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Scott 299c rectifier update.



 
 
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  #1  
Old March 3rd 05, 05:27 PM
Chad Wahls
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Default Scott 299c rectifier update.

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


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  #2  
Old March 4th 05, 05:41 AM
shiva
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"Chad Wahls" > wrote in message
...
> 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
>


Oh, jeesh, is your bias adjustment pot missing?
Have fun.
-dim (who's a bit curious why someone would take advice from strangers
rather than use a meter)


  #3  
Old March 4th 05, 05:50 AM
robert casey
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> 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!
>

Is this the bias rectifier only? As there is an adjustment pot,
the silicon diode giving you more voltage doesn't much matter
as the pot would be readjusted anyway. Higher negative bias
voltage will reduce quiescent current thru the tubes anyway,
so the danger is minimal if you didn't readjust.
  #4  
Old March 4th 05, 02:35 PM
Chad Wahls
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"robert casey" > wrote in message
k.net...
>
>> 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!
>>

> Is this the bias rectifier only? As there is an adjustment pot,
> the silicon diode giving you more voltage doesn't much matter
> as the pot would be readjusted anyway. Higher negative bias
> voltage will reduce quiescent current thru the tubes anyway,
> so the danger is minimal if you didn't readjust.


Done, did it last night, and the results came out good. No dropping resistor
needed. Should be done with the rebuild tonight, it is coming along quite
nicely and I can't wait to hear it.

This 299C is not even close to the schematics in the power supply arena, I
feel I have a real oddball. Makes it more fun to rebuild though.

Thanks for your help
Chad


  #5  
Old March 4th 05, 03:20 PM
John Byrns
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In article t>, robert
casey > wrote:

> Is this the bias rectifier only? As there is an adjustment pot,
> the silicon diode giving you more voltage doesn't much matter
> as the pot would be readjusted anyway. Higher negative bias
> voltage will reduce quiescent current thru the tubes anyway,
> so the danger is minimal if you didn't readjust.


It will increase the heater voltage somewhat, which IIRC Scott set at
about 10.0 volts per 12.6 volt tube heater.


Regards,

John Byrns


Surf my web pages at, http://users.rcn.com/jbyrns/
  #6  
Old March 4th 05, 03:35 PM
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!

  #7  
Old March 4th 05, 08:20 PM
shiva
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Default


"John Byrns" > wrote in message
...
> In article t>, robert
> casey > wrote:
>
> > Is this the bias rectifier only? As there is an adjustment pot,
> > the silicon diode giving you more voltage doesn't much matter
> > as the pot would be readjusted anyway. Higher negative bias
> > voltage will reduce quiescent current thru the tubes anyway,
> > so the danger is minimal if you didn't readjust.

>
> It will increase the heater voltage somewhat, which IIRC Scott set at
> about 10.0 volts per 12.6 volt tube heater.
>
>
> Regards,
>
> John Byrns
>


Hi John -
I can't remember who did the test, or how good the methodology was, but i
remember seeing lowest noise ratings for small signal toobs happening just
around 80% of recommended heater voltage - makes one wonder if all them
7005 regulators are a *good & useful thing*... (i was real happy when I
found out that 06 existed & were cheaper than dirt - now all those 'puter
PS's have one more part to donate!). It makes some sense, though...
OTOH, i'm amazed that no one said: Hey, them dc bal. pots are neat & all,
but how's about using a *bias pot!!!!*. The reason that bias was left
non -udjustable in most (fairly) high-quality consumer gear is, I think,
the sound notion that "if there's a pot, they'll spin it". That, and
(maybe?) higher consistency of toobs "back in the days", and a justification
for "use only Genuine replacement parts".
But there's no reason I can see to rely on a fixed resistor & keeping your
fingers crossed, instead of stickin' in a bias pot & a couple of 1-ohm
resistors at the cathodes of the OT tubes. Otherwise, the DC bal. pot
becomes ... another hum - buckin' pot?
-dim


  #8  
Old March 6th 05, 09:51 PM
Wbittle
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Default

Hi Chad,
You will need to install around a 5 ohm resistor right off the new
silicon bridge rectifier. This is to drop the voltage to 48 or lower
where it is attached to the preamp tube filament string. The bias
circuit in a 299C not only delivers negative voltage to the grids of the
four 7591A output tubes but 48 volts (original spec was 44 volts) to the
four 12AX7 preamp tubes who have their filaments in series. This allows
the tubes to have DC voltage on their filaments so as to reduce hum.
It is critical that you do NOT exceed 48 volts here. Since the amp
has both DC bias and balance adjustments, you will have enough
adjustment on the bias pots to take up a few volts either way.
The 299C will generally be biased correctly with around -21 volts at
the control grid of each 7591A. To check the actual bias correctly on a
299C, you will need to install a 10 ohm cathode resistor between pin 5
and ground of each 7591A and measure voltage drop across this resistor.
Spec is 35 ma per tube, so a voltage of .35 volt is what you want. You
can bias to .3 volt (or 30ma) and the amp will run much cooler with
almost no sonic effect.
Because this amp runs so hot and is so compact, you MUST replace all
the power supply capacitors as well as all the coupling capacitors.
Bill B.

Tube amp design, building, and service at:
http://home.alltel.net/wbittle1

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
>
>


 




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