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    Building, troubleshooting and testing of these amplifiers should only be
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Solid-state replacements for PAS2/3 rectifiers?

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If you are going to keep the stock filter network, you can safely replace the tube rectifier with a pair of 1N4007 diodes, or preferably UF4007 diodes. It will raise the B+ voltage a few volts, but, unless the "can" capacitor is in poor condition because of age, there should be no problems. It will have the beneficial effect of reducing the load on the transformer heater winding. If you are concerned about the filter capacitor, it's easy to install a CL200 "inrush limiter" in series with the power transformer primary.
 
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If you are going to keep the stock filter network, you can safely replace the tube rectifier with a pair of 1N4007 diodes, or preferably UF4007 diodes. It will raise the B+ voltage a few volts, but, unless the "can" capacitor is in poor condition because of age, there should be no problems. It will have the beneficial effect of reducing the load on the transformer heater winding. If you are concerned about the filter capacitor, it's easy to install a CL200 "inrush limiter" in series with the power transformer primary


I was thinking of replacing the stock filter network(esp. capacitors), due to the age (40-50 yrs) of the components. Any suggestions for a better replacement for low noise? (Any schematics?)
 
I was thinking of replacing the stock filter network(esp. capacitors), due to the age (40-50 yrs) of the components. Any suggestions for a better replacement for low noise? (Any schematics?)

How extensively do you wish to revise the preamp? Are you thinking of just the power supply, or do you wish to upgrade other boards and components for better performance? There are many upgrade choices for this preamp. In the last week, I've just completed one that I find to be, very, very good.
 
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Just a line amp (from scratch)

I have an enclosure, but the boards and most other components are badly oxidized, due to exposure to the elements(the PAS was given to me). ;)

I was thinking of new line amp boards (any recommendations?), and a new power supply board (like Sheldon Stokes' board, which permits elimination of the selenium & tube rectifiers). Plus an upgraded volume pot.

Basically all I will reuse is the(cleaned)chassis and power transformer to build a tubed line control amplifier, which I will use to drive, alternately, an ST70 and a PS Audio power amp. My first foray into tubed preamps!

-Joe

p.s. Did I mention I'm on a tight budget?
 
There are many good boards and power supplies available for this preamp. If you are on a tight budget, though, it’s possible to save some money if you feel confident stuffing the boards and doing some of the fabricating yourself. The picture, attached. is an example of this. I fabricated the power supply board myself, using a piece of FR4, a few turrets and a few “standoffs”, and other normal PS items. The large capacitors, 330mfd/400VDC, are surplus from “All Electronics” at a cost of $2.50 ea.
The phono board is original, with some components changed. The line stage is a "Norman Koren” designed board, sold by “Classic Valve Design”, that I stuffed, and added a custom built tone control board, just visible in the lower part of the picture.
As I stated, I’m very, very, pleased with the sonic results of this particular pre-amp so far.
I have additional changes planned for this unit in the future, including replacing the current PS with an active regulator, replacing the volume control with a stepped attenuator, and a few other changes.
I’d recommend that anyone looking to rebuild/modify A PAS preamp should spend some time studying Koren’s website. I’ve included a link below to part 1, follow it to part 2:

Spice and the art of preamplifier design, Part 1
 

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Thanks!
Of course, I’m not trying to “show off” my work, here. There are many folks that produce much more “polished” looking results than I do. I’m again trying to illustrate the point that with a well designed circuit as starting point, a little thought, some planning, and patience, will achieve excellent results without spending “big bucks”. A preamp is a component where individual sections and parts (line stage, power supply, volume control, etc.) can be improved incrementally, as money and time allows.
 
Using modern day solid state rectifiers has a real advantage for relieving stress on the power transformer. The issue is the filaments of the tubes exceed the transformer's ratings.

The 12X4 alone draws something like 300 mA (as measured on my PAS 3). The power indicator bulb draws another 83 mA.

Getting rid of the 12X4 will essentially fix the over current issue.

Modern day solid state rectifiers are quiet enough now that you can go that route with confidence.

On the downside, you can end up pumping a lot work into the PAS to fix all the shortcomings and there are many. By the time you are done you could easily have invested that same money in building a superior DIY preamp.

The is the road I am now taking and I am doing as little to the PAS as possible while I scratch build something else.

There are a number of places that offer replacement boards and upgrades. Just search using the terms "PAS 3 Rebuild" and you will get an idea of what is out there.

You probably should check out prices for all the parts and see where it takes you.

Lastly, many might disagree, but it is far more economical to build a solid state preamp and you will get superior performance compared to a tube amp version.

Building a tube power amp makes far more sense than a tube preamp unless you are simply looking for a bigger challenge than a solid state design - and you will get just that.
 
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