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I’m looking to build a vacuum-tube based linear amplifier. I’d like to do it all tube based, including the power supply. The original ARRL design I’m looking at (from the 60s) has two paralleled 807s delivering 150w, with 700v on the plates at 200ma. I was thinking perhaps I could double the output power to 300w by paralleling up four 807s. I guess that would draw twice the current.

The original power supply has a pair of 6DE4s for rectification, feeding a 5U4G. Assuming the transformer can handle the current draw, could I beef up the power supply to handle the four 807s by having 2 paralleled pairs of 6ED4s feeding a paralleled pair of 5U4Gs?

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closed as unclear what you're asking by Phil Frost - W8II, Marcus Müller, Scott Earle Mar 26 '18 at 3:37

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    $\begingroup$ I'm afraid a schematic would be worth a million words here. Also, this is not really the optimal place, I'd argue that electronics.stackexchange.com is better suited to these non-radio-related design questions. Also, why of all things, build a power supply out of tubes? Heating with electricity isn't exactly cheap! $\endgroup$ – Marcus Müller Mar 11 '18 at 11:04
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    $\begingroup$ (you can argue for tube amplifiers for audio for acoustic-aesthetical reasons, or on the extreme end of the power spectrum of audio amplifiers; you can argue for tube amps for megawatts of microwave generation; but you can't really argue for tube amps for PSUs on a technical level. They suck. The first thing the semiconductor diode replaced was tube rectifiers, there were kits to simply replace a diode rectifier with a semiconductor diode, by leaving the heater unconnected.) $\endgroup$ – Marcus Müller Mar 11 '18 at 12:01
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    $\begingroup$ Well it kind of is a radio related question, an rf amp power supply. Point taken about the characteristics of a tube based power supply. I just want to construct something entirely out of valves. For no other reason than I can. Solid state is probably more practical now, but darn it I just love tubes! $\endgroup$ – user2555403 Mar 11 '18 at 12:40
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    $\begingroup$ Without a schematic to add some specificity, this question reduces to "can I beef up a vacuum tube PSU by adding more tubes" and the answer is yes. Though as Marcus says, you may also need to upgrade the electrical service to your home. $\endgroup$ – Phil Frost - W8II Mar 11 '18 at 13:48
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    $\begingroup$ Adding more tubes in parallel might make it unstable, especially on the higher bands. Even if it did not, you would certainly need to change the values of some components, especially the RF tank circuit because the plate impedance would be half with 4 tubes vs. 2 tubes. Also, as Marcus and Phil correctly point out, we need a schematic. Without more detail, this question is in danger of being closed as unclear what you're asking. $\endgroup$ – Mike Waters Mar 11 '18 at 19:47
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I would encourage you to reconsider this project as it not simply a matter of paralleling all of the tubes to double the output power.

When you parallel more finals (807's in this case) the input and output impedance of the stage will decrease. This will require a redesign of the output circuit of the final stage at a minimum. If it has input filtering/matching circuit, which many old designs did not, this will require redesign as well.

You mention:

The original power supply has a pair of 6DE4s for rectification, feeding a 5U4G

This seems quite unlikely as the 6DE4 'TV damper' tubes are normally used for rectification in this application. I would infer that two of them indicate a full wave rectifier with a center tapped transformer. A 5U4G is a single tube, full wave rectifier. Why would one tube be cascaded into the other except in the case of a voltage doubler circuit?

You must also consider the filter inductors and capacitors must be increased in order to maintain the original ripple current. The filament transformers will also need to be doubled or have increased current ratings in order to support the additional tube filaments.

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  • $\begingroup$ Yup. More likely, they were in separate power supplies; perhaps bias, screen, and plate supply. $\endgroup$ – Mike Waters Mar 12 '18 at 2:50

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