OT Primary Impedance for Bassman Circuit

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Mmitch, Feb 12, 2020.

  1. Eugene Wallace

    Eugene Wallace Member

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    360-400 plate voltage, cathode biased and tube rec will tweedify anything.
     
  2. big mike

    big mike David Grissom Wannabee Gold Supporting Member

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    OT changes are relatively minor compared to circuit differences

    me thinks hanging your hat on the OT influencing it to be too ‘blackface’ is barking up the wrong tree
     
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  3. Steppin' Wolfe

    Steppin' Wolfe Member

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    “Tweed sound”..... I hold that just from 1956 through 1960 there are at least five different ‘tweed’ sounds, and I never expect all of these amps to sound like the others in the other ‘classifications’.
    Champ, Princeton
    5E3 Deluxe, cathode biased Tremolux
    Fixed biased Tremolux, Vibrolux
    5E5A Pro,5F4 Super, 5E7 Bandmaster, 5E8A Twin
    5F6A Bassman, 5F8A Twin

    The last two are closest to the BF thing due to the output section, imho. However, I have always been able to discern the difference between these preamps and the BF type of preamp.
    I have a Fender Pro Sonic that is loosely based on the 5F6A. That amp can mimic any of the tweeds above if one has the experience with the other amps to know what to do with the gain controls. Imho, it might be an interesting experiment to integrate the Gain 2 circuit into something like the circuit with which Mitch is working. It is a very usable mid range boost control. That along with a preamp gain and a good MV allows one to manipulate things to go fr9m very loose to a tighter Sonic.
     
  4. Mmitch

    Mmitch Member

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    I do indeed use a tube rect. For a while I was using a 5Y3, but recently went to a 5AR4. I like the tighter low end response. With the Allen TP40D (320-0-320) I am seeing around 430VDC at my plates and the bias is set accordingly. I use JJ 6V6S so it is able to handle the plate voltage, but ultimately the bias is set to give me 70% diss., which is what I do no matter what rectifier or power tube is installed.

    I hear you on the cathode bias thing but I really dont think that is a route I would like to go. The tweed bassman utilizes a fixed bias, so I am not convinced the bias type is the culprit. Plus - an adjustable fixed bias is the bees knees ;)

    I am really starting to take a different approach to this amp. I am accepting it for what it is, not trying to make it something it isnt. As I start to shed what I think it should sound like, I am loving what it actually is producing.
     
  5. Mmitch

    Mmitch Member

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    Yeah if I were I would be. Again, just asking for thoughts on the OT thing.

    The preamp is pretty bassman, aside from a couple of things. The cathode tonestack (with all BM value components), LTP, etc are all derived from the tweed bassman circuit. The power rail gets into more Deluxe territory in terms of values, but the preamp stuff is on the level.

    Having done a comparison, I will hang my hat on the speaker a little quicker ;) I AB'd my bassman with the amp for tonal differences. Running them AB revealed a lot. What revealed even more was plugging into the 410 cab and bypassing the 12" Weber. The speaker is where I am.

    But like I said before - learning to love it. After all - today is all about love hahaha
     
    Last edited: Feb 14, 2020
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  6. cxx

    cxx Member

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    You've got a 40-50 watt amp into a 20 watt OT with an impedance mismatch. I would guess that the OT would color the sound if you drove it hard. If it were me, and I had a HRD OT handy, I'd clip it in to see what It sounds like. If you were looking for civilized tones then it might not matter.
     
  7. Eugene Wallace

    Eugene Wallace Member

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    " The tweed bassman utilizes a fixed bias, so I am not convinced the bias type is the culprit."

    using this logic, the notion of getting the amp to have attributes of a tweed bassman is doomed to utter failure.
    you gottsta get creative on this one if you want tweed bassman tones.


    " Plus - an adjustable fixed bias is the bees knees ;)"

    nah, its just fixed adjustable bias. it's good for some things, not so good for other things, cathode bias is killer in the right application.
     
  8. HotBluePlates

    HotBluePlates Member

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    The output transformer impedance is not a tone thing so much as a power output thing. If you're using 6L6s with a 6.6kΩ output transformer (and typical blackface Fender voltages near 400vdc), the 6L6s won't draw as much current from the power supply or generate as much output power. That's it.

    "Impedance mismatch" = nonissue. The load presented by the OT is the load, whether or not it is the ideal load to get maximum output power.

    You have a good point that if @Mmitch could get more than 20w applied to the OT that it will color the sound. This happens with all transformers, where applying ever-greater power results in the OT pushing the sound towards the midrange, and distorting the bass. That is, at some point the OT core will be too small to get full power at some low frequency, cleanly.

    But this amp won't even get there. The OT is 6.6kΩ, and the power supply is not big enough to allow the output tube plates to swing more than 400v peak. Power = Voltage^2 / Impedance = 400v^2 / 6600Ω = ~24 watts. The output tubes will not be able to pull more than 400v / 6600Ω = 60mA peak, which is fine for 6V6s (and why this is a Deluxe Reverb OT) but nowhere near what 6L6s can do on the same power supply.

    Since the OP is also using a DR power transformer (sized to have the current available for 2x 6V6), this PT likely will have its voltage output collapse if a lower-impedance OT were used with 6L6s. That is, the PT will be the next thing that constrains power output because it's not stout enough for a 40w application.

    The above is why plugging 6L6s in a circuit using a 6V6 transformer set yields 6V6 power output. The typical 4kΩ OT isn't a matter of "matching impedance" with the 6L6s, but allowing the 6L6s to pull larger current. After all, the OT primary impedance is just like a resistor when a.c. signals are applied, and it limits current just like a resistor.

    If the power supply voltage is up around 450vdc and the 6L6s are allowed to swing 400v peak, the 4kΩ OT yields 400v^2 / 4kΩ = 40 watts. Hopefully it becomes obvious why Fender used a 4kΩ OT with two 6L6s to enable 40w of output while keeping the basic supply voltage same/similar as the ~20w amp using two 6V6s.
     
  9. EdFarmer

    EdFarmer Member

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    Curious . . . Did you get around to trying other speakers?
     

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