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Old 17th November 2005, 08:45 PM   #11
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I wish to do the same, can I use a 12AT7 and use one half as a preamp and the other half as the power amp? Would love to have a schematic...
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Old 25th November 2005, 04:22 PM   #12
scudinc is offline scudinc  United States
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Wouldn't a 12DW7 be better for that application?
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Old 30th November 2005, 06:49 AM   #13
Fuling is offline Fuling  Sweden
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Sorry, no schematic.
This was something I just threw together a couple of years ago.
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Old 19th December 2005, 01:51 PM   #14
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another thing you can try if your curious is grab some of the old 12v tubes
12k5 and 12al8 come to mind there.
they are tubes used back in the day designed for use in car radios if I remember right and most are designed to be run on 12v for everything from the heater to plate voltage and most also are able to handle a max of around 30v.

try going to www.sopht.ca tog et some more information or check in the internet for "space charge" tubes
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Old 19th December 2005, 03:28 PM   #15
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so with the 12DW7 you use the medium mu section as your preamp and the high mu for your output? or I guess it could be used both ways..


If I Can get ahold of one of those I'll try it!

I used my 12at7 and got the desired effect by changing the plate resistors and bias resistors. I used the schematic from a fender amp for the preamp section, it used a 12ax7 but it worked fine with the 12at7. I used a 220k resistor on the pre section and a 100k on the post section. Bias resistors were 1k for the first section and 1.5k for the second, with different capacitors. I didn't get as much gain as I would with a 12ax7 because the 12at7 doesnt have as much, but it worked fine for me and was more than capable of driving the 50c5 to clipping. One thing remains, I still have a little bit of power hum ( no buzz, its a hum)
What could be causing this? I have a rather large filter cap (330uf 200v)
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Old 27th December 2005, 03:37 PM   #16
testlab is offline testlab  United States
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You can clone the Supro Bantam. It uses a 12AX7A and a 50L6. The original schematic shows the filaments wired in series and the plates operate with the line voltage directly rectified(no transformer!). I would recommend that you put a 1:1 transformer on the power input.

If you want, you can use a 12L6 and put in a small filament transformer. The beauty of this amp is that the output tubes are really cheap, 2 to 3 bucks a piece. If you go to a hamfest you can probably get them even cheaper than that.

A schematic can be found in the Aspen Pittman guitar amp book.
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Old 27th December 2005, 04:59 PM   #17
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I think Im going to try a 12c5 instead, I wont need an isolation transformer because Im using a voltage multiplier to turn 24vac @ 2 amp into 180vdc @ 200ma. I also created a schematic for using an EL86/6cw5 instead for slightly more power. You can see it here : quick 50c5 to play around with , and mockup pics are on the next page.
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Old 27th December 2005, 05:33 PM   #18
testlab is offline testlab  United States
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No transformer does raise your risk of electrocution. You can pick up a 25 or 50VA for under $20.00. Please be safe.
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Old 27th December 2005, 10:06 PM   #19
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I said I'm going to use a 24 volt 2 amp transformer and use a 6 stage voltage multiplier to get it to around 180v at only 200 ma or so (rounding up). I have already shocked myself a few times while working on it, theres not enough current to do anything its mostly just an uncomfortable zap. I purposly designed the power supply this way for one so I wasn't wasting power and for two so I don't melt the plates on the tube during testing if something goes wrong.

I plan on using a 24vac wall wart to power my finished project
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Old 27th December 2005, 11:21 PM   #20
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Sorry, I misunderstood. Good luck.
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