Valve Amp with soild state front end

[Fiat1]

Can any direct me to any schematics that use valve output stages but soild state front ends?

http://www.lenardaudio.com/main/051_opal_const.html if you have a look half way down this page you will see these speakers amps, which are push pull amps using KT 88s which have solid state driver stages.

Something around this sort of power (100watts) is what I am after really.

[ashok]

Elektor magazine had published such a design long ago. I had a copy but haven't found it yet. Someone might be able to help out.
I'll search for it in the meantime.
Cheers.

[poynton]

Wireless World also published a design using the JLH Class-A to drive KT88s

Andy

[ray_moth]

That's an interesting link. I can understand their preference for a low-noise solid state front end and I agree that it's the output stage that gives it the 'tube sound'. (I could never understand why some people want to put a tube front end on an SS output stage.)

I think they're a bit confused over Classes AB1, AB2 and B.

[al2002]

Originally posted by Fiat1
Can any direct me to any schematics that use valve output stages but soild state front ends?

http://www.lenardaudio.com/main/051_opal_const.html if you have a look half way down this page you will see these speakers amps, which are push pull amps using KT 88s which have solid state driver stages.

Something around this sort of power (100watts) is what I am after really.

The first one I know of was published in Wireless World in April '76 issue. A very interesting and unusual design. They have published at least 2 mores since. Best to go to your local library and search through the 1990's issues.

[ErikdeBest]

National Semiconductors launched the LM4702 some time ago, which could probably be used as a single stage driver for high power output tubes.
http://www.national.com/pf/LM/LM4702.html

In the chipamp forum there are some people who already built PCB's for the LM4702 to operate as driver stage for SS output...surely someone has already tried it to drive tubes!

Erik

[SY]

Ray, the "low noise" thing baffles me. A tube front end can be less noisy than the thermal noise of the preamp feeding it. An ECC88, for example, will have an equivalent noise resistance of 300R. 300R at room temp and a 20kHz bandwidth should generate about 0.25uV of noise (that's consistent with experience using ECC88 in phono stages). That noise is about 120dB below 1W into 8R. Ahem.

[Johan Potgieter]

Sy,

Again "expanding" the thread of Fiat1 (hi, Fiat1, long time no hear - hope you are OK!).

You mention the tube noise only; what about that of the resistors in the circuit? OK, they are in parallel with tube rp's. Still, resistors used in ss circuits are quite lower, so the equivalent noise is?? I have never compared calculations (both can provide more than acceptable low noise), but am just wondering.

Ray,
Mmmmmm. Some say they want tube sound, but mosfets improve on tubes with output transformers, thus they imagine that should be better - at least less expensive per watt. Not something I fancy though. I am not even sure about this "tube sound" thing. I do not hear any good reason why that should exist. (Now please, not to start another topic here, unless Fiat1 changes it!)

Regards.

[ilimzn]

Originally posted by ray_moth
I could never understand why some people want to put a tube front end on an SS output stage.
I think they're a bit confused over Classes AB1, AB2 and B.

Who is confused about classes and what does it have to do with tube front ends (or am I reading you wrong)?

Anyway, regarding tube front ends to SS power stages, here's a clue: transistors of any kind have non-linear input, transfer and output capacitances. While their effect can be minimized with carefull attention to topology, in the end they act in some form at the very input of a SS amplifier, forming a nonlinear filter with the source and feedback network impedances, or worse, get multiplied by miller effect. Tubes have vacuum capacitances which is about as linear as you can get, hence this problem is avoided.
Then, if you don't like feedback, you can get quite a lot more linearity from a tube than a solid state device, especially from something like a CCS loaded triode. On the other hand, driving amperes of current without a costly transformer that has many of it's own issues, tends to be very difficult with tubes...

[SY]

You mention the tube noise only; what about that of the resistors in the circuit? OK, they are in parallel with tube rp's.

Let's take a worst case, a tube in cascode so that rp has no bypassing effect. For a typical 47k plate load (I'm still talking about an ECC88 type), the Johnson noise voltage 20Hz-20kHz at 40 degrees C is about 4uV. Referred back to the input, that's 4/33uV (assuming worst case gain), so the tube still dominates. This ignores excess noise in that plate resistor, but that's still likely to be fairly small and, when divided by the tube gain to refer to the input, will still be negligible compared to tube noise.