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Schematic for 12 AX7 6L6 SE amp

2/23/2005 10:46 PM
George S
Schematic for 12 AX7 6L6 SE amp
I Have a 10 watt PA amp that I would like to convert into a practice Amp. As a starting point I'd like to have a schematic for what I have, and then make mods from that point on.  
I've been up and down the WWW looking for info about a Knight KN-3210 but I come up with nothing. It looks like a pretty straight forward amp, and there must be something pretty similar that I can use. It's a tube rectified SE amp with a 12ax7 preamp. It has 3 inputs that all run into separate pots and then into the 12AX7. It has a single tone control.  
Let me know of any amp designs that I can use to Mod this into a nice little amp, or suggestions of where to find a schematic for a Knight PA.
2/25/2005 12:02 AM

I would suggest converting it to resemble the Blackface Fender Champ, using the 6L6 in place of the Champ's 6V6 shown in this drawing:  
You should be able to use the three control panel holes for the Volume, Treble, and Bass knobs. If you wanted to put in a midrange knob to use the fourth hole, you could replace the 15K resistor in the champ drawing with 5K to 10K fixed resistor in series with a 10K to 25K pot.  
Provided the unit was originally designed to use a 6L6 type, there are not many modifications to the Champ circuit which would be needed. I would suggest to add a 470ohm *screen stopper* resistor to pin 4 of the octal socket to improve reliability and help extend tube life. I would generally consider a 2watt rated 470ohm to be the minimum rating for this resistor, but 5watts are commonly used.  
Then on pin 8 there are the 470ohm 1watt cathode resistor and the 25uf/25VDC cathode bypass capacitor. These need to be upgraded for 6L6 use, to at least a 5watt and a 50VDC rating respectively. Also this is a time to consider the effect that changing the values of these components has on the overall tone of the amp. This particular 470ohms is how the bias of the tube is set, with modern 6L6 types many people are now preferring a somewhat higher resistance so the tube runs cooler and is expected to last longer before replacement is needed. Like 510ohms which is the next higher standard resistance value, or you could go to 560ohms or higher or even lower it a notch to 430ohms since each 6L6 can be quite different even when two of them are brand new from the same batch.  
So if you want to experiment or just have the capability to adjust the bias to accomodate different tubes, you may need a range of values for this component. Therefore if you are ordering parts for the modification, don't just get one 470, get a few at slightly higher ohms and at least one just a little lower. In the rectangular ceramic type resistors you can sometimes find the *non-standard* values of 450 & 500 ohms still being manufactured.  
Then once the idle current and overall operating point has been set by the cathode resistor, the cathode bypass cap (which is not an essential component but you will not get full gain without it) when it is added will boost gain across all frequencies unless the cap is below *about* 25uf. Smaller values reduce bass, although you may reach a point where an incremental reduction sounds more full and clear. So if you think the amp has too much bass (which can heavily depend on your guitar & speaker) or its a farty bass, you may like it a lot better if the bypass cap had fewer uF. For instance 10uf or 15uf seem to please more modern players more often, as well as old-timers who are not actually disappointed with the stock champ performance. If you use a polarized capacitor in this position, you need to make sure the + cap terminal connects toward pin 8 and the - terminal to ground.  
Also, while looking at cathode bypass caps, the 12AX7 has a 25uf on pin 3 that also can be reduced in value to stifle woofy bass, in this position there is no need to look for parts rated in excess of 25VDC.  
Now looking at the *filter* capacitors, on the old AA764 Champ they were 20uf/450VDC per section and were contained in the 3-section chassis-mounted aluminum can. Its much more sensible today to use individual caps, plus even the later silverface champ had more capacitance for the sections which were connected to the two highest voltage nodes, they were 40uf at the 360v & 350V points where the AA764 only has 20uf each.  
I *upgrade* further by using 47uf at each of these points, this is the closest modern standard vaue of microfarads. The 20uf is not really considered a standard value any more, the equivalent parts are now marked 22uf. All of these are OK with a 450VDC rating as long as your high voltage is lower than 450. Notice how the Champ looks like it goes no higher than 360V while in operation. Since a 6L6 was designed to handle higher voltages than a 6V6 it is likely that your amp may naturally have higher voltages than on the Champ drawing. You would still be OK but may be operating with less of an excess margin. You should be able to tell how that would compare to the original design, how much voltage are your original big filter caps rated for?  
450 or below is a good sign since these days getting replacement filter caps rated any higher, even for 500V, is much more limiting regarding suppliers and choices of uF values. Polarized electrolytic caps, once again need to pay close attention to the polarity when soldering them in.  
Then you have the power resistor which connects the first two cap sections, the 1K 1W. To use a 6L6 more reliably this would be better if it was 2W, but 5W is commonly used.  
Lastly, the champ only has a 5Y3 rectifier, if that's what you had originally OK, mainly I would just use what your chassis was originally wired for.  
There could be some more suggestions about rectifier changes depending on what you had originally, and how much higher the voltage is on your unit compared to a regular Champ.  
Also, make sure to read more than one document focusing on high-voltage safety techniques and procedures. You may need to work on this kind of apparatus more than one time, and a single fatal error in this field is absolute :(  
This technology can not be rebooted to make everything better ;)  
ps- if you can't find a schematic in less than a couple hours for an amp having this few components, you've already spent more time searching than it should require to draw your own :)  
If you are not that fast (yet) then get out the pencils & paper it looks like you need practice and the overall benefit will outweigh the time consumed even if it took 8 to 12 hours to make your own accurate drawing.
2/25/2005 9:29 PM

Thanks Mike,  
Great stuff. I think I'll just start over with the everything on this amp. New wire, caps and resistors.  
I've heard that putting in too much filtering capacitance in the power supply will result in more bass than is desired. I'm curious about the filtering of the power supply's found in these amps. I assumed that the values used for the filtering were more about cost impact to the amps than the tone quality. I guess I'd like to shape the sound with tone controls and cathode bypass caps instead of limiting the power supply capability, (but then I really don't know enough about either guitar sounds or amplifier design to say what my preferrence is). But I do have some 80-100 uf 450 volt caps that need a home, and I hate hum!
2/25/2005 8:58 PM

Use an 80 or 100uf for the first filter cap, or at least leave room for it if you want to see if you can hear the difference. I do like a 100uf for a SE6L6GC and prefer the improvement over the response of an 80uf.  
Its mainly the coupling caps (aka blocking caps) where the increase in bass is dramatic as the size is increased over the working minimum.  
With the filter caps you will get too little bass (along with too much hum) if they are too small, but once you get above the minimum size for full frequency response, then further size increases reduce sag to more of an extent without affecting bass very noticably.  
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