Dual bias vs single

[daz]

Sorry for all the threads, I know i'm probably wearing out my welcome here. And i try and post elsewhere when i can, but this is the only forum where i actually get great answers, or even any at all ! One of the things as you likely know that i've been battling for a while now is a harder brighter sound at higher volumes. I keep staring at my schematic trying to see what it could be compared to a jcm 2204 for example, but theres just no difference that i either haven't had it configured as before, or that i can't adjust to be the same like my variable NFB. The only difference which till now i kept telling myself it can't be that is the dual bias. So i lifted one of the 220k's and soldered it to the other one to make it single bias. The tubes are very well matched so that wouldn't be a issue if comparing dual to single bias, which i just did. I checked each tube when i tried single bias and they were within 1 MA, and i biased them to where i had em when dual.

That said, single bias sounds quite a bit different. I was taken aback to say the least. That had me thinking i either designed the bias circuit wrong, or dual bias sounds different. So the question is, looking at my schematic, does it appear i did it right or could there be an issue in the circuit, or is there indeed something with DB that can sound different ? Not sure if it has anything to do with my tone issue at volume, but it DOES sound smoother with single bias. I will have to wait to test it at volume due to personal reasons concerning my living situation.

[soundguruman]

Sorry for all the threads, I know i'm probably wearing out my welcome here. And i try and post elsewhere when i can, but this is the only forum where i actually get great answers, or even any at all ! One of the things as you likely know that i've been battling for a while now is a harder brighter sound at higher volumes. I keep staring at my schematic trying to see what it could be compared to a jcm 2204 for example, but theres just no difference that i either haven't had it configured as before, or that i can't adjust to be the same like my variable NFB. The only difference which till now i kept telling myself it can't be that is the dual bias. So i lifted one of the 220k's and soldered it to the other one to make it single bias. The tubes are very well matched so that wouldn't be a issue if comparing dual to single bias, which i just did. I checked each tube when i tried single bias and they were within 1 MA, and i biased them to where i had em when dual.

That said, single bias sounds quite a bit different. I was taken aback to say the least. That had me thinking i either designed the bias circuit wrong, or dual bias sounds different. So the question is, looking at my schematic, does it appear i did it right or could there be an issue in the circuit, or is there indeed something with DB that can sound different ? Not sure if it has anything to do with my tone issue at volume, but it DOES sound smoother with single bias. I will have to wait to test it at volume due to personal reasons concerning my living situation.

[ATTACH=CONFIG]27989[/ATTACH]

In a single bias, the two out of phase signals cancel each other and produce a drive limit to the output tube grids,
depending on the value of the two resistors.
So, the single bias is going to sound different because the frequency response is altered due to cancellations, and
the limit of signal that can reach the output tube grids, before cancellation occurs.
The smaller those 2 resistors are, the more cancellation occurs, and the more distortion is produced...
it's more of a "crossover" distortion that occurs, if you look closely...

With dual bias there is probably less distortion, and better frequency response, because that cancellation of out of phase
signals is not occurring, and the drive to the output tubes is not being limited by that phase cancellation.

Also, the single bias circuit is considered a method of current limiting, because the output tubes can only be driven "so far,"
by the output of the phase inverter. There's a limiter effect.

So on one hand, the dual bias is cleaner...
On the other hand there is no output drive current limit, depending on the values of components used.
The dual bias is more hi-fi sounding, I would venture...less distorted probably.

AND the dual bias allows balancing the output tubes better, producing less noise on the final output (less hum from unbalanced tubes, possibly).

Another consideration in dual bias is the loading of the phase inverter output, both capacitive (from your bias filter capacitors) and resistive (from your resisters to ground). This also affects your final grid drive characteristics, and in doing so effects the sound quality...
although builders of tube guitar amps tend to ignore this rather obvious alteration....they tend to consider the other components as having NO affect on the sound quality...even though it obviously DOES. (if you are listening?)

Like I have said many many times, the "sound quality" does NOT end with an oscilloscope...
There are dozens of nuances that a technician cannot "see" with test equipment.
Half the test for sound quality requires musically trained ears, and an electric guitar, not test equipment...
So, if you are using test equipment, and not listening, and not using an electric guitar, you have missed "half" the problem.
AND you have missed the part that is most important to: "a musician."

[elipsey]

Thanks for posting your observations, it's nice to hear someone's subjective impression of the difference. I have wondered about this too, but didn't really want mess with my bias circuit to find out.

I have been experimenting with a pre-amp circuit that's based on the 2204 (but with a lower power output stage). Are you able to get a satisfactory clean sound from this amp?

I ask because I have had trouble finding a compromise between having higher gain available along with a good clean sound without have some kind of "boost" or channel switch type of arrangement; the single gain/master arrangement doesn't get really clean for me, especially with humbuckers. Sorry if this is off topic...

[soundguruman]

Thanks for posting your observations, it's nice to hear someone's subjective impression of the difference. I have wondered about this too, but didn't really want mess with my bias circuit to find out.

I have been experimenting with a pre-amp circuit that's based on the 2204 (but with a lower power output stage). Are you able to get a satisfactory clean sound from this amp?

I ask because I have had trouble finding a compromise between having higher gain available along with a good clean sound without have some kind of "boost" or channel switch type of arrangement; the single gain/master arrangement doesn't get really clean for me, especially with humbuckers. Sorry if this is off topic...

It's not subjective and it's not off topic.
It's real, and it means everything to a musician...
although a person who is purely "technical" would fail to understand it. Would indeed miss it completely...
to a musically trained person...with ears...it's undeniable. Don't doubt it, this is REALLY important.

[soundguruman]

The Whole Thing About Playing a Marshall:

You have to change your habits, and become accustom to using the volume and tone controls on your guitar.
Volume & Tone on guitar will control everything about Marshall. From clean to overdrive. From soft to scraping...

You do not need two channels, you do not need a foot switch, you do not need an effects pedal, you do not
need a guitar on board preamp...

turn the guitar volume down, and the Marshall will be very clean almost like a twin reverb- that's your clean sound

turn the guitar UP, and the Marshall will overdrive- that's your overdrive sound.

No footswitch. You did that with your guitar volume control.

Now use the tone controls on the amp and guitar, and your 2 pickups
to produce your desired variations. That's all.

The first thing (get over it) if you don't like it (that way above), get another amp.
Either accept and play a Marshall like a Marshall, or change your amp choice.

Cause Marshall is going to operate the way I told you, and you may as well not fight it anymore. You may be piezoing in the wind...

But if you are willing to operate it that way,
you can discover that it is just as clean as any clean amp, and just as rude in overdrive as you please.
Or else you may as not have a Marshall, and need some other amp.

The basic operating hints are:

A. Use humbucking pickups.
B. Keep the amp cranked up very high.
C. Modify amp with good mods, different tubes, etc...
D. Attenuate the amp, after the power amp, not by turning down master volume.
Leave the master very high, to overdrive the power tubes.

E. Attenuate the amp, to the desirable volume level, that is suitable for the venue.
1. Use 1X12 speaker instead of 4X12 speaker
2. Use less efficient speaker.
3. Use Power Attenuator.
4. Use Speaker emulator.
5. Change pentode Output to Triode output. Easy.

All to preserve the ability to crank the Marshall wide open, without blowing your ears out.
Get a good sound at lower more consistent appropriate volume. Clean or overdrive.

If you are unwilling to work with it that way, you might be looking for another amplifier. That's what a Marshall "is."

There are lots of alternatives which might suite a particular player better than Marshall.

[elipsey]

It's not subjective and it's not off topic.
It's real, and it means everything to a musician...
although a person who is purely "technical" would fail to understand it. Would indeed miss it completely...
to a musically trained person...with ears...it's undeniable. Don't doubt it, this is REALLY important.

elipsey backs away slowly...
I was concerned that my question about pre-amp gain was off topic. I certainly did not mean to criticize OP in any way.

Perhaps you infer sarcasm where it isn't intended. I called his report "subjective" and that is a statement of fact, but hardly an insult. He told us what dual bias sounds like to him. I have already read about the technical merits of dual bias elsewhere, so I said "thank you", and meant it, because it's helpful to me to know what it sounds like.

[elipsey]

I'm not really here to make value judgements.

I do use the volume knob on my guitar to clean up the pre amp. It sounds like you have a similar experience with your Marshal amps, which is an interesting datum. In point of fact, my amp is not a Marshal. I built it from scratch with an eye towards the JCM800. For all I know they are nothing alike, that's why I asked.

I would be interested to hear about daz's experience with the pre-amp gain of the his 2204.

[Enzo]

Hey, elipsey, welcome to the forum. Don't worry about soundguruman, he is more about seeing his own words on the screen than being genuinely helpful or anything. Give it a day, most posters are only around a few hours a day.

[daz]

I'm not really here to make value judgements.

I do use the volume knob on my guitar to clean up the pre amp. It sounds like you have a similar experience with your Marshal amps, which is an interesting datum. In point of fact, my amp is not a Marshal. I built it from scratch with an eye towards the JCM800. For all I know they are nothing alike, that's why I asked.

I would be interested to hear about daz's experience with the pre-amp gain of the his 2204.

Same here, I built mine to be similar to a 2204. But i deviate a lot from the 800 in the preamp. From the cathode follower on back it's almost the same as a 2204 except the filtering, the new style of master i just implemented, and variable NFB. Not sure what you want to know about the preamp gain, but i can tell you theres no cold bias stage in mine so the gain is substantial, tho not metal. More like the amount you;d find on the drive channel of a peavey classic 30, or a bit more than a blues Jr at full drive. Maybe a Blues Jr with a slight clean boost to take it to more sustaining levels of OD.

[elipsey]

Cheers Enzo! It's good to be here.

[elipsey]

Thanks, that's exactly the kind of answer I'm looking for. I have two input jacks, one of which bypasses the first gain stage. I'm not sure that arrangement is any better then using the volume knob on my guitar. I do have a higher value (10K) cathode resistor on the second gain stage. Is that what you call a cold bias stage?

[daz]

I ask because I have had trouble finding a compromise between having higher gain available along with a good clean sound without have some kind of "boost" or channel switch type of arrangement; the single gain/master arrangement doesn't get really clean for me, especially with humbuckers.

And you likely won't find that. After spending years on this design and having every circuit arrangement imaginable, i've learned a lot about preamp design. One thing i learned is that great preamp distortion requires certain things which make it pretty much impossible to double as a clean tone just by turning the gain down. Preamp OD requires you shape the tone in a way that makes for a tinny clean tone but without which the OD tone would be flabby and unfocused. Some amps try and make the amp do a clean tone and OD tone that meet each other 1/2 way, each one compromising for the other. I find that foolish to my way of thinking, which is that you can either have 1 killer tone or 2 mediocre ones.

That said, my personal preference for clean tones are those which aren't pristine clean but instead have that on the edge quality that gets dirt when you dig in and has a kind of chime that pristine cleans don't have. I do love really good pristine clean too, but it's not my priority and if i have to have that i plug into an amp that does. But anyways, yeah....no preamp that has killer OD is going to have the kind of cleans you're looking for.

[Tooboob]

In a single bias, the two out of phase signals cancel each other and produce a drive limit to the output tube grids,
depending on the value of the two resistors.
So, the single bias is going to sound different because the frequency response is altered due to cancellations, and
the limit of signal that can reach the output tube grids, before cancellation occurs.
The smaller those 2 resistors are, the more cancellation occurs, and the more distortion is produced...
it's more of a "crossover" distortion that occurs, if you look closely...

I don't get it. Can you give me a hand?

In the dual bias drawing, there are 10uF caps on the 220K resistors attempting to bias the power tube grids. For single bias, you could tie both 220K resistors to the same cap. Now, if the 10uF caps were actually ideal and infinite value, I don't see any difference in the result at the grids. In the drawing, they are 10uF caps with an ESR that's probably fairly negligible. The PI output has to pass through caps that are maybe 450 times lower value, so I wouldn't expect that 10uF is too small for decent performance. I understand that 220K will load the PI outputs in the frequency range of interest (is this the cause of your "drive limit to the output tube grids"?) , but the load on each output looks the same in either case. The only interaction I see in the single bias case is past the 10uF capacitor, which looks like a suitably stable, low-impedance voltage source to me. Is the less than infinite capacitance of the 10uF cap and its ESR the cause of the cancellation of which you speak? Not counting ESR, anything that makes it through the .022uF cap should be down around 48dB or more at the 10uF cap.

How do the "cancellations" you refer to occur?

[GainFreak]

I didn't quite get it how dual bias could sound different than single one either.

[Dave H]

I don't get it...

Yes you do.