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Was: Re: PAS Transformer Replacement
|Posted on March 20, 2011 at 17:49:25|
In a post on Feb. 1, 2009 titled "PAS Transformer Replacement" (I'm not
going to revive a 2-year old thread), it was suggested that the Hammond
290AX transformer might be a usable high-quality replacement/upgrade for
the Dynaco PAS2/3 PA211 power transformer. The specs on the 290AX (the
real one, from Hammond, not the knock-offs) are:
Secondaries:325-0-325@81mA for B+,
6.3V@2A for filaments,
5V@2A for rectifier tube
Weigh: 2.85 pounds.
It's a drop-thru, meant to sit in a hole in the chassis, and be wired
from underneath. It can be made to fit in the unmodified chassis with a
custom pop-up bracket with 4 bolt holes that screws into the original
mounting holes, but you'll have to ditch the selenium rectifier for a
pair of diodes to make room.
That said, the physical mounting isn't the main issue, contrary to the
original post. If you're replacing the tranny because the old one is
dead, one of the drop in replacements from specialty Dynaco
parts houses like Triode will probably serve you, though there's some
mixed rap on the Triodes. If you are looking for a beefier tranny
because the filament current is sagging under load (a common observation)
you may want to consider the following first:
- replace the pilot lamp (240ma - 400ma draw) with an LED (30ma-50ma)
- replace the selenium rectifier (it drops 1.0-1.5V, consuming 100ma
under load) with a pair of fast recovery 3A rectifier diodes
- definitely replace those old 2000uf filament filter capacitors (may
have ESR of several ohms or more) with new low-ESR 3300uf 35V
- replace the power supply filter can with a new 20/20/20/20uf or
30/20/20/10uf can. This isn't in the filament current path,
but if the preamp is sagging under load, it may be drawing excess
current in the B+ path, and sucking it out of the 12V path due
to ESR in the big caps. Even better for a little more money,
there's a 80/40/30/20uf@525V can for $75.
- bypass the PS filter caps with 0.02uf 630V MKP film caps (because of
the wound construction of electrolytics, they have increasing R at
- check all the capacitors (don't bother with the pF caps) and replace
any that are out of spec by more than 50% or have ESR more than a few
tenths of an ohm (I know, more work than you want to do). Use PP
film caps rated at least 450V.
If your tranny is actually alive, 80% of you won't need a new one if
you do these. If you are among the 20% who are still having a problem,
the alternatives are problemmatical.
Sellers on Ebay sell Chinese-made toroid transformers as "drop-in"
replacements for PA211s. Not so fast - you'll need soft-start circuitry
to protect against inrush current 40 times greater than that for an
upright transformer. They also run just on the edge of saturation.
When they do saturate, they generate all kinds of nasty high amplitude
high frequency artifacts that can destroy your tubes. You need a filter
to block these, and a clamping circuit to avoid current saturation
that'll overheat the transformer and cause a meltdown. In short, to
use these, you need a regulated power supply. The sellers don't tell you
That brings us back to the Hammond 290AX. The filament supply on the
PA211 is 12V@800ma, very wimpy. The filament supply on the Hammond
290AX is only 6.3V@2A - 12V@1.05A if we can step up the voltage easily.
It already looks dubious, because that's only an extra 250ma over the
PA211 800ma, and we haven't considered the I^2*R losses in the voltage
step-up circuit. You'll need a pair of big caps and a pair of diodes,
just like what's already there, and a small .4ohm (or .39ohm) 2W resistor
to drop the resulting 12.6V to 11V or so, the level output by the original
selenium rectifier. The current passes through the additional series
cap with ESR probably in the .1ohm range, so the loss is 2A*2A*.1ohm=.4W.
The resistor is in series with the secondary, and the loss there is .8V*
2A=1.6W, total 2.0W. At 12V output, that's a parasitic loss of 167ma
from the circuit to use the 290AX's 6.3V filament winding. That means
we're only getting 250ma - 167ma = 83ma of additional current for the circuit out of this tranny.
The 290AX B+ current at 81ma is >8 times that of the PA211, but B+ current
wasn't the problem with the design of the PA211. The tranny runs 4 12ax7
@1.2mA plate current = 4.8mA plus rectifier tube and parasitic losses,
so we're probably covered with 10ma of the PA211, but a little more
headroom might be needed under load. I note that the replacement PA211
drop-ins from Triode are rated 660V@15ma, quite good enough.
The 2009 thread left the impression that the 290AX was a "better", i.e.
upgrade transformer. It IS better quality, but it won't solve the problem
of insufficient current rating for the filament winding, particularly
if you've modded the box with cathode followers or anything else that
consumes more filament current.
Most other alternative trannys will have the same issue with filament
winding at 6.3V, because triodes and most other tubes have 6.3V heaters.
Dynaco for some strange reason, wired the 12ax7 triode pairs in series,
then the tubes in series, so requiring a total voltage of 25.2, though
they dropped the winding voltage a bit to give the tubes longer life.
The problem that the further from source a series triode is, the more
it'll get staved for current under load is an issue for a re-design.
|RE: Was: Re: PAS Transformer Replacement, posted on March 21, 2011 at 17:49:01|
Joined: August 16, 2004
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.
|RE: Was: Re: PAS Transformer Replacement, posted on March 21, 2011 at 19:31:40|
Location: Central New Jersey
Joined: September 8, 2007
Does anyone know the real specifications from Dynaco on the original PAS-2/3 PA-211 power transformer, the reason I am asking is because I read the filament rating is 800ma but if you add up the filament current of the four 12AX7 tubes, 12X4 tube and pilot lamp, it adds up to over 1 amp.
Is the 800ma rating really a Dynaco specification or just a myth?
|RE: Was: Re: PAS Transformer Replacement, posted on March 22, 2011 at 02:34:10|
Joined: August 16, 2004
I would assume Joe or Bill would know the actual specs. You could contact Roy Mottran from Tubes4hifi.com, Triode Electronics, or Van Alstine. These are the guys that have been doing Dynaco forever.
I know Dynaco works on the cheap and unless there is something wrong with the math in calculating the full current draw I can't imagine Dynaco putting out a product that is destined to fail. Even on the cheap.
You do realize that the 12AX7's are in series in both the linestage and the phono sections. What does that do to the current draw in each section? Then they are paralleled back together. When doing th math along with adding in the 6x4 I get 1.2A current draw. So if the power tranny filament is only rated at 800MA you are way over spec.
In looking at the tube specs the 12X4 is only drawing 300MA(.3A). It seems that when adding up the current draw using a 12X4 then that is a much closer match to a 800MA filament winding.
Could all this have something to do with the voltage doubler? I know the doubler is not used for the rectifier.
|RE: Was: Re: PAS Transformer Replacement, posted on March 22, 2011 at 10:04:59|
The line and phono sections are in parallel, otherwise we'd need voltage
of 48V, not 24v. The triodes of each tube are in series; the tubes in
each section are also in series. From the 12ax7 spec, series wiring the
tube draws firstname.lastname@example.org (~10.5v, after the selenium rectifier). Series
divides the voltage; parallel divides the current. At 24V from the
voltage doubler, each section sees 24v divided between two tubes, so
each tube sees 12v. Each tube draws 150ma@12V, so each section draws
150ma@24V. The two sections in parallel draw 300ma.
The 12x4 draws 300ma@12v. The #57 12v 5W bulb draws ~400ma, less as it
ages. Let's count 300ma.
So I get:
300ma pilot bulb
That's not the whole story, because we have I^2*R losses in the voltage
doubler and selenium rectifier as well as the rest of the circuit. The
ESR in those old 2000uF filament filter caps could be a few ohms, so
we're losing 50-100ma right there.
Since heater current doesn't vary with load (we hope), it looks to me
like we're over spec @800ma on a constant basis.
See Norm Korens figures for this at
http://www.classicvalve.ca/docs/PAS-K_phonoboard_docs.pdf. (Scroll down
to near the bottom). He's an old hand at this stuff, too.
The HV winding isn't much better off: each section of 12ax7 (from the
spec) "typically" operates at a draw of 1.2ma. There are 4 tubes, 8
triodes, so the "typical" draw is 9.6ma. I think I miscounted in my
original post, and said it was only 4.8ma, oops! But maximum dissipation
at the plate is 1.0W@300V = 3.3ma (B+ isn't 300V, but more like 265-280V
at the plate for PAS). There are 8 sections, so under full load (250mv
square wave input at phono, at maximum gain), the draw /could/ be 26.4ma.
Of course,the voltage would sag badly, and the dissipation drop sharply
if you did that for longer than a transient (the PS caps will support
the voltage for brief transients). The HV winding is rated at 10ma reportedly.
The secondary windings aren't independent - they're on the same piece of
iron, and share the magnetic flux. If you overdraw one, it'll suck juice
out of the other(s). If you overload the HV winding, it'll draw down the
heaters. And that is where we're at, with the PA211.
|RE: Was: Re: PAS Transformer Replacement, posted on March 22, 2011 at 16:56:20|
Joined: August 16, 2004
That is exactly the reason I said to build an outboard power supply using substantial iron and much better filtering using much better quality parts. But on the other hand, I can't imagine that these engineers that originally designed this stuff, designed it to fail or not perform, unless we are both missing something.
|RE: Was: Re: PAS Transformer Replacement, posted on March 22, 2011 at 17:43:29|
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 email@example.com
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
|RE: Was: Re: PAS Transformer Replacement, posted on March 23, 2011 at 05:28:42|
Joined: August 16, 2004
|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.|
|RE: Was: Re: PAS Transformer Replacement, posted on March 23, 2011 at 07:53:22|
Location: Central New Jersey
Joined: September 8, 2007
Thanks for the info guys.
|RE: Was: Re: PAS Transformer Replacement, posted on April 19, 2011 at 23:11:44|
All of these figures are of great help, but do not solve the problem. The point of the thread was to find an ALTERNATIVE to the Triode. My experience with their iron has been documented elsewhere. If the 290 is not adequate what would would REALLY help would be to then find another alternative. It sounds like you have a great deal of knowledge that would be extremely useful in solving that problem. I, for one, would opt for Audiomanx's solution before throwing money into another Triode.
|RE: Was: Re: PAS Transformer Replacement, posted on May 3, 2012 at 10:37:41|
Joined: November 8, 2008
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, posted on May 8, 2012 at 10:28:33|
Joined: November 8, 2008
I have no information on the quality of the unit but there is a drop-in replacement also available from dynakitparts:
330-0-330 vac secondary @ 15 mA DC
10.9 vac secondary @ 1.25 A
Study of current Hammond offerings shows an alternative; I don't have a PAS on hand to measure but it might be usable. FWIW Van Alstine mounts the transformers outside their SuperPas. Here's the Hammond data:
117V primary 50/60 HZ
650V C.T. @ 46ma.
5V @ 2A (I would use this for the pilot lamp or LED)
6.3V @ 2A (Note: Filament voltage on PAS is 11 V DC)
Z6 3.0" high 2.5" wide 2.5" deep
Mouser part 546-273AZ $68 from mouser
Digikey part 273AZ-ND $61 from Digikey
|For the archives, posted on April 7, 2022 at 04:51:07|
Location: Greenville SC
Joined: February 25, 2007
For those searching these Dynaco Asylum archives for information, as I often do, the above post is incorrect.
Per the original PAS manual, the Dynaco PAS used a #53 pilot lamp, NOT a #57. The data sheet for the #53 lamp indicates a current draw of 0.12 A, 60% less than the current draw stated incorrectly in the above post.
That means that the four (4) 12AX7s are drawing 300 mA, the 12X4 rectifier tube draws 300 mA, and the #53 lamp draws 120 mA, for a total of 720 mA, which is well within the speculated 1A filament ratings of the Dynaco PA-211 transformer.
Edit: Note that the 120 mA draw of the #53 lamp is based on its 14.4 volt rating. In the PAS testing done by Pacific Audio Regenesis (link below), the actual draw of the lamp, at the voltage the PAS runs it, is 90 mA.
"Replace the power lamp with an LED?
A common recommendation, propagated on online forums, is to replace the power lamp with an LED to reduce the load on the transformer. Keep in mind that the lamp was part of the intended load, is operating at well under its rated voltage, and only draws about 90mA.
And the transformer is already operating with a good temperature rise margin. Removing the lamp will only lower the core temperature by about 1.3