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Author Topic: Fetzer-Valve/Tube Preamp  (Read 15322 times)

Raulgrell

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Fetzer-Valve/Tube Preamp
« on: May 27, 2007, 08:02:17 PM »
Inaugurating this discussion board, and bringing an idea for a pre-amp...

I was considering powering a tube with a tube emulator... The Fetzer valve (runoffgroove.com) as part of a solid-state pre-amp which then powers a tube preamp, which finally leads to a transparent as possible SS power amp and finally the speaker.

Has this been tried before? Does it sound viable?

joecool85

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Re: Fetzer-Valve/Tube Preamp
« Reply #1 on: May 27, 2007, 09:43:08 PM »
It does indeed sound viable.  I just wonder why you would want a SS preamp powering a tube preamp, then a tube poweramp?  I'd be interested to see a SS preamp/tube poweramp setup, you always see it the other way around.  I think a SS preamp could drive a tube poweramp real hard and get some good sounds, as well as be able to back off and get some solid clean tones.
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Raulgrell

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Re: Fetzer-Valve/Tube Preamp
« Reply #2 on: May 28, 2007, 07:56:43 AM »
I'd prefer a SS power amp in order to keep the design more simple in design and maintenance terms, as well as reducing it's cost and volume.

I'm sure there are a few ss power amp designs that are extremely faithful to the sound produced by the preamp. The idea I put forward would still allow for the overdriving of a tube (or possibly 2) to produce the warm harmonics one gets from tubes.

teemuk

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Re: Fetzer-Valve/Tube Preamp
« Reply #3 on: May 28, 2007, 09:35:23 AM »
I'd be interested to see a SS preamp/tube poweramp setup, you always see it the other way around.

People do it the cheap way around: Tube preamplifiers don't need expensive transformers (just one suitable for filament supply) since there are no problems with impedance matching or rail voltages. Basically, tubes can be even powered from very low rail voltages which is a typical configuration but changes the transfer curve to less soft. Required iron for all-tube amp with decent output power costs (and weights) like hell but when speaking only about tube costs a preamp tube like 12AX7 is quite cheap when compared to power tubes. It's quite nice marketing to stick one inside a SS amp (it provides two gain stages) and say the amp now has a "tube sound" - especially since there is no clear definition to what "tube sound" really is: For example, some HiFi amps and Mesa Boogies use tubes but sound nothing alike. I think you all get my point.

If you like to see some SS pre and tube power amp designs hunt for old british Vox amps, some Marshalls, Standels and especially some Music Man amps. Personally, I regard the cathode driven output stage configuration of the latter more interesting than the simple SS preamps of most hybrids.

Speaking about the topic of tube power vs preamps, my personal opinion is that you need the complex interaction of tube, transformer and speaker load to get some of that "tube sound" people fuzz about: Tube power stage clips a bit more softly than a conventional SS power amp and more importantly transformers (sometimes) saturate when overdriven - which forms a bandpass filter that has huge effect on clipped signals. The transformer will also add some distortion - I think in the form of odd harmonics. A speaker load is far from pure, constant resistance which is important since tube amps have high output impedance and poor damping factor: Generally, the speaker configuration will have more effect on frequency response in a tube amp than in a conventional SS amp. I think Vox had a nice idea when they introduced that "power amp overdrive" Valvetronix concept. Unfortunately they built those amps quite cheaply, didn't use real transformer and powered the 12AX7 push-pull "power" stage from +-15V rails.

I have been planning to build a class-A SE (or just class-AB PP) low power tube stage that would be transformer coupled to a load circuit that is a mockup of non-linear speaker impedance but basically converted to higher impedance than the 4-16 ohms of a conventional speaker. The circuit would have a potentiometer or switch that varies DC level over the output transformer ("saturation control"). The output signal would then be coupled to a linear, high power SS power amp stage. There are also few extra features that I have been planning to use, like a variable negative feedback circuit (similar to one used in Valvetronix) and switchable setup between different "speaker loads". I'm quite sure that all other stages than the last one before the "speaker load" could use either FETs or BJTs and still sound quite "tubey" - In fact, I'm pretty sure it would sound that way even if all stages were simply using FETs. Using MOSFET logic gates as gain stages also provides very soft clipping. Basically, it's all about voicing. Anyway, I'm not too happy about custom winding the required transformer - especially since I know nothing about transformer design - so I never have really started to design this stuff on paper or anything. Maybe this provides some food for thought to someone who is generally more interested in tube designs than I am, though.

d4v1d5hu13r

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Re: Fetzer-Valve/Tube Preamp
« Reply #4 on: May 21, 2008, 04:29:15 PM »
I agree with joecool. Cool thing is I basically have your setup you want, except 6v6 power tubes instead of a SS power amp. The Fender Super Champ XD has some solid state emulation before the preamp tube and then when you drive the preamp it sounds good. I would always go with a tube power amp though, because then you can crank it with out it mudding up easily and it will sound better loud. capish?

 

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