Hybrid anyone?

[soundbrigade]

Acting often on an impuls I am expecting two pairs of the good ole 2SJ50/2SK135 duo. BUT it was not my intention to look for a vintage MOSFET amp, but find a way to marry those two pairs to some vacuum.
There were an article in Elektor Electronics magasine in 97, though a component list for the main PSU was missing as well as setup instructions.
Anyway, here're schematics; can supply the "whole" article:

Input and driver (PCC88):
http://www.pastisch.se/EF/EE_9709_hyb_1.jpg
Current/OP amplifier (SJ50/SK135):
http://www.pastisch.se/EF/EE_9709_hyb_2.jpg
Heater supply:
http://www.pastisch.se/EF/EE_9709_hyb_3.jpg
Main PSU (no data ):
http://www.pastisch.se/EF/EE_9709_hyb_4.jpg

I found nothing on the web apart from this short thread on another DIY-forum (not as good as this one, and wrote a few lines) - http://www.diyaudioprojects.com/Forum/viewtopic.php?f=10&t=2774.

I have scematic for a Moscode, but that doesn't intrigue me at all ( ).
About the EE project (above) I must say that I am no big fan of SRPPS and could consider a pentode instead av the PCC88. A C3g could come in handy or an EL84/6BQ5 if necessary.

So why leave the bottomless vacuum tube quagmire?? Because of an impulse like a lightning on a clear summer day. I'm no vacuum fascist that requires tubes always and everywhere, I like to experiment and this is could be another project.

Any ideas, guys?!

[kheper]

You may wish to pull yourself out of the "bottomless vacuum tube quagmire" in order to drive a tube into A2.

Below from Electra-print is a circuit capable of driving the 211 into A2 with low distortion. The circuit consists of a "chip amp" and an inter-stage transformer. Chip amps, such as the TDA2030 (~$1 each), can push out about 10 watts of clean power, and the transformer serves to load the grid of the 211 at a constant, low DC resistance.



I cannot find it now, but someone from Electra-print wrote (words to the effect): the only important thing to the resulting sound in a "tube amp" is that the output tube push current through the transformer. Some seem to think that drive circuity (whether tubes or sand) does not really matter, only the output stage does? Huh?

http://www.electra-print.com/211a2.php

http://www.electra-print.com/techblog.php?blogid=7

http://www.electra-print.com/techblog.php?blogid=2

[soundbrigade]

I was more into the idea of using a tube as input/driver device and the complementary MOSFET pair as output.

There's an article at TubeCad: http://www.tubecad.com/2006/04/blog0058.htm

[kheper]

Yes. I was just showing the other side of the coin. There are some cheap tube drivers (tubes, parts and boards) on ebay that should be ideal to drive a sand output circuit.

[soundbrigade]

OK, I get it.

I have seen a few versions of amps with silicon input devices and vacuum output. One very interesting and design is a schematic from British ETI journal with a long tailed MPSA42 pair on the input that are DC-coupled to the output tubes - PP EL34.

[kheper]

KT120s are supposed to sound good in triode mode. A less dare-devilly way than with the 211 might be to run KT120s at 500V with a similar circuit for a reasonably powerful A2 triode amp.

http://www.audioasylum.com/cgi/vt.mpl?f=kandk&m=9336

[jukingeo]

...

I cannot find it now, but someone from Electra-print wrote (words to the effect): the only important thing to the resulting sound in a "tube amp" is that the output tube push current through the transformer. Some seem to think that drive circuity (whether tubes or sand) does not really matter, only the output stage does? Huh?

Really? Someone actually said that? The input and driver stages matter just as much...in fact even more so. Because the voltage amp tube does the actual amplifying, it affects the signal drastically. Also ask anyone with a guitar amplifier and they will tell you that the preamp stages very much affect the tone of the amp. In fact with guitars everything from the pickups you use to the input stage circuitry, to the tubes, to the output stage, output tranny, and yes even the speaker all affect the tone.

I am not one for using sand, but if I were boxed in a corner and had no choice, then I would say to go with a tube front end and a MOSFET output stage. But the only case where I would do something like this if I needed some really high power (more than a couple hundred watts) to drive inefficient speakers or for some commercial amplifier such as a theater or nightclub amp. Ampeg, a very popular bass amplifier, uses a circuit topology on this concept in their hybrid SVT amp. They do have a full tube SVT that puts out 300 watts of all tube power but with using SIX 6550's, it gets expensive to retube the amp, so Ampeg developed a much lighter and more powerful amp that uses the front end of the all tube amp, but has 8 MOSFETS for the output stage. It puts out slightly more power.

In terms of sound I honestly cannot tell the difference between the all tube and hybrid version...except when the amp is driven hard. However, Ampeg has an all transistor variant and THAT am is by FAR inferior to the Hybrid and All tube models. Same type of circuit, but just all sand. So in this case it would seem that the pre-amp stages seem to affect the sound more than the output stage.

All in all, even here my heart still belongs to the all tube version as they have proven to be the most reliable. It is just very expensive to retube.

[kheper]

"The state of the art op amp is so linear few distortion analyzers measure its distortion. They also are very uniform from type to type (in audio use). They are used in the highest quality sound recording equipment, exclusively, so I see no reason to not use it for an output tube driver. “You can’t do that, that’s a solid state amp in a tube amp!” is the usual response. Then we reply, “Can you visualize a CD player that is totally built with tubes and not one solid state device in it? Why it would be the size of a refrigerator!” CD players have many IC op amps in their design. So what’s the difference - nothing. But I still want a tube to drive my speakers. Tube voltage on an output transformer sounds alive, that has been the reason SE has increased in popularity over the last decade, not the caps, interconnects or tube sockets."

[Geek]

A topic I missed!

I repared an Alts that had a similar arrangement.

No schematic ever existed, but the designer who made it in Hong Kong was now a local and offered a hand!

I found nothing on the web apart from this short thread on another DIY-forum (not as good as this one, and wrote a few lines) - http://www.diyaudioprojects.com/Forum/viewtopic.php?f=10&t=2774.

Awwww, Gio is a good guy

[glen]

...

I cannot find it now, but someone from Electra-print wrote (words to the effect): the only important thing to the resulting sound in a "tube amp" is that the output tube push current through the transformer. Some seem to think that drive circuity (whether tubes or sand) does not really matter, only the output stage does? Huh?

Really? Someone actually said that?

There is a lot of truth that transformer coupling has an important advantages to driving speakers even with SS. I saved a pdf I could email you with the technical specifics, but a lot of folks are moving toward transformer output stages even in all S amps.

But back to the Op the lateral mosfets are some of the most linear of sand devices, but the key is the complimentary pairs aren't so complimentary. The industry got around that oversight with NFB.

But the smart thing to do is split the phases with an input transformer or dual triode, etc and drive a push pull pair of Sk135's and forget its compliment, you will get much better match than using the complimentary pairs. I have a modern commercial hybrid designed like this (only with V-mosfets), I wish it had MSK135s as I am sure it would have much less distortion and sound better. But I think it is one of the smartest ways to do a hybrid with a SS output.