Does the EZ81 need its own 6.3vac supply, separate from the other tube heaters? I want to build a Marshall 1974 mdl 18W amp, and have a salvaged PT that has a large capacity 6.3 vac heater winding, enough to handle the total heater currents. And an HV winding that was used for a PP 6V6 power section..... PTs for the Marshall are expensive, and I want to use what I have laying around. Thanks!
It looks OK to me.
The reservoir cap should not be more than 50uF.
Also, if the transformer is a big one, it may not have enough internal resistance, so you may need to add protection resistors in both anode circuits, see page 5 of this data sheet:
http://www.drtube.com/datasheets/ez81-philips1970.pdf
The reservoir cap should not be more than 50uF.
Also, if the transformer is a big one, it may not have enough internal resistance, so you may need to add protection resistors in both anode circuits, see page 5 of this data sheet:
http://www.drtube.com/datasheets/ez81-philips1970.pdf
The Marshall amp anode load should be close to what my "big" PT was seeing when it ran a PP 6V6 organ amp. Its heater winding supplied about 30 preamp tubes.... I have read comments about this "cathode to heater" voltage, but don't really understand its purpose. Is it and/or the protection resistance to protect the transformer?
I think the protective resistors (if they are required) are to limit the peak ripple current which could damage the EZ81. This page from Merlin looks very useful:
The Valve Wizard
The Valve Wizard
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The 'cathode to heater' voltage limit (which is 500V for the EZ81) is just to prevent the thin layer of insulation between the cathode and the heater from breaking down.
In a Marshall 18W you will probably reference the heater string to ground, or maybe 'elevate' it a bit. The B+ might be at 340V or something like that. So you will be well within the 500V limit.
In a Marshall 18W you will probably reference the heater string to ground, or maybe 'elevate' it a bit. The B+ might be at 340V or something like that. So you will be well within the 500V limit.
That's true for many rectifier tubes, but the EZ81 has an indirectly heated cathode.
The positive volts are on the cathode, not the heater. While I admit that, when building 3-3s, the only amp in which I used an EZ rectifier (all the others were GZ) I used a separate winding for heaters, if they guarantee a 500V separation, it should work fine on the same winding. After all, it's only because of the proximity that cathode/heater shorts are so common, and just add a bit of low frequency hum to the system, until the faulty tube/valve is found and replaced.
Yes, they often do have a separate heater winding, but I wonder if that's just transformer manufacturers not wanting to restrict the application of their PT to rectifiers of that type?
Even when on a separate heater winding there will be a strong coupling of any buzz etc. onto the other heater circuit through transformer action, I would imagine.
Right you are Malcolm, I assumed that it was the same as the GZ34. I'd still be leery of the the common filament string, especially with new production EZ81's.
Just to clarify, the EZ81 has an indirectly heated cathode. This is necessary for sharing the heater wiring with other valves, but it is not sufficient. You also have to check that the cathode does not share a pin with the heaters. The EZ81 does not, so you can have a common heater chain for all the valves.
On the other hand, a 5Z4 is also indirectly heated but it does share a pin between cathode and heater so it needs its own 5V heater supply. I assume this is because it gives some commonality with other octal rectifiers which are directly heated.
On the other hand, a 5Z4 is also indirectly heated but it does share a pin between cathode and heater so it needs its own 5V heater supply. I assume this is because it gives some commonality with other octal rectifiers which are directly heated.
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