Why does a Deluxe Reverb have more output power than a Princeton Reverb?

Discussion in 'Amp Tech Center' started by David Barnett, Feb 7, 2020.

  1. David Barnett

    David Barnett Doctor of Teleocity

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    A BF Fender Deluxe Reverb-Amp is rated at 22W into its load. A Princeton Reverb-Amp is rated at 12W. Both amps run push-pull 6V6s, with +420V B+ and similar idle currents and load impedances. So why does the Princeton give up the ghost sooner?

    Some have suggest it's the Princeton's cathodyne phase splitter, but I posit that the power supply (with a Champ power transformer) is packing it in. Maybe it's some of each?

    https://www.tdpri.com/threads/looki...for-my-5f11-build.1004733/page-2#post-9609460
     
    Last edited: Feb 7, 2020
  2. David Barnett

    David Barnett Doctor of Teleocity

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    [​IMG]

    [​IMG]
     
  3. hamerfan

    hamerfan TDPRI Member

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    Try a 12ax7 as PI in Deluxe and you will hear it roar!
     
  4. tele_pathic

    tele_pathic Friend of Leo's

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    I do agree that it is the output transformer. What would the output be if you dropped in a 25 watt op transformer in the princeton?
     
  5. schmee

    schmee Poster Extraordinaire Silver Supporter

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    It's hard to say. It's likely a combination of Power Trans, Output Trans and the PI as well as other minute details.
    I have built several PR clones, even with 6L6's and Super Reverb transformers. Some thoughts just based on playing with these amps
    :
    -A 6L6 PR with those big Super trannys is a loud beast, but it does have some robust grit that a Super Rev or even DR doesn't have. It's dirtier at volume than a Super which can tear your head off with clean!
    -A stock PR modded with only a large PT stays clean much much better (ie: better headroom) than a stock Champ size PT. It becomes dead quiet at idle also.
    -I have a PR with a DR output tranny in it, it helps, but it's not everything and doesn't quite sound like my DR headroom wise.

    Right now my favorite PR based amp has a Vibrolux PT and DR OT. It has the PR PI circuit, not the long tail PI. IMHO it's the PI that is the PR magic. Not as spikey as my DR, but plenty of headroom.
     
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  6. David Barnett

    David Barnett Doctor of Teleocity

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    But will it put out any more power before clipping?

    Gain and power are not the same thing.
     
  7. Ten Over

    Ten Over Tele-Meister

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    The phase inverter doesn't have anything to do with it as long as it can swing a peak voltage that is at least as large as the absolute value of the bias voltage. When the positive peak coming from the PI is equal to the absolute value of the bias, the two voltages cancel each other out and the net voltage on the power tube grid is 0V and that is right about where the power tube will clip. Both phase inverters can easily put out the 35V or so necessary to clip the power tube.

    The power that these amps put out is affected by the OT primary impedance and the amount of HT sag. As a simplified example, let's say that the HT is 400V at idle, the DeLuxe has a 6.6K primary, and the Princeton has an 8K primary. The minimum plate voltage at clipping for both amps is 50V. The amps operate in Class B at full power, so the one tube that is conducting is only using half the primary. Half the primary is one quarter the impedance, so the DeLuxe has a primary impedance of 1.65K. The DeLuxe miraculously doesn't sag at all at full power, so the peak voltage swing is 400V - 50V = 350V. Now that you have the peak voltage and the impedance, you can calculate the rms power and it rounds off to 37W.

    The Princeton HT sags to 320V at full power, so the peak voltage swing is 320V - 50V = 270V. The primary impedance is 2K, so the power rounds off to 18W.

    It's a simplified model, but it illustrates how the impedance and the sag affect power.
     
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  8. schmee

    schmee Poster Extraordinaire Silver Supporter

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    To me, if you want more gainy sound that AX7 will do it, but you are correct, no more headroom and maybe even less.
     
  9. David Barnett

    David Barnett Doctor of Teleocity

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    Thank you!

    This is the kind of information I was hoping to draw out. 100V of sag, wow.
     
  10. jsnwhite619

    jsnwhite619 Friend of Leo's

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    To add an self-educated attempt at understanding these things, I think a lot of people think that a bigger OT simply "handles" things better to all more headroom. They don't figure into it that the bigger one probably has a lower primary impedance, and will allow more current to flow through the circuit.

    Start putting bigger 100mA+ rated PTs on the smaller 6v6 Fender amps that were using the Champ-sized model, and you'll get a noticeable boost in horsepower without even touching the OT.

    Sent from my moto g(6) using Tapatalk
     
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  11. Dacious

    Dacious Poster Extraordinaire

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    Compare the iron in both transformers. The Deluxe transformer has a higher volt-amp rating. That's the power you can pull out of it, literally in watts.

    The Princeton power transformer started out as the Champ power transformer using a GZ34 as a higher voltage rectified output . It's lighter, less winds primary and secondary than the Deluxe so it won't power the tubes biased up to 22 watts. It's also light on for the heater current on the preamp and power tube heaters. Hence the later SFversion used a revised power transformer which upped the heater and power current rating.

    The output transformer is also smaller and lighter duty so if you did manage to drag 22watts out of the power transformer without cooking it, the output will saturate and most likely cook quickly.

    Randall Smith's original Mesa Boogie was a Princeton fitted with Bassman power and output transformer. That's what allowed it to push 45-50 watts. The Bassman units are beefier and larger by far.
     
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  12. tele_pathic

    tele_pathic Friend of Leo's

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    Jim at Lil Dawg amps can fit a Deluxe Reverb in a cabinet the size of a Princeton. Now, there's no tremolo. BUT the WonderDawg amp is basically a Princeton that puts out ~20-22 watts. And it sounds sooooo good.
     
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  13. King Fan

    King Fan Friend of Leo's

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    This is a great discussion, and I learned a lot. Before we close, though, I'm gonna chip in on the side of the poor little overworked Champ PT in the original PR. I've said before that there are two kinds of PR players: Those many who want a bigger, tighter, louder, more modern sound. And those few who want the original, magic, unique sound that made the amp famous.

    Fast country and many kinds of rock want a big, tight sound; bigger PT, OT, and speaker. But blues and ballads and '60s and lo-fi and some jazz and small-venue work want, basically, sag and compression. The original PT and OT (and maybe a non-efficient alnico 10") give us that.

    There's no wrong answer -- we all have different ears as we head out to play different music in different venues. But I'm gonna stick up, some of the time, for a PR that'll melt your heart instead of your face. :D
     
    Last edited: Feb 8, 2020
  14. gusfinley

    gusfinley Tele-Holic

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    What is the contribution of the Deluxe's choke vs. the Princeton's dropping resistor?

    Of course, the DC resistance on the Princeton's 1K resistor is always going to be 1K.

    The Deluxe's Choke appears to have a DC resistance of around 100 Ohms, but is this constant?

    Of course, there is also the plate resistance of the Tube Rectifiers to consider, but it doesn't look like their serial resistances are very dissimilar.
     
  15. The Ballzz

    The Ballzz Tele-Afflicted

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    Very well put, my friend! Once we find "just right" no "more" is needed!

    Just Sayin'
    Gene
     
  16. Dacious

    Dacious Poster Extraordinaire

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    Very little. The resistor is cheaper than a choke which is the main reason it's used in the student amp, plus real estate, choke is also a smoothing/noise defeating measure. Resistors create white noise in a circuit.
     
  17. Paul G.

    Paul G. Friend of Leo's

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    Even with bigger iron the cathodyne PI in the Princeton Reverb limits how hard the output is driven.

    The right answer is that they are different amps, and a combination of factors affects the output power.
     
  18. David Barnett

    David Barnett Doctor of Teleocity

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    Not having looked at it on a scope, this is only speculation, but - we're talking about "rated power" here, which is usually defined as the onset of output clipping. What the amp will do when pushed beyond that into overdrive is not really relevant to the original question. I have little doubt that the Princeton's cathodyne splitter can swing enough volts to push a pair of 6V6s to 22W at clipping.
     
  19. jimgchord

    jimgchord Tele-Afflicted

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    If the impedence of the primary is the same then only the point where it saturates would change
     
    Last edited: Feb 13, 2020 at 12:56 PM
  20. gusfinley

    gusfinley Tele-Holic

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    Mr. DB, is this a little like the Vox AC10 vs Vox AC15 output question?

    The vintage types of these have very similar output sections, but one it "rated" for 10 watts, while the other is 15.
     
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