Disappointed with AK 55C Buy
Dave Hurtle
and took a look at the power cord. The outer portion was frayed but
the insulator on the two inner wires was fine so I plugged it in. Lo
and behold it played! No hum, no burning smell, just radio. How
disappointing. That kinda takes the fun out of things.
It's complete. The cabinet is in fair shape with no veneer missing.
It needs new grille cloth. It has the original speaker but it's been
torn and repaired several times over the years.
The tone control doesn't work correctly. I'll start with contact
cleaner and see if it improves. The sound did drop down very low
after about 10 minutes of listening but overall it really looks like
it doesn't need much electrical restoration at all. I guess I can use
this as an opportunity to hone my refinishing skills.
I paid $50 for it. Here are some pics for anyone wants to see what it
looks like:
Robert Casey
> I bought an Atwater Kent Model 55C (Pooley cabinet). I got it home
> and took a look at the power cord. The outer portion was frayed but
> the insulator on the two inner wires was fine so I plugged it in. Lo
> and behold it played! No hum, no burning smell, just radio. How
> disappointing. That kinda takes the fun out of things.
I'd still do a recap before things get less disappointing...... :-(
Phil Witt
>No hum, no burning smell, just radio. How
>disappointing. That kinda takes the fun out of things.
Don't you just hate it when they do that? Your pictures make me want
one. I'm going to resist it, however.
Ken G.
sold it for 400$
Did you know that had the legs cut off ?
Pgonshor
changed so that cabinets without legs were in vogue. Too bad. I've added legs
back onto a Pooley cabinet. It can be done and makes the radio look more
original. Try a good lumber yard. They have pre-turned legs. I matched them
up to the Pooley cabinet picture from their catalog. Pretty close. Regards,
Dave
Jeffrey D Angus
Dave Hurtle wrote:
>I bought an Atwater Kent Model 55C (Pooley cabinet). I got it home
>and took a look at the power cord. The outer portion was frayed but
>the insulator on the two inner wires was fine so I plugged it in. Lo
>and behold it played! No hum, no burning smell, just radio. How
>disappointing. That kinda takes the fun out of things.
>
That's funny. I bought a 50's vintage push-pull 6L6 amplifier on eBay.
When it arrived, somebody had already gone through and re-capped
it, and even stuffed new caps inside the electrolytics. Where's the fun
in that?
Jeff
--
"They that can give up essential liberty to obtain a little temporary
safety deserve neither liberty nor safety." Benjamin Franklin
"A life lived in fear is a life half lived."
Tara Morice as Fran, from the movie "Strictly Ballroom"
Sven Franklyn Weil
> it, and even stuffed new caps inside the electrolytics. Where's the fun
> in that?
Well...if it was done correctly, someone did the dirty work for you? :)
Plug it in and enjoy it, man!!!
--
sven
Peter Wieck
>disappointing. That kinda takes the fun out of things.
On the AK55
Recapping:
1. There are very few caps of any nature on this radio.
2. The filter caps are 2mf. PAPER caps potted in a can. They MUST NOT be
replaced with electrolytic capacitors as the tolerance for these caps is -15% +
100%.
3. AK requires that the tolerance on these caps is +/- 10% by test.
Significant problems with the radio, up to and including destruction of the
field coil may be caused by too much capacitance per AK and a service bulletin
they distributed in the early 30s.
4. Keep in mind that a large-value potted cap is about the last thing that
might go bad in a radio (excepting poor storage or abuse). By my own experience
and the three AK55s that have passed through my hands, none of them had even
remotely bad filters. All megged out fine, and all tested within 2% of the
rated capacitance. This is also true of the two other mid/late 20s AC radios
that I still have.
So, your surprise is not uncommon.
Cautions:
a. If you MUST recap, use non-electrolytics that BY TEST fall within 10%
of the listed requirements. Even though these caps are expensive relative to
their smaller cousins, there are only two of them, as I remember.
b. DO NOT use electrolytics, please.
c. Watch the temperature of the power transformer and cap-can carefully.
This radio was designed and built in Philadelphia, PA against a nominal 110V at
the wallplate and at a time when PECo. had a hard time making 105V. If you have
say…. 125V at the wallplate, you might get excess heat where it should not
be. You also might consider a dropping resistor or some other tactic to reduce
the voltage to the radio. This will also help preserve your 45 output tubes (2,
as I remember).
Otherwise, a fine radio. Enjoy it.
Spammers successfully cancelled to date: 4227
Peter Wieck
Al-Khobar, KSA
Dave Hurtle
> I'd still do a recap before things get less disappointing...... :-(
I searched web and google after the fact and found that recapping
probably isn't necessary. Peter Wieck and others have reported that
theres a good chance these will work fine with no electrical
restoration. There are only a few caps in the entire radio and
they're encased in a tar pit. Due to the nature of the design they
need to be within very close tolerance to the original values.
Recapping will probably do more harm than good.
Dave Hurtle
> I restored a 55 . my cabinet had a different grille and no doors . I
> sold it for 400$
> Did you know that had the legs cut off ?
I wasn't aware that the legs were cut off until people read my post
and informed me. I probably would have passed on it if I knew that.
Dave Hurtle
> That's funny. I bought a 50's vintage push-pull 6L6 amplifier on eBay.
> When it arrived, somebody had already gone through and re-capped
> it, and even stuffed new caps inside the electrolytics. Where's the fun
> in that?
I just can't get the feeling that it's *my* radio unless I have to
repair it. Right now I'm not feeling love for my new purchase. I has
hoping to do a super job on the cabinet to compensate but I received
an e-mail response warning me of the potential problems if I strip it.
Dave Hurtle
> Recapping:
>
> 1. There are very few caps of any nature on this radio.
> 2. The filter caps are 2mf. PAPER caps potted in a can. They MUST NOT be
> replaced with electrolytic capacitors as the tolerance for these caps is -15% +
> 100%.
> 3. AK requires that the tolerance on these caps is +/- 10% by test.
> Significant problems with the radio, up to and including destruction of the
> field coil may be caused by too much capacitance per AK and a service bulletin
> they distributed in the early 30s.
> 4. Keep in mind that a large-value potted cap is about the last thing that
> might go bad in a radio (excepting poor storage or abuse). By my own experience
> and the three AK55s that have passed through my hands, none of them had even
> remotely bad filters. All megged out fine, and all tested within 2% of the
> rated capacitance. This is also true of the two other mid/late 20s AC radios
> that I still have.
>
> So, your surprise is not uncommon.
>
> Cautions:
>
> a. If you MUST recap, use non-electrolytics that BY TEST fall within 10%
> of the listed requirements. Even though these caps are expensive relative to
> their smaller cousins, there are only two of them, as I remember.
> b. DO NOT use electrolytics, please.
> c. Watch the temperature of the power transformer and cap-can carefully.
> This radio was designed and built in Philadelphia, PA against a nominal 110V at
> the wallplate and at a time when PECo. had a hard time making 105V. If you have
> say…. 125V at the wallplate, you might get excess heat where it should not
> be. You also might consider a dropping resistor or some other tactic to reduce
> the voltage to the radio. This will also help preserve your 45 output tubes (2,
Thank you, Peter. I did some searcing on the web last night and the
google RAR+P archive and found some good info you've written on the AK
55/55Cs in the past.
It's amazing that you can just plug these in after 70+ years and have
them work. My hat's off to Atwater Kent for creating such a beast.
Bill Turner
Alan Douglas
>GOOD LUCK. Bill Turner
Amen to that. Differences of opinion make a horse race, and mine
diverge from Peter's. A power supply is a power supply, and whatever
problems there may have been with 1932-vintage replacement
electrolytics, they're long gone. A modern electrolytic will sub for
a paper filter, any day, any value (within reason) and I see no
problem using a couple of 8µF to replace A.A. Kent's originals.
Now if you want to actually measure the leakage at working voltage,
to decide whether to replace or not, fine. I've measured dozens of
old paper caps, new and used, and have found very very few that leak
less than 10 to 20 microamps. One, interestingly, was a filter block
from an AK 89, so it's at least possible that AK used better quality
caps than some other makers. But I wouldn't bet my power transformer
on it. He made a lot of radios and not all his caps were wound from
the same roll of paper. Also, the 89 block happened to have been
sealed in a metal can with Bakelite terminal block, reasonably
moisture-proof, not likely with a cap potted in tar.
Now, as to household voltage, I posted a message last December
under the header "Household Voltage Standards, 1920s-1930s." It's not
too long so I'll copy it below. The last sentence summarizes it.
Cheers, Alan
previous post:
I found the definitive reference, a series of papers in the AIEE
(American Institute of Electrical Engineers) Transactions for 1927,
pp. 161 - 215. There's far too much material to copy here, and most
of it applies to high-voltage generating and distribution equipment,
but a few points stand out.
The original idea was voltage multiples of 100, measured at the
load, but generators were rated to allow 15% drop in transmission,
and later 10%. In 1911 the voltages began to be measured at the
sending end, accounting for the shift to multiples of 11, and in 1914
motor voltages were standardized at 110, 220, 440, 550 and 2200V, with
an allowable variation of ±10%. Appliances were not yet standardized,
but were often rated for a range of voltages.
Carbon-filament lamps could not be manufactured to a specific
voltage, but were always selected afterward, so that 110, 115, and 120
volt lamps were inevitably produced. Electric operating companies
were encouraged to standardize on different voltages, to create a
market for all the lamps produced. This manufacturing spread
disappeared with the introduction of tungsten lamps in the teens, but
household voltages remained at 110, 115, or 120 volts in different
areas (and often 125V in DC systems).
Over time, 115 volt lamps accounted for an increasing percentage of
demand, while 110V lamps declined in popularity. Specifically, 115V
overtook 110V in 1919. 120V lamps increased their share a little,
though some of those went to DC areas. Figures were given for 1926:
110V lamps accounted for only 12% of demand, 115V for 47%, 120V for
35%, 125V 4%, and all others 2%.
Accordingly, the proposed 1927 standards were for 115/230V. There
were various other factors taken into account, such as the effect of
three-phase 208/240 systems, how much existing distribution
transformers could be overexcited, and so on; anyone interested in all
of this will have to read the original papers.
In summary, 115V was predominant, even in the late 1920s.
Manufacturers would have rated their appliances to work over a wide
range, from 110V to 120V and for AC/DC radios, as high as 125V.
DumpsterDiver
and 100% effective insurance against the eventual capacitor failure that's
sure to occur. Fuses are cheap, but AK power transformers, chokes and
speaker field coils are tough and expensive to replace.
No matter how well it appears to function, there's ALWAYS the risk of a
small leakage current escalating into a thermal runaway short and a fratzed
power supply.
It amazes me how few people seem to know this simple
preventive measure. I discovered this independently ... THE HARD WAY ... a
LONG time ago, and I wouldn't even THINK of selling a "restored" set of this
type without a B fuse, re-capped or not. Curiously, I have never seen this
in print ... anybody else ever seen it?
DD
"Dave Hurtle" <da...@hurtle.com> wrote in message
news:db0d6d2f.02111...@posting.google.com...
Alan Douglas
DD suggested:
>Install a 0.125A fuse in the B+ line, right off the #80 socket. It's cheap
>and 100% effective insurance against the eventual capacitor failure that's
>sure to occur. Fuses are cheap, but AK power transformers, chokes and
>speaker field coils are tough and expensive to replace.
You bet. I wrote a little pamphlet 20+ years ago on restoring an
AK40, which Jim Fred sold (maybe still does). That's exactly what I
recommended installing. Great minds....
Cheers, Alan
Hagstar
>
> No hum, no burning smell, just radio. How
> disappointing.
I HATE those too- because any fool knows ALL the paper and 'lytic caps
are completely, hopelessly, and irredeemably bad and dangerous. So it's
more satisfying if that's apparent from the get go- that way you're not
tempted to be lazy, not recap if fully, and later burn your neighborhood
down.
John H.
Pgonshor
200-300 sets. I'm not saying it's not a good insurance policy, but that is a
lot of fuses to install to prevent the unlikely failed transformer. Other
opinions?
Dave
Hagstar
>
>
>
> I HATE those too- because any fool knows ALL the paper and 'lytic caps
> are completely, hopelessly, and irredeemably bad and dangerous.
Also encased in TAR- I forgot. Better fuse it and play it only closely
supervised.
John H.
Hagstar
>
> I'm not saying it's not a good insurance policy, but that is a
> lot of fuses to install to prevent the unlikely failed transformer.
Fusing is just good sense, to protect against all sorts of shorts. A Rat
Shack holder and fuse costs a few cents and a minute to install. What's
the beef ?
John H.
Bill M
>
>
> Fusing is just good sense, to protect against all sorts of shorts. A Rat
> Shack holder and fuse costs a few cents and a minute to install. What's
> the beef ?
>
> John H.
The "no parts at Rat Shack" beef is a different thread :-)
-ex
Sven Franklyn Weil
DumpsterDiver wrote:
> Install a 0.125A fuse in the B+ line, right off the #80 socket. It's cheap
> and 100% effective insurance against the eventual capacitor failure that's
If the set is recapped, what causes the capacitor to fail again? Is it
just an inherent design flaw with this particular set?
--
Sven
Jeffrey D Angus
It doesn't need to be an electrolytic on the B+
I could just as easily be a short in the output transformer, or an internal
short in a vacuum tube.
Bill M
No parts have 100% reliability. And there's a handful of other things
that can happen that would make the fuse be advantageous...anything from
a shorted tube to a loose wire or chunk of solder. Same as any other
radio.
-BM
Robert Casey
I think he is suggesting the fuse if he *doesn't* do a recap. Makes sense.
But fuses have voltage ratings, which tell how much voltage the fuse
can isolate once burned out. A B+ circuit may exceed that. Some fuses
have a little spring inside which pulls the conducting path apart when it
blows. Probably a good idea here. Quenches the arc.
Be sure to size the fuse so that a momentary power interruption doesn't
cause it to blow (as the rectifier, still hot, has to charge up the filter cap
fast). Twice normal current would be about right. Or use a slo blow
at the normal current draw.
Peter Wieck
>cause it to blow (as the rectifier, still hot, has to charge up the filter
>cap
>fast). Twice normal current would be about right. Or use a slo blow
>at the normal current draw.
RANT WARNING:
"Twice the normal current": Read "worse than useless" One might then rely on
the fuse to take the place of proper direct observation for potential problems,
and/or metering the actual current draw of the radio to be sure that it is
well-behaved, or other elementary diagnostic tools. Twice the normal current
on the B+ side would have to be dissipated somewhere, and that somewhere might
fail with spectacular and expensive results. So, any fuse that will permit 2X
the actual normal current for any length of time beyond the starting surge IMO
is worse than useless.
Slo-Blow Fuse: Marginal at best. A slow-blow fuse will permit overage at a
scale depending on the amount of the overage over time. So: at 110% of rating,
it might last 10 seconds, for example, and only 1/10th of a second at 200%.. In
any case, it can permit a substantial downline build-up of heat.
Dual-Element Fuse: This is the ticket. It can be tailored quite precisely to
the actual amount of current to be permitted. And it can handle substantial
overcurrent for tiny amounts of time. I keep (even here) fractional-amp dual
element fuses from 1/10A to 1.5A made by LittleFuse and Buss (accept no other).
Sometimes tough to find, but cheap at the price. '
I also do not put fuses in radios, but in outboard plug boxes (ask Bill Turner)
so that I can plug up to two radios in the same outlet, each one fused
differently, each one fused at what is exactly right for *that* radio.
NOTE: 1. Dual Element fuses do not take to rapid cycling. Each time the radio
is turned off, the fuse must be allowed to cool for several minutes. If these
fuses are cycled too rapidly they will pop.
2. Dual-Element fuses should be tailored to the actual draw of a radio, not
necessarily to nameplate current. But lacking other data, nameplate current is
better than nothing.
3. Dual element fuses commonly are available in 1/10 increments from 1/10A to
5A in the standard glass fuse configuration. TV Repair SUPPLY shops tend to
stock them, as well as those very few electronic repair shops still extant.
DaveW
pair of 6X5Gs, is rated for 117 volts.
That seems to split the difference between 110 and 120.
Regards,
DAve
Mark Oppat
paper filter caps ca'28-'30, I say "fuse, 'em and forget 'em" if they work.
too much grief recapping those, but I do always cut out the "quality
condensor" that is in the audio can. Improves the treble response, and
often, some volume too if it is leaky. You gotta find the right wire to
cut by trial. They are in the tar... just run the set and snip til it
sounds better. If it goes dead, you reconnect that and try another!
Mark Oppat
"Pgonshor" <pgon...@aol.com> wrote in message
news:20021111215516...@mb-cv.aol.com...
John Goller, k9uwa
dial...@webtv.net says...
>
>
>GOOD LUCK. Bill Turner
>
Yup....I agree with Bill....... Good Luck.......
I have rebuilt 5 or 6 or these things in the last couple of years....
all worked great after a total recap......I am careful about the values
I put in them....I do have Cap measurement equipment! Mine are all
played a lot by Non-RadioCollector Types....... hours and hours of each
of them.....
John k9uwa
Ken G.
never replaced the filters in them because they were all good & never a
problem .
I cant emagine any problem refinishing this radio i resinished most of
mine . Thats the reason they sold for so much because they were
beautifull . Try some antique shops who may have or know someone with
junk furniture that may have nice legs you could use on your radio .
They dont have to be original just of an antique vintage somewhat like
other radios have . it`s not a rare radio the final value will not be
affected by this .
ken
lbrt...@aol.com
> of the listed requirements. Even though these caps are expensive relative to
> their smaller cousins, there are only two of them, as I remember.
> b. DO NOT use electrolytics, please.
You fail to state or establish a credible reason why electrolytics of
proper value ought not be used.
But it is reasonable to presume that too-high filter values (any kind
of cap)may produce both higher inrush (possible risk to a marginal
field coil), and higher peak current heating (possible risk to a
marginal xfmr).
> c. Watch the temperature of the power transformer and cap-can carefully.
> This radio was designed and built in Philadelphia, PA against a nominal 110V at
> the wallplate and at a time when PECo. had a hard time making 105V. If you have
> say…. 125V at the wallplate, you might get excess heat where it should not
> be.
AK did not mfg radios for sole use in PHL, and these sets saw voltages
about as high as what we have today in many other areas, without
incident. It seems this "really-lower-voltage-then" myth dies hard.
But it is reasonable to presume that the condition (and therefore
reliable maximum current-carrying ability) of the period's xfmrs &
field coils may not be what it once was (enamel insulation
deteriorating a bit with moisture & time, possibly attacking the wire
as well, and so forth).
>You also might consider a dropping resistor or some other tactic to
reduce
> the voltage to the radio. This will also help preserve your 45 output tubes (2,
> as I remember).
How might this help preserve his 45 output tubes, specifically? For
that matter, is there any evidence that good-testing 45's have short
lives? However, insuring that the heater voltages are correct
(meaning not too low either from a misguided over-lowering of line
voltage) may be the single most important thing in any tube's life.
Often, the practice of running a radio at lower supply voltage can be
counterproductive in terms of tube life.
Uncle Peter
It's also a lot of work to recap a set.
For a few pennies, the peace of mind is worth it. What if it is a Zenith
11S474
or 10S464 and a 6X5 fails?
Pete
Uncle Peter
"Peter Wieck" <pf...@aol.comspam> wrote in message
news:20021111234448...@mb-dh.aol.com...
I usually place internal fuses in the my sets on the AC line,
only to insure the radios are ready for polite society once
they leave my possession. That day will come for all of us.
My own feeling is that a fuse at 2X normal current is ample for catastropic
failures--if the B+ shorts the set will draw appreciable current and the
fuse
will pop almost immediately. I've seen rectifier failures short the HV
winding,
and the fuses always do their job.
Regardless of the method used, a fusing is just common sense and cheap
insurance.
regards
Pete
Joe Bento
I have an 11S474. I hope to have Ken work with me on the cabinet
sometime next year when the weather warms up.
As to a fuse, do you use a 1A or so fuse on the primary, or a
high-voltage fuse on the secondary?
Joe
Remove the "nospam" to send e-mail
jos...@nospam.kirtland.com
Uncle Peter
"Joe Bento" <jos...@nospam.kirtland.com> wrote in message
news:fusdtu0t9i5cj97td...@4ax.com...
measure the current draw on the HV to decide on a value, I'd
guess around 100 mA or so. A one amp will barely carry the
11S474 line load, in fact turning the set off and on after it is running
will blow a 1 amp primary fuse. Mine is fused at 1 amp, but I
don't play with the power when it is running.
Pete
Bill M
My 2c worth...I keep a handy stock of 1, 1-1/4 and 1-1/2 amp fast blow
fuses and generally put them on the transformer primary.
One cannot assume that the filament string will not encounter a short,
especially if there's a 6X5 in the mix. There's too many things that
could possibly happen in any old radio to make fusing only the B+ seem
illogical if you're going to bother installing a fuse at all.
Although an amp or amp-and-a-half on the primary may work out to be
mathematically silly, take a screwdriver and short out most anything in
the radio and you will see that it blows instantly. Never had one blow
from turn-on surges.
The dual element guys are recommendable, however, if you experience
turn-on problems.
Somewhere in the process of selecting a fuse, a fellow should verify
that the AC current draw is where it is supposed to be...and that's
typically at or below the label rating. If not, thats an indication
that there are other problems in the set that haven't been resolved and
if my personal experience is any indication, a leaky power xfmr is often
the culprit in an otherwise restored set. The eventual tar boil will
not be prevented in such a case with only a secondary B+ fuse.
-Bill
Peter Wieck
>proper value ought not be used.
Put simply, an electrolytic that is within normal manufacturing tolerances may
be anywhere from -20% to +100% of its rating. AK requires +/- 10%. Electrolyics
can also drift considerably based on actual use conditions. In either case, the
tolerances simply do not meet the AK requirements.
>AK did not mfg radios for sole use in PHL, and these sets saw voltages
>about as high as what we have today in many other areas, without
>incident. It seems this "really-lower-voltage-then" myth dies hard.
Old man AK did design against 110V for a dead certainty. And he was fiercely
independent of "conventional wisdom" in the process, more-or-less ignoring his
cross-town rivals in much that he did. It is also a fact that wallplate
voltages today exceed 127V in some parts of the country, including an area
where we have our summerhouse in upstate PA served by PP&L. I use 130V lamps up
there when I can find them, 120V lamps simply fail too quickly.
>How might this help preserve his 45 output tubes, specifically? For
>that matter, is there any evidence that good-testing 45's have short
>lives?
Any lamp run at a higher voltage that its rating will fail proportionately
sooner. The most apt way of dealing with this situation is to measure the
actual voltage-under-load across the filaments. If it exceeds the filament
rating by any fraction the life of the tube will be shortened proportionately.
One does not willy-nilly do ANYTHING to a radio without good, observable
reason. You will note that I say "might" instead of "Should" as conditions
vary.
As to running under-voltage, that is also a matter of proportion. If a 5V tube
is running at 4.9V, that will have a healthy (if small) effect on tube life,
and should have no observable effect on performance. If the tube is so spavined
that it fails at 4.9 (or even 4.7V), then perhaps it is so marginal that a
rejeuvenation treatment might be in order.
Please remember that this discussion is about an AK55 TRF radio, made in 1929,
and NOT a Zenith (or other) superhet of the 30s. It is dangerous to reason from
the specific to the general in this case as the AK55 is in many ways a
transitional set from the battery sets of a few years earlier to the AC
superhets of even fewer years in the future. It is, withall, a remarkably
simple radio and a remarkable performer, despite the three-gang tuning cap, the
brass belts, and the massive parts.
Scott Robinson
Good general engineering practice-used in every German set I ever
hazd-is to put a slow-blow fuse at aobut 1.5x the RMS current draw.
NOw, fuse sizing is a little tricky,m becaseu the littel metric fuses
are rated with a differnet idea in mind> A 1A fuse of this type will
carry 1A for along time.
US practice is that a 1A fuse will blow in a few minutes at 1A.
The difference is about 25% in rating. the factor of 1.5 is for a US
style fuse; for a metric fuse you could use 1.2 or thereazbouts.
Any reasonable fuse is better than none. Exception: most AA5 sets are
sefl-fuesd by the resistor in series with the 35Z5 plate and the pilot
lamp seciotn of the 35Z5 heater and its boon companion, the #47 bulb.
Regards,
>
Mark Huffman
or Littlefuse prints them each year. All you need to know about fusing.
Also, weird fuses from unheard of companies can be way off specs. mark
"Scott Robinson" <s...@earthlink.net> wrote in message
news:spr-476A5B.1...@nnrp04.earthlink.net...
lbrt...@aol.com
>
> The difference is about 25% in rating. the factor of 1.5 is for a US
> style fuse; for a metric fuse you could use 1.2 or thereazbouts.
Irrespective of mfg, one might consider where the fuse is placed - and
beyond what PW opined. For example, note the original submitter
refers to a fuse in the B+ supply, while another suggests one in the
xfmr sec or pri. The B+ current will be fairly well established as
its metered value, while the peak current seen by a fuse in
transformer winding with a cap-input filter (though to a much lesser
degree with a choke input) will be several times higher than the
average or RMS current measured. It is not unlikely that the unseen
current peaks may cause xfmr fuses which are closely rated to fail
when placed in a xfrm ckt, yet do fine in a B+ ckt. Without
calculating and only as a hipshot, I'd venture that the ballyhooed
suggestion of 2x RMS AC draw for a xfmr fuse is probably about right
in many sets - and we see that many mfgr's of tube expensive gear went
this high as a matter of course.
One also may consider that there are many variations of fuses with
many different blow characteristics, to throw into the fray if one
wished to get carried away with fusing.