HOLY %#*%
John,
I'm with you on the calculations, and derating makes sense. If I go for a 1 deg C/W sink, I will need 4.5" of the $98 heatsink on the bottom of the page??? OUCH! That is some pretty expensive regulation.
I've never noticed the temp ratings on heatsinks, presuambly b/c the sinks I use dont dissipate any heat (flimsy little POS).
Is there a more economical method to supply the regulation without mountiung the regulators to a cast-iron bath-tub?
Also, if I supply regulated voltage, I would be facing the same delema, correct?
I guess this wont be that much of a problem for the other valves, as they draw much less current...
Hmmm
John,
I'm with you on the calculations, and derating makes sense. If I go for a 1 deg C/W sink, I will need 4.5" of the $98 heatsink on the bottom of the page??? OUCH! That is some pretty expensive regulation.
I've never noticed the temp ratings on heatsinks, presuambly b/c the sinks I use dont dissipate any heat (flimsy little POS).
Is there a more economical method to supply the regulation without mountiung the regulators to a cast-iron bath-tub?
Also, if I supply regulated voltage, I would be facing the same delema, correct?
I guess this wont be that much of a problem for the other valves, as they draw much less current...
Hmmm
Hi Guys,
I have all the parts and have been supposedly building a parallel 845 amp for some years now!!
I have been stumped by the heater supplies. I plan to use 300B as driver so usual AC hum from this is amplified by final stage :-(
I need 6 big DH supplies!!! Yikes! Hence my hesitation to plunge in - just about need a dedicated chassis for the LV supplies esp if heatsinked!
Any comments on what sounds good? I HATE hum!!
The reason I am stumped is all my heater transformers are designed for AC not DC and have inadequate headroom volts. Mostly they also sag further under load.
I have been thinking of trying Halogen light HF AC as a new approach - cheap! Any experience with these out there?
How is the DC sound versus AC sound debate going? Hum cancellation tecvhniques? Current reg versus volt reg? I just go round thinking in circles. Maybe iI should just build simple AC and try to fix hum - I have std humbuck pots for centre earth etc.
cheers
I have all the parts and have been supposedly building a parallel 845 amp for some years now!!
I have been stumped by the heater supplies. I plan to use 300B as driver so usual AC hum from this is amplified by final stage :-(
I need 6 big DH supplies!!! Yikes! Hence my hesitation to plunge in - just about need a dedicated chassis for the LV supplies esp if heatsinked!
Any comments on what sounds good? I HATE hum!!
The reason I am stumped is all my heater transformers are designed for AC not DC and have inadequate headroom volts. Mostly they also sag further under load.
I have been thinking of trying Halogen light HF AC as a new approach - cheap! Any experience with these out there?
How is the DC sound versus AC sound debate going? Hum cancellation tecvhniques? Current reg versus volt reg? I just go round thinking in circles. Maybe iI should just build simple AC and try to fix hum - I have std humbuck pots for centre earth etc.
cheers
Bryan,
This is why I decided to use plain 'ol CRC to supply mine, though it does have the disadvantage that the voltage will differ if the valve curent changes ie different valve, but at least my spares are the same
Yes, a voltage regulator has the same problems but there is a small cheat that is used in lots of commercial equipment:
Since the power dissapation is equivalent to the area under the graph * current, then reducing the smoothing capacitor value and introducing some ripple actually reduces the heat. You just have to make sure that the minimum voltage drop "dropout" voltage of the regulator is not "exceeded". You then rely on the regulator to smooth out the ripple.
This is why I decided to use plain 'ol CRC to supply mine, though it does have the disadvantage that the voltage will differ if the valve curent changes ie different valve, but at least my spares are the same
Yes, a voltage regulator has the same problems but there is a small cheat that is used in lots of commercial equipment:
Since the power dissapation is equivalent to the area under the graph * current, then reducing the smoothing capacitor value and introducing some ripple actually reduces the heat. You just have to make sure that the minimum voltage drop "dropout" voltage of the regulator is not "exceeded". You then rely on the regulator to smooth out the ripple.
Hello hifryer, welcome to the fray
Hmmm, you said "parallel".... That sounds like it's asking to be tried on AC with reversed phase on each valve. That's surely got to be worth a first try.
Don't even think about halogen lighting transformers until you have 'scoped the output.... then if you still think it's a good idea I'll go and screw my head in the vice.
There are ways of doing all this, it's just deciding how much iron and heat you can afford.
For heater transformers consider winding your own secondary on a large toroidal mains transformer. You can mess to get the voltage right. About 3 turns per volt. You might want to put a noise filter on the primary side.
Hmmm, you said "parallel".... That sounds like it's asking to be tried on AC with reversed phase on each valve. That's surely got to be worth a first try.
Don't even think about halogen lighting transformers until you have 'scoped the output.... then if you still think it's a good idea I'll go and screw my head in the vice.
There are ways of doing all this, it's just deciding how much iron and heat you can afford.
For heater transformers consider winding your own secondary on a large toroidal mains transformer. You can mess to get the voltage right. About 3 turns per volt. You might want to put a noise filter on the primary side.
I am with dhaen on this one. I use CRC. I have a 6 amp bridge rectifier with a 22000uF cap then a .8 ohm 10 watt resistor (should be bigger) and another 22000 uF cap. This is running off of a transformer with two (one for each channel) 12 volt 4 amp windings. Inrush current limiter on the primary.
It is desirable to feed the 845's as close to 10 volts as possible, +/- 5% is considered OK. In my case I get 9.7 volts on one tube and 10.1 on the other. There is 10 feet of wire and 1 connector in the path also. There is a cable from the power supply to the amp.
The bridge rectifiers must be heatsinked to the chassis. I had to let the smoke out of one to find this out.
Often heatsinks can be robbed from the processor chips in a discarded copmuter. I use these in many of my designs, but they may not be big enough in your case without a fan.
It is desirable to feed the 845's as close to 10 volts as possible, +/- 5% is considered OK. In my case I get 9.7 volts on one tube and 10.1 on the other. There is 10 feet of wire and 1 connector in the path also. There is a cable from the power supply to the amp.
The bridge rectifiers must be heatsinked to the chassis. I had to let the smoke out of one to find this out.
Often heatsinks can be robbed from the processor chips in a discarded copmuter. I use these in many of my designs, but they may not be big enough in your case without a fan.
Heater supplies:
Here's the direct link to mine: OnRyoku_LT-Supplies
Although not marked, the caps are 47mF *
*Remember mF= 1000 uF
Edit:
Realised that the CRC was used in my 300B heaters, not the 845 - sorry. Time goes by and I forget......
Here's the direct link to mine: OnRyoku_LT-Supplies
Although not marked, the caps are 47mF *
*Remember mF= 1000 uF
Edit:
Realised that the CRC was used in my 300B heaters, not the 845 - sorry. Time goes by and I forget......
Good Grief,
AC sure would be alot easier to do!
I'm looking at the data sheet for the MBR735 and can not find the voltage drop. Am I blind, or just dumb today?
For the LM338, what is the necessary input voltage to maintain regulation? is it 1.25VDC + Vout?
Looking down the chain at the 300B, I only have half the current and half the problem... YEAH! Another source of heat!
I may just have to go CRC filtering all the way around. I'm just worried that when I move out of this apt where to mains is 125-127VAC to another place with presumably 117VAC I will run into problems. I dont want to have to re-design multiple components of the amp a second time over....
UGH !!!
AC sure would be alot easier to do!
I'm looking at the data sheet for the MBR735 and can not find the voltage drop. Am I blind, or just dumb today?
For the LM338, what is the necessary input voltage to maintain regulation? is it 1.25VDC + Vout?
Looking down the chain at the 300B, I only have half the current and half the problem... YEAH! Another source of heat!
I may just have to go CRC filtering all the way around. I'm just worried that when I move out of this apt where to mains is 125-127VAC to another place with presumably 117VAC I will run into problems. I dont want to have to re-design multiple components of the amp a second time over....
UGH !!!
Don't panic....
Bryan,
For geographical mains variation you can arrange for parallel resistors to jumper in or out on the heater supplies.
For the HT supplies consider changing the value of the input cap of your CLC filter. With a jumper lead, you can attach an extra cap to either side of the choke for more or less output voltage.
Bryan,
For geographical mains variation you can arrange for parallel resistors to jumper in or out on the heater supplies.
For the HT supplies consider changing the value of the input cap of your CLC filter. With a jumper lead, you can attach an extra cap to either side of the choke for more or less output voltage.
Some notes on LM338. The LM338 has built in thermal protection. Max junction temperature is 125C. Dropout voltage decreases with higher temperature. TO-220 LM338s are speced for 25watts dissipation. Try sticking it on an old cpu heatsink or the like and letting it run hot. Test on a 3 ohm 100+ watt power resistor if you have one.
If you want to drop the regulator entirely, you could save over 50 watts of dissipation per heater by using a choke instead of resistors to drop the voltage. Apex Jr. sells an 8-11.2mH choke that would get you close. $20. Critical value is 3.1mH for an 845 heater.
If your currently getting 16-17V under load, choke input would give about 11V.
Your probably going to have 120V mains where you move to, rather than 117V.
If your currently getting 16-17V under load, choke input would give about 11V.
Your probably going to have 120V mains where you move to, rather than 117V.
Bryan,
You might want to take a look at the LT1084 regulator, it has lower drop out voltage than the LM338 and will operate comfortably with just 3V across it. To reduce the current through it and hence the dissipation you can parallel a large low value power resistor across the input to output pins - for example if you had a raw supply of 13V a 3.0 ohm resistor would source 1A. I recommend a 2.2 ohm resistor resulting in about half the dissipation in the regulator and half in the resistor. (Note above is for use with voltage mode regulation, not current mode.)
You should make sure you design for low line, i.e. the raw dc voltage at 10% low line should be just above the drop out voltage of the regulator at say 1.5V..
I use toroids for these applications because they usually have low winding resistance. Choose one that drops a volt or less at the rated load current and you may be able to get by with something under 12Vrms, use schottky diodes and you might even have enough head room with a 10Vrms transformer. Simulate in PSUD to be sure, and go conservatively higher if there is any doubt.
On heat sinks - figure the normal device has an thermal resistance tab to die of about 1 degree C per watt (refer to spec), let's say you are dissipating 20W (3A with 6.6V delta across regulator) and your chosen die temperature target max is 125 C assuming ambient of 25C.. So overall your thermal resistance die to ambient needs to less than 100C/20W or 5C/W - you already know that your thermal resistance tab to case is say 1C/W so the heatsink needs to be better than 4 degrees C per watt to ambient - which incidentally is not that large.
Add some margin for high ambient temperatures by subtracting from say 40 degrees C instead of 25 degrees C.
The above is only an example, by judicious design and the techniques applied above you can reduce the internal dissipation by half over the number listed above and use a 5 degree C heatsink or smaller.
Hope this is helpful.
Kevin
edit: fix minor typo
You might want to take a look at the LT1084 regulator, it has lower drop out voltage than the LM338 and will operate comfortably with just 3V across it. To reduce the current through it and hence the dissipation you can parallel a large low value power resistor across the input to output pins - for example if you had a raw supply of 13V a 3.0 ohm resistor would source 1A. I recommend a 2.2 ohm resistor resulting in about half the dissipation in the regulator and half in the resistor. (Note above is for use with voltage mode regulation, not current mode.)
You should make sure you design for low line, i.e. the raw dc voltage at 10% low line should be just above the drop out voltage of the regulator at say 1.5V..
I use toroids for these applications because they usually have low winding resistance. Choose one that drops a volt or less at the rated load current and you may be able to get by with something under 12Vrms, use schottky diodes and you might even have enough head room with a 10Vrms transformer. Simulate in PSUD to be sure, and go conservatively higher if there is any doubt.
On heat sinks - figure the normal device has an thermal resistance tab to die of about 1 degree C per watt (refer to spec), let's say you are dissipating 20W (3A with 6.6V delta across regulator) and your chosen die temperature target max is 125 C assuming ambient of 25C.. So overall your thermal resistance die to ambient needs to less than 100C/20W or 5C/W - you already know that your thermal resistance tab to case is say 1C/W so the heatsink needs to be better than 4 degrees C per watt to ambient - which incidentally is not that large.
Add some margin for high ambient temperatures by subtracting from say 40 degrees C instead of 25 degrees C.
The above is only an example, by judicious design and the techniques applied above you can reduce the internal dissipation by half over the number listed above and use a 5 degree C heatsink or smaller.
Hope this is helpful.
Kevin
edit: fix minor typo
Note 2: These specifications are applicable for power dissipations up to 50W for the TO-3 (K) package and 25W for the TO-220 (T) package. Power dissipation is guaranteed at these values up to 15V input-output differential. Above 15V differential, power dissipation will be limited by internal protection circuitry. All limits (i.e. the numbers in the Min. and Max. columns) are guaranteed to National’s AOQL (Average Outgoing Quality Level).
Heatsink isnt specified, but for TO-220 theta Junction to Case is 4C/W.
For TO-3 it is 1C/W.
Heatsink isnt specified, but for TO-220 theta Junction to Case is 4C/W.
For TO-3 it is 1C/W.
Hi Guys,
So, I've looked at things a little closer and I just think I may get away with the regulated filament.
I've changed the first cap to 10mF. Now under load I have 13.5VDC. I am regulating at about 3.05 amps which is giving me 9.8VDC on the 845 filaments ( I don't think my current set resistor is highly accurate) . Yes, the LM338 gets super hot, but I think I may be able to get an adequate heatsink for it. At this point I am burning about 10 watts, which will allow me a lot more options with the heatsink.
Do you think if I add more capacitance in front of the regulator (22mF) the input voltage will increase? I've modeled it in PSUD, and I don't see a voltage increase at the input cap.
Lastly, if I use a more accurate resistor to set the current to 3.25A, should the voltage on the filaments rise to near 10VDC?
I'm familiar with deisgning a tollerance of +/- 5% for voltages, but what should I ami for here, +/- 5% current or +/- 5% on the output voltage?
Thanks,
BK
So, I've looked at things a little closer and I just think I may get away with the regulated filament.
I've changed the first cap to 10mF. Now under load I have 13.5VDC. I am regulating at about 3.05 amps which is giving me 9.8VDC on the 845 filaments ( I don't think my current set resistor is highly accurate) . Yes, the LM338 gets super hot, but I think I may be able to get an adequate heatsink for it. At this point I am burning about 10 watts, which will allow me a lot more options with the heatsink.
Do you think if I add more capacitance in front of the regulator (22mF) the input voltage will increase? I've modeled it in PSUD, and I don't see a voltage increase at the input cap.
Lastly, if I use a more accurate resistor to set the current to 3.25A, should the voltage on the filaments rise to near 10VDC?
I'm familiar with deisgning a tollerance of +/- 5% for voltages, but what should I ami for here, +/- 5% current or +/- 5% on the output voltage?
Thanks,
BK
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