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06-22-2023, 02:00 PM
Hi Guys
When using toroidal OTs, as our amps do and PA66 kits do, there is absolutely no reason to use active bias controls (servos, differential balancing, etc). Standard preferred** bias-set networks are stable over the long term.
That advice goes for EI output transformers, as well.
You do not need matched tubes either. You simply have to have a separate bias control for the push- and pull-sides of the output stage. As mentioned frequently on this forum and in our books, DC idle current balance is slightly off from the hum-balance of the OT - just a few milliamps. So, set one side of the circuit by meter for a safe idle condition, then set the second half by ear for minimum hum.
POP is from 1996 and I made a last minute change to the circuits in the Toroidal Output Transformers chapter, to include active bias balancing after testing some methods for this and from hearing about the importance of balance in a toroidal OT. Frankly, the change to POP was primarily to impress Menno vander Veen, who I was about to meet. I'm like everyone else and there are people I admire and am a fan of their work. Menno is a fantastically smart guy at the cutting edge of audio transformer science, and a totally lovable and down to earth guy to boot. In Menno's own book at the time, he used a faulty passive bias-set scheme that lacked range resistors and his kits followed suit. He did not bother with active bias until his later "trans" amp adventures which were mostly SE and often used EI OTs.
The "trans" amp is an idea of using local feedback around the power tube to essentially give the stage a virtual-earth input - one that take current in rather than voltage, so "trans" refers to transconductance, not a car transmission, or any of the modern identity liberation meanings
This idea is actually quite old, then revisited in the 1980s and then by Menno in the 2000s.
**Preferred bias circuits are ones that have a raw bias supply that is separate from the bias-set network. For example, the bias circuits for the projects in TUT3 or TUT5 are preferred. The usual bias circuit found in a Marshall is not.
When using toroidal OTs, as our amps do and PA66 kits do, there is absolutely no reason to use active bias controls (servos, differential balancing, etc). Standard preferred** bias-set networks are stable over the long term.
That advice goes for EI output transformers, as well.
You do not need matched tubes either. You simply have to have a separate bias control for the push- and pull-sides of the output stage. As mentioned frequently on this forum and in our books, DC idle current balance is slightly off from the hum-balance of the OT - just a few milliamps. So, set one side of the circuit by meter for a safe idle condition, then set the second half by ear for minimum hum.
POP is from 1996 and I made a last minute change to the circuits in the Toroidal Output Transformers chapter, to include active bias balancing after testing some methods for this and from hearing about the importance of balance in a toroidal OT. Frankly, the change to POP was primarily to impress Menno vander Veen, who I was about to meet. I'm like everyone else and there are people I admire and am a fan of their work. Menno is a fantastically smart guy at the cutting edge of audio transformer science, and a totally lovable and down to earth guy to boot. In Menno's own book at the time, he used a faulty passive bias-set scheme that lacked range resistors and his kits followed suit. He did not bother with active bias until his later "trans" amp adventures which were mostly SE and often used EI OTs.
The "trans" amp is an idea of using local feedback around the power tube to essentially give the stage a virtual-earth input - one that take current in rather than voltage, so "trans" refers to transconductance, not a car transmission, or any of the modern identity liberation meanings
This idea is actually quite old, then revisited in the 1980s and then by Menno in the 2000s.**Preferred bias circuits are ones that have a raw bias supply that is separate from the bias-set network. For example, the bias circuits for the projects in TUT3 or TUT5 are preferred. The usual bias circuit found in a Marshall is not.
