09-10-2014, 03:04 PM
Ron
The way this radio is, unless the tubes will behave at higher voltage, the way it is now is the way.
If you look at 18 vs say 38-15, which have very similar tube Lineup in the receiving-detecting part, the sole difference s the Push-pull 42 stage in the 18 vs single 41 Class A in 38-15.
So they gave the Push-Pull 280V P-K whereas Class A needs only 200V. So they need to lower B+ for the rest of them in 18.
Hence the bleeder.
This said, the options were:
1. They could have used a separate winding on the transformer, better yet - bucking winding. Needs a second rectifier.
Expensive, costly.
2. Use a speaker with higher Field coil resistance. Possible if it exists. Still needs a bleeder but will dissipate less. A model H speaker has 3,200 Ohms.
3. Could use the lower voltage on the output tubes.
This is interesting as 42 tube shows 250V recommended B+ and so do other tubes in this radio, so I think, it is possible to find a compromise voltage where the pushpull still works well (how much power do we really need) and the other tube are still comfortable.
This would require lower rectifier voltage.
Again, at this point, unless the power transformer is gone, and one is looking to replace it, not an optimal solution.
4. Increasing the bleeder alone so it dissipates less will possibly stress the tubes a bit more, and the bleeder is still required.
5. Using tubes in PP with lower Plate votage and different output transformer. Like 90 or 111 do.
Unfortunately there are two ways to reduce the DC voltage: divide it, dissipating power (so do linear dropout regulators and resistors) or convert it.
The latter option will require a DCDC converter and presents the problems of other sorts, the very first being necessity to create a solid state high voltage DCDC which also will be noisy.
It is not easy to change the existing design.
Also the changes will violate the "original" status.
–---------
What I would try.
A speaker used in 18 (for 118 cathedral might not be an option) is shown to have 1.11 Ohm voice and 1.14kOhm field.
An H speaker from 111 (or 95, or..) has close enough voice and transformer to be used with the same pushpull, but has 3.2kOhm field.
That can be combined with higher value bleeder that will result in lower dissipation.
The performance will have to be evaluated after that, especially audio.
The way this radio is, unless the tubes will behave at higher voltage, the way it is now is the way.
If you look at 18 vs say 38-15, which have very similar tube Lineup in the receiving-detecting part, the sole difference s the Push-pull 42 stage in the 18 vs single 41 Class A in 38-15.
So they gave the Push-Pull 280V P-K whereas Class A needs only 200V. So they need to lower B+ for the rest of them in 18.
Hence the bleeder.
This said, the options were:
1. They could have used a separate winding on the transformer, better yet - bucking winding. Needs a second rectifier.
Expensive, costly.
2. Use a speaker with higher Field coil resistance. Possible if it exists. Still needs a bleeder but will dissipate less. A model H speaker has 3,200 Ohms.
3. Could use the lower voltage on the output tubes.
This is interesting as 42 tube shows 250V recommended B+ and so do other tubes in this radio, so I think, it is possible to find a compromise voltage where the pushpull still works well (how much power do we really need) and the other tube are still comfortable.
This would require lower rectifier voltage.
Again, at this point, unless the power transformer is gone, and one is looking to replace it, not an optimal solution.
4. Increasing the bleeder alone so it dissipates less will possibly stress the tubes a bit more, and the bleeder is still required.
5. Using tubes in PP with lower Plate votage and different output transformer. Like 90 or 111 do.
Unfortunately there are two ways to reduce the DC voltage: divide it, dissipating power (so do linear dropout regulators and resistors) or convert it.
The latter option will require a DCDC converter and presents the problems of other sorts, the very first being necessity to create a solid state high voltage DCDC which also will be noisy.
It is not easy to change the existing design.
Also the changes will violate the "original" status.
–---------
What I would try.
A speaker used in 18 (for 118 cathedral might not be an option) is shown to have 1.11 Ohm voice and 1.14kOhm field.
An H speaker from 111 (or 95, or..) has close enough voice and transformer to be used with the same pushpull, but has 3.2kOhm field.
That can be combined with higher value bleeder that will result in lower dissipation.
The performance will have to be evaluated after that, especially audio.
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