Dr. Z AMA: "Tube Amp Bias"

[@wgbearden7776]

As a car guy I appreciate both the names of these amps along with the idle metaphor explaining the biasing 👌

[@middle_pickup]

Bring these AMA vlogs back! I love learning from you, Doc!

[@mixn5919]

Dr Z seems like a guy who you could have a bombass barbecue with

[@mrGoesto11]

...and talk about amps. What a great day that would be.

[@elvis_chen]

and drink as much as beer they have, ah so satisfied.

[@gozer9556]

Hell yeah brother, amps and lawn darts!!

[@newgunguy4176]

He's the perfect guy to teach us.

[@jettramel]

Love this video, taking the mystery out of biasing so the layperson can somewhat understand it. I wish I had all this knowledge 25 years ago when I had a 71 & 73 Marshall heads. Looking forward to some more of these videos. Thanks Z.

[@brentstewart59]

Thank you sir for sharing your wealth of knowledge and de-mystifying this for us all. Dr Z you are both a gentleman and a scholar.

[@RoboRocker69]

I purchased a Weber BiasRite approx. 7 years ago and love it. Makes getting the plate voltage and current reading a breeze. Dr. Z is one smart cookie!

[@montag4516]

Generous, practical information offering here. Thank You Sir.

[@carlosanvito]

Great analogy relating fixed bias to idle adjustment - threading the needle to maintain plate current to a reasonable value. On a related subject, many years ago I was playing my guitar at a gig through my 1966 Traynor amp. Suddenly, it started to sound awful. I could see the plates glowing cherry red. I thought the bias circuit had failed, but actually, both grid caps on the 6CA7s had failed and were driving both tubes into solid conduction! The caps were waxed paper types. After replacing them, the amp was back to normal, with slightly worn tubes.

[@richardatanacio4049]

Thank u so much Doc. Your a sage of valves engineering.... Very rare nowadays...very generous

[@silviopimentel7247]

Just amazing how humble you are about the beautiful products you make without caring about all the knowledge you cher with us! Thank you so much. I definitely need to get myself one of your amp's no doubt 💯 about that! 👌✌

[@jazztime4847]

Thanks Doc, you're the best. I never stop learning.

[@midnightraiin4035]

that is about the best explanation i"ve heard on this topic.

[@David.S.]

check Uncle Doug's for fixed/cathode bias, if you want to go in greater detail..

[@seamanjive]

Thx Doc, another great presentation. Been evaluating my early Maz 18 NR head after installing JJ EL844s and the 5V4 rectified - it's working for me! Crunching up nicely.

[@GIBKEL]

Thank you so much for these info vids. Black box mystery’s explained for those of us without an electrical background. Only makes me want to understand more.

[@michel333100]

I would like to thank you very much for getting back on those questions I had concerning my valves that are mismatched.

[@timbrackeen3508]

Love the info man!!! Never get tired of the vids. Thanks for your time and knowledge

[@cantstartafire]

Thanks Bro! I really didn't get the concept until I saw this video! (btw, love my Remedy and Z best cab) Cheers!

[@watersignproductions]

He clearly comes from a time when cars had carburetors.

[@johnmarshall3903]

Dean is one of the nicest, most knowledgeable guys out there and the origional boutique amp builder before there was such a thing or the internet.

[@jessejohnson8087]

Thanks for sharing this info with us Doc--much appreciated

[@toneconsultant]

So informative! Thank you for sharing all that you share.

[@jaceyhorvath5704]

Thank you Dr. This clarifies things re next time I bias my Z28.

[@Rodedog55]

Thanks Dr. Z ! You are a fabulous teacher. Clear and Concise ! I love learning, . . . Thanks very much !

[@amp_mechanic]

Good info Dr. Z though I have run into some issues with people misunderstanding cathode bias. I have learned not to refer to cathode bias as self biasing or auto-biasing when talking with customers because it tends to give people the wrong impression. Cathode bias amps still need to have the bias set, as you said, you set them for a mid range tube. But that all depends on the tube supplier. And to some degree how old the amp is and how much components (including transformers) have drifted away from spec or even the wall voltage in your area. Because let's face it, most people don't replace or refresh parts until they break or blow up. I usually say to customers, cathode bias amps need to be checked and/or set for the tubes that are being placed in it. Cathode bias does tend to be more forgiving but it is not auto-biasing in the sense that you can throw any tube in (even in a mid-range) and not have problems. I think it at least should be checked as I've seen far too many amps that have problems that could have been avoided if it were checked when re-tubing. Anyway, great videos on here, that was just a thought I had and something I've run into with people misunderstanding cathode bias because they don't understand the term self-biasing in the same way as someone who works on this stuff.

[@GuteisFinger]

This is clear and so informative. Thank you.

[@dokebibees1012]

You make it easy to understand thank u

[@sthengr]

Thanks for the vid Dr Z!

[@chuckyz2]

They key factor is making sure you have the right tubes the amp is biased for. Lots of trust needed to make sure you don't damage your tubes or amp or get the specs right for the best performance. A couple of Eurotube meters will tell you all you need to know. Plate voltage and mA.

[@scottdunn2178]

I knew Ken Fischer since 1988, and learned everything I know from him. I sure miss the guy.

[@DrZAmplification]

a legend and an unbelievable human being

[@scottdunn2178]

@@DrZAmplification Yes he was. He spent literally hours on the phone with me when he was down with that "bs virus" answering my questions and teaching me about tubes, transformers, circuits, etc. And his twisted sense of humor 🤪

[@gabet3754]

Great explanation and analogies.

[@carnivaltym]

So informative! I'll be going to a tech, I don't wanna die! 😜 But, now I will sound more informed, at least! 🤣 You are a top man Dr Z!

[@johnsimms3957]

I prefer easily accessible test points and a mutimeter set to DC mV. This helps me to keep the tubes out of the danger zone. And then I fine tune the bias by ear.

[@nathanmontover1436]

awesome thanks!

[@johnsears4197]

Great explanation

[@DeadKoby]

As am amp tech, This was a little weird to understand, until someone was able to compare electrical current to water. By visualizing it as water, it made sense to me.

[@mfowler8808]

Good video Mike keep them coming. Mark Fowler

[@IndyRockStar]

i have 2003 JCM800 2203x. I just bought and installed a brand new matched quad of Genalex Gold Lions KT77. I let the tubes warm up, and the initial bias went fine all around 39ma w/ 438 plate voltage. Then all the sudden tubes 3&4 went from 39ma to 50+ma and tubes 1&2 stayed the same. Is this supposed to happen? I need help quick!.

[@nestormurrietachavez8586]

Tengo un fender con twin reverb de 135w* lleva 4 bulbos 5881 es posible usar el mismo vías para el kt66 si modifico la polarización del socket?

[@DrErickLopez]

What about Vintage amps that are neither fixed or self biased? For example, my 1963 Fender bassman has no bias pot. How would you bias that? How was it done back then?

[@DrZAmplification]

It is still technically a fixed bias, through non-adjustable. The bias supply has a fixed value of resistor in it that determines the bias current, instead of a pot (variable resistor). This was done as Fender tended to prefer a cold bias for reliability sake, and to extend tube life. Matching tubes also was not really a thing back in the day, and a colder bias will be more forgiving in variances in tubes.

[@DrZAmplification]

So in other words, they just didn't rebias the amps back then.

[@wmk0100]

? What can you say about grid biasing? I am learning about amp circuits and I built an amp that utilizes cathode "self" biasing. On a JCM 800 super lead 1989, it looks like the grid is the bias point.... Great channel by the way, I wish I started learning about this 35 years ago...

[@DrZAmplification]

Applying negative voltage to the grid is grid biasing, which is also known as fixed biasing.

[@Spidouz]

As a non-expert, I think if an amp needs to be biased manually, the best thing still is to directly have the plug in the back of the amp to make it possible without opening the amp and to plug easily the the right spot and without risking anything. It's not 100% idiot-proof, but it still makes the whole process a little bit more safe... or at least, that's the impression I get from :)

[@mindstorms44]

maybe it's me but I can't understand what your saying?

[@anthonysilva5312]

Any comments on cathode biased vs Class A? Thx Doc!🇨🇦

[@DrZAmplification]

cathode biasing is a method of bias, while class A is a method of output. they are only tangentially related based on the overall design of the power section.

[@TheAtheistworld]

Brother Z, do all self biasing amps use mid range 5 tubes, or it’s just your amps do? Cheers ❤

[@DrZAmplification]

can't speak for any one other than ourselves!

[@RadOGuitar]

for perfect adjust bias in "self bias" u must change cathode resistor. There is no small people doing adiustment inside ...

[@Toddler1967]

Does the Maz 18 Mk ii have a cathode bypass cap on the power tubes? If it doesn't, can you explain why?

[@DrZAmplification]

It does.

[@Toddler1967]

Thank you.

[@parkboy55]

Adjusting bias by ear is probably a bad idea for most people. If you adjust too hot, the tubes will not last as long and tone does not improve. Kevin O' Connor recommends biasing the output tubes to one-half of their dissipation rating in watts (from tube chart). NOS tubes will usually have a higher working dissipation rating than current tubes. Stay on the conservative side when using (nos tube chart) dissipation ratings or check with supplier to double-check dissipation rating of their tubes. Old (50s-60s) amps were designed for 110-117VAC so 120-125VAC will increase V+ voltages inside the amp and change the sound of the amp- possibly in a negative way.

[@DrZAmplification]

parkboy55 biasing via ear would be a micro adjustment after finding the proper range. As Dr. Z stated, it is his final step. Within the an safe range 50-70% idle dissipation, this could be up to 15mA which is a relatively wide range to make this adjustment. 50% of maximum dissipation while safe, can be too cold for some amps that rely upon power tube distortion for their tone. It is more common to see a colder bias like this in something like a high gain amp that relies on a clean power section with a cascaded preamp for its distorted tones.

[@matthewlapczynski5012]

What is a "medium range" tube? How do you measure the range of a tube? Thanks.

[@DrZAmplification]

Different resellers/distributors have relatively arbitrary ratings that change from who you buy from, that could be a 1-10 scale, or a letter scale, or whatever. They generally tell you how much headroom the tube has. We suggest medium, but it's entirely up to preference. For example, we match 6n14n based on their bias current at 300 volts. So that range could be anywhere from 38mA to 60mA. In general we look for tubes between 45mA-50mA, then match them within 1mA. MAZ amps can get higher than that sometimes, Z Wrecks can get lower, but generally, mid to high 40mA is our "medium" range based on the average bias current at 300v.

[@AB1Vampire]

Swamping resistor type bias (crude approach) vs Zener diode fixed bias -switched or w/regulator circuit for variable (more refined approach).

[@411Soulman1]

Seems to me, that Self Bias is the most common in 6 V6 tubes whereas fixed bias is most common in 6 L 6 tubes. What do you think?

[@DrZAmplification]

This is usually the case but either can be biased in either fashion depending on the goal.

[@mrGoesto11]

On cathode biased amps, using only 1 large cathod resistor for all the power tubes seems to be a good idea but why do some amps have separate cathode resistors? How close does the current have to be for each power tube before it's deemed in/out of spec?

[@DrZAmplification]

Some cathode biased amps are this way. Other's are not. Our amp line has separate cathode resistors, this is so the whole amp doesn't melt down if the amp loses bias, blows a tube, or has a faulty resistor. We measure our EL84s to 1mA, but 3-5mA is generally acceptable for a set of matched power tubes at idle.

[@michaelhicks9563]

Which bias method will give me some sag in my picking? My friend’s old deluxe reverb amp with fixed bias had beautiful saggy picking dynamics. All the amps I’ve owned were cathode biased without the desired sag.

[@DrZAmplification]

In theory cathode bias would sag more, but there are other considerations like how hot or cold it is biased, type of rectification, power supply filtering (value of capacitance, choke inductance if there is one) and plate voltage. Power supply filtering and rectification alone are more important in determining sag, bias method would come after those.

[@michaelhicks9563]

@@DrZAmplification Thanks for the info Dr!

[@aussie_philosopher8079]

Thanks z. Can someone tell me how do you know if the tubes you are buying are in the 1-10 category z mentioned . Is there ratings for the same tube type say El34 1 - El34 10. Do they draw slightly more or less current.

[@DrZAmplification]

Different manufacturers have different ways of rating their tubes. We buy tubes in the medium range (4-6) to stock our amps. Since in most cases EL34 amps are fixed bias, the current a tube will draw will be dependent on the range you set the bias voltage.

[@jimcatanzaro7808]

They sell fixed bias pcb boards on eBay now they work great

[@michel333100]

Hi, I would like to ask a question. I have a Ceriatone JTM45 bluesbreaker amp. In it I have two KT66 valves that are 10 milliamps apart from each other. Are these valves unusable being so mismatched? If it is still ok, how would you set the bias?

[@DrZAmplification]

You need to do a bias calculation based on your plate voltage and stay under 70% idle dissipation as recommended for that brand of tubes, and keep the stronger tube under that mark. While your DC bias may be off, that doesn't necessarily mean the tubes are unusable. They will wear differently, and the bias is likely to drift sooner than matched tubes, but as long as the AC output (signal to speaker) looks good on an oscilloscope (or more importantly, SOUNDS good to you), they are still usable. Just not the optimal situation.

[@russellesimonetta3835]

If I bought a production amp it would be a Z! I hate circus boards. When amp makers say circus boards allow consistency I say that means saving lots of money building them!!!

[@captainamerica9353]

Russell E Simonetta , yeah, funny how all of the old handwired amps that were a few hundred dollars back in the 50's and 60's can be worth tens of thousands now. But I don't know of any transistor amps that have appreciated in value.

[@fercholand]

Where should the volume be set during the bias. Does it affects?

[@DrZAmplification]

Biasing is done at idle, with no input, so volume level does not come into play.

[@stringsnapper7990]

There's over 375 volts inside the compartment. I've completed two fender Blues jr tone stack mods. Via bill audio.com if I have it right... I removed a fixed resistor and installed a variable to create an adjustable bias...Or per Dr Z, fixed but manually adjustable. Adjusting the bias was hairy. I watched billmaudio like five times and even he says what to do backwards... If I remember right. But if u think and use safe common sense it can be figured out. I did it twice. Something a bit worse is or was a cracked solder joint on a power tube. Be Careful!!! I've seen flames.

[@peterm3964]

Never heard of a resistor generating voltage . maybe try again .

[@mirage69]

Why would an amp builder choose fixed biasing over a self biasing circuit?

[@DrZAmplification]

A fixed bias amp general has higher clean headroom and can be more efficient which will translate to being higher powered and louder when we get up past 50 watts.

[@newgunguy4176]

Question. Why can't all amps be made as self biased?

[@DrZAmplification]

There are microcomputers you can install in amps to automatically regulate bias voltage and current but they are expensive additional considerations that in someways outweigh the cost to benefit. However, cathode bias, on top of self-biasing, is a type of self-limiting circuit that has limitations in terms of potential output. Fixed (adjustable) bias amps operate differently and allow the tubes to produce a higher amount of power in terms of wattage.

[@newgunguy4176]

@@DrZAmplification Thank you. I've avoided owning tube amps all my life because I don't know how to perform maintenance and don't know which techs are hacks. I keep reading about replacing tubes, adjusting bias and draining and replacing filter caps but I can't seem to find legit instruction on how to SAFELY perform these tasks. What's a good source of instruction that you recommend?

[@DrZAmplification]

@@newgunguy4176 Uncle Doug on youtube is one of the best and most knowledgeable resources online in terms of tube amps. You can learn a lot and we even reference his videos from time to time here at the shop.

[@newgunguy4176]

@@DrZAmplification Thank you so very much for your help. I really appreciate it. Blessings.

[@JJPsyc]

I love Dr Z amps, however, I am a little disappointed with customer service. I have sent messages with tech questions tnrough their website and received no reply.

[@DrZAmplification]

what email address are you using? Feel free to contact [e-mail]. We answer every single email we get, so there must be something going to a spam filter or an address no longer in use.

[@Lasse3]

wait, what is a carburetor?

[@azzazelo]

I didn't know Danny Devito made amps

[@nautilusmagellan7263]

I knew

[@Rufko]

Haha I'm sure Joe Walsh is his brother, he has the same funny way of talking... that is why he made him a signature amp...

[@munafo13]

What's up with the Galaxie making an appearance?!?!?! 😉

[@jamesha175]

right when the video started i thought those 2 amps were a single amp and i thought to myself, "what, is that Ted Nugent's amp or something?"

[@paulkielt9301]

Cathode biasing with just one resistor and capacitor is a total bullshit! You should have one resistor per tube to make it work properly. It removes hum in unmatch tube situations and balances heat much better, because those bias resistors can get very hot when they operate. When you see only one of them in cathode biased amps, it's just because companies are penny-pinchers! Unfortunately it's the same issue with fixed biased amps, they should have two bias potentiometers at least. That way you could even buy unmatched tubes and adjust them to your needs.

[@jimcatanzaro7808]

Tubes amps are not dangerous if you use test leads and have a brain if you are not trained or have a person to talk too for advice that knows what they are doing then don’t even attempt working on them period and make sure you use a varaic

[@thisiscrap34]

self-biasing or nothing for me.......dont want to die from trying to bias a EL34 amp!!! Way too dangerous!! AND DON'T LIE ABOUT IT!!!!