300B SE Driver Stage Understanding

[Wavebourn]

Why is it being misused?

Because grid current of an output power triode charges the cap each time volume of the music increases, so bias floats after the envelope causing dynamic distortions. That's why some people claim that they don't like 300B tubes; rather they don't like such distortions that exhibit an amp improperly designed.

Yes, although it is unfair to blame the cap for simply behaving as a cap.

Anyway, this design seems to replace a harmless cap (second stage grid) with an almost harmless cap (second stage cathode) yet leaves another cap being misused to drive the output stage.

Exactly.

 

[MoparWade]

Because grid current of an output power triode charges the cap each time volume of the music increases, so bias floats after the envelope causing dynamic distortions. That's why some people claim that they don't like 300B tubes; rather they don't like such distortions that exhibit an amp improperly designed.

So what would you suggest to rectify this design flaw?

 

[Wavebourn]

So what would you suggest to rectify this design flaw?

Vice verse, of course. No cap before 300B, but a cap between first 2 tubes.


Hers was my example:

http://www.diyaudio.com/forums/tubes-valves/183329-one-more-4p1l-se.html

 

[beancounter]

Vice verse, of course. No cap before 300B, but a cap between first 2 tubes.

How about direct coupling the first two stages, and transformer coupling the output triode?

With an appropriate driver and interstage

 

[45]

How about direct coupling the first two stages, and transformer coupling the output triode?

With an appropriate driver and interstage

Transformer coupling is a good idea if done right. Especially if the 300B is run with cathode bias.
No reason to spend millions. One that is appealing would be a 10Y + LL1692a in Alt S connection (i.e. 4:3.5 ratio with 18 mA max recommended primary DC current and 125H primary inductance). The 10/10Y can run as from data sheet at 425V/18mA and would be capable of driving the 300B down to 20Hz even at full throttle (which means 300B operating at 450V plate voltage and 80mA with 4.5-5K). However one needs to be careful as many 300B are not happy at this plate dissipation, especially those with punched or mesh plates......and this would require some 550-560V supply voltage to use cathode bias.

The first two stages could be easily DC-coupled with 560V supply but if they are RC-coupled is fine as well.

 

[MoparWade]

Transformer coupling is a good idea if done right. One that is appealing would be a 10Y + LL1692a in Alt S connection

So would the 10Y replace the second stage (half of the 6SN7)? Apologies if that's a dumb question. Sort of learning and building valve amps at the same time. Built an AB763 two channel guitar amp with great success but the circuit was already designed so as I continue to build these wonderful devices I want to eventually have a full understanding of what I'm doing. Now that the guitar is set I figured some home audio would be good. Imagine I'll end end up building several of both. As usual thanks in advance for any advice.

 

[45]

So would the 10Y replace the second stage (half of the 6SN7)? Apologies if that's a dumb question.

Yes. No need for apologies.

 

[MoparWade]

I love it. It's going on the list. It's nothing like anything I've considered before. I fear that I'm going to end up building two home audio systems now.