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Solid state eq in tube amps


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4/25/2000 12:53 PM
Chris Harden
Solid state eq in tube amps
I am designing a hybrid, but mainly valve amp, and I'm trying to make a descision about what sort of eq to use. If I use an op-amp, I can use much more adventurous circuits, but I don't know how using a Solid State device in the middle of the preamp might affect the tone. The circuit would have to have enough headroom so as not to distort at all. Has anyone had any experience with this configuration or got ideas on the subject of EQ and particually graphic equalisers in guitar amps?  
Any comments please.  
 
Thanks  
Chris
 
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4/25/2000 1:16 PM
kg
stephen conner has incorporated such a design in his "toaster" tube guitar amp. he did have some noise issues, but as far as i know he resolved them at least to his satisfaction.  
 
i don't recall his url off the top of my head, though.  
 
ken
 
4/25/2000 1:36 PM
Ray Ivers

Chris,  
 
I'm really into the hybrid concept, as well.  
 
The Stomp Box Cookbook, by Nick Boscorelli, has a lot of the info you'll need. You can use op-amps if you like, which of course are real versatile and can give you really hi-Q bandpass filters for your graphic if that's what you want; but he's also got some inductor-based circuits that use tubes and transistors as well - lots of different circuit choices here. An op-amp running from + and - 18 volts will have fairly decent headroom - I would recommend placing it in the vicinity of any effects loop you might have since levels there should be around -10 dB or so (I personally like placing a 'master EQ' section AFTER the loop but BEFORE the master volume, and then having any 'slave amp' send taken off the master volume pot - but that's just me). I got my copy of this book from JK Lutherie (513) 353-3320, guitar@jklutherie.com (I'm not sure if they have a Web site - try and see) - it cost 30 bucks.  
 
Any hybrid amp has so many more possibilities than a straight all-tube amp for a given price point and overall noise level, but careful design decisions have to be made to interface the two different topologies properly, or the tubes' sound can be diluted or lost altogether.  
 
Ray Ivers
 
4/25/2000 2:10 PM
Stephen Conner

Hi guys,  
 
My hybrid amp has been going for quite a while now and has proved to sound pretty good. I used a stack of op-amps to give two high-Q swept EQs. YOu can see the circuit at:  
 
http://homepages.strath.ac.uk/~cnbp111/toaster.html  
 
Steve C.
 
4/25/2000 2:15 PM
Chris Harden

Thanks for your comments, and I'd be interested to discuss designs with you if you want. A master eq section is an idea that I like, the Mesa Boogie Mark IV's have a five band switchable graphic just before the power amp. I think a simple pre-eq in an early gain stage, possibly even right on the input, combined with a five or seven band graphic as post-eq would give good flexibility. I intend to have an op-amp as the first input stage, so eq could be added to that without too much dificulty. In fact I have experimented breifly with using op-amp style negative feedback eq with valve amps, but although it did work, the sound wasn't particually good. I read somewhere that negative feedback on overdriven stages makes the clipping harder and more solid state sounding. I might put some switchable s/s distortion in (back to back diodes etc.) just to give a diferent option for sound.  
 
Another thing I am looking at is using a digital studio style, fx unit instead of using spring reverb etc. Any ideas on this?  
 
Hope to speak to you again soon.  
 
Chris
 
4/25/2000 2:49 PM
kg
quote:
"I intend to have an op-amp as the first input stage, so eq could be added to that without too much dificulty."
 
 
i must say that, in my opinion, this is a horrible mistake. the input stage, along with the output stage, represent the two most important places to have a tube, since they represent an electro-mechanical interface. put anything you like in between, but for god's sake leave the input and output alone! and to put an OPAMP there?!  
 
check out russel hamm's paper on precisely this effect here:  
 
http://www.geocities.com/SunsetStrip/Garage/5701/TVTR/tubes.html  
 
not ragging on you, just strongly advising ;)  
 
ken
 
4/25/2000 2:59 PM
Chris Harden

Thanks for the hint! I haven't actually started building yet, so it's not a problem. I'll try both, and see what difference there is. I have seen circuits using such a configuration, but I won't know what it sounds like until i try it. I just thought (I'm new to amp building) that it would be more flexible and easier to use a high impedance FET input OPAMP, the gain and frequency response of which could be easily varied. This could start quite a discussion on the topic.  
 
Chris
 

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