How to protect the OT?

[Gregg]

Gregg, that is a nice circuit, but it doesn;t protect against no load, it only protects against not having a plug in the jack. Bad cord, intermittent voice coil, or cab wiring problems won;t be protcted against.

You're right but it's one more line of protection. Maybe "unloading" the amp in the way you mention doesn't happen very often otherwise the big companies would have invented something against it or they're just saving on parts?

[daz]

Wait, since all the current from that point is also going through the OT's center tap, then I am back to a fast-blo, OT center tap fuse... like just about all Marshall amps used.

Bruce, i looked at 5 or 5 marshalls at schematic heaven just now and the only fuses i saw near the OT center were in series with the B+ BEFORE the OT CT, not in series with the CT. Is this what you mean? And if so, what would the right fuse value be?

[Steve Conner]

Hi daz, all

The precise placement of the fuse doesn't matter, as long as it's "upstream" of the OT center tap, all the current that passes through the OT must pass through it. It's typically 500mA for a 50W Marshall type amp, as they draw about 300mA at full blast. (Replacing it for 20A is barbaric )

The diodes are indeed "Amulets to the voltage gods", hey, I like that phrase For anyone who just has to know, here is an explanation of how they work in autistic detail!

There are actually two forms of inductance in the output circuit of a P-P amp. The first is the inductance associated with the transfomer core, and also any inductive load connected to the speaker terminals (for instance a speaker near its resonance) which I will call the magnetizing inductance.

The diodes can deal with flyback spikes from this inductance by returning them to the B+. If we assume the two halves of the OT primary perfectly coupled, then any spike that makes one half shoot up above the B+ will make the other half shoot down towards ground by the same amount. The diodes will clamp any negative excursions as soon as they get below ground, and therefore this clamps the spikes to a voltage equal to twice the B+.

In this respect they're doing the same thing as the diodes added to the output stage of BJT power amps, that prevent inductive loads from forcing the output voltage outside of the supply rails. They're also doing the same as the two diodes in a FWCT rectifier, taking the energy coming back from the load and "rectifying" it back into the B+.

However, the other inductance is leakage inductance, which is a measure of imperfect coupling between the primary halves. The above form of clamping doesn't work in this case, because it assumed perfect coupling. The only way the diodes can limit spikes in the leakage inductance is by breaking down in the reverse direction, and this is what can blow them.

Musical instrument OTs tend to have lots of leakage inductance, too.

[daz]

Thanks Steve. I will add a 500ma fuse and leave it at that....well, that is, assuming someone doesn't now chime in with reasons why that too is a waste.

on a side note, this got me to thinking about choke location. So i started looking at marshalls and noticed the choke came AFTER the first cap can and OT. So i tried it. Wow....it's been a long time since i have found a tweak that really advanced the amps feel/tone to any degree, but that really did. I guess with the two sides of the can tied together i have doubled the value and a tighter sound resulted. But it also for some reason seemed to get richer and now has more strat cluck in the bridge/middle position, something i always look for because when i get that to a good degree it's usually part and parcel of good tone in general. It's the way i determine a amps tonal value to me....the more cluck the better the overal tone. Maybe thats just me, but in any case that really was a great change. It did a lot of things to the tone that improved it, even seems to have more resonant and fluid mids, another thing i lust after. Maybe all this would not be noticeable to someone listening, but certainly to me as i play it. I'm still waiting for a chance to record a clip, but everyone upstairs has been home all the time lately.
Thanks Steve, a 500 ma fuse will go in both amps.

[pdf64]

500mA fuse? As long as it's quick blow between the OT CT and the B+. My experience indicates that a slow blow can allow the OT to get damaged before it fuses.

Re choke location
'thinking about choke location. So i started looking at marshalls and noticed the choke came AFTER the first cap can and OT. So i tried it.'
How do you mean? The schematic you posted showed the regular arrangement.
http://s24.photobucket.com/albums/c3...urrent=amp.jpg
Peter.

[daz]

Then I'll use a fast blow and if it blows w/o reason i'll up the value.

[tubeswell]

Hey daz, just to confuse ya, the 1987 schematic as a T500mA (T for 'Time' - i.e.; Slo-Blo)

[daz]

Well, if the regular 500 blows occasionally i'll just up it a bit or try the slo-blow. Any way you look at it i'm better off than with nothing.

[pdf64]

It's slow blow in the 1987 because it's feeding the downstream B+ as well as the OT, and so charging up the caps. Thats why to specifically protect the OT, a quick blow can be used between the B+ and OT CT. The downstream B+ should be on the input side of this fuse, not the output side. All it's feeding is the OT, not having to cope with the switch on surge current to the caps. Peter.

[daz]

So then i take it i should use a fast blo and put right after the rectifier?

[Hardtailed]

Just a thought to prevent the cap charging rush from blowing the fuse: a resistor accross the standby switch. High enough that the amp remains silent in standby, but just low enough that the caps start charging slowly while the tubes are warming up.

Of course, in the typical arrangement, the standby switch is after the first cap so there's still a risk.

Just a question... if you had to choose between a 1A fast blow, and a 500mA slow-blow.. what would be safest?

I currently have 1A fast blow types for my 50W Marshall project.

[pdf64]

'So then i take it i should use a fast blo and put right after the rectifier?'
No, for fast fuses you need to put it in series with the OT CT.
Or put a fuse in series with each cathode as I described above, which is better as it will blow quicker in event of red plating. And the amp will still function on the remaining output tube. Peter.

[black_labb]

sorry to bring this back to life, but i reread it after searching for something else (cant even remember what i was searching for anymore).

if a fuse (fastblow 500ma was reccomended for a 50w amp) was put between the OT ct and the b+ and then the fuse blew accidentally, would that not blow the tubes due to voltage on the grids but not on the plates?

i was under the impression that voltage on the grids but not the plates sends the tubes to an early death, or am i incorrect.



for a big bass amp build (8xkt88/6550 @~620v into a hammond 1650w) i'm planning on putting fb fuses on the cathode of each pair of tubes (which are individually biased) and to use flyback diodes to the cathode. this way if they short the fuses will blow and the amp should keep going. this and some b+ fuses (one for all of the b+ and one for the screens and preamp).

any issues anyone can see? i know its a bit over the top, but i am already putting bias adjustments/test points for each pair of tubes on the back panel so the cathode fuses should not be an issue/too much extra work. i also dont want to ever have to replace the 1650w, their quite pricey (and heavy)

[Steve Conner]

Hey black labb,

That's why they always take the screen supply from somewhere after the fuse. If it blows, it cuts off both plate and screen voltage. So it's OK unless you're building a high powered amp with separately fused supplies for plate and screen. Then you're in trouble if the plate fuse goes but the screen one doesn't.

You could add a couple of diodes so that if the plate fuse blew, the plate voltage would fall to the screen voltage instead of zero. Then you could use this as a reduced power mode, too.

[daz]

Ok, i'm about to show how stupid i am i suppose, but i have to ask. If you put a fuse on the OT CT and it blows, why would the screens no longer have voltage applied to them? The OT is not in series with the B+, so whats to stop the B+ volytage from continuing on to the screens?