Different kinds of amp bias?

[MasterEvan07]

Would a gracious and informed party be so kind as to illustrate cathode bias vs. whatever vs. whatever (as you can see I'm not even aware of all the bias options there are!).

What are their distinguishing features and sonic characteristics?

I'm on a new amp quest and would like to know what I'm really reading about when I check the spec list.

Thanks!

 

[HughesP]

You pretty much only end up with 2 types of bias methods in amps. But bias is certainly not the end all be all of amp tonal characteristics.

There is cathode bias (aka self-biasing) which used a big resistor on the cathode of the power tubes. This was a cheaper way, initially, because less parts are involved, though some will certainly prefer the sonic merits of it. Generally cathode bias is more "vintage" sounding, rounder tone, with looser bass. It is also less efficient, so you get less power. Typically, in cathode bias, amp makers like to bias the tubes really hot.

Classic examples are the vox ac30, ac15, tweed deluxe, 18 watt marshalls, etc...

Most cathode bias amps include a bypass cap along with the bias resistor which makes them brighter/tightens up the bass - basically making them more like a fixed bias amp, but not entirely (you need a REALLY big cap to completely get that sound and most are only around 25-50uF)

Fixed Bias (aka adjustable bias) uses a negative voltage supplied to the screens. This is more efficient, higher headroom, tighter bass. Some will say fixed bias amps are a little more harsh sounding, but that can be corrected elsewhere in the circuit. Tubes aren't biased as hot, generally, so they last longer while giving more power. They sound more "modern"...

Classic examples range from BF fender amps, tweed bassman, Marshall Plexi, mesa boogie amps, etc...

The biggest thing to understand is that these are bias methods - ways of making the tubes work. They can actually sound pretty similar, but there's a lot of other factors at play too that mess with people's perceptions of these things... how hot the tubes are biased changes the sound, and since fixed bias amps are generally around 70% or less of the tubes max dissipation , and cathode biased amps up around 90% or higher, that skews perception.

Likewise, a lot of the classic cathode biased amps don't have negative feedback, which has a much bigger difference to the sound/behavior of an amp than the bias method.

 

[MasterEvan07]

Very helpful thank you!

I have a 5E3 clone that takes pedals like nothing else...just fantastic dirty sounds, but it's looser in the bass. I want something snappy but still dirt friendly while having excellent chime and responsiveness. I've been leaning towards a Vox style circuit but don't know exactly how the bass responds because I've not had enough time with them.

But based on what HughesP said a good Vox-y amp with a bypass cap could have the bass response I want while retaining the chimey goodness Vox amps are known for.

 

[Timbre Wolf]

Very nice rundown, HughesP.

To add to this discussion, it is useful to also consider the phase-inverter type (long-tail pair the most modern and clean, cathodyne or split-load less clean [like in your 5E3], and paraphase is least clean), and also the voltages used in the preamp and power tubes. Of course, there's also the preamp type, negative feedback, etc.

Dave Hunter's The Guitar Amp Handbook is an excellent resource for learning more about these topics.

- Thom

 

[HughesP]

Very helpful thank you!

I have a 5E3 clone that takes pedals like nothing else...just fantastic dirty sounds, but it's looser in the bass. I want something snappy but still dirt friendly while having excellent chime and responsiveness. I've been leaning towards a Vox style circuit but don't know exactly how the bass responds because I've not had enough time with them.

But based on what HughesP said a good Vox-y amp with a bypass cap could have the bass response I want while retaining the chimey goodness Vox amps are known for.

A few days late in my reply... but just wanted to add in that generally a 5e3 and AC15 have the same bypass cap along the bias resistor. An AC15 will typically be tighter, though, not because of the bias arrangement or anything to do with the bias, but because the coupling caps from the PI to the power tubes are smaller in the AC15, cutting more bass before the signal gets to the tubes. That, and you've got the difference between el84 tubes vs 6v6 tubes, which brings in a host of other factors at the same time...

An AC15 might give you tighter bass, but it isn't because of the bias scheme! It's simply because there is less bass being amplified in these amps.

You could really tighten up the bass quite a bit on a 5e3 if you changed the .1uF coupling caps to .022uF, increased the cathode bypass cap on the 6V6 tubes from 22uF to 220uF (or even 1000uF), and beefed up the filter caps. It wouldn't necessarily as much like a 5e3 at that point (and won't sound like an AC15 either), but it would tighten up the bass considerably.

For really tight/modern bass, though, you really need to go fixed bias, but not everyone wants bass that is THAT tight. Similarly, the more power one has, the bigger the bass will be/easier to keep it tight.

Again, though, it depends what you want...

 

[teemuk]

http://ken-gilbert.com/images/pdf/fixbias.pdf

 

[MasterEvan07]

It's funny...my SuperDawg is tight whereas my Crema Wheat is quite loose even though they're in the same tweed family. Phase inversion, tubes, speaker configuration, coupling caps...so many things to consider how do you keep track of it all?

It's hard for me because there are things I want to know but at the same time there are things I don't want to concern myself with as there are other places I'd like to focus!

Ever the journey

 

[mad dog]

That is a good description, Mr. Hughes. I had a fixed/cathode switch installed on my Sewell Wampus Cat, which in stock form is cathode bias. Very interesting to compare. At least in that amp, fixed mode works much better for me. Not because fixed is always better I suspect, but because it works better to my ears with this particular circuit.

I think tube vs. SS rectifier has an equivalent, perhaps even more noticeable impact on amp tone. So many other factors to consider, it's really hard to generalize.
MD

 

[TNO]

For tighter bass in a 5E3 I like .02 coupling caps all the way across. The biggest improvement is changing the first filter cap (the one closest to the power trannie) from 16 uF to 32uF. Don't go higher than that with a 5Y3. Swapping the speaker for a Jensen Neo will tighten up the bass like you wouldn't believe and give more clean headroom.