[bigwig]

On 13 Dec, 18:49, John Byrns wrote:
In article ,





*bigwig wrote:
On 13 Dec, 07:52, "Edward Morris" wrote:
Hello all,
* * Thanks for all your help. *What a great bunch of people here! *Would you
recommend using a resistor between the red/yellow wire and the capacitor?

Edward

"bigwig" wrote in message

....
On 12 Dec, 22:00, Peter Wieck wrote:

On Dec 12, 4:04 pm, ":-:Ghost Chip:-:" wrote:

"Edward Morris" wrote in message

news

Hello,
I have a McIntosh C22 preamp. I'd like to replace the two diode B+
selenium rectifiers. One lead from the transformer attaches to the +
and -
leads together in the middle of the rectifiers and the other lead
attaches
to a 40mfd capacitor. One outside rectifier leg attaches to the bridge
rectifier that powers the filaments and the other leg ties into a
common
tie
with a number of resistors that tie into a number of capacitors.. Does
anyone have a ballpark guess what value of resistors I need for the
voltage
drop.

Thanks in advance,
Eddie

Refer to the schematic.
Replace SR1 and SR2 with 1n4007 diodes.
Replace SR3 with a 2 to 4amp 50V or higher diode bridge.- Hide quoted
text -

- Show quoted text -

Just for giggles, I would add ~10 ohms (or so, up to 20 ohms) of
resistance replacing single-element selenium diodes with silicon.
First, wall-plate voltages have climbed considerably here in the US in
recent years from a typical ~115V to as much as 125V as a steady-state
condition. Second, selenium diodes have a much greater voltage drop
than silicon diodes. So, a little bit of additional resistance makes
up for that reduced drop. Of course, one should check the loaded B+
after any diode replacement to make sure that it is well within
operating specifications anyway.

Others may have differing opinions.

Peter Wieck
Melrose Park, PA- Hide quoted text -

- Show quoted text -

Agreed,
* I would measure the HT with the selenium rectifiers in place (If
they still work). Then measure the curent in the red/yellow wire (be
carefull if you do this). Use a bit of good ole Ohms law to determine
what the equivalent load resistance on the supply is. Disconnect the
HT from the rest of the circuit. bodge up a resistor combination of
appropriate power rating and connect this to the now disconnected HT
(the letters HT should be enough warning) leave the heaters intact for
now as if you disconnect them the HT will rise. Now replace the
rectifiers with whatever you choose, as long as the voltage/current
ratings are ok. Measure the voltage, back to Ohms law again to work
out your series resistance needed.
* Thats the long winded way. I think Peters 10-20R ballpark figure
will be fine. There is another way to work it from the AC output from
the transformer taking into account diode drop, ripple current etc.
This is even more long winded and only really if you are designing
from scratch.
* I just looked at the diagram. Cant believe I just typed all that,
10R will be fine with IN4007s or anything else for that matter. The
only thing to watch is the heaters if you change to silicon.
Especially as you guys have a higher than designed for mains. In the
UK they were suposed to go down from 240V to 230V to get in with the
rest of Europe. My mains is 254V in the evening dropping to 250 or so
at "put the kettle on watch a crap soap time". I must admit that the
distribution transformer is at the end of my garden so thats the
reason. I work as a sparky by trade and I have seen as low as 200V in
some rural locations. Even seen one house with glowing line taps.
* *Matt.- Hide quoted text -

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I would stick it in series with SR1. SR2 doesnt really need the series
R.

Why would SR1 need a series R any more, or any less, than SR2? *The need, or
lack thereof is equal for both diodes. *If I were of a mind to use a series R I
would connect it in series with the "RED" wire from the PT where it connects to
the junction of SR1 & SR2.

There does seem to be a minor drawing error in the schematic where a short piece
of wire is missing.

--
Regards,

John Byrns

Surf my web pages at, *http://fmamradios.com/- Hide quoted text -

- Show quoted text -

oops yes both diodes. You could just put one in at the junction of the
two.