OTOMON Laboratory　(音門ラボ)/Product details page

DHT(Direct heat tube) is always the best tube which perform the beautiful sound with detail music note.

Using DHT tube as driving tube always bring the special sound because DHT have big current and low output impedance and its special tone in music. But using DHT tube as driving tube is not easy, there are some traditional methods but all of them has limited points.

Basically to drive well a DHT, you need a loading device that have high AC impedance and low DCR value and below is some popular methods that are being used.

1. Using resistor to drive DHT. This is the most simple method but the high DCR value make signal lost much and because of much current from tube, you need to use a very big resistor and actually this idea is worst one and can not be used in almost case.

2. Using choke to drive DHT

Using choke make low frequency very beautiful, very low signal loss but you need a very good and big coupling condenser to transfer AC signal to power tube. Using coupling condenser makes the sound colorful, the more coupling condenser using in your amp, the less true sound will become. As the tone of sound become more colorful, the audience will get more tired and boring as hearing music.That is why the true sound amplifier is always the target of almost audiophile, not a amplifier with bass and trebble control which help customer easily give the color to the sound.

3. Using cathode follower method

This is most popular method that most of tube amp manufacturers use because the amplifier using this method will have very beautiful response frequency technical data. The cathode of DHT will connect directly to grid of power tube and set bias to power tube. The advantage of this method is that it uses the much current in DHT to drive power tube. It is no need to use coupling condenser so the sound is not affected by coupling condenser. Because the coupling is directly connection from cathode to gird of power tube, so if the amp is working in pure class A, the cathode voltage must be minus voltage and amp will work in fixed bias.

The biggest disadvantage of this method is gain in driver tube is only 1:1, there is no gain in driver stage so the beautiful tone of DHT almost can not afford any role to the overal sound. By the way, because drive stage does not have any affect to the amplifier gain, so you need to use 2 stages before driving stage to make gain and total of driving and gain stages will be 3 or more, the more stages using in amp the more distortion the sound is.

4. Using interstage transformer, this is one of the best method until now that many audiophile like to use. It has advantage of using driving choke which has high AC impedance and low DCR value, lowest signal lost and also resolve problem of choke driving method. Audio signal from secondary winding will connect directly to grid of power tube, no need using coupling condenser. But even when you use a very good interstage transformer, physically it will be limited in range of music. The response frequency will be narrower than using coupling condenser. Even the distortion in sound is better than using coupling condenser, it is not as good as using direct coupling from cathode in cathode follower method.

5. What is different in O.U.D.D.C (Otomon Ultimate Direct heat tube Driver Circuit)

+ Hybird bias which has the advantage of both fixed bias and auto-bias and cut off disadvantage of both method.
+ The biggest characteristic of DHT is using heater as cathode. Not as indirect tube and penthode, there is a limitation of voltage between cathode and heater, with a DHT, you dont need to care about it. Using this characteristic, OTOMON LAB researched and developed a special circuit to drive a DHT that resolve all problems mentioned above.
+ Using choke to drive DHT but no coupling condenser,driver stage will connect directly to power tube.
+ Very low distortion with high power in driving stage make high power output in output stage.
+ Cancelling the hum, noise from high voltage make more clear sound in high frequency and richer sound in low frequency.
+ The gain in driving stage is gain of the tube, so there is no need much gain before driving tube. The beautiful sound in DHT tube will make a big affection to overall system.
+ No NFB in circuit