Two rectifiers

[Leevi]

I added an SS rectifier and a rectifier switch to a 50W Plexi clone and wonder
the huge difference between the cathode currents depending on which
rectifier SS or tube (GZ34) has been selected.

Measured values:

Tube rectifier: 25mA/tube on 482v B+
SS rectifier: 33mA/tube on 466v B+

/Leevi

[phsyconoodler]

Sure you don't have those mixed up?The tube rectifier should have lower voltage.

[Leevi]

Sure you don't have those mixed up?The tube rectifier should have lower voltage.

Thanks phsyconoodler, you are right, the voltages have been mixed.

Corrected values:

Tube rectifier: 25mA/tube on 466v B+
SS rectifier: 33mA/tube on 482v B+

/Leevi

[tubeswell]

I added an SS rectifier and a rectifier switch to a 50W Plexi clone and wonder
the huge difference between the cathode currents depending on which
rectifier SS or tube (GZ34) has been selected.

The 'internal' plate resistance of the tube rectifier means that it will drop some voltage and lose some current. Is that what you were thinking about?

[PRR]

That doesn't look right. What happens to G1 and G2 voltages?

[Leevi]

I wonder the big difference in the cathode current.
The negative voltage on G1 is same in both cases. There is on PT an own tap from which
the voltage is generated. G2 drops of course in the case of tube rectifier since the overall voltage level drops. Maybe I should mention that I'm using 10 Ohm resistors on the cathode.
/Leevi

[Leevi]

I solved the problem by changing bias setting circuit.
I'm using a DPDT switch for the rectifier selection and utilized it also for the
individual voltage setting for both cases. Hopefully this is a generic solution i.e.
it's valid even with other power/rectifier tubes?
/Leevi

[clyde]

The DPDT switch is the solution.  I use it switch between a 5Y3 and a 5AR4 and plug in the correct resistor values to set bias for each rectifier.  Well done.

[jeff]

Do you just switch between SS and tube or just short the tube rect with the diodes? (A or C)
Do you use seperate filter caps for each rect before the switch or switch between tube or SS before the first cap? (B)

Why are you using 10 ohm resistors on the cathode?

[Leevi]

I'm using the switch like in alternative A. The cap is the first filter cap.

Why are you using 10 ohm resistors on the cathode?

In order to make the bias setting easier. Normally I use 1 Ohm resistor but
I didn't find any. The cathode current = voltage over the resistor/10.

/leevi

[HotBluePlates]

Why are you using 10 ohm resistors on the cathode?

In the old days, a 10Ω resistor would have been most-typical instead of 1Ω.

Why?

The average tech might have a consumer- or service-grade VTVM, like an RCA VoltOhmyst. The lowest range on many of those meters was 1-1.5v full-scale. 40mV is barely budging the needle off the bottom of the scale on a 1.5v range, where 400mV (40mA through 10Ω) is low on the range but easily readable.

Even if your meter goes lower, the bigger voltage is easier to read accurately. The only trade-off is you mentally shift the decimal point knowing that this is the voltage across a 10Ω resistor.

[jeff]

Cool thanks.
How about the switch? Should you use A,B or C?

[kagliostro]

Sometime ago I was asking about SS to tube rectify switching
(I found a schematic and asked an opinion on this forum)

Mr. Merlin confirmed that this circuit will work fine, only move the standby switch after the reservoir capacitor as to prevent inrush currents that can demage the tube rectifier



K

[Leevi]

I have successfully used this in my 5e3 clones.
/Leevi

[jeff]

Thanks
 
Kagliostro, for some reason I can see your schematic, just a red X in a white box.

[kagliostro]

Hi Jeff

Now can you see the schematic ?

K

[jeff]

Yes, Thanks.

Why are we disconnecting the heater? Doesn't that mean when we switch we have to wait for the rect tube to heat up? Does this protect the tube?

[kagliostro]

I found that schematic sometime ago and asked about its reliability

Merlin answer is here

That method works fine. There is also no harm in turning off the heater when not in use (except having to wait for it to warm up). In fact, you have the benefit that the valve will last longer!
But: Don't put the standby switch BEFORE the reservoir capacitor. Most recto's do not like that.

K