Who knows about transformer operating temps?

[mort]

After an amp has been on for a coupe hours, is it normal to have ~150 degrees on the OT and ~190+ degrees F on the PT?

The amp in question is a '66 Princeton Reverb. There are no problems with the amp soundwise. Any amp I've ever played through after a couple hours has gotten pretty darn warm, but I was just considering it and thought I would ask. I took the temps shortly after the amp was powered off with a Raytek red light digital temp thingy. Should be fairly accurate.


Anybosy ever measure actual operating temps?

[R.G.]

If the readings are accurate, those are too hot for long life - maybe.

Generally, you want them under 130F if they are insulated with temperature Class 105 (used to be 'Class A') insulation. However, unless they are stamped with a class number, ('105', 'Class A', '130C', '155C', similar) there is no good way to know that they're not actually the higher insulation class.

The insides of a tranformer, the iron and copper, can work fine at temperatures that will instantly blister your skin. However, the insulation between windings is what gives up, and when the insulation goes bad, electrical arcs get even higher, copper melts, fuses blow, etc. So it's the insulation class that limits how hot they can get and work OK.

I would mistrust the temp reading first. Get a kitchen meat thermometer and hold it in contact with the core. What does it say?

[mort]

How would I take a reading of the core? Wouldn't putting something conductive into the windings be a pretty bad idea?

[Chuck H]

FWIW those amps were never designed to run overdriven for long periods of time. That said... That's the typical info re: most vintage amps. So... The standard has become "can you touch the transformer and hold your finger on it". I think you can safely continue contact with 150*. Not so much 190*. Maybe not the most scientific approach, but then most vintage amps weren't designed scientifically. Or to be tortured by continuous diming.

If you beat the crap out of the amp it will wear out sooner than it would otherwise. The Princeton Reverb is notorious for high Vp and that is only excacerbated by modern wall voltages. I think the greatest likelyhood of failure is a power tube failure. This could, of course, take out other components when it happens.

The best solution I have is to trim the Vp a little with a zener on the PT CT. If the tubes are running on a lower voltage you won't need the same current for the same level of clipping and the PT won't be as stressed. Same tone, just a couple of watts lower. You probably won't even hear a difference.

Running vintage amps on the edge for tone has always been hazerdous, and cool. Great tones to be had. Proceed at your own risk.

[mort]

I've never measured anything in this particular amp. I've always just played it. Maybe I'll go in for a peek here soon, just to see




fwiw, I had the amp on about 5 for this particular sitting. When I play with the band it usually sits on ~8 for about 3 hours(once every couple of months on average)

[Chuck H]

That's a good testiment to the validity of the "finger touch" test that has been adapted for vintage amps. I personally hate that ideology and over iron my builds that will be dimed to avoid such quandries. But it seems to be working, in most cases anyway. If you love the amp, "I" would trim the stresses just so that I could continue to enjoy it. As I said, you probably won't hear much difference. The amp would live much longer. Power tube life would increase. The worst that could happen is that you'll need to put a mic in front of it for gigs (which you probably do sometimes anyway). A simple line out could also boost gig volume too. This could be a box in line with the speaker cable. No internal mods required.

[J M Fahey]

Finger touch test rules.
Touch the laminations and the windings (even through the insulation paper).
Remember that's the *outside*; the inside will be appreciably hotter.
Even if copper can sustain that temperature and more, hot copper has much higher resistance and becomes even less efficient.
Or lossier, if you want it this way.

[JoeM]

After an amp has been on for a coupe hours, is it normal to have ~150 degrees on the OT and ~190+ degrees F on the PT?

The amp in question is a '66 Princeton Reverb. There are no problems with the amp soundwise. Any amp I've ever played through after a couple hours has gotten pretty darn warm, but I was just considering it and thought I would ask. I took the temps shortly after the amp was powered off with a Raytek red light digital temp thingy. Should be fairly accurate.


Anybosy ever measure actual operating temps?

My Princeton runs hot too, but after 47 years it's still going strong. I wouldn't want any new build that hot though.

[Alan0354]

That sounds awfully hot to me. 150 deg on the output is not the end of the world, 190 of PT is hot no matter how you cut it. You can't touch with bare hand. it's only 20 off boiling water!!!

I don't record the OT even get that warm on my Fenders.

[mort]

OK, I probed around a bit in the power section and this is what I found.

First off, the recto tube in place is a 5AR4/GZ34 where the schematic calls for a 5U4GB.

Main - 121vac

PT - 355vac (out to recto)

Output of Recto - 439vdc

neg bias voltage - -39vdc

Power Tubes(same readings on both)
P4 - 427vdc
P3 - 433vdc

Filter section(from input to output of filter section, or bottom to top on the layout)
a - 427vdc
b - 427vdc
c - 340vdc

*** re-measured, must have made an error)
a - 441vdc
b - 430vdc
c - 343vdc

*shouldn't the "b" reading be about halfway between "c" and "a"? Am I correct to think that one cell of the can cap is going [or already] bad, or would such a reading be likely attributed to the resistor between those two points? That particular resistor has been swapped out for a 5W 1K at some point in the past. I mention this because perhaps there was a reason that it was swapped for a higher wattage??


idk, does anyone see any cause for concern on either the recto tube type of the voltages at the filters?


http://www.thevintagesound.com/ffg/s...164_layout.gif

PT: 022722 8316635
OT: 022913 154A2 8316634

I also noticed that the neg voltage cap has been changed to a 47uf 100v cap. What sort of difference could this make in the circuit and could it possibly relate to heat in any way?

[R.G.]

The finger-touch test only verifies that the inside is not too hot. It's based on two different quirks. (1) The inside of a typical transformer with an external temp of less than 130F/54C is, on average, under 105C in the hottest spot. Not always, but on average, for average iron and copper EI transformers. (2) on average, humans will not hold their index fingertip on a metal surface which is over 130F. It happens that the lowest temperature rating for recognized insulation classes in transformers is 105C or Class A. Failing the temperature test doesn't mean it's going bad. It only means to investigate further.

There are other classes, all higher temperature. See NEMA Insulation Classes.

The iron in a transformer keeps its magnetic properties up the curie point of iron - 770C, and the copper works fine, albeit with a rise in resistance, up to its melting temperature, 1083C. So both the iron and copper will continue to work fine up till over 700C. If you used very pure water that did not conduct, then boiling water would make a great heat removal fluid for transformers. Since water picks up impurities and becomes conductive so easily, most really high power, high temperature transformers use some form of insulating oil to carry heat away.

Be that as it may. The limiting temperature on most transformer operation is => the insulation class <= . The insulation fails before the iron and copper do.

So if you can keep a finger on it, it's pretty sure to be OK. If you can't, that doesn't mean it's overheating unless you know that it has only Class 105/Class A insulation. It ** could be class 130, 155, or 180 and be fine at temperatures that will scorch your skin.

On the other hand, nothing proves a point like actual testing. If it's played three hours at a time at highish power for it and survives this many times, chances are it's not too hot.

There is no substitute for actually knowing the numbers. If I were wondering about this, I would get a thermistor and stick it on the trannies, then take their temperatures every half hour or so at the next practice. This would tell you not only how hot it got, but how hot it *could* get, by whether the temperature/time curve had flattened out.

You could also measure the internal temperature by accurately measuring the copper winding resistance. That's how the transformer testers do it.

[Gingertube]

Many Amps Manufactureres make the assumption that their amps will not be driven to 100% for 100% of the time and so cut costs on the Power Tranny. They use trannies designed for a 60 degrees C temp rise (108 degrees F) at 100% loading. A more conservative / reliable design would use Power Trannies designed for a 40 degrees C (72 degrees F) rise at 100% loading.
So while those temps you quoted look high they are fairly typical for a Commercial Amp.
No excuse for it in a DIY Amp.
Cheers,
Ian

[Prairie Dawg]

Good cheap infrared thermometers can be had for a smallish amount from Harbor Freight. Point it at the surface and watch 'er go. Used these to test the heat sink temp and thermostat function on an SWR bass amp. It didn't help-the amp was a POS. But I have used that thermometer many times since. You can also get a multimeter with a thermostatic probe and tape it to the transformer. Costs about $25 at the same Harbor Freight.

Also remember that your PT can absorb radiated heat from the rectifier tube.

[Ronsonic]

How would I take a reading of the core? Wouldn't putting something conductive into the windings be a pretty bad idea?

One way is to measure the resistance of the high voltage secondary when cold, then measure again when hot, then look up the temp coefficient of copper and try to remember algebra. Temperature Coefficient of Copper

Mostly, if you can hold your fingers on it for five seconds without crying that's within limits for a Fender.

[MWJB]

The voltages are not making your amp run hot, the plate current & overall current draw is. The colder you bias your amp, the cooler it will run, but the higher the voltages you will see.

It' possible that with EH/JJ 6V6s you can still continue to run the 5AR4/GZ34 if you wanted, if the amp is a '66 then this is probably what it came with. A 5Y3 would reduce B+ voltage & plate current & cool the amp, but really I'd suggest fitting a bias trim pot, adding a trim pot in series with the 22K bias load resistor on the bias board.

With EH power tubes try up to 23mA @ 470vdc max, I wouldn't go much higher on the plate current at any voltage, no matter how low with a 5Y3. JJ will take 30mA at 425vdc up to as much voltage as the amp can deliver at mid 20's on the plate current. If you want to stick to NOS, then you'll have to run them hot (<30mA) to keep plate voltage reasonable, or go with the 5Y3.

A Princeton Reverb has a PT rated for 2A heater current, stock tube compliment is 2.1A...borderline straight off the bat, they are likely to run on the hot side.