Understanding the cathode Biaset resistor and its removal

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Understanding the cathode Biaset resistor and its removal

Postby 20to20 » Wed Jun 23, 2010 9:36 am

I fully understand the purpose of the 15.6R but I have a question about the "proper" method of connecting the cathodes between output tubes. If the cathode resistor was removed, I assume the cathodes would need to be grounded. The second option would be a direct connection between them. Or would that create an overcurrent or a no current situation? In either case, what would be the effect on the biasing if the bias was set to 1.56v before removal of the R and the correct connection option? After the change, I'd guess the biasing would have to be set by reading pin 5 and setting to (-)32v.

TIA!
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Postby EWBrown » Wed Jun 23, 2010 10:03 am

An easier approach would be to remove the 15.6 ohm resistors and replace them with 10 ohm (or even 1 ohm) 1% and then adjust the bias for a reading of 1.0V or 100 mV, depending on which resistor you use.

The resistance is low enough to have no effect on the tubes' operation, in either situation.

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Postby 20to20 » Wed Jun 23, 2010 10:15 am

Ed, Thanks. I had read other posts about modding thr R value but hadn't found anything about simply removing it and what needed to be done after that. This question is more to the issue of the correct reconnection, to better understand the theory for future projects. I'm getting deeper and deeper by the day into this hobby and trying to figure it all out is coming slower than my new piles of parts! ...trannys do multiply like a box full of white mice.
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Postby mesherm » Wed Jun 23, 2010 10:36 am

There is really no reason to remove the cathode bias set resistor other than to change to a more convenient value or better quality. One of the first mods commonly done to an ST70 is to change the bias circuit so each tube can be individually biased. This would entail as a minimum, removing the 15.6 ohm common cathode resistor and installing a 1 or 10 ohm 1% 1/2 watt metal film resistor from each power tube cathode to ground. You also then need to run a wire from each cathode to a convenient front front socket pin for easy DMM access. Now you can measure what each tube current is but can only adjust each pair. You can swap tubes around and try for a better match or add additional components to allow balancing the bias between each pair of tubes.
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Postby 20to20 » Wed Jun 23, 2010 11:06 am

Mike, Thanks. It was probably a few of your posts, helping others with Kr mods, I'd read. I guess the root of my search is the question of why some outputs are K biased and some are G biased. I've seen circuits with K biasing that have Kr's and filtering and I've seen them G biased with Kr's and filtering. And I've seen them G biased with K grounded and yadda, yadda, yadda. Anyway, I think the answer to my question is to ground the Ks if the R is removed and set bias via pin 5 voltage. I had wondered how much of a change in operating current would result from grounding the K and I think Ed answered that, an d from that I realised the lower K resistance was a very small fraction of the total R through the tube, considering the high plate R.

Thanks!
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Postby mesherm » Wed Jun 23, 2010 12:09 pm

Cathode vs fixed bias is matter of personal preference and design goals. Sort of like choosing a car with manual or automatic transmission. Both methods have different strengths/weaknesses. I have built SE amps with DHTs with fixed bias and PP amps with cathode bias and vice versa and either method done right works fine.
With PP amps I prefer to have the power tube pairs running as close as possible to equal currents so that the net magnetic field in the OTs is zero. That is always my primary goal in deciding a PP bias scheme.
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Postby burnedfingers » Sun Jun 27, 2010 6:41 am

There are some that would argue that having a resistor (10 ohm per tube) will and does affect the sound quality of the amplifier. The 10 ohm resistor makes it easy for one to set and or check the bias current for each tube. Some amplifiers have a jack in which to insert a mA meter that allows one to set and or check the bias current.

One amplifier that does without the individual resistor or any resistor in the cathode leg is a Quicksilver mono block 8417 amplifier or the KT88 version. I have had a number of both of these amplifiers and have heard no difference by not having a bias/cathode resistor to ground. If one were religious in their belief that the resistor or resistor value makes a difference then one could alter the value to say a 1 ohm resistor and scale the reading down on their meter.
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Postby 20to20 » Sun Jun 27, 2010 7:17 am

There are some that would argue that having a resistor (10 ohm per tube) will and does affect the sound quality of the amplifier.


I guess an interesting experiment would be to put a low value pot. in there and swing it through the first 100 ohms and listen. See if 50R = 5v. when the pin 5 voltage is set to the value seen with 1.56v.
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Postby dcgillespie » Sun Jun 27, 2010 7:45 pm

The generally well known understanding of the common "Biaset" cathode resistor that Hafler patented in his amplifiers was that the voltage across it (when properly set) equaled that of a fresh carbon-zink D battery. Therefore, the accuracy of the typical meter of the day fell out of the equation, as you simply set the amplifier to read at the same point as the battery would read.

What is not as well known -- and what his patent primarily refers to -- is that the original value (11.2 ohms as determined for the MK III), also produces a noticeable drop in IM distortion at full power output. Using more, or less resistance causes IM to rise. The effect is not insignificant, as use of the proper resistance nearly cuts IM distortion in the MK III in half to a typical value of .5% at 60 watts output, versus that of grounded cathodes, or a higher resistance.

I have verified this information for myself, and Hafler documented the original information regarding it through an article he wrote associated with the introduction of the MK III. Each will have to determine for themselves how important this is to their own application, but it does at least provide an understanding as to the origin of the original value.

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Postby 20to20 » Sun Jun 27, 2010 10:13 pm

Dave,

Thanks Much! You hit the point of my question square on. I'd like to find that article. Any link to it? I'm getting closer to seeing the light on biasing and how voltages are developed on each tube element. When I understand how a negative voltage can appear seemingly out of nowhere in some circuits as opposed to being supplied like the bias voltage in the st-70, I'll be closer to feeling like I can start creating some output circuits. Thanks again!
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Postby dcgillespie » Mon Jun 28, 2010 12:59 am

I have not searched for a link to the article I referred to, but it is entitled "New Amplifier with KT88's" by David Hafler, and appeared in the January, 1958 edition of Radio-Electronics magazine on pages 58 & 59. He provides a graph in the article showing how the chosen value (11.2 ohms) reduces IM distortion from .5% to .2% @ 27.5 watts output, and from .8% to .5% at 55 watts output -- over that of simply grounding the cathodes, or using values up to double that of which was chosen.

As to your other question and where negative voltages can come from; regarding output stages and specifically audio work, if they are negative, they must be supplied. This of course is all as referenced to ground. Under quiescent conditions, as far as the tube is concerned, all it needs is for it's control grid to be more negative than it's cathode. Now whether this is created by referencing the grid to ground and then allowing the cathode to become more positive by way of a voltage drop across a cathode resistor, or by grounding the cathode and then supplying the grid with a negative voltage, it makes no difference. It could even be a combination of the two. In either case, the grid is made negative with respect to the cathode so as to control current flow through the tube.

When a cathode resistor is used, it is called cathode bias. When a negative supply is used, it is called fixed bias. Both approaches have drawbacks and benefits which go far beyond mere biasing of a tube under quiescent conditions. Understanding which is best used and when starts with the overall design goals for a given amplifier. High power and low distortion favor fixed bias, while economy and low maintenance favor cathode bias.

As for the ST-70, it is a fixed bias amplifier. Yes, there is a small amount of bias created by way of the Biaset resistor, but the vast majority is provided by the negative bias supply. And yes, you can adjust the negative bias voltage to the tubes, but it is still considered "fixed" as once set, that is the bias the tube operates with. With cathode bias, it is typically not adjustable in classic designs, and is not considered as fixed since the bias voltage will depend on the current flow through the tube at any given moment. That of course varies with signal conditions, and so is not considered as fixed in the same manner that use of an adjustable negative bias voltage is. I hope this helps.

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Postby 20to20 » Mon Jun 28, 2010 7:14 am

Dave,

You have just summed an entire chapter, clearly, in 3 paragraphs! I had discovered this seemingly backward naming scheme for biasing over the last few days. So now when I hear someone refer to a tube as "self biasing" I assume they are refering to a cathode biasing scheme and not some magic designed into the tube itself as a unique design.

When I said that I was mystified by how negative voltage can appear out of nowhere, I was refering to a circuit I found in an 1954 RCA TV vertical output. Here's a link to the graphic I have in my pix album.

http://diytube.com/phpBB2/album_pic.php?pic_id=93

You can see the hard grounded cathode but the grids are all decoupled from (+) 270v. So under this situation I assume the (-)25v. and (-)48v. is a result of the internal voltage drops of the tube or the nature of the grid resistor circuit. Why it is (-) isn't obvious (to me) just looking at the diagram.

I'm interested in those 6K6 tubes for an audio amp, too. I have 2 of them from the TV I bought just for the transformer. One of them was used for the audio output in this set.

Thanks Much!!
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Postby dcgillespie » Mon Jun 28, 2010 7:58 am

The term "self bias" invariably refers to the traditional term cathode bias, because the bias is created by current flow through (and therefore the voltage drop across) a cathode bias resistor, whose current is also flowing through the tube "itself". Hence the term. The specific tube does in fact play a role in the bias created with this method, as for example -- with all else equal -- a 6550 will develop a much greater bias voltage across a given resistor, than say an EL84 will across the same resistor.

In the example you cited for the negative bias created in the deflection circuit of your TV schematic, two points need to be made:

1. This example is precisely why I carefully said "regarding output stages and specifically audio work" in my last post, as there are examples -- typically in TV circuits -- where the bias can seem to come from nowhere.

2. In the deflection circuit of the TV schematic you provide, the bias is actually produced by the deflection signal itself, in combination with the coupling caps used at the two grid points you referenced. In the set, this signal is generated by the vertical deflection oscillator, and amplified by the stages shown. The simple explanation is that with no traditional bias provided, the signal applied drives the tubes into saturation, which causes a negative voltage to build up on the grid side of the coupling caps. This voltage is the negative bias value indicated.

This event can happen in audio amplifiers as well -- although you surely don't want it to. Most often, it can occur in output stages where the previous stage is RC coupled into the output stage, and the signal is of such magnitude that the output tubes are driven into saturation, and events similar to those above then off-bias the tubes and create a particularly nasty form of distortion. Most often, this is referred to as "blocking" distortion.

In the TV set, the oscillator is continuously running, and as long as it is, the output tube is protected by the bias created. If the oscillator is disabled, then the output tube can be destroyed just like in audio work when a bias supply fails, since no bias would then be present.

In audio amplifiers, blocking can be avoided by either (1) having enough power available such that the output tube grids are never driven positive, or (2) employing cathode follower driver stages that are direct coupled into the output stage. Without a coupling cap, the blocking event cannot be created.

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Postby 20to20 » Mon Jun 28, 2010 8:47 am

Dave,

The specific tube does in fact play a role in the bias created with this method, as for example -- with all else equal -- a 6550 will develop a much greater bias voltage across a given resistor, than say an EL84 will across the same resistor.


Sure. I'd guess do to the Ra being twice as high in the EL84?

The simple explanation is that with no traditional bias provided, the signal applied drives the tubes into saturation, which causes a negative voltage to build up on the grid side of the coupling caps.


That was my third guess, but tube saturation wasn't in my universe yet, and I couldn't see how the coupling caps could provide any amount of "charging" from the deflection signal to create much of a (-) voltage, as the ST-70 bias circuit does. Tube saturation is the key there.

I'm working on building a set of '50's R-E mags. Jan. '58 will have to go to the top of my list now.

Once again, fantastic post.! Thanks Much!
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