Vintage Princeton Reverb power transformer safe limits

Discussion in 'Shock Brother's DIY Amps' started by itsGiusto, Oct 13, 2019.

  1. itsGiusto

    itsGiusto Tele-Meister

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    I recently installed a switch on my silverface Princeton Reverb to switch between a tube rectifier and diode rectifier (I also included an extra filtering cap on the diode rectifier). I want to ensure it's running safely, and not overtaxing the power transformer (or causing any other issue). Two questions:

    1. What is the safe current draw limit of the transformer? How can I find this out?

    2. How can I measure the current running through the transformer? I'm unsure where I'd hook up my multimeter to measure this.
    Random thought, I could maybe take out the fuse from the amp, and attach the ammeter leads to the fuse terminals? I think I'd have to measure AC current if I did this.
     
  2. Paul-T

    Paul-T Tele-Meister

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    others more experienced will chip in, but that would indeed measure the primary current. Is that what you want?

    My belief is normally when we do rating it's for the secondary. You could work out the secondary via V1I1 = V2I2. Otherwise measure the current to the rectifier. Or measure the voltage across a 5% resistor in the supply chain and use I = V/R.
     
    Last edited: Oct 13, 2019
  3. RottenTheCat

    RottenTheCat Tele-Holic

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    70ma on the B+, same as a Champ. Same PT.

    In reality, we always asked that PT to do more, and it usually would.
     
  4. RottenTheCat

    RottenTheCat Tele-Holic

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    Primary current reflects all three secondaries. Not a valid test, but i wouldn't sweat SS rectifier conversion. Neither will it do much for tone or envelope IMHO. The 5y3 is loafing in that amp, almost no rectifier sag. Its all in the PT.
     
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  5. jhundt

    jhundt Doctor of Teleocity

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    please keep in mind that using a SS diode rectifier in place of a 5Y3 will increase the B+ voltage markedly, and could pose a threat to your 6V6 tubes, which are already being run at a pretty high voltage with the 5Y3. In fact they are being run right at (or over) the limit according to the data sheet. Many people have done this successfully, and I personally do not know anyone who has had a failure. But is is frequently mentioned as a point of concern when the subject of SS rectifiers in Princeton amps is discussed.

    You might be tempted to replace the 6V6's with 6L6's, which may be suitable at the higher B+ voltages provided by the SS rectifier. But then you will run in to the limitations of the 70mA PT capacity, which is really just barely enough for the two 6V6s. Here you run the risk of blowing the PT. Again - this has never happened to me or to anyone I know personally. But there is a lot of talk about it.
     
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  6. peteb

    peteb Friend of Leo's

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    How warm or hot does the PT get after being on for an hour or two?


    If there is over current, it will heat up.
     
  7. schmee

    schmee Poster Extraordinaire Silver Supporter

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    Well, with the SS rectifier the transformer is taxed less right? If it isn't too warm I wouldn't worry... at least any more worry than normal, the PR power transformers do fail often...
    Just use JJ's. My PR/Deluxes run 450-460 volts and JJ's have never been an issue at that voltage.
     
  8. itsGiusto

    itsGiusto Tele-Meister

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    What do you mean by V1I1 = V2I2?

    Hah, yeah, so far I hear absolutely no change. Oh well, liv'n'lern.

    Thanks. In my mod, I did two things to correct the voltage:
    1. I have two 12v zener diodes (reverse biased) after the diodes and before the switch.
    2. I have a separate bias pot for each the tube and diode rectifier.

    After biasing each the tube and diode rectifier, they're each having the 6v6 tube plates sitting at around 419v, for between 50 and 65% max dissipation (seems to be significantly different for left vs right tube). Should this solve the problem you were talking about?

    I see that it's only recommended for 400v on the plate, but when I bias it that way, the percent dissipation was way off, either tube or diode rectifier. What should I do? Is this safe?
     
  9. jhundt

    jhundt Doctor of Teleocity

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    may I ask: what exactly are you trying to accomplish with your modifications?

    so far, we have been answering your direct questions. But we have no idea what outcome you expect to obtain.

    As far as I can gather from the fragmentary evidence, you have decided for some reason to switch from a 5y3 rectifier to a solid-state rectifier circuit. Question #1: why do you want to do this?

    What are you trying to achieve with this modification? How do you expect this to improve your sound from the amp? What aspect of the amp's original sound do you hope to alter with this?

    Having switched to a SS rectifier, you have also taken some steps to lower the B+. Why? If you wanted the lower B+, why did you switch out the rectifier? This part is very confusing to me.

    You do know that Princeton amps are very highly regarded for their sound - just the way they came from the factory - don't you?

    And that they are very easily modified to be louder, sweeter, and even-more-fantastic than they were - but that most of those changes are based around an upgraded power transformer.

    I don't mean to hijack your thread. I just think that if you want any really useful advice or assistance, you have to tell us more about how the amp will be used, and what you are seeking with your modifications.
     
  10. itsGiusto

    itsGiusto Tele-Meister

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    I love my Princeton Reverb. But I always like to experiment, and in all my mods, I always leave it switchable so I can get the stock sound as well. Here are some factors of what I was trying to accomplish.

    1. I always want to learn what are the effects different components have on the final sound, so I wanted to hear how it would sound with a diode instead of a tube rectifier, and also how it would sound with additional filtering in the diode side. Idk, I guess I thought it'd be more of a pronounced effect, given that people who talk about Marshall amps always seem to give great attention to how much filtering happens in the power stage, and talk about how the varying filtering between 67 and 72 caused massive tonal shifts between different specimens during that time.
    I lowered the voltage using zeners and a separate bias pot so I could do a proper AB comparison of the two sounds, and also so I don't end up running the voltage too high, causing power tube problems, as you suggested might happen.

    2. The ground switch was just sitting there unused. I wanted to do something interesting with it!

    I'm totally willing to believe that the mod was pointless, and tube vs diode rectifier makes no difference in this amp. But I also want to know I'm running it safely, which is why I want to measure the current, and also why I now want to now if I should lower the voltage on the power tube plates.

    It looks like in the original schematic, it had the plates at 410v. So is that the safest voltage to run it?
    [​IMG]
     
    Last edited: Oct 13, 2019
  11. peteb

    peteb Friend of Leo's

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    The bias voltage also makes a difference. If both of those are in range, then you are set.



    What you really want to look at is the operating point of the tube. Use plate voltage times plate current to find the plate dissipation at idle and compare that to the max plate dissipation for your tube type. Shoot for 50-70% of max plate dissipation.



    That will only give a good idea what the PT is being put up to, but it is a good place to start.




    If I wanted to check on the PT I would plug the amp into a power monitor, check the current and compare to the fuse amperage for the amp. Others have said the current in the PT secondany is more important than the current in the primary, but I think the current in the primary is also important. That’s where the fuse is. That’s where the over current happens.



    You could probably measure the current in the primary or the secondary by shunting the current through a meter, without harming your amp, but it might or might not be worth the risk for what knowledge you are gaining.
     
  12. itsGiusto

    itsGiusto Tele-Meister

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    Cool, yeah, that was what I originally did. But then I saw that 6v6s are only rated up to 400v on the plate, and got a little scared that there was more to running it safely than just plate dispassion. I'm still a little unsure.

    What do you mean by "shunting"? I thought that for any ammeter to work, the current needs to be running through the meter.

    Also, I sort of assumed that the current in the primary is always equal to the sum of the current in all of the secondaries. Is that true?
     
  13. peteb

    peteb Friend of Leo's

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    You are right, shunting isn’t the right word. I mean to force the current thru the meter, like you say.




    I do not think it works that way. The purpose of the transformer is to boost or drop the AC voltage. The power on either side is supposed to be theoretically some what equal, minus losses. The secondary has higher voltages and so the current must be lower.


    How much current are thise PTs meant to produce?


    Some PTs are spec ed to put out 100 mA for a small amp. I see the 70 mA figure quote above.



    Just going by the one amp fuse in a Princeton reverb, and experience with a power monitor., the primary would peak out at between 500 mA and 1 amp. Could easily be 5x more current than the secondary.



    You could measure the primary at the fuse, that would be handy.


    Measuring the secondary current might require unsoldering each primary and measuring separately,


    If the PT is spec ed for 70 mA is that AC or DC? I don’t know. If it is the DC out, then measuring the idle bias current thru the power tubes, the basic bias calculation, would give a low estimate for DC current draw from the PT.



    Here is some numbers.


    The champ and Princeton PTs are supposed to be similar.

    A champ’s power tube draws 40-50 mA of DC current draw from the PT.

    A Princeton’s power tubes draw ussually around 20 mA each for 40 mA of DC draw from the PT and as low as 10 mA each for only 20 mA of DC current draw from the PT.
     
    Last edited: Oct 13, 2019
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  14. itsGiusto

    itsGiusto Tele-Meister

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    Right, of course. I just wasn't thinking.
    So if I know that the voltage is getting multiplied by roughly 5.5 or 6 by the transformer, then I can read the current using my fuse method, divide it by 5.5, and use that as the upper bound of what the current in the secondary is, I think.
     
  15. Dacious

    Dacious Poster Extraordinaire

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    Chassis out, the best and easiest and very accurate method of checking bias is transformer resistance method.

    Measure voltage at centre tap and plates, pull plug from wall and let the voltage die off in the caps.

    Measure each primary half resistance.

    Voltage drop/resistance = bias current per tube.
    .
    Bias_Resistance_Method.gif
    .
    Ignore 6V6 voltage - as long as you're using 6V6GT Leo and others run them 440-450 volts.

    My PR ran 420 volts B+.

    It's where you set the bias point and total plate dissipation that determines tube life.

    My Superchamp runs 6V6s at 400+ volts, and I'm getting 18 watts measured for the pair. Deluxe Reverbs run 22 watts out of them, with 450-460 plate volts.

    The limits to the transformer will become apparent - trying to pull too much power out if it will cook it. You will notice as you try to increase plate current, the B+ will drop away and your transformer will heat up. Also your tremolo will fade, try to run too much current the bias wiggle fails.

    If everything is in good condition a healthy one will put 420-440 volts on the plates depending on mains voltage.

    The differences between PR and DR apart from preamp and the extra channel is the PT and OT are much beefier and the DR runs tubes much harder. Yet they still last years. So you need not worry.
     
    Last edited: Oct 14, 2019
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  16. Paul-T

    Paul-T Tele-Meister

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    V1I1 = V2 I2
    Is the formula to find the current, I, in the secondary. But it's not very helpful as you have several secondaries. But just in case

    Primary Voltage x Primary Current = Secondary Voltage x Secondary current.

    But for a proper analysis, put the ammeter in series with the transformer, as mentioned earlier.
     
    Last edited: Oct 14, 2019
  17. peteb

    peteb Friend of Leo's

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    Here are some more sample numbers:



    Champ AA764

    V2 / V1 = 640 / 115 = 5.6


    the max current on the primary is near 0.5 amps.


    0.5 / 5.6 = 89 MA, about right for a champ PT.

    For reference the power tube is going to draw about half of that.



    Princeton reverb AA1164

    V2 / V1 = 680 / 115 = 5.9

    the max current on the primary is also going to be around 0.5 amps.

    0.5 / 5.9 = 85 mA, about right for the PT on a PR

    the power tubes are going to draw about half that.



    Bassman AA864


    V2 // V1 = 610 / 115 = 5.3


    the max current on the primary is going to be around 1 amp.

    1 / 5.3 = 189 mA, about right for the PT on a BM.

    the power tubes are going to draw around 1/3 to 1/2 of this.






    I am now seeing why my princeton draws less wall current at idle than my champs even though it has 2 power tubes. One reason is I have the princeton biased where the two power tubes draw less current togethor at idle than one champ power tube. makes sense.
     
  18. itsGiusto

    itsGiusto Tele-Meister

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    How exactly do I know if it's a 6v6GT? I'm running the original RCA 7409 tubes.
     
  19. Dacious

    Dacious Poster Extraordinaire

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    They're black smoked tubes? They're GTAs.

    Probably original. The ones in my Superchamp only just wore out - after 35 years. A Princeton is a doddle for them.

    Like I said, they been in there that long, they may wear out but aren't going anywhere soon.
     
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