Oct 5, 2016

E140 SE A2 amplifier Part 3

Tubes selection made, now it's time to mix up all these good things.


Before publishing amplifier schematic let's have a look to the driver limitations.

High frequency: 12B4A & Miller's effect

The input capacitance of the tube C, in conjunction with the source impedance Rg of the stage, forms a simple low pass RC filter with an upper -3dB cutoff frequency equal to:

           f = 1/(2π x Rg x C)     where     C = Cgk + Cga x (Gain12B4A +1)

The -3dB point with a 220K Rg resistor will be: f = 1/(2π x 220K x 39pF) = 18.8kHz
As I can neither reduce the load (90K _ See part 2) of the previous stage without lowering the E80CC gain nor increase Rg, some local NFB will help to keep a good overall bandwidth.
A 4 to 6 dB loop fine tuned with an FFT analyzer will give the desired bandwidth and distortion figure, definitely set by listening test.

Low frequency: inverted interstage transformer

Tamura IT is used with inverted primary / secondary coils.
In that way E140 grid current in the secondary cancels a portion of 12B4A current flowing in primary coil.
Unlike all Shishido's amplifiers working A2 all the time, this one can swing the E140 A1 where no grid current appears. This is why I have carefully chosen a 12B4A operating point where anode current do not exceed 10mA.
With such a primary current the transformer low frequency response is very good.
High frequencies are also very good without roll off with this Tamura transformer. I believe it is the way the transformer is wound that prevents attenuation when inverted connected.

Amplifier schematic



Power supplies

This amplifier requires both high and low voltage.
Low voltage is required to bias E140 grid through interstage transformer. Needs to be very steady and adjustable, a LM317 is the simplest way to achieve good regulation and very little ripple. It requires just a few parts and can be easily implemented using PCB's.



High voltage supply is a bit more complicated than usually mainly because the power stage high voltage (240V) is lower than the ones needed for previous stages (270/260V). It imposes to split the supply in two just behind the first choke keeping in mind the necessity of well calculated cells to insure good transient response. In that way I had to consider both ripple attenuation and time constant factor.

I choose choke input filter for low, almost perfect sine wave ripple and good regulation. Moreover it is less stressing for the power transformer than capacitor input filter.
Unlike capacitor input filter the ripple is not a function of current. It just depends upon voltage, choke and capacitor.
It can be calculated from formula:

           Vripple = η VHV    where    η = 1,2/LC   (L in Henries and C in µF).

In that case it will be 0,00048 x 260 = 0,125 Vcc or 0,044 Vrms and represent a 60 dB attenuation of the ripple behind rectifier (44 Vrms). Very efficient!



Amplifier is now complete. Sold to a German amateur.

15 comments:

  1. Thank you so much and i`m indeed very interested :-)
    Roland

    ReplyDelete
  2. i'm very interested too.. it's a month that i'm waiting for this 3rd part.
    that's not so much out there talking about A2 class.
    i think there is a big potential in signal grid flowing current based power amp.

    coming back to your post, interstage is - i suppose - the simpliest way affording A2 coupling.
    I'm very interested into a deep treating into the cathode follower source, where the driver tube use the g1-k impedance like a catode resistor and the power tube use the driver like a low impedance source.

    thanks in advance

    Edoardo

    ReplyDelete
    Replies
    1. Thanks for your support
      I know that few people trade with A2 and It stays quite a mysterious and seldom explained way of driving power tubes despite the very good final results in audio.
      I try my best to bring some level of clarity without bothering the reader with maths and before publishing a schematic I take a very long time making tests and measurements but the echo is very weak.
      Tossie's Valves' World (http://tomo.fine.to/) brought some interesting schematics you could use as basis of future amplifiers.
      Music friendly

      Delete
  3. I had years ago a Wavac 4304 Amp (Shishido Design) for some days and it was magic. They used a special Tango Interstage which is no longer being manufactured. The Interstage is the problem here (hf response with inverted secondary). Lets see which Interstage is being used here. So please go on with this blog :-) And thank you so much.

    ReplyDelete
  4. I like your blog , especially the A2 part !
    A 100TH se amp will be my next project .
    I have two Tungsram 10/400T tubes new in org. box like the E140 .
    regards
    Martin.

    ReplyDelete
    Replies
    1. Hi Martin, thanks for your comment.
      Eimac 100TH is a very good sounding tube when modulated full A2, mandatory with tubes having a µ of 35 and more.
      I made some tests in the past ( I have a few of these beauties ) and the best driver was a 6C19Pi cathode follower choke loaded with a Hashimoto custom made one (50H/1.5K)to get the proper bias @ 20/25mA.
      Lucky you are to have the Tungsram equivalent to the E140, very well made tubes.
      Best regards

      Delete
  5. May I ask how these tube sound in comparison to other DHT ? I think it could be as well a nice driver with a mu of 25 and an rp of 10k, no ?

    ReplyDelete
    Replies
    1. Sound quality of tubes is very subjective. Anyway this peculiar triode reminds me the overall qualities shared by the 10 and the RS242. Neutral and accurate. IMHO this tube can really make a good driver as its µ and p are very close to one of the best tube ever made for that purpose, the 6J5 and its relative the 6SN7.

      Delete
    2. And I will probably build another amp with this triode because I liked a lot what I heard. I sold it to someone that really love it. Probably change the transformer driver ( almost impossible to source nowaday ) for the more common cathode follower one. Stay tuned.

      Delete
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      Delete