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I recently bought an old two-box metal detector that uses radio tubes and runs on the old radio batteries. There are two boxes, One is a transmitter box, the other is the receiver box. one with 6 tubes and one 45V battery (but has a third, center + connection, so is - + + the center plus terminal is 22.5V ) Then, there are two seperate 1.5V batteries also. The other box has two tubes and a 45V battery (no center 22.5V connection) and one seperate 1.5V battery also.

My question is this: I am trying to change the battery system to rechargable batteries. I made a battery pack out of 5 nine volt batteries, but the rechargables charge up to 10 volts 50V total) initially each, then I made a separate battery pack for the 22.5V out of 1.2 AA batteries, which initally charge up to 1.364 volts each (16 come up to about 23.188V 15 to 20.46V) how can I run down the batteries to get to the correct voltage(s)?

Or, does anyone know if these old detectors will take those over voltages? all batteries are connected "in series".

Also, do you think the rechargables probable higher mHa ratings(250 for 9V and 2,000 for 1.2V) could create any problems? This is the first time I have done this, so kind of new at it!

I was going to use alkalines first, because the voltages are closer, but this machine eats batteries, so I am told and rechargable batteries are so much cheaper, in the long run.

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    \$\begingroup\$ A pic of the insides would help. Also measurements from all combinations of the terminals (my guess is it may be dual rail) On my phone at the mo so can't go into much detail, but basically provide as much info as possible. You can link to pics and we'll put them in for you. \$\endgroup\$
    – Oli Glaser
    Nov 29, 2012 at 12:02
  • \$\begingroup\$ Even if you rig up battery packs, what are you going to do when the tubes wear out, if they aren't worn out already? Obscure tubes like what these sound like are difficult to find today. Listing the tube numbers would help. \$\endgroup\$ Nov 29, 2012 at 13:12

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You will probably find it eats 1.5V batteries but the 45V current draw is quite low...

As for approximating the 22.5/45V voltages; I recommend erring on the low side rather than going over-voltage. The glassfets will take the over-voltage quite happily, but 50-year old capacitors, probably not...

And HT batteries were commonly used down to a fraction of their nominal voltage, so a stiff supply from rechargeables at 90% of rated voltage should be fine.

An alternative to rechargeables for the 45V supply would be a stack of cheap eBay lithium coin cells.

Also be aware that old battery equipment often used a quite alien design pattern to modern designers : there was no direct DC path from HT to earth; therefore the HT battery was permanently connected, not switched. The only current path from HT was through the glassfets, so switching their heaters off reduced the HT current to 0. (Check this with the heaters off; any leakage may indicate a faulty capacitor.)

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  • \$\begingroup\$ All batt2 are bought with chargers, Nimh rechargeables (see more comments below, please.) There is a rheostat at the end of the tubes, if I dial that to zero, will itBatts turn tube heaters off? Sent a picture of the box open, but it is on another page here, in the forum and Just don't know how to move it, but the picture is downloaded, under "how to power an old 2-box detector-2." Boost and buck converters? Duel rail? Will my separation of the 45V batt using 5-250mAh 9V batts and 22.5V, using 14-1.2V AA's, 3000mAH work??? Tubes are KEN-RAD B5E/IH4G Thanks, Vance Please see picture on forum? \$\endgroup\$ Dec 15, 2012 at 7:27
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To run this relic from a modern battery pack, I'd probably use a single off the shelf pack with boost and buck converters to make the separate voltages. The higher voltages will probably require little current. The 1.5 V is probably for the fillaments so will have substantial current, probably a few 100 mA.

One issue may be whether it is OK to connect the 1.5 V supply somehow with the other supply. Since this was a separate battery, the design may have made use of the fact that it was completely floating with respect to the other voltages. For efficiency, these tubes probably don't have separate heaters. They probably have a integrated cathode/heater. Sometimes these are just two connections with one serving both purposes, and sometimes the fillament is center-tapped for the cathode connection. Either way, it may be that neither side of the 1.5 V supply can be tied to the ground of the high voltage supply.

If the supplies do need to be floating, then I'd probably put the battery on the high curren one and make the 1.5 V from a straight buck converter. The higher voltage can be produced by a flyback converted.

Real current measurements and finding whether the supplies need to be isolated would help a lot.

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    \$\begingroup\$ Good points. It's also possible that the 1.5V supplies MUST be connected to the HT -ve supply in a particular way; it was a common hack to get the gate cough grid bias that way. With 2 separate 1.5V batteries, did one tube need a 3V heater, or was the second battery grid bias only? or is it for a rectifier heater and isolated from the rest? A few minutes with an ohmmeter... \$\endgroup\$ Nov 29, 2012 at 14:12
  • \$\begingroup\$ On the subject of boost converters, that's how I run my Minifon (wire recorder, or tube Walkman) and it's fine. \$\endgroup\$ Nov 29, 2012 at 14:17
  • \$\begingroup\$ Thanks for all the feedback. I appreciate it. I am not changing the way the 1.5V batteries connect. They will be run off of 1.5V D cells, or 1.2V rechargeables, however they were originally run. (if they will power it! Or, I will run a few D's in parallel for more mAh, leave voltage the same). The only real change will be on the 45V battery, with the 3 connections and specifically, on the 22.5V center connection, which I am separating out and using 1.2V AA rechargables. The big question is will that work? using 15AA's for the 22.5V and 5-9V's for the 45V tied to the same ground/minus- wire? \$\endgroup\$ Dec 15, 2012 at 6:47

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