Your journey with lower-watt tube amps -- Can a kit be good enough?

@alexberger 

The problem of 300B they need a driver with 150 volt peak to peak voltage swing. For 845 you need much more voltage swing.  

If you direct-couple with a CF that's no problem at all! Here's something to consider:

The grid of triode power tubes is the most capacitive of any kind of power tube. Triode power tubes are also very low mu (gain). For example the 300b is a bit over 2. The 6AS7Gs we drive in our OTLs is only 2.

So driving such a load from the plate of a driver tube is a task- hence the common use of an interstage transformer. But that part is expensive if its done right and it makes distortion and limits bandwidth.

When you use a cathode follower its input impedance is really high so its a lot easier to drive than a power tube and easier to drive than using an interstage transformer. So its a lot easier for the Voltage amplifier to drive it without distortion and the driver itself is inherently low distortion as well.

Since you are into DIY you might consider trying this. Its not that hard and like I said, the tube section to do it and the minus supply is usually less expensive than a good interstage transformer. As a general rule of thumb whatever the B+ Voltage is that you use now for the driver, if using a direct-coupled cathode follower you'll need a B- of equal absolute value. That will allow it to have the Voltage swing you need. A single 6SN7 section can easily swing 150V or more no worries. We've been doing it for decades. Usually you can obtain the negative supply if you use a bridge rectifier. That requires a power transformer with a center tapped secondary. 

I’m not a tube guy, but I am enjoying this thread a lot. Thanks @hilde45 for starting it, and thanks @atmasphere for your well thought out comments which, despite being nominally tube-oriented, hold a universal appeal especially regarding what we listen to, how we listen to it and what we seek to get from it.

My entire history with tube gear consists of a vague yearning for big-iron VTL monoblocks... someday... so I don’t have much to contribute to the subject other than, if you’re still considering the DIY route, this:

Unlike another poster earlier, I find PCB work easy. Insert component in its assigned spot, solder, done. Point to point on the other hand gives you a lot more latitude in how you structure your circuits in 3D space. Personally I find point to point more rewarding, but harder to keep my OCD impulses in check 😃

For what it’s worth, I would recommend going the DIY route, if only because the satisfaction of building something with your own hands is without peer.

PCB is easy to solder, except when it comes to repairs.  Usually that involves an extra step of having to unmount the board to get to the correct side, and it sometimes means having to bridge a trace or make other such repairs where you pulled a trace or pad while unsoldering and trying to replace a component.  

For looks, as well as the benefits of ease of repair, I like point-to-point using turret boards.

Speaking of good point-to-point wiring, I saw an amp built by a Russian builder that was so carefully laid out that there is hardly any wire in the amp, almost all connections being lead to lead (and short leads at that).  All of these connections were extremely tightly wound together before being soldered.  The dealer showed me an amp that had been running for years that this builder made while a bit drunk.  About a third of the connections weren't even soldered but they functioned perfectly.  Another repair professional charged extra to work on this Russian's amps because of the difficulty in removing components.  I owned one of these amps and I planned to replace the diodes in the bridge rectifier, but when I looked at what that entailed, I gave up.  The entire bridge rectifier was about the size of a thumbnail and was stuck in a tight space under other components that would have to be removed to get at it.  I could not even figure out how he got everything into such a tight ball.

@larryi Decades ago I put myself through engineering school by working as a repair technician at Allied Radio Shack. I saw equipment that clearly was not built with the ability to easily repair it in mind. Electronic components fail no matter how well you vet them or how well you rate them in the circuit. IMO/IME easy repair should always be high on the list when laying out a circuit. 

If you do it right and use good quality boards the repair time is about the same (if you do a neat job) whether circuit board or hand-wired.

Hi @atmasphere 

Thank you for your advice. I can try it if I build a new amplifier.

So. Do you think the issue is not voltage swing of driver but the current and power it should drive on capacitive load. And the power is function of the voltage swing in squire.