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A Lockin GU50 Push Pull Amplifier.

86.104.12.192

Posted on August 11, 2020 at 06:11:04
Tubelcain
Audiophile

Posts: 32
Location: Guernsey
Joined: January 6, 2008





This project started with A mate listened to my Kevin inspired Lundahl 300B XLS push pull amp a few years ago now. The usual "Wow get me some of that" how much, really! OK can you build me a cheaper version then. So I started with some used Standard Lundahl transformers and chassis and some GU50s and 6E6P-E of which I had acquired a lifetime supply of thinking that they would be a good investment as well as some great sounding valves/tubes for a future project.
As time passed my mate lost interest, my wife won't let me etc etc. You know how it is.
Years passed and I thought this was an interesting opportunity to experiment with a few ideas namely Menno Van der Veen's Tube Auto Bias Module & High Current Electronic Choke as the budget had now increased.

Lost interest myself and side-lined the project until the Covid19 lockin came along.

This gave me time and renewed interest in this project. I am so glad I did.

This amplifier is again Kevin inspired from his "Input Stage For A Two Stage PP Amplifier" Circa 2012. A Push Pull LL1682PP GU50 Triode @ 400v/95ma LL1671PP 6E6P-E @ 200v/22ma LL1540 input/phase splitter. Power is A Custom Toroidal, DSE12-12A bridge, with a 6CM3 soft start damper diode, Van der Veen EChokes & Motor Run Caps. The 6E6P Driver/voltage gain stage Kevin's Cascade Constant Current Sources & 13GF7 compactron series-pass one tube regulator on each channel. The GU50s have fixed bias supplied by the dedicated Van der Veen bias module. AC heaters with common mode filtration.

OK so my hearing is getting a bit old now and so are the 300BXLS. I built this and my reference amp so I am a bit biased I guess, but I will try to avoid "Proud Papa syndrome".

So with 30 hours on the above amp it to my ears anyway, bests my reference amp by a large margin, which I thought was already pretty good. This is a Push Pull LL1620PPAM 300BXLS @ 500v/65ma LL1671PPAM 6EM7 Drivers LL1637 Choke loaded 6SN7 LL1690 phase splitter. Separate power supply with Hybrid bridge and LL1638 chokes in common mode. As of Kevin's inspiration circa 2010 + CCS on the cathodes of the differential pairs & 6EM7 series pass regulators on Driver & Voltage stages.
As you can see this amp has some designer components to its construction of which I featured on this forum a few years ago now. Along with my thoughts on its sound quality.

My test Speakers are Tannoy Kensington's and an Oppo BDP-105 source.

Both amps have roughly 30 watts output but the GU50 amp sounds more powerful and with a much more detailed low frequency range. This manages to sound taut & tuneful but still natural & musical. The imaging is amazing and previously un-heard details just keep jumping out of familiar recordings. Recordings that used to sound confused are now more focused & beliveable. The background appears blacker with the ambience disappearing into a deeper sound stage. Instruments stand out with more tonal colour and are easier to define within the image, with more space around and better Micro & Macro dynamics. The high frequencies and particularly the soprano voice sound very smooth and detailed which can often offend my ears. Solo Piano is beautifully re-created you are there, with the right recordings of course.

I also auditioned this GU50 PP amp against my well-regarded PSE 300b Monobloc amplifiers which sounded quite amorphous in comparison but typical of most single ended amplifiers, nice in the broard midrange but soft elsewhere. Guess which will be staying in my system.

For a relatively budget design I am so impressed. In my opinion you will never know how good the standard PP Lundahl's sound until you have tried them with the ultra-accurate & quiet balancing of the push pull output stage by the Van der Veen Bias Module. And the shorter signal path 6E6P/6j49p/6j11p/D3a etc Drivers. With also the ultra-quiet Electronic Chokes doing great things with low ripple and rejection of intermodulation products in the time domain contributing to the inky black low noise background. I M H O If you want to take your Push Pull amps to the next level the above are the way to go rather than that expensive quad of output tubes.

I have read that output valves/tubes make least difference to the sound of an amplifier on a cost basis that is. I am sure however that some will disagree. For $8.00/pair Triode conected GU50s sound amazing and are well worth experimenting with in this "Roll your own" Valves/Tubes push pull circuit, (thanks again to Kevin for shareing it with us all) but equally other power Valve/Tubes will benefit similarly.

I guess that I will now have to build the 300BXLS with the Lundahl AM iron to see if this sound can be improved on further. It occurs to me also that a driver stage would be very interesting with the 6e6p and another fixed bias module, (as has done wonders for the output stage) would need an input/phasespliter transformer that could tolerate DC however, now there's a thought.

Such is the nature of our hobby!!

Enjoy The Music And Please Keep Shareing

Stay Well
Tubelcain.

 

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RE: A Lockin GU50 Push Pull Amplifier., posted on August 13, 2020 at 06:15:11
Francois Robichaud
Audiophile

Posts: 26
Location: Quebec, Canada
Joined: December 28, 2005
Tubelcain, thanks for sharing your project and bringing some insights on the GU-50. I have always shied away from fixed bias, but when I finally tried the same modules that you've implemented, my experience mirrored yours. Incidentally, your amp is more good looking than lockin, congrats on the very nice metal work.

 

RE: A Lockin GU50 Push Pull Amplifier., posted on August 31, 2020 at 08:55:01
nerdorama
Audiophile

Posts: 439
Location: Eastern WA state
Joined: February 2, 2002
That is a beautiful looking amp. I've been sitting on my stash of GU50's for quite a while. Thanks for the inspiration. Any chance you would share some pictures of the insides? It is always fun to glean a few ideas from the projects of others.
John
De gustibus non est disputandum

 

auto bias details, posted on September 1, 2020 at 08:03:29
nerdorama
Audiophile

Posts: 439
Location: Eastern WA state
Joined: February 2, 2002
I'd like to understand more about using the auto bias module, specifically with an interstage transformer. I assume you fed each half of the IT secondary from the bias outputs of the module. Did you connect the bias output directly to the IT or did you use some additional series resistance? The user info for the module doesn't address the use with IT's.
Thanks,
John
De gustibus non est disputandum

 

RE: auto bias details, posted on September 2, 2020 at 11:18:37
Francois Robichaud
Audiophile

Posts: 26
Location: Quebec, Canada
Joined: December 28, 2005
You are right. In a push pull design, you just take your secondary ground connection from the transformer, and hook it instead to the bias output from the module. In a SE design, I guess that the same should work, with the additional option of hooking the bias output through the grid resistor or choke.

 

RE: auto bias details, posted on September 2, 2020 at 15:15:04
nerdorama
Audiophile

Posts: 439
Location: Eastern WA state
Joined: February 2, 2002
For the auto bias controller to work with a PP transformer coupled stage it seems you need separate connections for each output tube, meaning an IT with separated commons on the secondary for the bias connections. Otherwise you wouldn't have independent bias control fro each tube. I was just asking in order to confirm my notion.
Thanks for responding,
John
De gustibus non est disputandum

 

RE: auto bias details, posted on September 3, 2020 at 06:45:57
Francois Robichaud
Audiophile

Posts: 26
Location: Quebec, Canada
Joined: December 28, 2005
Yes, correct. You need separate connections for each output tubes.

 

RE: A Lockin GU50 Push Pull Amplifier., posted on September 3, 2020 at 09:59:55
Tubelcain
Audiophile

Posts: 32
Location: Guernsey
Joined: January 6, 2008



Hi Nerdorama
Thank you for your interest the chassis came from aliexpress & GD Parts, milling & engraving in house.

Yes the planning and implementation of the amplifier is often the most interesting part of the build unfortunately new ideas come along and by the time I have built it, I want to start again.

My point to point wiring is not very neat but prefer direct runs and lower capacitance that way.

I like clean & dirty compartments, where mains and noisy components can live without contaminating the sensitive ones. I also like a spaced sub chassis so I can run heater wires etc there only for them to emerge as close to the tube/valve socket as possible. Another bonus is the chimney effect where the hot air rises from the tubes and sucks warm air out from the chassis below. I can then also hide the tube/valve sockets and there screws from view as I become OCD in my old age. Couldn't resist using the front panel as a thermal sink for the CCS.

No it doesn't give details of connection to an IT, I just replaced the standard fixed bias components on the IT secondary from the pad labelled bias.It is quite simple follow the detailed instructions and checks and worked first time. No I did not add any more series resistance as it implies you can. I presumed that the DC resistance of the IT coil would be sufficient, with the 1k grid stopper hard on the GU50 sockets.

As I mentioned before in my opinion auto bias will improve any amp regardless of the output tubes selected. (Assuming your amp has fixed bias and not cathode bias of course) It always amazed me that a simple half wave rectified/small smoothing capacitor and variable resister was deemed suitable to feed directly into the grids of the output tubes. The noise injected etc into the signal path must be much reduced, as well as all the other advantages stated.

GU50 Tube/Valve has proven to be an excellent choice as an output tube. It will stand a constant 40 watts in class A1 on the Plates/Anodes. A worthy more powerful substitute for any KT88/KT120/EL509 amp in triode mode but with 12.6volt heaters and currently available for a fraction of the price. (See attached link) I would however suggest you avoid the cheap ceramic sockets currently available and use the original Russian ones as I had two which trashed several tubes before I realised what the issue was, even when they are inserted correctly. Removing the top cap is easy and makes it look a little prettier IMHO.

I hope this gives you some background.
If you do decide to add a bias module and Echoke please let us know what you think.

Enjoy The Music.
Keep Sharing
Stay Well

Tubelcain

 

RE: A Lockin GU50 Push Pull Amplifier., posted on September 3, 2020 at 10:23:14
Tubelcain
Audiophile

Posts: 32
Location: Guernsey
Joined: January 6, 2008



Dear Francois.
Thank you for sharing your experience with The Van der Veen modules.

I wonder if you would be so kind as to share with us what tubes and transformers etc you used in your build. Or have you modified an ready-made amplifier?

Enjoy The Music.
Keep Sharing
Stay Well
Tubelcain

 

Schematic: A Lockin GU50 Push Pull Amplifier., posted on October 4, 2020 at 18:34:14
Spantou
Audiophile

Posts: 115
Location: Central Illinois
Joined: October 5, 2016
Looks very good and apparently sounds excellent. I am interested in building this design, if the schematics and operating conditions can be made available.

 

RE: Schematic: A Lockin GU50 Push Pull Amplifier., posted on October 4, 2020 at 18:49:32
Spantou
Audiophile

Posts: 115
Location: Central Illinois
Joined: October 5, 2016
I realize that this amp is assembled using several modules that are proprietary circuits, but I shall appreciate as much building information as could be made available.

 

RE: Schematic: A Lockin GU50 Push Pull Amplifier., posted on October 6, 2020 at 02:29:49
Tubelcain
Audiophile

Posts: 32
Location: Guernsey
Joined: January 6, 2008



Hi Spantou,

Thank you for your compliments. I am indeed delighted with the sound of this amplifier.
As amplifier design gets better I guess then they all begin to sound more alike. This design manages to have a more low noise, bass control of solid state the imagining & depth of single Ended & the frequency response extension of push pull. So what's not to like then. Some listeners prefer the older more euphonic rounder & warmer sound of traditional Tube amplifiers. This is definitely in the more high resolution precision, accurate camp and not warm & cuddly.
There was never a set schematic just some ideas gleamed from other amplifier designs. I am sure that you will also make an amplifier your own by the build choices you take just as I did and the parts you already have in your personal parts bin. I could hand draw my final schematic for you if you wish but I think it more useful to show you what influenced me in my current design. I hope this helps.


This is my original inspiration schematic I am sure that you can see how I added K & K Audio front end to the IT and adapted this for use with the 6E6P driver stage at 200v/22ma. I used Kevin's Cascode CCS with a well-regulated -25volts underneath. Be careful with 6E6p/6E5p/6J11p/DA3 they will oscillate given a chance, grid & screen stoppers essential.

To be honest if I was starting again with a blank canvass for a new amplifier I would very strongly consider using Kevin's 6J49p push pull boards plus power supply, with a Lundahl LL1692a IT. (See Kevin's blog page and KT150 PP amp circuit for inspiration) This combination has the balanced driver stage topology I like, then add the output valves of your choice.(Gu50 inexpensive and good choice) It all depends on you construction choices, budget & if you want all PCB or you like point to point wiring. Then add the Van de Veen bias board and E Chokes.

I am sure you have already researched these modules and know what they do & there advantages, they are indeed a very potent combination. You could also do a D H T version but I guess you will struggle with the noise element.

The voltage regulator on the IT could be solid state or tube, there are many 6EM7/6GF7 based one tube regulators on the net or others tubes. Or perhaps one of Kevin's Shunt Regulators, Morgan Jones Statistical regulator looks very interesting also. However solid state regulators will add more solid state sound characteristics!

So many choices I guess that's what makes our hobby so interesting. Good Luck.

Stay well
Keep Sharing.
Tubelcain.

 

RE: Schematic: A Lockin GU50 Push Pull Amplifier., posted on October 9, 2020 at 20:44:25
Spantou
Audiophile

Posts: 115
Location: Central Illinois
Joined: October 5, 2016
Tubelcain,

Thanks very much! This is just what I had hoped for.

Happy listening,

 

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