tube, SS recto switch sanity check

[Tonegeek]

I want to use a SPDT or DPDT center off switch to switch between a GZ-34 (ignore the EZ81 label) tube and SS rectifier with center being standby.
Will this work without damaging the tube? (see image)
Anything else I should be concerned about?

[selloutrr]

I would use a DPDT switch and tap the voltage before the rec tube.

you may have a voltage drop with the current diagram since the tube will be sucking voltage at all times.
It will also burn the tube down even if you don't use it in the circuit which is a down side.

Edit: note that taking the voltage after the rec tube may exceed some switches max voltage depending on the voltage of the PT. Even if it's not exceeding it can get close and greatly age the switch. The last thing you want is for it to fail and short out.

[selloutrr]

I adapted your drawing in MS paint it's crude but it'll work

If you can find a DPDT with a center off you could remove the standby switch.

[selloutrr]

You may find you need to add a stage of filtering to the SS rectifier to lower the voltage to match the rec tube before it goes to the circuit.

[Structo]

Yes, always better to switch AC than DC if possible.

DC arcs more on the contacts.

[Tonegeek]

I would use a DPDT switch and tap the voltage before the rec tube.

you may have a voltage drop with the current diagram since the tube will be sucking voltage at all times.
It will also burn the tube down even if you don't use it in the circuit which is a down side.

Edit: note that taking the voltage after the rec tube may exceed some switches max voltage depending on the voltage of the PT. Even if it's not exceeding it can get close and greatly age the switch. The last thing you want is for it to fail and short out.

This all seems logical except...
I don't understand how the tube can conduct when disconnected from the load mainly because there does not appear to be a return path in that case. Doesn't it work just like the SS rectifier part? If there is no load, it won't conduct. Just can't wrap my head around it. Is it shedding electrons even when idle? Still I do think your idea is better than switching the DC.

[Tonegeek]

Yes, always better to switch AC than DC if possible.

DC arcs more on the contacts.

+1

[selloutrr]

I would use a DPDT switch and tap the voltage before the rec tube.

you may have a voltage drop with the current diagram since the tube will be sucking voltage at all times.
It will also burn the tube down even if you don't use it in the circuit which is a down side.

Edit: note that taking the voltage after the rec tube may exceed some switches max voltage depending on the voltage of the PT. Even if it's not exceeding it can get close and greatly age the switch. The last thing you want is for it to fail and short out.

This all seems logical except...
I don't understand how the tube can conduct when disconnected from the load mainly because there does not appear to be a return path in that case. Doesn't it work just like the SS rectifier part? If there is no load, it won't conduct. Just can't wrap my head around it. Is it shedding electrons even when idle? Still I do think your idea is better than switching the DC.

in your drawing you are sending voltage at all times to the tube regardless of it being the rectifier you chose to use in the circuit.
the Tube - standby - SS switch in your drawing is being hit with DC current on both sides at all time.

both Tube and Solid state rectifiers do accomplish the same function of converting AC to DC. The SS diode rec is more efficient and creates less sag and voltage drop. The Tube Rec sucks a few additional volts and adds sag. you will a voltage difference when switching between the two different styles of rectifiers.

[martin manning]

in your drawing you are sending voltage at all times to the tube regardless of it being the rectifier you chose to use in the circuit.
the Tube - standby - SS switch in your drawing is being hit with DC current on both sides at all time.

Both sides of the switch are seeing DC voltage at all times, but current is only flowing through the selected rectifier. If the SS rectifier is selected the only drain on the system from the vacuum rectifier is the current drawn by its filament.

If you forego the combined standby/recto switch concept, you could use a SPDT switch to drop the junction of the diode strings on the vacuum rectifier's cathode and simultaneously open the filament circuit to eliminate that current draw and preserve the tube.

[Tonegeek]

in your drawing you are sending voltage at all times to the tube regardless of it being the rectifier you chose to use in the circuit.
the Tube - standby - SS switch in your drawing is being hit with DC current on both sides at all time.

Both sides of the switch are seeing DC voltage at all times, but current is only flowing through the selected rectifier. If the SS rectifier is selected the only drain on the system from the vacuum rectifier is the current drawn by its filament.

That' the way I see it exactly

If you forego the combined standby/recto switch concept, you could use a SPDT switch to drop the junction of the diode strings on the vacuum rectifier's cathode and simultaneously open the filament circuit to eliminate that current draw and preserve the tube.

Sounds like what you are saying is this, which makes sense to me and you could still do center off if you wanted to (see pic):
EDIT: only works with a directly heated cathode like GZ

[martin manning]

I was thinking like this. Center off would leave the rectifier filament cold, so you'd have to wait for it to warm up. That would make for a soft start though.

Note GZ34 is indirectly heated but the cathode is electrically connected to the heater. The arrangement shown works with either type.

[Tonegeek]

I was thinking like this. Center off would leave the rectifier filament cold, so you'd have to wait for it to warm up. That would make for a soft start though.

Note GZ34 is indirectly heated but the cathode is electrically connected to the heater. The arrangement shown works with either type.

I thought the Gz 34 was directly heated. My bad.

I want to use the option for rectifier type in the next amp I build.
Thanks for all the help!

[surfsup]

I am doing an option in an amp now and just noticed this thread. The switch looks simple. I have a question though, if I were to calculate my voltages with the tube recto vs the SS, and determined the difference, I can drop the V on the AC side with two Rs before the diodes that would not affect the tube because they are switched out. So looking at this diagram, Voltage on the AC side is equal to current * impedance...

Would the impedance used to calculate Vdrop on the AC side be the impedance of the transformer? How does one calculate PT impedance?

[img:580:523]http://chicagocadcam.com/ChrisHahn/supr ... ordrop.jpg[/img]

[Merlinb]

if I were to calculate my voltages with the tube recto vs the SS, and determined the difference, I can drop the V on the AC side with two Rs

If you mean what I think you mean, each R would have the same resistance as the internal anode resistance of the rectifier.

This would, however, be kinda pointless since the reason for a switchable rectifier is to select either the saggy lower-voltage tube rectifier OR the stiff higher-voltage SS rectifier. But if you add those resistors then you'll get exactly the same voltage/sag in both cases!

[surfsup]

Merlin I know you know your stuff so I'm just asking to clarify...

The purpose of the Rs would be to drop the voltage prior to rectification to match the rectified voltage of the tube, so both options SS or Tube would result in the same HV rectified VDC. As I understand it the Vdrop across each resistor would be:

V = I * X where I is current draw from the amp and X is impedance

If i switch out the tube, why would the impedance of the tube be used to calculate the voltage drop across the AC resistor for the SS option? I thought this would be the impedance of the transformer. I read that somewhere else 1000 google searches ago. I had this problem before and ultimately wound up testing different resistors until I got close to the rectified VDC I wanted but would like to know how to calculate this so I can save a few $ next time on high W high V resistors like this that cost $3.00 a pair. I have your book but its not in there either. Hard to find info on AC voltage drop for an amp.