parallel rectifier question

[kward]

I suspect the voltage equalizing resistors serve the same purpose as I used them for when I paralleled three series-pass tubes in an active regulated power supply. Because each tube will not deliver exactly the same output voltage, there is an opportunity for small hysteresis-like currents to flow between tubes, or one of the tubes will do more of the work than the other one because they can never be perfectly matched.

In my application where I used three 6550 tubes as series-pass elements in a regulated power supply, I used a 10 ohm 2 watt metal film resistor as the voltage equalizing resistance, where each resistor was hung directly on the output pin of each of the tubes. I would think generically, using something like 1% of the internal resistance of the device as the equalization resistance would work in most cases. You can then determine the wattage of those resistors needed given their size and the current flow each will see.

 

[Mr Boochie]

From people I know that own dual rectifier tweed-era Fender Bassman and Twin amps, you may need to buy matched pairs of rectifiers. I have never had the pleasure or luck to own either of these great amps (though I've played on a few tweed 5E6 circuit Bassmans). But, owners tell me one rectifier will inevitably wear out before the other. Balancing the load between the two, through all the dynamics of music, is a hard row to hoe.

Good luck with your restoration!

 

[gadget73]

I would think generically, using something like 1% of the internal resistance of the device as the equalization resistance would work in most cases.

cool. I was hoping it wouldn't need to be anything overly large. If it was big enough that it negated any regulation improvement that dual rectifiers might give then there wouldn't be much point to it.


appreciate the input folks. I'll fool with it and see what it has to tell me. I should probably work out the load lines for this to see if the screen voltage needs to be different too.

 

[dcgillespie]

It's very hard on the cathode to draw a lot of current from it when it's not up to full temp. If you use 6 volt tubes, you will want to make sure they are being operated well under max rating to ensure no harm is done to the tubes. For lack of any better information, you should assume that between the two windings, you only have 5 Amps of current capability. That would rule out the use of 5U4s. 5R4GBs would work -- but then B+ is less, and your regulation goes right out the window again because it is a high drop tube.

If you're really going after regulation at the point of rectification, and damn the torpedoes with regards to current flow, then consider a couple of 83 Mercury Vapor tubes. Yeah, I know -- Mercury and all, but they are nearly as good as SS when it comes to producing a consistent voltage drop over a very wide range of current draw levels.

Dave

 

[gadget73]

Don't need it regulated quite that tight, I was just thinking if I could get the 13% drop down to about half that it wouldn't upset me. With an EFB setup providing voltage to the screens, the existing 5Y3 socket will be empty. Its a relatively small matter to turn that into a parallel rectifier socket.

Unfortunately virtually nothing comes up for this amp when I've searched for it. Its a stout power transformer but I don't really know it's ratings. 5AR4's can be had for not unreasonable money. In parallel they'd be living a pretty easy life.

 

[KarlE]

Don't need it regulated quite that tight, I was just thinking if I could get the 13% drop down to about half that it wouldn't upset me. With an EFB setup providing voltage to the screens, the existing 5Y3 socket will be empty. Its a relatively small matter to turn that into a parallel rectifier socket.

Unfortunately virtually nothing comes up for this amp when I've searched for it. Its a stout power transformer but I don't really know it's ratings. 5AR4's can be had for not unreasonable money. In parallel they'd be living a pretty easy life.

If you want more information to look over, the Knight model 93SZ508 located in Sams 379-8 has an almost identical power transformer setup as your amp. https://www.radiomuseum.org/r/allied_rad_knight_93sz741.html

KarlE

 

[Tom Bavis]

I often find that heater windings are all wound with the same wire, regardless of the actual load. I've verified this with a 6-digit DMM that goes down to milliOhms. When winding a transformer, it costs less to wind multiple windings with the same wire rather than changing the wire spools for a thinner one (and heater windings aren't very long). So if there's a 5A 6.3V winding, the 5V windings MIGHT be the same wire. Of course, if one winding is parallel wires or if the winding BARELY fits in the window, then they may decide differently...

You can measure winding resistance without a 4-wire milliohmeter. You need a 1A variable power supply and a meter that can read millivolts. Voltmeter hooked to transformer leads, power supply connected separately. Increase current to 1A, read mV (mV reading is same as resistance in mOhms) and drop supply to zero amps before disconnecting. 100 mOhm is about a 3A winding, 50 mOhm about 5A. Some Tek scope transformers have 25-30 mOhm windings.

 

[gadget73]

My one supply that can do that much current won't go down that low in voltage. I lost my 6 digit meter a while back and haven't replaced it. I was honestly thinking of just doing this the brute force way. Voltage check with no load, voltage check with 2 amp load, voltage check with 3 amp load. If it doesn't drop too much voltage or get too hot I could just run with it.

 

[Tom Bavis]

Yes, I do that too. In fact there's a 6AS7 with clip leads on m bench - a quick connect 2.5A load for a 6.3V winding.