What is Tube Amp Sag? Chapter 2 in the Search for a Saggy Nirvana. Capacitors and Rectifiers

In Chapter 2 of our series on Tube Amp Sag we start our look at the role of capacitors and rectifiers. By the end of this chapter you will know what 60 and 120Hz hum sounds like, how capacitors can eliminate hum, and look at how capacitors contribute to tube amp sag. You might even learn how to make a capacitor!
This series of chapters on Tube Amp Sag will come out weekly so please subscribe and press the Notification Bell to be alerted as to each new chapter. I hope you enjoy and get something out of the series. Enjoy Chapter 2.
- Chris -

Chapter Two Index
00:00 Introduction to Chapter Two: Rectifiers and Capacitors. Part I.
01:00 Generating Alternating Current. Why not DC, direct current?
02:45 “Stop! What’s your business here, Mr Rectifier?” “I’m here to change your AC into DC”.
05:50 What does hum and ripple sound like? Sound samples of 50, 60, 100 and 120Hz
06:20 When can 100Hz or 120Hz hum sound good in an amplifier?
07:20 What is ripple, and how do capacitors reduce ripple?
09:45 Fenders choice of filter capacitor in the 6G3 Brownface Deluxe and the AB763 Blackface
10:50 Let's make a capacitor to understand how a capacitor works
19:40 Think of a filter capacitor as a special kind of battery
20:50 The consequence of increasing wall voltages to vintage amps and specifically filter capacitors
23:25 Another Dad analogy: How a filter capacitor supports the role of the rectifier.

[-dazz-]

Not sure if I'm missing something here, but at 14:40 you said you were going to simulate ESR, but then you put the resistor in parallel with the cap, not in series. Wouldn't that simulate leakage current instead of ESR?

[Subcritical]

As a mechanical engineer I approve this lesson!

[Waspfella]

There's nothing worse than not experiencing true tube sagginess . Keep up the good work Chris .

[golgothaassassin]

I Always learn something from something from your videos, great demonstration on the capacitor...

[michaelrobinson]

Thanks Chris. The nerdy world of what’s going on inside an amp is becoming clearer.

[Germinalx]

Volume level is greatly different…🤔 …than before. The previous video. But very well done. Marvellous!

[Jonathan_Doe_]

I know hum is bad, but the hum at the start of voodoo child (slight return) is kind of iconic, in a weird way. Like ‘woah if the background hum is that loud, sh!ts about to go down’

[martyncoe]

thanks for these videos i look forward to the next.

[montygore]

Very basic to some of us but still fun. Very informative for future techs.

[uptownphotography]

Part 2 was just as interesting as Part 1.
BTW: I am still studying for the pop quiz on Part 1...
Phil
NYC Area

[markusf.]

Hi Chris, great video as always, I really enjoyed it!

[michaelwhite]

Great explanation Chris. Marveling at your knowledge!! Keep them coming. Cheers!

[MonkeysGarage]

That 9.1% increase you spoke of for My power supply is pretty accurate. say an AA1164 Princeton schematic calling for a plate voltage is 410 volts. add 9.1% and and you get a true voltage of 455 volts.

The original Filter caps were rated for 450 volts. Now it shouldn't be a shock that wall voltages might even be higher than the 9.1%, say 124 instead of 120. A quick study should say that those 450 volt max caps are well beyond their lifting power. And, that should also give an indication to look at voltages at the bias cap and grid caps, etc and make sure the voltage demands aren't being exceeded.


Now, of course, this tidbit comes with a couple of questions. Newer transformers are wound with the higher voltages in mind, so no issue there. But if you are running a 1964 transformer, you are getting the full benefit of this higher current and voltage in your circuits. This makes higher plate values when you lower the current to set the bias on your output tubes. The question is, what would be a good and proper way to resolve this without ponying up for a new transformer? Perhaps higher resistance dropping resistors in the power supply area? And how would that effect the load on the transformer?

as a side note, the Princeton Reissue shows a plate voltage of 440 also, so apparently they didn't do alot of work on the transformers.

(don't think I didn't notice you calling me out as smarty pants) :p

[ivanduke]

Great channel !!! Keep up the wonderful work!!!

[robbieandkimberly]

I have a JCM800 that appears to be in excellent condition, I have re tubed it, I have re biased it, and i just can't get any upper mid frequencies the sound of the pick hitting the strings or, lower mid frequencies AKA low end thump, I suspect SAG, possibly the transformer, but then again what do i know, have you ever come across this?
Please help, Rob from canada

[edwardhannigan]

Hey Chris, for a newbie like me that was some learning curve..! Great video and so funny. I had my hand up a few times when you asked a question, so something must be sinking in..Take care and keep the info coming..Stay safe..Ed..uk..😀

[williamgombert]

Very intresting, what about placing a small film (0, 1 à 0, 2µF) cap over de electrolytic filter cap?

[moodyga]

rectifier sag. A rectifier it rectifiers something

[identifiesas.wheresmyche]

When you say its a 20uf capacitor or whatever, do you actually mean that or is it 0.20uf? I wouldnt ask this if it wasnt for the fact that im an hvac tech and our 20uf capacitors are GIANT, like bigger than a tube. No offense meant!

[dwightbehm]

Capacitors non polorized or electrolytic store's energy electrostatically Filter chokes or coils of wire store energy electromagnetically. Chris your pretty cool. Giving away knowledge in a practicle way that can be understood for the layman. Google sent me your video. You are much like uncle Doug of Danisonic but he never shows his face.