Slow Blow pre?

[tommypiper]

Yes, I too, have been wondering where to find a (final or reliable) version of the schematic. There seemed to be a lot of futsing and deliberate mystery about the details.

Has anyone in North America built one of these?

 

[Bluzzi]

I also was very into making one but I think the powers that be just don't want it out.

So I went on and 'am just as happy. I wouldn't want to take on a project that seems to be laced with confusion and vagueness.

Its too bad because tube projects are rare.

But there are some great mike pres now, API, 990 and NEVE styles. We are so spoiled!

jim

 

[gary o]

I thinks there 2 schematics within this thread so a point to point would be easy im a little confused about the massive power supply..tho I think it will from a standard tbe PSU.

 

[Bluzzi]

Again wondering if anyone else has built it?

Jim

 

[penquista]

i built a total of 5 units.
2 classic slowblows using ll1538xl, 2 of another version using 12sx7 and 6n6p, input tx is oep. they are in a 3U case with external psu. at the moment they are still in a provisory case but the frontplates are near completion. see attached pic.

the last one has 2 6sn7 SE stages (i can switch off one of the stages for less gain) into one 6n6p srrp.

i love all of them.

peace

christian

 

[mikeyB]

Hello Christian - they look nice - any chance of inside pics to get some point to pointers on the layout/wiring

With thanks in advance!

 

[Bluzzi]

Christian: Nice job! "Why build 1 when you can get 4 for the price of 1!". Inside would be nice! What connector did you use to pass voltages? Did you use a buffer for the VU meters?

From all the past discussions I think I want to try a dual channel one with 12SN7 using Fotis power supply. Anyone else that is into this? We could help each other out on it.

Jim

 

[FotisGR]

hi, im back... ;D
and i am happy to meet new and old friends...

 

[Bluzzi]

Fotis, so good to see you are still here! Thanks for stepping in. Now I can start to bug you with my silly questions!

On the power supply drawing you state to duplicate the TL783 circuit for channel 2 for the 48v part of the supply. What is the reason behind having to duplicate the circuit rather than feeding both channels from 1 source? Isolation?

Jim

ps: Do I have your permission to post your power supply schematic here?

 

[michal_k]

Hi, I think I'd be into building one as soon as I pass the rest of my exams

 

[DaxLiniere]

You know what? I'm up for another crack at this. A few years ago I bought half the parts, but never got around to buying the rest.
Alright guys! Let's do it!!

BTW, I couldn't find a correct PSU transformer anywhere, this was a major stumbling block for me. Chokes were very expensive too and not really the correct type even then.
Correct me if I'm wrong, but doesn't Fotis have a cousin who winds custom transformers?

Also, can I make a suggestion that Fotis' PSU is PCB based and some tube enthusiasts think this is not the best way.

 

[Bluzzi]

Dax, so glad you are back on this too. Of all the gear I have built my LA2A has that "magic" "je ne sais quoi". So this preamp intrigues me as its tube too.

I started to look at parts cost for the Power Supply and the 4700uf/400v are not fun to price! So from that perspective it already looks like this may be the most expensive DIY preamp on Prodigy!

The choke I found is Hammond one and its big and somewhat costly.

I am wondering if the 4700uf cannot be subbed?

I can try and find Power Transformer too. Are you 220vac down under?

Jim

 

[DaxLiniere]

yeah, the hammond ones are the ones I was looking at, but they're not that cheap over here.

Yes, we're on 240v (but 220v is fine)

 

[michal_k]

are you guys sure, that much cheaper, regulated PSU would spoil the sound of this pre? Maybe it's worth trying it with g9 style power supply before spending big bucks on choke and caps?

 

[DaxLiniere]

Yes, I believe so.

Making it with cheap PSU doesn't really make sense either. Cos if you do that, listen and hate it, then you'll never build the proper PSU.


BTW, looks like congratulations are in order for Sismofyt/Shalimar/Slowie/SlowBlow/OleFrostBerry (any more pseudonyms?!? )

See here: http://www.sismofyt.net/

 

[Bluzzi]

I'm open to all suggestions but I think we should listen to the guys that have built some of these already.

Jim

 

[michal_k]

I'm open to all suggestions but I think we should listen to the guys that have built some of these already.

Jim

sure thing, it was just an idea.

 

[Bluzzi]

A good idea but everyone thats made it says the PS has to be hefty! It would take someone to build one with hefty and adequate supplies and try both to see.

Any ideas on where to get 4700uf/400 electrolytics cheaper than $80.00 each? Price comes down after qty 10 (group buy?).

Jim

 

[michal_k]

hmm, just checked local suppliers and they're really hard to get. Found those instead:
http://allegro.pl/item667747708_1000uf_400v_105_c_40x60_snap_jamicon_hs_u.html

they're about 5eu each, connect 5 parallel and you're set for 25eu < 80 USD

or another crazy idea:
http://allegro.pl/item664607117_1000uf_250v_85_c_30x45_snap_samwha_hc.html
slightly more than 1eu, two in series for proper voltage x 9 for 4500uf = ~18eu, and imagine how cool it's gonna look  

ps. just noticed that there are 1000uf/400V caps on my PSU schematic  ???

jj-electronics does 940uf cap, they're excellent quality:
http://www.jj-electronic.com/pdf/MNH.pdf

I can also ask for an estimate on power supply transformer. It's about 85W so it should cost less than 25-30eu in less than 10 quantity.

 

[DaxLiniere]

Yes, I think group-buy is the solution here.

I'm definitely in for caps and trafo and chokes.

Incidentally, here are my construction notes from years ago:
(Please be aware that it's very messy and thoughts are all over the place)


********
Make a cage using copper shim sheet to go around the P2P components for extra shielding

Idea: use 2 gain pots - one for output gain and one inbetween the 2 gain stages (and maybe a 3rd stage as a pad at input?)

audio IN cap by Jensen PIO (1uf)
audio OUT cap By AURICAP (10uf)
cathode caps Black gate FK with Jensen PIO

"that there has been a typo in the 48V power supply drawing, if I remember correctly"
yes. reversed biased diode. stupid mistake. i did that one, i think

Ditch off the line-input & associated switch

B+ should be near 400V (350-360v is good)

Need chokes and PSU toroid
4x 470uF, 400V good quality caps (for PSU) - NOTE: should probably be 220uF, see below
HV regulator (TL783?)
PSU bleeders?

note, that the currently proposed 12V heater Vreg will _NOT_ power more than two channels though. actually, i'm thinking about ditching that Vreg for a 13V zener + 2N3055 combo. that'll drive eight channels without any mods ... hmm .. cheaper too than getting a 3-6A Vreg

I played also with cathode's cap and there was a big sonic diference. The best for my ears was a quite big 470uf/25V Black gate in parallel with a 1uf Jensen PIO. More capacitance = more fatness and a rise to mid-bass frequencies like harmonic enhancer. I think the balance will be 100-220uf but at 470uf my TLM103 sounds like a good tube mic

odd, jazzy liked the 12SN7 _with_ the cathode cap as well. i generally do not like it. the bass tends to get to phat and uncontrolled. but it's a nice sonic variation i guess. one could just throw in a switch. (Dax Q: What is cathode cap?)

Jazzy_Pidjay - Asymptot:
Mine has some differents features:

- 2k2 / 25w Bleeder, automaticaly off by relay when power up. Discharging the caps faster and the B+ dont stay in the cold tubes after power off.
"perhaps shouldn´t use a relay-switched bleeder to discharge the HV caps after power-off, but better use permanent bleeders (one for each HV cap), and that the heater elevation serves as a further bleeder"
yeah, forget that relay thingy. use low resistance (50k) high wattage bleeders one for each cap

and some like Fotis

- Retarded B+ by timer + relay
- Slow start 12v for heater

Chokes seem to hold some form of 'Magic Box' status, but there is nothing magic about them at all. They are simply a winding of wire on a magnetic former. If placed close to power transformers they can increse hum, not reduce it. Their laminations should always be at right angles, to the laminations of any transformer near them, to reduce magnetic coupling. A popular misconception about chokes is that they usually have to carry heavy currents.

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