Tube preamp into solid state power amp?

[damian]

Blundering in..

No one here has mentioned frequency response as far as I can see. Valve amps have lousy frequency response as well as more, and more gradual onset, distortion. That offers the light compression that some of these people find so appealing.

Most preamp gain stages are triode based, whereas the output stages in hi fi valve amps can be triode, tetrode or pentode, and of course different valves and output transformers will offer different sounds.

94 dB isn't particularly inefficient. How loud do you want this system to be ? Also be aware that a low power amp with efficient speaks will produce the same sound pressure as a big amp and inefficient speakers, the sound can be more interesting also. The cost of a smaller amp and louder speakers can be lower than building a monster amp.

Plenty of people have coupled 12AX7esque preamps to mosfet output stages. The result is ok.

Remember also the current draw on a class A stage is constant so you can't physically get sag on that circuit. I use valve rectifiers on class A stages but only because I'm too lazy to use standby switches.

Just some thoughts.

Edit: Just a couple of quick additions. Soundman I understand your trying to avoid writing a text on this but a couple of things you say are potentially misleading:

1. There is a lot more to output transformer frequency response than size.

2. KT88/90/6550B output valves are relatively common and will produce rather more than 88 watts at negligable distortion (in a quad). What is neg dist is relative of course. 0.5% THD in my books. Balancing a quad, sextet or octet of output valves isn't difficult. Nowdays there seems to be a cult built up around valve matching/tuning/boutique supply. Perhaps todays valves are very much inferior to the old ones, or perhaps marketing people have found a new gullibility to exploit.

3. Valves wear out if they are driven hard. I have amps from the 60's with all their origional valves, not so thelectroytics. Carbon resistors and some caps are much less long lived than valves as are some early semiconductors.

and finally regarding power and distortion figures, in the old days there used to be common standards, then marketing people discovered they could publish misleading numbers from dodgy tests. Happened with cars also. Very frustrating. Perhaps the answer to the origional question needs to begin with better definition of what is trying to be achieved ?

[soundman]

Danian there is much truth in what you say.

as to the frequency response.
The frequency response in valve circuits generaly is not down to the valves, many valves used for audio applications will perform just fine up into the low RF frequencies.
The limiting factor on valve power stages IS the transformer, and size is an unavoidable side effect of the engineering.
to get good low frequency performance out of a valve output stage you need a transformer with a large core of iron so it does not saturate thus you need a large amount of wire and there will be a significantly large inductance that will limit the high frequency response.
I had a rack of amps out of one of the cinemas here at one stage the output was a pair of KT66's and they did about 20+ watts into 8 or 16 ohms.
I measured the response of those amps as running from 16Hz up to about 8.5KHz. and yess the transformers were quite large, the output tranny's were twice the size of the supply tranny's.

now most guitar amps need nowhere near that sort of low frequency response so the transformers are significnatly smaller, frequency response can be expected to be more like 80 to 12.5K. That is no problem to a guitarist as the isntrument wont produce out of that range and besides most guitarists cant hear past 10K.

Now tarsformer design is much more complicated than that, but the reality is that there would be very few valve output tranny's that could be considered modern in design let alone cutting edge....and the inductance caused bandwidth problen will remain.
The only reason that the achieved bandwidth is available is only due to the use of negative feedback in the amp.
Interestingly in the cinema amps it was necessary to change the feedback as well as the impedance tapping otherwise the top end frequency response suffered very significantly.

Indeed most of the preamp stages in most plain audio amps are triodes or more frequently dual triode valves.....this is because that is all that is required.
A properly built triode preamp will perform to the capacity of the valve. A valve preamp can be very quiet, low in distortion, reasonably linear , and have quite respectable frequency response.
Oh and they generlay operate "A" class.

In a power stage it is necessary to insert extra plates into the valve to exert better controll of the current flow and thus improve performance in many ways in particular distortion. It is the controll of relativly high currents that valves struggle with. Transient response is the problem, achieving frequency resoponse isnt soo much of a problem (well less than the transformer) but achieving good distortion at the same time as high frequency rsponse is.


yep 94dB/W isnt too bad for a HIFI speaker, it would be OK for a pro audio speaker, I have seen HIFI speakers down in the low 80's and mid 80's is very common.

Yep there are plenty of 12AX7 & similar preamps out there combined with all sorts of solid state power stages, and as stand alone preamps for recording and the like......If its a valve preamp sound you want the result is pretty hard to beat, unless you start looking for esoteric valves.
Cheap & easy to build too.

As to the output of various valves.
if we are talking similar levels of distortion.
lets start with a reference of .01% which in a solid state amp is considered prior to onset of clipping, assuming no rail sag or other misbehaviour.

5% is a significant amount of distortion and will produce more measured power.

even from a valve amp 5% sounds distorted, if you are playing guitar or other instruments it will sound fine.
but if you were playing quality speach or pure type sounds it will sound distorted.
At one time 1% would have considered " respectable " for consumer electronics and would problay be considered listenable for a valve radio.

but remember I am trying to give an aples & apples comparison.
At 1% most solid state power stages will be starting to sound nasty, well at least anoying. Some solid state amps will not tolerate being driven near or into clipping for any length of time. The old 480 module ( and its relatives ) is a good example.

So if we look at typical 4 valve kT88/ 6550, output stages they will be sold as 100watt and will output a genuine 88 watts before clipping.
Now of course you can push the friendship by selecting valves and bumping up the rail voltage but thet is a....... um........ "short term" solution.

Balancing valves is an important & significant matter, I would go further batch testing of valves is and always has been a significant matter.
Because of the crude and mechanical construction of valves the performance can vary substantialy from item to item. In the hey day of valves most sellers had a valve tester and many valves came in packaging that allowed the valve to be pluged in for testing while the box was still sealed. Many "respectable" valve retailers would not let a valve out of the shop untested.

Some of the top shelf old valves were very much better than others, I would not say generaly thet modern valves as a whole are better or worse, but I would expect that the range of quality has narrowed. The realy top shelf stuff will be long gone, but likewise the inconsistent and unrelible cheapies will also have passed.

Yep... Valves wear out......end story.
This is another problem particularly with output stages, the power output can only be guaranteed with valves with close to spec emission.
as the valve wears out the power output and other performance will degrade.
It is not unreasonable to expect an amplifier will still be functioninf but delivering less than 50% output.

As for the expressions of power and distortion, you could do a long thread on that one alone.
The important thing is to make sure the poweroutput is expressed in the same trems and under the same conditions.

100 Watts RMS continuous, pink noise, both channels driven, at .001% distortion, into 4 ohms at 25 deg C.
Now that is a speck you have difficulty arguing with.

As for improving the performance of a tube output stage, the simplest thing is to replace the tube rectifier with a solid state one, there will be much less rail sag and better transient power output.

more later. cheers

[damian]

I wrote 0.5% but it may not have been clear.

Mass is required to avoid saturation, but interleaves, winding density and geometry affect frequency response, albeit more suttly (sp).

Extra elements affect efficiency.

600V on a KT88 doesn't bother it.

I've got a valve tester, and a big box of NOS valves. I've been at this a long time. I can assure you an amplifier with conservative voltages will run with a set of quality valves for many many years, but whatever. Doesn't matter.

We're probably saying similar things in somewhat different ways.

[soundman]

Indeed what you say is true.

the limitations are and have always been not with the valves themselves but with the surrounding components, predominantly with the transformers.

It is a rare and expensive valve amp that will have anything like a "state of the art " output transformer.
As the quality and the voltage involved in both the output and supply transformers increases the cost skyrockets.

Cosidering that the transformers is where an overwhelming majority of the cost of a valve power stage is, I can understand this is where the corners have always and will always be cut.

From what I gather most of the common valve amps run somewhere arround 350 to 450 volts in the output stage which is a long step down from 600V.

one of the other limitations is getting capacitors ( particularly now) that are suitable at those voltages.

All these and other reasons are why a valve power stage is much less viable than a solid state one.

so running a valve pre-amp and a solid state power amp is both a reasonable technical and cost option for someone wanting a " smoother more melow" sound.

cheers

[Isaac Hunt]

Over the years I have used many different preamps.Transistor,chip,battery powered chip,passive ,TVC and valve.
There are good and bad preamps of every type.
By far the best preamp I have found is my current one.This is a Supratek Cabernet which uses 300B valves.I use it with both valve and SS power amps in a triamped speaker.
Modern well designed valve preamps are low distortion and wide bandwidth with perfectly flat frequency responses.Valve technology has come a long way in the last 10 years-whereas SS amps have gone nowhere[maybe with the exception of chip and Class D type amps].You cannot compare new valve amps with old guitar amps.
The distortion factor is a bit of a myth.What is the point of .001 % distortion when your speakers are probably at least .5% distortion?Some of the worst sounding amps you can hear have ultra low distortion.

[byacey]

If you want to experiment with a high quality tube output stage, look into an ultralinear or Williamson output configuration. With a good transformer this output stage will provide a flat frequency response from about 15 hz up well past 100 Khz, albeit at the expense of some output power. This is accomplished by using screen taps on the output transformer providing negative feedback to the screens forcing the outputs to behave in a very linear manner. Damping factor of course isn't as good as solid state, but still pretty respectable for 55 year old technology.

The majority of the "Tube" or "Valve" sound is created in the output stages of the power amp. A good tube preamp stage operated in the linear region shouldn't be much different than a good solid state preamp aside from a little more Johnson noise.