OT Protection for Marshall 50W Tube Combo

[rf7]

If you were running two 8 ohm speakers in series to get a 16 ohm load for a Marshall 50W combo (because you wanted to also run a 16 ohm extension cab), what would be a good resistor to put across the output in case one of the speakers opens up when you are just running the combo speakers alone?

Is a 100 ohm/10W resistor fine?

Is there an issue with using a 1K ohm resistor instead?

[Chuck H]

Obviously your considering the parallel load the resistor will add. Traynor used a 470 ohm for years. I don't know if I (or anyone) has the experience to say what the maximum R value would be keep the amp alive, but that's what you want. Anything under that places an undue waste of watts on a "peripheral" circuit, right? I suppose if you were planning to market a product you could just start with a low value and then increase it until it fails to protect the OT, then back it down twenty percent or so. It's a hard call.

I've often wondered why no one makes a switching breaker (perhaps someone does but I've never seen it). Then you could place the breaker in series with the speakers. If one blew the current across the breaker would "switch" it and that switch could be sent to a dummy load. Then you would have protection and no peripheral loading. Just thinking out loud.

Marshall OT's are usually pretty stout. The rep Marshalls have for blowing up is primarily due to the fact that with a head it's much easier to accidentally or by equipment failure end up running with no load at all. That's as bad as it gets. I say use a 470R. There's still some additional load that wastes watts but I can't remember ever seeing a problem with the old Traynors blowing OT's like Marshalls do.

HTH

Chuck

[tubeswell]

All you need is something across the secondary to provide a bit of a load for a short time, and large eough that it doesn't suck too much current out of the speaker coils, but not too large that it causes the OT to self destruct. Anything between 220R and 470R (5W) ought to be okay.

[Enzo]

All in all I thnk of OT failures as rare. I don;t replace very many of them. But I do seem to find the failures in Marshalls a lot more than other brands when they do fail. I think of the Marshall OT as relatively frail then. I never see the old built-like-truck Traynors with bad OTs, and darn few Fenders really. I look at HArshalls and see no flyback diodes, nor any shorting contacts on empty speaker out jacks.

And why don;t they have a system to shunt an output with a safety resistor if absence of load is detected? Well, why single out that one particularly rare occurence? Think of all the other failures we'd want detection and backup circuits for. How many should we hang onto the purchase price... I mean design of the amp? Now a quick test. Grab a pencil and paper. QUick, draw a basic amp circuit - something like a 5E3. I don;t care about resistor values. Frankly I don't even care if ther are a few mistakes. Looks OK? Now QUick, draw up a circuit that does what you propose. NO I don;t expect engineering, just what would you use? The amp used common tubes and other components. This protector would require not only a senistive enough circuit, but also a relay, and that relay needs a power supply unless it runs off B+ or 6VAC. And what inobtrusive measure would be used to detect the open speaker? Can't detect loss of continuity without running something through it. And the OT secondary is in parallel with any resistance check. And if you are detecting output signal current (how?) to open if it falls to zero, how does the system differentiale between zero current due to open speaker and zero curent due to not playing a note at the moment?

Not picking on the idea, just thinking that it is a involved circuit and system to fight just ONE potential failure.

I imagine RG can come up with a much simpler way to implement such a thing than I can, but still it seems overly complex. And $$$.

[Chuck H]

FWIW another potential reason for the seemingly higher rate of OT failures in Marshall amps is the cultural proclivity for cranking them. Of course, many amps get cranked but perhaps more Marshalls if only because that is the custom. Certainly there must be some Marshall OT's that ARE frail. But I bet most are as good as anything else. Ahh, take the old Deluxe for example. Those blew OT's all the time WITH a load. Some say it's because they were frail and some say it's because people are always cranking them. But tweed Fenders don't suffer the same rep.

OTOH I used Marshalls the whole time I was playing out in my band days. Never had an OT failure. But they did seem to be broken for some other reason half the time.

I wasn't blithely pitching a circuit notion above. I was speculating that IF there were switching breakers it could be possible. I reasoned that with no speakers there would be a voltage spike (though I did say current) hopefully brief enough not to harm the OT but trigger the breaker, which, if it switched could default to a dummy resistor. I have actually considered a circuit like you describe above and also thought it to be too far to go.

[EFK]

Selmer used a 600 ohm 5W resistor on the 50W Treble N Bass amps.

[rf7]

Thanks guys, for the replies!

Ran a Marshall head without a load for a few seconds at a gig, when my speaker cabinet wiring fell apart inside. It sounded real nasty and I shut it down quick. Thankfully, the OT was OK.

And yeah, we replace more Marshall OT's than any other brand.

[mooreamps]

Thanks guys, for the replies!

Ran a Marshall head without a load for a few seconds at a gig, when my speaker cabinet wiring fell apart inside. It sounded real nasty and I shut it down quick. Thankfully, the OT was OK.

And yeah, we replace more Marshall OT's than any other brand.

I use 100 ohm loads myself. Also, the Marshall OT's look a little small for their advertised power rating compared to the Hammonds.

-g

[Chuck H]

I use 100 ohm loads myself. Also, the Marshall OT's look a little small for their advertised power rating compared to the Hammonds.

-g

Almost any OT looks small for it's rating compared to the Hammonds. When I use Hammond "off the shelf" models for guitar amps I downrate the wattage 50%. As in 10W Hammond OT in a 20W guitar amp application. No failures in over a decade of cranking these amps. And I've made every mistake too, like accidentally running without a load or into a dead short, both with the amp cranked and playing into it and still no failures.

[mooreamps]

So, some of these guys who do mod's on the Valve Junior using the 15 watt Hammond OT's is what, something of an overkill ?

-g

[Chuck H]

Well, I don't know which OT the VJ guys are using. Is it one of the 16XX models, one of the 125XX models or one of the new "guitar amp" models. It was a tech at Hammond that told me to half rate the 16XX model OT's. This was long before the new guitar amp OT's were offered and I don't know about the 125XX OT's. I believe the "actual" power handling would be different for all three. But the 16XX trannies are bulletproof, sound good and are relatively affordable for the home builder. Not so much for commercial though as you MUST be a dealer to get a price break. I guess that wouldn't be too hard to fake though. Just offer parts for your amps on a website or some such.

[guitarmike2107]

Well, I don't know which OT the VJ guys are using. Is it one of the 16XX models, one of the 125XX models or one of the new "guitar amp" models. It was a tech at Hammond that told me to half rate the 16XX model OT's.

I take it that is because he is assuming you are not using the full bandwidth of the OT. i.e. this is likely not a general rule of thumb accross the ranges.

[Chuck H]

Right. The 16xx OT's are rated at full audio band width. Since we don't use the full band width with a guitar amp (not exactly true, but...) these OT's cruise along with no problem even rated half. No big bass notes drawing big current and no high hats, snare drums and cymbals spiking all over the place. The 125xx OT's have a narrower band width and should probably be used conservatively due to their "universal" nature anyway. And the guitar amp line are supposed to be clones of the originals. In which case they should be used in amps such as they are designed for and not larger.