Gtr0 6/5/2018 12:28 AM
R3000 Diodes, output tube plate to ground.
Hi Gang.

Referencing this page at the TubeDepot website, or even referencing any modern Fender schematic, the R3000 diode is a 3KV HV protection diode that can be connected between plate and ground of the output tubes.

I have a Fender Hot Rod Deluxe and there are these same diodes in this amp as well.

I wanted to add some to an amp build I made some time ago so I ordered a handful from the only place I could find at the time - eBay.

I received them and while they say R3000 on them, they are a bit smaller in size in comparison to the R3000 in the HRD. The ones in the HRD are somewhere between IN4007 and UF5408 sized, where the R3000s I purchased are the size of an IN4007.

Does it matter in this case? Is an R3000 an R3000? or should I be concerned about the slightly smaller size?

Thanks!
 
Enzo 6/5/2018 12:45 AM
AN R3000 is an R3000.... if it really IS an R3000. Different factories might make them in slightly different shapes (packages). And I'd overlook that if I bought the parts from reliable suppliers. But you said the magic word: ebay. Who knows where they came from and if they are fakes or not.
 
loudthud 6/5/2018 12:56 AM
Lots of fakes on ebay. Is the seller in China or have very little feedback? The small case makes me think they are not genuine.
 
galaxiex 6/5/2018 7:24 PM
Agree with this ^^^^

And to add... are these protection diodes really needed?
I mean... Fender and everyone else built amps for ages without them.
Do we really need them today?
 
The Dude 6/5/2018 7:48 PM
I often just use 3 series 1n4007. I have a drawer of those, so it's easier than ordering something.
 
Enzo 6/5/2018 10:47 PM
I mean... Fender and everyone else built amps for ages without them.
Do we really need them today?
They built amps for ages with two-wire power cords and death caps.

They built cars without seat belts.

They wired houses with no grounds in the wiring.

They used to sell meat without inspecting it.


These diodes protect the OT from open loads on the secondary.
 
Gtr0 6/6/2018 12:10 PM
Quote Originally Posted by Enzo View Post
AN R3000 is an R3000.... if it really IS an R3000. Different factories might make them in slightly different shapes (packages). And I'd overlook that if I bought the parts from reliable suppliers. But you said the magic word: ebay. Who knows where they came from and if they are fakes or not.
Quote Originally Posted by loudthud View Post
Lots of fakes on ebay. Is the seller in China or have very little feedback? The small case makes me think they are not genuine.
The seller was located in Austria and has what I'd consider good feedback... 99.7%, 1 neg. of almost 500 tractions where feedback was left in the last 12 month...

here's the page... R3000 on eBay

Usually I do not buy from ebay if for no other reason than having a supply available from normal online retailers... I seem to have difficulty finding them in EU, even mouser either does not carry them or considers them to be obsolete. Either way, they are unavailable there. Perhaps I will just order them from the US.

Quote Originally Posted by The Dude View Post
I often just use 3 series 1n4007. I have a drawer of those, so it's easier than ordering something.
This is my backup plan.
 
g1 6/6/2018 12:17 PM
Can you identify a brand logo? Have you looked at spec sheets to see if they come in different physical size for different brands?
I'm guessing you have no issues or failures with the parts and are just wondering about the size?
 
Gtr0 6/7/2018 12:02 AM
Quote Originally Posted by g1 View Post
Can you identify a brand logo? Have you looked at spec sheets to see if they come in different physical size for different brands?
I'm guessing you have no issues or failures with the parts and are just wondering about the size?
I have not yet, but it is on my list of to do's... and I have not just sort of blindly tried them yet... I hope to establish some sort of legitimacy... especially more so after reading some of the replies. :-) I will know more later today hopefully.
 
g1 6/7/2018 12:25 PM
I asked because Fender started using them long ago, and may have bought a major stockpile. I'd expect the modern parts may have downsized since then.
 
Gtr0 6/8/2018 12:39 AM
Looking closer at the diode, on it is printed "R3000 PY"

In my searches for these markings, I came up with a datasheet from a company called PFS that lists this diode to be of the same size as your typical IN4007. Once I get to my workbench I will bust out the old micrometer and take a measurement and make a better determination...
 
g1 6/8/2018 1:22 PM
Diodes Inc. specs theirs at 2mm x 4mm body size, which seems smaller than a common 1n4007 to me. But measure to be sure.
 
Leo_Gnardo 6/8/2018 1:37 PM
Quote Originally Posted by g1 View Post
Diodes Inc. specs theirs at 2mm x 4mm body size
What worries me is 3000V can easily jump 4 mm, since the intended use is quenching voltage spikes in that magnitude.
 
Gtr0 6/8/2018 2:32 PM
Pulled out the micrometer... and the diode falls in specs to the datasheet - the diode was 5.25mm x 2.5mm

The big thing I am seeing is that a lot of these datasheets look the same, as in copied from each other... probably all supplied by the same manufacturer and changed subtly for each supplier. Some datasheets end the smaller diode at R2500 and starting at R3000 move to the larger package... some don't even offer the smaller package at all.

I think perhaps I will seek out a more legit supply.
 
nsubulysses 6/8/2018 3:05 PM
are these the ones you're looking for ?? GP02-30

https://www.mouser.com/ProductDetail...qzDBUV3A%3D%3D

I believe the way I ended up buying these was I ordered "R3000" from tube depot in the past, got it in the mail, read that it says GP02-30 on the actual diode, then looked it up on Mouser and ordered it from them, haha

edit: yes, zoom in on R3000 in the tube depot picture and you can see it says GP02-30
 
Enzo 6/8/2018 4:56 PM
Look at various brands of amps. ANy with flyback diodes. The diodes in each amp are doing the exact same thing, so if Fender uses R3000, Peavey SR2873, Bugera R2000, and so on, it's all OK.
 
Leo_Gnardo 6/8/2018 5:06 PM
Quote Originally Posted by Enzo View Post
Look at various brands of amps. ANy with flyback diodes. The diodes in each amp are doing the exact same thing, so if Fender uses R3000, Peavey SR2873, Bugera R2000, and so on, it's all OK.
And so on: I'll add MusicMan, and strings of diodes used in Ampeg SVT, V4, V2 & similar. Plus the highly prized Trainwrecks. Ken Fisher gives 'em a big recommendation in his Trainwreck Pages. No harm in pinching off the extension of hi voltage to just a tad beyond B+, cut the stress on output transformer insulation as well as tubes & sockets.
 
g1 6/8/2018 5:41 PM
Quote Originally Posted by Gtr0 View Post
Pulled out the micrometer... and the diode falls in specs to the datasheet - the diode was 5.25mm x 2.5mm
PY brand is Pingwei. Their datasheet for R3000 here: http://www.alldatasheet.com/datashee...ING/R3000.html
 
Gtr0 6/14/2018 12:13 AM
Quote Originally Posted by nsubulysses View Post
are these the ones you're looking for ?? GP02-30

https://www.mouser.com/ProductDetail...qzDBUV3A%3D%3D

I believe the way I ended up buying these was I ordered "R3000" from tube depot in the past, got it in the mail, read that it says GP02-30 on the actual diode, then looked it up on Mouser and ordered it from them, haha

edit: yes, zoom in on R3000 in the tube depot picture and you can see it says GP02-30
Those are the ones. However they are not in stock. And my experience with mouser is that even though they have a site in my country of residence (Belgium), they seem to ship everything from the US. I tried to order some KOA Speer resistors... the resistors were $12, shipping was $30! And that was the cheapest option. I would rather give that kind of money to a Mom & Pops retailer from the US than Mouser.

Quote Originally Posted by Leo_Gnardo View Post
And so on: I'll add MusicMan, and strings of diodes used in Ampeg SVT, V4, V2 & similar. Plus the highly prized Trainwrecks. Ken Fisher gives 'em a big recommendation in his Trainwreck Pages. No harm in pinching off the extension of hi voltage to just a tad beyond B+, cut the stress on output transformer insulation as well as tubes & sockets.
Yep, I first saw one of those Trainwreck pages where there were 3 IN4007 in series. And that is also a viable option. Just figure a single R3000 would simplify things.

Quote Originally Posted by g1 View Post
PY brand is Pingwei. Their datasheet for R3000 here: http://www.alldatasheet.com/datashee...ING/R3000.html
Thank you!!! I searched and searched. The diode falls in specs according to their datasheet. And I did buy a bag full... perhaps I will give them a go whilst looking for another supply.
 
J M Fahey 6/14/2018 4:15 AM
Since you have a bagful, nothing stops you from using 3 of them in series

So even if they are rebranded 1N4007 sold as theb real thing, you are still safe.

My main concern is not PIV but as mentioned above, creep voltage.

Dry air is an excellent insulator ... humid air or much worse: condensation on amp parts on a humid day is not, I would like to have 3 plastic bodies in series instead of just one.

Just curious about the Austrian ad: 38 Euro was the 1000 piece bag price or for a single one?

Just curious 2: please measure forward voltage drop with meter set to the diode scale ... and post it here of course.
 
R.G. 6/14/2018 6:21 PM
It's worth worrying a little about why those diodes are put there, and what they do. The general idea (I think) is that it prevents a sudden secondary-open or discontinuous-signal transient from making the OT primary think it's a flyback-ing inductor and flying its primary leads apart with high voltages that might arc a tube, but might also puncture the insulating film on the wires inside the trannie. If it punctures the insulating film, the trannie will either be toast immediately, or die more easily next time.

The OT primary in a P-P amp has two half-primaries held by a CT to B+. The diodes are arranged so that in normal operation they don't conduct, so the diodes have to have a reverse breakover voltage that is higher than 2X B+. Otherwise, the voltage on the turned-off output tube(s) would reverse-break and conduct on normal operation.

So the diodes that are active when a flyback/transient event occurs are always the ones which are on the half of the primary that happens to be headed towards ground, while the other half flies up above B+ by an equal amount. When the side headed towards ground gets below ground by enough to forward-bias the diodes there, the diodes conduct and clamp that half-primary to just a bit more than B+ across it. By transformer action, the other half-primary is also clamped to a bit more than B+, stacked on top of the real B+. That is, not hugely more than normal operation.

This is a great theory. Like most great theories, it's a bit too simple. Well, it probably works OK in many situations, but not all. The issue is that there is always leakage inductance on every transformer lead. This is inductance that by definition is not coupled to the rest of the transformer, and it still can cause voltage spikes on the high-side half of the PT primary that plate diodes won't clamp.

My preference has been to use some kind of clamping device across the whole primary or both half-primaries that will conduct at more than B+ across these diodes. Now both half primaries AND any leakage is clamped to just a bit more than the magnitude of B+, and only a bit more voltage than normal operation. But it does require some thought and application to select clamping devices of the right rating. And diodes are so very easy, no thought needed.
 
trobbins 6/15/2018 3:36 AM
For those protection diodes to conduct, there must either be some leakage inductance at play, or some dI/dt and then the valve stopping conduction rapidly. The other half-winding may not have any current flow if in class B action.

However it plays out, its better if practical to loop the protection current directly back to the winding (as per a shunt device across the winding), rather than through the diodes and back through the B+ supply to the winding mid-point. A MOV across each half-winding is a simple configuration.
 
R.G. 6/15/2018 8:57 AM
Quote Originally Posted by trobbins View Post
For those protection diodes to conduct, there must either be some leakage inductance at play, or some dI/dt and then the valve stopping conduction rapidly. The other half-winding may not have any current flow if in class B action.
I'm still on my first cup of coffee, so my parsing of this is definitely suspect. You're right, the catch/protection diodes only conduct when something tries to stop current flow in one of the windings on the transformer. There are two currents that could be interrupted, those being the magnetizing current that sets up the flux in the core so that transforming works, and the transformed current that ultimately is transferred to the secondary.

Opening the secondary pretty much affects only the current through the secondary leakage inductance, not the magnetizing current. This would create a spike that is transformed back through to the primary by transformer action. This transformation may be imperfectly carried to the primary windings due to the normal frequency response limits of the windings.

Opening a plate lead on the conducting tube will generate a spike from both the primary leakage and the magnetizing inductance. Lots of energy stored in the magnetizing inductance! But we're still dealing with imponderables about how much voltage gets generated from these different events.

The non-conducting half primary gets just as much voltage on a transient as the conducting one, as transformer action requires that all turns experience the same volts per turn (minus fuzzinesses like leakage and interwinding capacitances).

However it plays out, its better if practical to loop the protection current directly back to the winding (as per a shunt device across the winding), rather than through the diodes and back through the B+ supply to the winding mid-point. A MOV across each half-winding is a simple configuration.
I agree. I used MOVs in the Workhorse amps, but Steve ought to pop in here in a minute to remind me that MOVs have capacitance, while TVS devices have much less capacitance, and hence less chance of messing with the frequency response of the OT.
 
Gtr0 6/15/2018 10:54 AM
Quote Originally Posted by J M Fahey View Post

Just curious about the Austrian ad: 38 Euro was the 1000 piece bag price or for a single one?

Just curious 2: please measure forward voltage drop with meter set to the diode scale ... and post it here of course.
they were actually 6 or 7 Euros for a bag of 100


They all (the several that I have tested) measure at 1.5x Volts. Where In4007 and UF5804 measure at about .5V. Does that sound right??
 
J M Fahey 6/15/2018 1:38 PM
Quote Originally Posted by trobbins View Post
For those protection diodes to conduct, there must either be some leakage inductance at play,
Oh, the main primary inductance is there all the time and is way larger than any leakage one, by definition, so that is the most dangerous one.

or some dI/dt and then the valve stopping conduction rapidly.
Which happens twice every cycle on an overdriven amp.

However it plays out, its better if practical to loop the protection current directly back to the winding (as per a shunt device across the winding), rather than through the diodes and back through the B+ supply to the winding mid-point. A MOV across each half-winding is a simple configuration.
yes.
That said, MOVs must b e designed and rated, both for trigger/clamping voltage and dissipation ; while diodes take care of both points, automatically.
A nice feature I might add.
 
Tom Phillips 6/15/2018 2:19 PM
Quote Originally Posted by Gtr0 View Post
...They all (the several that I have tested) measure at 1.5x Volts. Where In4007 and UF5804 measure at about .5V. Does that sound right??
Attached is a copy of the common 1N400X data sheet and one for the R3000.
Note that the R3000 data sheet lists a 5V Maximum Instantaneous Forward voltage drop spec at 0.2A DC.
The 1N400X data lists Maximum Instantaneous Forward voltage drop as 0.93V typ and 1.1V Max.
The 1N400X data also lists Maximum Full−Cycle Average Forward Voltage Drop = 0.8V whereas the R3000 data does not include a similar spec.
[ATTACH]49330[/ATTACH]
[ATTACH]49329[/ATTACH]
We don't have an apples to apples comparison here. However, the data indicates that R3000 diodes can exhibit higher forward voltage drop than our familiar 1N400X rectifier diode. Therefore, based on the manufacturer's data, I don't think there is any problem with your diodes.

Cheers,
Tom
 
Mick Bailey 6/15/2018 2:38 PM
The forward drop of an R3000 is definitely 1.5v when tested with a DMM.
 
Tom Phillips 6/15/2018 2:43 PM
Makes me suspect that there are two diodes in series within the R3000 package.
 
Enzo 6/15/2018 3:39 PM
Years ago working on monochrome video monitors, the flyback transformer spits out 19kV for hte third anode cup on teh CRT, but it needs to be rectified. They used these "stick" rectifiers. About 1/4" diameter, and almost as long as a business card, they rectified all those kilovolts. Inside they were a stack of multiple little discs of diode.

[ATTACH=CONFIG]49331[/ATTACH]

Here is a nice article about the topic.
http://www.kronjaeger.com/hv/hv/comp/rect/index.html
 
J M Fahey 6/15/2018 5:18 PM
Oh, I saw the "gleich_gross.jpg" label on screen (sometimes my ISP connection is ssslllloooooowwwwwwwww and text loads before actual pictures, by a few seconds) and imagined the image might have referred to something like:

[IMG]https://vice-images.vice.com/images/content-images/2016/03/03/meeting-germanys-newest-gang-the-osmanen-germania-876-body-image-1457011641.jpg[/IMG]

Then I remembered "gleichrichter" is German for "rectifier"
 
trobbins 6/15/2018 5:45 PM
The smaller 7mm and 10mm disk size MOV's have pretty low capacitance (below 100pF), and relative to the typical shunt capacitance across a half-winding are imho benign. If it was a hi-fi amp, then that capacitance could be used as a zobel network option out well beyond 100kHz.

I'd suggest making the MOV 1mADC rated voltage noticeably more than B+, then a MOV across each half-winding would only start to load up each plate's waveform on voltage excursions at or beyond what the diodes would do to one half-winding at that time. MOV loading would get progressively heavier if the energy in the winding is sufficient to push the voltage higher - even 7mm disks appear to have a sufficient continuous power dissipation capability when I looked at the levels a few years ago. Loudthud presented some illuminating X-Y plots of plate voltage-current in a PP guitar amp showing the 'level' of voltage excursion beyond the +/- B+ span. Some detailed notes are in https://www.dalmura.com.au/static/Ou...protection.pdf.

MOV's aimed for 240Vac and 415Vac applications are quite common.
 
loudthud 6/16/2018 12:30 AM
It's important to recognize that the pics in trobbins paper were taken with the speaker connected. Speaker inductance (IMHO) is the dominate source of spikes that the R3000 diodes are meant to deal with. The speaker X-Y clips I posted in this thread:

http://music-electronics-forum.com/s...ad.php?t=35493

were actually created with a solid state amp so none of the spikes were coming from an output transformer. Spikes are clamped by an MOV to ground and diodes in series with the Drains prevent the Drain-Source diodes from clamping the spikes. MOSFETs are connected in a common Source configuration with Sources going to the +/- rails. Also see the pic in post 52 of this thread:

http://music-electronics-forum.com/s...t=39829&page=2

Plate Voltage spikes of a tube amp going 600V negative.
 
J M Fahey 6/16/2018 4:16 AM
I remember (loooooonnnggg ago) when I first scoped one of my Twin clones to see the overdriven waveform and saw something similar to this:

[IMG]https://www.stereophile.com/images/archivesart/Manfig03.jpg[/IMG]

the point being that leading edge overshot +V rails and ground (yes, saturating tube gets negative peaks) by 5% to 10% which is a lot.

Nothing like the nice gentle rounded top waveforms drawn on many books, of course, but the real thing.

Clamping diodes wont let saturating plate go below ground (besides minuscule diode drop that is) so other plate wont go above 2X +V (see saw action) so yes, under normal (loaded) conditions, diodes will clamp/chop that overshoot.

In my book, they will also avoid many kV peaks which can and will appear if speaker gets disconnected or is much higher impedance than expected.

Hos Ultimate Attenuator uses a one-size-fits-all 30 ohm load resistor.
Fine for 16 ohm amps, not so sure about 4 ohm ones.

A MOV "should" be rated around 3X +V , which is still safe, because if rated 2X it will be triggering all the time on overdrive.

In this case rating to be considered is peak voltage, so 1.4X the AC value printed on it.
 
trobbins 6/16/2018 5:45 AM
Quote Originally Posted by J M Fahey View Post
A MOV "should" be rated around 3X +V , which is still safe, because if rated 2X it will be triggering all the time on overdrive.
MOV's don't 'trigger', they have a very soft loading characteristic. A MOV rated at a 1mA level at 1.5x B+ would at worst just start passing a few mA at the very peak of that scope waveform. That MOV rating has quite a span for the 1mA level, so only the worst case part would pass a mA or so.
 
J M Fahey 6/16/2018 12:50 PM
Ok, then rate them so they pass MANY mA at, say, 1.5X Vpeak or they wont do much "protecting" at all.

In any case, I never use them, much prefer the very predictable and "automatic" reverse diode clamping action.
 
trobbins 6/16/2018 5:51 PM
JMF, I'm not about advocating you use MOVs, just commenting on any misconceptions that arise.

A MOV can be a good option for over-voltage protection of output transformers as its soft loading characteristic can usually be aligned to fit between normal anode working voltage levels and the likely design voltage stress rating of the winding. Luckily an OPT primary winding would be designed, manufactured and possibly even tested along the same lines as a power transformer, with respect to insulation system and creepage/clearance, as the working voltage of an OPT primary winding is often higher than mains AC (even for my 240Vac mains, and especially the rating to core). So as a ball-park, an OPT primary winding could be expected to have at least a 1.5 to 2kV withstand capability. If the MOV can exhibit a heavy loading resistance across the winding at the 1.5 to 2kV level then it is doing its job, noting that it would be progressively representing a lower and lower resistance if the winding had the energy to keep pushing its terminal voltage up towards that 1.5-2kV level.

A good perusal of typical MOV V-I curves shows that there is usually a range of MOV models (eg. VAC ratings) that can fit the objective of not noticeably conducting at levels up to 1.5-2x B+, and heavily conducting at 1.5-2kV.

I agree that the MOV 'design path' can be a bit too fuzzy for those that prefer tight tolerance protective actions, but note that most seem to live with using fuses.

Ciao, Tim
 
R.G. 6/16/2018 6:41 PM
Quote Originally Posted by J M Fahey View Post
In any case, I never use them, much prefer the very predictable and "automatic" reverse diode clamping action.
The problem is that plate diode clamping relies on transformer action, and it can't clamp leakage inductance flyback at all. The clamping diode is always clamping one side to ground and it's the other side that is flying up at a bit less than B+ times 2, plus any leakage inductance kickback.

Granted, the differences may be small in real world situations.

One advantage for the TVS style clamps is that they're more abrupt than MOVs.

Many amps have none of the above, and live long, happy lives.

Sigh. Mother Nature is such a mother.
 
J M Fahey 6/16/2018 8:05 PM
Good.

So far we are speaking words, it would be nice to put some numbers into it.

Please somebody find an OT datasheet showing rated primary inductance (many will state that) and leakage inductance ... I bet most will be silent, but maybe a few brave souls print it.
Probably some Hi Fi guys will.

I would not be surprised at finding leakage inductance is 10% or 5% of main inductance.

And then finding that although it IS there, (I have no doubt about that, only doubt is how much) , it probably is not enough to kill a mouse.

But lets get some numbers first.

Energy almacenated in an inductor is straight to calculate .... same as energy in a capacitor is.
Then well know whether we are facing a Lion or a furry kitty
 
trobbins 6/16/2018 8:41 PM
I have had a go at putting some numbers to spike energy from a few fault scenarios in the linked article:
https://www.dalmura.com.au/static/Ou...protection.pdf

Hi-fi OT's were very informative of parasitic L & C values, mainly due to the Williamson amplifier's demands - the Partridge datasheets in particular (eg. links in https://www.dalmura.com.au/projects/Williamson.html). A few transformer players, like Patrick Turner, have progressed the design and measurement of those parasitics. But yes OT's in guitar amps could well show larger parasitic L, but also likely lower winding inductance.
 
R.G. 6/17/2018 10:22 AM
I'm all for looking at numbers. Even where you don't know the underlying relationships and interactions, they can give you useful trends and projections.

First, a well designed hifi OT will have microscopically small leakage inductance. The "Goodness Factor" from the Golden Age of tube amplifers was simply the primary inductance divided by the leakage inductance. These were commonly 10,000 to 100,000.

These were made by very, very careful interleaving of layers. It is possible to make excellent estimations of leakage inductance by calculation based on numbers of turns and the physical arrangements of window size, winding width, and winding build height. It is possible to >maximize< leakage inductance by winding primary and secondary side by side, as you see in some power transformers; it's even more extreme in ferroresonant transformers with windings side by side and iron shunts between them. This is the degenerate case, and definitely not what you want in a signal transformer.

To minimize leakage, you intermix the wires. The extreme in this direction is to wind all wires multifilar. A bifilar OT primary is possible, and very useful for preventing pseudo crossover distortion from leakage causing issues in the handoff from one half cycle to the other in an AB pushpull. It would be ideal in terms of leakage for an OT to wind all of it multifilar, with the number of turns being just the secondary number of turns, but with maybe 25 primary wires wound side by side with the secondary turns, and then the many primary sections interconnected in series external to the winding. Aside from the practical impossibility of doing this, this winding style also maximizes interwinding capacitance, which may give other problems.

Back at leakage inductance, the math says that if you get a certain leakage for primary over secondary, the leakage is reduced by a factor of the square of the number of interfaces. Primary over secondary (or vice versa) is a factor of one. Split primary (or secondary) sandwiching the secondary (or primary) is two interfaces, and the leakage from the simple case is reduced by a factor of four. A split with three sections sandwiching two sections is four splits, and this leakage is reduced by a factor of sixteen over the simple one-interface case. This process keeps up until you can no longer do the physical work of winding the layers and splitting the windings into finer and finer sub sections, or until you get to multifilar.

So the question becomes what is your leakage inductance in that OT you have in your hand? You can measure it of course. But there isn't any good way to say how big it is as related to the primary inductance without either measuring it or knowing exactly how it was wound. In the early days of tube amps, guitar amps used much the same OTs as hifi stuff, as this was easy to get from the companies that made hifi stuff. Later, hifi and guitar amps diverged, and the rise of the MBA led to cheapening everything until the customers just would not buy them any more so when guitar amp makes had to start ordering special runs of OTs for just them, they started backing down on the interleaving to reduce cost. This led to some pretty bad OTs in both the sound and high leakage inductance, as the end result was one-interface winding.

The one-interface winding was common in some brands. Marshall was reputed to order off-the-shelf Radio Spares for a while. I have cut open some dead Marshall OTs and found single sections. It's likely to be the case in other brands as well. My amp-tech friend tells me that it's more common in his experience to have dead Marshall OTs. It's not clear that this is a causal link, but it's fun to specify.

Guitar amp OTs have lower primary inductance than hifi standard, as befits their lesser needs for low end, and most likely poorer interleaving. The exact numbers will vary a lot, depending on the size of the transformer's window and how it's wound, but it's hard to imagine a ratio of primary to secondary worse than 20:1, that giving you a 5% leakage number.

Guitar OTs also typically have smaller primary inductances. I've heard numbers of 20H to 50H for guitar OTs, but have not measured one in a loooooong time. Let's say for the sake of discussion that the OT in question has a primary of 40H and a factor of 2%. So the leakage inductance might be 800mH. That seems grossly too big to me. We could just as easily say it was 1% or 0.5%. So you'd get a couple of hundred millihenries.

From there, the energy is just E = 1/2*L*I squared.

But now we get down to the issue of what damage gets done. Nothing happens until the wire insulation gets punctured. Puncturing depends on voltage, not energy. Energy gets into play once a puncture has been made, and damage per puncture is proportional to energy. Even a much smaller inductance can generate a nearly unlimited voltage. The real limits on V = L*di/dt for generating a puncture is the winding self-capacitance. It limits the rate of rise and hence the voltage. My quick look just now at magnet wire insulation showed film strengths of between 50l0V and 2500V.

A lot depends on the details of where the voltage happens and what happens after the first puncture. Magnet wire mostly insulates between adjacent turns in layer wound coils. In random wound coils, physically adjacent turns may have large voltages between them, so the voltage stress may be both larger and vary from unit to unit.

The first puncture most likely does not kill the transformer, and one or a few punctures may not ever kill it. But any puncture of the layer insulation will happen at a point of high voltage stress, and like arcing on tube sockets - and for the same reasons! - further arcs will be easier to start at the position of a previous arc. So one arc is a question of voltage, not energy. The second through ten thousandth arcs are where the damage adds up.