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RE: Was: Re: PAS Transformer Replacement

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In Reply to: RE: Was: Re: PAS Transformer Replacement posted by sbalfour on March 20, 2011 at 17:49:25

Another way around all this mess is the minimum, to build an outboard filament supply on another chassis with an umbillocal cord connecting the filament supply to the preamp chassis or you could go one step even better and build a much better full power supply using chokes and caps for the filtering on a separate chassis and have the proper voltages with more than enough current to supply the preamp.

This could all be done very easily and it will look great. I have done this for many of my friends and clients and all were pleased with the results. You could even build a fully regulated HV B+ supply and a fully regulated filament supply on another chassis.

The probelm with any mod is room to put the new components and there is very little extra room on Dynaco amps and preamps.

Here are just two of the outboard power supplies I built with supplied chassis. These were both for Dynaco Pas projects.

Follow Ups:

RE: Was: Re: PAS Transformer Replacement - sbalfour 17:43:29 03/22/11 (2)
Wow! Nice iron, dood! When I started to build my own project amp,
I did exactly that - a separate outboard power supply. My stereo rack
space limits what I can do with the existing equipment, though. For
existing boxes, everything needs to fit in the box.

But I need to make a correction to my last post in the thread, adding up
the current draw on the PA211 12V winding. The 12ax7 spec says 150ma@12.6v
for each 12ax7, and we're drawing from a 12V winding, so current needs to
be rated at the winding voltage. There are 4 12ax7's, each drawing 150ma
at the winding voltage, and it doesn't matter series or parallel arrange-
ment in the circuit. So that's 600ma for the 12ax7 filaments. Plus 300ma@12v for the rectifier, and at least 300ma@12V for the lamp =
1200ma or 1.2A. And there's still the I^2*R losses to consider. These are all resistive loads and don't depend on signal, so the current is
RMS. That indeed looks grim for a tranny rated 800ma@12v. (Norm Koren's
figures that I referenced are also incorrect - yes even the high and mighty can have a bad hair day).

Now somebody check my numbers and tell me why we aren't all having a
meltdown?
Stuart
- RE: Was: Re: PAS Transformer Replacement - openmind 10:37:41 05/03/12 (0)
  Mr. Balfour's first set of calculations were correct; unfortunately the second statement that "series or parallel doesn't matter" is incorrect. Since the two tubes on each board are in series, the same current flows through both tubes.
  
  My understanding is that Dynaco intentionally ran the PAS at 11 volts per heater in order to reduce noise and increase tube life. If the current is linear with voltage, it becomes 137.5 per board or 275 mA for all the 12AX7 tubes together. That puts us at 875 total if the bulb uses 300 and the rectifier uses 300.
  
  This is still over 800. That said, Frank Van Alstine said getting rid of the bulb could improve the sound and I believe Dave Vorhis said the filament filter capacitance should be as large as possible.
  
  Since Dynaco made their own transformers and they are not notoriously unreliable, I suspect they did not skimp. Either the transformer is providing more than 800 or the preamp is using less than we think.
  
  This still doesn't solve the problem of poor spewer! If I find a solution I will append the thread.
- RE: Was: Re: PAS Transformer Replacement - xaudiomanx 05:28:42 03/23/11 (0)
  If the case is that the filament winding is really off, these trannies and maybe entire preamps should all be in the garbage. I still can't see the techs at the original Dynaco factory being grossly off to actually be in line for destruction.

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