I have to ask about this idea of placing diodes on the PT HT leads to the Rect tube.

[catnine]

I understand how if a rect tube shorts out then you have ac to the filter caps and they can blow and then blow the PT because the diodes will not allow the voltage to flow back to the PT HT windings . What I don't understand is how the diode can keep the amp working and for two reasons that have yet to be explained. If the filter caps blow you are done anyway , and lets say the caps don't blow how does the HT voltage reach the heaters or rather find a path to the B+ ?

The rect tube changes a/c to d/c now wouldn't it sort of depend on what shorted in the rect tube to begin with?

What am I missing here? I have had a rect tube long ago short in a sf champ and the main fuse blew . I put in another fuse and powered up and the fuse blew again. I went to a local dealer who sold amps and guitar and all the varied stuff and he told me I needed a new 5Y3 . I put it in with a new fuse amp worked.

So just what part of a rect tube shorts in the first place . Fender never put diodes in the HT leads and if not why not . I have not read about adding diodes even on re -issue models of the old point to point fender amps that still use rect tubes. I am certain fender knew about tubes shorting out and that they would not risk replacing PT's for the cost of a few diodes

[Enzo]

You are missing that the added diodes prevernt that AC from reaching the filter caps, even when the tube shorts.

A rectifier tube is really just two diodes with their cathodes connected together. They happen to be tube diodes rather than solid state ones, but they are still diodes regardless. Current flows through them one direction and not the other.

DO you understand how two or three diodes work, like Fender uses in amps like the Twin Reverb and many others? A diode is just a one way valve for current, so putting three of them in series, in a row, just means the current has to pass through threee one way valves to get where it is going instead of one. But the current can;t go backwards. One diode will stop that, so will three. Or two. They put three in series because the important characteristic of a diode is how much reverse voltage it can withstand. AFter all, it lets current through one way, but when AC is applied, it has to block the reverse path. SO that reverse voltage it needs to block. Three in series is a way to increase the reliability and reverse voltage rating.

But forgetting voltage for a moment, if one diode is used, and it shorts, it basically becomes a piece of wire. Current flows both ways then - in other words AC passes. Now three diodes in a row. If one shorts and becomes a wire, there are still two more to prevent reverse current.

Now remember diodes in series. The AC from the transformer is normally applied to the plate (anode) of the rectifier tube. The tube is a diode and acts like it. SO if it shorts, then AC passes through. But if we put another diode in series with that plate (except this one is solid state) then if one of them - tube or solid state, which one doesn't really matter - shorts out, the remaining one still blocks the reverse current.

SO with these protector diodes, that AC does NOT reach the filter caps, so they are NOT blown anyway.

Are you clear on what a "short" means. That is...short... for "short circuit." And that essentially means that two points in a circuit are connected together when they ought not be. There is no "what happens when a tube shorts." There are various possibilities. All a short in a tube means is that two or more points in the tube become connected together when they should not be. IN a simple tube like a 5Y3 there are not as many possibilities as in a EL34 or something. One or the other of the two plates can short to the cathode. Or both of them for that matter. Darned unusual but I guess possible would be the two plates shorting together but not to the cathode. And since the cathode and heater are the same thing in this tube, it is concievable the two ends of the heater could short together, though I've never seen that happen.

SO most common is plate to cathode - anode to cathode - which would indeed send AC right on through. Now this usually blows fuses, but the reason it does is that the filter caps revolt, and the shorted half tube leaves the other diode half now right across the transformer winding. SO if we put a protection diode in series with the plate lead, then if the tube shorts plate to cathode, that diode prevents reverse cirrent, and the shorted tube has become that piece of wire passing the rectified AC - which of course is DC - on to the filters. The shorted tube in and of itself won;t blow the fuse, because the AC is blocked from the caps by the backup diode, and the backup diode also prevents the remaining tube diode from being right across the transformer windinig. So that is why the amp will keep working.

Fender never installed backup diodes, nor did 99% of all the other brands. WHy not? The percentage of amps that will experience a shorted rectifier tube is low, and usually the fuse will blow before any real damage is done. But mainly it would complicate assembly of the amp and increase costs. The PT is really the last thing on the list I would worry about. If you short out the secondary winding, it is going to take out the fuse.

[trobbins]

Just to add a bit more discussion to Enzo's response, If one diode in a dual tube shorts, then it would cause a large secondary current to flow each half cycle, which would normally take out the primary side fuse. The electrolytic main cap would see a large ripple current as the voltage source of the half winding charged and then discharged the cap - which would heat up the cap. The cap would also see high ripple if just one diode open-circuited, and you didn't notice the effect of a drop in B+, and continued to play the amp.

You may notice switching noise if you place ss diodes in series with a valve diode, as the ss diodes can cause a more abrupt halt of current in each half secondary.

Ciao, Tim

[R.G.]

To add even more discussion to it ( as anything worth doing is worth overdoing ) :
I always use what I call backup diodes if I use a tube rectifier. Yes, manufacturers are interested in lowest possible cost and especially lowest possible labor cost. But as a hacker, I can afford to tinker. The diodes are very cheap insurance, even balanced against the unlikelihood of a rectifier short. They convert a fuse-blowing disaster into something that's hard to notice. I like that kind of failure.

On the switching noise:
Maybe you'll have switching noise. Maybe not. Depends on the imponderables of the tube rectifier's forward resistance and turn off speed versus the speed/sharpness of the SS diode cutoffs and whether or not you snub them.

General purpose SS rectifiers are slow to turn off because the junction fills with charge carriers that have to be swept out when it turns off, and this happens abruptly in many cases, slamming off and causing the wires to ring with the sudden current stop. It can make a blip of RF that you hear as a hum/buzz if your circuit picks it up. It doesn't always happen.

And my experience has been that it doesn't happen with a tube rectifier because the tube rectifier cutoff is slow enough to let the SS diodes stay with it as it ceases conduction, without slamming off. You might hear it if the tube shorts.

You can also use fast turn off or soft turn off SS diodes, which won't do the slam off, or insert an R-C on each diode to snub the slamming off and prevent RF creation.

[catnine]

Yes Enzo i fully understand what a short is . I did say I know why the diodes will prevent a/c from reaching the filter caps and how this would protect the PT . I did not consider a short plate to cathode then yes the amp would continue to operate yet at the same time half the HT winding one side of the centertap will send a higher Voltage to the rest of the amp and if long enough may kill a filter cap or a power tube or preamp tube if you are already running close to rated spec on the amp to begin with. Would this not alter ripple affect. I always place a small value fuse between the pin 8 of a 5Y3 before the first filter cap in case there is a short in the rest of the circuit. I have seen fuses added in place of diodes and after the cathode but they will not stop A/C from reaching the filter caps .

If the rect plates short together then I suppose the PT is toast unless the line fuse blows which is what I would hope would happen.

Still as far as the PT windings are concerned there is nothign to protect the 6.3 volt heater circuit or the 5 volt rect heater other than their amp ratings which one would hope would open the line fuse.

[big_teee]

Sounds like all this shorting, is a good reason to just go Rectifier Tubeless, Like Marshall does.
B_T

[R.G.]

Still as far as the PT windings are concerned there is nothign to protect the 6.3 volt heater circuit or the 5 volt rect heater other than their amp ratings which one would hope would open the line fuse.

Long story lurking in this. The mains fuse is not there to protect the PT or anything else inside the box. It's there to prevent a fault inside the amp from starting fires and creating electrocution hazards. The life of the PT is NOT what requires an AC mains fuse or why it's there.

Suffice it to say that you cannot guarantee that the 5V heater or 6.3V heater windings will blow the mains fuse. There is a solution: fuse the 5V heater winding and 6V heater windings with fuses of their own. In fact, a fuse on each secondary which is rated for the max load on that winding is the way to protect the PT from overloads. I highly recommend fusing each secondary, with two fuses on each secondary that is center tapped. Secondary shorts, rectifier shorts and so on are not common, so most people get by without ever losing an amp to this kind of failure. But protection is so cheap, why not do it?

[catnine]

Switching noise ? Is this if the rect tube shorts or is this an affect that may be heard when using the amp and there is no shorted rect?

[catnine]

Long story lurking in this. The mains fuse is not there to protect the PT or anything else inside the box. It's there to prevent a fault inside the amp from starting fires and creating electrocution hazards. The life of the PT is NOT what requires an AC mains fuse or why it's there.

Suffice it to say that you cannot guarantee that the 5V heater or 6.3V heater windings will blow the mains fuse. There is a solution: fuse the 5V heater winding and 6V heater windings with fuses of their own. In fact, a fuse on each secondary which is rated for the max load on that winding is the way to protect the PT from overloads. I highly recommend fusing each secondary, with two fuses on each secondary that is center tapped. Secondary shorts, rectifier shorts and so on are not common, so most people get by without ever losing an amp to this kind of failure. But protection is so cheap, why not do it?

Most amps really don't have the room for all these fuses. I have seen this done on some site years ago . Yes if the amp is a build and designed for all this protection and has the room and they would have to be near the PT in either fuse holders inside the amp or fuse holders like most amps use for the main line fuse.

[trem]

My AC30CCH has all the fuses that RG mentioned. Kind of impressive for a "mass produced" amp........But then again, being made in China, and getting shipped world wide, those fuses may be required by countries other than the USA where I live.
Best

[catnine]

Long story lurking in this. The mains fuse is not there to protect the PT or anything else inside the box. It's there to prevent a fault inside the amp from starting fires and creating electrocution hazards. The life of the PT is NOT what requires an AC mains fuse or why it's there.

Suffice it to say that you cannot guarantee that the 5V heater or 6.3V heater windings will blow the mains fuse. There is a solution: fuse the 5V heater winding and 6V heater windings with fuses of their own. In fact, a fuse on each secondary which is rated for the max load on that winding is the way to protect the PT from overloads. I highly recommend fusing each secondary, with two fuses on each secondary that is center tapped. Secondary shorts, rectifier shorts and so on are not common, so most people get by without ever losing an amp to this kind of failure. But protection is so cheap, why not do it?

Most old amps say a SF fender champ , the 6.3 volt heaters are on one side grounded to the chassis , so you would have to remove the ground add a twisted pair and they are twisted for a reason then place the fuse either right near the PT of the pilot lamp , then if you have a 50 volt bias tap add another fuse. On these old factory builds there is just no place for all these fuses and trying to do so may well create a short or hum no matter if you you the snap in fuse holders in the amp of twist cap type and drill holes . So yes the diodes save form running A/C to the caps and rest of the amp and may save the PT yet if any other winding shorts for what ever reason even a rare one you are out shopping for a PT . Has anyone got a kit from any well know builder that hass all of this , I mean kits you buy to build or buy built ? Allen amps, Weber, Hoffman , Mission?

[GregS]

I highly recommend fusing each secondary, with two fuses on each secondary that is center tapped.

Is there any value to have only the centre tap fused? I've seen a number of designs where only the centre tap (which connects to ground) of the HT winding is fused. It seems to me that if the centre tap fuse blew the effect would be the HT winding would now be floating instead of the centre being at 0V. The same voltage difference would still be at either end of the winding though which would make this fusing method ineffective. Is this correct?

Thanks,
Greg

[trobbins]

GregS, The CT placed fuse is the better option for any secondary side positioned fuse - due to higher likelihood of a B+ fault, than a rectifier valve shorting fault - and it normally conducts AC (rather than DC) -and if it blows then there is no power transfered through the rectifier diodes to the B+ line.

In general, there are fuse options that can be used in space limited equipment. Eg. there are fuses with wires extending from the end to allow soldering in-line - perhaps with heatshrink placed over it. There are also little round bases with plug in fuses like a small W04 bridge rectifier. I've even soldered a 5x20mm clip on to a tag board, and the other end to a wire, and the fuse effectively hangs in place due to the force of the clip ends.

Ciao, Tim.

[R.G.]

Most amps really don't have the room for all these fuses.

I think a more correct way to say it is that the designers of most amps didn't make room for fuses. Some ingenuity may be needed.

Is there any value to have only the centre tap fused? I've seen a number of designs where only the centre tap (which connects to ground) of the HT winding is fused. It seems to me that if the centre tap fuse blew the effect would be the HT winding would now be floating instead of the centre being at 0V. The same voltage difference would still be at either end of the winding though which would make this fusing method ineffective. Is this correct?

A fuse can only interrupt current that flows through it. A single fuse in the CT is better than nothing, but there are failures where it will not protect the windings. Effective for some, not others.

All of the options depend on whether you want protection or to play the odds. Shrug. Pick your own poison.

[catnine]

I read somewhere today that the same value fuse placed in the D/C end is less likely to blow as fast as it will if placed in the A/c side . I think the idea behind this is the current . In a 12 VDC car you are working with much higher current 6VDC even more current .