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Some Discussion on Amp EQs

Discussion in 'Amps and Cabs [BG]' started by dunwichamps, Jan 21, 2013.


  1. dunwichamps

    dunwichamps Commercial User

    Jul 23, 2012
    Owner: Dunwich Amps
    5. Hiwatt DR103 EQ (Plate Driven)

    Hiwatt designed an EQ that is somewhat similar to a basic FMV circuit but include a number of extra components, and difference circuit. This EQ has a different response then the FMV. The traditional DR103 design had the EQ driven off the 2nd gain stage's plate. The plate is one of the terminals of a vacuum tube. It is the higher impedance of the 2 output terminals (the other being the cathode). This is similar to a Fender FMV as seen in the schematic below.

    [​IMG]

    The EQ is a bit more advanced then the regular FMV but it has a similar structure. Unlike the FMV, the controls go (from top to bottom), Treble, Mids, and Bass respectively. The pot value for the mids is 100k whereas Marshall's had a 25k and Fender had a 10k.

    Here is the input and output signals of the EQ when all the of the knobs are at noon.

    [​IMG]

    This EQ has a similar amount of insertion loss as a Fender FMV, as they are both plate driven. However the EQ loads down the high end a bit more than the fender FMV does. The mid cut is not as sharp in this design and is more broad looking.

    Now lets take a look at the 3 control sweeps

    Bass control sweep

    [​IMG]

    Mids control sweep

    [​IMG]

    Treble control sweep

    [​IMG]

    The EQ has some similarities to the Fender response but differs in someways. The mid cut is not nearly as strong and you cant cut as much mids. The mids also only really controls the mid range unlike in a Fender which also affects bass and treble to a much greater degree. The location of the mid cut is in a different location as well. The Bass and Treble responses are more similar tho.

    Here is the EQ with max on all controls

    [​IMG]

    There is a pretty big mid cut centering around 250 hz or so and a lot of bass and treble there.
    I wanted to include one last section about the Hiwatt EQ which is also to show more detail on how the impedance of the driver stage affects the response of the EQ. This is another way of highlighting the difference between the plate driven Fender FMV and the cathode driven Marshall FMV.

    6. Hiwatt DR103 EQ (Cathode Driven)

    According to the Hiwatt schematics on mhuss.com (I would say pretty much the authority on vintage Hiwatt information) a few rare models of the DR103 featured a cathode driven EQ. In this design the unused triode section of the V2 preamp tube is converted into a cathode follower and is DC coupled to the V2 gain stage. This is identical to what is done on the Marshall scheme.

    Here is the scheme I created

    [​IMG]

    Now what does the addition of the cathode follower do to the EQ response. Here is a comparision between the plate driven and cathode driven Hiwatt EQs with the controls all at noon

    [​IMG]

    The Red and Dark Blue Lines correspond to the cathode driven and the Light Blue and the Green lines correspond to the plate driven. As you can see there is a significant decrease in the insertion loss and loading of the signal in the cathode driven EQ. In some cases there is between 5 to 10 dB increase in the output signal. That is a considerable gain in the signal. At noon on the controls tho there is not a huge difference in the shape of the EQ beyond the increase in the output signal.

    Here are comparisions between the two EQs, just looking at the output of each while the controls are swept

    Bass Sweep

    [​IMG]

    Mids Sweep

    [​IMG]

    Treble Sweep

    [​IMG]

    So generally the cathode driven EQ has much more output then the plate driven EQ. There are some differences in the responses in terms of shape but not a great deal. The reason being is that I did not adjust the cathode driven EQ to be properly impedance matched with the cathode follower. One way to kind of show what this would do is to decrease the 100k resistor on the left of the tone stack to 33k.

    [​IMG]

    Now there is serious differences between the cathode (blue) and plate (green) driven tone stacks. Decrease the 100k resistor to 33k resulted in a tone of more low mids where the center frequency of the mid cut moved up to 700hz or so. This is now looking a lot more like the Marshall EQ response. Ok so you might say well why dont I just make the 100k resistor in the plate driven stack 33k? Well I certainly can do that but at the cost of increased insertion loss and more mid cut.

    That looks like this

    [​IMG]

    So now the mids are being cut even more and there is even more loss in the tone stacks but the mid cut center frequency is not ~700 hz, nearly the same as the cathode driven stack. So whats happening here? Well the EQ is now loading down the tube gain stage even more and since its a fairly high impedance that will decrease the signal going into the EQ and subsequently the output signal as well.
     
  2. JimmyM

    JimmyM

    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    No wonder Entwistle liked them ;)
     
  3. dunwichamps

    dunwichamps Commercial User

    Jul 23, 2012
    Owner: Dunwich Amps
    I believe his preamp actually had the CP103 EQ which is different compared to this one. Its a bit simpler and only has Bass and Treble but I could be wrong

    http://hiwatt.org/Schematics/OxPre.gif
     
  4. JimmyM

    JimmyM

    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    You're likely right.
     
  5. dunwichamps

    dunwichamps Commercial User

    Jul 23, 2012
    Owner: Dunwich Amps
    Father of Fires posted before I think to a site which already analyzed this tone stack in even more detail. Includes transfer functions.
     
  6. 4Mal

    4Mal Supporting Member

    Jun 2, 2002
    Columbia River Gorge
    Thanks for the Fender stack plots Charlie. I don't have the Duncan Tone Stack calculator. No wonder I like the 666 setting and 2-10-2 really looks to be a decent approximation to me. As always, If ya dig it, its good...
     
  7. dunwichamps

    dunwichamps Commercial User

    Jul 23, 2012
    Owner: Dunwich Amps
  8. Bassmec

    Bassmec

    May 9, 2008
    Ipswich UK
    Proprietor Springvale Studios
    From when I started at Polydor studio 2 in may 1977 our Mr Entwistle had various pre amps some built by Steven Gelf with Sowter high impedance 1 in and three output transformers, to drive the multiple Hiwatts, but later had a German engineer called Peter Struven custom design a dual pre amp Rig in console format, for the later Sunn power amps. The name of Peters company was Stramp.
    I showed a picture of them in my matamp pre heads up thread on here yesterday as it happens.:bassist:
     
  9. dunwichamps

    dunwichamps Commercial User

    Jul 23, 2012
    Owner: Dunwich Amps
    did he run the hiwatts at slaves for those preamps or just cascaded it into the amp? Ill admit I know little to nothing about his rig since I am primarily a guitar player
     
  10. Bassmec

    Bassmec

    May 9, 2008
    Ipswich UK
    Proprietor Springvale Studios
    As far as I can remember he didnt use the Hiwatt pre amp eq much, he allways wanted them set flat because they where only really used as slave amps with a bit of gain but then he wasn't a big fan of using much EQ at all, as evidenced by this picture of how he left his Stramp pre amps graphic EQ's set:
    jae-76_amprack.jpg
    I spent hours and hours watching his right hand technique desperately trying to get his all three finger style with accents on flick backs etc. he was amazingly good.:bassist:
     
    astheshipsails likes this.
  11. esa372

    esa372

    Aug 7, 2010
    Great thread!

    This caught my attention:
    According to the Duncan Calculator, the Alembic F-1X (which is a modded FMV) can be set to have an almost ruler-flat EQ:

    [​IMG]

    The settings have to be pretty extreme to get it, however:
    Bass - 0.3
    Middle - 5.5
    Treble - 0.0


    It does appear impossible to get a mid-boost from it, but that's why we have the SF-2. :D


    Again, great info in this thread - thanks!
     
  12. dunwichamps

    dunwichamps Commercial User

    Jul 23, 2012
    Owner: Dunwich Amps
    if u decrease the range on the graph by selecting auto y u will see more detail
     
  13. esa372

    esa372

    Aug 7, 2010
    Yes, a closer look:

    [​IMG]

    ...but it's still nearly as flat as flat can be. That roll-off from 100Hz to 20Hz is within one-half of a decibel, and there certainly no problem adding lows with this tone-stack!

    Anyway, I just wanted to show that it is possible to get a flat mid-response with an F-1X preamp.

    :D
     
  14. dunwichamps

    dunwichamps Commercial User

    Jul 23, 2012
    Owner: Dunwich Amps
    yea doable it seems at the expense of tons of loss
     
  15. esa372

    esa372

    Aug 7, 2010
    I'm not sure what you mean by "loss" (signal loss, perhaps?), but is sounds great!

    :bassist:
     
  16. dunwichamps

    dunwichamps Commercial User

    Jul 23, 2012
    Owner: Dunwich Amps
    the output signal is ~27 db less than the input signal. So your signal level drops a ton. In guitar world that might be more significant since generally clipping is wanted.
     
  17. esa372

    esa372

    Aug 7, 2010
    Ah... well, luckily, I don't live in that world.
     
  18. dunwichamps

    dunwichamps Commercial User

    Jul 23, 2012
    Owner: Dunwich Amps
    The signal level can always be increased with recovery gain. Its just something that factors into the design.
     
  19. Bassmec

    Bassmec

    May 9, 2008
    Ipswich UK
    Proprietor Springvale Studios
    At the moment we are trying to make this Duncan:
    [​IMG]
    To put in this.
    [​IMG]
    All Times 2 for glorious stereo.
    Sowter want £152 per channel for the midrange inductors alone, do you wind inductors by any chance or know a man who does.:bassist:
     
  20. dunwichamps

    dunwichamps Commercial User

    Jul 23, 2012
    Owner: Dunwich Amps
    I buy them from Wilco Inductors. you can get some up to almost 1H in small sizes

    Used some in a mid boost pedal, FET based design

    [​IMG]
     
  21. Primary

    Primary TB Assistant

    Here are some related products that TB members are talking about. Clicking on a product will take you to TB’s partner, Primary, where you can find links to TB discussions about these products.

     
    Apr 13, 2021

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