Hi guys, I hope you don't mind me asking an audio-related question.
I have electrostatic headphones (1 micron thick Mylar membrane that is
charged to around 550 V, between two stators that get opposite polarity
signal, about 750 V peak to peak on each stator). After considering
various amp designs and reviews, I decided to go with this DC coupled
hybrid:
http://www.headwize.com/projects/showfile.php?file=gilmore4_prj.htm
I've all the parts, and it simulates well (the feedback arrangement
where the NFB not only goes to the same side but also, through the LTP
input stage coupling, to the opposite side in phase thus getting
distortion in common mode on the output, works very well).
My question is regarding possibly replacing the Q13/14 MOSFET third
stage, which does most of the voltage gain in the circuit, with triodes.
Other than the need for different supply voltages to correctly bias
tubes in that place, are there any possible downsides? The main problem
I've been having in simulation is the loss in gain (I was trying with a
6DJ8 model).
Any suggestions for appropriate triodes, and best operating points for
them in this circuit?
Thanks in advance.
amps-request@contesting.com wrote:
> Send Amps mailing list submissions to
> amps@contesting.com
>
> To subscribe or unsubscribe via the World Wide Web, visit
> http://lists.contesting.com/mailman/listinfo/amps
> or, via email, send a message with subject or body 'help' to
> amps-request@contesting.com
>
> You can reach the person managing the list at
> amps-owner@contesting.com
>
> When replying, please edit your Subject line so it is more specific
> than "Re: Contents of Amps digest..."
>
>
> Today's Topics:
>
> 1. Re: The maximum amount ?of 811As and 572Bs in parallel
> (Peter Chadwick)
> 2. Re: "Tubes 201" - How Vacuum Tubes Really Work (Peter Chadwick)
> 3. Re: "Tubes 201" - How Vacuum Tubes Really Work (Tom W8JI)
> 4. Re: "Tubes 201" - How Vacuum Tubes Really Work (Tom W8JI)
> 5. Re: "Tubes 201" - How Vacuum Tubes Really Work
> (Karl-Arne Markstr?m)
> 6. Re: Looking for info: SRF7053 device (Peter Chadwick)
> 7. Re: "Tubes 201" - How Vacuum Tubes Really Work (Peter Chadwick)
> 8. AMP without HV transceiver (Hsu)
> 9. Re: Fw: "Tubes 201" - How Vacuum Tubes Really Work (GGLL)
>
>
> ----------------------------------------------------------------------
>
> Message: 1
> Date: Tue, 25 Jul 2006 11:22:34 +0200 (CEST)
> From: Peter Chadwick <g3rzp@g3rzp.wanadoo.co.uk>
> Subject: Re: [Amps] The maximum amount ?of 811As and 572Bs in parallel
> To: R L Measures <r@somis.org>, g3rzp@g3rzp.wanadoo.co.uk
> Cc: amps@contesting.com, Hsu <Jbenson@sohu.co>
> Message-ID: <21071101.1153819354246.JavaMail.www@wwinf3103>
> Content-Type: text/plain; charset=UTF-8
>
> Distributed amplifiers are a totally different ballgame to putting tubes in
> parallel. They're not efficient - one, done in the early 1960's used 2
> amplifiers in push-pull, each using eight 4CX250Bs. It had 1100 volts on the
> plates, and a forced air cooled wideband transformer at the output, delivered
> 1 kW PEP from 2 - 27MHz (and caught fire at 28!) and drew 5kW from a 3 phase
> line at 0.9PF. That would push anyone's electricity bill!
>
> The technique has been used in GaASFET amps at low power, running from 20 to
> 50GHz from memory.
>
> Half the power is dumped in the laod at teh sending end: I seem to recall
> some porposals of using a second transmission line as a delay line and a
> hybrid coupler to get that power out.
> 73
> Peter G3RZP
>
> ------------------------------
>
> Message: 2
> Date: Tue, 25 Jul 2006 11:41:55 +0200 (CEST)
> From: Peter Chadwick <g3rzp@g3rzp.wanadoo.co.uk>
> Subject: Re: [Amps] "Tubes 201" - How Vacuum Tubes Really Work
> To: craxd1@verizon.net, amps@contesting.com
> Message-ID: <13282403.1153820515809.JavaMail.www@wwinf3103>
> Content-Type: text/plain; charset=UTF-8
>
> Two things make grids emit. One is heat. You try to minimise that by flashing
> of gold etc, although if you boil that off, it's not there, so you're left
> with the basic material. Unfortunately, in oxide coated cathodes, barium and
> strontium can boil off the cathode, and land on the grid. Then you have a
> nice low work function emitter. The other is secondary emission, and for
> that, a carbon coating is useful. (all this from care and feeding and the RCA
> tube manual)The first tube that I know of with a gold flashed grid was the
> 6J4, gg recieving triode of around 1943/44. That has a gold flashed grid
> because the grid - cathode spacing was pulled down, and gold flashing helped.
> Often used in Wallman cascode with a triode connected 6AK5, although a lot of
> people used 1/2 a 6J6 for the gg stage (why only half? - the idea of the
> cascode is that the low load impedance on the first stage minimises the
> voltage swing and thus the Miller effect. So a lower impedance with the two
> halve
s o
> f the 6J6 in parallel would seem to be indicated) However, the gold flashing
> wasn't considered necessary for the frame grid pentodes such as the
> EF183/6EH7 and EF184/6EJ7, and they ahd very close grid cathode spacings,
> giving a very high gm.
>
> 73
> Peter G3RZP
>
> ------------------------------
>
> Message: 3
> Date: Tue, 25 Jul 2006 06:09:27 -0400
> From: "Tom W8JI" <w8ji@w8ji.com>
> Subject: Re: [Amps] "Tubes 201" - How Vacuum Tubes Really Work
> To: "Keith Dutson" <kdutson@sbcglobal.net>, <amps@contesting.com>
> Message-ID: <001d01c6afd2$6e006370$640fa8c0@radioroom>
> Content-Type: text/plain; format=flowed; charset="iso-8859-1";
> reply-type=original
>
>
>> I am totally lost on this excessive grid current
>> discussion. If there is a
>> resistor or fuse tied between the grid and chassis and it
>> blows, the grid
>> obviously has a potential above or below ground. Which is
>> it? Can it be
>> either depending on the fault?
>>
>
> Keith,.
>
> There are two major types of faults. One is RF generated
> where the grid has excessive drive from RF (alternating
> current) drive, the other is a hard fault caused by a
> flashover. There are multiple things inside a tube that can
> cause a hard fault, and they are very common.
>
> When tubes are designed the designer sometimes tries to work
> a gap into the tube in a harmless area, knowing there has to
> be a safe point for hard faults caused by gas. The point
> where a 3-500Z arcs is either from the bottom of the anode
> to the grid support cone, although I have seen a few fire
> off the top of the grid structure to the anode.
>
> Arcs generally have very low resistance, and when an arc is
> struck it can continue at much lower voltage than the
> initial breakdown voltage. During an arc molten metal or
> ions from gas can be involved. Eimac and other companies
> actually use intentional arcs to clear faults, rounding
> metal points inside the tube to restore voltage holdoff to
> acceptable levels. Arcs can also help getter or clear gas by
> the tube by changing it into harmless compounds that
> harmlessly bond with elements.
>
> If the tube was absolutely gas free, which large power tubes
> never are, and if it was a soft fault caused by drive a
> floated grid in a SOFT fault would go negative. However, a
> fuse (or worse yet a resistor) is a very poor way to protect
> for soft faults. Soft faults require electronic protection.
>
> If the tube has a hard fault, the grid has a direct
> conductive path to the anode. During the arc, even if you
> open the grid, it will simply pull up towards anode voltage
> and turn the tube on. Every tube book I have looked at warns
> about this condition!
>
> If you open the grid during a hard fault two things happen.
> Whatever you are opening has to hold off nearly full supply
> voltage. Fuses and resistors normally can't take this
> voltage, and neither can the grid-cathode path.
>
> If by some strange stroke of luck the arc quenches just
> after the grid path opens, you still have a grid hanging
> free. That's something every good engineering book I have
> warns about doing in high voltage high power tubes, and I
> put the text up on my website from three of those books. One
> of those books, by Giocoletto, is one of the most
> comprehensive tube design books around. When you float or
> have a very high grid resistance in a power tube, stray ions
> that are always present will collect on the grid and
> collect. They actually pull the grid positive, and the
> additional electrons striking the plate from increased
> cathode current free more positive ions to return to the
> grid. A runaway situation develops, and the tube goes into
> another fault!
>
> This isn't something I'm just making up. It is a common
> thing. That's why several other people piped up an all said
> the same thing as I have been saying, and why I can point to
> engineering text that has the same warnings. I don't have to
> say "the author said that but really meant this". It isn't
> an issue if I am right or wrong, but rather one of asking
> actual tube design engineers or reading scholarly
> peer-reviewed texts on the subject.
>
> It's a lot like the moly anode with zirconium coating for
> gettering Eimac used in the 3-500Z. People don't have to
> assume, guess, and say what their "opinion" is. They can
> pick up a phone or send an e-mail and ask a manufacturer
> before reaching false conclusions. It's like the 50-amp
> fault protection transistors that never really were even in
> the grid circuit, but the person espousing that stuff for 20
> years never even bothered to look at a schematic!!
>
> 73 Tom
>
>
>
>
> ------------------------------
>
> Message: 4
> Date: Tue, 25 Jul 2006 06:15:03 -0400
> From: "Tom W8JI" <w8ji@w8ji.com>
> Subject: Re: [Amps] "Tubes 201" - How Vacuum Tubes Really Work
> To: <craxd1@verizon.net>, <amps@contesting.com>
> Message-ID: <002e01c6afd3$36864fd0$640fa8c0@radioroom>
> Content-Type: text/plain; format=flowed; charset="iso-8859-1";
> reply-type=original
>
>
>> Tom, are you saying Terman was lying in the 1st edition of
>> his handbook? I don't need to re-write anything, I have it
>> here in print. I have a question, how can an element being
>> bombarded by or gaining electrons be positive (over 0 V,
>> +0.1 volt DC or greater)? Is Termans satements about
>> equilibrium hog wash?
>
> Hi Will,
>
> No, I'm not saying Terman is lying or that Terman spews
> hogwash.
>
> 73 Tom
>
>
>
>
> ------------------------------
>
> Message: 5
> Date: Tue, 25 Jul 2006 12:10:34 +0200
> From: Karl-Arne Markstr?m <sm0aom@telia.com>
> Subject: Re: [Amps] "Tubes 201" - How Vacuum Tubes Really Work
> To: <g3rzp@g3rzp.wanadoo.co.uk>, <amps@contesting.com>
> Message-ID: <000001c6afd3$6473aaa0$bc01a8c0@speedbox>
> Content-Type: text/plain; charset=iso-8859-1
>
> Peter wrote:
> "(why only half? - the idea of the cascode is that the low load impedance on
> the first stage minimises the voltage swing and thus the Miller effect. So a
> lower impedance with the two halves of the 6J6 in parallel would seem to be
> indicated)"
>
> This was the subject of considerable discussion in the VHF community in the
> 50's.
> It appears that the outcome was that the increase of interelectrode
> capacitances by paralleling two sections offset the higher gm of using two
> 6J6 sections.
>
> I have a 6AK5/6J6 cascode 2m converter (SM5VL design) hidden somewhere in the
> more ancient layers of the "junk-box". It may be interesting to evaluate this
> converter using modern instrumentation and methods and verify the difference
> between one or two sections.
>
> 73/
>
> Karl-Arne
> SM0AOM
>
>
> ----- Original Message -----
> From: "Peter Chadwick" <g3rzp@g3rzp.wanadoo.co.uk>
> To: <craxd1@verizon.net>; <amps@contesting.com>
> Sent: Tuesday, July 25, 2006 11:41 AM
> Subject: Re: [Amps] "Tubes 201" - How Vacuum Tubes Really Work
>
>
>> Two things make grids emit. One is heat. You try to minimise that by
>> flashing of gold etc, although if you boil that off, it's not there, so
>> you're left with the basic material. Unfortunately, in oxide coated
>> cathodes, barium and strontium can boil off the cathode, and land on the
>> grid. Then you have a nice low work function emitter. The other is
>> secondary emission, and for that, a carbon coating is useful. (all this from
>> care and feeding and the RCA tube manual)The first tube that I know of with
>> a gold flashed grid was the 6J4, gg recieving triode of around 1943/44. That
>> has a gold flashed grid because the grid - cathode spacing was pulled down,
>> and gold flashing helped. Often used in Wallman cascode with a triode
>> connected 6AK5, although a lot of people used 1/2 a 6J6 for the gg stage
>> (why only half? - the idea of the cascode is that the low load impedance on
>> the first stage minimises the voltage swing and thus the Miller effect. So a
>> lower impedance with the two halv
es
> o
>> f the 6J6 in parallel would seem to be indicated) However, the gold
>> flashing wasn't considered necessary for the frame grid pentodes such as the
>> EF183/6EH7 and EF184/6EJ7, and they ahd very close grid cathode spacings,
>> giving a very high gm.
>>
>> 73
>> Peter G3RZP
>> _______________________________________________
>> Amps mailing list
>> Amps@contesting.com
>> http://lists.contesting.com/mailman/listinfo/amps
>>
>>
>> --
>> No virus found in this incoming message.
>> Checked by AVG Free Edition.
>> Version: 7.1.394 / Virus Database: 268.10.4/396 - Release Date: 2006-07-24
>>
>>
>
>
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|