• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Is single ended Class A better than push-pull?

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If you're looking for 30W PP, then you're entering a different ball game. You're slightly out of the range of EL34 (although in practice you'd never notice the power deficiency), 6550 would probably be about right. You'd have a 450-500V HT. Alternatively, 845 and 211 could easily achieve the power, but the HT would be far higher. Time for some choices about what you feel competent to make.

Oh, and as to the original question, I'm with Johan Potgieter and SY; I'd like the amplifier to be engineered so that distortion is as inaudible as possible. That usually means PP triode or UL. I can accept the use of SET with loudspeakers that are so efficient that a 4W amplifier is barely ticking over.
 
Arnold, if you don't want to do a from-scratch design, one good candidate would be a UL-connected version of the Joe Curcio Stereo 70 mod. It was published in an early issue of Glass Audio and was a popular project. Full power supply regulation, constant current cascode diff amp input stage using an ECC88, EL34 output. Probably good for 25 watts, close to your target. With a slightly heftier power supply and KT88/6550 outputs, you could do 50 watts easily with the same 4300 ohm load. The only circuit change would be adjusting the bias string to give a bit higher grid bias.
 
Ex-Moderator
Joined 2003
I'd second the Curcio suggestion. Realistically, most of the 6550 designs are based on the classic designs of the 50s and 60s, athough the Curcio design adds a certain amount of silicon support circuitry. Provided that the design is based on one of these classic designs, you really can't go far wrong. A more adventurous modern design will probably require more experience to build. The key issue, as always, is the quality of the output transformer...
 
Depends.....

SE is best for high-efficiency speakers where PP does not quite produce the quality sonics. (why? I have no idea after building a few high quality amps). SE needs a better power supply and careful attention to the output transformer.

On the other hand, PP is great for lower efficiency speakers and does produce more power using the same tubes in same Class of operation. From an engineering stand-point, I like PP better. Best PP to date was a set of 6P5 driving a 6SN7 to 45 tubes. Sonics are wide open, relaxed without any fatigue, huge soundstage, powerful & tightly controlled bass, detailed & just wonderful sounding.
 
Oh, 6550?

I should read all posts before posting.

I am almost finished building a 6550 PP amp. UL means Un-Linear to me. Triode does yeild 30 watts with 400 volts plate to cathode. Remember 450 volts is needed with cathode bias.

My design is a 1/2 a direct couple 6SN7 (other 1/2 is for other channel) driving a 6SN7 twin-tail phase splitter. I much prefer Sylvania 'T' plate design from the 1940s & early '50s. I also understand the Wing C Svetlana 6550C is a good tube, but stay away from the older design with the round holes in the plates. It is reported the slotted plate holes are the good tubes.

The screen power supply is critical. I use a pair of 0A2 with a .1uF cap across both tubes to reduce noise. Then, the output is routed to a 12 HY choke with a 35uF polyproplyene cap on the choke output to the screen. This is unusual, but the choke isolates any 0A2 regulator noise & the 7ma to 38ma screen current does not effect regulation.

Use good iron such as MQ 465. In a pinch, the Dynaco ST70 audio iron can be used with a limit of 75- 80ma per tube idle current for about 40 watts output.

If you go triode connected 6550, bias voltage is much higher. However, the 6SN7 to 6SN7 will drive the 6550s.

A better tube especially for triode connected is the EI KT90 type I or II.
 
Hey-Hey!!!,
I will jump in with SY in a lot of ways. I have built a few PP amps, all with multigrid finals, all class A. I even discovered what I believe to be a new circuit. I named it E-Linear. It is a means of doing plate-to-grid feedback around the finals by attaching the diff amp driver plate loads to the U-L taps.

The PP amps have always sounded quite different from the SE amps I have compared them to. Power is small compared to AB1/high voltage designs, but power is not the goal. Power is a side effect.
regards,
Douglas
 

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SY said:

The inefficiency is just irresponsible. Might as well get the biggest damn SUV money will buy.



Yes, energy use is something to be considered. If you have an amp sucking 500 watts and you run it 2 hours a day for a year, you'll burn 9.2 Gjoules of energy (500x60x60x14x365). Hop on a flight to anywhere and you burn that in less than an hour (23Mj/passenger kilometer), but most of us hardly give that a second thought. But then, why worry. Better we follow our leadership's post 9/11 advice; (paraphrased, but accurate) our way of life will be fine, keep shopping.

Sheldon
 
SY said:
That will save gym fees, true.

You know, someone should design gym equipment that's fitted with generators. With a generator, the greater the load applied, the greater the resistance it has to turning. This would allow you to dial in the ease or difficulty of the piece of equipment.

I've always thought that a rowing type machine connected to a flywheel and a generator would be great not only for generating power but would also give a great near full body workout. Much better than a treadmill or statoinary bike.

se
 
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