micksupratek@gmail.com
After 25+ years of using supra@supratek.com.au as my email address , it has become cluttered with spam , which my current email provider is useless at removing.
This has led to missed and confusing emails , and I apologise if anyone has been disadvantaged by this.
I'm now using micksupratek@gmail.com as my primary audio email address.
Please take note and advise anyone you know who is in the Supratek circle.
On other news , I'm slowly building a few preamps - just finished a Dharma , starting another and then will do a couple of Granges, and a pair of Malbec power amps. This will likely take a long time, so no promises on orders.
Dharma with NOS 6J5 tubes and Power supply with NOS 6F6G, 5V4G tubes.
All are using genuine early 20th century NOS tubes from my tube stash , apart from rectifier tubes , which are becoming harder and harder to find.
I've also treating myself to some new power amp builds . Designs which I've wanted to try for decades, but have been constrained by time and the demands of Supratek.
These are the power amps I'm using in my system at the moment.
I've had a half dozen CV1076 (DA41) for at least 30 years, but never had the opportunity to do something with them.
https://www.r-type.org/exhib/aaa1214.htm
They are somewhat between a 300B and a 211 triode . From the transmitting triode family with a thoriated tungsten filament , they have the same "lighthouse" radiation of the 211, 845 etc tubes. They get quite hot, but look very cool! They were designed as Class B tubes for high power PA audio , but we want to use them for Class A high fidelity!
The circuit was originally designed by a Japanese guy back in the fifties , and although not popular , quite a few Japanese and some Western tube diy guys have built modified versions of it.
It uses a small power tube like a 6V6, 6K6, 6F6 etc as a cathode follower, with the cathode tied directly to the grid of the CV1076.
This way it can provide a POSITIVE grid bias and deliver grid current to the CV1076 so that it can run in Class A2 . 6-8 watts .
It sounds glorious , makes 300B SE amps sound like mud. Has a real 3D soundstage, deep imaging and detailed but still very natural and organic sound.
One of the best SE amps I've built , very happy with it.
Of the "conventional" SE tubes, the 2A3 is probably the most renowned. As Joe Roberts said, back in the 90's - I've never met a 2A3 amp I didn't like" .
My Loftin White 2A3 SE amp is another beautiful sounding amp- just the bees knees for female vocals and music that is not too complex and dense.
I'll do a write up of that later, but here's a description of my 6B4G parallel push pull amp .
The 6B4G is the 6.3V version of the 2A3 - the 6B4G has 6.3V filament v 2.5V of the 2A3.
This amp has seen many variations as I've used it as a test mule for various driver stage designs.
This version I've made it as simple as possible - one driver tube , an interstage transformer and the output tubes and transformer.
The driver tube is the Russian 6E6P - a high conductance/high gain tube with low impedance that can swing a lot of drive volts . The Inter-stage transformer is 1:1+1 and provides more voltage than the 6B4G will ever require, and will easily drive the 6B4G stage into Class AB2.
The 2A3/6B4G tubes have very high transconductance and require separate biasing for both phases of the push pull output stage.
In addition, the output transformer is a toroid design , and they dont like any imbalance between the two primary windings . With the separate cathode bias configuration, balance between the windings can be adjusted as low as 1 mA .
This amp starts to clip at 20 watts , which is far more than my high efficiency speakers need, but it has good solid bass , and the typical 2A3/6B4G sweet sound.
Such a simple circuit gives an unencumbered , pure sound . The interstage does produce a bit of ringing , but the typical loading to reduce it does more harm than good , its minimal, and virtually inaudible.
I've had some rare and very expensive Tango X-10S output transformers for at least 40 years.
These were most often used for 211 tube SE amps and sometimes 845's, although the 10K impedance is a bit high for 845 tubes.
I made a number of 211 amps with them, and then they sat on the shelf for a good number of years.
I found some very ancient VT25 tubes in my stash, that were used in aircraft radios post WW1 .
I realised that the VT25 tubes and the Tango OPT's could be used to make a very nice Class A2 6 watt SE amp , with a 3A/167 (WE437) tube driving an interstage transformer.
This build is underway, but will be a long-term project - I'll do another write up when it's finished.
The Loftin White circuit goes back to the 1930's , nearly a hundred years old! It was pretty much forgotten till the Japanese DIY guys started using it again, around 1970.
It's an all direct coupled circuit, no capacitors between stages. It's tricky to set up , but sounds great.
It really works well with the 2A3 tube , highlighting the purity and beautiful tone the 2A3 is capable of.
It needs a single high gain driver tube, the 12AX7 is often used , but the octal 6SL7 is a true audio triode and sounds much better . The ancient 6SF5 is also a good choice.
Not much power, you're lucky to get 3 watts from it, but you'd be hard pressed to find a more intoxicating and beguiling sound.
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| Simple 6SL7/6B4G Loftin White SE amp . Power supply not shown. |
