A 6SN7 direct coupled preamp & regulated supply for everyone's perusal. - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 22nd March 2008, 09:58 PM   #1
G is offline G  United States
diyAudio Member
 
G's Avatar
 
Join Date: Apr 2002
Location: Belleville, IL.
Default A 6SN7 direct coupled preamp & regulated supply for everyone's perusal.

Hi all. I wanted some opinions and vetting of a preamp & supply. Here ya go:
Attached Images
File Type: gif 6sn7 foreplay.gif (5.8 KB, 1323 views)
__________________
Gavin
  Reply With Quote
Old 22nd March 2008, 10:00 PM   #2
G is offline G  United States
diyAudio Member
 
G's Avatar
 
Join Date: Apr 2002
Location: Belleville, IL.
Here is the regulated supply:
Attached Images
File Type: gif 6sn7 foreplay reg psu.gif (7.6 KB, 1208 views)
__________________
Gavin
  Reply With Quote
Old 22nd March 2008, 11:58 PM   #3
diyAudio Moderator Emeritus
 
ray_moth's Avatar
 
Join Date: Jan 2004
Location: Jakarta
Suggestions for the preamp:

I'd connect a resistor of, say, 1Meg between the slider of the volume control and ground, to prevent unpleasant noises in case the track has imperfections that could cause momentary open circuit.

With the resistor values and B+ voltage shown on your schematic, the plate current in each triode will be only about 2.5mA, which is rather low. As you say, the plate voltage of the first triode will be 75v, which I also think is too low.

If you can increase B+ to 300v, using the same resistor values you will have a plate current of 5mA in each triode and a plate voltage on the first triode of 150v. This is a better operating point and both triodes will be dissipating only 750mW, which is well within their ratings.

Of course, there are all sorts of clever things you could do with a 6SN7 to achieve similar ends, including SRPP and mu-follower. My suggestions are just aimed at improving the results with your own perfectly good topology.

Suggestions for the PSU:

If you use the 125-0-125v @100mA winding end-to-end, you'll have 250v @ 50mA. This give 350v with a cap inout filter but will only let you draw 30mA (you can only use 60% of the transformer's AC current rating with a cap input filter). That's not going to be enough. I would get a 250v tranny with 100mA rating.

You will need two OD3 VR tubes in series in each regulator. Driving the VR tubes from a constant current source, as you have in your schematic, is a nice refinement but probably not really necessary unless your line voltage is highly variable.

I can't comment much on the CCS, since I don't know the device you're using, except to say that you need to set it to deliver 20mA for each channel. That allows 10mA for the preamp and another 10mA for the VR tubes in each channel. 5mA is the absolute minimum current for an OD3 to work so, to allow for low line voltage, you need to feed it more than that under 'average' conditions. It is happy with up to 40mA, so 10mA is a reasonable figure. That's why you need a bigger tranny.
  Reply With Quote
Old 23rd March 2008, 12:20 AM   #4
diyAudio Member
 
Join Date: Mar 2008
Quote:
If you use the 125-0-125v @100mA winding end-to-end, you'll have 250v @ 50mA. This give 350v with a cap inout filter but will only let you draw 30mA (you can only use 60% of the transformer's AC current rating with a cap input filter). That's not going to be enough. I would get a 250v tranny with 100mA rating.
I was just interested in this calculation. Is it just a rule of thumb? and is it dependent on the type of rectifier? Also, what causes the limitation, inrush current of the cap?
  Reply With Quote
Old 23rd March 2008, 12:37 AM   #5
diyAudio Member
 
stoolpigeon's Avatar
 
Join Date: Feb 2007
Location: Laputa
How is it direct coupled?

sp
  Reply With Quote
Old 23rd March 2008, 12:49 AM   #6
diyAudio Member
 
Join Date: Mar 2008
No coupling cap between U1 and U2
  Reply With Quote
Old 23rd March 2008, 02:35 AM   #7
diyAudio Member
 
zigzagflux's Avatar
 
Join Date: Oct 2006
Location: Charlotte, NC
Quote:
Originally posted by ray_moth

If you use the 125-0-125v @100mA winding end-to-end, you'll have 250v @ 50mA.
Hello Ray:

I have to disagree with you on this one. The transformer is rated 40VA, and comes with a 6.3V 2A aux winding. That leaves 27.4VA left for the secondary, which at 250V is 110mA. It looks like its rated for a full bridge operation.

Also, he actually doesn't have a cap input filter, it's more of a hybrid, riding the fence between choke and cap input. His transformer current is about as nice as you can get without the harsh turn-off of the choke input supply. So the transformer looks pretty perfect to me.

Of course, if he pursues a 300V supply instead, it would be useless, but looks good for 150V. PSUD confirms, at least.

Cheers.
  Reply With Quote
Old 23rd March 2008, 04:02 AM   #8
diyAudio Moderator Emeritus
 
ray_moth's Avatar
 
Join Date: Jan 2004
Location: Jakarta
Sorry, zigzag, I misread the schematic! I now see that the cap is, indeed, 0.33uF, not 33uF. In that case he has a choke-input filter, to all intents and puposes, and he must be using the full winding already. Becasue it is a choke-input filter, he can probably get 225vDC @50mA.

A pair of OC3 VR tubes in series would give 210v B+, allowing 15v to be dropped across the CCS and providing for a somewhat better op point for the 6SN7s than a 150v B+.
  Reply With Quote
Old 23rd March 2008, 04:14 AM   #9
G is offline G  United States
diyAudio Member
 
G's Avatar
 
Join Date: Apr 2002
Location: Belleville, IL.
Quote:
Originally posted by ray_moth
Sorry, zigzag, I misread the schematic! I now see that the cap is, indeed, 0.33uF, not 33uF. In that case he has a choke-input filter, to all intents and puposes, and he must be using the full winding already. Becasue it is a choke-input filter, he can probably get 225vDC @50mA.

A pair of OC3 VR tubes in series would give 210v B+, allowing 15v to be dropped across the CCS and providing for a somewhat better op point for the 6SN7s than a 150v B+.

I don't understand what you are saying Ray. The transformer is 125 - 0 - 125 @ 100mA not 50 mA. The transformer does not have two 50mA windings on the secondary. It has one center tapped 100mA winding. Am I missing something?
__________________
Gavin
  Reply With Quote
Old 23rd March 2008, 05:39 AM   #10
diyAudio Moderator Emeritus
 
ray_moth's Avatar
 
Join Date: Jan 2004
Location: Jakarta
What it is, G, is that the 100mA rating assumes that you willuse the tranny as it was designed to be used, with the center tap as negative and a diode on each end of the winding to give the positive. This will give 125v @ 100mA.

Using the full winding with a bridge rectifier, as in your schematic, will give twice the voltage but only half the current, so you get 250v @ 50mA.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
capacitor coupled vs. direct coupled ouput hugobross Solid State 11 21st January 2011 01:56 AM
single endedtransformer coupled 6sn7 mic preamp gurpreet Tubes / Valves 3 15th February 2009 08:55 AM
6sn7 direct coupled and transformer coupled neazoi Tubes / Valves 3 23rd May 2008 03:30 PM
Regulated PowerSupply for Frank's 6SN7 Preamp Vidi Tubes / Valves 2 10th December 2006 12:23 AM
Can someone explain parafeed & direct coupled & a.c. coupled? mwmitchell Tubes / Valves 2 11th September 2002 01:10 PM


New To Site? Need Help?

All times are GMT. The time now is 02:05 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2