Bill Fitzmaurice

You probably wouldn't be able to tell them apart. Back in the day we all found it odd that you couldn't hear any major difference between a Bandmaster, Bassman, Twin Reverb or Dual Showman when played through the same speakers. That was long before we knew about decibels and logarithmic hearing response.

It's not complicated, if you know how to use speaker modeling software, like WinISD, the process takes less than ten minutes, if you have the specs. IMO every speaker should make them available. Every driver manufacturer does, but that opinion isn't shared by the cab manufacturers. 🙄

It would be a simple matter to calculate the OBC 112 frequency response, sensitivity and maximum output using the driver Thiele/Small specs, but since there's no way of knowing what they are you're back to comparing it to other cabs the old fashioned way, trying them side by side.

The amp rating is moot, the transformer only powers the pre-amp tubes. I agree that the only reason it would blow is if there's something downstream is drawing a lot more current than it should be.

Based on the size I agree. You may find numbers on it if you pull it.

No. First off the position of the volume control doesn't indicate the power output. Depending on the level of the input signal halfway on the volume control could be full power, could be 1/10 power, could be clipping. Then there's the matter of the impedance load, which isn't constant. A nominal 4 ohm load will have an actual impedance anywhere between 3 and 30 ohms, depending on frequency. Power varies with impedance, so with 20 volts output the power can range between 13 watts at 30 ohms to 133 watts at 3 ohms with the same volume control setting depending on the note being played. Lastly, perceived sound levels are logarithmic with respect to power, not linear. To sound twice as loud doesn't take twice the power, it takes ten times the power. As for measuring power output, to do so requires simultaneous measuring of voltage and current, which requires two separate meters and a method to extrapolate the result, which will still vary with frequency. Engineers don't even try. We measure voltage, which is constant into any impedance load. We know the voltage output that an amp can cleanly deliver, and the voltage limit that a speaker can cleanly handle.

It works for their material. He's no slouch. I saw a video of him playing 'Roundabout' with 'Yes'. Not everyone can play like Chris did. Geddy nailed it, including the tone.

The first thing you need to do is to get schematic. That in hand it should be easy to determine what's required for a transformer. Without it you're up a certain creek sans paddles.

I assume you say that because at its limits the amp is clipping. That can cause a problem for tweeters, because clipping creates abnormal high frequency content that can over-power a tweeter. But it has no effect on woofers.

The 2225 was more of a PA woofer, with weak highs. The king of bass drivers back in the day was the Altec 418. Even today it would be competitive.

I liked my '65 Bassman amp well enough, but the speaker was gosh awful. I'll forever be thankful to Leo Fender for that, as it's what prompted me to study loudspeaker design, which eventually became my profession. 😉 If I was going to do a total rebuild of a Bassman I'd ditch the Fender tone stack, which was fine for guitar but not for bass, in favor of the Ampeg B-15 design.

I can't tell if those are threads or just ribs but either way it appears that the horn is stripped to the point that it won't hold. I'd use epoxy to secure it. There should never be a need to separate them, as a replacement driver and horn would come as a unit. Do not use superglue, it can destroy some plastics.

Get a book on how to repair valve amps. You may even find it free as a download with a search. They're very easy to work on, and the older the model the easier they are.

They do have a shortcoming, he has to add more coins between songs.

Tweeters don't hiss, amps do. That said, the response of the BGH25-8 rolls off above 8kHz, where there's no useful electric bass content, but plenty of hiss.

A blown tube can take out other components and cause a blown fuse. If you were to just remove a tube it wouldn't stop the amp from working, although if one does that because they notice a tube is ready to go they should remove two, from opposite ends of the amp, to maintain balance.

It should have. There are two banks of three tubes, those three tubes being parallel wired. The amp should still work even with only two tubes, on from each bank, in place. Four tube models, like the Twin and Showman, had two banks of two tubes. With one or two tubes removed the difference in how they sounded was imperceptible.

The reason I said deep and loud is that one can make a small cab that goes deep without boom. But doing so one sacrifices sensitivity, so while it's an option for home hi-fi and auto sound it's not for electric bass. The sensitivity issue can be offset by having a lot of power, and a driver that has both the electrical and mechanical ability to use it, but that sacrifices midrange response.

Boom isn't caused by cabs that are big, it's caused by cabs that are too small for the drivers within. One common shortcoming of commercial cabs is that they are too small, a marketplace concession to the desire of players to have a smaller rig to haul. But just as our instrument necks are long and our strings are fat our cabs need to be large if one wants to go deep and loud without boom.

It's logical to assume that the air volume of the room is the main factor, especially as decibels measure the intensity of an acoustic pressure wave. But cabin gain is something else entirely. This will help:

Anywhere between one half and twice the speaker rating is appropriate for the amp size. One half is usually enough to drive the speaker to its full mechanically limited output. More power gives more amp headroom, while too much power will cause distortion that will encourage a sensible person to turn it down. The mechanical limit of your Eminence Kappa 15 loaded Orange in the critical 50-80Hz range is 100 watts, so even a 200w amp has sufficient headroom.

What you've hit upon here is Pressure Vessel Gain, more commonly referred to in audio as Cabin Gain. It has to do not with the volume of air but the dimensions of the room. There is a sensitivity gain at a rate of up to 12dB per octave as frequency goes down below where the longest room dimension is one-half wavelength. In a living room that's 5 meters long cabin gain starts around 34Hz. In a car that's 2 meters long it starts at 85Hz. Cabin gain is what allows the silly high bass levels achieved in auto dB competitions. A club is too large to have any cabin gain, but in an ear canal it covers almost the entire audio range.

With isobarik the result of using two drivers is halved Vas, the volume of air having the same acoustic compliance as the driver suspension. This allows halving the internal volume of the cabinet, not counting the space taken up by the second driver and the air space between the two drivers, without any loss of low frequency response. As always there is a trade off. In this case since the cone area and excursion radiating to the outside air is that of only one driver the maximum output is the same as with one driver in the net doubled cab volume. One could stuff the chamber of an isobarik, with the same result as with a standard alignment, lowered Q. Isobarik was a reasonable alternative to huge enclosures fifty years ago, when driver Vas was generally much larger than today. For instance, the fifteen inch Altec 421 8LF Vas was 600 liters. The modern equivalent Eminence 3015LF Vas is 159 liters.

That's because stuffing doesn't make the cab look bigger. It lowers the cab Q, which can tame a midbass hump. Making the box larger does that, but making the box larger also lowers the speaker cutoff frequency, stuffing does not. Some 25 years ago a well respected audio expert measured some data and made the conclusion that stuffing made a box act as if it was larger. His conclusion was erroneous, because he didn't measure enough data to reveal what actually occurred. He published his conclusion, and based on his reputation alone much of the professional audio engineering community accepted it at face value without testing to make sure it was true. In both audio and nuclear arms treaties you can trust but you also must verify. 😉