• 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.

On Line Tube Learning for newbies....

Uncle Doug has a youtube channel explaining very clearly and simply how tube amps work.

+1 Gotta love Uncle Doug.

A couple of short videos and now I know more about how tubes work than I did after reading 2 books.

He assumes the viewer knows nothing. Very refreshing and unique.

How cool is it that OPTs actually do double duty as filter chokes for the B+? I had no idea.
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2019-12-28 9:16 pm
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.


2019-12-28 9:16 pm
Selecting components
* Capacitors
* Resistors

This is a great starting point: https://www.waynekirkwood.com/image...alls_Grant_Wurcer_Analog_Dialog_17-2_1983.pdf

He also has a mine of information on capacitors and the differences between caps - measured rather than surjective opinion: Index of /images/pdf/Cyril_Bateman

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.


Paid Member
2015-12-31 4:57 pm
Hope you will give my channel a look. I'm creating content on learning about tube amps and explaining how my DIY projects work as part of the process.

Skunkie Designs Electronics - YouTube

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.


2019-12-28 9:16 pm
A little more advanced - PSRR in differential and Circlotron/PP forms:

First let’s look at “aikido” with a simple differential stage:
Aikido Differential Amplifier

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

Also bumped onto this for the SE folks: Lowering SE Amplifier Noise (page 2)
yes, there is an internal cathode resistor represented by 1/gm...and is cathode current dependent, a look at data sheets will show...

plate resistance rp, otoh is plate voltage dependent, again looking at data sheets...

mu =gm x rp, but mu just like the pi in a circle does not represent a physical representation...

mu in a triode is quite low, except perhaps the 6jd5 which can be in the 300 range..

mu in a pentode is so high that it is not used in any calculations and is largely ignored...