In this clip I complete an objective test comparing Fixed vs. Cathode Bias 6V6 power tubes in the same amp. Part 2 of the clip deals with the theory and output power differences between the two. headfirstamplification.com
I liked the fixed bias sound the best. It just sounds rawer with more harmonics and bass. The cathode bias sounds a little more compressed and maybe a little tighter. But it also sounds like it's being held back. The fixed bias also sounds like it has a better signal-to-noise ratio. This is a really good video!
I hadn't considered why I may want one or the other but hearing the clips makes me think it's an option you should have to switch between both, especially if recording! I was tempted to say the fixed sounded better for dirt [ more mid range ] and the cathode for clean [ less mid , more top & bottom ]
Awesome video. I've searched for something like this and not really found anything that covers guitar tone. Love the plexi mode on that. It's warm & crunchy. Just the perfect amount of saturation. And the clean is beautiful.
This video is perfect for me right now, thank you! I have a Princeton Reverb clone I'm tinkering with & wanted to put a cathode bias switch into the circuit. I'm learning as I go & have managed to not kill myself with cheeky capacitors so far, which is nice. I've played several gigs using different amps over the years, mine, using other people's, amps provided etc., I've have always suspected I leaned towards cathode bias sound (& feel), nearly, every time. Now, thanks to you, I can quickly hear the difference on the same amp. I will definitely put that switch in now. 🤘
Good starter instructional video, especially having the tones and the theory side by side. For me, I would like to see you expand the very last bit with the oscilloscope and delve into what happens with a signal on both, not just the max power, but in terms of the bias point "changing" with cathode bias vs fixed during a dynamically changing set of conditions i.e playing a guitar :); more of exactly "why" the sound differences occur, cathode is "softer", "squishier" etc, why ?
Because -Vdc on grid allows for lower V on cathode (thinking at idle) so higher potential to plate. So a higher wattage potential in fixed bias, esp heard in compression of lows in cathode mode.
Thank you for this......it answered a lot of questions I had.....I did prefer the fixed bias as far as sound goes....it seemed tighter,and punchier.....great stuff...thank you again...
In the old days, it was all about more power. Then the world changed and we couldn't drive a 100W amp into a 4x12 cabinet in a small club hard enough to sound good. So cathode bias had some nice advantages: cheaper (no need for that negative grid bias circuit), self-biasing (no need to adjust the bias when changing tubes), tubes could last a little longer, and lower power giving the overdrive tone a little quieter. Now it's shifting back because distortion tones are being created with pedals for more control into a clean amp. So it all depends on what you're trying to do.
The difference is subtle, at least watching this on my phone 😂, but there's absolutely something there. Sounds like fixed bias is stiffer and a bit more aggressive and cathode bias a bit rounder and more compressed. Absolutely worth experimenting with!
What an excellent video. I'm about to purchase a cathode biased "metal" amp: the Badcat Lynx-x. While cathode bias may lack low-end tightness, there is a spongyness and liveliness that attracts my ear :)
Great amp. I have the Cougar 50 which seems to be a similar circuit made in Asia. Of course the Linx will be made with better components, looking at my schematic, the bias circuit on appears to be some sort of hybrid design. I don't hear this type of biasing being discussed often, and I have limited knowledge about it. I'm assuming it is some sort of compromise to tighten the bass. I hadn't had an issue. I don't think you will be disappointed.
I think you are right about the design being hybrid is a way to compromise the low end tightness. However, as time has passed since I wrote this comment, I have changed my mind and I'm getting a swedish handbuilt 20 watter "Olsson amps Little Hill". This one is also cathode biased and I tried it a few dags ago in a shop. It was tight as hell and you dont even need a tubescreamer. Sure if you wanna play meshuggah you'll need some pedal in front but for normal metal and rock its not needed. Really impressive quality. The sound is that of a hot rod marshall and the clean channel is very warm. Extremely articulate overall and the tightness in the gain channel made me feel like a "faster player" ☺ . Im sure the Lynx is a fine amp with its own thing going on but for my needs the Olsson amp seems to be on another level completely.
@@jonashjalmen5232 I just checked out a video of that amp. I'll agree with you on the sound of it. It looks similar to the Bad Cat. Over the decades, I have spent more money trying to save money than if I would have gotten expensive gear in the first place. Buy it nice or buy it twice.
@@qua7771 Getting new gear is always fun and will inspire you in different ways and sometimes when the gear is bad its just a learning experience in which you can declare " i dont like this type of tone" :) I've been using AXE FX II for a couple of years now and while that's an incredible product I have finally grown tired of it. But the good thing about it is that it allowed me to find "my sound". After experimenting with all the various amp models I usually end up with the same kind of tone and the biggest factor that decides your sound characteristic is actually the cab!!
@@jonashjalmen5232 I couldn't agree more. (Cab and speakers), I found that out late too. When I started playing, we did have the learning resources and technology we do today. I wasn't able to research gear effectively until the internet era. A few years ago I found the tone I liked and wished I had found it sooner (A Marshall 1960 cab with two V-30's and two H-75's through the Bad Cat, after rolling tubes). I use a Bad Cat unleash attenuator after learning that every amp has a certain volume that it sounds best at. I can dial back the volume while getting "that" tone. I can honestly say that I am satisfied, and can now concentrate on things that are less expensive. It was a long learning experience. I have always played Les Paul's, and super Strat type guitars. I am now in the process of building a Tele after returning one that just wasn't for me. I have never owned a Tele. I cannot wait until it is completed to play it. I hope the single coils sound good through my rig. Otherwise, I do have a few other amps. Cheers.
Cathode bias sounded more compressed with added harmonic content. Fixed bias seemed to have more headroom and clarity. Now I want to convert my sc20h to fixed bias, if that's even possible 😅
That's all I build is lower watt cathode biased amps, I love the sag and dirty bite it gives. Tubes do not last as long though. I have a couple fixed biased amp as well, tubes seem to last alot longer
An interesting comparison. I have often wondered if there was an audible difference between the two bias modes. The 47uF cathode bypass capacitor would have a reactance of ~41 ohms at 82Hz, which may or may not be significant. May be worth trying a 470uF and seeing if it makes a difference. Good luck with your experiments. I agree that for maximum power, fixed bias is the way to go. Cheers, Steve.
Inspired by your video I actually ended up putting in fixed bias circuitry in a cathode biased amp I'm finishing up with a switch between the two. I got pretty much the same result as you got, cathode biased was sort of swampier and looser in the bottom end and fixed bias was tighter and snappier. That amp has a tube rectifier though so it's still gonna sag a bit of course. Haven't tried it out properly yet (sleeping kids 😂) but it was well worth experimenting with!
When you had both cranked, do you think there was any power tube clipping? I have done the test, and I think when the power amp is clipping is when the difference is most noticeable. If the power amp is clean, of course there is still a considerable difference in feel, and the extended low end in fixed bias, like you said. Great video!
Great vid. Beware though that fixed bias is noticeably louder than cathod bias, so your ears are deceived by the "louder is better" rule. Once levelled out Cathod Bias is not that bad (and not that different)
Found your video as per live chat 12/1/23. I'm moving and hope to have workspace. Was wondering if experiment with single ended amp(keep volume down) fixed and/or cathode bias was worth it?
The only time I would use cathode bias is with a post phase inverter master volume as in some Dr. Z amps. I'm thinking of building a Plexi style amp but running 2 KT88's with cathode bias and ppimv. Unless someone can convince me that is a bad idea ?
Very interesting video thanks, I'm interested in ordering your pcb but I only have build one hot roded plexy from ceriatone, you know big board and so forth. Do you think these basic skills are enough for your pcb project including midi? Thanks!
If you've built before, even from a Ceriatone kit, then you're ready. We have a good community group on FB too for help from myself and others. Go for it!
Hi Jason, when selecting the correct value cathode bias resistor do you work that out mathematically or fit a value you have seen before / you think will be suitable and then adjust by measuring the voltage at the cathode bias resistor?
There are some online calculators for this, which give a good starting point. Then I adjust once built - aiming for 100% dissipation in cathode bias mode.
I hear a slight difference. How the notes bloom, or would say compress. I definitely think that fixed mode is for tighter approach but the other feels more forgiving kinda. There’s no better, both have their mojos. A great build by a master! Awesome ! Can you tell why so many guitar amplifiers don’t use Toroidal Power Transformers ? it bugs my mind because due to its construction it should be less prone to noise... Thanks!
Thanks for the feedback. Re toroidal PT's, a well laid out and deigned amp will have near zero noise with conventional transformers. For me, I wouldn't choose one over the other for noise reasons. There's a good response to this question from user Fuchsaudio here, www.thegearpage.net/board/index.php?threads/torroidal-transformers-vs-transformers.68141/
Headfirst Amplification Oh i see ! but it’s not me or someone saying that.. it’s in their datasets eu.mouser.com/datasheet/2/177/1182-736858.pdf lo ripple & light weight. could argue that n core could be more lighter. i was looking for something that could be used in rack preamp build. that’s the idea, maybe you have something coming up on that side... look up some cool idea . nasa point to point mode soldano slo ;) diy-fever.com/amps/soldano-preamp-mk2/
Fixed bias designs give the greatest output power for a given output tube - while the cathode bias is usually preferred in hi-fi applications because it exhibits the typical "classic tube sound". Another advantage of cathode bias is that you don't have to use matched tubes in push-pull output applications which can be useful if your only source of operational tubes are from discarded gear being sold off as a bargain in garage sales.
