6sn7 pre amp

ask your general tube related questions here

6sn7 pre amp

Postby 6B4G » Tue Jan 21, 2014 2:55 am

Hi again now that my se amp is running great I wanted to build a pre amp.
I have looked around and thought that the blue velvet looks ok.
You have to bare in mined I want to use what material I have on hand as times are tight.
Bellow I have the preamp and a psu that I put together (on paper) using psud2.
can you please have a look at the psu and tell me if it is ok.

Thanks guys
Attachments
img075.jpg
img075.jpg (45.38 KiB) Viewed 4958 times
6sn7 blue v.jpg
6sn7 blue v.jpg (75.87 KiB) Viewed 4959 times
User avatar
6B4G
KT88
 
Posts: 287
Joined: Sun Jul 06, 2008 1:02 pm
Location: Greece

Re: 6sn7 pre amp

Postby DeathRex » Tue Jan 21, 2014 7:09 am

Put the choke first. With the 1.2K resistor there it will have to drop 40 volts dc at 1.92 watts and 240 volts AC at 40.3 watts. You'll need a 50 watt resistor there, so put the choke first.
At first I wanted to be a ET. Now I are one.
User avatar
DeathRex
KT88
 
Posts: 1014
Joined: Wed Nov 18, 2009 8:13 pm
Location: Cortez, CO

Re: 6sn7 pre amp

Postby 6B4G » Tue Jan 21, 2014 7:55 am

Sorry I made a mistake on the transformer it is 250-0-250 100mA off load 260-0-260
I'm using 1/2 of the transformer 0-260v 50mA no load
also once I changed it on the psu2 program it didn't like it
rectifier PIV exceeded. Also shows me a voltage of 226.
I was looking for 204v
What would the ratting of the resistors be?
I also have a Hammond 193J 10H 82 ohm or a 3.6H 300mA 24 ohm choke
The 193 is quite large and I prefer not to use it.
amI correct with saying 2 X 6sn7 = 40mA load?
Attachments
img076.jpg
img076.jpg (56.95 KiB) Viewed 4943 times
User avatar
6B4G
KT88
 
Posts: 287
Joined: Sun Jul 06, 2008 1:02 pm
Location: Greece

Re: 6sn7 pre amp

Postby kheper » Tue Jan 21, 2014 1:10 pm

6B4G wrote:Sorry I made a mistake on the transformer it is 250-0-250 100mA off load 260-0-260
I'm using 1/2 of the transformer 0-260v 50mA no load
also once I changed it on the psu2 program it didn't like it
rectifier PIV exceeded. Also shows me a voltage of 226.
I was looking for 204v


If I were you, I'd use the whole 260-0-260 with a Full Wave rectifier, not a Full Wave Bridge - as is shown in the schematic.

What would the ratting of the resistors be?


P = 226 * .04 = 9W

10W is the minimum. I'd double it. 20W should be fine.

I also have a Hammond 193J 10H 82 ohm or a 3.6H 300mA 24 ohm choke
The 193 is quite large and I prefer not to use it.
amI correct with saying 2 X 6sn7 = 40mA load?


Figuring 10ma per triode section, 40ma is correct.
User avatar
kheper
KT88
 
Posts: 1252
Joined: Wed Dec 21, 2005 10:14 pm
Location: Philly, PA

Re: 6sn7 pre amp

Postby DeathRex » Tue Jan 21, 2014 5:34 pm

kheper wrote:
6B4G wrote:What would the ratting of the resistors be?


P = 226 * .04 = 9W

10W is the minimum. I'd double it. 20W should be fine.


He's not dropping 226, he needs to go from 226 to 204. Increase the second resistor to 470. Power for first resistor will be 1.92 watts, so 4 watter. Power for second resistor will be 0.752 or a 2 watter. You can get 2 resistors that are the same, but the values have to be about 910 ohms at 3 watts. Might have to adjust them though.

You can series 2 diodes to increase the PIV, but you'll need 8 diodes. Use the full secondary if you can, then only need 4.
At first I wanted to be a ET. Now I are one.
User avatar
DeathRex
KT88
 
Posts: 1014
Joined: Wed Nov 18, 2009 8:13 pm
Location: Cortez, CO

Re: 6sn7 pre amp

Postby 6B4G » Wed Jan 22, 2014 1:03 am

Thanks again I've made the changes below and I also found some 100μF 400v caps in my stash.
I have done as you asked but also made some more changes to incorporate the 100μF caps.
Would that be better than the 33μF caps?
Resister I will be a ceramic 5 watt. Is that ok?
Attachments
psu.jpg
psu.jpg (553.8 KiB) Viewed 4885 times
User avatar
6B4G
KT88
 
Posts: 287
Joined: Sun Jul 06, 2008 1:02 pm
Location: Greece

Re: 6sn7 pre amp

Postby DeathRex » Wed Jan 22, 2014 9:54 am

Why not use the whole secondary and center tap of the transformer? You'll get better regulation.
At first I wanted to be a ET. Now I are one.
User avatar
DeathRex
KT88
 
Posts: 1014
Joined: Wed Nov 18, 2009 8:13 pm
Location: Cortez, CO

Re: 6sn7 pre amp

Postby kheper » Wed Jan 22, 2014 2:21 pm

DeathRex wrote:
kheper wrote:
6B4G wrote:What would the ratting of the resistors be?


P = 226 * .04 = 9W

10W is the minimum. I'd double it. 20W should be fine.


He's not dropping 226, he needs to go from 226 to 204.


Err... A slight error in the rithmatics. That's what happens when calculating without thinking...
User avatar
kheper
KT88
 
Posts: 1252
Joined: Wed Dec 21, 2005 10:14 pm
Location: Philly, PA

Re: 6sn7 pre amp

Postby kheper » Wed Jan 22, 2014 2:35 pm

6B4G wrote:Thanks again I've made the changes below and I also found some 100μF 400v caps in my stash.
I have done as you asked but also made some more changes to incorporate the 100μF caps.
Would that be better than the 33μF caps?


2 100μF caps could be substituted for the 3 33μF. You only need one resistor to drop the 22V. Depending on the choke, my calcs suggest only ~1/2 of the total resistance in the first schematic is needed. LCRC rather than LCRCRC.
User avatar
kheper
KT88
 
Posts: 1252
Joined: Wed Dec 21, 2005 10:14 pm
Location: Philly, PA

Re: 6sn7 pre amp

Postby 6B4G » Thu Jan 23, 2014 2:27 am

Now I'm confused
User avatar
6B4G
KT88
 
Posts: 287
Joined: Sun Jul 06, 2008 1:02 pm
Location: Greece

Re: 6sn7 pre amp

Postby DeathRex » Thu Jan 23, 2014 9:05 am

To get good filtering, you can use 3 33uf caps or 2 100uf caps.
Using PSU II simulate for 2000ms after reporting a delay of 3 seconds to see the ripple.
On the power supply with 3 33uf caps I got a ripple of 600mv on C2 and 60mv on C3. A little too high for me.
On the power supply with 2 100uf caps i got a ripple of 130mv on C2. Still a little high.
Looking at Dick Olsher's power supply for the Blue Velvet, he is going to have very little ripple. You need very little ripple in a preamp.
To get the least amount ripple, use a regulator, tube or transistor. ICs are a little easier to use.

Here's one on ebay, seems to have most everything already:
http://www.ebay.com/itm/HV400-Variable- ... 338575e6dd
At first I wanted to be a ET. Now I are one.
User avatar
DeathRex
KT88
 
Posts: 1014
Joined: Wed Nov 18, 2009 8:13 pm
Location: Cortez, CO

Re: 6sn7 pre amp

Postby 6B4G » Thu Jan 23, 2014 11:57 am

Here's one on ebay, seems to have most everything already:
http://www.ebay.com/itm/HV400-Variable- ... 338575e6dd


Now that's something I can use LOL
The cost is much lower than I could build one and no stuffing around.
Thank you very much for that.
User avatar
6B4G
KT88
 
Posts: 287
Joined: Sun Jul 06, 2008 1:02 pm
Location: Greece


Return to tube 101

Who is online

Users browsing this forum: No registered users and 15 guests