• Tube mono-block amp with SIX 6L6 outut tubes

    From bitrex@21:1/5 to Chris on Fri Dec 28 20:18:07 2018
    XPost: sci.electronics.design

    On 12/28/2018 06:32 PM, Chris wrote:
    On 12/28/18 03:11, tubeguy@myshop.com wrote:
    I found a schematic of a stereo amplifier containing TWELVE 6L6 audio
    output tubes. But aside from sharing the power supply, it's two
    mono-blocks with each having SIX 6L6 output tubes. Unfortunately, I was
    not able to get any real detailed information on this.

    I compared it to several guitar amp schematics which use FOUR 6L6 tubes
    in push-pull parallel, and this is very similar. It just has 3 tubes on
    each side of the phase, rather than two. The plates are all wired
    together and cathode / grid components are duplicates to each tube.

    They did use as bias adjust potentiometer on each tube, which I have not
    seen on the guitar amps.

    Obviously the purpose is to increase wattage output.

    I'm not intending on building this, but it's interesting and makes me
    wonder if someone could use EIGHT 6L6 tubes, or TEN?
    (Of course adding more tubes mean bigger audio output transformers and
    heftier power supplies.

    Another question that comes to mind is the impedience of the primary on
    the audio output transformer. Assuming I could find a transformer that
    would handle the wattage, would the impedence be the same as those used
    with FOUR 6L6 tubes, or would that change since there are two more tube
    plates connected in series.

    Anyhow, looking at this schematic makes me think that any PPP amp with
    FOUR tubes could have more output tubes added, as long as power supply
    current is available and a suitable audio output transformer is
    obtainable.




    If you want to see an interesting audio amp, look up the Editors and Engineers Radio Handbook from the mid 60's. There's a design that
    which uses 6 or 8 6080 double triodes to build a direct coupled
    amplifier, no output transformer and dual power rails, positive and
    negative. Never built built it here, but a very original design. Could probably update that to use power mosfets...

    the two big issues with OTL tube amps is the intrinsic impedance
    mismatch of a tube cathode vs a tube plate in a totem pole arrangement
    looking into the load, and related lack of any intrinsic power supply
    noise rejection in the output stage the way a push-pull
    transformer-coupled output stage has when both halves have similar
    output impedances working into the same reflected load.

    Global negative feedback can't do anything about the second and there
    isn't usually enough open-loop gain available to do a good job of
    correcting for distortion caused by the first across the audio band.
    often leading to a kinda poor-performing amp.

    --- SoupGate-Win32 v1.05
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  • From bitrex@21:1/5 to bitrex on Fri Dec 28 20:20:10 2018
    XPost: sci.electronics.design

    On 12/28/2018 08:18 PM, bitrex wrote:

    If you want to see an interesting audio amp, look up the Editors and
    Engineers Radio Handbook from the mid 60's. There's a design that
    which uses 6 or 8 6080 double triodes to build a direct coupled
    amplifier, no output transformer and dual power rails, positive and
    negative. Never built built it here, but a very original design. Could
    probably update that to use power mosfets...

    the two big issues with OTL tube amps is the intrinsic impedance
    mismatch of a tube cathode vs a tube plate in a totem pole arrangement looking into the load, and related lack of any intrinsic power supply
    noise rejection in the output stage the way a push-pull
    transformer-coupled output stage has when both halves have similar
    output impedances working into the same reflected load.

    Global negative feedback can't do anything about the second and there
    isn't usually enough open-loop gain available to do a good job of
    correcting for distortion caused by the first across the audio band.
    often leading to a kinda poor-performing amp.

    These issues can be addressed but it requires more work than I'm
    guessing they put in in the mid 60s (can't immediately find the article
    in question online)

    --- SoupGate-Win32 v1.05
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  • From Tauno Voipio@21:1/5 to Chris on Sat Dec 29 20:27:21 2018
    XPost: sci.electronics.design

    On 29.12.18 01:32, Chris wrote:
    On 12/28/18 03:11, tubeguy@myshop.com wrote:
    I found a schematic of a stereo amplifier containing TWELVE 6L6 audio
    output tubes. But aside from sharing the power supply, it's two
    mono-blocks with each having SIX 6L6 output tubes. Unfortunately, I was
    not able to get any real detailed information on this.

    I compared it to several guitar amp schematics which use FOUR 6L6 tubes
    in push-pull parallel, and this is very similar. It just has 3 tubes on
    each side of the phase, rather than two. The plates are all wired
    together and cathode / grid components are duplicates to each tube.

    They did use as bias adjust potentiometer on each tube, which I have not
    seen on the guitar amps.

    Obviously the purpose is to increase wattage output.

    I'm not intending on building this, but it's interesting and makes me
    wonder if someone could use EIGHT 6L6 tubes, or TEN?
    (Of course adding more tubes mean bigger audio output transformers and
    heftier power supplies.

    Another question that comes to mind is the impedience of the primary on
    the audio output transformer. Assuming I could find a transformer that
    would handle the wattage, would the impedence be the same as those used
    with FOUR 6L6 tubes, or would that change since there are two more tube
    plates connected in series.

    Anyhow, looking at this schematic makes me think that any PPP amp with
    FOUR tubes could have more output tubes added, as long as power supply
    current is available and a suitable audio output transformer is
    obtainable.




    If you want to see an interesting audio amp, look up the Editors and Engineers Radio Handbook from the mid 60's. There's a design that
    which uses 6 or 8 6080 double triodes to build a direct coupled
    amplifier, no output transformer and dual power rails, positive and
    negative. Never built built it here, but a very original design. Could probably update that to use power mosfets...

    I found it in my copy, 16th edition, 1963.

    The amplifier is not direct coupled, there are several stages
    with coupling capacitors. The final is series-connected with
    3 series pairs of 6082 triodes in parallel and direct feed to
    a 16 ohm speaker. The power supplies are + and - 140 V.

    --

    -TV

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  • From Les Cargill@21:1/5 to tubeguy@myshop.com on Sun Dec 30 11:51:22 2018
    XPost: sci.electronics.design

    tubeguy@myshop.com wrote:
    I found a schematic of a stereo amplifier containing TWELVE 6L6 audio
    output tubes. But aside from sharing the power supply, it's two
    mono-blocks with each having SIX 6L6 output tubes. Unfortunately, I was
    not able to get any real detailed information on this.

    I compared it to several guitar amp schematics which use FOUR 6L6 tubes
    in push-pull parallel, and this is very similar. It just has 3 tubes on
    each side of the phase, rather than two. The plates are all wired
    together and cathode / grid components are duplicates to each tube.

    They did use as bias adjust potentiometer on each tube, which I have not
    seen on the guitar amps.

    Obviously the purpose is to increase wattage output.

    I'm not intending on building this, but it's interesting and makes me
    wonder if someone could use EIGHT 6L6 tubes, or TEN?
    (Of course adding more tubes mean bigger audio output transformers and heftier power supplies.

    Another question that comes to mind is the impedience of the primary on
    the audio output transformer. Assuming I could find a transformer that
    would handle the wattage, would the impedence be the same as those used
    with FOUR 6L6 tubes, or would that change since there are two more tube plates connected in series.

    Anyhow, looking at this schematic makes me think that any PPP amp with
    FOUR tubes could have more output tubes added, as long as power supply current is available and a suitable audio output transformer is
    obtainable.



    The largest production tube MI amp I am aware of is the
    Mesa Boogie 400+. It uses ( I believe ) 12 power tubes,
    essentially 6 push-pull pairs ).

    SFAIK, it exists only to compete with the Ampeg SVT.


    I wouldn't want to lift it.

    --
    Les Cargill

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  • From Big Bad Bob@21:1/5 to Tauno Voipio on Mon Dec 31 11:14:33 2018
    XPost: sci.electronics.design

    On 12/29/18 10:27, Tauno Voipio wrote:
    On 29.12.18 01:32, Chris wrote:
    On 12/28/18 03:11, tubeguy@myshop.com wrote:
    I found a schematic of a stereo amplifier containing TWELVE 6L6 audio
    output tubes. But aside from sharing the power supply, it's two
    mono-blocks with each having SIX 6L6 output tubes. Unfortunately, I was
    not able to get any real detailed information on this.

    I compared it to several guitar amp schematics which use FOUR 6L6 tubes
    in push-pull parallel, and this is very similar. It just has 3 tubes on
    each side of the phase, rather than two. The plates are all wired
    together and cathode / grid components are duplicates to each tube.

    They did use as bias adjust potentiometer on each tube, which I have not >>> seen on the guitar amps.

    Obviously the purpose is to increase wattage output.

    I'm not intending on building this, but it's interesting and makes me
    wonder if someone could use EIGHT 6L6 tubes, or TEN?
    (Of course adding more tubes mean bigger audio output transformers and
    heftier power supplies.

    Another question that comes to mind is the impedience of the primary on
    the audio output transformer. Assuming I could find a transformer that
    would handle the wattage, would the impedence be the same as those used
    with FOUR 6L6 tubes, or would that change since there are two more tube
    plates connected in series.

    Anyhow, looking at this schematic makes me think that any PPP amp with
    FOUR tubes could have more output tubes added, as long as power supply
    current is available and a suitable audio output transformer is
    obtainable.




    If you want to see an interesting audio amp, look up the Editors and
    Engineers Radio Handbook from the mid 60's. There's a design that
    which uses 6 or 8 6080 double triodes to build a direct coupled
    amplifier, no output transformer and dual power rails, positive and
    negative. Never built built it here, but a very original design. Could
    probably update that to use power mosfets...

    I found it in my copy, 16th edition, 1963.

    The amplifier is not direct coupled, there are several stages
    with coupling capacitors. The final is series-connected with
    3 series pairs of 6082 triodes in parallel and direct feed to
    a 16 ohm speaker. The power supplies are + and - 140 V.


    sounds like a gimmick. Tube plate/cathode currents are just way too
    small to properly drive a speaker without an impedence matching transformer.

    Had they thought of it "back then" they could've used toroidal
    transformers with PWM push-pull amplifiers modulated up to 70 or 80
    percent as a class 'G' amplifier. By the time that was invented, it was
    all transistors, though. [and no benfit to using tubes].

    I suspect that the push-pull configuration's advantages [and
    disadvantages] disappear when you use a transformerless configuration,
    and new ones take their place.

    If you want high power, you should consider using KT88's and a matching transformer from Hammond. A single pair should get you at least 100W
    RMS. I saw a stereo amplifier built using these at a state fair once,
    in the 70's, basically a clone of the GE tube manual's reference design.
    It was on an oversized chassis, though. I would've preferred it as 2 "monoblock" amplifiers that could fit together on a shelf...

    even the high heater current requirements of a KT88 would be smaller
    than 6 or 8 6L6's [and is physically smaller]. But I suppose it looks
    'cooler' to have all of those bottles grouped together.


    --
    (aka 'Bombastic Bob' in case you wondered)

    'Feeling with my fingers, and thinking with my brain' - me

    'your story is so touching, but it sounds just like a lie'
    "Straighten up and fly right"

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  • From Big Bad Bob@21:1/5 to bitrex on Mon Dec 31 11:25:08 2018
    XPost: sci.electronics.design

    On 12/28/18 09:16, bitrex wrote:

    All amps of that scale should have active protection
    circuitry/monitoring of currents, biases, and temperatures via
    microprocessor to avoid potentially catastrophic faults.

    I think it works ok without "all that". fuses where needed of course,
    to avoid things catching on fire. But tubes are amazingly strong at
    absorbing conditions that transistors would melt under. Sure the plates
    turn pink [like when half of a push-pull transformer melts away] but if
    you don't push them to the edge of physics they generally take it ok and survive getting a new output transformer. Or if a single tube fails in multi-pair configuration, the others will take up most of the slack
    without too much bad behavior. You'll probably hear the bad quality
    sound at high volumes, but without extended operation "that way" the
    other tubes should survive.

    the more you have paralleled, the less impact a single tube failure will
    have on the other tubes.

    Even briefly overvolt or overcurrent on a typical transistor, and you'll
    be replacing it VERY soon. I made the mistake of designing a circuit
    that operated close to the maximum Vceo (these 60V transistors should be
    able to handle 45-50V right?), and the transistors never lasted long
    under load (replaced 3 times, and 3 blown fuses that were supposed to
    protect them). Replaced with transistors that had twice the Vceo and no problem.


    --
    (aka 'Bombastic Bob' in case you wondered)

    'Feeling with my fingers, and thinking with my brain' - me

    'your story is so touching, but it sounds just like a lie'
    "Straighten up and fly right"

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Tim Williams@21:1/5 to Big Bad Bob on Mon Dec 31 22:32:00 2018
    XPost: sci.electronics.design

    "Big Bad Bob" <BigBadBob-at-mrp3-dot-com@testing.local> wrote in message news:bsGdneTGabiI8LfBnZ2dnUU7-aHNnZ2d@earthlink.com...
    I think it works ok without "all that". fuses where needed of course, to avoid things catching on fire. But tubes are amazingly strong at
    absorbing conditions that transistors would melt under. Sure the plates
    turn pink [like when half of a push-pull transformer melts away] but if
    you don't push them to the edge of physics they generally take it ok and survive getting a new output transformer.

    A lot of sweep tube datasheets contain the rating, "short term overload duration: 220W for 60s" or something like that. Transistors can only dream
    of such abuse, gone in milliseconds!

    But the purpose of that rating must be understood. These were TV tubes, and the sweep tube in particular often took a beating as other tubes heated up
    and other signals stabilized: horizontal oscillator and sync, and the damper diode especially -- its high cathode voltage isolation takes a long time to warm up.

    There was definitely no money in adding a protection circuit! Burn a tube,
    pop it out, take it down to the corner drugstore and buy a new one for a
    buck or a few.

    Nowadays, with both tubes and transformers being rather pricey, let alone
    the repairman -- the balance changes, and especially with how little
    hardware is involved in adding a protection circuit (if one does not mind
    that it contains silicon), it's well worth it.

    Tim

    --
    Seven Transistor Labs, LLC
    Electrical Engineering Consultation and Design
    Website: https://www.seventransistorlabs.com/

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  • From Tim Williams@21:1/5 to Big Bad Bob on Mon Dec 31 22:39:02 2018
    XPost: sci.electronics.design

    "Big Bad Bob" <BigBadBob-at-mrp3-dot-com@testing.local> wrote in message news:R6mdnYGk2c0E97fBnZ2dnUU7-LPNnZ2d@earthlink.com...
    sounds like a gimmick. Tube plate/cathode currents are just way too small
    to properly drive a speaker without an impedence matching transformer.


    So what, nothing comes out?...

    They were definitely below the maximum power point (at clipping), if that's more accurately what you meant.

    6AS7/6080, 6S33S and the other regulator tubes have peak cathode current in
    the ballpark of an ampere, so a modest number of tubes offers a modest power output, say 20 or 40W, preferably into a higher load like 16 ohms. The efficiency is poor, with more heat dissipated in the heaters alone, than delivered to the output.

    If you put dozens in parallel, the efficiency keeps going up as you get
    closer to matching, but now your whole system consumes multiple kilowatts...

    There was also the Philips "SEPP" with a pair of EL86? driving a 100s-ohms voice coil at good efficiency, but those speakers are so rare that this is practically a unique case.

    Tim

    --
    Seven Transistor Labs, LLC
    Electrical Engineering Consultation and Design
    Website: https://www.seventransistorlabs.com/

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From bitrex@21:1/5 to Tim Williams on Tue Jan 1 01:32:52 2019
    XPost: sci.electronics.design

    On 12/31/2018 11:32 PM, Tim Williams wrote:
    "Big Bad Bob" <BigBadBob-at-mrp3-dot-com@testing.local> wrote in message news:bsGdneTGabiI8LfBnZ2dnUU7-aHNnZ2d@earthlink.com...
    I think it works ok without "all that".  fuses where needed of course,
    to avoid things catching on fire.  But tubes are amazingly strong at
    absorbing conditions that transistors would melt under.  Sure the plates
    turn pink [like when half of a push-pull transformer melts away] but
    if you don't push them to the edge of physics they generally take it
    ok and
    survive getting a new output transformer.

    A lot of sweep tube datasheets contain the rating, "short term overload duration: 220W for 60s" or something like that.  Transistors can only
    dream of such abuse, gone in milliseconds!

    But the purpose of that rating must be understood.  These were TV tubes,
    and the sweep tube in particular often took a beating as other tubes
    heated up and other signals stabilized: horizontal oscillator and sync,
    and the damper diode especially -- its high cathode voltage isolation
    takes a long time to warm up.

    There was definitely no money in adding a protection circuit!  Burn a
    tube, pop it out, take it down to the corner drugstore and buy a new one
    for a buck or a few.

    Nowadays, with both tubes and transformers being rather pricey, let
    alone the repairman -- the balance changes, and especially with how
    little hardware is involved in adding a protection circuit (if one does
    not mind that it contains silicon), it's well worth it.

    Tim


    Yep, the OP is talking about monoblock amp with 6, 8, ten power tubes.
    The power supply iron and output transformer will be large, possibly
    custom, and not cheap.

    $5-10 worth of microcontroller or ICs and relays to at the least monitor
    tube cathode currents and grid voltages and cut the HT if things start
    going tits-up compared to a melted half of an output transformer that
    might cost $3-500. Big BUMMER!

    --- SoupGate-Win32 v1.05
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  • From upsidedown@downunder.com@21:1/5 to BigBadBob-at-mrp3-dot-com@testing.l on Tue Jan 1 14:21:45 2019
    XPost: sci.electronics.design

    On Mon, 31 Dec 2018 11:14:33 -0800, Big Bad Bob <BigBadBob-at-mrp3-dot-com@testing.local> wrote:

    On 12/29/18 10:27, Tauno Voipio wrote:
    On 29.12.18 01:32, Chris wrote:

    <clip>


    If you want to see an interesting audio amp, look up the Editors and
    Engineers Radio Handbook from the mid 60's. There's a design that
    which uses 6 or 8 6080 double triodes to build a direct coupled
    amplifier, no output transformer and dual power rails, positive and
    negative. Never built built it here, but a very original design. Could
    probably update that to use power mosfets...

    I found it in my copy, 16th edition, 1963.

    The amplifier is not direct coupled, there are several stages
    with coupling capacitors. The final is series-connected with
    3 series pairs of 6082 triodes in parallel and direct feed to
    a 16 ohm speaker. The power supplies are + and - 140 V.


    sounds like a gimmick. Tube plate/cathode currents are just way too
    small to properly drive a speaker without an impedence matching transformer.

    Search for 6082 "50 ohm" and you find quite a lot hits with single
    pair driving 50 ohms.

    With tree pairs, the load could be 16 ohms.

    --- SoupGate-Win32 v1.05
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  • From upsidedown@downunder.com@21:1/5 to tiwill@seventransistorlabs.com on Tue Jan 1 14:34:54 2019
    XPost: sci.electronics.design

    On Mon, 31 Dec 2018 22:39:02 -0600, "Tim Williams" <tiwill@seventransistorlabs.com> wrote:

    There was also the Philips "SEPP" with a pair of EL86? driving a 100s-ohms >voice coil at good efficiency, but those speakers are so rare that this is >practically a unique case.

    Usually closer to 1000 ohms.

    These speakers were sold for a few years, but later on, they went on
    sale. Adding an 8:1000 ohm transformer and you could cheaply build a
    good quality speaker box for an ordinary 8 ohm tube or transistor
    amplifier :-)

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Kevin Aylward@21:1/5 to bitrex on Tue Jan 1 15:27:45 2019
    XPost: sci.electronics.design

    "Big Bad Bob" wrote in message news:bsGdneTGabiI8LfBnZ2dnUU7-aHNnZ2d@earthlink.com...

    On 12/28/18 09:16, bitrex wrote:

    All amps of that scale should have active protection circuitry/monitoring
    of currents, biases, and temperatures via microprocessor to avoid
    potentially catastrophic faults.

    Even briefly overvolt or overcurrent on a typical transistor, and you'll be >replacing it VERY soon. I made the mistake of designing a circuit that >operated close to the maximum Vceo (these 60V transistors should be able to >handle 45-50V right?), and the transistors never lasted long under load >(replaced 3 times, and 3 blown fuses that were supposed to protect them). >Replaced with transistors that had twice the Vceo and no problem.

    Well... overcurrent on a mosfet output device is usually pretty safe for
    quite a while. Second breakdown is the problem with bipolar. Their rating at high voltage is much lower than their power ratings would imply.

    I designed mosfet amp in the early 80s. I tested it by putting in a full
    level signal with an output s/c. The only protection was a zener across the gates to limit the current to the device ratings. I left it cycling with its
    90 Deg heatsink thermal cutout for 3 days. No problems.

    -- Kevin Aylward
    http://www.anasoft.co.uk - SuperSpice http://www.kevinaylward.co.uk/ee/index.html

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Tauno Voipio@21:1/5 to Tim Williams on Tue Jan 1 21:00:36 2019
    XPost: sci.electronics.design

    On 1.1.19 06:39, Tim Williams wrote:
    "Big Bad Bob" <BigBadBob-at-mrp3-dot-com@testing.local> wrote in message news:R6mdnYGk2c0E97fBnZ2dnUU7-LPNnZ2d@earthlink.com...
    sounds like a gimmick.  Tube plate/cathode currents are just way too
    small to properly drive a speaker without an impedence matching
    transformer.


    So what, nothing comes out?...

    They were definitely below the maximum power point (at clipping), if
    that's more accurately what you meant.

    6AS7/6080, 6S33S and the other regulator tubes have peak cathode current
    in the ballpark of an ampere, so a modest number of tubes offers a
    modest power output, say 20 or 40W, preferably into a higher load like
    16 ohms.  The efficiency is poor, with more heat dissipated in the
    heaters alone, than delivered to the output.

    If you put dozens in parallel, the efficiency keeps going up as you get closer to matching, but now your whole system consumes multiple
    kilowatts...

    There was also the Philips "SEPP" with a pair of EL86? driving a
    100s-ohms voice coil at good efficiency, but those speakers are so rare
    that this is practically a unique case.

    Tim


    The final triodes (6080) are special high-current tubes. The amplifier
    designer rated the thing at 25 W into a 16 ohm voice-coil speaker.

    --

    -TV

    --- SoupGate-Win32 v1.05
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  • From bitrex@21:1/5 to Kevin Aylward on Tue Jan 1 18:02:06 2019
    XPost: sci.electronics.design

    On 01/01/2019 10:27 AM, Kevin Aylward wrote:
    "Big Bad Bob"  wrote in message news:bsGdneTGabiI8LfBnZ2dnUU7-aHNnZ2d@earthlink.com...

    On 12/28/18 09:16, bitrex wrote:

    All amps of that scale should have active protection
    circuitry/monitoring of currents, biases, and temperatures via
    microprocessor to avoid potentially catastrophic faults.

    Even briefly overvolt or overcurrent on a typical transistor, and
    you'll be replacing it VERY soon.  I made the mistake of designing a
    circuit that operated close to the maximum Vceo (these 60V transistors
    should be able to handle 45-50V right?), and the transistors never
    lasted long under load (replaced 3 times, and 3 blown fuses that were
    supposed to protect them). Replaced with transistors that had twice
    the Vceo and no problem.

    Well... overcurrent on a mosfet output device is usually pretty safe for quite a while. Second breakdown is the problem with bipolar. Their
    rating at high voltage is much lower than their power ratings would imply.

    Ya, BJT amps tend to have oversized output devices as compared to what
    their rated maximum "RMS power" output would imply. 75 watt-rated
    devices in 25 watt amps. The problem is risk of second breakdown when
    working into reaactive loads, not de-rating BJTs appropriately when
    they're handing significant powers into reactive loads common newbie
    mistake

    I designed  mosfet amp in the early 80s. I tested it by putting in a
    full level signal with an output s/c. The only protection was a zener
    across the gates to limit the current to the device ratings. I left it cycling with its 90 Deg heatsink thermal cutout for 3 days. No problems.

    -- Kevin Aylward
    http://www.anasoft.co.uk - SuperSpice http://www.kevinaylward.co.uk/ee/index.html

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Tim Williams@21:1/5 to Kevin Aylward on Wed Jan 2 00:16:56 2019
    XPost: sci.electronics.design

    "Kevin Aylward" <kevinRemovAT@kevinaylward.co.uk> wrote in message news:Pdedne3rNa5uG7bBnZ2dnUU7-aHNnZ2d@giganews.com...
    Well... overcurrent on a mosfet output device is usually pretty safe for quite a while. Second breakdown is the problem with bipolar. Their rating
    at high voltage is much lower than their power ratings would imply.

    Used to be true -- modern MOSFETs are more than current-dense enough to
    exhibit 2nd breakdown. I shouldn't actually say modern, because apparently SuperJunction process has... PTC source connections or something? I haven't seen one without a square SOA yet I don't think. So by now, it's actually previous generation that you have to watch out for. I forget if lower
    voltage (SJ goes away under ~400V I think it was?) processes are still
    prone.

    Tim

    --
    Seven Transistor Labs, LLC
    Electrical Engineering Consultation and Design
    Website: https://www.seventransistorlabs.com/

    --- SoupGate-Win32 v1.05
    * Origin: fsxNet Usenet Gateway (21:1/5)
  • From Kevin Aylward@21:1/5 to All on Thu Jan 3 21:25:34 2019
    XPost: sci.electronics.design

    "Tim Williams" wrote in message news:q0hkvr$7dm$1@dont-email.me...

    "Kevin Aylward" <kevinRemovAT@kevinaylward.co.uk> wrote in message >news:Pdedne3rNa5uG7bBnZ2dnUU7-aHNnZ2d@giganews.com...
    Well... overcurrent on a mosfet output device is usually pretty safe for
    quite a while. Second breakdown is the problem with bipolar. Their rating
    at high voltage is much lower than their power ratings would imply.

    Used to be true -- modern MOSFETs are more than current-dense enough to >exhibit 2nd breakdown. I shouldn't actually say modern, because apparently >SuperJunction process has... PTC source connections or something? I
    haven't seen one without a square SOA yet I don't think. So by now, it's >actually previous generation that you have to watch out for. I forget if >lower voltage (SJ goes away under ~400V I think it was?) processes are
    still prone.

    Exicon are the audio mosfets of choice today, apparently. They are laterals.

    http://www.exicon.info/

    They have the usual power limited SOA.

    I don't know who actually makes them, but my guess is someone like XFAB.

    Standard fab vendors will make any asic for any fabless company, even if the asic is just the one big transistor!

    -- Kevin Aylward
    http://www.anasoft.co.uk - SuperSpice http://www.kevinaylward.co.uk/ee/index.html

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  • From tubeguy@myshop.com@21:1/5 to tauno.voipio@notused.fi.invalid on Thu Jan 3 16:54:33 2019
    XPost: sci.electronics.design

    On Fri, 28 Dec 2018 14:19:50 +0200, Tauno Voipio <tauno.voipio@notused.fi.invalid> wrote:

    On 28.12.18 10:45, upsidedown@downunder.com wrote:
    On Thu, 27 Dec 2018 21:11:35 -0600, tubeguy@myshop.com wrote:

    I'm not intending on building this, but it's interesting and makes me
    wonder if someone could use EIGHT 6L6 tubes, or TEN?
    (Of course adding more tubes mean bigger audio output transformers and
    heftier power supplies.

    Tubes may age at different rates, so the characteristics might be
    quite different after a long time. It might not make sense to replace
    a single (failed) tube and you may have to replace the whole set of
    tubes with new tubes preferably from the same manufacturing batch.
    This can be quite expensive :-)


    In the 1960's, I made a guitar amplifier with six EL500's. The tubes
    are not characterized for linear use, and they were not matched in
    any way. Each tube had an own cathode resistor, and there was no
    evidence of any overload from unbalanced operation.

    The tubes had a tendency of parasitic oscillation, so I added a
    ferrite bead on the control grids and an inductor-resistor parallel >combination on each plate (for constructions, see nearest ham handbook).

    Back in the late 60s early 70s, I had several (self refurbished)
    mono-block power amps that used four 6L6 output tubes. I did not even
    know about matching those tubes. I just put in any tube marked as a 6L6.
    I recall having both the glass (GC) types mixed with the black metal
    ones. I always had good sound and lots of power. I do recall that
    replacing the metal cased ones with 6L6GC did increase my power though.
    But until I could afford new tubes, I used what I had. And even with the
    new tubes, I never matched them.

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