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Old 17th November 2003, 08:43 PM   #1
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Default Any designs for tube/chip hybrid guitar amp?

The thread on the gainclone guitar amp got me thinking...there must be a simple way to build a 2 or more stage tube frontend for a chip? Something like 2 tubes, 1 overdriving the other, no xformer, feeding a power amp chip. Is it as easy as I think? Already been done? Any info or links appreciated.
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Old 22nd November 2003, 12:09 AM   #2
svokke is offline svokke  Belgium
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It won't be that hard, but don't forget that when interfacing from tube=>chip you need to protect the chip properly against high voltages on the input. If you use just a capacitor+resistor from plate to chip input, I think there will be a large voltage on the input pin when turning on the amp. This won't be for long, but it could be high enough to kill the chip.
For a DIY preamp, you could use the preamp of an ax84 project
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Old 22nd November 2003, 12:43 AM   #3
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how about putting a zener across the GC input to absorb any potential spike? you'd probably need some resistance after the zener to prevent wierd behavior, 200R or so should work. resistance after the zener idea credit to one of the many Nelson Pass articles I've read.
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Old 23rd November 2003, 06:13 PM   #4
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Thanks for the tips.

I'm not sure that excessive input voltage would only be a problem at start up. The max differential input V spec for a 3886 is 60V. Wouldn't most tube amps be higher than that across the output transformer?

Anyways, I did some searching and looking at commercial tube amp schematics. What I was looking for was anything with a line out connection before the output transformer. None do, it's all DI after the tranny.
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Old 24th November 2003, 12:27 PM   #5
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Quote:
What I was looking for was anything with a line out connection before the output transformer. None do, it's all DI after the tranny.
All you need is the preamp section, without the output stage. For good sounding overdrive, check out the front end of Marshall JCM800 series amps. Take the output from just before the inverter valve. As this is a high impedance point, buffer it with an opamp with reverse biased diodes from opamp input to each supply rail to prevent excessive input (may also need to reduce level also). As you are only running preamp valves, supply to these could be provided from the chipamp supply using voltage doublers for say +-50V or so.

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Old 24th November 2003, 12:56 PM   #6
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I do not agree with Centauri's opinion that the JCM800 preamp section will give yopu 'good sounding overdrive'. That particular amp definitely needs it's output stage for that signature sound.

I you need overdrive: look at the MesaBooghie rectifier tube preamp section or the modern Fender blues amps. For clean: I would suggest a preamp section of a vintage fender. Look at the Soldano preamp section. It will be a good choose and does not need the opamp buffer with protecting diodes. Simply use the FX-out (from a cathode follower split in 1k/100K to lower Vout and Rout).

I suggest looking at www.sound.au.com for a article about variable output impedance. you can make a high output impedance amp LMxxxx series.

Goodluck,
Thijs
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