Hello all,

I have been a long time lurker, but I finally (hopefully?) have an
intelligent question to ask.

I am in the process of restoring my father's old 100W pure pentode 8417 PA
that he built as a teenager. After gathering some experience restoring my
old 6V6 PA and building my own line stage, I have a reasonable grasp on
what I am doing.

The first order of business is designing a PSU for the output tubes. After
destroying many HT transformers, my father implemented a direct plug-in
voltage quadrupler for 560V of HT. While I do still have this box of
death, I am quite certain those old electrotytics will explode upon
connection. I am going to build a more conventional design.

I have settled on an LC filtered design for the good regulation, but I am
having trouble finding a transformer with enough capacity. I'm looking at
300mA of current draw per channel. I did find one at Audio Electronic
Supply (P/N PT270), which has a pair of 540V @ 600mA windings. That gives
me about 450V of HT, good for over 70W of ultralinear power (if/when I get
new OPTs). Has anyone had any experience with AES's transformers? I
would like a bit more HT (maybe 500V), but I would settle for this one.

Thanks,
Russ

Russ,

Why not just re-cap the existing PSU? Just replace the electrolytics. If it
has any selenium diodes in it (like in a bias supply), replace them with
silicon units. Replace any carbon composition resistors that may be in there
with metal oxide or ceramic resistors. Then light the fire, so to speak.

There's nothing wrong with using a voltage doubler/tripler/quadrupler. If I'm
not mistaken, the likes of McIntosh et. al. use voltage doublers.

If you're hell-bent on building your own, then I would suggest a CLC design to
get a higher HT. I don't think regulation is going to be that big a deal
unless you are driving the thing way past class A. And that, my friend, is
going to be so loud that you will likely not hear the distortion nuances
introduced by a bit of power supply sag.

Best regards,

Steve Robertson

"Russ W. Knize" wrote:

Hello all,

I have been a long time lurker, but I finally (hopefully?) have an
intelligent question to ask.

I am in the process of restoring my father's old 100W pure pentode 8417 PA
that he built as a teenager. After gathering some experience restoring my
old 6V6 PA and building my own line stage, I have a reasonable grasp on
what I am doing.

The first order of business is designing a PSU for the output tubes. After
destroying many HT transformers, my father implemented a direct plug-in
voltage quadrupler for 560V of HT. While I do still have this box of
death, I am quite certain those old electrotytics will explode upon
connection. I am going to build a more conventional design.

I have settled on an LC filtered design for the good regulation, but I am
having trouble finding a transformer with enough capacity. I'm looking at
300mA of current draw per channel. I did find one at Audio Electronic
Supply (P/N PT270), which has a pair of 540V @ 600mA windings. That gives
me about 450V of HT, good for over 70W of ultralinear power (if/when I get
new OPTs). Has anyone had any experience with AES's transformers? I
would like a bit more HT (maybe 500V), but I would settle for this one.

Thanks,
Russ

On Wed, 06 Aug 2003 12:52:06 -0400, Steve Robertson wrote:

Why not just re-cap the existing PSU? Just replace the electrolytics.

Because it has no transformer.

On Wed, 06 Aug 2003 12:52:06 -0400, Steve Robertson wrote:

If you're hell-bent on building your own, then I would suggest a CLC
design to get a higher HT. I don't think regulation is going to be that
big a deal unless you are driving the thing way past class A. And that,
my friend, is going to be so loud that you will likely not hear the
distortion nuances introduced by a bit of power supply sag.

I have been considering the Pi filter as well. The only problem is that
with this transformer, I am looking at 700VDC which is a bit much. They
do have a 510V @ 400mA that I could buy two of, which would give me more
like 660V, which is the max for the 8417s. The only thing that bothers me
about the Pi filter is the EMF generated by leakage flux due to the
current spikes required to drive the first cap. Perhaps I am just being
paranoid. Is this really an issue with properly bypassed hexfreds? Should I go
with a tube rectifier? Any thoughts?

Thanks for the advice,
Russ

Russ W. Knize wrote:
On Wed, 06 Aug 2003 12:52:06 -0400, Steve Robertson wrote:


If you're hell-bent on building your own, then I would suggest a CLC
design to get a higher HT. I don't think regulation is going to be that
big a deal unless you are driving the thing way past class A. And that,
my friend, is going to be so loud that you will likely not hear the
distortion nuances introduced by a bit of power supply sag.


I have been considering the Pi filter as well. The only problem is that
with this transformer, I am looking at 700VDC which is a bit much. They
do have a 510V @ 400mA that I could buy two of, which would give me more
like 660V, which is the max for the 8417s. The only thing that bothers me
about the Pi filter is the EMF generated by leakage flux due to the
current spikes required to drive the first cap. Perhaps I am just being
paranoid. Is this really an issue with properly bypassed hexfreds? Should I go
with a tube rectifier? Any thoughts?

Well, both the capacitor-input (pi) and choke-input (L) filter
topologies have their advantages and disadvantages. You've nailed the
main disadvantage of the pi filter; those high-current spikes can be a
bugger. They can even cause audible buzzing on transformers that aren't
potted. But if you're building your power supply in a separate
enclosure, you should be alright.

On the L filter side, it can be a nuisance because of mechanical humming
of the filter choke. Furthermore, since such cores are generally gapped,
there will be a lot of 60 Hz. magnetic fields in the vicinity. This is
not normally a problem in line-level amplifiers, but can be a bear to
correct for in high gain (phone, mic or instrument) amplifiers, pentode
input stages seem especially susceptible to this.

The other thing to be aware of is that the regulation issue isn't "cut
and dried." The L (choke-input) filter does have better regulation over
most of its operating range, staying pretty close to about 0.9 times the
RMS voltage from the transformer. Except at low currents, where it
skyrockets to up to 1.4 times the RMS value!

If you design for a C-input filter, you'll generally end up with about
1.2 times the RMS voltage under load. So a 510v transformer will give
you about 600 volts under load, or about 715 volts with no load (as
during warmup). To do the same thing with a choke-input filter, you'd
need a transformer of about 665 volts RMS. That means, with no load, the
output voltage will be a whopping 930 volts!

On the other hand -- your transformer's current rating can be a lot
lower for a given DC current output, using the "L" filter topology. This
is because the peak currents are a lot lower. Power loss (and therefore
transformer heating) is proportional to the square of the current
integrated over a cycle, so capacitor-input (pi) filters tend to be a
lot more demanding of power transformers.

Vacuum tube rectifiers are, for me anyway, just a convenient way of
adding series resistance to capacitor-input filters (in order to limit
peak currents, and therefore keep both mechanical and induces buzzes
down to a dull roar). There are people who claim that rectifier tubes
have a "sound", but I could never hear a difference between a tube
rectifier and a silicon rectifier with the same series resistance. The
main difference is, as I say, the convenience of having the "resistor"
socketed and above chassis, where it can vent its heat reasonably elegantly.

For choke-input filters, I don't see any advantages to vacuum diodes, in
fact I consider them a liability. In one case I wanted to use some 866AX
merc rectifiers just because they looked cool, but the hassle wasn't
worth the trouble. When the filament transformer arced over, I said
"enough of that nonsense!" and changed to a convenient, trouble-free,
compact, and inexpensive silicon diode string. Problem solved.

Cheers,
Fred
--
+--------------------------------------------+
| Music: http://www3.telus.net/dogstarmusic/ |
| Projects, Vacuum Tubes & other stuff: |
| http://www.dogstar.dantimax.dk |
+--------------------------------------------+

On Thu, 07 Aug 2003 01:08:19 +0000, Fred Nachbaur wrote:
Russ W. Knize wrote:
On Wed, 06 Aug 2003 12:52:06 -0400, Steve Robertson wrote:
If you're hell-bent on building your own, then I would suggest a CLC
design to get a higher HT. I don't think regulation is going to be that
big a deal unless you are driving the thing way past class A. And that,
my friend, is going to be so loud that you will likely not hear the
distortion nuances introduced by a bit of power supply sag.

I have been considering the Pi filter as well. The only problem is that
with this transformer, I am looking at 700VDC which is a bit much. They
do have a 510V @ 400mA that I could buy two of, which would give me more
like 660V, which is the max for the 8417s. The only thing that bothers me
about the Pi filter is the EMF generated by leakage flux due to the
current spikes required to drive the first cap. Perhaps I am just being
paranoid. Is this really an issue with properly bypassed hexfreds? Should I go
with a tube rectifier? Any thoughts?

Well, both the capacitor-input (pi) and choke-input (L) filter
topologies have their advantages and disadvantages. You've nailed the
main disadvantage of the pi filter; those high-current spikes can be a
bugger. They can even cause audible buzzing on transformers that aren't
potted. But if you're building your power supply in a separate
enclosure, you should be alright.

I may end up having to do this anyway. Is there a "rough figure" to the
amount extra capacity a transformer should have to keep the buzzing under
some degree of control?

On the L filter side, it can be a nuisance because of mechanical humming
of the filter choke. Furthermore, since such cores are generally gapped,
there will be a lot of 60 Hz. magnetic fields in the vicinity. This is
not normally a problem in line-level amplifiers, but can be a bear to
correct for in high gain (phone, mic or instrument) amplifiers, pentode
input stages seem especially susceptible to this.

I have read this as well. How are Hammond's chokes in this regard?

The other thing to be aware of is that the regulation issue isn't "cut
and dried." The L (choke-input) filter does have better regulation over
most of its operating range, staying pretty close to about 0.9 times the
RMS voltage from the transformer. Except at low currents, where it
skyrockets to up to 1.4 times the RMS value!

Right, that is an issue that bothers me, but I rarely see it addressed.
Sometimes I see shunt resistors put in, but that is wasteful.

If you design for a C-input filter, you'll generally end up with about
1.2 times the RMS voltage under load. So a 510v transformer will give
you about 600 volts under load, or about 715 volts with no load (as
during warmup). To do the same thing with a choke-input filter, you'd
need a transformer of about 665 volts RMS. That means, with no load, the
output voltage will be a whopping 930 volts!

On the other hand -- your transformer's current rating can be a lot
lower for a given DC current output, using the "L" filter topology. This
is because the peak currents are a lot lower. Power loss (and therefore
transformer heating) is proportional to the square of the current
integrated over a cycle, so capacitor-input (pi) filters tend to be a
lot more demanding of power transformers.

Vacuum tube rectifiers are, for me anyway, just a convenient way of
adding series resistance to capacitor-input filters (in order to limit
peak currents, and therefore keep both mechanical and induces buzzes
down to a dull roar). There are people who claim that rectifier tubes
have a "sound", but I could never hear a difference between a tube
rectifier and a silicon rectifier with the same series resistance. The
main difference is, as I say, the convenience of having the "resistor"
socketed and above chassis, where it can vent its heat reasonably elegantly.

Speaking of series resistance, how does one calculate Q for a Pi filter?
I know how to do it to critically dampen an LC filter, but I have not seen
anything for a PI filter. Playing with Duncan's PSU Designer shows that
the Pi filter does resonate, though it takes less C and R to squelch it. I do
like the fact that I can get away with less overall C in a Pi filter to
achieve low ripple and low Q. It allows me to afford higher quality caps.

For choke-input filters, I don't see any advantages to vacuum diodes, in
fact I consider them a liability. In one case I wanted to use some 866AX
merc rectifiers just because they looked cool, but the hassle wasn't
worth the trouble. When the filament transformer arced over, I said
"enough of that nonsense!" and changed to a convenient, trouble-free,
compact, and inexpensive silicon diode string. Problem solved.

Yes, I was following your formerly 300A amp project. Was kinda looking
forward to the blue glow, but I agree about the annoying filament
requirements of the 866As. I have also ruled them out for another project
in the back of my head....

Russ W. Knize wrote:

On Thu, 07 Aug 2003 01:08:19 +0000, Fred Nachbaur wrote:

[...]

Well, both the capacitor-input (pi) and choke-input (L) filter
topologies have their advantages and disadvantages. You've nailed the
main disadvantage of the pi filter; those high-current spikes can be a
bugger. They can even cause audible buzzing on transformers that aren't
potted. But if you're building your power supply in a separate
enclosure, you should be alright.


I may end up having to do this anyway. Is there a "rough figure" to the
amount extra capacity a transformer should have to keep the buzzing under
some degree of control?

I don't think there's a real rule of thumb for this. For this project
http://www.dogstar.dantimax.dk/tubestuf/ampindex.htm the transformer
buzz was just noticable enough to be annoying between tracks, until I
added a bit of series resistance (R1 in the schematic he
http://www.dogstar.dantimax.dk/tubes...cs/pwrsupm.gif ). I've
since then increased R1 even further (from 10 ohms to 22 ohms) and
decreased the input capacitor from 47 uF to 22 uF; mainly to decrease
transformer heating over continuous use, but it also reduced the buzz
even further. (The two effects are, of course, closely related).

On the L filter side, it can be a nuisance because of mechanical humming
of the filter choke. Furthermore, since such cores are generally gapped,
there will be a lot of 60 Hz. magnetic fields in the vicinity. This is
not normally a problem in line-level amplifiers, but can be a bear to
correct for in high gain (phone, mic or instrument) amplifiers, pentode
input stages seem especially susceptible to this.


I have read this as well. How are Hammond's chokes in this regard?

In this regard I think they're no better or worse than anyone else's.
For the mechanical hum, this tends to be dependant on your chassis also
(acting as a sounding board), and for the magnetic induction into other
circuitry, it's the gap itself that causes it.

The other thing to be aware of is that the regulation issue isn't "cut
and dried." The L (choke-input) filter does have better regulation over
most of its operating range, staying pretty close to about 0.9 times the
RMS voltage from the transformer. Except at low currents, where it
skyrockets to up to 1.4 times the RMS value!


Right, that is an issue that bothers me, but I rarely see it addressed.
Sometimes I see shunt resistors put in, but that is wasteful.

I can see how it offends your engineering sensibilities, but power
efficiency is usually one of the last concerns on a vacuum-state
designer's mind. Compared to the total filament power of a typical amp,
the bleeder resistor is the least of your efficiency worries! ;-)

The bleeder also has the benefit of discharging the capacitors for
safety's sake whilst working on the unit. But there's a more efficient
way of accomplishing this, any time I build a supply over about 400
volts I use a second pole on the main power switch to engage the bleeder
upon power-off.

You can also address the choke-input regulation problem by using a
delayed-start power supply that doesn't come up until the filaments have
heated up. This is tough to do elegantly, though, unless you also use a
slow-start scheme (e.g. a vacuum diode such as a sweep damper diode,
etc. strictly used as a slow-start device).

[...]


Speaking of series resistance, how does one calculate Q for a Pi filter?
I know how to do it to critically dampen an LC filter, but I have not seen
anything for a PI filter. Playing with Duncan's PSU Designer shows that
the Pi filter does resonate, though it takes less C and R to squelch it.

I don't know. I suspect you might have to go back to the Laplace
transform for the circuit (and I haven't done any of those since school,
where I had to do them to prove I could!)

Before PSUD, I basically flew by the seat of my pants when designing
PSUs. I had starting points based on experience, and would sometimes do
a time-constant calculation or two, and that was about it. Fine tuning
was done "in the field."

Now with PSUD, I fly by the seat of my pants on a simulator first. ;-)

I do
like the fact that I can get away with less overall C in a Pi filter to
achieve low ripple and low Q. It allows me to afford higher quality caps.

Indeed. I'm sure that this was the prime consideration in vintage
circuitry also, because in the 40's - 60's high-capacity electrolytics
were very large and quite pricey.

For choke-input filters, I don't see any advantages to vacuum diodes, in
fact I consider them a liability. In one case I wanted to use some 866AX
merc rectifiers just because they looked cool, but the hassle wasn't
worth the trouble. When the filament transformer arced over, I said
"enough of that nonsense!" and changed to a convenient, trouble-free,
compact, and inexpensive silicon diode string. Problem solved.


Yes, I was following your formerly 300A amp project. Was kinda looking
forward to the blue glow, but I agree about the annoying filament
requirements of the 866As. I have also ruled them out for another project
in the back of my head....

Hehe, you have indeed been lurking for awhile! Glad you decided to jump in.

I might one day go back to 866's for something, "just because." It would
have to be one of those rigs that looks more like a cappuccino machine
than an amplifier, where "The Look" is as important as "The Sound." Some
custom neon tubes would be nice... and maybe an NE-2 based light
chaser... and

[omigod, what am I saying???]

Cheers,
Fred
--
+--------------------------------------------+
| Music: http://www3.telus.net/dogstarmusic/ |
| Projects: http://dogstar.dantimax.dk |
+--------------------------------------------+

Russ W. Knize wrote:

I have read this as well. How are Hammond's chokes in this regard?


From my experience, I have never had a problem with a humming choke from Hammond. However, the transformers are another story. I have had hum issues usually when I'm at 1/2 the current rating or more.

On other issues, I like choke input supplies for their higher current potential, good voltage regulation, and predictability, as long as you pull the minimum current for it to work. I use this formula from the Radio Amateur's Handbook:

L = V / Ima where (L) is the minimum rating in Henries for a choke input power supply to behave. (V) is voltage and (Ima) is the current in milliamps.

Best of luck with your project.