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Thoughts on "self biasing" vs adjustable bias amps


AB3CX MIke

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About 5 months ago I placed an order with Decware for their Zen Tori amp.  I was happy to know at the time that it was "self biasing",  and the company has great reviews. However, 2 weeks ago learning that my unit was  still months from being built, I took the plunge for a used  MC-275 Mark VI available in my area.  Of course it is not self biasing.  I'm very happy with the way it sounds. Reading up on the topic of "self biasing" I learned that the cathode bias resistor used  is just an approximation, and that the tubes gtherefore operate at lower power and the bias resistor generates heat, and that the circuit design is in many ways less effective than one with the bias manually set. Despite that, the Decware amps are quite respected. Wondering if the experts out there have any comments.  Decware does make amps that have manually set biasing as well. 

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It all depends on who you ask.  Cathode bias works very effectively and allows for totally plug and play operation with no tweaking ever needed.  If the tube draws a little more current the bias increases to maintain the status quo.  If tube emission decreases a bit causing less current flow, the bias will decrease accordingly.  It is impossible to maintain rock solid operating parameters with any circuit arrangement as the tubes themselves change over time with use.  As for the cathode resistors generating heat, that should not be a problem in any properly designed amp, and the resistor value is chosen to set a particular operating point.
 

My recommendation to prospective tube amp buyers is to go with something which suits your needs.  If you don’t want to ever be bothered with making adjustments, a cathode biased unit is definitely the way to go.

 

Maynard

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On 4/30/2021 at 3:24 PM, AB3CX MIke said:

2 weeks ago learning that my unit was  still months from being built, I took the plunge for a used  MC-275 Mark VI available in my area.  Of course it is not self biasing.  

 

 

 

    Did McIntosh change something I'm not aware of in the later versions? McIntosh as never produced an amplifier that required manual biasing  that I know of. They do not utilize a cathode resistor so they would be considered a fix bias amp. But the output design is very different then anything on the planet and is self balancing fixed bias to a degree. 

 

       To me any amplifier that does not have the ability to test and adjust the bias is always off by some degree. Even those modern Primaluna jobs with the logic board controlling the bias. Nothing wrong with cathode biasing I think it has a really warm natural tone to it kind of rolls with the punches! With Push pull amps closely matched quad of output tubes are very much advisable.       

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On 4/30/2021 at 3:51 PM, tube fanatic said:

  It is impossible to maintain rock solid operating parameters with any circuit arrangement as the tubes themselves change over time with use.

Tubes drift over time

Self bias addresses this continuously

 

Low tech solutions frequently are better

Microprocessor monitored and controlled bias may be more accurate, but you have introduced a point of failure with much greater likelihood than a resistor; a piece of wire.

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I have no issue adjusting / checking bias.  I use the VHT bias meter when I check my amps bias.   Makes it super easy and safe.   Although I really do appreciate not having to do that with my Quicksilver headphone amp as it it self biasing .   

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On 5/7/2021 at 11:40 AM, Bubo said:

Tubes drift over time

Self bias addresses this continously

 

Low tech solutions frequently are better

Microprocessor monitored and controlled bias may be more accurate, but you have introduced a point of failure with much greater likelihood than a resistor; a piece of wire.

 

 

 Self bias takes care of tube drift? Please explain how it does that continoulsy? 

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58 minutes ago, NOSValves said:

 

 

 Self bias takes care of tube drift? Please explain how it does that continoulsy? 

Cathode Bias

This type of bias circuit is a little different than the one we’ve just been discussing. In order to explain it both clearly and succinctly, I’m going to pull a quote directly from Gerald Weber’s wonderful tome, Tube Amp Talk for the Guitarist and Tech.

“There are two common ways of biasing the output tubes — either fixed-bias or cathode-bias. In a fixed bias amp, a constant negative voltage is injected to the grids of the output tubes while the cathode is grounded to the chassis. In a cathode-biased amp, the grids are grounded through a grid return resistor and a resistor is placed between the cathodes and ground. This creates a slight positive charge on the cathodes. Basically, the tubes can’t tell the difference between having the grid negative with respect to the cathode or the cathode positive with respect to the grids, so either way can be used to bias an amp, however they do sound much different.”

What a great explanation. Of all the ones I researched, this was, by far, the most “guitarist-friendly,” IMHO. Heck, even I could understand it without too much brain ache.

Cathode bias is often referred to as “self-biasing,” as the design ensures that the tubes — wait for it — bias themselves. Thus, no biasing is required when replacing tubes — but using a matched set of replacement output tubes is, once again, highly recommended for obvious tonal reasons.

Amps with cathode-biased power-tube circuits are lower-output ones — 30 watts or less. EL84s are a very common tube in such designs, 6V6s are, too.

https://www.sweetwater.com/insync/what-is-tube-amp-bias/

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1 hour ago, NOSValves said:

 

 

 Self bias takes care of tube drift? Please explain how it does that continoulsy? 

Tube Amp Bias Types:

There are a few different ways that the bias is set on an amplifier and some are more involved than others, though it is hard to say that one is really better than the next. Here are few of the more popular types found in tube amps:

Cathode Bias

Cathode bias amps are the least work intensive, as they do not really require biasing at all in a general sense. Many people actually refer to them as "self-biasing" for this reason. Without getting into too much technical detail/jargon, basically through a series of design characteristics - mainly a resistor - these amps balance themselves against the current draw of a given set of power tubes. For this reason, they will normally function pretty well with a wide range of tubes without needing any adjustment. It is basically "plug and play," however, we would still recommend buying a matched set of tubes for optimum performance. Cathode biasing is very commonly found in lower powered amplifiers and even more so in amps that use EL84's. Unfortunately, the design is a bit difficult to incorporate in higher powered amps, which is one reason for the limited use in tube amps.

Adjustable Fixed Bias

Adjustable fixed bias certainly sounds like an oxymoron, but it is actually the best way to describe this type of biasing. Basically, the bias is pre-set to a fixed point based on the performance of a given set of tubes. Normally a tech will take a series of readings while the amp is idling in order to establish the proper amount of current a tube should be drawing and then via an internal adjustment pot he/she will set the bias based on this information. It is actually not an extremely difficult process, but can be very dangerous if you do not know what you are doing, as it often requires a person to take internal readings while an amp is plugged in and turned on. With this method of biasing it is recommended that the bias be checked after replacing the power tubes, unless the new set of tubes is identical to the set being replaced.

Non-Adjustable Fixed Bias

With this type of biasing, as the name would imply, the bias can't be easily adjusted. It is pre-set to a fixed point by design to work with a pretty wide range of tubes. As with cathode biasing, it is basically plug-and-play, however the results will vary slightly given the different operating characteristics of each set of tubes. For this reason, it may be advantageous to look for tubes that are graded or matched based on different characteristics. Being that it is difficult to optimize the amp for the set of tubes like with the other two methods, it is more important to optimize the tube to the amp in terms of operating characteristics.

https://www.humbuckermusic.com/pages/tube-amp-bias-article

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Tubes which are matched for dc operating conditions only (I.e. for setting bias) can be very dissimilar under ac conditions (i.e. signal present).  The only way to do it correctly is with a curve tracer.  Small signal tubes can be grid leak biased.  This eliminates the need for a cathode resistor/bypass cap and truly ensures self adjusting operation for the life of the tube.  I use it almost exclusively.

 

http://aeaaudio.com/matching-tubes-with-a-curve-tracer/

 

You can also look up work by Daniel Schoo on using curve tracers.

 

Maynard
 

 

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On 5/12/2021 at 2:30 AM, Bubo said:

Basically, the tubes can’t tell the difference between having the grid negative with respect to the cathode or the cathode positive with respect to the grids, so either way can be used to bias an amp, however they do sound much different.”

What do you mean with „the sound is different“? Is it meant in principle, or in the sense that a self-biasing amp looks for the best operating point, and the manually adjustable amplifier can be fine-tuned a bit? It's sometimes done that way with guitar amplifiers. A somewhat "hot" idle current gives the sound more warmth and fullness, but the tubes have a shorter life. Conversely, a "cold" setting makes the amplifier sound (deliberately) harder and dirtier with longer tube life. These are all factors that should not play a role in hi-fi.

The circuit of my MC275 MK4 is self biasing. MC traditionally has a relatively low anode voltage, I mean around 450V. And the circuit together with the special output transformers is unique.
I would be interested to know why the new MC275 has a manual bias.

 

On 5/3/2021 at 6:51 PM, NOSValves said:

McIntosh has never produced an amplifier that required manual biasing  that I know of.

I have a MC2102 from 2001 and it needs to be adjusted. Maybe because 4 KT88 per channel is a bit much for self biasing? But there is only one adjustment screw per channel so in any case a matched octet makes sense.. BTW it is a very unfortunate procedure. You have to unscrew the amp bottom and put it on the back side where the speaker terminals are on a soft pad. Inexperienced people have very quickly caused a short circuit if you get to the wrong pin of the V4 with the voltmeter, it unfortunately happened to me 16 years ago although I knew what I did but I slipped and a diode was fried.
It weighs a murderous amount of 88 pounds. That's why it's an ordeal for customers who don't know anything about electronics if you have to send the amp in for this bias adjustment. Even worse I estimate that almost no one will be in the know of this measure. It's only described in the service manual but not in the owners manual.
On the other hand, Mcintosh can't say it's a lifetime adjustment, because it has to be done every time you change the tubes in the MC2102.
By the way, I suspect this missing readjustment is one reason…   (among others perhaps, like the power transformer is not bigger than that of a MC275, or because parallel switched KT88 do not sound as good as only one per phase…yes more output current but less subtile?) …why its sound is sometimes rated less good in forums than the MC275.
Ok, the sound signature is slightly different in comparison to a MC275 but many people might hear it after all these years and probably some tube changes without having been adjusted in bias. And it top form it is a very good sounding amp.

 

The pic is out of the MC2102 service manual

96207945-5500-4862-A971-F7A3F2DF0DB7.png

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KT,

I find that some of the best applied audio work is done by musicians and  instrument repair technicians, many musicians themselves.

 

They are not afraid to experiment, and push the limits of designs, in  pursuit of the sound they are looking for. Possibly, biasing can and does have an audible impact on an electric guitar etc....

 

As to design, there are design fads like women's fashion.  If it sells, someone will make it.

 

Ex. Class D is a solution to a density and heat dissipation problem in surround amps, that has worked its way down to 2 channel, lots of watts for the money.

 

Mc making their adjustment customer unfriendly was probably done to discourage DIY, in the old days Mc had factory days when people could bring the gear to the local store to have a factory tech look at it....

 

Tubes are consumed and they do drift, so how best to manage it ?

 

El Paso Tube amps does a lot of good repair and eval videos, was on youtube may still be there.

 

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I find it ironic that people are drawn to tubes and turntables for the hobby / tinkering factor, yet the first thing that many wants to do is get things like self-biasing amps so they don't have to actually tinker with anything.  Just get a CD player and a solid state amp.  :)  

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8 hours ago, Paducah Home Theater said:

I find it ironic that people are drawn to tubes and turntables for the hobby / tinkering factor, yet the first thing that many wants to do is get things like self-biasing amps so they don't have to actually tinker with anything.  Just get a CD player and a solid state amp.  :)  

This topic is fatiguing...

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On 5/13/2021 at 7:21 AM, KT88 said:

What do you mean with „the sound is different“? Is it meant in principle, or in the sense that a self-biasing amp looks for the best operating point, and the manually adjustable amplifier can be fine-tuned a bit? It's sometimes done that way with guitar amplifiers. A somewhat "hot" idle current gives the sound more warmth and fullness, but the tubes have a shorter life. Conversely, a "cold" setting makes the amplifier sound (deliberately) harder and dirtier with longer tube life. These are all factors that should not play a role in hi-fi.

The circuit of my MC275 MK4 is self biasing. MC traditionally has a relatively low anode voltage, I mean around 450V. And the circuit together with the special output transformers is unique.
I would be interested to know why the new MC275 has a manual bias.

 

I have a MC2102 from 2001 and it needs to be adjusted. Maybe because 4 KT88 per channel is a bit much for self biasing? But there is only one adjustment screw per channel so in any case a matched octet makes sense.. BTW it is a very unfortunate procedure. You have to unscrew the amp bottom and put it on the back side where the speaker terminals are on a soft pad. Inexperienced people have very quickly caused a short circuit if you get to the wrong pin of the V4 with the voltmeter, it unfortunately happened to me 16 years ago although I knew what I did but I slipped and a diode was fried.
It weighs a murderous amount of 88 pounds. That's why it's an ordeal for customers who don't know anything about electronics if you have to send the amp in for this bias adjustment. Even worse I estimate that almost no one will be in the know of this measure. It's only described in the service manual but not in the owners manual.
On the other hand, Mcintosh can't say it's a lifetime adjustment, because it has to be done every time you change the tubes in the MC2102.
By the way, I suspect this missing readjustment is one reason…   (among others perhaps, like the power transformer is not bigger than that of a MC275, or because parallel switched KT88 do not sound as good as only one per phase…yes more output current but less subtile?) …why its sound is sometimes rated less good in forums than the MC275.
Ok, the sound signature is slightly different in comparison to a MC275 but many people might hear it after all these years and probably some tube changes without having been adjusted in bias. And it top form it is a very good sounding amp.

 

The pic is out of the MC2102 service manual

96207945-5500-4862-A971-F7A3F2DF0DB7.png

 

 

 

Yeah someone around here was nice enough to send me a manual for a MC2000 these later year 4 or more tube per channel McIntosh amps have bias controls and test points. The vintage models I'm used to working on could have been modified to do the same but that would be a sin to any true McIntosh lover ;) With these newer units only being around 20 years old I serious doubt many have needed repair McIntosh amps are really reliable even the vintage ones will keep working and working long after they should be for there own good! 

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On 5/14/2021 at 12:43 AM, Paducah Home Theater said:

I find it ironic that people are drawn to tubes and turntables for the hobby / tinkering factor, yet the first thing that many wants to do is get things like self-biasing amps so they don't have to actually tinker with anything.  Just get a CD player and a solid state amp.  :)  

 

I bought tubes because they sound way better than SS with my speakers.  I was never drawn to the tinkering factor, just the sound.  Same reason I bought the speakers in the first place.  

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