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10-27-2026, 12:44 AM
Hi Guys
Wow! How did everyone not see this post?
Vintage amps were based on hifi circuits from the various tube manufacturers, and the guitar use was considered more to be a portable PA than a dedicated guitar amp. Many units had inputs for microphone, accordion, bass, guitar and even phono - that would have to be a ceramic pickup.
In a cathode-biased amp designers then and now seem intent on getting maximum power out of the tube and have to run the tube flat out at idle. Some of those old amps will have low-enough B+ and high-enough OT primary impedances to keep the tubes in class-A, but those are few and far between. Most self-biased amps are just very high bias class-AB.
To stretch the idle power, designers will add the plate and screen power ratings together to arrive at the idle current, and eventually to the Rk value. Considering that most such amps go out of
class-A to reach full output AND share Rk among many tubes, there is no point risking tube failures by having idle currents be so high. There are many ways to improve the hum issues and tube life problems for these amps, as TUTs discuss.
In those early days of electronics, mains voltages were generally lower than they are today. On today's mains, old amps will have higher voltages all around because the primary on the PT might be rated for 115Vac instead of 117Vac or 120Vac - double these for everywhere other than Japan and North America. An easy way to correct the mains expectation of the PT is to buck the voltage down. This will help preserve the amp and tubes.
As I have said here and elsewhere, I am not a fan of self-biased power stages and only use them as a switchable option for a different tone. In that mode I set Rk high to keep tube dissipation modest. I also have a very clean supply so hum is not an issue if there is an option to use a shared Rk for a push-pull tube pair.
One thing you might want to try with self-biased output tubes is to use a much higher value for Ck. 22-100uF is really inadequate to keep Vk constant. You have to get into the mF range for it to work more ideally.
Wow! How did everyone not see this post?
Vintage amps were based on hifi circuits from the various tube manufacturers, and the guitar use was considered more to be a portable PA than a dedicated guitar amp. Many units had inputs for microphone, accordion, bass, guitar and even phono - that would have to be a ceramic pickup.
In a cathode-biased amp designers then and now seem intent on getting maximum power out of the tube and have to run the tube flat out at idle. Some of those old amps will have low-enough B+ and high-enough OT primary impedances to keep the tubes in class-A, but those are few and far between. Most self-biased amps are just very high bias class-AB.
To stretch the idle power, designers will add the plate and screen power ratings together to arrive at the idle current, and eventually to the Rk value. Considering that most such amps go out of
class-A to reach full output AND share Rk among many tubes, there is no point risking tube failures by having idle currents be so high. There are many ways to improve the hum issues and tube life problems for these amps, as TUTs discuss.
In those early days of electronics, mains voltages were generally lower than they are today. On today's mains, old amps will have higher voltages all around because the primary on the PT might be rated for 115Vac instead of 117Vac or 120Vac - double these for everywhere other than Japan and North America. An easy way to correct the mains expectation of the PT is to buck the voltage down. This will help preserve the amp and tubes.
As I have said here and elsewhere, I am not a fan of self-biased power stages and only use them as a switchable option for a different tone. In that mode I set Rk high to keep tube dissipation modest. I also have a very clean supply so hum is not an issue if there is an option to use a shared Rk for a push-pull tube pair.
One thing you might want to try with self-biased output tubes is to use a much higher value for Ck. 22-100uF is really inadequate to keep Vk constant. You have to get into the mF range for it to work more ideally.
