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Marty Dippel
 
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On Thu, 03 Mar 2005 11:27:12 -0600, Chad Wahls wrote:

I have a Scott 299C that I am getting ready to recap and do a general
restore as it is getting noisy and amazingly has all the original caps.

This is in a listening system so I'm gathering all my eggs before proceeding
because I don't want to thake the system down for long.

I am going to replace the selinium rectifier with silicon, do I need to put
a dropping resistor in to compensate for the efficiency of the silicon or
should I be fine with a direct replacement? BTW this one has bias and
balance for each pair of 7591's much like the early 299D, seems I have a
weird one!

I know I could experiment but surely someone has done this many of times and
has a value needed if any.

Any help would be greatly appreciated.

Chad



I have a 299 (looks like a late model, actually more similar to the 299B
than the 299) and I've gone through it, doing the same thing you're doing
to your 299C.

With the selenium rect. still in place, the voltages on the various stages
it supplied read somewhat less than the values indicated on the schematic.
I replaced it with a silicon FW bridge and again measured the voltages,
which are now almost exactly what the schematic indicates.

Why this should be so remains a mystery:

1. The selenium rectifier has considerably more drop than does silicon.
I'd expect to see another 20V greater output, and that's about what I
saw.

2. The schematic was produced in an era when the line voltage here in the
US was closer to 115V than the 125V I'm measuring currently. So I'd
expect to see higher voltages from the selenium rects than the
schematic indicates- -but instead they were lower, as I indicated.

Could be the selenium was degenerating, allowing more reverse current now
than the day it was made, possibly contributing to the lower voltages that
I was seeing- but then I'd expect Silicon rectifiers to produce a MUCH
greater B+ -and instead I found it to be within a volt or two of the
values on my schematic.

Your results may vary, of course. If it was my amp, I'd replace the
selenium and bring it up on a variac to see what you're getting.

Ignoring the fine points, I'd say that as all the components
(tubes, caps) are operating at or below their ratings, you'll probably be
OK. It's always a fine idea to go through and measure voltages, Q points,
etc. just to prove to yourself that each stage is operating normally.


You can find schematics on the vintage H.H. Scott pages, at
http://www.hhscott.com/database/vhhs_00011.html



This is a fine amp indeed. I really like the idea of having controls for
EVERYTHING: DC bias, DC balance, and AC balance.


I used axial lead caps for mine- but since then, FP "cans" are again
available. One of the FP units in mine (yours, too?) has a POSITIVE
COMMON can - I know of no way to replace this except by using axial lead
caps.

Enjoy your Scott, Chad! It's one of the finest amps from that era!