Telefunken V72 Power Transformer
- Thread starter CJ
- Start date
Help Support GroupDIY Audio Forum:
we were able to drop turns and increase wire size,
pri went from 3,200 T of #34 to 2500 T #34, and we still have over 100 henries with the good core instead of 8 henries like the original
sec went from 4600 T of #43 to 2700 T of #38, a vast improvement as far as DCR and mechanical strength and winding ease as well as making for an easier hookup,
heater went from 112 T of #24 to 80 T split of #26,
Telefunken went overkill on the heater wire with the #24, as we only need 1/2 amp, you can cheat the wire chart on current when your wire length is so short,
so we save space which means we can use bigger wire for the secondary,
since we are ditching the voltage gobbling selenium rectifier stack in favor of a more modern silicon bridge, we will end having way too much sec voltage unless we tweak the turns ratio and reduce sec turns,
V 72 circuit wants about 250 volts B+ after all the filters,
so we need about 300 before the filters,
so we divide 300 by about 1.3 and get 230 VAC, this is 60 volts lower than the stock sec voltage rating, so you see how high the B+ would have been if we used tue stock turns ratio,
this means fewer turns which means lower DCR and bigger wire,
prototype takes 1 day, 2nd unit takes about 2 hours, next we do in and output iron,
here is the new print for the pwr trans>
pri went from 3,200 T of #34 to 2500 T #34, and we still have over 100 henries with the good core instead of 8 henries like the original
sec went from 4600 T of #43 to 2700 T of #38, a vast improvement as far as DCR and mechanical strength and winding ease as well as making for an easier hookup,
heater went from 112 T of #24 to 80 T split of #26,
Telefunken went overkill on the heater wire with the #24, as we only need 1/2 amp, you can cheat the wire chart on current when your wire length is so short,
so we save space which means we can use bigger wire for the secondary,
since we are ditching the voltage gobbling selenium rectifier stack in favor of a more modern silicon bridge, we will end having way too much sec voltage unless we tweak the turns ratio and reduce sec turns,
V 72 circuit wants about 250 volts B+ after all the filters,
so we need about 300 before the filters,
so we divide 300 by about 1.3 and get 230 VAC, this is 60 volts lower than the stock sec voltage rating, so you see how high the B+ would have been if we used tue stock turns ratio,
this means fewer turns which means lower DCR and bigger wire,
prototype takes 1 day, 2nd unit takes about 2 hours, next we do in and output iron,
here is the new print for the pwr trans>
Attachments
just some regulation math on both the orig. V72 and DIY pwr transformers,
regulation specs can be at any pwr level you want, these V72 transformers run quite a bit lower in plate current than the max value listed, so we can compute regulation at these lower values to see what it does to the spec.
Full Load Specs: (taken from xfmr label)
Original V72 Pwr XFMR Regulation: 11.3%
DIY V72 Pwr XFMR Regulation: 8.8%$
the DIY xfmr has lower DCR which is why the regulation is better.
The lower sec voltage of 230 instead of 280 means less output power which works against the lowered DCR as output power is in the denominator of the formula:
Regulation = Copper Loss/(Output Power + Copper Loss)
but the lower DCR is more dominate than the drop in output power so we still get better regulation.
Here are the regulation specs for both transformers taken at the reduced plate current of 1.4 ma for both EF804s tubes, there is no bleeder resistor so all the power goes into the tubes as there are no bleeder resistors or other paths to ground,
heater current is 340 ma instead of 500 ma so we see a reduction in copper loss which will also improve regulation
Regulation of transformers in V72 circuit:
Original pwr xfmr : 7.6% (down from 11.3%)
DIY pwr xfmr 7.2% (down from 8.8%)
regulation improved more on the original xfmr with the reduced load because the DCR is higher, so when sec current on the HV winding drops from 9 ma to 1.4 ma, there is a larger percent change in the regulation.
just for kicks we computed regulation with a bleeder resistor installed from B+ to ground,
Regulation on DIY transformer with a 220K bleeder resistor: 6.8%
so not much difference from 7.2% without a bleeder to 6.8% with a bleeder,
and also, since the second tube in the V72 uses a plate choke, the plate current draw seen by the transformer will have very little variation, and since the first tube only draws 0.6 ma, total plate current variation will be pretty small so regulation is not really an issue, thus, a bleeder resistor is not going to do much,
also see that the heater circuit draws .34 * 6.3 = 2.14 watts
and the B+ circuit only uses 0.0014 * 280 volts = 0.392 watts,
so the steady power draw of the heater circuit helps keep the primary current very steady when audio is being passed through the tubes.
Note: for 120 VAC operation of the V72, just drop the primary turns down to 1,364 T,
the sec windings can stay then same. flux will stay the same as voltage and turns drop by an equal percentage.
OPT next,
regulation specs can be at any pwr level you want, these V72 transformers run quite a bit lower in plate current than the max value listed, so we can compute regulation at these lower values to see what it does to the spec.
Full Load Specs: (taken from xfmr label)
Original V72 Pwr XFMR Regulation: 11.3%
DIY V72 Pwr XFMR Regulation: 8.8%$
the DIY xfmr has lower DCR which is why the regulation is better.
The lower sec voltage of 230 instead of 280 means less output power which works against the lowered DCR as output power is in the denominator of the formula:
Regulation = Copper Loss/(Output Power + Copper Loss)
but the lower DCR is more dominate than the drop in output power so we still get better regulation.
Here are the regulation specs for both transformers taken at the reduced plate current of 1.4 ma for both EF804s tubes, there is no bleeder resistor so all the power goes into the tubes as there are no bleeder resistors or other paths to ground,
heater current is 340 ma instead of 500 ma so we see a reduction in copper loss which will also improve regulation
Regulation of transformers in V72 circuit:
Original pwr xfmr : 7.6% (down from 11.3%)
DIY pwr xfmr 7.2% (down from 8.8%)
regulation improved more on the original xfmr with the reduced load because the DCR is higher, so when sec current on the HV winding drops from 9 ma to 1.4 ma, there is a larger percent change in the regulation.
just for kicks we computed regulation with a bleeder resistor installed from B+ to ground,
Regulation on DIY transformer with a 220K bleeder resistor: 6.8%
so not much difference from 7.2% without a bleeder to 6.8% with a bleeder,
and also, since the second tube in the V72 uses a plate choke, the plate current draw seen by the transformer will have very little variation, and since the first tube only draws 0.6 ma, total plate current variation will be pretty small so regulation is not really an issue, thus, a bleeder resistor is not going to do much,
also see that the heater circuit draws .34 * 6.3 = 2.14 watts
and the B+ circuit only uses 0.0014 * 280 volts = 0.392 watts,
so the steady power draw of the heater circuit helps keep the primary current very steady when audio is being passed through the tubes.
Note: for 120 VAC operation of the V72, just drop the primary turns down to 1,364 T,
the sec windings can stay then same. flux will stay the same as voltage and turns drop by an equal percentage.
OPT next,
