Should I fuse the power transformer secondary?

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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!


Re: Should I fuse the power transformer secondary?

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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.



Re: Should I fuse the power transformer secondary?
On Fri, 27 Mar 2015 21:11:48 -0700, Ralph Mowery  

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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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Re: Should I fuse the power transformer secondary?
wrote:

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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!


Re: Should I fuse the power transformer secondary?
On 28/03/15 12:11 PM, Ralph Mowery wrote:
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Well there's still the old mains series light bulb ?

Re: Should I fuse the power transformer secondary?
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

Re: Should I fuse the power transformer secondary?

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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.




Re: Should I fuse the power transformer secondary?
boomer# snipped-for-privacy@none.com wrote:
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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


Re: Should I fuse the power transformer secondary?
boomer# snipped-for-privacy@none.com wrote:


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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

Re: Should I fuse the power transformer secondary?
Jon Elson wrote:

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** 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.  


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


...  Phil  





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