GU50 Lockin Push Pull Power Amplifier.
Twenty months ago through this forum I introduced you to My Kevin inspired Lockin GU50 Integrated amplifier. Faced with a second covid lock down and A Reference amplifier which had been well usurped (sound quality wise) I set about using all that lovely Lundahl amorphous core iron to construct a clone of GU50 Lockin Integrated. I thought I would share my experiences with you during the build and the finished sound quality of going Amorphous Core again.
GU50 Push Pull Power Amplifier consists of Lundahl LL1620AM PP Triode wired GU50 400v@95ma LL1671AM PP Triode wired 6E6P DRU 200v@22ma LL1690 Phase splitter input transformer. As my previous and Kevin's (Input stage for a 2 stage push pull amplifier) During lockdown the Van de Veen Auto Bias board seemed to be unavailable so I purchased a Tes ABS-Q instead. https://www.audioamp.eu/ This looked to all intent and purpose the same as the Van de Veen unit and certainly appears to perform with the same accuracy for 60% of the price. This did the fixed biasing duties very ably as before. The power supply was as previous with a custom wound mains TX + DSE12-12A bridge 6cj3 Damper diodes to the Van de Veen Echokes. Additional smoothing by some 390uf + 20u motor run caps. The 1671 HT being fed by a 6EM7 one tube series pass regulator +5uf motor run caps. This amp has no feedback and my usual common mode interference filtering of the AC filament/heater lines. K&K Audio cascade CCS on the cathodes of the 6E6Ps.
The Casework & fittings was sourced from Aliexpress engraving & milling in house.
I started listening with a 100R resister as the cathode sink for the 6E6P tubes, this however was changed to a Kevin sourced Cascode CCS with a -30v sink, as I knew that this could be well improved upon.. I had also been thinking about the DC balance aspects of this circuit with the one CCS under the cathodes as before. I was always disappointed when I re tested the current flowing through the 6E6P differential pairs. Even matched pairs kept on changing their DC current measurement from day to day by many ma. There always seemed to be a dominant tube. I had considered buying a second Tes fixed bias unit to replace the CCS biasing arrangement. Decided that a replacement input transformer which would tolerate dc on its windings and still have good phase splitting capabilities were few and far between. Maybe a LL1660S!!! Not only that the Tes bias control unit only went down to 2v and the bias point for the 6E6P is around 3.5v so rather too close for comfort. So I decided on a scheme where each cathode had its own Cascode CCS & 1R test resister to ground negative supply and the cathodes were linked for AC balance by parallel Polypropylene 10uf/100nf capacitors. Of which I prefer the sound of compared to the single CCS. It was worth the effort but has translated to a small improvement in perceived depth of image and overall clarity & tonal colour. At least the DC current balance in the interstage transformer now stays spot on each Valve/tube and satisfies my increasingly O.C.D. tendencies. This is also how I reviewed the above amp.
So how does it sound compared to its standard Lundahl iron forbearer.
The reference gear was my trusty Tannoy Kensington SE & Oppo BDP105.
Balanced output.
Please refer to my previous comments re this amplifier.
I had forgotten that the LL1690s were new and needed some time to run in also. The other iron was well run in but after 50 hours the 1690 are still opening up and making quite a difference. Indecently the 6E6P driver tubes really seem to come alive sonically after you light them up with 20ma +.I am currently running them at 22ma which is about 45% of what they're capable of. I would turn up the current even more if my regulators would allow.
Considering that the Power Amp uses the same basic circuit & tubes some interesting sonic differences appeared. Generally the sound appears even more focused but a little bit warmer & with more bass extension & detail still. Guess this is the famous amorphous core sound. This amplifier is now even more modern sounding and if you like your amps warm and romantic sounding this won't be for you. Personally I adore there ultra-accurate rendition of a live event they produce & there transient punch.
I am still amazed at the quality that Push Pull Triode GU50 produce. For about $12 at the moment you can buy a Valve/Tube & an original Russian socket. I even like their rugged military looks. (Note do not use the cheap ceramic sockets you see, they will trash your tubes) They are a true pentode which can turn out very respectable triode curves, some equate them to a bigger EL-34. Just choose a PP 300b schematic with ac heaters they will be very quiet also. It makes me smile after I sold my used quad of 300b xls.
Comparing the prices of any KT/88,120,150,170, 300B etc they have to be the bargain of the century. Especially now the very worrying scenario re the supply of Russian tubes will now be in doubt due to the recently introduced embargo. The 6E6P tubes are also still out there for a few Pounds/Dollars but I notice they are starting to dry up & go up. Aren't we audio amateurs so lucky we can use these high quality Cold War tubes and we are not limited to current production. Buy them whilst you can.
As I write I have been asked to build a Pal a version of this amp. Probably a PP GU50/6E6P Kevin inspired capacitor coupled version. I look forward to completing this and listening to the results. I will of course report back on this forum.
Stay well,
Keep sharing,
Enjoy Your Music.
Tubelcain.
