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Old 27th January 2006, 02:06 AM   #1
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Question help to tame a 12ax7/6AQ5 circuit

Hi,

I just built a guitar amp out of an old stereo radio from the sixties. this uses one 12AX7 for the preamp section (in series) and two 6AQ5a tubes in parallel on the output. it works. clean sound until I get about 1/4th or so up in volume, and then it starts breaking up. that sweet tube break up. However, I would like a bit more clean volume before i get the distortion going. I don't know much about tubes and how to bias them and so on. I got the basic schematic from a website called Adams amps. http://amps.zugster.net/projects/micro-champ
he has one called a micro-champ. my schematic is similar except I use two of the 6AQ5s. any suggestions are appreciated.
you can email me if you want to see my schematic. mysolace@juno.com

Ds
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Old 27th January 2006, 10:20 AM   #2
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How much volume are you expecting?, it's only a VERY low power amplifier (in guitar terms).
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Old 27th January 2006, 06:02 PM   #3
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Default tame the 12AX7

This amp is plenty loud! Over 80db at ~9watts. A great little practice amp. Just want to get rid of some of the distortion and have a more clean signal at lower volume. some people have suggested i change some resistors on the preamp section. i'll do this. and look at it with a scope.
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Old 27th January 2006, 09:02 PM   #4
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Default Re: tame the 12AX7

Quote:
Originally posted by DavidAmp
This amp is plenty loud! Over 80db at ~9watts. A great little practice amp. Just want to get rid of some of the distortion and have a more clean signal at lower volume. some people have suggested i change some resistors on the preamp section. i'll do this. and look at it with a scope.
The volume per watt is more dependent on the speaker than the amp, although the high distortion levels of a valve amp sound louder than a clean transistor amp.

If it's distorted at low volume levels, it's most likely the preamp which is distorting?, but using two triode preamps sounds a bit excessive for a single ended amp driven from a guitar?. Try bypassing the first triode.
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Old 28th January 2006, 08:07 PM   #5
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Question taming my 12AX7 tubes

I found the answer to my distored (over driven ) signal on the AX84 website. there is a tube theory doc there that explains how to bias this tube. based on my calculations, the bias is off. Second stade is at zero which is at saturation. I will make the needed changes to the resistors on the cathodes and see if that makes a difference. I'm betting it does.
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Old 29th January 2006, 09:09 AM   #6
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Default Re: taming my 12AX7 tubes

Quote:
Originally posted by DavidAmp
I found the answer to my distored (over driven ) signal on the AX84 website. there is a tube theory doc there that explains how to bias this tube. based on my calculations, the bias is off. Second stade is at zero which is at saturation. I will make the needed changes to the resistors on the cathodes and see if that makes a difference. I'm betting it does.
Have you tried measuring the anode voltages?. For that matter, post the cirsuit of your preamp section.
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