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Thread: Help me understand an Ampeg SVT schematic

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    Help me understand an Ampeg SVT schematic

    Hello,
    i was looking at this preamp, and i don't anderstand several points, here is the link : https://www.schematichell.com/ampeg/svtpreamp.pdf

    So in channel one, what happens after the tone stack, with the signal taken at the plate AND at the cathode, what's the goal, and what 's the coil in the midrange circuit?

    Many thanks

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    Last edited by big_teee; 08-31-2019 at 11:29 PM.

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    Quote Originally Posted by tepsamps View Post
    Hello,
    i was looking at this preamp, and i don't anderstand several points, here is the link : https://www.schematichell.com/ampeg/svtpreamp.pdf

    So in channel one, what happens after the tone stack, with the signal taken at the plate AND at the cathode, what's the goal, and what 's the coil in the midrange circuit?

    Many thanks
    Following the bass/treble tone stack, the output feeds bootstrapped V3B at pin 7...the bootstrap connection is the 1M connecting to the junction of the two cathode resistors 560 ohm and 7.5k. The output from the plate connects that gain stage to the next gain stage in V4B, at pin 7. That stage feeds the cathode follower of V4 directly at pin 2 from plate pin 6. The cathode follower output goes two places....it mixes the output signal of Ch 1 with the output sigal of Ch 2 via 100k resistors, feeding the input of cathode follower V5. The other path from V4A cathode is to the Midrange control, which sits between the lower portion of the cathode resistors of V3B and V4A. The Inductor forms a resonant 'bell-shaped' curve of the midrange, which ties to ground thru the capacitors off the coil taps via Mid-Range Select Switch. All of the tone circuits in this preamp are Baxendall type circuits, where at the center of the tone pot, you get flat response.

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    Many thanks !
    I think i don't anderstand this midrange control...
    What are those swithes with 4 poles ? The inductor is a choke with 4 legs ?
    There is no attenuation between the stages, no grid leak resistor, why so many gain stages and CF to have a clean sound? Channel one is clean, channel two gets dirty at loud volume?
    It's not usual for me

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    Quote Originally Posted by tepsamps View Post
    I think i don't anderstand this midrange control...
    What are those swithes with 4 poles ? The inductor is a choke with 4 legs ?
    There is no attenuation between the stages, no grid leak resistor, why so many gain stages and CF to have a clean sound?
    The two gain stages and cathode follower after the tone stack of channel 1 are controlled by a negative feedback loop to reduce the gain. The mid control is part of this feedback loop.

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    Quote Originally Posted by tepsamps View Post
    Hello,
    i was looking at this preamp, and i don't anderstand several points, here is the link : https://www.schematichell.com/ampeg/svtpreamp.pdf

    So in channel one, what happens after the tone stack, with the signal taken at the plate AND at the cathode
    No, itīs taken at the plate only, the cathode is a feedback point.
    , what's the goal,
    to put it simply, to make a "1 band Graphic Equalizer"
    To make it more useful, frequency is switch selectable.
    and what 's the coil in the midrange circuit?
    The coil selectable taps and the selectable capacitors let you select different frequencies.
    What are those swithes with 4 poles ? The inductor is a choke with 4 legs ?
    See above.
    why so many gain stages and CF
    Whatīs EASY to make with Op Amps is HARD/COMPLEX to make with tubes

    Just look at a Tube Op Amp, and this was literally a NASA level design, used in missiles defending USA from evil Reds and their Nukes, not kidding:



    the Ampeg circuit is a somewhat simplified version, missing 1 triode.

    So you have 2 gain stages, right side of V3 (call it V3b) and the full V4 (call it V4a and V4b)

    Actual voltage gain stages are V4a and V4b , combined gain is huge as you imagine, BUT itīs knocked down big time using NFB (negative feedback)
    Why would anybody do that?
    Well, you can do beautiful things tailoring NFB to your needs, one is very sharp and controllable gain at a given frequency.

    In principle gain is knocked down to about 6 (50k pot and 8k V3b cathode to ground) BUT the pot has an LC (inductor capacitor net) connected to its slider which grounds (kills) a given frequency, that at which the selected LC combination resonates.

    Imagine these 3 situations:

    * pot wiper fully right: LC attenuates "Op Amp" output by grounding the 47k/6k8 junction so you have a midrange notch.

    * pot wiper fully left, now it kills NFB at V3b cathode, increasing gain, you have a midrange boost.

    * pot wiper centered: now LC slightly attenuates output, also slightly attenuates NFB (itīs 25K away from each) at V3b cathode or it grounds it increasing gain, 2 ways to say basically same thing, both effects balance each other , net result is flat response.

    Now you have an adjustable cut/boost at a single selected frequency.
    Also understand why Ampeg did not offer a full 6 band graphic equalizer

    FWIW this is a typical Tube Op Amp user:



    Notice the *Analog* (Op Amp) computers at 1:00

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    Great Fahey, superbe explanation, i anderstand better, thanks.

    And what about the power amp ? https://www.schematichell.com/ampeg/...eramp6146b.pdf

    All i anderstand is that there is a recovery stage from the previous cathode follower and a cathodyne PI.
    Each branch of the cathodyne feeds a gain stage, (why?), but what's the use of V2b and V3b ?
    Whare are those 2 diodes (no zener) to ground ?
    And what's the balance control ?
    Many thanks

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    Quote Originally Posted by tepsamps View Post
    And what about the power amp ?

    Each branch of the cathodyne feeds a gain stage, (why?), but what's the use of V2b and V3b ?
    Whare are those 2 diodes (no zener) to ground ?
    And what's the balance control ?
    The gain stages provide more output swing than the cathodyne and increase the power amp's loop gain.
    V2b and V3b are directly coupled cathode followers to drive the three output tubes.
    The balance control adjusts the balance of the cathodyne outputs.

    Do you mean the 2 diodes across the input?

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    Quote Originally Posted by Dave H View Post
    The gain stages provide more output swing than the cathodyne and increase the power amp's loop gain.
    V2b and V3b are directly coupled cathode followers to drive the three output tubes.
    The balance control adjusts the balance of the cathodyne outputs.

    Do you mean the 2 diodes across the input?
    OK, thanks, what does the directly coupled cathode followers bring to the power tubes? What about if there are not there ?
    Yes the 2 diodes across the input ...

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    And what are those 4 poles switches in the preamp? plzzzz!

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    Quote Originally Posted by tepsamps View Post
    OK, thanks, what does the directly coupled cathode followers bring to the power tubes? What about if there are not there ?
    Yes the 2 diodes across the input ...
    I'm guessing but the directly coupled cathode followers could be there to provide power tube grid current to take into class AB2 for more power output.

    The diodes could be a limiter to prevent the power amp being driven too hard.

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    Quote Originally Posted by tepsamps View Post
    And what are those 4 poles switches in the preamp? plzzzz!
    The bottom left one as drawn in the Ultra Low position is a low pass filter to cut out treble.

    The top one in the left Bass Cut position is a high pass filter to cut bass.
    In the centre position its flat.
    In the right Ultra Low position it look like it could be a Parallel T notch (band stop) filter to remove mid range.

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    Quote Originally Posted by Dave H View Post
    The bottom left one as drawn in the Ultra Low position is a low pass filter to cut out treble.

    The top one in the left Bass Cut position is a high pass filter to cut bass.
    In the centre position its flat.
    In the right Ultra Low position it look like it could be a Parallel T notch (band stop) filter to remove mid range.
    Actually i don't anderstand the schematic of these switches. Not what purpose they serve. What kind of switch have 2x4poles with 3 positions? What kind of switch (hardware) can be used there?

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    Quote Originally Posted by tepsamps View Post
    Actually i don't anderstand the schematic of these switches. Not what purpose they serve.
    Well, thatīs your privilege. You can "not understand" anything you want. We live in Democracy here.
    Of course, studying a little Electronics doesnīt hurt, and helps understanding how things work.
    What kind of switch have 2x4poles with 3 positions? What kind of switch (hardware) can be used there?
    Designers design.
    Then manufacturers manufacture.
    If components can be ordered from a standard supplier. such as Mouser, Digikey, etc., then they do that.

    If component is a custom one, Factories such as Ampeg are large/important enough to order custom made components.
    They have the required "buying power".

    Didnīt find that exact switch, but as examples of NOT usually available, CUSTOM ORDERED parts:

    https://www.fullcompass.com/prod/195...-for-svt-3-pro


    or:



    in general, you can custom order *anything* you want, as long as you can pay for it.
    Custom switches, pots, speakers, transformers, whatever.

    As a side note, thatīs why Enzo often suggests: "buy the part from the Amp Factory"

    Maybe it looks more expensive than average cost, but they have it; OEM supplier will make them for less but insist on a minimum order of, what? ... 1000 parts minimum?

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    Quote Originally Posted by Dave H View Post
    I'm guessing but the directly coupled cathode followers could be there to provide power tube grid current to take into class AB2 for more power output.

    The diodes could be a limiter to prevent the power amp being driven too hard.
    Is can’t operate in AB2 due to the cllamping action caused by the 47k grid stoppers. My guess is the direct coupling is to eliminate the RC HPF, and preventing blocking/crossover at low frequencies.

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    Quote Originally Posted by SoulFetish View Post
    Is can’t operate in AB2 due to the cllamping action caused by the 47k grid stoppers.
    I thought of that but the cathode followers should be able to supply a couple of mA of grid current through the 47k stoppers.

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    Quote Originally Posted by Dave H View Post
    I thought of that but the cathode followers should be able to supply a couple of mA of grid current through the 47k stoppers.
    The problem is, 1mA across 47k would cause a voltage drop of 47 volts. I don’t see how the grids could be driven into positive grid voltage

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    Quote Originally Posted by SoulFetish View Post
    The problem is, 1mA across 47k would cause a voltage drop of 47 volts. I don’t see how the grids could be driven into positive grid voltage
    I agree, the grids won't be driven far positive with a only few mA of current.

    As you can see I don't know much about AB2 It was just the first thing that came into my head when I saw the cathode followers. They must be there to buffer the previous stage from the 47k grid leak resistor so as not to reduce the voltage swing available for the power tube grids.

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    I think the 'why cathode followers' and 'why DC coupled cathode followers' are really 2 different questions.

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    And I would like to add the question "why these extremely large grid stoppers?"
    Does anybody know if previous versions had lower values?

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    Quote Originally Posted by g1 View Post
    I think the 'why cathode followers' and 'why DC coupled cathode followers' are really 2 different questions.
    You'll have to enlighten me

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    I can't, but just commenting on the answers I'm reading.
    I'm fairly sure the plain old 'cathode follower' part of the question is mostly due to the drive loss when dealing with many power tubes (lets say more than 4). We see this with SVT, Hiwatt 400, etc. whereas Fender uses it for Super Twin, but interstage drive transformer for PS400.
    The 'DC coupled' part is the question I'm hoping to hear answered.

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    Last edited by g1; 09-03-2019 at 12:16 AM. Reason: 400 not 200 Hiwatt
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    Quote Originally Posted by tepsamps View Post
    Actually i don't anderstand the schematic of these switches. Not what purpose they serve. What kind of switch have 2x4poles with 3 positions? What kind of switch (hardware) can be used there?
    Here's one such switch that came to mind:

    https://www.stewmac.com/Pickups_and_..._Switches.html

    Likely also available from the usual sources.

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    Quote Originally Posted by Helmholtz View Post
    And I would like to add the question "why these extremely large grid stoppers?"
    Does anybody know if previous versions had lower values?
    I’ll have to double check, but I believe the early ones had 47k ( or similarity large) grid stoppers as well.

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    The 'DC coupled' part is the question I'm hoping to hear answered.
    DC coupling eliminates the risk of blocking distortion.

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    Quote Originally Posted by Bloomfield View Post
    Here's one such switch that came to mind:

    https://www.stewmac.com/Pickups_and_..._Switches.html

    Likely also available from the usual sources.
    Good ! But no ! It could be but the ampeg switch has a center shaft that is a contact actually and connects side 1 to side 2.

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    If the RCA tube manual RC-28 is to be believed, the maximum grid #1 circuit resistance for fixed bias operation is (not a typo) 0.05 MegOhm per tube. This is especially important at high plate Voltage to prevent thermal runaway. With three tubes in parallel, this would create an impossible to drive grid circuit impedance unless a direct coupled cathode follower is used. The direct coupled CF allows a higher much easier to drive impedance on the CF's grid.

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    Quote Originally Posted by tepsamps View Post
    Good ! But no ! It could be but the ampeg switch has a center shaft that is a contact actually and connects side 1 to side 2.
    I see at least one typo on the schematic in that area. The schematic shows the 2 sides of the switch connected, but have you verified that with your meter? I'm a bit surprised they would use a different switch there than the other positions.

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    Quote Originally Posted by g1 View Post
    I see at least one typo on the schematic in that area. The schematic shows the 2 sides of the switch connected, but have you verified that with your meter? I'm a bit surprised they would use a different switch there than the other positions.
    When I was trying to figure out how they worked I assumed that the broken line between the wipers of the two sides indicated that they were ganged not electrically connected. I thought they would operate like the 2 row, 4 contact switches in post #24, i.e. type #1232.

    Click image for larger version. 

Name:	Switch 1232.png 
Views:	10 
Size:	10.9 KB 
ID:	55075

    Edit: The diagonal connection between top row contact 3 and bottom row contact 2 is a wire link.

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    Last edited by Dave H; 09-07-2019 at 12:00 PM.

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    I just realized the schematic in the first post is a piazza drawing which may or not be accurate. I prefer to always use the factory drawings.
    Here is a link for the factory service manual. The parts list states that the ultra-low and the 2 ultra-hi switches are all the same part number.
    https://music-electronics-forum.com/...4&d=1542072839

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