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04-11-2025, 12:37 PM
Hi Guys
The original post was about the effect of the primary impedance not secondary loading or matching. Two different issues.
To begin with: for a given PSU voltage and current capability, there will be an optimum OT primary impedance that will allow maximum audio output power to be tapped from the PSU. You can increase the primary impedance and draw less power from the PSU.
The secondary load match is another issue, and is what your references are talking about exclusively. They leave out a few points of context.
As most people know, guitar and MI circuits began as hifi circuits meant to be "mobile PAs" that you could plug anything from guitar, bass, accordian, organ, radio, etc into and have them be amplified. It was expected that if the amp or speaker distorted that one would turn down the Volume control. As guitar amps became more focused on making guitars sound good, output transformers became smaller for a given power rating BUT the OEM expectation was still a clean output.
In the original context of design and use, "clean" was the emphasis, so clipping the output stage or mismatching the secondary load was not on anyone's mind. But...
Along came players who wanted to be louder in a given space, or found that the overdriven sound fit into their musical expression. Eventually guitar amps were voiced to sound good when overdriven, and often sounded quite thin when not, and new preamp arrangements allowed distorted sound to be created without overdriving the power amp. Even with this new design paradigm, players will push the envelope and on a modern stage, a Master Volume amp will still be driven to clipping.
An example of amp clipping is easily heard on the live album for the Heaven and Hell tour. The beginning of 'I' starts relatively quietly but distorted. Then for the chordal section the guitar volume is dialed up and you hear the sound get louder but there is a "sizzle" on top. That is PA clipping.
Guitar amp OTs are band-limiting inasmuch as they will not provide full power at low-frequencies but will at mid- and maybe high-frequencies. This reduced size makes the OT more vulnerable to damage by overload. The tubes are natural power limiters but you could easily assemble and output stage where the OT is in peril almost all the time. Most OTs are not designed with much margin for abuse - abuse that is considered normal operation in MI. So, one way to assist OT reliability is to use a full-power-bandwidth unit, which also makes the dynamic tone more consistent, at least as far as the OT's contribution goes.
To protect the OT from other mishaps, some amps have clamping diodes on the primary side, from the driven end(s) to ground. This is a brute force method that results in shorted diodes if they are sized to ever come on at a useful point. Do they save OTs? No one can say for certain.
I prefer a benign suppression approach of simply tethering the driven end(s) of the OT primary to ground using resistors. These never fail and they work continuously over the audio cycle to sap energy from the parasitic elements of the OT. Yes, they consume some audio power as well, but an insignificant amount. If the load becomes discontinuous, or the player has set an extreme secondary mismatch, the universe remains stable.
The original post was about the effect of the primary impedance not secondary loading or matching. Two different issues.
To begin with: for a given PSU voltage and current capability, there will be an optimum OT primary impedance that will allow maximum audio output power to be tapped from the PSU. You can increase the primary impedance and draw less power from the PSU.
The secondary load match is another issue, and is what your references are talking about exclusively. They leave out a few points of context.
As most people know, guitar and MI circuits began as hifi circuits meant to be "mobile PAs" that you could plug anything from guitar, bass, accordian, organ, radio, etc into and have them be amplified. It was expected that if the amp or speaker distorted that one would turn down the Volume control. As guitar amps became more focused on making guitars sound good, output transformers became smaller for a given power rating BUT the OEM expectation was still a clean output.
In the original context of design and use, "clean" was the emphasis, so clipping the output stage or mismatching the secondary load was not on anyone's mind. But...
Along came players who wanted to be louder in a given space, or found that the overdriven sound fit into their musical expression. Eventually guitar amps were voiced to sound good when overdriven, and often sounded quite thin when not, and new preamp arrangements allowed distorted sound to be created without overdriving the power amp. Even with this new design paradigm, players will push the envelope and on a modern stage, a Master Volume amp will still be driven to clipping.
An example of amp clipping is easily heard on the live album for the Heaven and Hell tour. The beginning of 'I' starts relatively quietly but distorted. Then for the chordal section the guitar volume is dialed up and you hear the sound get louder but there is a "sizzle" on top. That is PA clipping.
Guitar amp OTs are band-limiting inasmuch as they will not provide full power at low-frequencies but will at mid- and maybe high-frequencies. This reduced size makes the OT more vulnerable to damage by overload. The tubes are natural power limiters but you could easily assemble and output stage where the OT is in peril almost all the time. Most OTs are not designed with much margin for abuse - abuse that is considered normal operation in MI. So, one way to assist OT reliability is to use a full-power-bandwidth unit, which also makes the dynamic tone more consistent, at least as far as the OT's contribution goes.
To protect the OT from other mishaps, some amps have clamping diodes on the primary side, from the driven end(s) to ground. This is a brute force method that results in shorted diodes if they are sized to ever come on at a useful point. Do they save OTs? No one can say for certain.
I prefer a benign suppression approach of simply tethering the driven end(s) of the OT primary to ground using resistors. These never fail and they work continuously over the audio cycle to sap energy from the parasitic elements of the OT. Yes, they consume some audio power as well, but an insignificant amount. If the load becomes discontinuous, or the player has set an extreme secondary mismatch, the universe remains stable.
