A fellow where I used to work is/was a brilliant circuit designer. Power supply group. EE education in Russia. I asked him if he did any audio circuits in his EE training curriculum, specifically power tube output stages. He said the only one they analyzed the theory of, was a resistive loaded plate capacitively coupled to an OPT.
I remember some hype around using PA line distribution transformers as an OPT for a single ended design. Cheap and readily available. But, no gap to allow it to handle the DC bias, as when designed, they werent expecting a DC bias.
So, what aspect of a resistive loaded, capacitively coupled to an OPT output stage is so bad, that you never see it (?) as a viable single ended design? Is it because you lose efficiency; a 10W SE amp done with the "right" gapped OPT becomes only 5 with the scheme?
So what? Low power tube amps for guitar seem to be popular; perhaps its possible to get that SE tone without having to spend $150 on an OPT designed to work in that circuit. Just move the mic a little closer to the speaker.
I remember some hype around using PA line distribution transformers as an OPT for a single ended design. Cheap and readily available. But, no gap to allow it to handle the DC bias, as when designed, they werent expecting a DC bias.
So, what aspect of a resistive loaded, capacitively coupled to an OPT output stage is so bad, that you never see it (?) as a viable single ended design? Is it because you lose efficiency; a 10W SE amp done with the "right" gapped OPT becomes only 5 with the scheme?
So what? Low power tube amps for guitar seem to be popular; perhaps its possible to get that SE tone without having to spend $150 on an OPT designed to work in that circuit. Just move the mic a little closer to the speaker.
Well, for one thing, a 400VDC supply suddenly needs to be a 600-800VDC supply. ;-) To change the load (say from 2.5K to 5K) you have to redesign the whole power supply. My Williamson amplifiers would need an 850VDC supply instead of 450VDC. You're throwing away a lot of heat and power.
Not so uncommon as you wrote ...
Capacitively coupled opt is called "parafeed", search on this forum or look at:
https://www.tubecad.com/2014/09/blog0308.htm
Resistive load would be extremely inefficient, so usually choke load or totem pole is commonly done.
Capacitively coupled opt is called "parafeed", search on this forum or look at:
https://www.tubecad.com/2014/09/blog0308.htm
Resistive load would be extremely inefficient, so usually choke load or totem pole is commonly done.
Try to do a quick design using a common 6BQ5/EL84 with 300v on the plate and biased to 50mA with a 5K load. What plate load resistor value would you use, and what primary Z for the OPT would you use? What PS B+ would you start with?
"Try to do a quick design using a common 6BQ5/EL84 with 300v on the plate and biased to 50mA with a 5K load. What plate load resistor value would you use, and what primary Z for the OPT would you use? What PS B+ would you start with?"
OK, simple design - use a 10K resistor and a 10K transformer, to get a net 5K load. Then you get half the power and the resistor gets the other half. And you need another 500 volts, for a total 800v power supply.
OK, simple design - use a 10K resistor and a 10K transformer, to get a net 5K load. Then you get half the power and the resistor gets the other half. And you need another 500 volts, for a total 800v power supply.
Why would you need that kind of voltage if you only wanted a couple of miserable watts?
70 volt transformers aren’t a 5k load, even at 5 watts. Load impedance go down, voltage requirements go down with them. The problem is then a 6BQ5 is not the right tube. Need something that can swing more current on a 250-300V power supply, to match the needed 1k plate load. It’s a job for a big fat sweep tube. Some of them can be had for cheap, just like the transformer.
70 volt transformers aren’t a 5k load, even at 5 watts. Load impedance go down, voltage requirements go down with them. The problem is then a 6BQ5 is not the right tube. Need something that can swing more current on a 250-300V power supply, to match the needed 1k plate load. It’s a job for a big fat sweep tube. Some of them can be had for cheap, just like the transformer.
I haven't seen a single high-quality line to speaker transformer. They are all designed for PA use.
You are not going to get audiophile boutique hi-fi out of it. No one expects it to. “Move the mic a little closer to the speaker” implies guitar amp. A three watt guitar amp (for practice, in an apartment) is often too loud.
Its just that using the right quality magnetic components is one of the keys to getting a good distorted guitar sound. Unless maybe you want it REALLY grungy.
It sounds like an experiment to me, not something somebody is going to gig or record the next hit rock single with.
If someone wants perfection, they just have to shell out.
If someone wants perfection, they just have to shell out.
The original premise is wrong. He's willing to get 5W from the amp and thinks that takes a $150 SE OPT.