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Old 31st October 2004, 10:49 PM   #21
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Quote:
Originally posted by silurato
(he said it's a "Brosky's idea", but i don'know him).
It is Broskie, John. Check at:


Tubecad
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Old 1st November 2004, 04:19 PM   #22
sajti is offline sajti  Hungary
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Hi,

my friend made power amplifier, with 6AS7G. He used fixed bias push-pull class AB output stage. For this settings the driver needs separated 350V psu, to gives enough voltage swing.
The driver was Williamson style with two E82CC...
With 180V HT the output was some 10-12W

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Old 13th November 2004, 07:58 PM   #23
kevinkr is offline kevinkr  United States
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As I recall the svetlana design was done in house by Eric Barbour years and years ago in order to demonstrate interesting and novel uses for the Svetlana 6BM8 as well as the SV version of the 6AS7. Note that what follows is opinion only - I have not heard this design. Honestly if you look at the (svetlana) curves for this tube the linearity of both sections is just awful, and I have heard a few very mediocre sounding amplifier designs using this type.. I would think a driver stage based on the ECC99 or 5687 would be an interesting alternative. I have heard both SE and PP amplifier using the 6AS7 and they were all good performers.. Strikes me that an interstage transformer with a 1:2 step up ratio with a high transconductance/low Rp driver tube like the 5842/5687 would do the job nicely.

Just my two cents worth

Kevin
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Old 13th November 2004, 08:15 PM   #24
jleaman is offline jleaman  Belgium
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as this is my first tube amp i have strong ideas and strenth that since it is my only tube amp i could build it with no problems and do a great job working with a schematic of course. i could build something nice.. just a little time money and planning.. I just dont know what to build. maybe the 6AS7's or the 813's im not scared of voltages as im pretty brave and smart with my fingers and aware of all the hazards so yeah.. anyone got some advice on what one to start with as i want to order some tubes..
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Old 14th November 2004, 12:19 AM   #25
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Quote:
Originally posted by JasonL
as this is my first tube amp i have strong ideas and strenth that since it is my only tube amp i could build it with no problems and do a great job working with a schematic of course. i could build something nice.. just a little time money and planning.. I just dont know what to build. maybe the 6AS7's or the 813's im not scared of voltages as im pretty brave and smart with my fingers and aware of all the hazards so yeah.. anyone got some advice on what one to start with as i want to order some tubes..
Jason,

I really want to advise you against building an 'electric chair' amplfier like the 813 as your first entry into vacuum tube project construction. Your survival as a living being depends on more than being brave, and being aware of the hazards. You need to have good high voltage construction techniques under your belt and you need to have an automatic ritual procedure developed from years of working with potentially dangerous electrical equipment to know where to put your clever hands and when. With 1000 volts or more at the current stored in an amplifier power supply capacitor bank you don't usually get a bad shock with your first mistake, you get death. I'm not kidding!
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Old 14th November 2004, 12:23 AM   #26
jleaman is offline jleaman  Belgium
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so building tesla coils is not safe ? i have build lots of well over 3k volt coils and never been shocked and i have build lots of 5k caps to go with them. : O ) I think i can manage a tube amp.. also ill be running my 813's at 600v
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Old 14th November 2004, 12:51 AM   #27
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Actually Jason, building Tesla coils operating from Neon Sign Transformers CAN BE A LOT SAFER than tube amps with 1 kV DC plate power supplies. It isn't the voltage that kills, it is the total energy available to the victim. A typical NST powered TC may store about 1 to 1.5 Joules in the primary circuit tank capacitor. A 125 uF cap bank in a 1000 volt transmitting tube monoblock amp PS stores 62.5 Joules. That is 40 to 60 times more energy! A stereo amp would likely store twice this with a 250 uF cap bank. That's 80-120 times more energy.

125 uF charged to 600 volts (to use your own tube supply example) stores 22.5 Joules. That's still almost 20 times that stored by a 15,000 volt, 60 mA NST using a 0.01 uF tank circuit cap.

The formula for stored energy is 1/2CV^2. That's one half of (the capacitance in farads times the voltage squared).

My 805 stereo amp has 250 uF charged to 1600 VDC during start sequencing. That is a seriously scary 320 Joules. I am expecting a loud explosion if a spider ever crawls up in the chassis. That size discharge would vaporize the end off a screwdriver. BTW, I have extensive Tesla coil experience as a previous manufacturer of commercial coil systems for science museums and industry up to 20 kVA in size. I spent a lot of time experimenting with NST powered coils as well.
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Old 14th November 2004, 06:54 AM   #28
kevinkr is offline kevinkr  United States
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Hi Jason,
I've been building tube amplifiers for over 20yrs, and despite extensive experience and an exemplary track record for safety I have never built an amplifier that operated at over 700Vdc because I am scared of working on devices operating at such high voltages. I agree totally with rcavictim on this issue. It only takes a moment of inattention to experience a potentially fatal event.. I have designed regulated supplies for friends who felt they could handle the safety and construction issues involved at up to 1.4KV, but I have no plans to follow suit.

I will look at posting something in a couple of days based on the 6AS7, and a tbd driver tube if you are interested and not too annoyed at all this concern..

The 6AS7 is and remains an excellent choice in the <10W class.

Also I will say that I strongly prefer amplifiers based on 45's/2A3's/300B's to the largest 845/211/572X I have heard.. And all of these designs operate on supplies of 400V or less. Not safe, but "safer." I have a good performing, moderately complex 45 based design on my website at www.kta-hifi.net if this floats your boat.. lol

Another thought would be a DC darling using the 1626, very simple and sounds wonderful.. Power is only a couple of watts at best.

Not trying to sound preachy, but I would be remiss not to point out the risk and my own feelings on such an issue.
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Old 14th November 2004, 07:36 AM   #29
jleaman is offline jleaman  Belgium
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all i have to do now is find a good 6AS7 schematic then.. who's got one ? wanna share. ?
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Old 14th November 2004, 04:07 PM   #30
kevinkr is offline kevinkr  United States
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hi Jason,
I will look through my collection of schematics and figure out what seems appropriate.. I may also come up with a new design, but you may have to do a little work to optimize the design - if you are interested.

Kevin
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