I wonder whether anyone can help. I am building Mullard 3-3 (my first attempt at an amp build) in the UK, and have realised that I bought a 300V C.T. power transformer, when it seems I should have bought one rated at 600 V C.T (According to the article I am using: www.r-type.org/articles/art-003h.htm ).
The transformer I have is a Hammond 369BX (39 VA, 300V C.T. @86 mA, 6.3V C.T. @ 2A). This has taps for various input voltages including 120V and 240V. My question is, if I connect my 240V supply to the 120V taps, will this give me the 600 V C.T. that I need (I guess so) and is it okay to do this, or will it have some adverse effect that I haven't thought of?
My next question relates to the heater supply, which I expect will also double in voltage. Can I simply use the centre tap to take me back down to the 6.3 Volts I need?
Lastly, in the drawing on the article above, the transformer has two secondary windings for the heaters. It makes clear that if there is only one winding for the heaters it is okay provided it has a 2A rating, but I am intrigued; is there any particular reason why the circuit is shown with a separate winding for the EZ80 heater (a winding with no centre tap), and another for the EL84 and EF86 (that one with a centre tap)? This partly relates to my query in the previous paragraph, because if I have used the centre tap in the way I have described, I no longer have a real centre tap to connect to 0V as shown in the diagram. Is that crucial, and if so I would be intrigued to learn why.
I hope that makes some sense, and many thanks for any assistance. This site has already provided me with much inspiration and guidance which is much appreciated.
Ian
The transformer I have is a Hammond 369BX (39 VA, 300V C.T. @86 mA, 6.3V C.T. @ 2A). This has taps for various input voltages including 120V and 240V. My question is, if I connect my 240V supply to the 120V taps, will this give me the 600 V C.T. that I need (I guess so) and is it okay to do this, or will it have some adverse effect that I haven't thought of?
My next question relates to the heater supply, which I expect will also double in voltage. Can I simply use the centre tap to take me back down to the 6.3 Volts I need?
Lastly, in the drawing on the article above, the transformer has two secondary windings for the heaters. It makes clear that if there is only one winding for the heaters it is okay provided it has a 2A rating, but I am intrigued; is there any particular reason why the circuit is shown with a separate winding for the EZ80 heater (a winding with no centre tap), and another for the EL84 and EF86 (that one with a centre tap)? This partly relates to my query in the previous paragraph, because if I have used the centre tap in the way I have described, I no longer have a real centre tap to connect to 0V as shown in the diagram. Is that crucial, and if so I would be intrigued to learn why.
I hope that makes some sense, and many thanks for any assistance. This site has already provided me with much inspiration and guidance which is much appreciated.
Ian
DO NOT use the 120V tap on 240V! It will burn out the primary winding.
It may be best to cheat a little. Use a bridge rectifier to produce the HT you require. 300V X 1.414 = 424V Probably 400V in the end. Sounds like the transformer is completely wrong as the VA is not enough at that voltage.
The circuit only calls for 325V so watch the voltage.
It may be best to cheat a little. Use a bridge rectifier to produce the HT you require. 300V X 1.414 = 424V Probably 400V in the end. Sounds like the transformer is completely wrong as the VA is not enough at that voltage.
The circuit only calls for 325V so watch the voltage.
Don't apply 240V to the 120V primary tap!
You could consider ditching the EZ80 in favor of a SS bridge rectifier and increasing R15 to bring B+ back where it belongs. The HT winding sees about 90mA of AC current in this case, which might just squeak by since the heater load is reduced. It's close, so the transformer is going to run near its temperature limit. Good ventilation is an absolute necessity if you do this.
Regarding the heater winding CT: It's probably sufficient to fake a CT with a couple of 100R resistors stretched across the 6.3V winding, with the junction taken to ground.
Edit: On second thought, R15 probably makes the transformer utilization 'form factor' somewhat better than the standard 1.8X for capacitor loaded bridge rectifiers, which means that HT secondary current should be substantially less than 90mA.
One more thing -- Add a snubbing network on the HT winding to suppress switching transients if you use a SS rectifier. 1K and 0.001uF in series generally does the job.
You could consider ditching the EZ80 in favor of a SS bridge rectifier and increasing R15 to bring B+ back where it belongs. The HT winding sees about 90mA of AC current in this case, which might just squeak by since the heater load is reduced. It's close, so the transformer is going to run near its temperature limit. Good ventilation is an absolute necessity if you do this.
Regarding the heater winding CT: It's probably sufficient to fake a CT with a couple of 100R resistors stretched across the 6.3V winding, with the junction taken to ground.
Edit: On second thought, R15 probably makes the transformer utilization 'form factor' somewhat better than the standard 1.8X for capacitor loaded bridge rectifiers, which means that HT secondary current should be substantially less than 90mA.
One more thing -- Add a snubbing network on the HT winding to suppress switching transients if you use a SS rectifier. 1K and 0.001uF in series generally does the job.
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Thanks for the quick and helpful responses, which save me from having to ditch (or fry) my new transformer! Once I've digested I'll proceed with a SS rectifier.
The reason why you might want to use a separate heater winding for the EZ80 is so that the heater-cathode insulation is not strained so much. Instead, the secondary winding insulation in the transformer takes the strain. EZ80 was designed to share a heater supply with the rest of the circuit, so it has thick insulation which is why it takes a long time to warm up.
Given a 300V RMS transformer to power a circuit designed for 325V your best bet might be to use a choke input PSU and get around 270V. A bit low, but 420V from a cap input supply will be far too high. You can't simply drop off the excess voltage using resistors as that could lead to poor LF response.
Given a 300V RMS transformer to power a circuit designed for 325V your best bet might be to use a choke input PSU and get around 270V. A bit low, but 420V from a cap input supply will be far too high. You can't simply drop off the excess voltage using resistors as that could lead to poor LF response.
The transformer can still be used if the rectifier is configured for hybrid bridge operation (silicon negative side , valve positive side)
316a
316a
Thanks, the hybrid bridge rectifier appears to be a simple fix. Wouldn't the Bplus voltage be almost the same as expected with the original circuit? Id still have very close to 300V rms across the ez80.
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