300b lovers

For example, floating paraphase phase inverters instead of split-load inverters or Mullard long-tail pairs. The phase division isn’t as precise, which is why they dropped out of favor, but the drive capability is much stronger than the other two types.

@lynn_olson A simple way to improve this circuit is to use the bias supply for the KT88s as a B- voltage and then use a 2 stage constant current source to set the operating point of the differential driver. This improves the differential effect quite a lot and gets rid of the need for slightly different plate load resistors- they can be matched instead.

To this end we use bipolar supplies in our amps; B+ and B- have the same absolute value. No balancing is required in the differential amplifier and often the plate voltages are within 3-4 volts of each other. This improves the CMRR quite a lot which in turn reduces distortion (there is the additional benefit of more gain and wider bandwidth as well). If the input stage is built in a similar manner, even orders will cancel throughout the circuit, resulting in a dominant 3rd harmonic which can mask higher orders.

The harmonics will be found to fall off at a faster rate (than seen in SET circuits) as the order of the harmonic is increased; they will follow an exponential decay based on a cubic function. This works really well for the human ear (smoother sound and greater detail, both on account of reduced open loop distortion)!

To eliminate a frequency pole caused by a coupling capacitor, we’ve been using a differential cascode circuit as the sole source of gain in our OTLs. Because the gain is increased in that single stage of gain, so is the CMRR and overall differential effect. This allows one to use a cathode follower driver direct coupled to the output section. In our OTLs the power tubes are in turn direct coupled to the loudspeaker. If you were to use an output transformer, a pair of DHTs are easily driven- linearity is such that no feedback need be used. Bias is obtained from the driver circuit, so if bias controls are used, they are in the grid of the driver tube rather than the power tubes.

If feedback is desired, it can be wrapped around the circuit and applied in a manner identical to how its done in opamps- using resistor divider networks that mix the feedback with the incoming signal at the grids of the input stage. This technique vastly reduces distortion that the feedback signal would otherwise encounter, which in turn means the feedback is more effective at its job, generating less higher ordered harmonics (caused by non-linearities in the feedback nodes traditionally used in both tube and solid state amplifiers). By doing this a wider range of speakers can be used.

Here's a question for @atmasphere - Though I'd welcome the comments from anyone else on this thread -  these are all related to the question of what causes a power tube to wear out - 

1. Does it matter what volume a power tube is played at? Does that effect tube life?

2. Does it matter if a tube is cooled, say, by a small fan nearby?

3. If a tube is powered up but not making music does that "cost" tube life just as if you were playing music through it?

4. What is harder on a power tube? Turning it on and off, let's say twice during a day (two listening sessions totaling three hours) or letting it stay on, let's say for an eight hour period?

1. Does it matter what volume a power tube is played at? Does that effect tube life?

@markusthenaimnut 

If the tube is operating class A1, the power its making won't matter. If operating class A2 or A3 the higher power levels will probably affect tube life. If the tubes are running class AB then they will run cooler, which could translate to longer life, but higher power will shorten that. You didn't mention the load but that affects things too- its rare that the speaker actually loads the output transformer correctly for a given tap; transformers transform impedance in both directions so a load too low on the output transformer will be a load too low on the output tube(s) as well. That will reduce tube life as more of the power made by the output section will be dissipated in the tube(s) rather than the load!

2. Does it matter if a tube is cooled, say, by a small fan nearby?

It helps! During WW2, 6L6s were used due to shortages to get amazing power levels by being water cooled.

3. If a tube is powered up but not making music does that "cost" tube life just as if you were playing music through it?

Always- how much depends on the class of operation and other variables such as dust on the envelope and so on.

4. What is harder on a power tube? Turning it on and off, let's say twice during a day (two listening sessions totaling three hours) or letting it stay on, let's say for an eight hour period?

Tubes wear out no matter what you do. They also draw power... The longer the off time the easier it is to answer a question like this. If we're talking about an indirectly heated power tube, the turn on should include time where no B+ is applied until the power tube cathode is properly warmed up. We supply a Standby switch for this purpose on our amps. If the tube is directly heated it won't matter. So if it were me I'd shut the amp down when not in use, even if for only an hour.

Great response, Ralph, answering questions I have long wondered about.  My Sachs monos put out a fair amount of heat so I mounted a couple of noiseless computer fans at the back of my rack to dissipate the heat to a degree.  

The Statement ... or Blackbird ... or whatever it’s called, is pretty sensitive. Don told me 1.5 Vrms to full clipping, and my gain calculations indicated a bit more sensitive than that. Since there is no RC loading anywhere, and all the cathodes are bypassed, all tubes run at full gain.

For a 6SN7 that’s a mu=20, and for a triode-connected 6V6 that’s a mu=8. Since the 300B grids take 80 volts to reach zero bias, and can go positive to +20 above that, that’s 100 volts of swing for each side, about 3x what a pentode requires. Since this is a PP amplifier, full power requires +100 volts on one grid, while the other receives -100 volts. The input+driver provide 20 * 8 = 160 gain, excluding transformer losses, which are typically 5% or less.

Imagine those 200 volt peaks at 30 kHz, with no local or global feedback, at less than 1% distortion, with another 3 dB of headroom above that. This is why driver design for DHTs is difficult, and not in the Radiotron Designers Handbook.