Tube Pre Amplifier for SEWA 7 Watt ClassA MOSFET

[MIKET]

Interesting

Doug L said "I was doing a paper design inspired by your requirements:
CSS loaded ECC99 with an EDCOR 10k:600 interstage transformer parafead between Cathode and Plate (Anode).
Current tentatively set to about 10 Ma"
I am interested in how this turns out.
I will pursue the 6C45Pi for now since I have a few in my tube stock.
Another possiblity might be a 12B4, it has a gain of about 7 but could improve with a transformer in front of it. I was thinking possibly something like this Jensen 1:1.4 transformer:
http://www.jensen-transformers.com/datashts/11p41.pdf
Its a bit pricey but very nice specs.

 

[Peter M.]

What about something like this?

http://home.pacifier.com/~gpimm/Active_loads_and_signal_current_control.html

http://home.pacifier.com/~gpimm/26_battery_biased_mosfet_linestage.gif

http://home.pacifier.com/~gpimm/

 

[Shoog]

Forget about the tube buffered LM3875 approach. Its not the buffer he uses in his commercial project, and its strickly limited in performance.
A CCS loaded 12B4 with a step up xformer infront sounds very promising.

Shoog

 

[MIKET]

Another option for a preamp

How about an Ultrapath 6C45?
http://www.geocities.com/dmitrynizh/Ultrapath.htm
This is a definate possiblility, ultra-simple which agrees with SEWA's design goals.

 

[Tyimo]

Hi!
Check this out!
This monster pream could drive everything......!😀 😀

Greets:

Tyimo

 

[MIKET]

Hi!
Check this out!
This monster pream could drive everything......!😀 😀

Greets:

Tyimo

It may drive everything but with a typical cd player it won't drive SEWA to 7 watts. The gain of this tube is about 2.4 per section, see data sheet http://www.jogis-roehrenbude.de/Rus...dedDatasheetMB.pdf#search="6c33c-b datasheet"

 

[MIKET]

If you want to go SRPP this would be a better choice

Look for "A SRPPed 6H30 Line Preamplifier" on this page http://digilander.libero.it/paeng/high_sigma_vacuum_tubes_part3.htm
This is a higher gain SRPP using a fairly low impeadence tube.
datasheet at: http://www.jacmusic.com/ehx/datasheets/6H30EH.jpg

 

[Tyimo]

The gain of this tube is about 2.4 per section

Thanks MIKET! It was only a nice dream.......

SRPPed 6H30 Line Preamplifier

It is nice!! I like it! Thanks! 🙂

Tyimo

 

[lineup]

RJM Parallel Triode Line Stage

😎
What you tube people and SEWA owners think of this RJM circuit?

Parallel Triode Line Stage.

Primitive ART and a nod of appreciation to Conrad Johnson.

If I needed a line stage, this is the circuitry I'd use.
The parallel tubes lower the noise and output impedance.
With four 6DJ8 triode sections per channel, the output impedance is 850 ohms.
Gain is about 26 dB.

The gain is too much for a line preamp,
but what about the idea, of parallelling fairly cheep triode tubes
to get better drive in the output?

Here is the circuit:

An externally hosted image should be here but it was not working when we last tested it.

And here is the link to RJM project page:
Parallel Triode Line Stage


lineup

 

[planet10]

Hi halo,

CJ does seen to get kudos for their paralleled line stage. Benefit -- a 1/4 the output impedance and 4 x the current drive.

The downside is that you get the average of the transfer curves of 4 tubes (just like paralleling any device) which tends to average out the lowest level of information.

So far i for single stage (what i'd like to see) are the totem pole designs or the idea of a 12b4... the last migt not have low enuff output impedance... this leads to thinking 2 stages, gain stage followed by a follower, but that doesn't have the elegance...

dave

 

[lineup]

Hi halo,
....
The downside is that you get the average of the transfer curves of 4 tubes (just like paralleling any device) which tends to average out the lowest level of information.

So far i for single stage (what i'd like to see) are the totem pole designs or the idea of a 12b4... the last migt not have low enuff output impedance... this leads to thinking 2 stages, gain stage followed by a follower, but that doesn't have the elegance...

dave

thanks planet10

And when we parallel 4 transistors, Output Stage MOSFETs for example,
many are happy about output feature / figures
and
they do not understand, that C-iss, input capacitance will be 4 times higher
and of course will require more of a previous stage transistor

For the price of 4 x 6DJ8 / E88CC / 6922
I will get 2 other more expensive high class Triodes
that may have higher current drive ( lower output impedance )
and also better audio quality.


thanks again

lineup 😎
i walk on - a little bit more wise, with tubes
and with a little bit more heavy weight of my knowledge
... this my burdon to I have to carry with me
----------
For in much wisdom is much grief;
and he who increases knowledge increases sorrow.
[ Ecclesiastes 1:18; Author unknown ~250 years before Jesus Christ ]
😎

 

[Ryssen]

Why not a Cathode follower?🙄

 

[planet10]

Cathode follower has no gain, so it would have to be used with a preceeding voltage gain stage. Ideally the cathode of the follower would be a CCS or a significant nagnitude B-.

You then have basically a "ForePlay" style preamp in front. Certainly an option.

dave

 

[lineup]

post #53
http://www.diyaudio.com/forums/showthread.php?postid=999317#post999317

Design Goals:
1. Gain 20-40, this will alow a cd player to drive the MOSFET to full output.
2. Low output impedance. Wide Band performance.
3. Low parts count. We are using this to drive SEWA, otherwise would be wrong from a purist standpoint.
4. Output Swing 20V p-p minimum to assure we can get all 7 watts out of the MOSFET.
5. SE design no NFB.............thats my goal, but don't think it's unreasonable.

I think even Mad_K, the master of puristic simplicity, could live with a 2 tubes pre amp.
This is his comment on a Kaneda legendary 5 transistor schematic. ( Kaneda Schematic here )

Looks nice,
but I think it has about 5 times too many transistors in the signal path 😉 😉

yes
looks as a good alternative, planet10
and, by the way, this is how I build my transistor pre amps
always a follower output buffer
to reduce load onto and make it easy for voltage amplifier to operate as good as possible.


I suggest we keep away all thoughts on global negative feedback.
Personally I wouldnt dream of using anything like this, in a tube amp.
If I wanted to reduce some harmonic distortion using a feedback loop,
I wouldnt go for Them Nice Tubes in the first place.
What I want by sometimes using tubes is a character of sound
that only tubes can fully give.

Those who wants the '100% distortion-free sound', whatever this is
have plenty of other good alternatives for THE SEWA.


lineup 🙂 leader of The Mad_K official fanclub

 

[johndiy]

linu up keep up the good work😉

john

 

[lineup]

linu up keep up the good work😉

john

Thanks john - I work hard to become a master - but long way to go, still .....
I like to read your posts, johndiy
they have more of a positive funny twist and spirit, and you show you have good audio understanding,
what we can not say about everything posted at this board.
I mean the positive and friendly approach in respect of others.
See my opinion here:
I managed to set a nice end commentary to discussion in
Class B - high feedback
http://www.diyaudio.com/forums/showthread.php?postid=1006773#post1006773

lineup - far away from you

 

[Ryssen]

Sitting here listening to my newly made Sewa amp,I am driving it with my Mu folower with 6C45 and 6888,I have in a box as a preamp.I think it´s working fine can´t see what the problem should be of driving the mosfet.If it shouldn´t be so fine should the frekvensy response be bad or what,maybee I could test it with RMAA and see.

 

[MIKET]

Care to share

Ryssen, care to share the schematic with us.
I think this should be a great choice, I like the 6C45 a lot, and have used a Solid State CCS, but a pentode on top is a very good choice indeed.

 

[Geek]

Hi guys,

Did some benchwork tonight on I design I've been meaning to get to.

So far, it seems to fit the bill, is simple, provides great performance, ONE TUBE per channel and can be built with commonly available parts. Schematic attached with the values I used in the test.

Transformer can be had from Radio Shack, their 12VCT, 2A seconday piece. 12V back-to-back. One ECC88 filament on one 6.3V leg, the other on the other 6.3V leg, CT grounded.

Since the SEWA has a Zin of 100K || 250p, I did the tests with a 2:1 margin, or 47K || 470p. Here's some pics of the results.


5KHz Squarewave

15KHz Squarewave closeup

Full tilt - that's 160V P-P at 1KHz

 

[lineup]

Since the SEWA has a Zin of 100K || 250p, I did the tests with a 2:1 margin, or 47K || 470p. Here's some pics of the results.

The input impedance of SEWA Rev_d ( see attached schematic )
which i think is latest published version,
is the sum of 3 impdances:
- R2 100 k , this is needed to bias SEWA to 1/2 supply voltage
- input impedance of MOSFET Q1, this is probably so high we can put almost infite value = not needed to count this
- R9 100 k, this is not really necessary, but is good for charging input cap
as well as terminate signal wire closer to input jack

100k in parallel to 100 k, 100k//100k = 50 kOhm
Rev D as seen in picture has got an input impedance a bit lessthan 50 k
say 47 kOhm


Now R9, if we do not want to exclude it ( I dont )
can be made having higher value,
say 470 kOhm .... ( or even maybe 1 MOhm )
will give
Zin is 100//470 ... <= 82.4 kOhm

and this Zin, input impedance, will better fit your circuit, Geek