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Amp Stuff => EZ Board postings => Topic started by: Geezer on June 17, 2005, 10:08:45 pm
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Hoffman Amplifiers
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> How do you drop down screen voltage ??
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trescat
Junior tube assistant
Posts: 13
(3/9/04 4:30 am)
How do you drop down screen voltage ??
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At present the screen voltage is around 12 volts higher than the plate voltage, I am just a bit concerned that it is too high, the plate is 405 VDC and the screen is 417 VDC, on a 6v6 tube, are these voltages too high for the 6v6 ? and isn't it best to have the screen set at a lower voltage than the plate ?? I already have a 470 ohm screen grid resistor installed ???Thanks in advance
Trevor
Tiny Daddy
I will work on all amps
Posts: 540
(3/9/04 8:03 am)
Re: How do you drop down screen voltage ??
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What circuit are you using? Usually the screen gets voltage at a point farther down on the power supply divider, on the second set of filter caps.
6bq5
Junior tube assistant
Posts: 14
(3/9/04 1:19 pm)
how do you drop screen voltage
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hello there, it's not unusual for the screens to be a bit higher than the plates---it depends on the resistance on either pri. leg of the output transformer and of course the screens draw a lot less current than the plates do. i don't think i've seen quite the difference you have, but it's not uncommon for there to be 2-5 volts difference.
rh
trescat
Junior tube assistant
Posts: 14
(3/9/04 10:16 pm)
how do you drop screen voltage
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Tiny Daddy,
Its a princeton 5F2 circuit
Cheers
Trevor
Tiny Daddy
I will work on all amps
Posts: 542
(3/9/04 10:33 pm)
Re: how do you drop screen voltage
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There seems to be too much voltage available for the 5F2 circuit. Are you using a tube rectifier? I would modify the power supply divider network by adding resistance in series with the choke and add a resistor to ground across that last filter cap so that the screen gets around 300 Volts.
eddiejg
Senior tube assistant
Posts: 245
(3/9/04 10:33 pm)
how do you drop screen voltage
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Trevor-
On the 5F2, it is normal to have the higher screen voltage.
If you look at the schematic for the 5F2, it is set up so the screen gets the b+ from right after the choke & the plate gets it from the other lead on the OT.
If you look at the 5F2A, you will see that Fender made some small changes/improvements & one of those changes was to move the screen B+ after a 10K dropping resistor.
You might want to make this modification to your amp.
-Ed
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