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Is Princeton Reverb actually 12 watts?

Discussion in 'Shock Brother's DIY Amps' started by itsGiusto, Oct 29, 2019.

  1. itsGiusto

    itsGiusto Tele-Holic

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    Everyone says the Princeton Reverb is 12 watts. But...
    Each of the 6v6GT tubes in the Princeton Reverb are rated at about 12w. Putting these together in class AB, wouldn't the resulting amp be about 24w?

    For example a Marshall 50w uses two EL34s in class AB. Each of those is rated at 25w, and the resulting amp puts out 50w of power.
     
  2. Dacious

    Dacious Poster Extraordinaire

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    No. Power is based not only on the tube's potential but also the power supply. The PR uses a Champ power supply which can only supply around half the current a Deluxe Reverb power trans can. The more you try to crank up current to increase plate dissipation and power, the more B+ voltage will drop and all you will do in the end is overheat and cook the power transformer. Also, the PR output transformer is light-on for delivering more. It will 'saturate' or basically bottleneck the wattage as heat, sound strangled and - bzzzzzzt!

    People can and do hotrod PRs - the original Mesa Boogie was a PR fitted by Randall Smith with Bassman power and output transformer and a 12" speaker.

    The VA rating of the power transformer - volt amps is volts by current - helps to determine how much power you can ultimately pull. The output transformer is again worked out by winds of certain gauge wire around iron and you need more iron and winds for high power - a TR output transformer is like a brick, the PR"s is like a pencil sharpener.

    I get what you're saying about 50 watt Marshalls, but I have a 20 Mini Jubilee that has a pair of EL34s at 300 volts B+ in cathode bias which puts out 20 watts. To increase power would simply require new transformers for 450 volts l in fixed bias like my 4120 JCM800, bigger B+ dropper and grid biasing and would be quite easy to pull 50 watts like the 87 2554 Jubilee combo.
     
    Last edited: Oct 29, 2019
  3. Paul G.

    Paul G. Friend of Leo's

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    My car is rated at 170 hp. But if I put in a bigger turbo, bigger fuel injectors, higher flow fuel pump and reprogram the ECU it could make 400 hp.

    But it doesn't.
     
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  4. sds1

    sds1 Tele-Afflicted

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    So you may be conflating two different measurements, as I have been doing for the past 3 years.

    Power tube plate wattage is a factor of plate voltage and plate current.

    But the 12W rating of a Princeton is a measure of clean audio power into a resistive dummy load, taken at the speaker jack, given an input at some stated frequency and amplitude. The math here is AC RMS ^2 / dummy load.

    For example, in the case of the Princeton Reverb, you would connect an 8R dummy load, inject 100mV @ 200Hz into the input, scope the speaker jack tip, and adjust gain pot on amp until the waveform just begins to distort. You should see around 9.8VAC RMS at the speaker jack tip, or 12W.

    On the Deluxe Reverb (same power tubes, same B+, bigger OT, less rectifier sag) for the same test you'd measure 13.3 VAC RMS at the speaker jack tip for 22W.
     
    Last edited: Oct 29, 2019
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  5. printer2

    printer2 Poster Extraordinaire

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    One thing to keep in mind also, 12W plate rating for the tube is not the level of dissipation it is run at in the amp, more like 70%. Actually if the bias is lower there is more room for a signal. But then the sound may not be as full.
     
  6. CK Dexter Haven

    CK Dexter Haven Friend of Leo's

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    For years I have been told by a variety of people that a PR in good shape at full cry ( 5% THD I suppose)is putting out @ 18 watts, I always took this w/ a grain of salt, but it seems a common belief.
     
  7. Milspec

    Milspec Poster Extraordinaire Silver Supporter

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    With the right speaker efficiency it can sure sound like 18 watts. The question asked is a very understandable one unless you are a tube nerd that understands all that stuff. I had a Gibson Skylark that was rated for 6 watts yet ran dual power tubes (6BQ5) which made me ask the same question...would have expected something more like a Blues Jr., but nope, it struggled to make that 6 watts.
     
  8. Phrygian77

    Phrygian77 Friend of Leo's Gold Supporter

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    The power tubes' max plate dissipation really has nothing to do with output power. Tube amps are really inefficient, class A amps especially. The Vibro Champ I just build consumes about 50 watts from the mains, but it puts out maybe 4-5 watts (haven't scoped it yet).

    I've been able to get about 15-16 watts out of my PR builds with a 100mA PT and a Deluxe Reverb OT. The 1k screen node resistor, vs a choke, probably shaves off a couple of watts also.
     
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  9. schmee

    schmee Doctor of Teleocity Silver Supporter

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    My Princeton Rev has (at this point) a Super Reverb Power tranny. It's huge compared with a stock PT. Headroom for days. Even surprised me.
     
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