WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
Building, troubleshooting and testing of these amplifiers should only be
performed by someone who is thoroughly familiar with
the safety precautions around high voltages.
Just a word of caution for OTL owners that think up-sizing the output cap is a non-risky thing todo.
Having a PSU cap and an output cap without any resistance between them except the valve will measure happily with no signal. Your tube will show the idle current no problem (ie no plate resistor for example).
As soon as you have large transients or larger signals, the caps act as power reserves. So each 330uF 450V cap will happy put ±10mA to the tube current. What may read 50mA through the resistor at the cathode supply, may actually be putting out far more (depending on capacitance).
In a LTspice sim, it's very easy to demonstrate this and have an idle current of 30mA but have a peak signal transient of over 140mA given an over sized grid signal.
So check your schematic first or you may end up reducing the live of your valves or putting your speakers/headphones at risk.
It's interesting that different types/brands have different secondary and third harmonic distortions adding to the noise and the bi-polar electrolytic is far less (although he also covers the 'diode' threshold effect) noisy than polar caps. He also covers the impact of bias on noise.
Have to agree that Uncle Doug is great. He really breaks stuff down without making me feel dumb for not already knowing it. And he sounds a bit like Adam West (Original Batman)
Also Marks videos on Blue Glow Electronics Youtube are great. Sometimes he gets a little rambly and can jump too quickly between sections but I still learnt heaps watching his videos.
David Tipton on Youtube also has some great repair videos of old radios and quite often explains his equipment and methods and shows a full repair. Not just a puff of smoke and finished. He shows mistakes and all and how he traces his mistakes.
I watch your videos and really like how you explain things. Keep up the good work
Maybe you could help me with a new to me tube amp. It’s the Monoprice IIIP Bluetooth model with EL84 tubes. I’d like to see what kind of high end tubes would work in it.
A great little page- covering OTL. The reason this is good is it shows how to design and select the right output impedance. Both calculating it and then applying it. It covers all the otl designs (not just circlotron) and does a comparison between atma-sphere and corvi mkII designs.
Now let’s look at more advanced, there’s reference to Broskie’s Brazilian OTL but he provides the full schematic for study. Ignore the solid state followers, but if you follow the first link this should be straight forward: Fancy Box and OTL Downshift
If you've not seen this: Understanding Audio then it's possibly a good place to start it has some very good explanations about things like negative feedback, cathode degeneration and what that does to both input impedance and output impedance.