February 11, 2022 at 1:04 pm #18158
I’m just wondering if anyone else has experienced this, and if I should be concerned.I’m working on a Deforest Crosley York/Mayfair 851a.b
- On the schematic there is no value as to what the B+ should be at the rectifier but it measures ~320 V DC.
- The voltages I measure at the 5 locations given on the schematic are 20 to 40 volts higher than they should be.
- All resistors are within range or they have been replaced.
I’ve considered reducing the B+ at the rectifier by about 20 volts but that would still leave the voltages high at some points so I’d possibly have to increase the size of other resistors from what the schematic calls for and at that point I’m confused as to why this is all happening and what the best solution might be.
(An interesting discovery during this project is that my isolation transformer puts out about 124 Volts with 117 going in. I adjusted the voltage with my variac for the measurements above.)
Attachments:February 11, 2022 at 3:26 pm #18161
2 points/ questions.
1: You said you have a variac. What input voltage is required from the variac to get the voltages on the schematic ? If it is way off what might be expected from the electrical supply then your transformer probably has a short in the primary, resulting in effect, less windings on the primary and there by increasing the turns ratio, thus increasing the secondary voltages.
2:: If the transformer is rated for 110 volt and works at 30 cycles or less, then I have always been led to believe secondary voltages will go up as frequency goes up.
February 11, 2022 at 5:46 pm #18163Ralph SpracklinCVRS Member
- This reply was modified 1 year, 9 months ago by John Greenland.
The write-up/article below, is around 90 plus years old Maybe it will give you some added insight into this tube.
How are your voltages at the rectifier tube pins, with the 280 tube pulled? Also set your variac to where you read 115 Volts, at the primary, with all tubes pulled. It seems your Variac may very well be putting out higher voltages than you are setting the Variac for.
Rectifier Tube 280
THE 280 is a full -wave rectifier tube for use with receivers requiring a maximum rectified voltage of 350 volts or less and a current of 125 milliamperes or less, or a maximum rectified voltage of 400 volts with a maximum current of 110 milliamperes. It has a five volt filament winding requiring a current of two amperes. CHARACTERISTICS OF THE 280 RECTIFIER Filament voltage 5.0 Filament current, amperes 2.0 AC voltage per plate (RMS) 350.0 Max. load milliamperes for 350 volts on plates 125.0 AC voltage per plate (RMS) 400.0 Max. load milliamperes for 400 volts on plates 110.0 Maximum overall length, 5 5/8 inches. Maximum diameter, 2 3/16 inches. Socket, standard UX. The power transformer used with this tube should have two secondary windings, one of high voltage for the plates and one of low voltage for the filament. The high voltage winding should be accurately center -tapped so that both plates get the same voltage. The filament winding need not be center- tapped but a center -tap is desirable if the tube is to deliver a high current. If the load current is taken off on one side of the filament that side will be operated at a higher temperature because more of the rectified current will flow through this side. When a center -tap is used the load current is divided equally between the two legs of the filament and the heating will be uniform. Line Voltage Variations . The primary should be wound for 115 volts but should prefer- ably have taps for 107.5, 115 and 122.5 volts so that compensation may be made for line voltage fluctuations. If the primary is not tapped, it should be wound for about 115 volts and a variable resistance should be put in the primary circuit to cut the voltage down to that required in case the line voltage should rise above 115 volts. The actual effective voltage across the high voltage secondary depends on the DC voltage desired, and may vary from 220 to 400 across each half of the winding, that is, from 440 to 800 volts across the entire winding. In Fig. 1 is shown a typical rectifier circuit incorporating the 280 tube, together with the filter circuit and the voltage divider. The average output characteristics of such a circuit are shown in Fig. 2. These curves show the variation in the DC output voltage with changes in the load current for different values of RMS input voltage per plate of the rectifier tubes. The output voltage is as measured at V in Fig. 1. The full lines are for condenser input to the filter and the dotted lines for choke input. The difference between these inputs is indicated on the diagram in Fig. 1. These curves may be used to estimate the maximum output voltage when the current drain and the input voltage are known. Suppose, for example, the effective AC voltage per plate is 350 volts and we wish to draw a current of 100 milliamperes. The 350 -volt curves, tell us that the voltage is 345 volts at V, with condenser input, and that it is only 250 volts with choke input. The curves do not take account of the drop in the choke coils so that the useful voltage will be less. Suppose the total resistance of the two chokes is 400 ohms. If the current is 100 milliamperes the drop will then be 40 volts. This leaves 305 volts for the case of condenser input and only 210 volts for choke input. Condenser Requirements As indicated in the rectifier circuit, the capacity of the first condenser is 4 mfd. If the capacity of this condenser is increased, the voltage will be slightly greater, and if the capacity is less the voltage will be less, but changes in voltage with the condensers ordinarily used are so small that they may be neglected. The capacities and resistances associated with the voltage divider are not specified because they would depend on the circuit with which the rectifier is to be used. This is especially true of the resistances. It may always be stated that the capacities of the condensers should be as high as practicable and that not less than one microfarad should be used in any position. In selecting condensers it should be remembered that the voltage may rise to the peak of the voltage applied to each plate of the rectifier tubes and that the condensers must stand this voltage for brief periods. The peak voltage is 1.41 times the RMS voltage, so that if the applied voltage per plate is 400 volts, the voltage across the condensers might rise to 564 volts. This would occur when the load is removed without turning off the power to the transformer. Even when there is a bleeder current which remains when the load is removed, the voltage will rise, although not to the same value. This rise in voltage is indicated on the curves in Fig. 2. At 10 milliamperes, for example, the voltage across the first condenser, when the voltage per plate is 400 volts, RMS is 510 volts. Only the first three condensers will be subjected to the maximum voltages. Surges This voltage rise does not take surges into account, but only the rise due to regulation. When there are surges the voltage may rise to much higher values instantaneously, and surges will occur when there is a sudden change in the current. For these reasons the voltage rating of the condensers should be at least twice as high as the highest steady operating voltage. Moreover, care should be taken that the power is always turned off before any change is made in the circuit, such as removing a tube or changing the position of some lead on the voltage divider. It would seem that the condenser next to the rectifier tube would be subject to the greatest stress, but in most instances it is the third condenser that breaks down first, if the three con- densers across the entire line are of the same rating. This rup- ture, no doubt, is due to high voltages generated in the amplifier due to static crashes and other disturbances of a similar nature, and is superinduced by the heating of the condenser by the con- stant flow of normal signal current through it. While it is customary to put the two filter chokes in the posi- tive side of the line, as is done in Fig. 1, they could also be placed in the negative side. This is sometimes done in commercial sets. Or one could be placed in each side of the line, and this method, too, has been adopted by some manufacturers. When the load current is high it is common practice, especially when push -pull amplification is used, to tap the current for the power tubes from the junction of the two chokes. This diverts the larger part of the load from the second choke so that it becomes more effective in filtering the remaining current.
RalphFebruary 13, 2022 at 9:32 am #18167
John Greenland, thanks for the information. The radio is stamped with 25 cycles on the chassis so if that’s what is increasing the voltage that makes sense. I can input about 100 AC into the transformer to get the specified voltages.
Ralph Spraklin, thanks for the extensive information on the 280.February 13, 2022 at 11:39 am #18168
I would be be curious if either of the club members who has much more knowledge on these 25 cycle transformers than I, can give some idea of what to expect when running a 25 cycle transformer from 60 cycles. I am sure someone has had that experience and has a fix.May 8, 2022 at 9:44 pm #18516Dan MiddletonCVRS Member
Les, For the radios designed for 25 Hz I always end up adding a resistor between the rectifier and the first filter capacitor. The value is selected by how much you need to reduce the B+. I recently worked on a Stromberg Carlson 180. This unit has 400 volt B+ and an idle current of a whopping 200 ma. It required a 200 ohm 50 watt power resistor to drop off 45 volts. The resistor runs blazing hot. I attached a photo. I built a cage around it for safety. One end has 450 volts DC.
I have an AC powered TRF I’m working on that was putting out 220 volts DC instead of 150 volts. It needed a 2,000 ohm 10 watt resistor. The current draw is only 25 ma. It runs warm.May 9, 2022 at 5:23 am #18517Ralph SpracklinCVRS Member
Dan, As I am still learning, there is much about electricity and radio circuits I still have not fully grasped. So let me ask, what to some may be a dumb question.
Once one knows what the required voltage drop should be. What formula (simple I hope) do you use to determine the necessary resistor value in Ohms as well as the required wattage value??? I realize that once I know the appropiate value range, I can then try lowering or raising the resistor value to achieve what I am looking for.
May 9, 2022 at 5:18 pm #18519
- This reply was modified 1 year, 6 months ago by Ralph Spracklin.
Perhaps another way to check the transformer is by measuring the heater circuit voltage. All the tubes except the 80 are connected to a 2.5 volt tap on the transformer.
With all the tubes in, the current draw on this winding is a bit above 12 amps.
What voltage do you need from the A.C. line to get 2.5 volts on the heater circuit with the tubes in their sockets.
I would think, that if that line voltage seems too low compared to normal line voltage then quite likely you have a transformer primary problem. thus messing up the turns ratio.
John G.May 9, 2022 at 9:00 pm #18520
Thank you Dan Middleton, Ralph Spracklin and John Greenland for your recent posts. I experimented with a dropping resistor as suggested but the heat it generated was worrisome. I rencently learned of using a ‘bucking transformer’ from Gerry O’Hara so I’m going to experiment with that.
Attachments:September 27, 2022 at 2:15 pm #18984johngillespieCVRS Member
Another thing to consider is the voltmeter you’re using to take those voltage measurements. Your radios schematic voltages may have been taken with a 1000 ohms/volt meter back in the day. Since that type of meter put a considerable load on the circuit being measured their readings will be lower than yours especially if you are using a relatively new meter with much higher ohms/volts. A VTVM or a modern digital meter puts virtually no load on the circuit being measured. As a result, your readings will be higher by comparison.
ps: I also use Bucking transformers on all my vintage radios, just to help reduce line voltage stresses.
JohnOctober 3, 2022 at 6:31 pm #19009Don RootForum Participant
I am new here and it seems that you have not resolved the problem.
i have not seen that you have mentioned any indication of the rated AC voltage in .. from info on the back.
normally it is near the frequency.
we need to keep in mind that the old standard was more like 110 volts and now it is often 120 volts so if a radio desighed for 110 is really getting 120 , all voltaged inside will be up by 10% .. that could be the problem/situation
i assume that you have double checked your readings with another meter?
does the radio work? does the bias on the cathode of the output tube pull the voltage on the plate down enough?
many radios have been made for 25 to 60 Hz ,using a transformer good for 25 cycles. I dont expect the rated frequency will cause your high voltages.
you did not mention where the voltages came from , but if the B+ is high , all the DC will be high.
are you using that isolation transformer putting out about 124 Volts out to feed the radio? if so you can expect some high voltages inside.
radiodonOctober 4, 2022 at 12:48 pm #19017Lee WhiteheadCVRS Member
The bucking transformer is the way to do it. Is there room to mount it in the cabinet?
LeeOctober 8, 2022 at 1:05 pm #19041
Thanks to John, Don, and Lee for the recent responses to my query. I don’t usually work on radios during the summer – I’d rather be in my garage on my wood lathe. In winter I will get back to the radios. I have experimented with the bucking transformer and that’s the route I’ll take when I’m back at it.November 21, 2022 at 1:56 pm #19307Ken FarrellCVRS Member
Just a thought, but what is the DC voltage out of the rectifier with the radio volume turned up louder? At higher volumes the voltages will drop due to higher current draw from the extra current drawn from the audio output stage.
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