M610 Microphone Pre schematic

[Winston OBoogie]

Anyway, it sounds as if the easiest route would be Winston's #2, a standard output trafo with resistor load and double HT.  That way, it sounds like I won't have to worry about the inductance of the primary? Should be a large-ish core? 

BTW, what kind of voltages/current should this thing see?

Oops, just saw this.

Maybe try a 9661 first, wired just like on the schem.  You could ask Cinemag what they think the approx L is at 7mA, or just try it and see if you like the low end.  If not, then go to the trouble of creating a higher H.T. for the output plate and cap couple it.

Most of these pre amps had about the same supply voltage and the 610 was no exception - 300V.  Don't remember the H.T. current draw off hand but probably in the region of 13 -14mA per channel. 

 

[mitsos]

Thanks John!

 

[Winston OBoogie]

As an update to the 610A schematic I drew and Mitsos was kind enough to post in the group Gmail account, here's a slightly different input stage arrangement.  This is from an earlier original 610 module. 
Refer to the schematic in Gmail for differences and, really sorry about the HUGE picture size!

 

[mitsos]

Awesome!! Thanks again! I'm hoping my workload will slow down (in a good way, not the dreaded economic downturn kind of way) so I can get one of these together soon!

btw, how's the output trafo coming along?

 

[mrclunk]

You guys get any further with the output transformer on this?

Did any one try the Cinemags?

I scored a UA 2-610 frontpanel off eb@y a would like to build the original 610 circuit into it.

I see from this thread the problematic part is the output tran, i'm going to email Mr Sowter to ask if he does anything suitable.
best
paul

 

[rodabod]

You'll just need to shop around for something suitable. Mr W.O'B has given all of the details which you need, and even if he had not, you can still calculate these from looking at the circuit.

Sowter should definitely be able to help, but keep in mind that you'll be paying a fair bit for a transformer like that unless you find something suitable second-hand.

 

[mrclunk]

Yeah thanks i've assimilated the info, even if some is over my head.

The closest i've found on the sowter site is:-
http://www.sowter.co.uk/specs/9705.htm
http://www.sowter.co.uk/specs/9040.htm

also found this in another thread.
Same transformer used in Urei 175 compressor
http://www.twin-x.com/groupdiy/albums/userpics/UA176-177-OutputXfmr.jpg

 

[rodabod]

The closest i've found on the sowter site is:-
http://www.sowter.co.uk/specs/9705.htm
http://www.sowter.co.uk/specs/9040.htm

Looks like you've possibly found some fairly decent subs with inductance in the right ballpark. Halving inductance is going to raise your -3dB point by ~ an octave since the inductance itself begins to look like a lower impedance load towards lower frequencies (and thus less LF "drops" onto your transformer where you want it to land).

 

[Biasrocks]

The closest i've found on the sowter site is:-
http://www.sowter.co.uk/specs/9705.htm
http://www.sowter.co.uk/specs/9040.htm

These are specified for single ended outputs, don't we need a transformer designed for push-pull in this circuit?

I'm interested in finding an equivalent for the Drip UA175B I'm building, which uses the same transformer as the 610.

Drip has specified the Sowter 8650e or 9990s, Hammond 1650g which will take DC on the primary but none of them
specify the inductance.

Mark

 

[mrclunk]

I'm not sure how to calculate the inductance myself, perhaps someone could point us in the right direction.
I'm googling...

I've found some old UTC/Triad catalogs which show several promising looking transformers but again no inductance. I suppose its pointless to list as it relates to several variables.

 

[rodabod]

You either measure the inductance, or find it on a spec. sheet.

From a quick glance, the output stage is not push-pull, assuming I am looking at the correct schematic.

 

[mrclunk]

Ok Thanks.
I think its a single ended output.
Lundahl has some good info on their transformers and help explain a few things.

http://www.lundahl.se/tubes.html
http://www.lundahl.se/pdfs/datash/1689.pdf
http://www.lundahl.se/pdfs/datash/1680.pdf
http://www.lundahl.se/pdfs/datash/1630.pdf

 

[CJ]

holy jeezus. was winston here or is that post date screwed up?
he was supposedto be in a paris shootin gallery, last time i checked...

 

[mitsos]

Just to add some more to this pile of info...
From:
http://www.groupdiy.com/index.php?topic=23825&highlight=175b

talking about the PA5946

[quote author="NewYorkDave"]the drawings I have indicate that it's a UTC A-19 run backwards.

correct
[/quote]

I think I've seen these on ebay before.. might be worth a try to get an "original."

 

[mrclunk]

Didn’t think about running Trans backwards...

I've been pouring over UTC catalogs and found several possible subs for the PA-5946.
Top of the list are:
LS-27 (Plate to line, Pri:15K  Sec:50,125/150,200,250,333,500/600. +23dBm, 8mA DC on Pri

A-25 (Similar specs to LS-27)

The output of this circuit also looks the same as the RCA BC-2b amp.
Although not the original the LS-27 has been used successfully in this circuit.
http://www.larryseyer.com/index.php?option=com_content&task=view&id=59&Itemid=39

Lundahl makes a replacement. (LL1680)http://www.lundahl.se/pdfs/datash/1680.pdf

Tons of info on the RCA BC-2b in the forum.
http://www.groupdiy.com/index.php?topic=33228.10
http://www.groupdiy.com/index.php?topic=26675.80

 

[mrclunk]

Recieved an email from Sowter today RE output transformers:-

I recommend our type 1160.  See:
http://www.sowter.co.uk/acatalog/SOWTER_TRANSFORMERS_OUTPUT_TRANSFORMERS_12.html#a503

This is a new development with improved bandwidth relative to our other types.  It has the advantage that you can configure the secondary windings to give a ratio of 10:1 or 5:1.  Obviously this gives you a choice of output level and output impedance.  It also means the load seen by the 12AY7 can be varied.  I suspect the sound is improved with a high anode load which would be achieved using the 10:1 configuration.

 

[mrclunk]

I've slowly been piecing together our 2-610. 
Just building one channel at the moment as the output transformers £100!
Once i've got it going i'll order the parts for channel 2.

Please could you guys cast an eye on my psu design.
Circut for my PSU.

Its a simple affair, my thinking is get it going first and then refine the supply later if neccesary.
Never 'designed' a valve supply before so your thoughts would be really appreciated. (Also, i'm too young to die!)
I've basically copied the the HT supply from NYD's MILA, the heater supplies from the PM660 and i forget where the phantoms from...

At the moment it'll only be powering one channel but for two channels should i have a separate crc stage feeding each channel for better separation?

Also i've re-draw a version of the 610 schematic but without the eq section. Think it's ok...?

I've some p2p layouts drawn up too, i'll post them once its up and running, bit scrappy at the mo.

best
Paul

 

[Kingston]

Nothing wrong with the PSU plan. Might as well build it like that already.

But some points of interest anyway:

1. no film caps needed for heater DC filtering, completely pointless there. They are just light bulbs. But since you're filtering already, might as well go straight to 4700-10000uF (16V is fine).

2. B+ filtering is a bit too under specified. It'll work, but it's like an assembly line design where they needed to save money and space everywhere. You don't need to care about that, and will be interested in high performance only, so might as well go straight to 100-220uF (non-critical). Adding another RC or even LC stage might be a good idea (just spread the 2.2k resistance to two stages). Hammond still makes cheap chokes that the guitar amp guys often use. Anything around 40-100mA, 10-20H, 200-1500ohm series resistance would do. Also, Landing at the exact 300VDC is non-critical. Won't really make a difference even if it ends up at only 250V.

That 2.2k might not be an optimal value, I'm too lazy to calculate the actual quiescent current draw, but just experiment until you land to a reasonable B+ voltage.

If you decide to use higher value caps for B+, use lower resistance draining resistor, like 100k. (is that a 220k or ohm on the schematic now, I can't tell what the letter is)

You could also take the preamp B+ 10uF stabs straight to 47uF, but these are all very non-critical.

 

[mrclunk]

thanks Kingston, really appreciate your comments.

Already bought the caps but i have some spare so i can parallel them up for better filtering.
I've the Duncan amps psu designer software at home so i'll add an extra RC stage and see what values it suggests. I like the idea of a choke too, i see most of the good old gear used them.

Almost finished wiring it all up, just have to wait for a 12ay7 to arrive and then we'll find out if the 610 schematic works...
Don't think anyones built this circuit yet so we shall see....  :-\

 

[mitsos]

I love this... thanks kingston for the breakdown... I'm itching to build one of these but haven't been able to splurge on an output trafo yet.. Most likely going Cinemag, unless someone can spare an A-19....

Good luck mrclunk, can't wait to hear about this!