[planet10]

Peter Mcalister (of Mcalister Audio) told me he believes self-biasing tube amplifiers sound worse than those with fixed bias.

And likely for everyone like him, there is one that believes cathode bias sounds better/

And to the con that tube amps tend to have low damping factor (they don't have to be tube amps either take the FirstWatt F1 & F2 for example), that is an advantage with the right speakers.

dave

[cujobob]

From the Mcalister site...

"Fixed Bias and Self Automatic biasing Explained;
Cathode/ automatic/ self biasing amplifiers use a large resistor impedance between the cathodes of the output tubes to ground. This raises the cathodes above the ground potential in effect creating the required negative grid number one voltage on the output tubes that determines the current flow and class of operation. Amplifiers with this type of bias do not require any adjustments when output tubes are replaced and are less expensive to the manufacture as no additional negative power supply or controls are required- just a cathode resistor and by-pass capacitor. With cathode biased amplifiers the speed and rise times are much slower due to the large value cathode resistor that is in series with the audio output. Further a large value electrolytic cap in parallel with this cathode resistor has to be used for a shunt AC path ground return and impedance reduction. Electrolytic capacitors in the signal path further degrade the sound quality. Fixed negative bias supplies connect the output tube cathode to ground and have an additional negative voltage power supply. The negative voltage is adjustable and fed to the control grid of the output tube that determines the current flow and class of operation."


Not saying this is 100% accurate, just sharing an opinion.

[tanchiro58]

Edcor makes inexpensive iron up to 100w. Hammond makes some decent iron.  James, Electraprint, O'netics (Level I maybe II), Silk and some of Lundahl & Magnequest are all in the medium price range.  Tamura, Hashimoto, Lundahl amorphous, Magnequest's higher lines, O'netics (level II/III) are alll getting quite pricey.

Do not forget the Nippon Core make a better iron too. It is also depending on the winding method of the best quality of wire (copper vs silver). For example, I can see some of the small japanese manufacturing products of the R-core output transformers which is costly but pleasant to listening.

[Niteshade]

There is nothing wrong with cathode or DC biasing. How it sounds/works is highly dependent upon how the tubes are matched to the output transformers and the specifications of the power supply. I have found that the best sound from a tube amp is largely dependent on operating the power tubes near their center values for loading and plate voltage. Cathode biasing can work excellently, but the rest of the circuit has to be made to accommodate it. Likewise with DC biasing. Sounds like a, "Well...duh! That's obvious!" kind of answer. That's not true. There are many different ways of doing things. That's why all tube amp brands sound differently.

Note: The best way to make a tube amp sound slow, lifeless and sterile is to add ample amounts of negative feedback. No matter what else is done with the circuit- spiffy coupling caps, ultra-expensive iron, etc.. NFB in generous quantities will rob the system of all positive attributes.

[ST-2A3]

Class A to me is a pure waste.

Not to me. I live the sound of my 2A3 amp driving my pair of LaScala speakers.

Class AB is extremely efficient and is able to sound as good as class A.

The speakers mentioned above, are very efficient (105db/1W/1m) and do not require a Class AB PP amp. As to AB sounding as good as Class A, that is very arguable, and I do not think we will come to any conclusion this year. 

When tubes are transistors are biased for efficiency correctly, there is very little heat output.

A lot of the heat of the tube is the filament heater. It is putting out its maximum amount of heat all the time.

Lower heat also means longer component life. Tubes can be made to auto bias as well. Prima Luna has a neat bias system and there are other methods as well. 

Well with a Class A SET amp, bias adjustments are not needed.

[Freo-1]

I've owned a lot of tube and solid state amplifiers over a long time span, and while both can sound excellent, it's harder (a lot harder) to get the solid state unit to sound as good as tubes.  The tube topologies are easier to implement, require less devices to swing large voltage rails, and are more forgiving overall.

I've posted this before, but may be wort re-visiting:

http://www.spectrum.ieee.org/archive/1640

For most real world budgets, I am convinced that the best amplifier performance is obtained by the DIY approach.  In my case, I took a old Harmon Kardon Citation II, field stripped the unit, and installed a 5687 triode front end, a more stable bias circuit, better power supply, and the results are amazing.  I'm able to use NOS Tung Sol tubes with the individual bias pots, and I would put the performance of this unit against any other at 60 WPC.  The HK has some of the best iron ever made.

I was so impressed with this, that I had a line stage preamp made, using a Freed power transformer, a period choke, and all high quality parts used throughout, with a 6DJ8/5687 front end.  It's quick, clean dynamic, quiet, and sounds better than the majority of the line stage units out there that are manufactured.

So, if one really wants to get great sound without a second mortgage,  suggest you bone up on circuits, learn how to solder, and cruise yard sales/flea markets, etc. One of the biggest costs is casings, along with the iron. You can spend a lot on caps, but you do not need to, as good caps are out there that will not break the bank.

Of course, a friend who can help you with this is most helpful as well.   

[ST-2A3]

From the Mcalister site...

"Fixed Bias and Self Automatic biasing Explained;
Cathode/ automatic/ self biasing amplifiers use a large resistor impedance between the cathodes of the output tubes to ground. This raises the cathodes above the ground potential in effect creating the required negative grid number one voltage on the output tubes that determines the current flow and class of operation. Amplifiers with this type of bias do not require any adjustments when output tubes are replaced and are less expensive to the manufacture as no additional negative power supply or controls are required- just a cathode resistor and by-pass capacitor. With cathode biased amplifiers the speed and rise times are much slower due to the large value cathode resistor that is in series with the audio output. Further a large value electrolytic cap in parallel with this cathode resistor has to be used for a shunt AC path ground return and impedance reduction. Electrolytic capacitors in the signal path further degrade the sound quality. Fixed negative bias supplies connect the output tube cathode to ground and have an additional negative voltage power supply. The negative voltage is adjustable and fed to the control grid of the output tube that determines the current flow and class of operation."


Not saying this is 100% accurate, just sharing an opinion.

An "Ultrapath" configuration will take the cathode resistor out of the output of the amp. This also takes the electrolytic bypass cap out of the output as well...

[Ericus Rex]

I love tube amps. I hate biasing tube amps. 

A well designed tube amp using well tested and matched tubes should give you no biasing woes.  I look at my bias only every year or so and so far it hasn't drifted much.

[Niteshade]

The Ultrapath design is really nice! That's based on a great design philosophy. Class AB can sound just as good as Class A (to my ears), especially if its using a circuit designed without loop feedback. I would caution against stereotyping push-pull amplifiers. There are many,many implementations of that kind of circuit and the outcomes are all different. It has the most potential because there's no power limiting factor since multiple output tubes can be used.

Agreed- With 105db speakers, a single ended amp amp is all that's necessary.

BTW: Did you know there's a push-pull amplifier design that doesn't use a phase splitter? It works by allowing the power tubes to equalize each other through a cathode resistor. The non-driven power tube is referenced to ground and is cathode driven. http://www.triodeel.com/compact.html

My findings: When the theory behind this circuit is implemented, the results are stunning!

Biasing: What about a combination DC/Cathode biasing?

[Bill Baker]

Good quality output transformers, especially higher powered ones are quite expensive.  If you in into cheap chinese iron (not all bad mind you) then made it can be done more competitively.  

 Good iron is the heart of any tube product. There is so much more to transformers than the 'average' person recognizes. Mains transformers can make or break a tube product in terms of noise floor. A good mains transformer designed for the specific application with proper shielding will provide a much quieter presentation than most off the shelf pieces.

 When prototyping and designing our Purity preamp, I tried a few off the shelf transformers. Most performed at a decent level but none reached the level of performance we required. There are some very good off the shelf units out there but you can not always obtain the parameters you require for a particular application. Having transformers customer designed for the unit was the only way we could obtain our design goals. The headroom increased and the noise floor dropped considerably.

 Output transformers-------- Yes, good output devices can be very expensive. When you are looking at manufacturers like Hammond and the likes, they make decent products but none I have seen have the bandwidth required to produce a full range output. Most standard output transformers are rated at only 30Hz. This is fine if your system and speakers are not capable of full range but why purchase a true full range speaker if your amplifier cannot deliver You can often judge the quality of a good output transformer by it's weight compared to it's rated power output. If you took an average off the shelf rated at 100 watts output, let's use a hammond for example, and a high quality unit designed for full output, you will see a difference in weight alone of several pounds.

 We have been through this as well with the R&D on our new mono blocs. We had to go back to having custom transformers design to meet our goals.

 So yes, good tube amps can be expensive. Sometimes it is truly a matter of "you get what you pay for".

 I won't get into solid state because I honestly do not know enough about the design to comment. I do know that quality solid state watts are much cheaper than equivalent tube watts but some for some of us there simply is no substitution for tubes.

[Browntrout]

Hello all.
           I think valves should really only be used as they were originally intended to be used. Class A, single ended and transformer coupled with no feedback.
           As for the pros and cons for tube amplifiers.......pros..... they make real music.......cons.......eerrrr...... I should be listening to my stereo right now...does that count as a con?
  

[Freo-1]

Good quality output transformers, especially higher powered ones are quite expensive.  If you in into cheap chinese iron (not all bad mind you) then made it can be done more competitively. 

 Good iron is the heart of any tube product. There is so much more to transformers than the 'average' person recognizes. Mains transformers can make or break a tube product in terms of noise floor. A good mains transformer designed for the specific application with proper shielding will provide a much quieter presentation than most off the shelf pieces.

 When prototyping and designing our Purity preamp, I tried a few off the shelf transformers. Most performed at a decent level but none reached the level of performance we required. There are some very good off the shelf units out there but you can not always obtain the parameters you require for a particular application. Having transformers customer designed for the unit was the only way we could obtain our design goals. The headroom increased and the noise floor dropped considerably.

 Output transformers-------- Yes, good output devices can be very expensive. When you are looking at manufacturers like Hammond and the likes, they make decent products but none I have seen have the bandwidth required to produce a full range output. Most standard output transformers are rated at only 30Hz. This is fine if your system and speakers are not capable of full range but why purchase a true full range speaker if your amplifier cannot deliver You can often judge the quality of a good output transformer by it's weight compared to it's rated power output. If you took an average off the shelf rated at 100 watts output, let's use a hammond for example, and a high quality unit designed for full output, you will see a difference in weight alone of several pounds.

 We have been through this as well with the R&D on our new mono blocs. We had to go back to having custom transformers design to meet our goals.

 So yes, good tube amps can be expensive. Sometimes it is truly a matter of "you get what you pay for".

 I won't get into solid state because I honestly do not know enough about the design to comment. I do know that quality solid state watts are much cheaper than equivalent tube watts but some for some of us there simply is no substitution for tubes.

This is EXACTLY why I went after a H/K Citation II, and used that as the baseline for the power amp. The output iron in that unit is as good as it gets.

[ST-2A3]

The Ultrapath design is really nice! That's based on a great design philosophy. Class AB can sound just as good as Class A (to my ears), especially if its using a circuit designed without loop feedback. I would caution against stereotyping push-pull amplifiers. There are many,many implementations of that kind of circuit and the outcomes are all different. It has the most potential because there's no power limiting factor since multiple output tubes can be used.

Oh I am not dissing PP. I have a Dynakit ST-70/PAS combo in our den, driving a pair of B&O speakers. It sounds good, but after getting the LaScalas at such a great price, I just had to build a 2A3 SET amp and I very pleased with the combination.

BTW: Did you know there's a push-pull amplifier design that doesn't use a phase splitter? It works by allowing the power tubes to equalize each other through a cathode resistor. The non-driven power tube is referenced to ground and is cathode driven. http://www.triodeel.com/compact.html

Now that looks very interesting  and I have the iron from a Fisher X-100 amp, that uses 7189 tubes. Now I have a circuit to build with them! Thank you for that link.

[Niteshade]

Building new amps from old ones is a FANTASTIC hobby! Saves wheelbarrow loads of money. Just be careful about how the secondary winding speaker impedances are selected. The H/K, Fisher, Scott, etc... do not all have a straight forward setup due to phase inversion switches, etc.. Use a DMM on the lowest scale to determine the correct 'common' lead and the available impedances. Take the time to repair weathered cloth covered leads w/ heat shrink tube or lacquer coatings. Polyurethane works wonders on old wire.