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Step 8Fixing the half-wave rectifier problem
But wait--the 35W4 tube is only a single diode , so the rectification is half-wave , rather than full-wave.
Is that bad?
Well, yes. As the name implies, half-wave rectification only uses one half of the AC waveform, and blocks the other half. Power transformers are really designed to be symmetrically loaded. The flux field collapses as one peak falls, and the transformer expects an equal load--and an equal amount of magnetic force from the complementary peak. Without a load on half the cycle, the collapse of the field causes the transformer core to become saturated much more quickly than normal. That puts a "standing" DC voltage on the transformer. The N-68X, being a small transformer, isn't designed to handle this.
Half-wave rectification isn't quite as much a big deal on your household "mains." The current draw is small compared to the available current. The resulting asymmetry only changes the total waveform fractionally. But even that could be enough to create noise in other devices...
When I first installed it, I tried to use the N-68X with the circuit, as-is. But it immediately became obvious that the transformer became too hot, considering a current draw less than 30 watts.
Solving the problem
The best solution is to rectify twice -- once with a solid-state bridge rectifier to shift the negative voltage over to positive; then rectify again with the 35W4 tube. That will eliminate our asymmetry, since there will no longer be any negative voltages for the tube rectifier to block.
See the fifth illustration for this "combination" technique... Note that the output of the combination is full-wave , despite passing through a single diode rectifier after the bridge. So there's more current potential for the amp circuitry than before. Plus it's probably quieter, too.
And note that the peak voltages of the tube rectifier (diode) are lower than the solid-state bridge. Note also that half-wave rectification need not be done with a tube diode--a solid-state diode functions just as "well" for this application.
Where to insert the SS bridge
There are two good options:
Option A ) between the isolation transformer and the entire amp circuit. Since rectified AC (pulse DC) holds the same potential as regular RMS AC, the total voltage doesn't change.
If the filaments were fed solid-state rectified and filtered DC the voltage would be too high, because the total voltage would approach the peak voltage, rather than being an average. And the filaments would fail. However, the filtering caps come after the tube rectifier, so that's not a problem.
In addition, the SS rectifier could be mounted back on the iso module. Since I didn't do that initially, I placed it on the chassis.
Option B ) after the filaments, and feed the tube rectifier only (only the DC parts of the amp cause asymmetry.) This would work fine. But it also requires a bit more rewiring.
I chose the first option...
Why include the tube rectifier at all?
The bridge produces all the rectified current the amp needs...why keep the 35W4?
-- Leaving the 35W4 will keep the peak DC voltages at a lower level than the more efficient SS bridge by itself. The 50C5 power tube wasn't designed for plate voltages much higher 120V. Since AC peak voltage is higher than it's RMS value, rectification circuits tend to output a higher DC voltage (theoretically 1.414 times higher than the RMS.) But as stated previously, tube diodes are less efficient.
-- All the tube filaments are still connected in series, so removing the 35W4 would have created a new problem--how to drop the voltage on the series string of filaments (the remaining two tubes) by an additional 35V.
Leaving the 35W4 tube in place solves both these issues.
Necessity
Is all this absolutely necessary? Well, with a large enough Isolation transformer, maybe not.
A 100 or 150VA rated transformer could safely deal with halfwave issues for a <50 watt amp, I'd say.
Is that bad?
Well, yes. As the name implies, half-wave rectification only uses one half of the AC waveform, and blocks the other half. Power transformers are really designed to be symmetrically loaded. The flux field collapses as one peak falls, and the transformer expects an equal load--and an equal amount of magnetic force from the complementary peak. Without a load on half the cycle, the collapse of the field causes the transformer core to become saturated much more quickly than normal. That puts a "standing" DC voltage on the transformer. The N-68X, being a small transformer, isn't designed to handle this.
Half-wave rectification isn't quite as much a big deal on your household "mains." The current draw is small compared to the available current. The resulting asymmetry only changes the total waveform fractionally. But even that could be enough to create noise in other devices...
When I first installed it, I tried to use the N-68X with the circuit, as-is. But it immediately became obvious that the transformer became too hot, considering a current draw less than 30 watts.
Solving the problem
The best solution is to rectify twice -- once with a solid-state bridge rectifier to shift the negative voltage over to positive; then rectify again with the 35W4 tube. That will eliminate our asymmetry, since there will no longer be any negative voltages for the tube rectifier to block.
See the fifth illustration for this "combination" technique... Note that the output of the combination is full-wave , despite passing through a single diode rectifier after the bridge. So there's more current potential for the amp circuitry than before. Plus it's probably quieter, too.
And note that the peak voltages of the tube rectifier (diode) are lower than the solid-state bridge. Note also that half-wave rectification need not be done with a tube diode--a solid-state diode functions just as "well" for this application.
Where to insert the SS bridge
There are two good options:
Option A ) between the isolation transformer and the entire amp circuit. Since rectified AC (pulse DC) holds the same potential as regular RMS AC, the total voltage doesn't change.
If the filaments were fed solid-state rectified and filtered DC the voltage would be too high, because the total voltage would approach the peak voltage, rather than being an average. And the filaments would fail. However, the filtering caps come after the tube rectifier, so that's not a problem.
In addition, the SS rectifier could be mounted back on the iso module. Since I didn't do that initially, I placed it on the chassis.
Option B ) after the filaments, and feed the tube rectifier only (only the DC parts of the amp cause asymmetry.) This would work fine. But it also requires a bit more rewiring.
I chose the first option...
Why include the tube rectifier at all?
The bridge produces all the rectified current the amp needs...why keep the 35W4?
-- Leaving the 35W4 will keep the peak DC voltages at a lower level than the more efficient SS bridge by itself. The 50C5 power tube wasn't designed for plate voltages much higher 120V. Since AC peak voltage is higher than it's RMS value, rectification circuits tend to output a higher DC voltage (theoretically 1.414 times higher than the RMS.) But as stated previously, tube diodes are less efficient.
-- All the tube filaments are still connected in series, so removing the 35W4 would have created a new problem--how to drop the voltage on the series string of filaments (the remaining two tubes) by an additional 35V.
Leaving the 35W4 tube in place solves both these issues.
Necessity
Is all this absolutely necessary? Well, with a large enough Isolation transformer, maybe not.
A 100 or 150VA rated transformer could safely deal with halfwave issues for a <50 watt amp, I'd say.
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Just make sure the new ground comes from the bridge, not from the old wiring setup, which is the "neutral" side of the mains plug...
This is an 8 amp, 400V version. Eight amps is overkill, though. Half that (4 amp) would be more than enough.
(Radio Shack has the equivalent, although you can find them cheaper elsewhere...)
I've got several Teisco-ish guitars: Norma, Kingston, Kimberly, Maxitone, Sekova. Fantastically odd guitars! No amps, though. That really looks nice.