V 72 Stuff

[rotation]

Hi!

I am thinking of diying V72 preamp. I would buy choke from Sowter, but can not find transformers.
Do i really have to get original transformers? Is there anything similar at lover price than Sowters ouput at around 80 euros?
What about the input transformer? I dont find it nowhere..

Thanks in advance!
Miha

 

[CJ]

Anything but original will sound different due to lams, aging, etc.

That in mind, why not try anything?

I went with a K-241-D inny and HS 50 outtie.

Then found out this is exact pultec iron, so i have a Pul-a Funken 72.
Works fo yo.

 

[rotation]

Well, i know it won't be like original this way.
I'v read several times that V72 is very very good preamp, like Rolls Royce for cars. Do you think it is worth to give it a try with Sowter's choke and good in and out iron? Or do something else where choke is not needed?
If you can suggest something else for diying, pelase do so. What i want to do this time is high end, old school preamp with not more than 45dB of gain, without any filters.
Thanks for help!
Miha

 

[CJ]

it goes togeter in an afternoon, and all the feedback means yiou can use any cap you like, and it won't change.

The original uses a Bosch distributor condenser for one of the caps,
go figure.

 

[helterbelter]

[quote author="CJ"]
I will have to knock the voltage down a bit, since therer is going to be less drop with the solid state rectifiers.
[/quote]

A friend of mine suggested to put a few 1N5408 diodes in series to get (about) the same voltage drop. But ehhh... How big is the voltage drop of the original rectifier ?

 

[analag]

[quote author="guavatone"]Good one CJ. A While back Ollie(i think) told me he wouldn't go higher than 10uF on those PS caps. -I couldn't see why, but I didn't feel like questioning God.[/quote]

Ollie is smart, he knows going too far beyond 8uF affects the unit. I confirmed this through simulation.

 

[CJ]

The output section has a choke, so its a pi filter, with the cap between the choke and output.
So you are changing the corner freq with a different value.
I doubt the filter cap on V1 makes much diff.

 

[madriaanse]

so eh CJ, did I hear you say you're gonna manufacture those transormers and choke and sell 'em, no? :green:

M.

 

[CJ]

They still sell the one piece lams?
Even the pwr trans has one piece lams, less 60 cycle into space from those sharp cornered EI lams.

 

[edanderson]

They still sell the one piece lams?

yeah but good luck in getting them in anything other than steel. they're called M lams. you can try markus over at tubeland in germany (tubeland at tubeland.de). he would probably sell you some lams and bobbins.

ed

 

[jensenmann]

CJ, whats going on? Did you recieve my mail/pm?


:sam:

 

[CJ]

I have 25 PM's, I am trying to work up the nerve...
Evrything is fine, back at it full force, i will have my internet connection back full time tomorrow.

Had to do some work on the coil winder to get it to wind back and forth automatically, so that I could wind your stuff quicker and better.

I had to pay off all my debts, but now I am rollin in the green.
So full speed ahead, look for NYD passive coils and other goodys in the future, API outies and that stuff.

Rebuilt the LDR matcher for Kubi, so that was the stall out the stereo T4's.

I was also gone for ten days on a 3000 mile backroad extravaganza up in Eastern Oregon.

Steens Mountain is insanely beautiful.

Sorry again for the hang ups, guys.

 

[jensenmann]

:thumb:

good to see you here

:guinness: :guinness: :guinness:

 

[martthie_08]

umm, sorry to be kind of misusing this thread, I just happen to have a V72 on the bench, maybe one of you tube experts can help me out? The V72 (not "a") came in with supposedly busted input iron, so I suggested the owner to let me make a HI-Z DI out of it, as that input transformer is getting seriously hard to get (repaired) and there is tons of other Siemens preamps in the studio. After I checked the unit through it turned out that the transformer was OK after all, it was just some broken solder joints in the feedback between the two stages and some heavy oxidations. :thumb:

I'd still like to add an instrument input with a switched input jack which bypassed the input transformer. Space is tight in there but I can make it fit in place of the power on indicator lamp. Now the problem is I don't really have a clue on how to wire this up.

As a quick test I detached the input tranny completely and put the input signal straight onto the grid of the first ef804s with the 400K (reads 425K / measures 446K) grid leak to ground. A short test with my Rickenbacker was rather underwhelming: tons of noise (ok, cassette cover was not in place), very little gain, almost no top end. Can anyone point me in the right direction?

 

[ioaudio]

you need to add a cap for your di input, look at gyrafs g9 schematic.
maybe better if the cap is only in-circuit when the di is used and out when the transformer is connected.
-max

 

[CJ]

thats gonna be tite with that tuschel conn in there.
the jack is the hardest thing to solder, unless you gut the pwr supply.
:twisted:

if you rack it, you will have tons of space for wiring up an aux hi z input.
(di)

 

[martthie_08]

Max, thanks for your reply, I had a quick look at the G9 and tried a 0,22uF cap in the signal path before the grid leak resistor - things are better, but still too much "waterfall" noise and only little gain. Next thing would be to try a 1M grid leak resistor this should bring the level up a notch as well. I never had the V72 on with the cover removed, maybe things will get better if I put it back on, will see tomorrow.

CJ, The PSU will stay as it is, my plan was to mount the jack on the front in place of the lamp is, next to the output trafo. It is really tight in there, I'll take some pics when I get it done!

thanks, Marten

 

[jensenmann]

wasn´t the lamp for voltage stabilizing or was it in the V72a?

 

[ioaudio]

good call, but it´s the -a version.
this version uses the heater.

http://www.irt.de/IRT/publikationen/braunbuch/V72.PDF

http://www.irt.de/IRT/publikationen/braunbuch/V72A.PDF

 

[martthie_08]

ok, I can't get the HI-Z input working properly, I'll come back to this when I understand a bit more about tube circuits.

Once I got the the V72 back in the casing it started acting up again, might be a loose connection so I'll have to do some error tracing. I started with the power supply (all caps original), the B+ seems fine with voltages at 346VDC after the bridge rectifier, 324VDC after R26, 301VDC after R27, 283VDC after R25 and 265VDC after R13.

One thing boggling my mind: The factory installed caps have metal pins which go through the plastic mounting board and are then turned to physically hold the caps in place. (see pic below) Question: are these also the electrical connectors for the caps? I am asking because there are no wires on the "-" side and there is no continuity to 0V.

Anyways I am thinking about a recap for the PSU, shouldn't hurt. CJ, I remember you posted some nice pics of Jens' V72 PSU rebuild, is there any chance you could get these back up again for me to see? If you need webspace let me know, my .mac webspace got expanded last week to 10 gig so space is no issue!