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Re: Schematic for low watt amp.


 :
5/11/1998 1:49 AM
M. Helin
Re: Schematic for low watt amp.
 
If you want _really_ low power, I just sent the boss a gif of the 'Moonlight' amp - designed specially for apartment dwellers.:-) Approx 1 watt - 6SL7 P-I driving 6SN7 in p-pull.
 
 
Stephen,  
Is it possible to use 12AT7 in Moonlight in the place of 6SN7 push-pull pair (with the same component values, just as in the schematics of Moonlight, with the same OT)?
 
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5/12/1998 4:46 AM
stephen delft

Is it possible to use 12AT7 in Moonlight in the place of 6SN7 push-pull pair (with the same  
component values, just as in the schematics of Moonlight, with the same OT)?
 
 
You might be able to use a 12AT7 with the same OT, but it won't be an optimum match. If you want to use small 9-pin tubes, then a 5687 would probably be a better choice. At a guess, you'd get about 2 watts into a 6-8k load, from around 270v B+ and about the same 22mA idle current as the 6SN7. Try around 500 Ohms cathode resistor. You may also need to reduce the grid resistors to 100k each to stop the o/p tube running red hot and unstable when driven hard.  
 
## Note that 5687 has different pin connections from all the 12AX/AT/AU7 series. It also has higher heater current, and the glass gets hotter. NOS are still relatively cheap and available. Around $4.oo or so, depending on where and when. Try Angela, Triode,Antique El. .  
 
You could drive it with a 12AX7 and much the same PI resistor values as my schem. Try as shown, or adjust the 470 Ohm for about 1mA per plate.  
 
I don't know what it would sound like. Maybe the same...maybe different. Some of those old octal tubes seem to do something nice to the sound.  
 
Try it any way you want, and report back....  
Cheers, Stephen
 
5/13/1998 2:36 AM
M. Helin

Thanks, I checked the data sheets and found that 12AU7 is close enough to the 6SN7 and much easier to find here in Europe than a 5687. I'll measure the cathode current and adjust the resistor value(s) if needed.  
 
Mikko
 
5/15/1998 10:38 AM
Doc

Stephen:  
 
I just found your Moonlight Amp schematic posted. I like it. I've always wanted to try a 6SN7 p-p output stage, but never got around to actually building it. I'm wondering if you put yours together with a 15k output transformer like what's shown. Did you notice a power increase when connecting an 8ohm speaker to the 4ohm tap, rather than the 8ohm tap?  
 
Reason I'm curious....Plate resistance is around 7k for a 6SN7 triode with those voltages. Optimum load would be twice that, for each tube section, or 28k plate-to-plate. Connecting an 8ohm speaker at the 4ohm tap would be in that range.  
 
Doc
 
5/17/1998 4:39 AM
stephen delft

Doc, Thanks for the kind words. Do you have the latest version of this schem? Tony kindly did some tidying up of my original, and I added some notes.The latest gif includes text about alternative preamp tubes, and is called "moonli_2.gif"  
Should be up on Steve's site now, or soon.  
 
You said "Plate resistance is around 7k for a 6SN7 with those voltages. Optimum load would be twice that for each tube section, or 28k  
plate-to-plate."  
I may be misunderstanding you here...I checked RDH p577 "...load resistance from plate to plate.... ...twice the plate resistance of one of the valves...."  
- Which for this 6SN7 would be about twice 7.5k = 15k??  
Actually on this occasion I worked out the 'best' p-p load with a guitar, a good 12inch speaker, a scope, and my ears. I think it sounds best with  
a lower plate load than this (less power but more rounded clipping)  
Looks like it's flying against good theory, but the situation is complicated by speaker impedance not being a steady resistive 8 ohms.  
I may be inviting the dreaded 'excess peak plate current' but my SN7 doesn't get substantially hotter when cranked than it does when idling, so I went with what sounded good.  
Alternatively, around 13k p-p gives nearly as smooth clipping and a higher output of about 1 watt, which was the original target. So I have put an extra tap on the Moonlight OT, giving 13k from an 8 ohm load - people can choose which they prefer.  
 
Triodes are quite load-tolerant. 15-16k p-p still works ok (16 ohm on 8 ohm tap of an 8k p-p tranny) but it's getting further from the warm sound that I was aiming for. Someone else might prefer it this way. You are welcome to try any load you like. Let me know what _you_ think sounds best.  
Cheers, Stephen
 
5/11/1998 11:46 AM
Bill Harbour

Contact Bruce Collins (kb0tj@aol.com). I just finished building his design the SoulKicker which has 1 12AX7 and 1 6V6. It's similar to the Champ. It's a single ended ClassA amp. It was my first amp project and it worked the first time. Bruce has excellent schematic and layout drawings. He's just about to produce the amp in kit form. He was also very supportive in answering my every question.  
 
My amp probably puts out about 6 watts and into 2 10" Fender Gold speakers in a closed back cabnet sounds huge. It's my best sounding amp and I built it!  
 
Good luck...tell him Bill Harbour sent you.  
 
Bill
 

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