Slow Blow pre?

[Bluzzi]

[quote author="DaxLiniere"]Not essential, but VERY highly recommended.
Do a little search of the forum for a post that I contributed to that had VU somewhere in the title.

NB: The 3k6 series resistor is IMPERATIVE![/quote]

Sorry Dax I did a search but there is a lot of VU mentioned out there.
What does the 3k6 resistor do?

jim

 

[mickey76]

the 3k6 story is explained here

Maybe this is useful as a buffer...

but we would have to find a way to power the opamps from the existing PSU or find some suitable opamps...

Is this really worth the effort?

 

[sismofyt]

not imo. vus look nice but aren't really usefull. you don't need to use a balanced buffer either. just take the signal before any output trannie if you're using that. powering? you only got one option and that is to regulate the P48 down to 24-36V and use a generic opamp. needs to be well filtered so that crap don't run back into the mics. but it's doable. don't try using the elevated heater psu though

the lot could be fitted onto a couple of small pcbs mounted behind the meters

but try the meters _without_ buffers to check if that 0,000000032% extra distortion bothers you. note that the majority of cool gear made in the last eighty years didn't use buffers ..

 

[ubxf]

Hello,
Is there a big difference between those tubes(12sn7 vs 12sn7gt)? and is halo getters vs rectangular getters any concern. Sorry for the newbie questions but i'm gathering the parts for this project and this came up.
francois

 

[mickey76]

the GT suffix stands for Glass Tubular (in usa tubes) and will most likely be smaller than the non GT. Pinout and specs should be identical although some people on the net believe GT tubes to be slightly inferior sometimes... I don't know if any of this is true for the 12sn7 though... so wait for a better informed response ;-)

the getters are used during production of the tube to burn up any remaining air for a perfect vacuum. I don't see how its shape would influence the sound of the tube since it doesn't serve any pupose after leaving the factory

[edit] I just talked to a friend and he disagrees... getters continue to withdraw gasses during the tubes operation and getters on the side or top of the tube do a better job than the more common bottom ones... so there you have it ;-)

 

[DaxLiniere]

Wow, you learn something every day!
In everyone's photos I've seen non-octal tubes, but when I looked up the 12SN7 all I could find are octal tubes.
And I assumed the GTs were a different beast.
The getters information was great too! Those are the kind of details I remember in life. People's names and birthdays.... that's another story. (A tale of shame....

Thankyou!

And thanks very much to Sismofyt for the trafo information. I don't know how to do any of those tests, but now I know where to start! THANKS!!

 

[sismofyt]

[quote author="ubxf"]Hello,
Is there a big difference between those tubes(12sn7 vs 12sn7gt)? and is halo getters vs rectangular getters any concern. Sorry for the newbie questions but i'm gathering the parts for this project and this came up.
francois[/quote]

not so much in my experience. i've found much bigger difs between 6J5s (half a 12SN7 with 6V heater) much fun in swapping 6J5s ('G's with the large ST-12 coke bottle are my favs) for 2C22s, 7193 or L63s

i prefer NOS american 12SN7s .. GT or not. RCA is fine and dandy. not expensive either

 

[sismofyt]

[quote author="DaxLiniere"]Wow, you learn something every day!
In everyone's photos I've seen non-octal tubes, but when I looked up the 12SN7 all I could find are octal tubes.
And I assumed the GTs were a different beast.
The getters information was great too! Those are the kind of details I remember in life. People's names and birthdays.... that's another story. (A tale of shame....

Thankyou!

And thanks very much to Sismofyt for the trafo information. I don't know how to do any of those tests, but now I know where to start! THANKS!![/quote]

octals are _the_ shit. the novals you see are either am ECC82 or an 5687 (fotis)

my name is ole frost berry - 201174, 186cm / 69kg, beautifull eyes and a twisted but friendly mind. my sister is not yet married and dinner is at six

let me know if you need better guiding in testing those jap babies

 

[DaxLiniere]

So how do you feel about 12SN7 octals for both gain stages? Is that going to colour the sound too much then? I assume the 5687 is "cleaner" in this circuit? I suppose two sockets could be wired in parallel for ease fo tube-swapping.

(Yes, I know the package doesn't determine the amount of colour :wink: )

 

[sismofyt]

[quote author="DaxLiniere"]So how do you feel about 12SN7 octals for both gain stages? Is that going to colour the sound too much then? I assume the 5687 is "cleaner" in this circuit? I suppose two sockets could be wired in parallel for ease fo tube-swapping.

(Yes, I know the package doesn't determine the amount of colour :wink: )[/quote]

i like to use different tubes / loadings (kondo-san style), but in this case i have good experience with using a 6SN7 for this job. i'd recommend using dual pots (just duplicate the exact stage) so you can push the second tube stage without excessive output levels. you can play with different tube loadings and / or cathode bypass on both tubes

the 5687 is not cleaner as such. it's the configuration that makes the colour not so much the tube. actually, it's _always_ how the tube is loaded, not what brand. hence my dislike for 100$ TFKs. i can get a two-dollar tube to sound much more satisfactory. to me that is

i prefer using an octal rather than the 5687 in this pre. fotis likes the 5687. great. this baby is highly configurable .. i guess that's why it's so popular

you could wire the sockets parallel, but you might run into oscillation. you probably need seperate gridstoppers, but it's doable. heating requiremetns are different (the 5678 is one thirsty sucker), but with a standard Vreg, i don't see any probs. just make sure the tubes draw the same amount of current, 15mA'ish

not to cofuse or throw in too many choices, but just about any tube with a medium to high mu can be used here. you got thousands to choose from

 

[ubxf]

Thanks mickey76 and sismofyt, i got my tubes now i still need the power transformer and the choke. So here is the next newbie question is there an
noticable difference between different brands of choke?
Do i need to get a Lundahl or a generic from Digikey would do ?
francois

 

[ubxf]

I found some 12sx7 can i just use them in the place of the 12sn7 or do i need to adjust the value of some components.
Thanks
francois

 

[sismofyt]

[quote author="ubxf"]question is there an noticable difference between different brands of choke?
Do i need to get a Lundahl or a generic from Digikey would do ?
francois[/quote]

yes there is if it's not up to spec. it might even blow up. as long as it's around 10H, can take at least 100mA and have a DCR around 100-200 ohm all is fine. this is (or was in the dayz) a very common type of choke for tube gear. you can find great fancy potted ones at ebay or surplus shacks. this is good place:

http://www.surplussales.com/

since you're in the us, you can get a cheap hammond. a 159P is what you need, 25$ at mouser.com (allthough not stocked, but you can easily find it in the us)

i have not tried the lundahl chokes. i know another builder have used them with good result. they're quite expensive though. i would think you should use the LL1673:

http://www.lundahl.se/pdfs/datash/1673.pdf

it's better to use this one than the LL1638 because it has a too low DCR resistance with those big caps

85$ at k&k:

http://www.kandkaudio.com/prices.html

eta:

the sowter CB14 would also do nicely and easy to get for people in europe:

http://www.sowter.co.uk/

40£ online

 

[sismofyt]

[quote author="ubxf"]I found some 12sx7 can i just use them in the place of the 12sn7 or do i need to adjust the value of some components.
Thanks
francois[/quote]

nope, no adjustments. you got lucky:

http://www.diyaudio.com/forums/showthread.php?s=71b8446e02d2c7e71ea415142124bed9&threadid=74134&highlight=

actually, you could be so bold to try AC heating. with proper p2p you can get a very low hum level especially with those SXs. elevate and decouple as usual. you might even like the sound better

 

[Flatpicker]

[quote author="sismofyt"]http://www.lundahl.se/pdfs/datash/1673.pdf

it's better to use this one than the LL1638 because it has a too low DCR resistance with those big caps...[/quote]
Well, that's cool cause I actually have one of those. :thumb:

 

[mikeyB]

[quote author="FotisGR"]I have for sale some quality toroid trafos and chokes for this project if someone wants to buy.

The trafos are multivoltage (250VAC , 50VAC and 13VAC) 150VA enough for 10 channels psu and the chokes are 10H/150mA - 65Ohm.
These are that i use on the pictures of my project.
The prices are 45Euros for main trafo and 25Euros for each choke.
If i can remember from Jazzy's order the delivery cost was about 10Euros (from Greece to France).

I will come back later...
fotis[/quote]

Have you still got these to sell, whats the secondary currents, can I pay by PAYPAL?
Also Fotis - did you continue with single channel version of G9 - psu looked similar to this from what I can remember!

 

[mikeyB]

Sorry Fotis - forgot to ask for transformer and choke dimensions. I am interested in both the 80 and 150VA transformers.

 

[sismofyt]

[quote author="Flatpicker"]Well, that's cool cause I actually have one of those. :thumb:[/quote]

super. they look neat:

 

[sismofyt]

ok, it's up. still not finished the schematics. this week. promise. anyway, there's a good bit of reading. enjoy

http://www.sismofyt.net/slowblow.html

 

[Flundran]

Absolutely great. Many thanks, sismofyt :thumb: