Should I fuse the power transformer secondary?

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.

Reply to
Ralph Mowery
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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!

Reply to
boomer#6877250

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.

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Reply to
RobertMacy

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

Reply to
Ron D.

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

Reply to
Rheilly Phoull

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.

Reply to
Ralph Mowery

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

Reply to
mroberds

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!

Reply to
boomer#6877250

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

Reply to
Jon Elson

** 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.

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

... Phil

Reply to
Phil Allison

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