FET Buffer Help

For the three and more legged things
User avatar
colin.hepburn
Shed dweller
Posts: 2304
Joined: Sat Jun 02, 2007 1:24 am
Location: Scotland Aberdeenshire

Post by colin.hepburn » Mon Jul 01, 2013 12:48 am

Hi James
Yep I have been intending to try some sort of valve buffer at some point
The sand buffer works very well with the pioneer amps warm sound

I have yet to try it with the JE labs EL34 SE Amp as this buffer works well I thought the valve version could be an idea but can’t find the website I got the valve schematic from now so hoped that someone in the know could advice on that schematic if its worth trying
Colin
The Blues man ImageImage

JamesD
Old Hand
Posts: 607
Joined: Tue Oct 09, 2007 4:26 pm
Location: North Yorkshire

Post by JamesD » Mon Jul 01, 2013 2:16 am

Looking at the 6922 circuit I can't see any reason why it wouldn't work providing the 6922 does bias up at +2.2V on the cathode...

On the other hand I can't see any reason why it should bias at +2.2V on the cathode except serendipity...which means if anything goes wrong or it doesn't start in quite the same way as usual that its likely to run away and blow the sand up at least...

I guess the most likely failure mode is an intermittent on the pot loosing the ground connection and reference for the grid - if that happens there is a real chance of blowing some sand up...

But I would like a positive reason for the 6922 to bias at +2.2V on the cathode before I built the circuit... I think we need some other eyes on this one too...

Looking at the 6922 anode curves, its a pretty non-linear region to run the 6922 at and the +2.2V doesn't correspond to the given conditions Va=90V and Ia=5mA, suggesting that the built circuit doesn't correspond to a typical 6922 either. All in all I would give this circuit a miss...

James.

User avatar
Nick
Site Admin
Posts: 11409
Joined: Sun May 06, 2007 10:20 am
Location: West Yorkshire

Post by Nick » Mon Jul 01, 2013 8:07 am

I have built something very similar. Cathode follower with voltage follower on top and constant current load. Works fine, very low distortion. There is in effect no way for the valve to distort. Its basically the circuit Allan Wright used on his line output stages. He said it originated with Tek. It very much doesnt have a sound, but I found there was a touch of SS about the audio after it had been through it. I would use a 1M between the grid and ground though instead of relying on the pot, as James suggests.
Resistance isn't futile it's V / I.

User avatar
colin.hepburn
Shed dweller
Posts: 2304
Joined: Sat Jun 02, 2007 1:24 am
Location: Scotland Aberdeenshire

Post by colin.hepburn » Mon Jul 01, 2013 7:01 pm

Thanks James/Nick
Thanks both for the heads up on this schematic if used as a buffer I would omit the pot and replace with resistor

The SS version I built came from the gain clone amp brigade but removing the two filter caps and resistor on the output and replacing it with a 100k resistor to ground after the 4.7uf and it works fine

I think I came across the valve version in the same please I think possibly on DIY Audio but can’t find it now

But as the SS Version works well I was just wondering if the valve version would be a contender or if I should look at a different circuit with other valves any suggestions here on a schematics

Valve i have are 6922 and 12ax7 Russian versions 6SN7s/6SL7 I believe the 12ax7 out as not having the current for a buffer i may have to get more valve :)
Colin
The Blues man ImageImage

JamesD
Old Hand
Posts: 607
Joined: Tue Oct 09, 2007 4:26 pm
Location: North Yorkshire

Post by JamesD » Mon Jul 01, 2013 7:21 pm

Colin,

Sorry, I suffered from some brain fade in analysing the 6922 circuit. As Nick points out, the CCS in the cathode fixes the anode current and with the grid at ground and the anode voltage some handful of volts under B+ it should always settle with 2-3Volts on the cathode...

I still think the current is too low for a 6922 to be really happy but Nick has much more experience with them than me... In this circuit I would use a 6C45pi at around 12-15mA - its really sweet at this point and very open and detailed.

The Russian 6SN7 are nice cathode followers too, run at 8-10mA or more they are very neutral and correct.

As Nick says, one can always hear the sand after a while so I really would go for an ECF valve and use the triode to low noise stabilise the screen for extra goodness :D

James.

User avatar
Nick
Site Admin
Posts: 11409
Joined: Sun May 06, 2007 10:20 am
Location: West Yorkshire

Post by Nick » Mon Jul 01, 2013 9:31 pm

I still think the current is too low for a 6922 to be really happy but Nick
Cant remember for certain, but I think I used about 5ma @ 90v on the 88.

I had previously did the same with with a pile of 6n6p's one in the middle, one on the top as a voltage follower, on on the bottom as a current source, more distortion, but sounded better. Ali had it for a bit I think. Someone else has it now.
Resistance isn't futile it's V / I.

User avatar
colin.hepburn
Shed dweller
Posts: 2304
Joined: Sat Jun 02, 2007 1:24 am
Location: Scotland Aberdeenshire

Post by colin.hepburn » Wed Jul 03, 2013 2:11 am

Thanks again for the advice for the 6C45pi
One other thing which Leds are best for ether valve 6922/6C45pi
Is it Just the standard 5mm 2.1Vf Ifmax=15mA for 6922 and a higher current led for the 6C45
Colin
The Blues man ImageImage

User avatar
Nick
Site Admin
Posts: 11409
Joined: Sun May 06, 2007 10:20 am
Location: West Yorkshire

Post by Nick » Wed Jul 03, 2013 8:07 am

colin.hepburn wrote:Thanks again for the advice for the 6C45pi
One other thing which Leds are best for ether valve 6922/6C45pi
Is it Just the standard 5mm 2.1Vf Ifmax=15mA for 6922 and a higher current led for the 6C45
the LED sets the voltage, its the resistor that sets the current to match the voltage.
Resistance isn't futile it's V / I.

JamesD
Old Hand
Posts: 607
Joined: Tue Oct 09, 2007 4:26 pm
Location: North Yorkshire

Post by JamesD » Wed Jul 03, 2013 9:52 am

For the 6C45pi, these leds from Rapid at just over 7p each are nice.
http://www.rapidonline.com/Electronic-C ... cd-55-0864

They have a datasheet showing Vf with current and at 5mA its about 1.8V and at 15mA its 1.95V

As nick says, the resistor sets the current!

J

User avatar
colin.hepburn
Shed dweller
Posts: 2304
Joined: Sat Jun 02, 2007 1:24 am
Location: Scotland Aberdeenshire

Post by colin.hepburn » Wed Jul 03, 2013 3:43 pm

Thanks James/Nick :)
In the middle of putting an order in to rapid so will add some of the LEDs in
Colin
The Blues man ImageImage

Post Reply