Audio Quad 6SL7gt

After successfully building a couple of DIY tube preamps, I was inspired by the simple way the Stereo 20 was built!

How brilliantly thought up by the people at Harold J. Leak with a pertinax plate, solder taps and hard wiring...simple and durable! 

At the same time, I was captivated by Peter Walker's ideas... the genius who created the Quad. 

How he managed to calculate the circuit for a complete amplifier with only 2 EF86, 2 KT66, 13 resistors and 6 capacitors in 1953 is still a mystery to me? 

The great enthusiasm in 2004 made me give my DIY amplifier the name  Audio Quad 6SL7gt 

I later changed the name on my amplifier, but the Letra transfer gold letters are still there! 

A jpg of the power-bank and the DC heaters for input and drivertubes

One wonders if this DIY amp will last the next 50 years? 

After buying the Leak Stereo 20 at a flea market in Frederiksberg in 1992, my perception of what good hi-fi sound was changed forever. 

Until this point in 1992, the doctrine of the bipolar transistor, and its superiority in terms of the number of amps it could deliver, had been paramount. This even though the power fets ​​from Hitachi 2SJ50/2SK135 with their only 7 Amps and their tube-like sound reproduction had made their way into the hi-fi world. 

A transistor amplifier should ideally be added to Edgar Villchur's AR3a that could deliver 30 Amps....lesser couldn't do it!!

 But yes.....lesser could do it.... 

It took some time before I realized that this old 1959 construction from H.J. Leak still delivered hi-fi in a sound quality, that only very few transistor amplifiers in the past could or can match in our present. 

If you think about it, there aren't many electrical appliances that still meet today's standards after more than 60 years! 

As mentioned, the mounting style inside the Stereo 20 was the way forward after what I had been thinking . I began designing a suitable fiberglass plate with individual copper tracks, which would form partial electrical connections to the shown points on the pertinax strips with solder pins. This was one of my first attempts to imitate the Stereo 20’s mounting style. 

The result was mechanically 100% stable while the cosmetic design with the pertinax strips probably doesn’t take first place. 

All copper lines, wires  and components were layout made in the CadInt program 

When you, as a DIY electronics guy, start devising circuits and designs for an amplifier, you're bound to make mistakes, and I also managed to finish the heat sinks for the EL84 cathode resistors, even though upon later inspection I could see they were never going to be used. 

Back to the design. My wish was :  “The same sound as the Stereo 20 and double the power”....not because I needed the higher power, but because I wanted to “see” if 4 EL84s in double push-pull could sound as excellent as a single set of EL84s in the Leak Stereo 20? 

Before we get to the circuit that I wanted, there was just the obstacle of some good output transformers. 

Fortunately, 20 years ago Denmark had a company near Roskilde called "VRT Transformer". 

I knew the owner of the company because I had delivered glasses with correction to him. The owner of VRT Transformer Mr. Larsen has now been retired for almost 20 years . But he knew everything about winding technology. I asked him back then if he could wind an OP-transformer based on the recipe which can be found at the link below. 

I had found this address about how to wind a Leak output transformer and Mr. Larsen knew exactly how the windings should be in the coil to achieve a result with a –3dB point above 56kHz 

All about Leak:

www.44bx.com/leak/8615tx.html 

Luckily I got Mr. Larsen to wind a couple of 4K OP-transformers for my project with 4 x EL84, but at the same time also wind a set of 8615's for the later DIY project with Leak Stereo 50/60. 

Measured on Raa: the 4K transformers went from under 18Hz to > 56kHz –3dB..... I was sure of the quality from VRT Transformer when the first 1992 project with the Conrad Johnson MV40 copy went beyond expectations....no photos of the Conrad Johnson copy from that time 

It was the OP-transformers now for the diagram that experiments and measurements came up with 

First of all I had some brand new American 6SL7gt tubes lying around. They had an octal socket and they looked nice....6SL7gt was chosen as the input tube. 

I had some Philips ECC83 NOS which were chosen as driver tubes (initially). 

Then 8 x EL84 (4 per side) Sovtek 

(they had not been chosen today after 2022, but 20 years ago they were ok). 

I wanted a super stable adjustable negative bias for the eight EL84. 

And at last 2 pieces of Philips 5V4G rectifier tubes. 

Learned from the experience of the CJ MV40, the "rubber diodes" ..i.e. rectifier tubes sound and play very well, but suffers from the fact that they cannot deliver enough current in a sudden attack in the music. 

A solution to this problem we solved by limiting the first decoupling electrolytic to 47uF/450V followed by a resistor of 10 Ohm ..found by experiment. After the 10 Ohm resistor a battery of 6 x 150uF/450V = 900uF which gives the output tubes the opportunity to draw the current they need without "running out" of energy. The ROE 150uF/450V capacitors fits in here!

I took my amplifier to the professor from the Technical University of Denmark, and he suggested that I tried E80CC instead of ECC83 to get a stronger drive for the output tubes, and with very few adjustments the tube was changed to E80CC special and it sounds great! 

By turning the phase on the input tube with 470p / 2K2 in parallel over R5 / 150K (lead-lack) we ensured that the amplifier does not come close to self-oscillation. 

This 6SL7gt amplifier is completely stable, even though it goes up high in frequency! 

My final conclusion is that I have put together a really good EL84 amplifier that reproduces in high fidelity sound quality, and that the eight EL84's drives most speakers, even if the sensitivity is below 90dB/1W, which is the case for my Acoustic Research AR11. 

Edit:

The Leak 8615 Output transfomer was used in all Stereo 60 power amplifiers, and in late production TL/25 Plus power amplifiers. The 8615 was a revision of the earlier type 3925 transformers used in the Stereo 50 and early production TL/25 Plus power amplifiers.

In December 2001, Ray Whitcombe (who was employed as a Design Engineer at Leak in the 1960's) provided the following details on the winding arrangements inside the 8615 transformer. This data has been obtained by dismantling an 8615 transformer, and would enable any proficient audio transformer company to re-wind a this transformer from new.   

Alt om Leak: 

www.44bx.com/leak/