an online calculator for the common cathode phase splitter it is easy to see that R13 in the LEAK TL12 should be 51kΩ ~ Using the calculator entering values µ=19 and ra=13kΩ for a 6SN7 with Va≈100V and the resistor values from the TL12 schematic you should see a better balance with R1=51kΩ ~ The number 1 can often look like a 7

There are some LEAK mods which "potential divide" the anode load of the first stage valve but this method is not suitable as it does not improve the S/N or reduce the distortion ~ In fact the distortion can be worse due to overdriving the grid ~ With an un-bypassed cathode the local feedback means the cathode follows the grid input allowing large voltage swings at the input without producing distortion due to grid overdrive

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