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 Post subject: PACO E-200-C with 5u4GC rectifier
PostPosted: Jan Wed 12, 2022 6:13 pm 
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I've seen about a dozen of these so, I apologize for rehashing. I just picked up a Precision Apparatus E-200-C, one of the very later variants. I think my serial number is somewhere in the 450,000 range. It seems to have the same basic circuit as the 1091 drawing, with a 6au6 and 6u8, but I have a 5u4GC for the rectifier. It seems like a hell of a lot of rectifier tube for this, and with some quick checks after recapping, it is putting out somewhere near 320V DC.

Does anyone have a schematic or a unit with this rectifier? I'm wondering if it was a modification by a previous owner as I can only find info with a 5Y3 rectifier tube, and if the 1091 drawing is fine to work with. So far I haven't noticed any other differences, but I just started on the thing last night.


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 Post subject: Re: PACO E-200-C with 5u4GC rectifier
PostPosted: Jan Wed 12, 2022 8:04 pm 
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You are right, it is supposed to have a 5Y3GT rectifier, no wiring changes needed for it.


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 Post subject: Re: PACO E-200-C with 5u4GC rectifier
PostPosted: Jan Wed 12, 2022 8:27 pm 
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Thanks. I just got looking at the tube charts for both and from what I can tell, they seem to be interchangeable, same pin out and similar specifications. Previous owner must have had one on hand instead of a 5Y3, but given prices that I'm seeing for the 5u4GC right now, that seems like a pretty expensive swap.


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 Post subject: Re: PACO E-200-C with 5u4GC rectifier
PostPosted: Jan Wed 12, 2022 8:32 pm 
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The 5U4G could be a LOT more expensive than a 5Y3GT due to its greater draw for the filament. That extra ampere could spell disaster for the transformer.


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 Post subject: Re: PACO E-200-C with 5u4GC rectifier
PostPosted: Jan Wed 12, 2022 9:05 pm 
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Joined: Jun Sat 09, 2007 8:14 am
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People do all sorts of not a good idea subs. I bought a B&K 700 from a TV shop that had an 80 in it instead of an 83.

RRM


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 Post subject: Re: PACO E-200-C with 5u4GC rectifier
PostPosted: Jan Thu 13, 2022 1:03 pm 
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Location: Long Island NY
You'd be surprised how common it is that somebody will use whatever they have on hand to save a few cents. 5U4 tubes were used in many transformer operated TV sets from the end of WW-2 until the early 1960s. Some larger sets had two. Many years ago they were as common as dirt in any TV repair shop.

The specs are not entirely the same, as noted the 5U4 filaments draw a full ampere more than those in a 5Y3. While this did not cause catastrophic failure of the power transformer, it would cause the transformer to run warmer. That might have been okay when the transformer was new, but that insulation is now probably somewhere between 60 and 70 years old and additional heat stress will not improve its longevity.

Some may suggest replacing the 5Y3 with a pair of diodes or a "plug in" solid state rectifier. I generally do not recommend this, first because the B+ voltage will be a lot higher and may need you to add dropping resistors to bring it back into line. The peak currents in the HV secondary of the transformer will be higher and it will run hotter without dropping resistors, but if you add the resistors you're simply moving the heat to the outside of the transformer so that may not help it much. The simplest and most satisfactory course of action in my opinion is to simply put the generator back on a 5Y3.

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 Post subject: Re: PACO E-200-C with 5u4GC rectifier
PostPosted: Jan Thu 13, 2022 1:39 pm 
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Whether or not putting in a 5u4 for a 5Y3 stresses the power transformer more, depends on how conservative the design of the power transformer is in the first place.

If you load the 5V winding on the power transformer with a lower resistance load (as the 5u4 does compared to the 5Y3) the additional heat generated depends on the resistances of both the primary and secondary windings.

Lets say the resistance of the secondary is Rs Ohms and the resistance of the primary is Rp ohms, you can think of this as one resistor, where the impedance of the windings are transformed into each other by the square of the turns ratio.

If the primary is 115V and the secondary 5v, you can imagine that the primary resistance has vanished and that the secondary resistance is now Rs + Rp' Ohms where:

Rp' = Rp x (5/115)^2

Clearly then the heat dissipation in the transformer, as a result of increasing the filament current from 2 amps for the 5Y3, to 3 Amps for the 5U4 (1 amp increase) is;

Extra power loss as heat with 5U4 = (1 amp)^2 x (Rs + Rp(5/115)^2)..........That old I squared R chestnut.

If the power transformer is a decent physical size with low range resistance windings this sort of increase in load current is very well tolerated as its only an additional 5W of load.

But, if the transformer designers scrimped on the transformer size, with a small bobbin and stack, necessitating a smaller gauge wire, with higher range primary and secondary winding resistances, and the transformer was running hot with the 5y3, the extra heating caused by the load of the 5U4 heater could push it to failure.

You could therefore measure the primary and secondary resistance of your transformer and calculate the additional power losses using the 5U4. Likely if its less than 4 or so watts it will not be a problem as the additional transformer heat will be tolerated.

Another method to solve the dilemma is to put a temperature sensor on the power transformer. If the temperature is only modestly elevated using a 5U4, say maybe less than 7 to 10 degrees, its probably fine.

One way to think about this is to compare two 115 to 5V transformers. One is the size of your living room with 1/4 inch diameter wire and extremely low winding resistances, one transformer fits in the palm of your hand and has moderately resistive windings. For the large one with incredibly low resistance windings, you could add loads of many 100's of watts, its temperature would barely even elevate. The small one would start to get hotter with an additional few watts of load.


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 Post subject: Re: PACO E-200-C with 5u4GC rectifier
PostPosted: Jan Thu 13, 2022 2:08 pm 
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My take on this? Keep it simple. Just use the original designed-in tube, the 5Y3. It will outlive most of us at this point in time.

One factor that is a wild card here is the aforementioned 50+ year old transformer. I think we can be pretty sure that it was never designed for, nor envisioned to last, this long. So, I'd tend to leave it the way it was designed, especially since the cost of a brand new shiny NOS 5Y3 is unlikely to be more than all the time and fussing spent to try and modify the thing.

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 Post subject: Re: PACO E-200-C with 5u4GC rectifier
PostPosted: Jan Thu 13, 2022 2:41 pm 
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Barry H Bennett wrote:
My take on this? Keep it simple. Just use the original designed-in tube, the 5Y3. It will outlive most of us at this point in time.

One factor that is a wild card here is the aforementioned 50+ year old transformer. I think we can be pretty sure that it was never designed for, nor envisioned to last, this long. So, I'd tend to leave it the way it was designed, especially since the cost of a brand new shiny NOS 5Y3 is unlikely to be more than all the time and fussing spent to try and modify the thing.


Not bad advice, since it appears that vintage transformers have a penchant for running hot and the insulation is aged and degraded.

And if the line voltages goes up to around 120 to 125V the situation is worse.

I was surprised to find that a pre WW2 115V line transformer (that is not faulty) running on 100V @50 Hz had peak primary (off load) magnetization current of 1 Amp. Where as a similar sized & power rated modern core power transformer from Hammond, it is under 100mA.

Old American (60Hz) power transformers appear to run hot on 120V @ 60Hz. On 100V line voltage @ 50Hz (which in terms of peak currents and peak flux is equivalent to about 120V @ 60Hz) it is very similar.

It had led me to wonder if the transformer cores could have possibly been like this from new. Or, conjuring up a theory that the cores may have somehow degraded and lost permeability. If you replaced any of the old transformers with a modern one from Hammond, it will always appear to run as cool as a cucumber.


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 Post subject: Re: PACO E-200-C with 5u4GC rectifier
PostPosted: Jan Thu 13, 2022 4:55 pm 
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Location: Long Island NY
Quote:
Extra power loss as heat with 5U4 = (1 amp)^2 x (Rs + Rp(5/115)^2)..........That old I squared R chestnut.

If the power transformer is a decent physical size with low range resistance windings this sort of increase in load current is very well tolerated as its only an additional 5W of load.


That's the heating in the tube, not the heating in the transformer. Heating of the transformer winding is its resistance times the current squared. A complete discussion would include the losses in the iron and the primary but they can be neglected here.

I happen to know the Precision E-200 generator has a relatively small power transformer as it only supports two small receiving tubes aside from the 5Y3. Therefore I will take a wild guess and say the 5-volt winding has about half an ohm of resistance. With the 5Y3, it will dissipate 0.5(2^2) = 2 watts as heat. With the 5U4 it will dissipate 0.5(3^2) = 4.5 watts of heating. So it must've gotten warmer with the 5U4, but obviously it survived. I just wouldn't tempt fate by running it that way now. The cost of a new replacement transformer and the labor to install it would likely exceed the value of the apparatus by a considerable amount.

Quote:
I was surprised to find that a pre WW2 115V line transformer (that is not faulty) running on 100V @50 Hz had peak primary (off load) magnetization current of 1 Amp. Where as a similar sized & power rated modern core power transformer from Hammond, it is under 100mA.

Old American (60Hz) power transformers appear to run hot on 120V @ 60Hz. On 100V line voltage @ 50Hz (which in terms of peak currents and peak flux is equivalent to about 120V @ 60Hz) it is very similar.

It had led me to wonder if the transformer cores could have possibly been like this from new. Or, conjuring up a theory that the cores may have somehow degraded and lost permeability. If you replaced any of the old transformers with a modern one from Hammond, it will always appear to run as cool as a cucumber.


It's the insulation that determines the operating temperature of a transformer. The old 1920s and early '30s organic insulation systems could not tolerate very high temperatures so most of those had to be designed to run cool to the touch. By the early 1940s, synthetic insulation systems which could tolerate much higher temperatures safely were coming into use so transformers got smaller, lighter, cheaper--and hotter. Hot running is not necessarily a bad thing in a transformer, as long as it is not so hot that the insulation degrades prematurely. Some modern insulation systems like Class N or Class H are designed to be used where winding temperatures can reach 200 or 220 degrees C, respectively. 220 degrees C is 428 degrees F, so don't try holding your hand on a transformer like that! As an aside, I judge transformers not by their temperatures but by other effects. A good transformer to me should be perfectly silent, give good output waveforms, and not leak magnetic flux. Ones that buzz, leak magnetic fields, and have highly distorted waveforms don't impress me a whole lot.

The other thing that has to be remembered is that transformer steels have come a long way too. Modern practice is to use grain oriented, high silicon steel in quality transformers as it gives the best flux density with the lowest eddy current losses. But modern lamination steel formulations developed over the years and some are quite recent, and as always some manufacturers sacrificed efficiency for lower cost. Lossy, inefficient steels will of course require higher magnetization currents and will get hotter. But I do not know of any mechanism that would change the composition of lamination steel unless it got so hot that it de-alloyed.

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Thomas A. Edison


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 Post subject: Re: PACO E-200-C with 5u4GC rectifier
PostPosted: Jan Thu 13, 2022 8:30 pm 
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Joined: Jun Sat 09, 2007 8:14 am
Posts: 5247
Location: Melbourne, Florida
As has already been mentioned: use the tubes things were designed for.

RRM


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 Post subject: Re: PACO E-200-C with 5u4GC rectifier
PostPosted: Jan Thu 13, 2022 9:46 pm 
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Retired Radio Man wrote:
As has already been mentioned: use the tubes things were designed for.

RRM


Part of the fun is finding substitute tubes to solve problems and improve performance. Newer tubes were developed over the years and I adapted earlier sets to use them and get the benefits. I often suggest different tubes on my posts on this site, to solve problems.


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 Post subject: Re: PACO E-200-C with 5u4GC rectifier
PostPosted: Jan Thu 13, 2022 10:28 pm 
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BikenSwim wrote:
Retired Radio Man wrote:
As has already been mentioned: use the tubes things were designed for. RRM


Part of the fun is finding substitute tubes to solve problems and improve performance. Newer tubes were developed over the years and I adapted earlier sets to use them and get the benefits. I often suggest different tubes on my posts on this site, to solve problems.

Ok. What's the problem with a 5Y3 as the rectifier in this generator ? :mrgreen:

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 Post subject: Re: PACO E-200-C with 5u4GC rectifier
PostPosted: Jan Thu 13, 2022 10:59 pm 
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Chris108 wrote:
Quote:
Extra power loss as heat with 5U4 = (1 amp)^2 x (Rs + Rp(5/115)^2)..........That old I squared R chestnut.

If the power transformer is a decent physical size with low range resistance windings this sort of increase in load current is very well tolerated as its only an additional 5W of load.


That's the heating in the tube, not the heating in the transformer. Heating of the transformer winding is its resistance times the current squared. A complete discussion would include the losses in the iron and the primary but they can be neglected here.



Yes, well spotted, the formula was incorrect should have been:

Extra power loss (in the transformer) as heat with 5U4 = (3^2 - 2^2) x (Rs + Rp(5/115)^2)
( I effectively typed it as (3-2)^2 , or 1^2 in the example, in error, must have been late when I typed it in !).

So the additional heating if the apparent total secondary resistance, to include the contribution of the primary is 0.5 Ohms, going to the 5U4 increases the additional power loss in the transformer by the modest amount of 4.5 - 2 watts , or 2.5 watts extra heat as you say.

Ignoring eddy currents and hysteresis, it is the transformer's DC resistance(both primary and secondary) and the thermal resistance of the entire object that largely determines the temp rise.

On the subject of changes in the transformer core over time. It is known that the metallic lattice can change over time in its crystalline makeup with thermal cycling in some cases for certain metals. The effect can be observed mechanically with springs, though not specifically for laminations with permeability changes. This could potentially be one mechanism how aging in the laminations could alter their permeability. Another theoretical mechanism could be increased eddy current losses if the laminations short together over time.

Testing good numbers of vintage power transformers, they run very close to core saturation. Its is worse here with 50Hz line frequency. If the frequency goes from 60 Hz to 50 Hz, it is equivalent in terms of the peak flux, increasing the line voltage by a factor of 1.2 or say 110V to 132V. This is why in 50Hz localities Early American transformers tend to run very hot, because off load, when the core is pushed to far up its B-H curve and the primary magnetization currents climb very high increasing the primary I^2 R losses and the hysteresis losses go up too. Most of the time I have to run old American power transformers here on 98 to 100V on 50Hz.

Some years ago I saved and stored a power transformer from a 1939 vintage American TV set. It appeared to have what amounted to a deteriorated iron core. Its windings were still good. As a result its off load magnetization current was very high and it had extremely high external radiated magnetic fields to the extent that it deflected the CRT's beam enough to see a loop pattern on it with no raster deflection. I will see if I can dig it out and perform some more tests on it. If the results are interesting I will post it as a separate topic.


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 Post subject: Re: PACO E-200-C with 5u4GC rectifier
PostPosted: Jan Sat 15, 2022 12:32 am 
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I think I have a 5y3 in my desk so I fully plan to replace that 5u4 if I can ever get back to it, or get one ordered up if I don't have it. I don't mind some minor mods here and there, but not something like this.


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