what limiting resistance for 5U4GB with 50uF reservoir?

[tubeswell]

So I was looking through Merlin's article here and wondering about how to go about this.

http://www.freewebs.com/valvewizard/fullwave.html

On the JJ5U4GB datasheet here http://www.jj-electronic.com/pdf/JJ-5U4GB.PDF (200kB file too big to attach sorry), it appears (correct me if I'm wrong) that the total limiting resistance presented to their 5U4G rectifier putting out 400V with 40uF input capacitor is 52R. Have I read that right?

In which case, if I wanted to up the input cap to 50uF, would I need a total limiting resistance of 5/4 proportionately more (i.e.; 65R) to keep the rectifier within 'safe' operating specs?

Or should I just run the rectifier at 50uF anyhow?

(PS I know this will lower the B+ some, but such an outcome is partly what I'm after)

TIA

[FYL]

The 5U4GB is a very rugged valve, able to withstand 1A continuous / 4.3A transient. You may safely use up to 80µF for C1 in a CLC or CRC filter at MI voltages and currents - say B+ < 500V and I < 250 mA with a standard PT.

Play with PSUD2 and see for yourself how it'll behave in your circuit...
http://www.duncanamps.com/psud2/index.html

[tubeswell]

The 5U4GB is a very rugged valve, able to withstand 1A continuous / 4.3A transient. You may safely use up to 80µF for C1 in a CLC or CRC filter at MI voltages and currents - say B+ < 500V and I < 250 mA with a standard PT.

Play with PSUD2 and see for yourself how it'll behave in your circuit...
http://www.duncanamps.com/psud2/index.html

Thanks FYL. If only I had a PC...

[Merlin]

The 5U4GB is a very rugged valve, able to withstand 1A continuous / 4.3A transient.

Just to clarify, it's about 300mA RMS (basically your DC load current), 1A peak ripple, 4.6A transient (i.e., inrush current).

Having said that, you probably will get away with 50uF, since your transformer probably has over 60R impedance.

[PRR]

> is 52R

INcluding PT winding resistance, which as Merlin sez is probably in that zone already.

If you custom-wound, you would give this number to your winder, might use one size smaller than "best" and save a penny as well as your rectifier.

For existing PT: Measure CT to one end for secondary resistance. You could do some math to reflect the primary resistance. But since primary loss is usually similar to total secondary loss, and tube-amp PTs have several secondaries, primary times 1.5 is probably close-enough.

The cap value hardly matters (just like there is no "maximum cap"). See Sylvania 5u4 data page 5. They plot AC Volts each plate against minimum safe plate supply impedance. Doesn't say what cap or what load. A super-small cap or super-small load might get away with a little less, but then why you using such a big tube? A 5U4 will usually be worked very close to "worst case", and this chart tells you what to use for that worst-case.

[tubeswell]

At the moment I am running a Tweed Bassman type clone with a "5F6A spec" custom-wound PT (325-0-325 @ 200ma, 6.3V 5A, 5V 3A and a separate 50V bias winding). The DC resistance of one side of the High-Tension winding is 34.4R and the DC resistance of the primary is 4.3R. The PT is for 230VAC mains, so I make the secondary to primary turns ratio to be 1.413:1

So RT in this case = 34.4 + (1.413 x 1.413) x 4.3 = 156R (Did I sum that together the right way? i.e. adding the 34.4 to the (1.413 x 1.413) first, and then multiplying it by the 4.3??)

I have a JJGZ34 and 2 x JJ6L6GC. The reservoir cap installed is 50uF. I have decided I want to try some lower-power output tubes (JJ6V6S) and scale back the rectifier a bit, hence I was thinking about the 5U4G.

[FYL]

So RT in this case = 34.4 + (1.413 x 1.413) x 4.3 = 156R (Did I sum that together the right way? i.e. adding the 34.4 to the (1.413 x 1.413) first, and then multiplying it by the 4.3??)

Precedence, precedence...
 

The formula is:
Rt= Rs + (n^2 * Rp)

With your values:
Rt = 34.4 + (1.413^2 x 4.3) =  42,9852467 ohms

[DummyLoad]

At the moment I am running a Tweed Bassman type clone with a "5F6A spec" custom-wound PT (325-0-325 @ 200ma, 6.3V 5A, 5V 3A and a separate 50V bias winding). The DC resistance of one side of the High-Tension winding is 34.4R and the DC resistance of the primary is 4.3R. The PT is for 230VAC mains, so I make the secondary to primary turns ratio to be 1.413:1

So RT in this case = 34.4 + (1.413 x 1.413) x 4.3 = 156R (Did I sum that together the right way? i.e. adding the 34.4 to the (1.413 x 1.413) first, and then multiplying it by the 4.3??)

I have a JJGZ34 and 2 x JJ6L6GC. The reservoir cap installed is 50uF. I have decided I want to try some lower-power output tubes (JJ6V6S) and scale back the rectifier a bit, hence I was thinking about the 5U4G.

try a 5V4.

[tubeswell]

So RT in this case = 34.4 + (1.413 x 1.413) x 4.3 = 156R (Did I sum that together the right way? i.e. adding the 34.4 to the (1.413 x 1.413) first, and then multiplying it by the 4.3??)

Precedence, precedence...
 

The formula is:
Rt= Rs + (n^2 * Rp)

With your values:
Rt = 34.4 + (1.413^2 x 4.3) =  42,9852467 ohms

Yes that's why I asked - I had already thought about that, but the end result of yours seems reediculous. (No offence intended)

I have checked those figures again and doing it the 'precedence' way I get ~43R.

(Edit - I was curious about the seemingly low DC resistance of the primary, so I checked another PT from the same bloke, and it was also lowish (around 7R). I put this down to the fact that the winding wires look quite big and the trannies are humongous.)