Simple question,
I have never tried this I guess someone has..
Is it possible to fix bias an output tube close to bias point and then autobias it to the correct value? So you have fixed bias not adjustable and its auto biased into correct operation?
I guess this dosen't work or I would have seen it done?
Regards
M. Gregg
I have never tried this I guess someone has..
Is it possible to fix bias an output tube close to bias point and then autobias it to the correct value? So you have fixed bias not adjustable and its auto biased into correct operation?
I guess this dosen't work or I would have seen it done?
Regards
M. Gregg
The result would be to effectively and significantly reduce the value of the cathode bias resistor over that traditionally used for pure cathode bias. From a pure performance standpoint, fixed bias usually delivers the best overall performance, although cathode bias can deliver very good performance as well when designed correctly. However, if any deviation from pure fixed bias represents a move from optimum, then the combination of the two bias schemes would have to represent something less than optimum conditions.
One way to easily try your idea would be to combine my Enhanced Fixed Bias (EFB)(tm) design with the addition of an appropriately small cathode bias resistor between the output of the EFB regulator and the cathodes. In this scenario, the EFB regulator would be set for (possibly) a couple of volts less bias than required, and then the small cathode resistor could make up the last few volts.
In the end, I'm afraid that to have enough flexibility with such a design so as to allow the cathode bias portion let each tube properly self adjust to an optimum bias condition, the cathode bias resistor would have to be so large as to largely negate the qualities that the fixed bias portion was attempting to provide. All you can do is try it and see. If you do, let us know how it works out!
Dave
One way to easily try your idea would be to combine my Enhanced Fixed Bias (EFB)(tm) design with the addition of an appropriately small cathode bias resistor between the output of the EFB regulator and the cathodes. In this scenario, the EFB regulator would be set for (possibly) a couple of volts less bias than required, and then the small cathode resistor could make up the last few volts.
In the end, I'm afraid that to have enough flexibility with such a design so as to allow the cathode bias portion let each tube properly self adjust to an optimum bias condition, the cathode bias resistor would have to be so large as to largely negate the qualities that the fixed bias portion was attempting to provide. All you can do is try it and see. If you do, let us know how it works out!
Dave
I don't want to use a servo....I am wondering about Grid bias and standard auto bias combined..
Regards
M. Gregg
If I can get a reduction of resistor to quarter of the value or even half the value it will be useful..
Regards
M. Gregg
This is sometimes known as mixed bias. I have used it for an EL34 UL PP output. My 'fixed' bias was variable. If it is really fixed then you can't take it too far as it won't adjust for valve variations and ageing. Halving the cathode resistor value is about as far as you can go, unless you can test and select the valves and hope they don't age too quickly.
looks interesting, I guess it could be one dual pot to just shift the Fixed value..(If needed)
Just reducing the resistor would reduce the discharge time of the capacitor after transient..worth a try...
Thank's for your thoughts...
Regards
M. Gregg
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I use fixed bias when I want to get the all the available power from the output tubes, and no cathode resistors at all; however one needs to adjust the bias time to time to mantain the correct anode current (or use servos).
Cathode bias (auto bias) is nice as it don't ask for periodic bias adjustments, and it overloads nicely (sound wise); however output power will be smaller, and large cathode resistor bypass capacitors are required to mantain good low frequency response as well.
Because these caps are in the signal path and large values are required (the lower the cathode resistor, the larger value the caps should have to maintain low frequency response), normally electrolytic caps are used, and I really hate to use these in the signal path.
So in the end, I tend to use fixed bias.
Cathode bias (auto bias) is nice as it don't ask for periodic bias adjustments, and it overloads nicely (sound wise); however output power will be smaller, and large cathode resistor bypass capacitors are required to mantain good low frequency response as well.
Because these caps are in the signal path and large values are required (the lower the cathode resistor, the larger value the caps should have to maintain low frequency response), normally electrolytic caps are used, and I really hate to use these in the signal path.
So in the end, I tend to use fixed bias.
Thank,s Printer2.
You know I have looked at some of these schematics in the passed and never noticed the two used togeather...strange
Regards
M. Gregg
Thank,s Printer2.
You know I have looked at some of these schematics in the passed and never noticed the two used togeather...strange
Regards
M. Gregg
WAD KEL34, 88 and 80 series used such a method to bias the output tubes.
LOL so it is quite common when you look for it..
Thanks so much for the information...It would be interesting to read the reason they did this...
Regards
M. Gregg
LOL so it is quite common when you look for it..
Thanks so much for the information...It would be interesting to read the reason they did this...
Regards
M. Gregg
Local feedback & increased reliability of the product are the first things that come to my mind..
But it would be nice to find out what their reason was...
The World Audio Design is interesting I wish I could find the old magazine of the design of this circuit...I had it in the past..
I have found K5881 and their 300B + the pre amp not the KEL 34...the search is on..LOL
Regards
M. Gregg
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Going to rip them out of the amp I have at some day. Fender took their amps in a clean direction for a time, not necessarily what most want in a guitar amp.
I remember playing around with a Fender Bassman amp head I got for cheap. It was kind of a rough one, silver face.
Stock, with fixed bias, I drove it into overload and observed the waveform on a scope. You could plainly see the sharply squared off shoulders of the waveform as the amp went into clipping. It tried to make the incoming sine wave into a sort of square wave.
I then took away the negative bias supply and biased the 6L6's with a shared, bypassed cathode resistor. Drove the amp into overload and noticed that now the waveform no longer flat-topped, but would round off at the shoulders as the amp went into clipping. Almost like a compressor, less like a fuzzbox.
I returned the fixed bias and put a shared cathode resistor of smaller value from the 6L6 cathodes to ground. (I wish I could remember the exact value of the resistor.) The amp's clipping behavior was now intermediate between pure fixed bias and pure cathode bias. There was more of a flat top to the the clipped sine wave as in fixed bias, but the shoulders were rounded like they were in cathode bias.
So with "mixed bias" even the clipping behavior was intermediate between pure fixed bias and pure cathode bias.
Mixed bias' power output was lower than with fixed bias, but a bit higher than cathode bias, as expected.
Was a fun experiment. I eventually blew up that Bassman and sold it for parts.
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Stock, with fixed bias, I drove it into overload and observed the waveform on a scope. You could plainly see the sharply squared off shoulders of the waveform as the amp went into clipping. It tried to make the incoming sine wave into a sort of square wave.
I then took away the negative bias supply and biased the 6L6's with a shared, bypassed cathode resistor. Drove the amp into overload and noticed that now the waveform no longer flat-topped, but would round off at the shoulders as the amp went into clipping. Almost like a compressor, less like a fuzzbox.
I returned the fixed bias and put a shared cathode resistor of smaller value from the 6L6 cathodes to ground. (I wish I could remember the exact value of the resistor.) The amp's clipping behavior was now intermediate between pure fixed bias and pure cathode bias. There was more of a flat top to the the clipped sine wave as in fixed bias, but the shoulders were rounded like they were in cathode bias.
So with "mixed bias" even the clipping behavior was intermediate between pure fixed bias and pure cathode bias.
Mixed bias' power output was lower than with fixed bias, but a bit higher than cathode bias, as expected.
Was a fun experiment. I eventually blew up that Bassman and sold it for parts.
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