PT Mythbusting

[jordan86]

I have read/heard the notion that if want to experiment with running bigger glass in a Princeton (5881/6L6) that it is advised to use a SS rectifier in place of the tube rectifier, to "free up that current". Read it on RobRobinette's site and heard straight from the mouth of a VERY reputable builder. After doing more research, I'm perplexed as to HOW this is the case.

My 290AX supplies 2.25A on the 6.3v heaters, 100ma on the HV secondary, and 3A on the 5v rectifier heater. A little more stout than a "stock" Princeton PT. According to this calculator I would need over 3A on my 6.3v heaters. Not a chance. If I pull the V2 (the reverb driver) I get down to 2.79. Closer, but not that close. The one thing this calculator does not account for is the rectifier current. Gives the option to select a rectifier tube or diode, but that doesn't effect the heater current. Makes sense. https://thesubjectmatter.com/calcptcurrent.html

So my mythbuster question....does using a SS rectifier REALLY help the undersized PT provide more current?


The only explanation I can think of is that I am no longer using those 2A of the 3A available on the rectifier tap with my GZ34 out of the equation. But does that really get "freed up" though or more specifically said, "reallocated" to the 6.3v heaters? That would imply that my PT is cumulatively capable of 5.25A? Never heard of it working like that.

[sluckey]

So my mythbuster question....does using a SS rectifier REALLY help the undersized PT provide more current?

Yes. The SS rectifier puts out more B+ than a tube rectifier. Increased B+ means the tubes will run a bit hotter which translates as more current.

[shooter]

another + in my book, using a FWB SS provides better filtered DC (less hum).  It's also simpler "design" in "new" construction
PS "sag" might be a thing for a musician, but a good musician can overcome that with talent, or pedals 

[sluckey]

using a FWB SS provides better filtered DC (less hum)

How so?

If comparing to a one hump half wave recto, then sure. But if comparing to a conventional 2 diode full wave recto, then the dc output has the same number of humps.

[jordan86]

So if I am interpreting you correctly, Sluckey...An SS replacement doesn't actually tax the PT for less current. Just ups the voltage over what a tube would provide to create more power without requiring additional current from the PT?

I wouldn't be looking at a total rebuild of the rectifier circuit. Just a plug in SS, like a copper cap or similar.

[sluckey]

ALL CURRENT COMES FROM THE PT. Tubes do not generate current.

[kagliostro]

A x V = W

if you have the same available A (current) but you increase a bit the V (voltage) as result W (power) will increase

swapping from Vacuum Tube Rectifier to Solid State Rectifier will not increase the voltage the transformer feeds, only save a bit of it because Vacuum Tube Rectifier has a higer resistance than Solid State Rectifiers and is on that (vacuum Tube) recistance that you have the drop of voltage

Franco

[jordan86]

Appreciate the conversation and input here. Yes, I am tracking on that. The PT is the source of everything. Voltage and current. I also understand that more voltage with the same available current equals more power. My question is more so related to the PT's spec'd current limitations.

I guess the clarifying question is...If I use a plug in SS rectifier, what happens with the "available" 3A on the rectifier heaters? Does removing the current draw from the tube rectifier free up the PT to provide more than spec'd current on the 6.3v heaters?

Maybe that's just something I could reach out to Hammond on?

[sluckey]

I guess the clarifying question is...If I use a plug in SS rectifier, what happens with the "available" 3A on the rectifier heaters?

The current demand drops to zero, but the "available" 3A is just hiding inside the PT, just waiting for you to put a load on it. (or not)

Does removing the current draw from the tube rectifier free up the PT to provide more than spec'd current on the 6.3v heaters?

Not enough to even measure. The PT will run a bit cooler though.

Maybe that's just something I could reach out to Hammond on?

Yes. The guys at Hammond are very helpful.

[jjasilli]

Actually, the PT is not the source of anything.  If it were, we wouldn't need the power company or batteries.  We could power everything with transformers!  Batteries or solar cells, etc., can power amps; but for our purposes all power comes from the Power Company, through your wall outlet, through the PT and through the tubes.


Per Ohm's Law, a resistor in series in a circuit does 2 things simultaneously: it drops voltage and limits current.  A tube rectifier has more resistance (though here we call it impedance due to the presence of AC current) than does a SS rectifier.  Hence, a tube rectifier will drop more voltage and limit more current than does a SS rectifier (which drops only about 1V). 


With less B+ on the plate, the tube will draw less current - all other things being equal.  But we can make a tube draw more current, at lower voltage, by biasing it hotter. 


We can pull as much current as we want, from -0- to infinity (and beyond, for Buzz Lightyear).  If we substitute the load of the tube with a short circuit we can in theory pull an infinite amount of current through the PT and the rectifier.  But if we allow more current to be pulled than the fuses, wires or components can handle, they may be damaged or fail.


Whatever current anything draws, it draws from the Power Company (or some other actual source of Power).

[shooter]

 

If we substitute the load of the tube with a short circuit

In the Navy we called it min volts, max smokes 

[PRR]

> what happens with the "available" 3A on the rectifier heaters?

It can support "some" more power on another winding. You can always suck more current through a transformer, but it heats-up and dies young. Stop sucking 5V 3A, that may be 10%-15% less total load. You can't reallocate all of that on another winding, but some of it.

"Myth"? Thing is, a transformer should serve a decade or a century. So it takes gross abuse to "bust the myth" in the attention-span of the innernet.

[trobbins]

Typically the heater windings are the outermost windings on the PT core.  The 5V winding may be the outermost winding - whether that is observable not sure.  If 5V winding is not conducting current, then a 6V winding under/over it will be operating cooler, and so can be allowed to operate at slightly higher current for the same temperature rise (as would occur if 5V 3A was being supplied). 

Also loading the 6V winding with higher current will droop the voltage available at the valve heater terminals, and that would slightly reduce the current draw from each 6V heater.

So there are two subtle effects that likely allow the 6V winding to support a higher than rated heater loading, without venturing in to the VA rating capability and what happens on the primary side of the transformer.

[vampwizzard]

Multiple secondaries have to add up to the overall loaded VA. When you use a SS rectifier you aren't drawing rectifier filament current out of that dedicated winding. You have more VA left in the bank to use elsewhere. The limiting factor becomes the heat because I^2*R adds up real quick.

On the 290AX the secondary ratings show 61.7VA available. Drop the filament current with a GZ34 and you get a little bump to VA and if you take out a tube filament altogether you're only using 46.7VA. Hammond usually does 30% derate off max KVA on their transformer heat runs (the larger ones at least) so youre probably going to be ok running the 6L6's in that configuration.

[jjasilli]

Multiple secondaries have to add up to the overall loaded VA. When you use a SS rectifier you aren't drawing rectifier filament current out of that dedicated winding. You have more VA left in the bank to use elsewhere.

1.  I'm not seeing how VA applies, because it's Current not Power at play.  It's specifically the Current component of electricity that burns things out, or causes the amount of voltage drop across a specific resistance (or impedance).  If VA is 10, that could be: 1V @ 10A; or 10V @ 1A. 


2.  In terms of current handling, each secondary winding seems to function as if it were a separate, standalone transformer (ignoring complexities such as heat build-up which increases static resistance; and magnetic, inductive and capacitive reactance among the closely knit windings).  Each winding has its own current handling capacity.  It is not immediately clear how the disuse of one winding will enable some other winding to handle more than its rated current.  (Except maybe that the tranny will run cooler if some windings are unused???)

[vampwizzard]

1.  I'm not seeing how VA applies, because it's Current not Power at play.  It's specifically the Current component of electricity that burns things out, or causes the amount of voltage drop across a specific resistance (or impedance).  If VA is 10, that could be: 1V @ 10A; or 10V @ 1A. 


2.  In terms of current handling, each secondary winding seems to function as if it were a separate, standalone transformer (ignoring complexities such as heat build-up which increases static resistance; and magnetic, inductive and capacitive reactance among the closely knit windings).  Each winding has its own current handling capacity.  It is not immediately clear how the disuse of one winding will enable some other winding to handle more than its rated current.  (Except maybe that the tranny will run cooler if some windings are unused???)

Overvoltage will certainly blow out the windings.. thats why we short circuit (hi pot) test them but current is the larger factor only because of its relation to heat and its squared relation (I^2*R). Transformers are given maximum and design VA based on how they "heat run", so you'd get equivalent heating at equivalent VA.

This link is somewhat advanced but has the transformer design calcs and describes them the theyre designed.. by constraints.
https://ecee.colorado.edu/~ecen5797/course_material/Ch15slides.pdf

Transformers are electromagnetic devices and in this case the first constraint is that core stack. We imagine the design is optimized for that core size to meet the second constraint, operating temperature. The overall temperature is summed over all currents running through windings. The longer calc is early on in that slide show but when you use multiple secondaries all that matters is the sum over all secondary winding contribution.

[jjasilli]

Somewhat advanced.   


Anyway my, perhaps limited, understanding of overvoltage is, once again, it's the current that's the problem, not the voltage.  Too much voltage dropped across a component or device will cause commensurately too much current to flow.  It's the current flow that causes issues, not the voltage charge in-and-of itself.  (For all I know, maybe some things are damaged by a mere charge even w/o current flow; but I don't think transformers are one of them.)


Dan Torres opines that an amp sounds better if its PT is relieved of filament supply duty, and a separate filament tranny is installed.  You, PRR and possibly Bill Monroe are taking a similar position.  If true, what is the scientific explanation?  (I'm willing to settle for less heat.  But math equations with large Greek symbols are persona non grata.  )

[shooter]

But math equations with large Greek symbols are persona non grata.

then you'll never get an "answer" 
the math folk are the end of the line, if they say Yes we have to believe, or crank the formula n prove them wrong.  I'm sure there's an imaginary number in there somewhere 

[vampwizzard]

But math equations with large Greek symbols are persona non grata.

then you'll never get an "answer" 
the math folk are the end of the line, if they say Yes we have to believe, or crank the formula n prove them wrong.  I'm sure there's an imaginary number in there somewhere 

 

You're not wrong.. inductance/impedance calculation is a M*@&#$)er. I designed several hundred dry type 600V transformers in the prior EE career.. the short answer is that VA (energy, potential energy) is the rosetta stone. Whatever you change, youre bound to the VA rating on the transformer. Gaining an amp at one voltage equals a different new available current at a different voltage, converted through VA.

there are two ratings on conductors.. voltage and current. THHN is good for 90 degrees C up to 600V and electrical codes dictate how much current theyll let you pass through it and maintain that temperature compliance.

On a transformer you're using a different insulator. Nomex is an important one. Sand casting is another one.. you just fill the damn thing with sand. When we power rate the transformer we see how hot it gets under full load and check for continuous steady state heat in its enclosure and for the hottest spot in the windings to make sure that the insulation degrades over 30 years. A lot like the effect on biasing hot.. itll work if you cook it but for how long?

You'll just have to trust me. Or not. 

[HotBluePlates]

> what happens with the "available" 3A on the rectifier heaters?

It can support "some" more power on another winding. ... You can't reallocate all of that on another winding, but some of it.

... current is the larger factor only because of its relation to heat and its squared relation (I^2*R). Transformers are given maximum and design VA based on how they "heat run", so you'd get equivalent heating at equivalent VA. ...

Guys, the OP's power transformer is a Hammond 290AX meant for 2x 6V6s (so 1x 6L6 oughta be fine).  The high voltage winding is rated for 100mA, so I'm not sure why we need to worry about switching to a solid-state rectifier to gain anything.

Separately, this is Hammond's replacement for the 125P1B that went in both the 60's push-pull 2x 6V6 Princeton amps, and Fender's 1x 6V6 Champ/Vibro Champ amps.  Hammond's part has a B+ winding resistance of 277Ω (not counting reflected primary resistance).

I can tell you from measuring my 1965 Vibro Champ that the original part has a B+ winding resistance of 378Ω, and is under-loaded when running only 1x 6V6.  To get 6.3vac on the heaters I had to dial the line voltage down to just under 107vac, at which point B+ was ~370vdc with a 5Y3 and the 6V6 pulling ~39mA of cathode current.

Pushing the line voltage up to 110vac yielded a B+ of 388vdc and 40.3mA of cathode current.

Meanwhile, my 1964 Deluxe Reverb landed on 6.3vac and B+ below schematic values with a GZ34 and a line voltage of 120vac on-the-nose.

Point being, the original part was very under-loaded with only 1x 6V6, so 1x 6L6 (drawing heater & plate current equivalent to 2x 6V6s) should be no sweat. Moreso in the OP's case where the B+ winding is lower resistance (and so will already deliver higher B+ voltage than my 1965 transformer).

[vampwizzard]

(so 1x 6L6 oughta be fine)

The way I read it was to replace 2x6v6's with 2x6L6's. Everything else you said I agree with.

[shooter]

 
me too
went to fender docs and there are 2 types SE 5xx n PP aa964

[ac427v]

I don't see RobRob specifically addressing running 5881 tubes in a Princeton Reverb. But if that's what you want to do, I think his general comments are helpful. One additional suggestion. If you use the plug in silicon rectifier, use the low voltage secondary wires on your Hammond 290AX power transformer. In my build they are 275-0-275 volts. With a plug in rectifier and TungSol ri 6V6GT tubes I get 385 volt B+. The lower secondary voltage allows you get the heat reduction from not using the 5 volt windings and tube rectifier without getting the extra current demands of running bigger tubes at a silicon-boosted voltage.

[pdf64]

Even if the HT winding was able to fully utilise the 15VA freed up from the rectifier heater winding, it still seems somewhat (woefully) inadequate to supply a p-p pair of 6L6 at maybe 450Vdc.
So yes, lower voltage taps seem pretty much mandatory.

[SILVERGUN]

I have a 290AX here so I checked it because I don't remember it having dual secondary HT options.
It doesn't. So, I'm sure this was a "recent" upgrade.
jordan86, I'm just posting this in case you look at yours and only see the 650VCT option.

[HotBluePlates]

... Guys, the OP's power transformer is a Hammond 290AX meant for 2x 6V6s (so 1x 6L6 oughta be fine).  ...

The way I read it was to replace 2x6v6's with 2x6L6's. Everything else you said I agree with.

Doh!!  Thanks, I had tunnel-vision on the "sensible move" and was thinking a tweed Princeton!  No idea why, since this PT went in the blackface Princeton (2x 6V6) amps (and OP mentioned the reverb driver).

Yeah, bad idea to try to use this PT for a 2x 6L6 amp.

Princeton - 290AX: 100mA and 277Ω winding resistance
Deluxe Reverb - 290CX: 207mA and 84Ω winding resistance (FWIW, my 1964 DR's 125P23B has a measured 220.4Ω of winding resistance)
Super Reverb - 290D2X: 300mA and 44.5Ω winding resistance

Lower current rating and higher DCR compared to a PT intended to run 2x 6L6 will cause this amp to sag into the ground.

[kagliostro]

Talking about the non use of the 3A winding

If we don't use that 3A some W of power are still available from the transformer and teorically we can take more current from the other windings

Someone rightly told that those 3A are not full available on the other windings

the core of the transformer can afford to feed those further 3A of current without became too hot, so where is the problem ?

Simply the problem is the thickness of the wire of the other windings

When a transformer is planned the thickness of the wires for each winding is thinked to suffice the current the specific winding is planned to be able to affort and no more

--

Teorically, if you add some windings to the 5V (3A) winding till it can give you 6.3V of output (instead of the 5V) you can take from that winding around 2.38A of current (5V x 3A = 15W --- 15W : 6.3V = 2.38A)


I've seen someone do something like that (without dismanting the transformer, but it was a very skilled guy and the transformer had been constructed in a way that permitted to do that)

Franco

[Soulfetish]

Talking about the non use of the 3A winding

Someone rightly told that those 3A are not full available on the other windings

the core of the transformer can afford to feed those further 3A of current without became too hot, so where is the problem ?

Simply the problem is the thickness of the wire of the other windings

Exactly right. For instance, I put a dedicated secondary winding in my PT to run my driver tube from a dedicated, separate +/- power supply for a directly coupled output stage. It was wound for a high(ish) potential-low current supply for 12AU7.
If I ran the transformer with the main HT and Heater windings open, and tried to pull the full VA of the Transformer through that winding, the winding would overheat excessively. I imagine it wouldn't take long for the insulation to breakdown leading to a shorted turn and then the PT is toast.

When a transformer is planned the thickness of the wires for each winding is thinked to suffice the current the specific winding is planned to be able to affort and no more

--

Teorically, if you add some windings to the 5V (3A) winding till it can give you 6.3V of output (instead of the 5V) you can take from that winding around 2.38A of current (5V x 3A = 15W --- 15W : 6.3V = 2.38A)


I've seen someone do something like that (without dismanting the transformer, but it was a very skilled guy and the transformer had been constructed in a way that permitted to do that)

Franco

This would almost be too easy if you were using a Toroid. Probably talking about a few extra loops around?

[jordan86]

Thanks for all these replies. Helpful for me to read through all the more experienced opinions and perspectives.

1) Yes, I do have a newer 290AX. I believe they redid it a few years back, merging the features of two older models into one. I do have the dual HT options. I like the amp with both feels, and am grabbing a DPDT switch to be able to use both.

2) Yes, I am looking to run two 5881's. Or possibly 6L6's (but more likely 5881's). Not just one in SE.  Robinette does specifically mention 6L6's on his Princeton page. Still seems kinda crazy to me that a stock Princeton could survive that. David Allen of Allen amps was the one who gave me the OK on 5881's so long as I use a SS rectifier.

3) I also wrote Hammond a few days back with my model number and the specifics of what I am trying to achieve. Based on my calculations, I need about 2.8A on the 6.3V filament. They gave me the green light on that so long as I do not use the 3A rectifier winding. My 2.8A calculated filament draw would be with 5881's (although I guess 6L6's would draw the same on the heaters) and pulling the 12AT7 reverb driver.  I would probably only do this in a live situation where I need max clean headroom. In that scenario, my amp would likely be mic'd offstage while I am running verb and other effects from my pedalboard. So losing the spring verb in amp would not be an issue.

And speaking of max headroom, I took my Princeton over to a friend's house two nights ago. He just recently gutted his 1990's Deluxe Reverb and rewired with the Mojotone small parts kit. So nearly identical in terms of circuit. My PR has the 290AX, Allen's TO20B, the stokes mod, and a few lower bypass cap values. I was delighted that my PR stood toe-to-toe with his DR in terms of clean volume/headroom, but with fatter mids and that Princeton sweetness the big boys don't have.