Hi guys,
Can you please explain if it's best to have a centre tap on a valve transformer?
I am looking to build a buffer preamp with an EZ81 Rectifer feeding 1 or 2 (yet to decide) 12AU7 or 6NP1-EV valves.
I have come across the following Transformer in the UK which has a 300v HT (no CT) and 6.3v heater winding which is centre tapped.
Spec
Comprises a 300V HT secondary.
The 6.3V heater winding is centre tapped.
Constructed using high grade grain orientated M6 0.35mm laminations, for lower loss.
Double insulated.
Flash tested at 4.6KV,
Separate primary and secondary windings for safety.
European primary voltages selectable from 220V, 230V, 240V and 250V.
Secondaries: 300V 320mA, 6.3V 3A centre tapped
Voltages quoted are at full load on all secondaries.
Better than 10% regulation.
The second option is
Primary voltage: 220, 230, 240 and 250V.
Secondary: 250V @ 80mA. 6.3V @ 2A Centre tapped.
The alternative is a Hammond Transformer such as the 369EX which is Centre tapped.
Can you please explain why a centre tap is or isn't necessary and weather the amps above are suitable?
Thanks
Can you please explain if it's best to have a centre tap on a valve transformer?
I am looking to build a buffer preamp with an EZ81 Rectifer feeding 1 or 2 (yet to decide) 12AU7 or 6NP1-EV valves.
I have come across the following Transformer in the UK which has a 300v HT (no CT) and 6.3v heater winding which is centre tapped.
Spec
Comprises a 300V HT secondary.
The 6.3V heater winding is centre tapped.
Constructed using high grade grain orientated M6 0.35mm laminations, for lower loss.
Double insulated.
Flash tested at 4.6KV,
Separate primary and secondary windings for safety.
European primary voltages selectable from 220V, 230V, 240V and 250V.
Secondaries: 300V 320mA, 6.3V 3A centre tapped
Voltages quoted are at full load on all secondaries.
Better than 10% regulation.
The second option is
Primary voltage: 220, 230, 240 and 250V.
Secondary: 250V @ 80mA. 6.3V @ 2A Centre tapped.
The alternative is a Hammond Transformer such as the 369EX which is Centre tapped.
Can you please explain why a centre tap is or isn't necessary and weather the amps above are suitable?
Thanks
in the old days, center taps were common place, today we have better options....
hybrid greatz rectifiers are what i use...
hybrid greatz rectifiers are what i use...
IMHO rectification topologies all sound a little different and in my personal preference
bridgde < centre tap < half wave. Of course sound is not always the main priority, there are other considerations too, which often disqualify the half wave.
In practicality terms, there is no easy way to have a valve rectified supply without using a centre tapped transformer as most valve rectifiers contain two diodes with common cathodes in the same valve. This makes the bridge connection not very elegant. The hybrid connection appears to be a reasonable compromise if one is not suspicious towards semiconductor junctions 🙂
bridgde < centre tap < half wave. Of course sound is not always the main priority, there are other considerations too, which often disqualify the half wave.
In practicality terms, there is no easy way to have a valve rectified supply without using a centre tapped transformer as most valve rectifiers contain two diodes with common cathodes in the same valve. This makes the bridge connection not very elegant. The hybrid connection appears to be a reasonable compromise if one is not suspicious towards semiconductor junctions 🙂
Last edited:
in the old days, tube rectifiers like the 5y3 and 5ar4 to name a few, used the center tapped traffo, the thing with center tapped traffo is that transformer utilization is not 100%, in that the two halves of the center tapped traffo take turns in supplying currents to the rectifier plates, so half of the time one half of the traffo secondary winding is doing nothing. not the best situation then, but today we can use the traffo for better utilization...and so the full wave bridge is what i use in my tube amps...
sure, and so we design our amps to have the best sound possible to our delight...
and once you realize that you have options, then the better and faster you can make up your mind...
sure, and so we design our amps to have the best sound possible to our delight...
and once you realize that you have options, then the better and faster you can make up your mind...
Draw-out a non-CT winding and your two-diode rectifier tube.
A single winding favors a four-diode rectifier.
And if done with thermionics (tubes), it really wants three rectifier heater windings (usually in three bottles).
Using a CT winding, you need twice the total turns but only half as many diodes; and the two diodes can be common-cathode which is very synergistic with thermionic technology.
It's not about the sound. There may be "a sound" but first you need a plan which will come together and work.
Attachments
With both of your transformers you'll need a hybrid rectifier bridge, as Tony has said yet. Add two silicon diodes, connect their anodes and tie them to ground. Connect both cathodes to the EZ81 anodes and to the secondary winding, and connect the EZ81 cathode to the 1st filter cap.
Btw, my ears aren't that golden that I could discern between different rectifier topologies by the sonic quality of the appliance. At least not in a DBT.
Best regards!
Btw, my ears aren't that golden that I could discern between different rectifier topologies by the sonic quality of the appliance. At least not in a DBT.
Best regards!
Hybrid Bridge (Graetz) Rectifier
The system that Tony was intimating is described in this page:
The Valve Wizard
The system that Tony was intimating is described in this page:
The Valve Wizard
You need a centre tap if your rectifier arrangement needs a centre tap. An EZ81 used on its own in the conventional circuit is an example of this. This is how things were done 60 years ago.
You don't need a centre tap if your rectifier arrangement does not need a centre tap. Nowadays it is common to use a bridge (i.e. four silicon diodes). This makes more efficient use of the transformer.
Either way will work fine. You just have to buy the right transformer to suit the circuit, or choose the circuit to match the transformer you already have.
You don't need a centre tap if your rectifier arrangement does not need a centre tap. Nowadays it is common to use a bridge (i.e. four silicon diodes). This makes more efficient use of the transformer.
Either way will work fine. You just have to buy the right transformer to suit the circuit, or choose the circuit to match the transformer you already have.
Your proposed buffer needs 20mA at most.
You are buying a pickup truck to fetch a few packs of beer.
You want something smaller and cheaper.
A doubler off an isolation transformer and a small SMPS for heaters would be a lot less cash... Or use a 12V SMPS to power a boost converter for B+...
Like this: AC-DC Converter AC 110V 220V 230V to DC 12V 6-8A Isolated Switching Transformer | eBay
into this: 150W Inverter Boost Board Transformer DC-AC Converter 12V to 110V 200V 220V 280V | eBay
Like this: AC-DC Converter AC 110V 220V 230V to DC 12V 6-8A Isolated Switching Transformer | eBay
into this: 150W Inverter Boost Board Transformer DC-AC Converter 12V to 110V 200V 220V 280V | eBay
Last edited:
thanks for the links...
building traffos with lower voltages than a ct type would is really a joy,
and using a full wave voltages doubler even yummier....
makes building traffos with less attention to creepage and insulation, what is not to like?
You can use what ever rectifier that you want, with whatever transformer that you want to use.
IOW a 250-0-250 tranny with a full wave rectifier (2 diodes) will give you about 353 volts peak.
that same tranny with a bridge rectifier (4 diodes not using the ct) will give you about 707 volts peak. Depending on the diodes that you use, and the input filter choice, the total voltage out will be less by the loss incurred in the rectification and smoothing.
IOW a 250-0-250 tranny with a full wave rectifier (2 diodes) will give you about 353 volts peak.
that same tranny with a bridge rectifier (4 diodes not using the ct) will give you about 707 volts peak. Depending on the diodes that you use, and the input filter choice, the total voltage out will be less by the loss incurred in the rectification and smoothing.
- Status
- Not open for further replies.