Jasonpatrick
Friend of Leo's
I was showing my friend Robs calculator and he asked me what class B fixed bias was and I was like, I don’t know. I can’t think of any examples?
Class B fixed bias…
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elpico
Tele-Afflicted
Class B amps are amps designed with high power ouput as their primary goal.
Musicman, the company Leo worked at after he sold the "Fender" brand, made a line of class B amps that are probably the best known examples in the guitar world. Those amps made 100w with a single pair of 6L6, and only cheated a *little* bit to do it. They were made in a few formats including a 1x12 version with one of those beast EV speakers handling the 100W output
Musicman, the company Leo worked at after he sold the "Fender" brand, made a line of class B amps that are probably the best known examples in the guitar world. Those amps made 100w with a single pair of 6L6, and only cheated a *little* bit to do it. They were made in a few formats including a 1x12 version with one of those beast EV speakers handling the 100W output
elpico
Tele-Afflicted
At lower signal levels class B has higher distortion than AB. At high signal levels it's similar to AB.
Push pull class A is the cleanest, but very inefficient. You'd need a lot of 6L6 and a real boat anchor transformer set to make 100w. The idea with Class B is to trade off some increased distortion in exchange for better efficiency.
Push pull class A is the cleanest, but very inefficient. You'd need a lot of 6L6 and a real boat anchor transformer set to make 100w. The idea with Class B is to trade off some increased distortion in exchange for better efficiency.
In the Music Man manual the two models RD and RP boast of their distortion, warm sound of tubes, cool running, low noise, and high power. - with no need to use matched tubes.
- The D stands for = distortion, R= reverb, P= phaser........ no C for clean.
"...from gentle warmth ...to all-out raucus rock sound."
The cooler running of class B, with no loss of high power, meant that tubes could last longer.
The 75W Fender, with the 15" EV speaker, produced at the same time, 1980, is possibly the cleanest sounding amp Fender ever built.
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Pete Farrington
Tele-Afflicted
I suspect both those Musicman and Fender amps relied on being loaded with the particular STR387 type 6L6 to achieve rated performance.
i wrote that based on their reputation.
then i read the reviews and watched the demo videos.
much different than the silverface fenders.
the distortion is SS and not respected. Nobody plays the distortion in the demo videos.
these amps are used for clean or as a pedal platform.
that is what 700 volts does to the sound, more volts is tighter and cleaner.
i doubt it has 700 volts nor the sound of 700 volts.
and
MM amps are not biased at cutoff and they are not class B. cold class AB.
they have transistors for the pre amp including the PI to deliver a clean signal to power tubes. the power tubes provide some tube warmth.
The MM amps were Leo and Forrest taking amps to the next evolution after the bf/sf amps.
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Music Man Amps
Any fans of Music Man amps? I remember back in the day Music Man amps were really popular. I think Johnny Winter and many others endorsed them. Now I'm seeing a Music Man 112 for sale for $350. Anyone still playing through their Music Man? I'm hesitant about picking one up since it may be hard...
www.tdpri.com 
That is the music man.
My comment was about the Fender 75 amp.
The Fender 75 shows 250 volts on the plate.
I don’t know how that could be.
The Fender 75 may have a beautiful clean. To my ear, the MM amps have an overly clean sound. They lack harmonics.
The MM 65RD and 100RD share the same circuit.
It is unclear how they get 100W out of one and 65W out of the other.
The fender 75 offers an interesting comparison with the MM amps. The MM were developed starting in the early 70’s. Leo was not allowed to resurface until 75 under the CBS deal. The fender 75 was available in the early 80’s and that was kind of the middle period for MM.
Both amps feature a seperation of duties. The clean and the dirt were separate channels. When amp designers do that they tend to make the clean extra clean.
That is why many people including myself prefer non master volume amps where the clean and the dirt are on the same channel. Add volume to get dirt and to get more dirt.
Although I have to admit that I don’t crank my amps above 6 or 7. My amps in that range probably don’t sound a lot different than MM amps at the same volumes.
elpico
Tele-Afflicted
The Musicman amps are low distortion opamps all the way from the input to the power tube grids. I don't think anyone would question that an amp with an all tube preamp colours the sound more than an all opamp one.
The question asked was about Class B though. Power amp design, not preamp. The musicman amps happen to have very clean preamps yes, but that's not a "Class B" thing. Tons of opamp based solid amps out there. The unique thing about these was mating that very clean preamp to a tube output stage, which *adds* distortion and colour compared to doing an all solid state amp.
The question asked was about Class B though. Power amp design, not preamp. The musicman amps happen to have very clean preamps yes, but that's not a "Class B" thing. Tons of opamp based solid amps out there. The unique thing about these was mating that very clean preamp to a tube output stage, which *adds* distortion and colour compared to doing an all solid state amp.
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This would be true if the clean pre amp were mated to a regular tube output stage, but it is not.
This clean pre amp is mated with a cold biased high voltage clean tube output power stage.
The OP was about whether there were any examples of class B fixed bias amps.
When one was given, you falsely claimed that
Yet still you harp-on about "clean", and have nothing to say about the primary subject, a failing I now also make.
When one was given, you falsely claimed that
but I noted that MM claims a list of attributes about the amp, of which cleanness is not mentioned. Therefore, it most definitely was not their primary goal.
Yet still you harp-on about "clean", and have nothing to say about the primary subject, a failing I now also make.
elpico
Tele-Afflicted
At a time when full solid state amps had been a thing for ages. This amp is from 1980-ish. If they were trying to make the lowest distortion amp on the market they made a strange mistake by putting tubes in it.
My understanding is that musicman amps originally used 6CA7 but the supply dried up. The circuit voltages make a lot more sense when you think of the tubes they were designed with.
Pete, the 250vdc to the plates is in the reduced power mode. In full power, there will be something around 500vdc to the plates.
The screen grid voltage is very low in those MM amps.
The MM amps were designed by Tom Walker and refined by Bob Luly.
elpico
Tele-Afflicted
For context here's a 6CA7 datasheet that has some class B operating point data. Output power @ 5 or 6% THD is given for a range of supply voltages between 400 and 800Vdc.
The 100w example listed has 6% THD. Typical 2x6L6 class AB datasheet curcuits are usually around 2% @50w.
The 100w example listed has 6% THD. Typical 2x6L6 class AB datasheet curcuits are usually around 2% @50w.
This design example is also found on EL34 datasheets.
But note that the idle plate current is 25mA, meaning an idle dissipation of 20W or 80% PDmax.
IMO this is rather hot class AB.
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elpico
Tele-Afflicted
Amplifier class describes how many degrees the tube is conducting when it's producing it's rated output though. We don't know what it's class is from it's idle characteristics, at least not in a real amp. In a theoretically "perfect" amp though, yeah zero current at idle would be ideal.
A conduction angle at full power measurement isn't given in the table, but if Tung-Sol engineers felt comfortable labeling that operating point class B, I can only imagine it was close enough to 50% that they felt B was a better description than AB. Obviously it's not an "ideal" class B amp though, agree with that.
A conduction angle at full power measurement isn't given in the table, but if Tung-Sol engineers felt comfortable labeling that operating point class B, I can only imagine it was close enough to 50% that they felt B was a better description than AB. Obviously it's not an "ideal" class B amp though, agree with that.
This calculator: https://www.vtadiy.com/loadline-calculators/power-stage-calculator/
shows that there is a (small) class A range contributing 3.4W, so not textbook class B.
shows that there is a (small) class A range contributing 3.4W, so not textbook class B.
elpico
Tele-Afflicted
And does amplifier class mean "what do the tubes do at idle"? or "what do the tubes do at low signal level"? Not in the slightest.
What you're talking about is efficiency, not class.
There's a *maximum* efficiency a theoretically "perfect" class B amp could achieve. There is no *minimum* however.
This amplifier, having non-zero dissipation at idle, is clearly less efficient than the unattainable, non-buildable "ideal" class B amplifier, but that tells you nothing about it's operating class. The amp could have 100% dissipation at idle and still be class B.
Only conduction angle at rated output determines the class.
Now I certainly don't have the high power 800Vdc supply and NOS Tung Sol 6CA7s you'd need to say for sure what the conduction angle was at that operating point. I can only guess from the data given, and it looks like they may have chosen that idle point to make the low volume performance tolerable then counted on bias shift to bring it down to 180 degree conduction at rated output. Note that the input signal for max output is slightly higher than the idle bias.
If someone has the required hardware, do share your results though. Again, I can only trust the Tung Sol engineers knew a lot about this stuff. They were innovators over there, seems to have been a technically sharp place.
What you're talking about is efficiency, not class.
There's a *maximum* efficiency a theoretically "perfect" class B amp could achieve. There is no *minimum* however.
This amplifier, having non-zero dissipation at idle, is clearly less efficient than the unattainable, non-buildable "ideal" class B amplifier, but that tells you nothing about it's operating class. The amp could have 100% dissipation at idle and still be class B.
Only conduction angle at rated output determines the class.
Now I certainly don't have the high power 800Vdc supply and NOS Tung Sol 6CA7s you'd need to say for sure what the conduction angle was at that operating point. I can only guess from the data given, and it looks like they may have chosen that idle point to make the low volume performance tolerable then counted on bias shift to bring it down to 180 degree conduction at rated output. Note that the input signal for max output is slightly higher than the idle bias.
If someone has the required hardware, do share your results though. Again, I can only trust the Tung Sol engineers knew a lot about this stuff. They were innovators over there, seems to have been a technically sharp place.
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