In some of my previous postings I’ve mentioned that my Leak Troughline Stereo tuner displayed an unacceptable bass-heavy tonal balance. This was described in my post:
http://theartofsound.net/forum/showthread.php?t=3007.
To put this in perspective, I found I could equalise the tonal balance by using a –2/+2dB ‘tilt’ plus a bass roll off –3dB at 150Hz. With this setting the tonal balance was very similar to that of the Quad FM3 I use. Otherwise voices were unnaturally chesty and deep.
Following my initial post Neil, suggested that I might try changing the output cathode follower; erroneously stated to be an ECC88. I did so; in fact I did some tube ‘rolling’ here, all of course to no avail as I was changing the wrong valve! Having now found out that the cathode follower is in fact an ECF80, I have replaced the original Mullard valve with an NOS Mullard ECF80.
Mirabile dictu – it’s worked; not only is the tonal balance much better, the output level has increased to 63mV for full deviation. Before, the output was only ~22mV compared to 100mV for the Quad.
I had thought that my mistake over the output valve might have been due to the fact that my tuner is designated a Leak “Trough Line Stereo”, whereas Neil’s tuner is designated a Leak “Trough Line III”. To further confuse the situation, the rear of my tuner is actually marked “Leak Trough Line 3”. In order to try and resolve matters I did some research. To be precise, the Trough Line 3, has the following valve line up:
V1, cascode RF triode ECC84; V2, triode mixer ECC85; V3, triode oscillator and pentode IF ECF80; V4, pentode limiter EF80 and V5, 2nd pentode limiter and cathode follower ECF80.
The ‘Trough Line Stereo’ incorporated minor circuit revisions, as well as incorporating a simple stereo decoder. The circuit changes are as follows:
ECC88 replacing the ECC84;
C6 1000pF
R5 33Ω
C29 200pF
C40 50pF
C41, 42, 43, 44 150pF
C47 0.47uF
C53 1uF
and
R38 47kΩ.
There is also a bypass capacitor of 5000pF (C58) fitted between one of the anodes of the EZ80 rectifier (V7). Curiously the HT rail is now marked as 148V.
http://44bx.com/leak/Leak/Circuits/TroughLine2.gif
and
http://www.44bx.com/leak/Troughline3Stereo.html.
Encouraged by the improvement in tonal balance (the difference between the Leak and the Quad FM4, I now use, is less than 1dB, though there is still a bass ‘hump’ around 150 –200Hz which now gives an acceptable 'warmth'), I intend to replace all the valves in the signal path by NOS Mullards. I wont replace the EM84 tuning indicator, nor the EZ80 rectifier, since these are not part of signal path (though again some might challenge the latter). So far I have obtained NOS Mullard samples of all but the ECC85, but the search continues. When I have finished ‘fettling’ the tuner, I will do a full comparison with the Quad FM3, FM4 and, if possible, with a Quad FM2 (valve) tuner.
So thanks Neil for the advice, despite sending me off on a fool’s errand regarding the ECC88!
As before – to be continued
Regards