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Old 25th February 2006, 04:08 PM   #1
mikee55 is offline mikee55  United Kingdom
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Default minimalist valve pre for follower

Hi everybody, I can't do a search because my PC has died,and I'm using my daughters PC which is a pentium 2 and is seriously slow.She's only six years old and likes using Paint.
Anyway,hi,can anybody tell me of a schematic that uses a valve for a pre to drive Rod Elliotts Class A Mosfet Follower?I just want a minimal component outlay and what would be nice,a 6922 or similar running on the same voltage as the Follower.I know of a Gainclone that uses this valve but don't know if it can drive a Follower.Has this been done and does it sound any good?Many Thanks all.I was reading a lot of threads and posts at this site,I've come over from the Audio Asylum and enjoy reading your messages.Can't wait to get my PC fixed.I upgraded the heatsink on the Duron cpu to one that uses copper and dropped it on the chip.What a silly thing to do.I was rushing the job.I'll never learn.I've got one coming off ebay,but it will be only a 600mhz instead of 1100mhz.Oh well,looking forward to your replies.

Thankyou.mikee55
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Old 26th February 2006, 03:40 PM   #2
kevinkr is offline kevinkr  United States
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I'm not sure what the voltage is that the Rod Elliot follower operates on, but with a 6922 you would want about 80V across the tube itself (plate to cathode) for good linearity plus the drop across the plate resistor or ccs - so it's doubtful that the supply for the follower could be used. I suspect that even with a ccs you are talking about a supply voltage in excess of 100V depending on the voltage swing you need into the follower.
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Old 26th February 2006, 04:38 PM   #3
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The 6GM8/ECC86 is a low voltage (30 V. on plate max.) relative of the 6DJ8/ECC88.

6GM8 data sheet here.
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Old 26th February 2006, 05:06 PM   #4
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You need a pre with with maximum 1k ohm output impedance to drive the input capacitance of the mosfet.
To get the full output power the preamp has to deliver minimum 12V rms. I don`t think that is possible with tubes at only 40V b+ and still have low distortion.
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Old 26th February 2006, 05:21 PM   #5
lineup is offline lineup  Sweden
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this circuit uses +-24 Volt = 48 Volt, regulated with LM317, LM337
Tube is ECC88, which is same type as 6922, E88CC, 6DJ8

it is called 'Buffer For CD player'
http://valveaudio.tripod.com/schematics.htm

uses some kind of feedback, i havent figured out how much gain is
maybe you can use some part of this circuit
in your design


Even if TRUE Valve People (the tube maffiosos)
do not want to go lower than 100 Volt circuit for this tube 6922,
there are quite a many examples of doing it this low voltage way just the same.
With good results.

Will probably not give too bad tube sound,
even if ultimate performance of this tube is 100 volt or higher.

See for example this Tube Buffer for chip amp,
using +-35 Volt = 70 Volt
http://www.customanalogue.com/diytub...lone/index.htm


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Click the image to open in full size.
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Old 26th February 2006, 07:08 PM   #6
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Yes, but you also want some gain, so a tube buffer doesn`t do any good without a gainstage in front of it, at least not with a mosfet source follower.
A mosfet sourcefollower has a voltagegain of slightly less than 1, so the preamp has to do all the voltagesving.
The chip-amp has loads of gain so it doesnt need all that voltage sving at the input, this is why it is possible to use low b+ (with good results) in some cases.
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Old 26th February 2006, 08:17 PM   #7
mikee55 is offline mikee55  United Kingdom
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Default So how to add gain?

Hi all,thanks for replies.
Please,how can I add some gain?Could I use a transistor instead of the Power amp IC,and feed the follower with that?If I had to increase the voltage,what parts do I change to run it off of a higher voltage?Also I bought a Sfernice 250k volume pot,can I still use this for changing volume?,as it was expensive,and the solidstate amp it was installed in,died a week later,(I loved that amp,r.i.p),it was old and heavily re-wired and modded.I don't make habit of damaging things,tho. .
So,more thoughts please,and thankyou for your time and effort.

Thankyou.Mikee55
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Old 26th February 2006, 08:44 PM   #8
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I think this one is good:
http://www.headwize.com/projects/sho...ciuff2_prj.htm
Or this one, not tubes but simple and much lower voltage:
http://www.passdiy.com/pdf/brideofzen.pdf
Power follower99:
http://www.audiodesignguide.com/Powe...wer/index.html


Don`t play with high voltages if you are not sure about how to do it safely.
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Old 26th February 2006, 08:46 PM   #9
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You can use a step up microphone transformer to provide the VOLTAGE gain needed. The stepped up voltage from the trafo drives the vacuum triode cathode follower. A JFET with a high transconductance wired as a source follower drives the trafo, as step up mic trafos must be driven by impedances of 150 Ohms or less.
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Old 26th February 2006, 09:07 PM   #10
mikee55 is offline mikee55  United Kingdom
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Default Come again!

Sorry for my ignorance,can you draw a picture or somthing please.Thankyou.Mikee55
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