About Tube Amplifier Construction





Why Tubes and Not Transistors?

Tubes are the most linear amplification devices available and the most linear tubes are the directly heated triodes (DHT). Tubes are expensive and the efficiency is poor and that is the negative side of it, but the circuits are simple and no feedback needs to be applied even in single ended designs. The tube principle with two plates in vacuum and electron movement controlled by a grid in between the plates has a nice feel to it, can it be simpler? Transistors work in microcosmos with electrons and holes! A very small charge will alter the behavior dramatically and so will local heating (not controllable), this does not feel as nice as the tube. I think that the vast improvements in solid state amplifiers when these are left powered on for a very long time indicates that local heating is a problem. It does not sound good until everything is hot! My tube amplifier sounds good from start and reaches magic in about 90 minutes. Tubes are however microphonic, but that can be controlled.
 
It is probably possible to get good results with transistors, but it is very time consuming and costly to make several PCB's. Tube equipment can be hardwired and is more interesting for the hobbyist. I also like the tube amplifier sound (or rather lack of sound), and I have not heard any solid state amplifier that can compete with good tube amplifiers. This includes expensive Krell and Mark Levinson equipment, but I have not heard the Pass amplifiers enough to include them on this list. However I have heard amplifiers with transistors that sound great, Pathos amplifiers use tubes for voltage amplification with coil loaded transistors in the output stage and the sound is very tubelike.


Sweet Sound but Correct?

There are still some who think that the tube amplifier renaissance is only nostalgic, but I doubt that anyone of them have spent much time listening to good acoustic recordings reproduced by tube equipment. The sound is much more realistic than when solid state amplifiers are used. There are good solid state amplifiers and there are bad tube amplifiers, but the best tube amplifiers sound better (more realistic) than the best solid state amps. The conclusion: realistic=correct! The reason why some (the majority) prefers solid state is that you get more power/buck, more punch and maybe better bass or at least more control with lousy loudspeakers, but the sound stage realism is not as breathtaking. It seems as if the HiFi enthusiasts can be divided into two groups, one searching for SNAP and SLAM (solid state group) and one searching for 3D illusion and sense of music (tube group). The musical taste plays an important role, with electrical rock music SNAP and SLAM is essential and coloration does not matter as much, but with acoustical music, coloration must be low and illusion is important. If you like to listen to Carl Orff's Carmina Burana or other large scale music at realistic levels all is needed! I have not found a HiFi system capable of this, but tube amplifiers and large horns probably come closest. You will probably have to build a suitable house too.

Solid State Amplifiers Have Better Data?

True, but is it significant? There are still many reviewers who think that solid state equipment produces a more correct sound, based on the measurements. I have heard to many solid state amplifiers with good measurements, but awful sound to buy this argument, I think we measure the wrong parameters. Take the CD-players for instance, they are all perfectly flat from 1Hz to 20kHz and with almost no distortion, but still sound very much different.

Feedback, Good or Bad?

There is no easy answer to this, but if you can get a good result without feedback this is clearly the best alternative. If not, a reasonable amount of feedback may be used.
 
Feedback normally influences the sound both positively and negatively, make sure that the positive influence is dominant.
 
One negative aspect of feedback is that the clipping behavior is more abrupt (grid voltage is increased at clipping).
 
If the feedback signal is perfect, with no time delay or phase errors and if no distortion is added in the preceding stage(s) I can see no negative influences from the feedback, apart from the clipping behavior, but this is not easy to achieve.
 
If we take a KT88 in triode mode with 440V at the anode and 40V at the cathode this leaves us with 400V anode-cathode (B+ 752V with 3.9k resistive load at 80mA). If the cathode resistor (500R) is left without bypass capacitor, the load voltage will be reduced with 35V at 145mA (grid = 0V) and increased with 35V at 30mA (grid = -80V) which is the same as if the load was changed from 3.66k (0V) to 5.07k (-80V). This is clearly not perfect feedback, as the loadline is curved and the load conditions not as good as without feedback. This type of feedback is discussed in an article I have found on Dan Cheever's site, and it actually adds harmonics from 5th and up.
 
With split load transformers, used in my designs, the load is not altered by the feedback. The reason: it is the load itself (distributed load) that is used as feedback signal, which means that the load voltage is not altered.
Interesting?
 
A cathode follower with bypassed bias resistor is also an almost perfect feedback device.




If we take the signal from the output transformer secondary and use this as feedback signal in a preceding stage some interesting phenomena occurs.
 
If we for instance take a single end triode design with 5%THD in the output stage and 2%THD in the driver stage (common figures in
good designs), the THD is reduced without feedback to around 3% (distortion cancellation from opposite phase driver stage). If global feedback from the output to the input is applied the following occurs:
 
The transformer output signal is some degrees out of phase at low and high frequencies, enough to add high order harmonics.
 
The driver stage is in phase with the feedback and will add harmonics, also to the intermodulation distortion products (normally 3 times higher than harmonic distortion).
 
This means that the feedback fail in the two most important criteria for a good feedback design:
Perfect anti-phase feedback
Zero distortion in the preceding stages in the loop
 
Probably feedback will not improve the performance in this example, 3% THD is not much.
 
If feedback must be used, use local feedback and only in the stages that need it (the output and driver stage), and use loadlines with low distortion.
 
I have tried my Jadis DA-30 with and without feedback, and it sounds way better with feedback! Without feedback the bass is not far from terrible, but the higher frequencies a little bit more crisp and detailed. The high frequency level is however a bit lower and the overall impression is a soft but misty sound. When I changed my output tubes from 6550B to the much better KT88 Classic, the feedback seemed to work even better with high frequency as crisp with feedback as the 6550B without.
 
Apart from feedback there are two other forms of distortion reduction. The first is push-pull circuits (or balanced operation), and the second is driver stage cancellation, which is much the same as push-pull but with two different tube types in anti-phase. I think the driver stage cancellation shall be avoided, it is better to make each stage as good as possible.
 
I have studied the driver stage cancellation a little more, and this method has on major advantage compared to push-pull action. The distortion cancellation mainly occurs at large voltage swing, leaving the harmonic spectra untouched at lower levels.
 
If you are interested in amplifier distortions, check out the links below.


Amplifier Distortions
Dan Cheever's Audio Site
Negate The 2nd distortion
Triode Amps and Stereophile Trojan Horses

Transformers, Good or Bad?

Without doubt the transformers influence the sound, but it also makes life easier for the tubes. High load impedances can be obtained with reasonable supply voltage and if GOOD transformers are used this probably compensates for the transformer imperfections. The output transformer is the most critical part of the tube amplifier design, but it is necessary (OTL is discussed further down). Spend the money here if you wish to be a successful designer.
 
When I was younger I thought transformers were bad, with poor frequency response and phase irregularities. Then I bought an MC transformer and all the troubles with my moving coil pickup vanished, no noise, no hum. The performance was not initially as impressive as some MC amplifiers, but when I got used to it I found that the positive influence on the sound was dominant. The transformer influence the sound in a 'subtractive' rather than 'additional' way, you do not hear what it does not do and this is in the long run much more pleasant than devices adding their own sound. Then I bought my Quad ESL-63's and there are transformers in them too, but the sound is near perfect and when I bought my tube amplifier with transformers a lot more detail appeared than from my previous (good) solid state amplifier. Transformers can't be that bad after all!
 
To answer the question, I think transformers are GOOD.


Single-ended or Push-pull?

Single ended operation is probably best if the design is good and the power supply is very good. Single ended design without feedback and distortion cancellation does not produce any disharmonic distortion. Push-pull circuits have much better PSRR (Power Supply Rejection Ratio) and when driven in Class A draws the same current all the time, which makes PSU design much easier and cheaper. The push-pull designs also have hysteresis distortion from the transformers and some 'cancellation distortion' because the tubes can never be identical, both of these distortion types are disharmonic. Even though single end has a nice feel, I think it is possible to make great amplifiers with both single ended and push-pull design. I will publish an interesting approach (at least I think so) to reduce cancellation problems in PP designs soon (May, 1999). I have given my solution some more thoughts and I am not sure that it helps, so unfortunately I will probably not publish this (July 1999).

Most Important?

The loadlines and the iron used are the most important factors in tube amplifier design. In triodes the 3rd harmonic is decreased twice as much as the 2nd (3dB reduction of 2nd -> approx. 6dB reduction of 3rd) and this means of course that the design goal must be as low 2nd order harmonics as possible. Low 2nd harmonics is achieved by high load impedances and high idle current. The 300B data below shows how much lower the distortion gets (particularly the 3rd harmonic) with high load and how little the output power is reduced. Apart from the loadline, the power supply and tube choice is also very important.




I have tried some load conditions for the WE300B using the Glass Ware SE AMP CAD.

Below are eight scenarios with 300B at 425V, 80mA into different load impedances. The red on yellow is the one I intend to use.

Load imp.

5120 Ohm

5000 Ohm

4000 Ohm

3000 Ohm

2800 Ohm

2600 Ohm

2400 Ohm

2200 Ohm

Power output

8.31 W

8.44 W

9.63 W

11.0 W

11.3 W

11.6 W

11.8 W

12.0 W

Z out (8 Ohm)

1.63 Ohm

1.66 Ohm

1.87 Ohm

2.20 Ohm

2.28 Ohm

2.38 Ohm

2.49 Ohm

2.61 Ohm

2nd (dB)

-31.3 dB

-31.1 dB

-28.8 dB

-25.7 dB

-25.0 dB

-24.1 dB

-23.3 dB

-22.4 dB

3rd (dB)

-54.3 dB

-53.6 dB

-46.6 dB

-39.1 dB

-37.4 dB

-35.7 dB

-33.8 dB

-32.0 dB

2nd (%)

2.7 %

2.8 %

3.6 %

5.2 %

5.6 %

6.2 %

6.9 %

7.6 dB

3rd (%)

0.2 %

0.2 %

0.5 %

1.1 %

1.4 %

1.6 %

2.0 %

2.5 %

 









At 3k load the output power is higher than at 5.12k, but the 3k load does not come close to the 5.12k load distortion figures even at 7W (3k, 7.42W: 2nd -28.3dB, 3rd -44.4dB).

Below are four scenarios with 300B at 425V and different bias currents into a 5.12k load impedance. The red on yellow is the one I intend to use.

Current

80 mA

70 mA

60 mA

50 mA

Power output

8.31 W

8.37 W

8.48 W

8.34 W

Z out (8 Ohm)

1.63 Ohm

1.70 Ohm

1.78 Ohm

1.89 Ohm

2nd (dB)

-31.3 dB

-29.9 dB

-27.9 dB

-25.3 dB

3rd (dB)

-54.3 dB

-48.9 dB

-43.2 dB

-36.5 dB

2nd (%)

2.7 %

3.2 %

4.0 %

5.4 %

3rd (%)

0.2 %

0.4 %

0.7 %

1.5 %

 









300B Reviews

300B Comparison Review (Steven R. Rochlin)
The Karma Sutra of DHT (Dr Gizmo) 

EL34 and KT88 Reviews

When I ordered my JJ (Tesla) 300B's from Robert Losconi, I also ordered a set of KT88 for my Jadis DA-30 (Class A, triode coupled at 405V, 75-80mA). I have used these tubes for a couple of weeks now and I am very impressed by their performance. Previously I used Golden Dragon KT88 Classic, a tube I have been perfectly happy with, but aging has made them a bit lacking in detail and high treble response. The JJ KT88 bettered the performance (of my old GD) quite a bit. The sonic character is a bit laid back, but with tremendous detail, terrific soundscape and sound decay going on for ever (or so it seems). The bass is a bit thinner than with the GD, but very tight and well controlled. Playing sample records gives very different sound for each track, indicating a great transparency to the recordings. I think this tube is fantastic and reveals the recorded music in a detailed and musical way. Who needs 300B with KT88 like these? It is hard to believe at the moment that it can be better, but this remains to be seen. The JJ KT88 comes highly recommended.

Audionow (Fred Whitlock)

Not Much Power?

True, not much power/buck, but VTL makes a 1250W amplifier! If you wish to listen to rock music at 120dB and is not bathing in money choose solid state or use horns, but if you are satisfied with 100dB not much power is needed (2x15W with 88dB loudspeakers). It is more important how the amplifier behaves near the maximum power and well designed tube amplifiers with no feedback are superior in this area.

Which Class (A/AB/B)?

The most linear amplifier construction is Class A. There are good Class AB amplifiers too, but these are all high power and probably stay in Class A at normal listening levels. My conclusion: use Class A for tube amplifiers < 50W.

OTL?

I have not heard an OTL tube amplifier, but the reports are often very favorable so my opinion is not final. I am not convinced that this is the right way to go, as the operating conditions for the tubes are not the best. These designs rely on heavy feedback and I suspect that they will produce quite a lot of high order harmonics. It is also necessary to use protection circuits or very large output capacitors (10000uF/250V), you do not want 100-200V on the loudspeakers if one tube fails! I think the only sound use for OTL amplifiers is direct driven ESL's, but as I indicated above this may change.

Direct Drive for ESL?

This is the best amplifier/loudspeaker combination available, but I think a more realistic approach would be to use a tube amplifier with step-up transformers (one less transformer). The reason? A 5kV Class A tube amplifier needs a lot of power and the power supply will be expensive! Maybe there is a perfect tube for this somewhere, but I have not found it yet.

Where Can I Learn More?

The best book about tube amplifiers I have read is: 'Valve Amplifiers' by Morgan Jones. Very good, but not too complicated valve theory.
 
There is also lots of information on the internet, I have put some links below.

Book Reviews (Andrew Jute)
Duncan's Book Page
John's Valve Book and Magazine Page
Alan Kimmel's Tube Audio Laboratory
Bonavolt's Tubes for Newbies
Steve's Vacuum Tube Related Web Site
Summit Amps
Tube Cad Journal
Vacuum State Electronics (Allen Wright)
Valve Basics (Harry Lythall)

MT Audio Design Logo


This site uses frames, but if you have found this page via a search engine and do not see any frames, you can use the button below to get to the main frame of the MT Audio Design site.