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12at7 transformed amp
12at7 transformed amp
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Old 27th February 2013, 07:05 AM   #11
Clemford is offline Clemford  United States
diyAudio Member
Join Date: Aug 2011
Location: Florida
Originally Posted by gingertube View Post
Buzz generator - a (typically 12AX7) triode stage running at very low current (cold biased), designed to clip one half of the signal really hard (asymetrical distortion) thus introducing a lot of second and other even harmonic distortion.

You can recognize it from its position in the signal chain (always the last preamp stage of a high'ish gain preamp) and by a really high value cathode bias resistor which is never bypassed with a capacitor. In this case a 33K which is what they use in SLO Amps as well, same as last stage in a Trainwreck but in that case the cathode bias resistor is 10K.

It is an "icon" of higher gain preamps - to make it sound good, you add one of these cold biased stages with and unbypassed cathode resistor of between 4K7 and 33K as the last preamp stage. With 33K it has a gain of about 2 and clips half of the output so really it has an effective gain of 1, only changes the sound and not the loudness
Never actually heard it called a "buzz generator" before but it is appropriate.

Too much "buzz"? try 10K in place of that 33K.


PS You may be getting a sonic change from the 2nd half of the 12AT7 as well. This is used for the concertina phase splitter, before overdrive sets in the output impedance of a concertina (at both cathode and anode) is approximately 1/gm. An AT7 has 4 times the gm of an AX7.
Thanks GT for that info. Good to know that changing around the 33K will better dial in the distortion if needed. I could always use a temporary pot to find the best value. The amp just has way too much unusable gain for my liking with the 12ax7 and it's nice to know how to tame it in case a 5751 tube is too much gain compared to the 12at7. That said, what really needs fixin on this amp is the over compressed tightness when cranked up. Even on the clean channel it compresses a bit too much which must mean it's the last stage of tube 2 and with the lower gain of the 12at7(am I correct here? it seems opposite by your description if the 12at7 has 4 times the gain{gm?}) it is less compressed. From what I've heard, the el84's get compressed naturally so maybe theres no way of controlling it other than to use a lower gain tube in the phase inverter position. Hopefully if the 5751 is more similar to a 12ax7 than a 12at7 that it won't reintroduce the compression back into the amp.

Last edited by Clemford; 27th February 2013 at 07:13 AM.
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