Whatever the plate current is, 88.9mA, 83mA, etc., for fixed bias circuits, you do not subtract the bias voltage from the plate voltage to get the plate-to-filament/"cathode" voltage.
But that is what nicoch58 stated in post # 23. I think he forgot that the circuit is for fixed bias, not self bias.
At 83mA and 450V, the plate dissipation is 37.35 Watts, just over the revised Western Electric 36 Watt maximum.
And the Western Electric 300A data sheet lists 70mA maximum, for fixed bias operation. The 300A and 300B difference was the 300A had a locator pin on the side of the bakelite. Also, some other companies data sheets list a 70mA maximum for fixed bias operation. 83 mA is 13mA above the maximum spec.
300B tubes are expensive, operate them any way you want.
But that is what nicoch58 stated in post # 23. I think he forgot that the circuit is for fixed bias, not self bias.
At 83mA and 450V, the plate dissipation is 37.35 Watts, just over the revised Western Electric 36 Watt maximum.
And the Western Electric 300A data sheet lists 70mA maximum, for fixed bias operation. The 300A and 300B difference was the 300A had a locator pin on the side of the bakelite. Also, some other companies data sheets list a 70mA maximum for fixed bias operation. 83 mA is 13mA above the maximum spec.
300B tubes are expensive, operate them any way you want.
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I have read this was one of the major problems people had with this design. The potential of eating through tubes. I'll have to dig up that thread. The other issue was the size of the power capacitors at 1200uf. Interestingly the schematic and the picture underneath the amp are different. It looks like the caps in the picture are wired in series while the schematic shows parallel.
Don't have the money to throw away 300b tubes. I would be interested in any advice on improving these areas. Hopefully will be getting the first shipment of parts next week. Building will hopefully start soon...
Don't have the money to throw away 300b tubes. I would be interested in any advice on improving these areas. Hopefully will be getting the first shipment of parts next week. Building will hopefully start soon...
The requirements for most tube rectifiers that have large capacitor loads is: They need resistance in each plate lead. Just check your particular rectifier, and and the uF of your input capacitor. Then go to the rectifier data sheet and check the minimum resistance per plate lead for that large capacitance.
The DCR of most transformers are usually not enough to meet the minimum required resistance.
An example of how to do this (as an illustration). A example transformer has 120V in, and 2 x 360 volts out (360V from the center tap, which is 720V center tapped). The 120V primary has 5 Ohms DCR. The turns ratio is 1:3, so multiply the 5 Ohms by 3. That reflects 15 Ohms to the 360V winding (and 15 Ohms to the other 360V winding).
The example transformer 360V to center tap DCR is 30 Ohms. Add the 15 Ohms and 30 Ohms = 45 Ohms. Suppose the input capacitor is 50uF, and the tube requires 75 Ohms in series with the plate for such a large capacitor. 75 Ohms - 45 Ohms = 30 Ohms. You need to add a 30 Ohm resistor in series from each 360V lead to its respective plate lead.
The DCR of most transformers are usually not enough to meet the minimum required resistance.
An example of how to do this (as an illustration). A example transformer has 120V in, and 2 x 360 volts out (360V from the center tap, which is 720V center tapped). The 120V primary has 5 Ohms DCR. The turns ratio is 1:3, so multiply the 5 Ohms by 3. That reflects 15 Ohms to the 360V winding (and 15 Ohms to the other 360V winding).
The example transformer 360V to center tap DCR is 30 Ohms. Add the 15 Ohms and 30 Ohms = 45 Ohms. Suppose the input capacitor is 50uF, and the tube requires 75 Ohms in series with the plate for such a large capacitor. 75 Ohms - 45 Ohms = 30 Ohms. You need to add a 30 Ohm resistor in series from each 360V lead to its respective plate lead.
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mjstriker5,
The Klipsch Cornwalls you have should work well with a 300B amp that is not tweeked for max power, but that instead is tweeked for finesse.
Unless you like your music super loud, and want the neighbors to complain.
After all, summer is coming, and you may choose to open the windows to get fresh air.
Play the right music, and the neighbors may ask you to turn it up a little.
The Klipsch Cornwalls you have should work well with a 300B amp that is not tweeked for max power, but that instead is tweeked for finesse.
Unless you like your music super loud, and want the neighbors to complain.
After all, summer is coming, and you may choose to open the windows to get fresh air.
Play the right music, and the neighbors may ask you to turn it up a little.
Not looking to play loud. Have that in another set up...
Was able to pick up some CV378 tubes for a reasonable price. Found there datasheet here:
http://www.r-type.org/pdfs/cv378.pdf
Looks like the maximum capacitance is around 50uf. Still scratching my head I regards to the Cary amps having 1200uF. Was this really gross negligence as stated in other forum posts or does it work out. Also noticed in the pictures under the hood show a very different set up in regards to the power supply. Connections to the rectifier tube don't make sense to me. Looks like pin 4 is connected to pin 6 while pin 5 has one leg of the HV supply. Also noticed several diodes were added. Looks like some sort of hybrid power supply. Does that alleviate some of the stress and allow for larger capacitors?
Last thing is the minimum resistance of the CV378 is 75 ohms. Once I get the transformer I should be able to see if resistors are necessary.
Thanks again for everyone's insight.
Was able to pick up some CV378 tubes for a reasonable price. Found there datasheet here:
http://www.r-type.org/pdfs/cv378.pdf
Looks like the maximum capacitance is around 50uf. Still scratching my head I regards to the Cary amps having 1200uF. Was this really gross negligence as stated in other forum posts or does it work out. Also noticed in the pictures under the hood show a very different set up in regards to the power supply. Connections to the rectifier tube don't make sense to me. Looks like pin 4 is connected to pin 6 while pin 5 has one leg of the HV supply. Also noticed several diodes were added. Looks like some sort of hybrid power supply. Does that alleviate some of the stress and allow for larger capacitors?
Last thing is the minimum resistance of the CV378 is 75 ohms. Once I get the transformer I should be able to see if resistors are necessary.
Thanks again for everyone's insight.
I'm wondering if that schematic is misdrawn. I really don't see how you can use 1200uF with a GZ37, but maybe I'm wrong. Also, I see a lot of 450VDC caps in that picture, which would be pretty dodgy with a 465VDC B+. It's hard to tell from the photo but perhaps those larger caps are in series *after* the resistor. I don't know.
For a center tapped HV secondary:
If there is a choke input filter, the voltage is 0.9 times the 1/2 winding of the transformer HV secondary voltage. There will also be the voltage drop of the tube rectifier.
And lots of capacitance After the choke is OK for a true choke input filter (no cap before the choke).
But before the output tube warms up, the B+ can rise to as high as 1.414 times the 1/2 winding of the transformer HV secondary.
Plan your electrolytic capacitor voltage rating accordingly.
With a non center tapped HV secondary, and a bridge rectifier, i.e. one tube and 2 solid state diodes, the voltages are:
0.9 x the HV secondary (choke input filter)
1.414 x the HV secondary (choke input filter at start up; and cap input filter at and after start up)
Again, plan the electrolytic caps voltage rating accordingly.
If there is a choke input filter, the voltage is 0.9 times the 1/2 winding of the transformer HV secondary voltage. There will also be the voltage drop of the tube rectifier.
And lots of capacitance After the choke is OK for a true choke input filter (no cap before the choke).
But before the output tube warms up, the B+ can rise to as high as 1.414 times the 1/2 winding of the transformer HV secondary.
Plan your electrolytic capacitor voltage rating accordingly.
With a non center tapped HV secondary, and a bridge rectifier, i.e. one tube and 2 solid state diodes, the voltages are:
0.9 x the HV secondary (choke input filter)
1.414 x the HV secondary (choke input filter at start up; and cap input filter at and after start up)
Again, plan the electrolytic caps voltage rating accordingly.
I haven't seen any schematics showing any. I am now thinking the pictures of actual amps that I found show some of the necessary updates mentioned. It looks a choke was added marked C-3x. Also the two large caps are indeed placed in series not parallel. Also there are 4 diodes placed off the rectifier. I presume these things allow for the higher uF. I just need to figure how they are all incorporated.
The choke is the main question. What I am thinking is they placed two 47uF 450 VDC caps in series off the rectifier then a choke then two 1200uF 450 VDC caps in series. Would that make sense?
I have already ordered a choke with the other transformers which will arrive tomorrow. I figured I would already be incorporating one.
I touched based on this above but I am not sure they moved to a true choke input filter because it seems there is 47uF before everything. With the current voltages and the probable need to reduce B+ I will likely add the choke regardless.
Thanks again for everyone's guidance and insight
Cary Audio used to have a "parts department" where they sold stuff like their own transformers (Audio Electronic Supply). It closed a number of years ago, but I was surprised when I contacted them long after it was closed and they were like, "Yeah, we've got one of those hanging around here in the corner, you want it?"
First, I must state clearly I am no electronics expert, only a hobbyist with extensive experience with my Cary 40M Special monoblocks, which I have owned for over 20 years and had carried out extensive mods to make them sound good. A few points I want to make:
1. My 40M/KT88 was roughly produced at the same time as the 300B in question and they share many common parts and design concepts. To wit: 2 x 450V/1200 uF caps, 6SL7 driver tubes, step-up transformer from tube filament winding to bias. I suspect Cary had their hands on a big supply of these parts and designed their amps around those parts.
2. To use caps of such high value behind a tube rectifier is not a good idea, is it?
3. My 40M uses SS bridge rectifier and 2 x 1200 uF in series for filter – nothing else. I have since rebuilt an outboard CLCLC filter and I can tell you the improvements are striking. I used the first C to tune the B+ voltage. The B+ set by Cary was way too high to falsify their claim that the amp operates in Class A. Cary also has a tendency to maximize output power at all costs. A 300B outputting 15W? Another Cary 300B output was 20W. I think they used special 300B, like Sophia? But the high output probably butches the sound quality.
4. In message #29, 6A3sUMMER said that the reflected resistance to the secondary was 5 x3 for a turn ration of 3. I think it should be 5 x 3 x3 = 45.
1. My 40M/KT88 was roughly produced at the same time as the 300B in question and they share many common parts and design concepts. To wit: 2 x 450V/1200 uF caps, 6SL7 driver tubes, step-up transformer from tube filament winding to bias. I suspect Cary had their hands on a big supply of these parts and designed their amps around those parts.
2. To use caps of such high value behind a tube rectifier is not a good idea, is it?
3. My 40M uses SS bridge rectifier and 2 x 1200 uF in series for filter – nothing else. I have since rebuilt an outboard CLCLC filter and I can tell you the improvements are striking. I used the first C to tune the B+ voltage. The B+ set by Cary was way too high to falsify their claim that the amp operates in Class A. Cary also has a tendency to maximize output power at all costs. A 300B outputting 15W? Another Cary 300B output was 20W. I think they used special 300B, like Sophia? But the high output probably butches the sound quality.
4. In message #29, 6A3sUMMER said that the reflected resistance to the secondary was 5 x3 for a turn ration of 3. I think it should be 5 x 3 x3 = 45.
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