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Question about Hybrid Tube/SS Bridge Rectifier
#1
I have some power transformers that are made specifically for full wave bridge rectifiers. In some cases, I may want to use a 5V filament transformer and hybrid tube/ss diode bridge rectifier. I was wondering what to expect as far as a sag and voltage drop is concerned. Will it drop as much as voltage as the tube alone?  How would I approach calculating it? 

Thank for the help!
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#2
Hi Guys

This seems like a multip-part question, or maybe the actual Q is not defined? It would be helpful to have specifics about the transformers in question, and the circuit goal.

As far as mixing tube and solid-state rectification goes, TUT2 showed some example circuits. Dual-rectifier tubes form a half bridge that can take in AC and produce the positive end of a DC output. You can use solid-state diodes to form the negative output side of the bridge.

An alternative is to use solid-state for the main bridge, then use the dual rectifier in series with the positive output. A 3-way switch can allow for straight-SS, one tube rectifier, or 2-tube diodes in parallel. This gives you highest-voltage, most saggy and medium sag outputs, respectively.

The ultimate regulation and thus sag is determined by the PT, with contributions by the tube rectifiers.

You can generate a low-voltage from a high-voltage, or a high-voltage from a lower voltage, by using an auxiliary PT to do that directly from the mains, or as a back-to-back connection from the main PT. I do this in my preamps and hybrid PAs. If the main-PT sags under load, the cascaded PT outputs will also sag, which you can either tolerate and make useful, or accommodate by regulating the down-stream DC outputs.
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#3
Thanks for the reply. It's helpful. The question was pretty nebulous. I was planning on trying to use a single tube rectifier with a pair of diodes for negative half of the bridge.

I build quite a few of low wattage Tweed style amps which usually have a tube rectifier with a 315-0-315 high voltage secondary. I happen to have some power transformers of basically the same VA with 300V secondary for a bridge rectifier that I thought I'd try they hybrid rectifier with. I was just pondering what the differences in sound would be between the two setups. Probably the acid test would be to stop wondering ad build one!
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#4
Hello Makinrose

I do not believe there would be a difference in sound between the two power supplies as they both employ a valve rectifier.

The traditional design with a centre-tapped winding on the high-voltage winding has the standard dual-rectifier . The out is taken alternately from one side of the centre-tap through a single diode, so the loss and sag effect is related to a single tube rectifier.

In a bridge, were it all valves, there would be two valves in series and thus twice the sag effect and much lower output voltage. Replacing the rectifiers on the negative side of the bridge with solid-state diodes bring it back to a single tube rectifier sag and voltage loss. The addition of the solid-state diode in series with the tube adds insignificant voltage loss or sag effect. The wonder might be if the harder turn-on of the solid-state diode might alter the tone?

If one considers that there is a generously sized filter capacitor following the rectifier, it could be assumed there would be little impact for the hybrid rectification compared to pure solid-state or even pure tube. Also, as most members of this forum have a copy of one or more of the TUT volumes, we are likely to add that small series resistor between the rectifier and first filter cap that Mo. O'Connor suggests, as this tiny component adds multiple benefits, one of which is to filter the possible solid-state turn-on noise.

Do you find there to be a true sonic difference between the same amplifier circuit powered from a valve-rectified supply or one with semiconductors? Most players do not hear a difference, especially when invoking distortion. My impression is that most players believe they should hear a difference and claim that they do so they do not appear silly if others present claim to hear a difference.

Valve rectifiers are quite a liability in a working musician's amplifier, especially if it a combination amp with speaker. As KOC mentions, you can have the sound of the valve rectifier without having to suffer its mechanical detriment to reliability. It is embarrassing when your amp goes silent because the valve rectifier vibrated out of its socket; worse if it breaks and there is no replacement valve or amp.
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#5
(08-20-2023, 12:32 PM)Sherlok Ohms Wrote: Hello Makinrose

I do not believe there would be a difference in sound between the two power supplies as they both employ a valve rectifier.

The traditional design with a centre-tapped winding on the high-voltage winding has the standard dual-rectifier . The out is taken alternately from one side of the centre-tap through a single diode, so the loss and sag effect is related to a single tube rectifier.

In a bridge, were it all valves, there would be two valves in series and thus twice the sag effect and much lower output voltage. Replacing the rectifiers on the negative side of the bridge with solid-state diodes bring it back to a single tube rectifier sag and voltage loss. The addition of the solid-state diode in series with the tube adds insignificant voltage loss or sag effect. The wonder might be if the harder turn-on of the solid-state diode might alter the tone?

If one considers that there is a generously sized filter capacitor following the rectifier, it could be assumed there would be little impact for the hybrid rectification compared to pure solid-state or even pure tube. Also, as most members of this forum have a copy of one or more of the TUT volumes, we are likely to add that small series resistor between the rectifier and first filter cap that Mo. O'Connor suggests, as this tiny component adds multiple benefits, one of which is to filter the possible solid-state turn-on noise.

Do you find there to be a true sonic difference between the same amplifier circuit powered from a valve-rectified supply or one with semiconductors? Most players do not hear a difference, especially when invoking distortion. My impression is that most players believe they should hear a difference and claim that they do so they do not appear silly if others present claim to hear a difference.

Valve rectifiers are quite a liability in a working musician's amplifier, especially if it a combination amp with speaker. As KOC mentions, you can have the sound of the valve rectifier without having to suffer its mechanical  detriment to reliability. It is embarrassing when your amp goes silent because the valve rectifier vibrated out of its socket; worse if it breaks and there is no replacement valve or amp.
I'm not sure that's entirely accurate.  Dual rectifier tubes were used in some equipment to increase the current capability and reduce the sag. It's a common thing to pull one of the rectifier tubes from Fender's Low Power Twin to increase the sag effect.  In my experience sag or compression is most important in amp that don't have much in the way of overdrive in the pre-amp.  The smoothing effect is important to the feel of the amp for many players.  That is not to say that the sag needs to come from tubes but there is definitely a difference in feel with an amp under relatively clean or semi clean tones with and without some sort of means to achieve sag and without.
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#6
Hi Guys

Post-2 paragrapph-3 describes the use of parallel and nonparallel tube rectifers and the resulting sag produced.

In a way, saying that the parallel rectifiers "allows more current" is correct but also a bit misleading. The current has to be available from the PT to begin with, so what is really happening with parallel tube rectifiers is a reduction of the net diode resistance and thus a reduction of the sag. It is an attempt to overcome the imperfection off the device itself; current is increased for any given amount of sag compared to the use of a single rectifier.

It should also be noted that Fender was still following hifi rules and a hifi ethos for its amp designs up until the 1970s. Eliminating or reducing sag by adding a second expensive rectifier tube falls into that ethos. That players would remove one rectifier tube shows the difference of attitude between the designer and the user Smile
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#7
(08-24-2023, 08:59 AM)K O'Connor Wrote: Hi Guys

Post-2 paragrapph-3 describes the use of parallel and nonparallel tube rectifers and the resulting sag produced.

In a way, saying that the parallel rectifiers "allows more current" is correct but also a bit misleading. The current has to be available from the PT to begin with, so what is really happening with parallel tube rectifiers is a reduction of the net diode resistance and thus a reduction of the sag. It is an attempt to overcome the imperfection off the device itself; current is increased for any given amount of sag compared to the use of a single rectifier.

It should also be noted that Fender was still following hifi rules and a hifi ethos for its amp designs up until the 1970s. Eliminating or reducing sag by adding a second expensive rectifier tube falls into that ethos. That players would remove one rectifier tube shows the difference of attitude between the designer and the user Smile

I did leave out the critical important point about the power transformer. Thanks for clarifying.

   I've seen the dual tube rectifier setup on a few old Fenders but mostly in old hi-fi and in some organs.  

Just an aside:  The only original Low Power Tweed Twin I've worked on once belonged to a guitarist in Waylon Jenning's band and someone had removed the tube rectifiers and added SS diodes---I'm guessing to make it more reliable on the road. It raised the voltage quite a bit though.   I ended up reducing the voltage to the correct levels with some zener diodes and adding a sag resistor---basically what was in TUT2.  It worked out really well.
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