Should I fuse the power transformer secondary?

[boomer#...@none.com]

I have some very old tube amplifiers, which I used regularly until the
early 1980's for a home made stereo. Because the homemade speakers were
huge, I stopped using this system due to living in a small apartment,
and put this stuff in storage. I recently moved and brought these amps
home. I want to power them up again. There are three identical mono
amps, which have four 6L6 output tubes, two 5U4 rectifiers and several
other tubes. When I used them, I had one amp on each channel L&R, and
the 3rd amp was connected to a center channel output and driven thru a
reverb delay to simulate a large auditorium.

These amps and the huge speaker systems really put out excellent sound,
and I loved that "tube sound". The output transformers were HUGE and
could really cover those low bass notes.

Anyhow, I'm ready to power up these amps once again. First I checked
each amp for any possible shorts, where some wires may be frayed, but it
does appear that anything has gone bad in that regard. The tubes should
all be fine, since they generally do not degrade with time. But I am
concerned about the filter capacitors.

I should note that the filter caps were unsual, in the fact that they
were electrolytic cans with an OCTAL base (plugged into a octal tube
socket). Each of them have 3 caps in one can. I recall somewhere in
the early 1970's, I found an electronics store that still had three of
them, so I bought them and replaced all of them. But that's close to 40
years ago.

While I suppose replacement might be a good idea, I'll never find
anything to match, and I am aware that many of those older caps seemed
to last forever, unlike these poor quality newer ones. So, the urge
exists to just plug them in and see what occurs.

However I am aware that a bad filter cap can ruin a rectifier tube
quickly, and possibly do other damage. Therefore, as a precaution, I am
considering putting a fuse on each of the B+ wires where they exit the
power transformer. I'm taking a wild guess that a ONE amp fuse should
be sufficient, or maybe uit should be smaller?????

Or, can I just put ONE fuse on the center tap?

From the best of my recollection, the power transformer is around 500
volts center tapped.

The fuse(s) would be right at the secondary of the transformer, before
the 5U4 rectifier tubes.

I'm asking this to get opinions on both if this is practical, and will
protect the rect tubes as well as other parts, and also what size fuse
to use. I'm also not sure if it's best to fuse both high voltage leads,
or just the center tap.

Thanks for all feedback!

[Ralph Mowery]

<boomer#687...@none.com> wrote in message
news:f2achad9erqh3jjmd...@4ax.com...

You may have trouble with the fuses as the cpacitors will draw a lot of
current on startup even if good. Maybe the slow heating up of the tube
filiments will prevent that.

Usually people take a varac and bring the voltage up slowly on the older
equipment. Youprobably don't have one, so that may be a problem for you.

[RobertMacy]

On Fri, 27 Mar 2015 21:11:48 -0700, Ralph Mowery
<rmower...@earthlink.net> wrote:

>> ...snip,...one of the best descriptions...just to keep Aioe happy

>
> You may have trouble with the fuses as the cpacitors will draw a lot of
> current on startup even if good. Maybe the slow heating up of the tube
> filiments will prevent that.
>
> Usually people take a varac and bring the voltage up slowly on the older
> equipment. Youprobably don't have one, so that may be a problem for you.

yes, 2nd the recommendation on the variac. Either that, or use a solid
state relay to turn it on. That way you usually get the turn on occurring
at zero crossing instead of possibly at the PEAK AC mains voltage. Don't
know if true, but heard that using a variac and ramping up votlage on
those old caps will actually 'reconstitute' it. Anybody know?


If you're very nervous about the caps, measure them with an LCR Meter.

You can get a trial free one at Bob Masta's site: well, free in the sense
it piggy backs on your soundcard.

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[Arfa Daily]

"RobertMacy" <robert...@gmail.com> wrote in message
news:op.xv7j4dve2cx0wh@ajm...

> On Fri, 27 Mar 2015 21:11:48 -0700, Ralph Mowery
> <rmower...@earthlink.net> wrote:
>
>>> ...snip,...one of the best descriptions...just to keep Aioe happy
>>
>> You may have trouble with the fuses as the cpacitors will draw a lot of
>> current on startup even if good. Maybe the slow heating up of the tube
>> filiments will prevent that.
>>
>> Usually people take a varac and bring the voltage up slowly on the older
>> equipment. Youprobably don't have one, so that may be a problem for you.
>
> yes, 2nd the recommendation on the variac. Either that, or use a solid
> state relay to turn it on. That way you usually get the turn on occurring
> at zero crossing instead of possibly at the PEAK AC mains voltage. Don't
> know if true, but heard that using a variac and ramping up votlage on
> those old caps will actually 'reconstitute' it. Anybody know?

There is a slight problem with doing that in that the 5U4 won't get going
until it has sufficient heater volts to achieve at least a degree of
emission. By that time, the variac would be turned up quite high already.
Depending on how good the 5U4 is, the rectified voltage across the filter
cap(s) may still remain quite low due to the reduced emission of the 5U4
because of its heater not being up at normal temperature. I do agree though
that if the amps have been standing that long, and the caps are 'specials'
that the OP doesn't want to 'lose', it might not be a good idea to let it go
from zero to full HT across them in the few seconds that the 5U4 would take
to reach full emission.

If it was me, I think I would initially remove all the tubes, and solder two
diodes across the vacated 5U4 socket. Then bring the amp up on a variac to
about 60 % full line voltage over a period of about a half hour, and then
leave it there for a further half hour. Then turn off and refit all the
tubes except the 5U4, turn back on still at 60 % and let it run for a few
minutes, before carrying on ramping up to the full line voltage over a
period of say a further 15 minutes. It should then be ok to turn off, remove
the temporary diodes, refit the 5U4 and just go for it with a full voltage
switch on.

Arfa

[Ron D.]

Do a search on capacitor reforming.

You may replace the rectifiers with silicon replacements but there are issues in doing do. A LARGE series resistor is selected to be placed in series and you loose the slow warm-up

[Rheilly Phoull]

Well there's still the old mains series light bulb ?

[Ralph Mowery]

"Ron D." <Ron.D...@gmail.com> wrote in message
news:492d2fde-e49d-4654...@googlegroups.com...

The other tubes should also be removed. If not you may run into what is
called cathode stripping. That is what happens when you apply the plate
voltage before the filiments come up to temperature.

[Arfa Daily]

"Ralph Mowery" <rmower...@earthlink.net> wrote in message
news:36KdnbBbzKE4u4rI...@earthlink.com...

I did suggest that the other tubes were removed during the low voltage input
phase for this exact reason.

However, almost all 'modern' tubed amplifiers apply full HT without issue.
Many larger guitar amplifiers have plate voltages well in excess of 400
volts, and for the last 30 years or more, that is almost universally derived
from a silicon diode rectifier, so is applied to the tubes pretty much
instantaneously. As to replacing the OP's tube reccies with silicon diodes,
and thus the issue of needing a series resistor, I would agree if the solid
state replacement was going to be permanent, but I only suggested
*temporarily* replacing the 5U4 with diodes to allow a progressively
increasing voltage to be applied via a variac. A series resistor should not
be an issue for the time involved before the diodes were removed and the 5U4
refitted.

Arfa

[mrob...@att.net]

boomer#687...@none.com wrote:
> I should note that the filter caps were unsual, in the fact that they
> were electrolytic cans with an OCTAL base (plugged into a octal tube

> socket). [...]

>
> While I suppose replacement might be a good idea, I'll never find
> anything to match, and I am aware that many of those older caps seemed
> to last forever, unlike these poor quality newer ones.

Why not buy an octal plug, solder some modern capacitors with the right
ratings to it, and use that? Even if you just make one of these, and
it messes up the "sound", you can use it in all 3 amps to tell you if
you have any other problems. You can then try tricks with one of the
old octal filter caps to bring it back, while still having the "modern"
one to fall back on. If your "modern" cap blows, you're out a couple of
dollars of easy-to-get caps, rather than lots of dollars of tubes or
other components.

Matt Roberds

[boomer#...@none.com]

On Sat, 28 Mar 2015 05:39:38 -0700, RobertMacy <robert...@gmail.com>
wrote:

>On Fri, 27 Mar 2015 21:11:48 -0700, Ralph Mowery
><rmower...@earthlink.net> wrote:
>
>>> ...snip,...one of the best descriptions...just to keep Aioe happy
>>
>> You may have trouble with the fuses as the cpacitors will draw a lot of
>> current on startup even if good. Maybe the slow heating up of the tube
>> filiments will prevent that.
>>
>> Usually people take a varac and bring the voltage up slowly on the older
>> equipment. Youprobably don't have one, so that may be a problem for you.
>
>yes, 2nd the recommendation on the variac. Either that, or use a solid
>state relay to turn it on. That way you usually get the turn on occurring
>at zero crossing instead of possibly at the PEAK AC mains voltage. Don't
>know if true, but heard that using a variac and ramping up votlage on
>those old caps will actually 'reconstitute' it. Anybody know?
>
>
>If you're very nervous about the caps, measure them with an LCR Meter.
>
>You can get a trial free one at Bob Masta's site: well, free in the sense
>it piggy backs on your soundcard.
>

I dont have a Variac, but might invest in one if they are not too
costly. I'll have to see if Ebay has any new or used ones, depending on
the price.

After reading this whole thread, I began to think that I do have some
old power transformers from old tube type TVs, as well as some Octal
tube sockets I know these TV transformers probably are not around 500V,
(from what I recall, most were 200 to 350V). But anyhow, I suppose I
could just take the transformer, some power diode rectifiers, and
connect the output to the octal sockets. Plug these caps into the octal
socket, and power it up. At least that way nothing else can be damaged
in my amplifiers. Plug in caps were a rarity, but for doing these
tests, that would be to my advantage.

But I suppose a Variac would still be best, or at least the series
lightbulb someone mentioned.

This way I can leave the caps powered for an hour or so and see how they
react. I'd still put a fuse across at least the primary of the power
transformer in case of a dead shorted cap.

Thanks for all who replied!

[Jon Elson]

boomer#687...@none.com wrote:


>
> Or, can I just put ONE fuse on the center tap?
>
> From the best of my recollection, the power transformer is around 500
> volts center tapped.
>

Probably not a good idea. The typical electronic fuses are meant to
interrupt up to 250 V AC, and will likely arc over with 500+ V with a strong
DC component to them. A slo-blo fuse might do better as they have a spring
to pull one end away from the other. But, I would not expect it to work
reliably.

Jon

[Phil Allison]

Jon Elson wrote:

> > Or, can I just put ONE fuse on the center tap?
> >
> > From the best of my recollection, the power transformer is around 500
> > volts center tapped.
> >
> Probably not a good idea. The typical electronic fuses are meant to
> interrupt up to 250 V AC, and will likely arc over with 500+ V with a strong
> DC component to them.

** Absolute rot with the example in question.

For a fuse to arc end to end takes a very large current, hundreds of amps, not just a higher than rated voltage.

Standard 20mm and 3AG fuses, quick and slow blow, are fitted to countless tube amps in the HT and AC secondary circuits.

> A slo-blo fuse might do better as they have a spring
> to pull one end away from the other.

** Some do, most do not and it is not an advantage at high voltages.


... Phil