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Old 21st May 2006, 07:45 PM   #11
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cogsncogs's Avatar
Join Date: Jul 2003
Location: Wayne, West Virginia
so this tube doesn't have enough gain for a guitar?
As you may know guitar amps have a pre-amp stage as you are dealing with a low-level signal. You will need one more gain stage. One single (section) tube, no matter how configured, whether pentode or triode will have sufficent gain for your purpose.
Using both sections of a 12AX7 with a gain of 40x each section = 1600 which should be more than enough for some 'overdrive'.

But all of those tubes are intended for a much higher supply voltage.
Hmmm, 'cept for the 50C5, Max ->150V plate, 130V screen. They were used in AC operated table radios.

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Old 21st May 2006, 08:03 PM   #12
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Join Date: May 2005
Location: USA
Blog Entries: 7
Originally posted by ThSpeakerDude88
a 50c5 right now..

how do I apply constant current sourcing to tubes?

I tried a 12au6 today instead of the 14gt8, it has enough gain to start to push the output tube, but still not enough drive. Only when I use an overdrive pedal , or booster which only boosts the volume, can I get it to break up nicley. Otherwise its not loud enough to distract a loud conversation.. much. It also stays mostly clean which I'm not quite sure about... I've never heard the real silvertone so I don't know how its supposed to sound but shouldn't it have enough gain to push the output tube to break up? I'm using the basic parts of the silvertone 1430 circuit ( not line operated of cource, with proper traqnsformer, and I have a seperate power supply for the filiments. )
Go Here for a preliminary design of a 50C5 amp I did. Pay attention to the first stage, which uses CCS loading over passive, resistance loading. In this case, as with yours, the main problem is getting enough current to drive the 50C5's Miller, input, and stray capacitance. Passive resistance loading with Rp= 100K, and a Vpp= 120Vdc isn't going to get the job done by any means. A CCS will allow for more driver plate current, and enough current to meet the "Rule of Five" for the 50C5 grids.
There are no foxes in atheistholes
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Old 21st May 2006, 11:20 PM   #13
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Location: Wayne, West Virginia
Miles is correct, I was thinking gain only. 120V for the driver stage won't cut it using a resistor as the plate load. A CCS plate load (or a much higher B+) will solve that problem. But still it would be good idea to have a single stage pre with possibly a volume control between the the pre and the driver, i.e. the other half of the 12AX7.

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