Why did original 5F6-A use rectifier tube instead of diodes

Discussion in 'Amp Tech Center' started by itsGiusto, Sep 24, 2020.

  1. itsGiusto

    itsGiusto Tele-Meister

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    I don't know much about the history of electronics, but in the 5F6-A (released in 1958, i think) schematic, they're using a solid-state diode for the purposes of having a bias voltage:

    [​IMG]

    So they clearly had solid state diodes back then, and knew how to use them. So why didn't they use them in place of the rectifier tube as well? Why did it take fender a few more years (maybe roughly 1963?) until they started using diodes in things like the Twin? Why did it take until 1965-ish for Jim Marshall to come along and decide that solid-state diodes were a suitable replacement for the rectifier tubes in this style of circuit (I think he first did it in the JTM 100)? Were the 2 to 4 extra solid-state diodes necessary to replace the tube significantly more expensive back then than including a rectifier tube, extra octal socket, and transformers with dedicated rectifier heater filaments, as well as costs of drilling an extra socket hole?
     
    Last edited: Sep 24, 2020
  2. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    Not quite the same. Check out images of a tweed bassman for sale. You'll see that little greenish bluish blob to the left of a circuit board. That's a selenium rectifier, and a bunch have been replaced by now because they've failed. That guy had 40-70V going through it, unlike 300-400 of B+.

    The Selenium guys have stacked plates, with a certain voltage drop per plate. I don't know what it is as they aren't used anymore but it's something that can be looked up.
     
  3. itsGiusto

    itsGiusto Tele-Meister

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    Ah, so you're saying that because there was a dedicated bias tap, they didn't have to rectify as much voltage, which meant they could use something like a selenium rectifier? And the selenium rectifier wouldn't have worked on the high voltage?

    I still wonder, though, why Marshall would have gone on producing amps with tube rectifiers into 1965 to 1967 instead of solid state rectifiers, when the technology was clearly available. Was it cheaper?
     
  4. Paul G.

    Paul G. Friend of Leo's

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    Because it was 1958.
     
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  5. itsGiusto

    itsGiusto Tele-Meister

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    In my question text I mention that specifically there is a diode in the circuit, so I'm wondering why the inconsistency.
     
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  6. Dan_Pomykalski

    Dan_Pomykalski Tele-Meister

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    Classic Paul...
     
  7. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    That is my understanding.

    There's more than just production price when you build things too. Setting up jigs and redoing tools/chassis/diagrams/schematics/training assemblers for a parts change like that all adds up. It's completely normal to see tech lag a few years while the new thing becomes more standard.
     
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  8. Dacious

    Dacious Poster Extraordinaire

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    Jim Marshall was a drummer, not a tech. Leo used a tube rectifier in the amp he copied, and no doubt because transformer suppliers would have made them that way for stereos and consoles, TVs that's what he bought.

    Tube rectifiers are more expensive and the extra windings in the transformer make it heavier and costlier. So the moment reliable diodes became available Leo went there for the Bassman, Twin Reverb higher powered amps, so did Jim. Early 60s is when they started to appear.
     
    Last edited: Sep 24, 2020
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  9. Ronzo

    Ronzo Tele-Holic

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    Selenium rectifiers kept the TV repairmen in my childhood financially afloat. I was told by my Mom not to talk to the TV guy anymore when I asked him why he charged us for four of them when there were only two in the TV’s chassis.
     
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  10. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    My guess...
    The amp companies would have to get their marketing guys to "sell" the *no rectifier tube* concept. The selenium rectifiers were large and they were also stinky when they failed. Silicon diodes were just coming on the scene.

    As transistor radios and stereo devices became more popular, I would think it wouldn't be so difficult to sell SS rectifiers in the guitar amps.
     
  11. Ronzo

    Ronzo Tele-Holic

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    Sorry. I’m sure I probably missed it. But where do you see a diode in the schematic you posted?

    EDIT: Never mind, I spotted it.
     
  12. Javier668

    Javier668 Tele-Meister

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    Cause tube rect is bullet proof and cool
     
  13. elpico

    elpico Tele-Afflicted

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    Without a time machine we'll never know for sure, but in addition to whatever financial and reliability factors were at play, you know they sound different right? Maybe it took them a while to get used the sound of the solid state rectified amps. Maybe they had huge cases of tube diodes on the shelf they needed to use up. Maybe they just didn't feel like experimenting or changing the design when it was already selling so well. You'll never know.
     
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  14. Esquier

    Esquier TDPRI Member

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    I asked Electrician buddy what those big ugly things inside a panel at "the Plant" were and he told me they were Selenium rectifiers. Huge things and now obsolete compared to what is used nowadays. Maybe there are still gargantuan ones at a dam...
     
  15. ThermionicScott

    ThermionicScott Poster Extraordinaire

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    The usual story is that Leo Fender was waiting to find out how reliable the new silicon diodes would be before installing them in his bread-and-butter amplifiers. Even when he did start using them, he used extra ones to spread out the load, just to be on the safe side.

    He might have kept using tube rectifiers, which were a known quantity, but the late 1950s high-powered tweed Twin was too much amp for even the tough 5AR4 tube, so he had to do something.
     
  16. wabashslim

    wabashslim Tele-Afflicted

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    Yes they were. When I was a young teen one of those blew up in my bedroom, along with a big filter can, and I swear I still occasionally got whiffs of it during my remaining 5 years in that house. Imaginary?
    The stain on the nearby wall always bled through the paint too.
     
  17. NTC

    NTC Tele-Meister

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    Someone mentioned TV's. I remember seeing the selenium rectifiers from old tv's and they were huge. The tube would have taken less space than two of those. They also had rather high losses. High voltsge and current silcon diodes that were reliable were not available yet. The tube rectifer plus transformer with 5V winding were probably still cheaper at production time and definitely lasted longer. Fewer returns = higher profits.
     
  18. BobbyZ

    BobbyZ Doctor of Teleocity

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    It's interesting to note that even after CBS took over Fender in 65, when good diodes where available, they still used tube rectifiers in many amps. Even in the Bandmaster Reverb head, the BF Bandmasters didn't use a tube.
    They could've eliminated tube rectifiers anytime, and saved a few bucks.
     
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  19. itsGiusto

    itsGiusto Tele-Meister

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    Yeah, that's true. I have a 72 PR with a tube rectifier.
     
  20. Jon Snell

    Jon Snell Tele-Holic

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    Valve rectifiers were and still are, useful because their characteristics lend themselves to a soft start device when warming up, giving the output valves a change to get to emission before there is full voltage and power available, removing the high current inrush.

    Silicon diodes back in the 60s and 70s were limited to the reverse voltage limits of around 900volts peak, BY127, which equates to around 350volts AC, many amplifiers had two diodes in series, to cope with the voltages.
    The current was limiting as well at around 800mA peak so there is a switch on surge that mostly exceeds that value. That makes them unreliable compared to the 1N5408 and 1N4007 range of 1000volt diodes.

    Selenium has many drawbacks.
    Expensive to manufacture, high forward voltage of approximately 15volts per section with a current limitation of 50mA per square centimetre and there must be many bulky sections for a high working voltage and last but not least highly toxic!

    Valves are and will always be a classic component. Semiconductor diodes are efficient but have no charisma.
     
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