How the cathode resistor cleans up vacuum tube amplifier performance

[@psyolent.]

Mate this video is spot on. The theory then backed up by not only the prac but the measurement was sensational. Love your work.

[@RobsFixitShop]

Finally, a good explanation for what that capacitor does.

[@JohnAudioTech]

Demonetized again. Can't get it reviewed until it has 1000 views. KZbin needs to fix this shit.

[@raindogred]

I don't think they will "fix" it.. they appear to be trying to save money by ripping off creators... biting the hand that feeds, as it were..great business model huh??. Maybe you should start a patreon like mr carlson and aussie dave. I would gladly pay a few bucks to ask specific question about brick walls i run into with solid state audio circuits that i often run into. You could run a tech support/troubleshhoot service on patreon..make videos if you get on a certain subject if you get some requests, or even just give answers in comments. I would gladly pay for some of ur time.

[@CoolDudeClem]

Been saving this video so I have something good to watch while I have my dinner. In my experiments with an ECC83 (which is the same as a 12AX7), I also found that a 4.7k cathode resistor seems to provide the right amount of bias.

[@erin19030]

I built a tube tester based on your concept. I can plug in any tube and with jumpers I can hook the tube and resistors in circuit while controlling bias voltage vs plate current, thus calculating Transconductance. I also inductively couple a sine wave in on the grid and measure the plate signal for a gain measurement in Siemens.

[@williswet]

Fantastic demonstration John. I'm happy that you included the cathode bypass cap in this tutorial. I love it when you do vacuum tube videos. Keep up the great work.

[@NeverTalkToCops1]

This is too good! Multiple Radio Shack meters, ancient Rigol scope performing FFTs, and an excellent demonstration.

[@aerofart]

It’s nice to see someone creating breadboard circuits with vacuum tubes. I plan to build some of these circuits myself.

[@charlesshaw9090]

I haven’t done, or seen, tinkering like this for 30 odd years! Fun and informative.

[@paulkielt9301]

After trying different value resistors on my preamp tubes, I figured out that placing a 1.2 Volts rechargeable battery as a replacement it works the best. Now my preamp tubes get between 1.2 and 1.3 Volts on the cathode to ground and they are happy! The batteries recharge themselves when my amp is turned on, and there is no issues. I don't have to worry from now on about resistor and bypass capacitor values no more.

[@SuperFredAZ]

How does a positive power supply, recharge the battery?

[@jaysterdude]

You could also use some LEDs instead of a battery and enjoy the light show while you play :)

[@ReyciclismoMTB]

@@jaysterdude Actually I was thinking that exactly, I have something on a board right now.

[@bobwasmycat]

You just cured a 6 year issue. Thank you so much! And I trust wood and rubber insulators also. Just like my discharge tool, or wire insulation and a wall... couldn't help it. I saw pencil Karens...

[@OIE82]

That, was a great video that was explained well. I am trying to understand the differences between transistors and tubes in audio circuits and haven't built a tube circuit yet. Thank you uncle John.

[@ericrawson2909]

Great video. I knew that cathode bypass caps improved bass relative to treble, but was unaware that they reduced distortion.

[@gerrabath]

Because of the relatively low gain the 12AU7 is normally used as a phase inverter in double ended amps.

[@st2nh]

The best explanation to cathode Bias -THANKS

[@ElPasoTubeAmps]

John, nice video. I especially like the part where you showed the gain measurement of the triode. I have recently gone thru my TV7 tester and did a full calibration which, fortunately, turned out well. No repairs were necessary. But it got me thinking... instead of purchasing a 6L6 cal tube, why not make a simple circuit to measure the gm of a handful of the old metal 6l6'S and do it yourself. I have a second tube tester that I used for comparison to do my adjustments. I see across the Internet where people build tube testers and some of them look really good but I am thinking really basic. Just a chassis with the sockets and PS and banana jacks for accurate instruments to measure the plate current and bias voltage along with driving signal for the grid. I guess the way I am thinking right now is why I noticed your gain measurement of grid voltage vs plate voltage. Anyway, just wanted to say thanks for posting.

[@SPWoz]

Thanks for the tutorial - I appreciate your time and effort to share your expertise.

[@iblesbosuok]

What would happen if we substitute the 4.7k cathode resistor with a 3.9 volt zener? Magnificent video. Damn magnificent video. Cheers from Indonesia

[@danaolson2871]

That would defeat the feedback.

[@MarkTillotson]

The use of a pencil as a pointer to high voltage circuitry worries me - pencils conduct electricity very well!

[@calescapee9642]

Chopsticks conduct pretty good too!

[@zooleeka22]

I just read an old (2008) article from Gyula Sipos (IC engineer from Hungary ). Tubes vs Transistors in HI-FI amplifiers. The transistor amplifiers can be better in THD measurement if they are using one or two input signal. Over three different input signals the tube amplifiers are better in THD, those are make less harmonics than transistor amps. His experience. The production quality of the tubes in 1950's was much better than the transistor's in 2000-s. Deviance of parameters in branded tubes were around 10% in 1950-s. Deviance in transistors are huge in 2008 still. Pairing is not doable in amateur's workshop. The simple hfe measurement with DC in one operation point worths nothing. Deviance from vendor-to vendor, from transistor to transistor. Transistors are more complicated than tubes. Those have around 70 parameters which is only known by producers. Every parameter connected to every else parameter. For example just Hfe changes with temperature, current, voltage, frequency and of course different from vendor and series. The silicon temperature changes measurable to 50kHz. The transistors are much better on hi freq above MHz, they can easily oscillate in Mhz region. The combination of non linearity and complexity and hi freq oscillating the harmonics can produce frequecy product around the audible sound [freq intermodulation], which is unnatural sound and harmful for listener.

[@squiertelecaster6854]

I noticed the Radio Shack Ohm wheel tool in the back, I have the same one, good times!

[@wansolve2036]

Very interesting! I had always thought that Plate Voltage was purely an applied voltage, not a derived voltage determined by the grid bias. I'll have bench this out now and experiment! Thanks!

[@jimhenderson2308]

It’s my understanding that the voltage is applied to the circuit the vacuum tube is in which means the voltage at the plate could be the applied voltage but the voltage drop across the cathode to plate is derived. Cathode follower is when the useful load is placed on the cathode side of the tube. The tube acts to transfer small voltage fluctuations occurring at the grid the occur in larger form across an element in the circuit having cathode to plate continuity. The heater is not the main source of heat but all the mobile negative charge carrier bombardment going on within the tube is my guess. If there wasn’t a load in the circuit containing the tube the circuit would 100% inefficient with no useful output.

[@hendryparman3782]

John, I'll would like to add when you add the catode cap over the catode resistor, you short circuit the AC content but lifting the DC that why the amplification gets better.

[@charlesshaw9090]

Hendry parman the DC does not change when you use a cathode capacitor, other than it stabilises it if the cap is very large.

[@ufohunter3688]

Interesting video. I don't see the value of using vacuum tubes anymore. Power hogs that have now be refined to micro-meter size JFET transistors for amplification of weak signals, and BJT transistors for power output stages. Fun Fact: Apparently, vacuum tubes where discovered to be EMP proof. This is based on U.S. taking apart the MIG 23 that defected to Japan from Soviet Union in the early 80's, and finding all the circuits around the inside of the aircraft, to be vacuum tube based! It wasn't however due to lack of advanced IC design. The central brain-box, composed of traditional micro-electronic ICs circuits, but encapsulated inside a steel box, with shielded cables going to various external circuitry. They can take a kicking, and keep on ticking.

[@DaGhost141]

They look really nice, tube amps are relatively easy to build and understand, and the sound you get can (!) have a certain "warmth" to it. It's similar to the comeback of vinyl. A good DAC and Player are superior in terms of sound quality, but the sound of the vinyl itself can be pleasing too.

[@mkshffr4936]

6% seems kind of high even for a 12AU7. Of course that is a lot of output swing with not a lot of supply voltage. It would have been interesting to see the results at more reasonable output or more appropriate B+. A good video would be optimizing operating points.

[@jankowalski3541]

Pencil core is a conductor :-)

[@mr.amp0076]

Thank you for explaining that.... I was thinking as same as for the emmiter capacitor.. For transistor.. Same applies for tubes... But in my book they include a capacitor. I think that is for gain... But it increases distortion... They forget to mention.... Thanks john... I appreciate

[@martinda7446]

Ranjan....I haven't watched this video yet...Nevertheless the emitter resistor does indeed introduce local feedback. The bypass capacitor connected across removes this feedback for AC signals restoring gain etc. The DC conditions are stabilised by the resistor - always remember there are DC conditions and signal conditions to look at in a circuit.

[@martinda7446]

PS Often the emitter / cathode resistor is split into two values and only one is bypassed by the large value cap (to cover low frequencies) to compromise or have global feedback injected. Edit; And with valves it is of course a simple way of biassing the tube by lifting the cathode above ground potential, called 'autobias'.

[@MichaelBeeny]

Johns circuit only shows one stage of gain. Most amplifiers use multi stages of gain. To reduce distortion overall negative feedback is used from the output transformer right into the first amplifier valve (tube) This flattens the frequency response and reduces distortion and noise and generally makes the amplifier more stable, all at the expense of gain.

[@martinda7446]

Just to add though feedback is not a magic cure all -well almost -but you gotta keep an eye on phase changes. Keeping the feedback signal at 180 degrees out of phase is not always easy. If at any frequency it approaches the in-phase signal condition you have oscillation. Excessive amounts of feedback can cause instability, as can smaller amounts without frequency compensation. The more complex the circuit, the more phase trouble you can expect. The quality of the amplifier used to be measured by its ability to remain unconditionally stable with the largest amount of feedback, proving the design. Plus of course feedback can lower output impedance, raise input impedance along with extending response, lowering noise and distortion etc, all very welcome and invaluable - if done with care.

[@OptomodMix]

16:22 So what would casue a 22u/25V polarised cap to blow from the cathode of a 6V6, also in parallel is a 1W/470r resitor and a 104/63V cap?

[@Michael-lk1ox]

Hey, do you have any videos about how to build a full amplifier circuit on breadboard?

[@adaminsanoff]

Very educational. Makes me buy some tubes to experiment with.

[@larryshaver3568]

if you put a capacitor in the circuit with a smaller value would the grid be getting less current?

[@blazrepas8709]

A thumbs up for not oscilating on posotive feedback :D

[@henrykpycha1637]

Where did you derive an adjustable bias-supply voltage for the grid @ @ ?

[@stevafidanovski]

Excellent as always. Looking forward for next video.

[@filippf12]

Hi there i have buld an stereo tube amplifier with output tube a 829B at 520v on anode my question is what resistant must i use at the cathode. In some schematics i have see diferent values from 220ohms to 380ohms. I think the higher the resistance to the cathode as you lower the power output but i don't want to reduse the output power

[@nightbutchers]

have you done a vid on making a multi channel amp . 2.1-5.1 ch?

[@usertubeification]

Where did you derive an adjustable bias-supply voltage for the grid @ 5:40 ?

[@yrulooknatme]

YOU MEAN YOU get back the gain you lost by putting in the cathode bypass capacitor?

[@Cole-ek7fh]

but how does it sound with a guitar going through it?

[@jaymay9709]

I'm not clear on the 1% pilot signal that is being referenced on the FFT. How is that being generated?

[@michatroschka]

thats a reference signal from the oszilloscope, i assume

[@andresmartinelli5276]

Hello John! I am just think on a topic it would be good for a new video! and that is how to properly make a power supply for this amps you test. Treating dual rails, for both mono and stereo amp boards, and perhaps using a voltage regulator too!

[@aljenkins9748]

...same Harmonic Profile generated here by a tube (12AU7) and by a transistor (KSC1845) in the week before vid of nov 08,2017 . You prove on the scope that Harmonic Profiles are a result of the circuit design and not the result of discreet audio tube vs transistor components . i see it w my own eyes - - same profile , same intensity . Other things like threshold noise may differ - - but i can forget my cherished view that tubes r inherently "sweeter" . . . .............shalom,a.j.

[@gabet3754]

What’s the FFT?

[@arthurharrison1345]

Thanks for this excellent video.

[@georgebetts3953]

Awesome! Thanks, helped me a lot!

[@garymoore3497]

Surface does not use cathode resister bias. Surface uses lamp bias instead .

[@ki4dbk]

Nice work.

[@erin19030]

Good demo!

[@linandy1]

Great video !

[@mikehill3426]

You chose a cathode resistor based on your desired bias voltage. Is this good practice? You could choose a cathode resistor based on some other criteria and set the bias with additional components.

[@mikeduino4596]

Remember, the grid had to be negative by 3.4v with respect to the cathode. That resistor raised cathode by 3.4v, now pulling the grid to ground it is biased with the plate voltage at half the supply... giving maximum output signal swing in both directions! This technique, called "self bias" is used with jfet biasing (using a source resistor) This eliminates the need for dual supply. He did it absolutely correctly.

[@silasfatchett7380]

@@mikeduino4596 Maximum signal swing in both directions happens at 2/3 of supply voltage, not 1/2. (Which is why John's signal clipped on the positive excursion before clipping on the negative excursion.)

[@mikeduino4596]

@@silasfatchett7380 If what you say is true, then a circuit biased at 1/2 of the supply voltage would clip first on the negative output excursion. Right? This post is so old I don't remember the exact circuit. I designed and built a tube preamp with a single 12au7 and biased it pretty close to 1/2 B+ and it works fine. I used 150v B+ and have about 22Vp-p before any clipping. With a gain of approx. 18db I'll NEVER get close to clipping with a line-level input.

[@punkpop101]

Nice video. Your voice aounds exactly like Tobey Maguire. Spiderman is a guitar amp tech it seems!!!

[@HillsWorkbench]

Good vid!

[@anti_globalista]

Great one.

[@VandalIO]

This is black magic

[@ambient5]

nice

[@JoleenHurley]

The moral to the story is dont bypass cathode resistors or if you do just partially bypass cathode resistors. If u dont need the extra gain then let the cathode resistor clean up some distortion

[@danielsaturnino5715]

I play guitar. Leave my distortion alone :)

[@secod2022]

USE EF86 INSTEAD THIS DONT CLIPPING

[@kornami8678]

Bias.

[@TheXxPSYCHO]

Why do you need so much voltage for the 12ax7? Isn't plate voltage like 12.6v?