Biasing combination bias

[jpfamps]

1968 chassis. Here's a schematic example: http://www.webphix.com/schematic%20h...b568_schem.pdf

OK, so you are saying that you can balance between the cathode and fixed bias feels. Makes sense, but from what I've seen, on the Fender examples anyway, it does introduce some odd distortion. Has anyone analyzed this at all?

As pointed out about the AC50 used it, and these sound fine.

From my experiences I don't think that their is anything inherently "wrong" with combination biasing.

A key feature of the Fender system (which incidentally was fairly quickly abandoned by CBS) cf the AC50 is the 5uF cap between the 150 ohm resistors on each power valve cathode.

I assume this was included to try to provide some form of feedback at AC.

Not withstanding the fact that this cap is now a 40 odd year old non-polarized electrolytic, it never seemed to me to be a very good idea because when one half of the amp is driven into class B loading it will charge up and upset the biasing. I'm not sure where where Fender got the idea for this from as I've not seen it on any other amp.

I always remove this cap from Fender amps of this vintage.

Regardless, when I converted my AB668 Pro Reverb to BF spec I left the resistors in to see what happened. The amp sounds great, but as you might expect compresses more than the stock BF circuit.

[EFK]

I do very much appreciate the responses guys - some of it is over my head, but I did get my answer here as to how to bias such a setup, and I'm planning on trying it just for the sake of "hmmm how does THIS sound..."

[mooreamps]

I sure do hope that was a tongue-in-cheek response.

No one here is interested in anyone else's "proprietary magic". Perhaps you can tell us WHY you use combination bias.

There should be nothing "magic" about it. I use it for a few reasons.

1. I use it in conjunction with my high power / low power switch. 6 watts driven Class A ; 36 watts driven Class A/B.
2. If the bias power supply should fail, or cold solder joint on the bias wire on the power tube socket, ect ....... The tube would fall back to Class A bias, and not go "red-plate" on you.

-g

[mooreamps]

I do very much appreciate the responses guys - some of it is over my head, but I did get my answer here as to how to bias such a setup, and I'm planning on trying it just for the sake of "hmmm how does THIS sound..."

To my ear, I think the tone is very close to the same. It's just that you don't get the "depth" in the playing response you get in Class A/B verses Class A. Also, both modes can still sound pretty loud enough.

-g

[jrfrond]

2. If the bias power supply should fail, or cold solder joint on the bias wire on the power tube socket, ect ....... The tube would fall back to Class A bias, and not go "red-plate" on you.

-g

Considering the quality of new power tubes, this is a very good reason to employ this bias scheme, possibly the BEST reason.

[R.G.]

There should be nothing "magic" about it. I use it for a few reasons.

Indeed, there should be nothing magic about it, because there isn't. It's simply one way to get a desired voltage between the grid and cathode. It's a bit more complicated than either just (fixed) grid bias or just cathode bias because you have to think about the interactions.

1. I use it in conjunction with my high power / low power switch. 6 watts driven Class A ; 36 watts driven Class A/B.

1. Good for you.

2. If the bias power supply should fail, or cold solder joint on the bias wire on the power tube socket, ect ....... The tube would fall back to Class A bias, and not go "red-plate" on you.

2.a. On the other hand, we haven't yet seen a rash of deadly bias supply failures, cold solder joints, or ects (?? Did you mean "etc."?) killing amps over the last sixty to eighty years. So maybe it's not a *huge* problem, eh?
2.b. And combination bias or cathode bias does make you vulnerable to the failing cathode resistor bypass cap failing and killing tubes too. One could argue that by using both a fixed bias with a filter cap on it and a cathode bias with a cathode bypass cap on it, one has added new ways for the bias to fail. Now the amp doesn't work right if either one fails, so the failure rate goes UP. Whether other parts - like the output tubes, output transformer, power trannie, power rectifier, etc. (!) are damaged depends on other things in the amp's design. Which leads inevitably to
2.c. The tube - or tubes - might well go "red-plate" on you, depending on the power supply voltage and the design of the fixed and cathode biases for the tube operating point in the power supply. Tube "red-plating" is a function of how much heat is generated in the tube's plate, which is determined by the voltage across it and the current through it. Loss of bias lets the tube draw more current. Loss of all bias makes it draw a lot. In a mixed bias amp, the fixed bias section provides some of the back-bias on the grid, and the standing current in the cathodes provides some more. There is then a mix of fixed and cathode resistor bias. That mix can be anywhere between 100% fixed and 0% cathode to 0% fixed and 100% cathode.

Those two cases are merely the extremes of the continuum of biasing. Mixed bias is the more general case, and uses more parts than either of the others, while providing some intermediate mix of responses to bias changes. Note that a fixed bias amp has a melt down if you lose the fixed bias to either a short or an open. A cathode biased amp has a melt down if the cathode capacitor shorts, like it does if you inadvisedly mount it next to the very hot cathode power resistors.

Using both forms of bias, you'd have to do some fairly elaborate design or have a biggish bone pile of test victims to demonstrate that all failures of one or the other parts of the bias could be guaranteed to NOT let the tubes red-plate. It's rather like needing to know how a fuse will blow and not blow to be able to do a good design for selecting a fuse for protection, isn't it?

Just off-hand, I can't think of a good way to design (meaning, think it out first, not just easter-egg in parts in a victim chassis) a mixed bias amp and prove that losing one or the other bias could be guaranteed not to overheat the output tubes. Obviously, if 90% of the back-bias is by the fixed bias source, losing the cathode bias is not likely to be deadly. And if 90% of the bias is by cathode bias, losing the fixed bias source probably won't kill the tubes. But how about 80-20? 70-30? 40-60? 50-50? Which one is more deadly to lose at which mix? Does 50-50 kill the tubes if *either* bias source dies?

The answer to that last one lies in knowing about biasing class. If you have the amp already biased near class A - actually, I guess that is more properly *un*biased into Class A, then the power supply voltage must be low-ish to keep the tube dissipation down, and losing a bias source in a mixed setup is probably not critical. But if you have a bias setup with the tubes running very close to class B, you generally do that to run the power supply voltage up and get more power out of the tubes. That being the case, any substantial increase in standing current at all is likely to make the tubes overheat.

Wow! It's almost like the Devil is in the details, isn't it? And it being hard to generalize about this without handwaving! Who'd have ever thought that technology could be so doggone picky about details?

As a parting idea, doing something once does not guarantee that you can do it again successfully unless you actually know what you're doing. The most common last words of a not-too-bright redneck are "Hey guys! Watch this!".

The second most common words are "That's nothing, watch this..."

[mooreamps]

While that not being the case.. It runs -12 volts class A with only one power tube driven ; and a total of -48 volts class A/B with both power tubes driven...... I could have done the class A and class A/B using only fixed bias. But, something I read about omitting the cathode pass-by cap on the power tube was suppose to give it a little warmer sound. If you saw my print, you would have seen this....

-g

[Chuck H]

Cmon guys... I'm never going to be able to find a culinary link for "Gary's beef with R.G."

On that note... The only culinary related statement from Gary thus far has been "boiled in oil". Not sure I can use that. I don't think even Merlin (the magician) can do much for deep fried beef.

Gary, what do you think of rare blackened beef tenderloin sliced thin and served with a sauce of goat cheese and chanterelle mushrooms?

[mooreamps]

I don't think even Merlin (the magician) can do much for deep fried beef.

That's his problem, not mine......

[EFK]

Personally I would pass on the goat cheese just because, well, yuck. The rest of it sounds great!

[EFK]

Hey Tubeswell if you're still out there - I found an old thread from a few years back dealing with the same topic. Did you ever institute this method of bias in your 5G9? If so, would be curious to hear about your impressions of it and what difference (if any) it may have made.

[jimboyogi]

I have been trialling combination bias, and I posted some CRO screen shots on another forum which may be of interest.

Here's the link - OffsetGuitars.com • View topic - Building the RhoneyTone Senior (or Rhoneytone gets LOUD)

[R.G.]

While that not being the case..

Can you clarify? Did you mean "that's not how my amp works" or "I disagree with your technical analysis, R.G."?

It runs -12 volts class A with only one power tube driven ; and a total of -48 volts class A/B with both power tubes driven......

OK. Do you also modify the B+ when you switch between the single tube and dual tube conditions? What is the static and full signal dissipation of the output tubes in both cases? And what then happens to output tube dissipation when you lose one or the other bias provision in each mode? It's technical considerations like this that make for robust designs. It's part of good design practice to know these things before you start wiring up hardware.

I could have done the class A and class A/B using only fixed bias.

Yes, you could have. Or combination bias. Or cathode only. It's a continuum. You can use any mixture of them.

But, something I read about omitting the cathode pass-by cap on the power tube was suppose to give it a little warmer sound.

And as we know, everything one reads is the absolute truth, written stone. And everything on the internet is absolutely true. Out of curiosity, (a) where did you read that, (b) did this same nota bene happen to say what it meant by "warmer"? Was that warmer meaning treble loss and/or bass boost, or extra even-order distortion? Or did it say? And did you happen to measure what inserting/removing the cathode bypass cap actually did to a test signal by any chance?

If you saw my print, you would have seen this....

Probably so. But you never either post schematics or explain any of your "magic" so it's impossible to say, isn't it? That may be because you're afraid someone will steal your "magic"; but it comes across looking like a young boy who says "my hidden special secret things are better than anything anyone else has, but I'm not showing you because it's a secret!", and who may or may not have any secrets at all.

Gary, I keep thinking that you must have something useful and interesting to say. And I keep hoping that you can be led into saying it by asking you about things that seem to be missing or easily misunderstood in what you do say. Try, Gary, try. I believe in you!

[jrfrond]

OK so what have we gotten from this so far? Let's see:

1) Bias is bias, whether it is fixed or combination (though I HAVE, in the case of the aforementioned '68 chassis Fender's, saw negative harmonic effects on a sine wave, which was also palpable when checked with speakers with a guitar; it was a bit cold and ugly-sounding).

2) There is NO 100% failsafe biasing system. Perhaps the best bet would be UNBYPASSED cathode-biasing, leaving the possibility of the capacitor shorting out of the equation.

3) "Magical" designs are usually nothing more than tweaks, and everyone seems to have their own version.

4) Goat cheese doesn't seem to be very popular on this forum. I happen to love it.

[cminor9]

I liked goat cheese until I went on a field trip with my son to a farm. As it turns out, goat cheese smells just like goats. I don't enjoy goat cheese nearly as much as I used to.