Tube Pre Amplifier for SEWA 7 Watt ClassA MOSFET

[lineup]

Hello people.

I hardly ever find a chance to start a new topic in Tubes.
Tubes are nice and interesting!

This Project is about build a Pre Amplifier
to match the input of SEWA 7 Watt Class A MOSFET Power Amp.

If you post here, please RESPECT OUR TOPIC.
And find out some basic knowledge about SEWA MOSFET amp
before you make your posts and suggestions.
The SEWA thread is GIGANTIC, so try the last pages
for uptodate information about latest version of The Circuit.

Thank you.

/lineup


Reference link:
Pass Labs >SEWA - Seven Watt Amplifier
SEWA - Seven Watt Amplifier

[Geek]

Great idea to start a topic here

I have a design in mind, but without an actual SEWA present, I can't say if it will work to perfection.

If I am correct, we need 25V P-P driving some fairly hefty Cin on the MOSFET. Driving the capacitive load isn't an issue for what I have in mind, but keeping it stable with no feedback loop.

If there is a SEWA owner within reasonable driving distance of Chilliwack, BC, I'd like to meet up with you

Cheers!

[lineup]

Originally posted by Geek
Great idea to start a topic here

I have a design in mind, but without an actual SEWA present, I can't say if it will work to perfection.

If I am correct, we need 25V P-P driving some fairly hefty Cin on the MOSFET. Driving the capacitive load isn't an issue for what I have in mind, but keeping it stable with no feedback loop.

If there is a SEWA owner within reasonable driving distance of Chilliwack, BC, I'd like to meet up with you

Cheers!

Thanks Geek.

My own thinking, based on what little i have seen and read about tupe preamps, is 6922 / E88CC / 6DJ8
Not a very original thought, yes,
but this these tubes has made a lot of good and many people happy.
And is easy to find and buy, to a resonable price in most countries.
-------------

This article have a very valid title:
THE SUITABILITY OF THE 6DJ8 FOR AUDIO

Russia’s Best
Today I see many premium numbers with exactly the same internals as their commercial version, sometimes with a gold "flash plate," which the marketer adds to the pins after the fact. Since the gold rubs off on the narrow line of contact with the first insertion, its value is questionable. With a superior beefy structure, the 6922s I have recently bought from Russia are the best I have ever seen from that country, and certainly deserve a premium label.

About The Author ROGER MODJESKI

Roger Modjeski received a BS in Electrical Engineering in 1973 from the University of Virginia and was a teaching assistant while studying for his Masters at Stanford in 1975.
He has been active in electronics since the age of five watching his dad build a Heathkit Williamson.
Prior to starting Ram Tube Works and Music Reference in 1980, he opened a high-end retail store where he met and later worked for Harold Beveridge.

His other interests are music theory, piano, singing, building musical instruments, solar energy, architecture, and understanding how things work.

This article attempts to correct a few misconceptions
about some of the popular dual triodes
used in audio amplification.


With comparison of diffeent variations of 6DJ8. Original Article Link ( recommended! )

THE SUITABILITY OF THE 6DJ8 FOR AUDIO
http://store.electron-valve.com/suof6dforau.html


/lineup

[planet10]

Originally posted by Geek
If there is a SEWA owner within reasonable driving distance of Chilliwack, BC, I'd like to meet up with you

Gregg... you are a SEWA owner... one of the things we didn't get aeound to talking about enuff... i have (most of) the parts for 4 monobloks (need Mosfets & thru hole passive parts) -- 2 are yours. All part of the toob-i-fication project. We did talk about using the Al tube for chassis & heatsink which was the missing ingredient...

dave

[johndiy]

umm lets see some schematics tube driving sewa old chaps

john

[Zen Mod]

both systems of 6DJ8 in parallel.CCS-ed from down (choose your poison - you can also put some nice Ni-MH in cathode ; )),screaming current, low Rp .......
it must work like this

[Mad_K]

Originally posted by Geek

If I am correct, we need 25V P-P driving some fairly hefty Cin on the MOSFET.

Cin is typically 250pF because of the common drain connection, look at the Crss curve of the mosfet for this figure (about 250pF for IRFP240 @ 15VDS)

[Mad_K]

Here it is. If you are planning to use the full 7W potential, I would use 30V rails and 5K6 for R1 to keep distortion down

[Geek]

Hi Guys,

Originally posted by planet10


Gregg... you are a SEWA owner... one of the things we didn't get aeound to talking about enuff... i have (most of) the parts for 4 monobloks (need Mosfets & thru hole passive parts) -- 2 are yours. All part of the toob-i-fication project. We did talk about using the Al tube for chassis & heatsink which was the missing ingredient...

dave

...

Uhhh.. I think I remember what I forgot to pack it in the scramble to make the Greyhound?

Originally posted by Mad_K



Cin is typically 250pF

Very cool. I assume this takes into consideration Mr. Miller's contribution?

Anyway, this is what will drive it. Yes, it's an ECC88 and if you want to use a 6922/E88CC, it'll need changes.

I haven't done the gain stage yet... backed up on projects.

It uses lower voltage and higher current. I've used this as a straight buffer and is stable into a 4.7uF load, as well as complex weired inductances. It'll also drive fair good. Not as good as two trioded 6EJ7's in mu-follower, but it'll swing 25V P-P across 2K resistive+capacitive okay.

So, three tubes. One 6DJ8 for each channel and a third dual-triode for the gain stage. I'm thinking 6CG7.

[Mad_K]

Originally posted by Geek



Very cool. I assume this takes into consideration Mr. Miller's contribution?

I think we can disregard him for this Common Drain circuit