Tube Rectified Amps: Standby via Switch on Rectifier Heater - Good Idea?

[JoeM]

I have a Princeton that's been mine since sometime in the 70s. For most of that time it's had a 5U4, which comes on fairly quick. No standby on that amp, and it's never had a problem. So I dont think it's a big deal.

[catnine]

All this talk about tube rectifiers brings me back to the ideas many have of placing diodes in series with the PT HT leads with the rect tube to protect the PT if the rect tube fails .

Of all the fender amps I've had over the years and most were small using a 5Y3 and most were well over 30 years old and had all the original componants in them down to the caps and resisters . They all had the original tranny's too.

I do realize back then a 5Y3 was better than the ones we have today yet I use NOS 5Y3's . Perhaps it's because of the newer rect tubes there is the problem . I never saw an old fender with all these safety add on's . I haven't seen amp kits either with them added on. Sometimes I feel it's over kill and new thinking rather than neccessary .

[R.G.]

All this talk about tube rectifiers brings me back to the ideas many have of placing diodes in series with the PT HT leads with the rect tube to protect the PT if the rect tube fails .

Of all the fender amps I've had over the years and most were small using a 5Y3 and most were well over 30 years old and had all the original componants in them down to the caps and resisters . They all had the original tranny's too.

I do realize back then a 5Y3 was better than the ones we have today yet I use NOS 5Y3's . Perhaps it's because of the newer rect tubes there is the problem . I never saw an old fender with all these safety add on's . I haven't seen amp kits either with them added on. Sometimes I feel it's over kill and new thinking rather than neccessary .

I'm pretty sure that building owners objected strenuously to being forced to put in sprinkler systems, fire escapes, and fire extinguishers. They all had seen buildings last centuries without burning. And only other people's buildings burned, anyway.

To belabor the point, fire extinguishers are overkill in buildings almost 100% of the time. Most fire extinguishers are never used - ever. But it's not in buildings that aren't burning that you need a fire extinguisher, is it?

I personally have fire extinguishers in my garage, kitchen and cars. I've never had a fire to put out with them. I think of them as a kind of amulet that keeps fires away.

But meanwhile, back at tube amps. I *have* seen lots of old tube amps die for various reasons. Some die because the filter caps short and kill the PT as well. Some have the PT just die. In how many of those did the rectifier short and kill the PT? Or have the rectifier arc, soft short, kill the filter caps with a blat of AC, or punch through the insulation in the PT windings with the arc transients? Hard to say after the fact, isn't it?

It's not necessary to have cars or airplanes, when horses and boats worked for thousands of years.

Anyway, no one has to put protection *anything* in their amp. A couple of bucks and some soldering for a protection against declining-quality tubes isn't by any means necessary.

I just like safety amulets.

[catnine]

R.G I see your point yet how much protection is enough in a tube amp? I can see how the diodes protect the PT in case the rect tube goes south . What about the 5 volt heaters for the rect tube that if the rect tube shorts out they also go to the PT or is it since they are 5 volt and higher mA rated that they can handle a short possibly blowing the line fuse ? Then there are the 6.3 volt heaters that have nothing to do with the rect tube . Then I have read about diodes in some amp to protect from High Voltage flyback if there is not load on the OT secondary. I have seen diagrams showing every lead coming off the PT fused on some sites.

I bought a 78 SF champ from a friend and it blew the line fuse when I installed a new one so I took it to a guitar and amp shop and was sold a 5Y3 and a fuse fixed the problem yet I have no idea exactly what problem was in the 5Y3 so this is not to say the PT could have not been damaged .

I see most of the Fender and other amp brands of today don't use a rect tube at all yet the only other protection they install is a line fuse , some have a fuse on the B+ before the filter caps on the circuit board .

I installed a 1/4 amp fast blow fuse on both my builds between the rect pin 8 and the first filter cap so if I add the diodes the PT is protected, not sure about the 5 volt and 6.3 volt they are rated in amps so perhaps I need something in there too , perhaps a small fast blow fuse rated at the actual current draw .

Yes I only brought up the diodes in series with the rect and PT , then I wonder about the rest which seems never to be mentioned when talking about the rect to PT diodes. That's the issue I have. Isn't this sort of like having the fire extinguishes 25% changed ?

[pdf64]

The thing is to have sufficient info about the parts used to judge which circuits may be able to tolerate likely overload scenarios, and which won't; and to fuse them accordingly.
Anything can fail, it's a matter of assessing what is the likelihood and consequence of the various failure modes. If it's reasonably likely and has significant consequence, then a fuse (or some other mitigation for that failure mode) may be a good idea.
Pete

[frus]

read this:

The Valve Wizard

also, you can see on the last picture the way to protect the PT against a rectifier short with fuses. The series diodes that you mentioned serve not only to protect the PT (which you can accomplish with fuses), but for the amp to continue playing in case of a tube rectifier short

[Enzo]

We talk about adding a pair of 2 cent diodes to protect agains a shorted rectifier tube because that is a lot more likely to happen than a problem with a 5v heater winding. One reason the 5v winding doesn't get diodes or something is that it has no path to ground. If the rectifier tube heater could somehow short end to end, it would be a dead short across the 5v winding, but I have never in my almost 60 years of electronics seen a tube with a heater shorted end to end. If a rectifier tube shorts something TO its heater, then the heater winding won;t be at risk, the other thing it shorted TO is at risk, because that other thing probably includes a path to ground. By it very design, having hundreds of volts to ground is within the specs of the transwformer 5v winding.

To put it into the fire extinguisher analogy, I keep fire extinguishers around for all manner of fires. MY house/shop can catch fire since it is ful of combustibles. One thing I do not keep an extinguisher for is magnesium fires. Or phosphorus. There are esoteric things I COULD worry about, but where on earth could I have a magnesium fire in my house?

[R.G.]

R.G I see your point yet how much protection is enough in a tube amp?

It depends on your definition of "enough". There are some predictable things that can kill a tube amp. They are more or less likely to happen. A tube amp owner has to decide how to handle these things. He/she can:
1. Ignore them, not even knowing what they are. This is what I'd call the "Ostrich Maneuver".
2. Chose to find out, and accept the risk of the cost of repairs and the loss of use of the amp til it can be repaired.
3. Make a value judgement on whether it's worth it to them to spend money now to avoid the amp breaking down, or to lessen the severity of the failure when the inevitable happens, versus gambling on it not happening to them while they own the amp.

If you think about it, tube sockets are one of these little protection judgement decisions. It's a lot more expensive and difficult to unsolder a tube to replace it if it's soldered in. The early makers of tube amps realized that tubes are so very likely to fail that they ought to be in sockets for easy and cheap replacement.

Deciding what protection is worth having depends on value judgements about (1) how likely the fault is and (2) how affordable the protection or disaster amelioration is to them. If the fault will essentially never happen, protection can be dispensed with. If something is very likely to happen (like tubes dying) then it makes sense to figure out the most practical and least-total-cost way to either prevent them from ever happening, or to minimize the cost if they do happen.

Think of it as insurance - a cost you pay hoping never to use it, and a dead loss to you if you never do. I had homeowners' insurance for 23 years before I filed a claim for roof damage from a storm. When the storm came, I was glad I had insurance.

I can see how the diodes protect the PT in case the rect tube goes south . What about the 5 volt heaters for the rect tube that if the rect tube shorts out they also go to the PT or is it since they are 5 volt and higher mA rated that they can handle a short possibly blowing the line fuse ? Then there are the 6.3 volt heaters that have nothing to do with the rect tube .

Each failure probably has a unique protection that's practical and inexpensive compared to not having it. In the case of the 5V rectifier winding, a shorted tube does something different than it does to the HV windings. The 5V rectifier heaters being shorted (if that's the only fault that happens) won't kill the filter caps, but it *can* overheat the secondary winding before the primary fuse can blow. Gary Moore helped me with the math on this one. Since the 5V heater is not rectified, putting diodes on it won't help.

What *will* help is having a secondary fuse for just the 5V heater winding, rated for just a bit more than full rated current. That way if the rectifier tube shorts its heater winding, the fuse pops and the damage stops. Repairs are to replace the rectifier tube and the fuse. If the fuse isn't there, you can be replacing the PT, the single most costly part in the amp (outside of possible rarity value). Likewise, I think someone who wants to minimize his spending on repairs might want to put fuses in the heater windings, for the same reason. A $0.50 fuse holder and fuse plus some rewiring is the cost compared against a - what? $100 PT? $200PT? Plus labor? What do those things cost today? That seems like a good tradeoff to me. Notice that the solution is different for protecting the PT's HV windings against a bum rectifier tube than it is protecting the PT against a shorted heater winding.

Then I have read about diodes in some amp to protect from High Voltage flyback if there is not load on the OT secondary.

I actually prefer to do that with a MOV across the primaries, but the idea is to keep flyback pulses from the inductance of the OT from puncturing the wire insulation and killing your OT - the second most expensive single part in the amp. Again, a few diodes or a $0.50 MOV versus an unlikely-but-possible dead OT when someone cuts your speaker cable during sound check. Diodes or MOVs to make this a cheap repair seems like a good trade-off to me. Notice again - different possible failure, different protection circuit.

I have seen diagrams showing every lead coming off the PT fused on some sites.

Clever guys. What that does is ensure that a secondary load going crazy will not kill the PT. $0.50 fuse spent for sure, versus $100-200? spent for repairs and amp being down for a week/month? in the repair shop, but not very likely? It's a tradeoff.

I bought a 78 SF champ from a friend and it blew the line fuse when I installed a new one so I took it to a guitar and amp shop and was sold a 5Y3 and a fuse fixed the problem yet I have no idea exactly what problem was in the 5Y3 so this is not to say the PT could have not been damaged .

It's possible. Not all car wrecks result in injury or death. Not all rectifier failures kill the PT, maybe not very many. It kinda gets down to what Dirty Harry said - do you feel lucky?

I see most of the Fender and other amp brands of today don't use a rect tube at all yet the only other protection they install is a line fuse

A line fuse is not there to protect the amp or anything inside it. It is there (and effectively required by standard or law) to protect the people and building against fires and/or electrocution if and when something inside the amp dies and causes shock hazards or fires. There are in fact many failures inside a tube amp where the line fuse simply can't protect anything inside the amp. It might, and if it does, that's extra goodness above and beyond its purpose. But it's there to protect the people and building, not the amp.

This is a good illustration of a bit more protection philosophy. To do a good job of cost-effectively protecting a complicated setup - like a guy playing guitar in a room with other people in a building, perhaps during a lightning storm, you have to (1) identify the possible failures, (2) make some reasonable guess about how likely they are to happen (3) make some reasonable guess about how you could prevent damage or limit it to some level that's affordable to fix and (4) make the cost-value decision of how much to spend now to avoid a theoretical problem that only has a probability.

some have a fuse on the B+ before the filter caps on the circuit board .

That's OK, and it might protect the PT and rectifier against a shorted cap (by the way, electrolytic caps have a 100% chance of failing; the only question is how soon?). I personally think a fuse in each half of the HV winding would be better, as this will protect the PT against bum rectifier tubes as well as shorted caps. In my opinion, using one fuse after the rectifier is shortsighted and wishful thinking. You get much better coverage back on the winding itself.

I installed a 1/4 amp fast blow fuse on both my builds between the rect pin 8 and the first filter cap so if I add the diodes the PT is protected, not sure about the 5 volt and 6.3 volt they are rated in amps so perhaps I need something in there too , perhaps a small fast blow fuse rated at the actual current draw .

My personal opinion is that an expensive and/or rare PT would need one fuse on each section of winding that could be overcurrented. One primary one on the 5V heater winding. If the HV and heater windings have a hard center tap, one fuse in each side. The fuses should be rated for the full RMS current of the winding, and since PTs are rugged and withstand momentary overloads very well, the fuses should be slow blow or time delay types.

It's worth thinking about the difference between the fuses on the HV windings and back-up solid state diodes on the rectifier tube. A rectifier tube can have several different internal shorts. Some of these would let AC through the rectifier without shorting to ground. In that case, the HV winding fuse would protect the PT from damage, but the backup diodes would let you keep on playing. That can be a good thing if you are getting paid for a gig, or don't have the money/time for a repair at the moment.

Yes I only brought up the diodes in series with the rect and PT , then I wonder about the rest which seems never to be mentioned when talking about the rect to PT diodes. That's the issue I have. Isn't this sort of like having the fire extinguishes 25% changed ?

As Enzo notes, there are different kinds of fires, and different extinguishers. And there is no substitute for knowing and thinking. To do a good job of minimizing the money coming out of your pocket for amp repairs, you need to know (1) what kinds of failures might happen, (2) how likely they are (3) what might prevent them or at least minimize the cost and time of repairs and then decide whether the cost of prevention is worth avoiding a more expensive cure. You're getting a lot of items 1, 2 and 3 for free in this forum. What you decide to do about them is up to you.

I notice that I'm doing a lot of numbering of alternatives. This reminds me that there are 10 kinds of people in the world - those that understand binary that those that don't.

[catnine]

Ok this is the chassis that my SF champ sits in . I kept all the leads short as possible . So there is room for diodes between the PT HT leads is I cut the diode leads short enough and move the leads a bit . On the far left top inside the chassis there is one of the normal sized fuse holders to give an idea of how little room I have to think about adding more fuse holders without mounting them through new holes drilled in the chassis with grommets . I can't put them near the tube sockets . I am going to try to add the fuse holders it possible and just add the diodes . If this were a normal SF champ chassis there would be a lot more room to work , this chassis is just deep enough for the tube sockets to clear the front and back of the chassis . It all looks good on paper but in actual practice it's not always easy or even possible .

There are many amps that are packed even many old production fenders and especially small SE amps like fender champs. I look at narrow panel tweed 5E3's and even in them there is not much room to add all the desired protection .

[R.G.]

...It all looks good on paper but in actual practice it's not always easy or even possible .

There are many amps that are packed even many old production fenders and especially small SE amps like fender champs. I look at narrow panel tweed 5E3's and even in them there is not much room to add all the desired protection .

Yes, that's right. Sometimes the real world won't let you do all the stuff you'd like to, or makes it so expensive that you decide not to do it.

I suppose that one way to look at it is that if something you want to do is impossible, then that's the same as being infinitely expensive - no amount of money ( or time, or effort ) will make it be that way.

Things like reliability, ruggedness, fault tolerance, and failure protection are always better and cheaper if they're designed in to start with, not pasted on. We do what we can.

[catnine]

I guess all I can do is add the diodes and hope things hold up . so far they have for 5 years . My other build is a 71 Fender Music Master bass amp chassis and cab that I got blown so I built it into a 6G2 input and tone stack spliced to a 5E3 PI and power section with an adjustable fixed bias I used a weber 5E3 eyelet board and made the adj fixed bias board as a separate small eyelet board and mounted it where the cathode bias resister and cap went . It is pretty well packed in there as well .I even added a 9 pin hole for the PI and then tried diodes for the rect but the beefed up Allen Amps Champ PT had to high of HT voltage so I drilled another hole for an Octal to fit the 5Y3 and that was a close fit since it's near the PT.
this is that chassis below and below that is the view of the back chassis in cab and the Allen Amps huge Champ PT and his huge OT that can both power a pair of 6V6GT's with a 5Y3 or a pair of 6L6GC with a GZ-34. It took quite a bit of planning to get it all in and not have noise issues . I even installed two speaker jacks so I could use the 4 or 8 ohm taps . I need to get a NOS 5Y3 for that build what I have in there now is a sovtek 5Y3 so called and have 410 VDC on the plates yet when I used the champs NOS 5Y3 the voltgae only dropped 10VDC yet so far the amp has been working for 9 years yet I don't gig anymore and don't crank either up in this old apt Tought of trying a JJ 5Y3 since Allen Amps uses them and Euro tubes likes them they seem reliable to me as sellers .


If I had known at the time when I built it that Hoffman sold blank boards I would have made my own and much smaller . I did that with a few other builds of the same exact circuit and used smaller F&T filter caps like the champ has. I just drew the layout on paper so everything would fit and drilled the board and mounted the eyelets and made the board half the size. Best I can do on that build is add the diodes. I never unplug the speaker from the jacks so unless the speaker blows I doubt this would hurt the OT . If I tried to add more fuses in either amp I would most likely creat a short just trying to fit them in without them at some point touching something else. The champ chassis I got for Allen Amps one he stopped selling quite a while back , if I would have got a wide panel tweed chassis then they are much deeper front to back so there is much more room than this chassis and this one had all I needed to build a SF champ just enough eyelets and came with the board and it had the three pots for the vol,bass and treble.

[R.G.]

To anyone who's thinking about protecting their PT (or OT!) with fuses and having a similar problem, the EU style 5x20 fuses are much smaller than the normal AGC series. You can get holders that mount to a panel with one screw. The body is less than an inch long, and about 1/2" high.

Littelfuse MetricOMNIBLOCK

There are other brands that are similar. That's just the first link I found.

[R.G.]

To anyone who's thinking about protecting their PT (or OT!) with fuses and having a similar problem, the EU style 5x20 fuses are much smaller than the normal AGC series. You can get holders that mount to a panel with one screw. The body is less than an inch long, and about 1/2" high.

Littelfuse MetricOMNIBLOCK

There are other brands that are similar. That's just the first link I found.

[catnine]

R.G. when you talk about Full rated RMS that means if my PT is rated at 100mA on the HT secondaries the fuse should then be a 100mA slow blow fuse and if the heater is rated at 3 amp then it should be a 3 amp slow blow fuse or in both cases even a bit below that since that's max and not what the circuit is actually drawing . Say it's 5Y3 rated at 2 amp wouldn't a 2 amp slow blow be better just to be on the safe side? This is of course provided you are never going to use a rect tube rated over 2 amp.

It's not easy to find fuses that rate @100mA so I would imagine the diodes would be better in any amp for the PT HT secondaries and it would be easy to locate 2 or 3 or 5 amp fuses to use with the fuse holders.

[R.G.]

R.G. when you talk about Full rated RMS that means if my PT is rated at 100mA on the HT secondaries the fuse should then be a 100mA slow blow fuse

I do apologize for this, but it's a bit more complicated. It's common to rate tube amp PT high voltage windings in "DC output current" instead of the more common RMS of normal transformer practice. Probably the heaters are rated in AC RMS current. This is useful to someone picking a replacement transformer, but it obscures the issue with the HV DC current.

When you rectify an AC secondary and feed that into a capacitive filter, the current feeds the cap in spikes. The spikes are quite a bit larger than the average DC current, and have a different RMS (or heating-equivalent) in the winding. That makes the transformer windings heat up more than they would with just the DC output current flowing in them. For a full wave center tap circuit like a tube rectifier setup, each half of the winding will see up to 1.2 times the DC output current, even though both are conducting. So for a transformer rated for 100ma DC output, each half-secondary for the high voltage would be protected with a slow blow fuse of about 120ma. 125ma will be OK, and may be easier to find.

if the heater is rated at 3 amp then it should be a 3 amp slow blow fuse or in both cases even a bit below that since that's max and not what the circuit is actually drawing .

Since secondary fuses are to protect the PT, I would make them be at the rating, so that temporary transients don't cause the fuses to blow unnecessarily. So if the PT's 6.3Vac heater is rated at 3A, using a 3A fuse is not dangerous to the PT. You might use a smaller one if you're **sure** your amp will never get above your fuse but below the 3A max, but the PT is OK at 3A, so I'd use the rated current.

Say it's 5Y3 rated at 2 amp wouldn't a 2 amp slow blow be better just to be on the safe side? This is of course provided you are never going to use a rect tube rated over 2 amp.

If you're talking about the 5V rectifier heater winding, and the PT's 5V winding is rated at 3A I'd use a 3A fuse because people often just swap any old rectifier tube into a socket. Since the PT is OK at 3A, I'd go for 3A, since the idea is to protect the PT.

It's not easy to find fuses that rate @100mA so I would imagine the diodes would be better in any amp for the PT HT secondaries

Careful - you're mixing apples, oranges, and grenades. Fuses for protecting the PT from overcurrents, like a shorted cap, do a different thing than the series diodes. The series diodes protect the PT from a rectifier tube with an internal short. The fuse protects it against other things.