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Old 20th November 2004, 11:16 AM   #1
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Default help for tube noob

i want to see if i can swop the solid state pre amp in a pa amp to a tube pre amp for guitar work, but i dont know if the power supply would hold it, i cant see any markings apart from 130v 0.3a, 6.3v 3.5a, and 6.3v 0.7a

now i assume the 130v 0.3a powers the tubes

the 6.3v 3.5a goes to the tubes so is probly heater power

the 6.3v 0.7a goes through a rectifier to the s.s. pre amps, a simple pair of discrete transistors by the way, of witch there are three, mic 1, mic2, and aux.

Aux works fine for line level input from a cd player, and mic is low impedeance (600ohm) microphone.

there are 4 el84 tubes and one 6a08? poss 6aq8, cant read too well, the printing isnt to great.

do you think the power supply would hold up with another tube for the pre amp? thanks for the help so far, see you later, steve.. ..
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Old 20th November 2004, 11:36 AM   #2
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130V @ 300mA is respectible current. By use of a full-wave doubler, you can get the voltage that the tubes will be really happy with and still have lotsa current left over for the EL84 variety
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Old 20th November 2004, 11:44 AM   #3
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ok call me stupid if you want, but, full wave doubler???
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Old 20th November 2004, 11:48 AM   #4
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Quote:
Originally posted by psychosteve
ok call me stupid if you want, but, full wave doubler???

Greg means a voltage doubler something like the one below i think. Ignore the component values.
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Old 20th November 2004, 12:07 PM   #5
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why?

i know i dont know much about tube design, but if the tubes ive got work well how they are wired up,

why not use the same for another tube to use as a pre amp? is the voltage doubler aplicable for all the tubes?

and if so would i not be better off using a different transformer?

and would it improve the sound or give me more power?

sorry audiousername the pic doesnt come up, so i cant see it exept as thumb nail.
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Old 20th November 2004, 12:19 PM   #6
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psychosteve,

I'm not sure why Greg suggested a voltage doubler, apart from wanting to get more voltage?

Seriously, 130V ain't much, though it definitely can be made to work. Well, as you say, it's working now.

It seems like he was thinking of getting a higher voltage for the small-signal valves and leaving the lower voltage for the EL84s. Perhaps wait for him to post what he had in mind. I would have made some suggestions but it's past midnight here and I'm sleepy
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Old 20th November 2004, 12:22 PM   #7
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and would it be possible to use this circuit with the two 100k resistors from 240v line taken out, there by running 130v strait into the tube, or by dropping the values realy low???
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Old 20th November 2004, 12:25 PM   #8
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sorry bud didt realise it was gone midnite for you, i should pay more attention to where ppl are from, cheers for the help, see you later, steve.. ..
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Old 20th November 2004, 12:27 PM   #9
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The 100K load resistors are the plate load resistors for the 12AX7 sections. Valves are transconductance devices. A change in their grid voltage produces a change in the plate current. This change in current is developed across the plate load resistor (the 100K resistor). Lowering its value reduces gain (and usually increases distortion), removing it makes the circuit basically useless. 'night
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Old 20th November 2004, 05:30 PM   #10
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130V sounds way low for a EL84 amp. Can you take some measurements of the voltage after the rectifier or on the plates of the EL84s (being a noob PLEASE follow safety guides ) and post them for us? For guitar I would want around 300V on the EL84s. The current sounds like more than enough, You could run three or four 12AX7s in the preamp and make a real screamer.
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