I'm working on my first direct from schematic build (see below) and have a power supply related question for Asano/LW.
I plan to use a Hashimoto PT-160 power transformer with the following specs:
That said, is there anything I need to consider in the PS when adding a second channel for a stereo configuration. Seems pretty straightforward, but figured I'd ask.
Many thanks in advance!
Jonah
I plan to use a Hashimoto PT-160 power transformer with the following specs:
P.120V
S.320V-280V-0-280V-320V AC140mA
0-2.5V-6.3V 3A 2 circuits
6.3V 2.5A
5V 3A
I should note that I plan to use a 5V4G in place of the 83V as a rectifier; however, the current schematic is asking for 380V and my transformer only outputs 320V. I'm curious to know - for this project and moving forward - what effects this lower voltage will have on rectification and if there are any adjustments I need to make down stream in the power supply. Perhaps there is a better rectifier tube choice...That said, is there anything I need to consider in the PS when adding a second channel for a stereo configuration. Seems pretty straightforward, but figured I'd ask.
Many thanks in advance!
Jonah
Attachments
Actually, if your xformer outputs 320V, the rectified voltage will be higher than 380V.
OTOH the 320V might be an unloaded figure, and will drop when loaded.
Best plan of attack is to build up the rectifier and reservoir caps, then put on a load with some resistors that draw the expected load current from the tubes and see what you get.
You might actually get very close, close enough for not having to worry.
Jan
OTOH the 320V might be an unloaded figure, and will drop when loaded.
Best plan of attack is to build up the rectifier and reservoir caps, then put on a load with some resistors that draw the expected load current from the tubes and see what you get.
You might actually get very close, close enough for not having to worry.
Jan
Thanks for this!
In the schematic, the plates on the 83V/5V4G rectifier are looking for 380V and they will be getting 320V from the PT secondary. I imagine the voltage will be lower than expected on the way out of the rectifier. Are you suggesting that I see what it reads on the way out and adjust the value of the dropping resistor so that I measure the spec'd 460V at the resevoir caps?
In the schematic, the plates on the 83V/5V4G rectifier are looking for 380V and they will be getting 320V from the PT secondary. I imagine the voltage will be lower than expected on the way out of the rectifier. Are you suggesting that I see what it reads on the way out and adjust the value of the dropping resistor so that I measure the spec'd 460V at the resevoir caps?
The resistor before the first filter capacitor can be eliminated if the power transformer resistance is high enough, A 5AR4 rectifier has a higher peak current rating and less voltage drop, would increase voltage further. And a larger first filter cap would increase voltage slightly But a direct-couple circuit like this needs high B+ so you may have to settle for a bit less output power. Without the 50 Ohm resistor, I estimate you'd get 395V with 5V4, 410V with 5AR4, 425V with solid state rectifier.
That makes all the sense in the world re: voltage drop over the rectifier tube. For no good reason, I'm hesitant to go with solid state rectification, but will keep that in my back pocket if I need more B+. How much resistance in the PT - ball park - would I be looking for the eliminate the 500 Ohm resistor?
The RCA data sheet says 100 Ohms per plate minimum, higher if input capacitor exceeds 40 uF. You are below maximum voltage rating for 5V4 (375VAC for cap input), and well below maximum current. I would guess from the current rating that your transformer is around 60-75 Ohms end to CT. Should be OK without the 50 Ohm resistor as long as the input cap stays at 20 uF (increasing it would only raise output by a fraction of a volt anyway!).
I can not see enough detail in the schematic, but I believe the resistor is 500 Ohms 10 Watt; Not 50 Ohms 10 Watt.
Aproxomate: 60mA 2A3 load, plus the input tube 0.4 mA load, plus the voltage divider 4mA load = about a 65mA load.
500 Ohms x 0.065 = 32.5V drop, but it will be much larger, because of the transient current to the first capacitor filter is much higher than 65mA.
I estimate a transient current of at least 130mA. 500 Ohms x 0.130 = 65 volts drop.
I bet the actual transient current is at least 3X, or 195mA, for a likely 97.5V drop.
So changing from 500 Ohms to 100 Ohms should get you close to the B+ you want.
Or, as suggested earlier, use solid state rectifiers, and adjust the 500 Ohm resistor to get the B+ voltage you want.
There is no magic to power supplies, just math.
Aproxomate: 60mA 2A3 load, plus the input tube 0.4 mA load, plus the voltage divider 4mA load = about a 65mA load.
500 Ohms x 0.065 = 32.5V drop, but it will be much larger, because of the transient current to the first capacitor filter is much higher than 65mA.
I estimate a transient current of at least 130mA. 500 Ohms x 0.130 = 65 volts drop.
I bet the actual transient current is at least 3X, or 195mA, for a likely 97.5V drop.
So changing from 500 Ohms to 100 Ohms should get you close to the B+ you want.
Or, as suggested earlier, use solid state rectifiers, and adjust the 500 Ohm resistor to get the B+ voltage you want.
There is no magic to power supplies, just math.
Just use solid state rectifiers. I have done that for a couple of decades.
2A3, 300B, 45, 6L6GB and GC, KT66, KT77, KT88, 7591, 807, 5881, EL34 . . . I never had an output tube failure with my solid state rectifier B+ circuit.
Are you worried about noise from solid state rectifiers?
Most of those above tubes and my amplifier designs they were in had less than 100uV hum and noise into an 8 Ohm load resistor.
2A3, 300B, 45, 6L6GB and GC, KT66, KT77, KT88, 7591, 807, 5881, EL34 . . . I never had an output tube failure with my solid state rectifier B+ circuit.
Are you worried about noise from solid state rectifiers?
Most of those above tubes and my amplifier designs they were in had less than 100uV hum and noise into an 8 Ohm load resistor.
It is indeed a 500 Ohm resistor. Sorry for the low res image.
It is and thanks for your insight. Makes perfect sense when you walk me through it... It's all just new to me... tracing the flow through a schematic and the interactions between components.
Thanks Tom!
I've used them in the two previous amps I built and they were great. Just wanted to try something different and keep it as true to this design as possible.
So just to be clear - and circling back to my original question - is it safe to say that the rectifier tube will not behave drastically different in this circuit if it's being supplied less that what is specified in the design/schematic? 380V is spec'd but my PT will only feed 320V...
The rectifier tube doesn't matter as long as you stay within the ratings. But even with solid state diodes and removing the 500 ohm resistor, you won't get enough voltage, not to run the 2A3 at 60mA anyway. 320-0-320 gives you about 450 volts unloaded. A small drop for the diodes and more for the choke, and you'll be down around 430 or so. You'll probably end up with around 420 volts on the plate of the 2A3. With a 5V4 rectifier you'll lose another 30-40 volts, at least.
While using a vacuum rectifier such as this with 320 volt each side of the CT you can expect 1.25 to 1.3X the 320 colts as DC.
Change that 500R at the 5V4 cathode to 100R, that will get you closer to what you are looking for.
The rectifier input cap can be safely increased from 20 microF to 40 microF. That will get a few more volts.
Some designers instead of the 100R in the cathode put a pair of 100Rs in the rectifier plate leads. Helps to reduce ringing in the PT windings.
The 5V4 is the best of what used to be reasonably priced vacuum rectifiers. I used many during my time in research in the 50s & 60s.
Next best choice was the GZ34 / 5AR4, but they were expensive. The 5R4 looks nice to many, but they are made for service up to 40.00 feet.
Not many of our projects will ever get there, And it is not a good rectifier for most applications.
Change that 500R at the 5V4 cathode to 100R, that will get you closer to what you are looking for.
The rectifier input cap can be safely increased from 20 microF to 40 microF. That will get a few more volts.
Some designers instead of the 100R in the cathode put a pair of 100Rs in the rectifier plate leads. Helps to reduce ringing in the PT windings.
The 5V4 is the best of what used to be reasonably priced vacuum rectifiers. I used many during my time in research in the 50s & 60s.
Next best choice was the GZ34 / 5AR4, but they were expensive. The 5R4 looks nice to many, but they are made for service up to 40.00 feet.
Not many of our projects will ever get there, And it is not a good rectifier for most applications.
After reviewing these posts and the data sheet, I'll definitely go with the 5AR4. The combination of less voltage drop, higher peak current and plate voltage seems to make this an obvious choice in terms of specs.
Maybe I'm misunderstanding this... so the 320V secondary will get me about 415V DC before the cathode resistor, correct? If I need 460V DC after the cathode resistor, how would lowering the value of the resistor get me there?
I did find another Hashimoto transformer that might work, feeding 420V to the 5AR4 and making an adjustment to the cathode resistor... its just... more expensive...
P.120V
S.480V-420V-240V-0-240V-420V-480V AC170mA
0-2.5V-5V 5.5A 2 circuits
6.3V 3A
5V 3A
Oye... it's a lot to absorb. Thanks again for all of your contributions.
I would go with the 420V Hashimoto, much easier to pad down than to try and eek out a few more volts with the 320V transformer, partial choke input for example could be an option. (Vary the value of the input capacitor before the choke to fine tune the voltage, start with a couple of uF and increase until you reach target voltage range.)
Many people still think the DC voltage available from a capacitor input rectifier will be root of 2 times the PT HV.
But things are never as they seem, the final voltage under load will be somewhat less. There are significant voltage drops
in both the PT winding resistance & the rectifier tube itself. Worse still, the voltage drop does not depend on the average
DC current but rather the peak during the time the filter cap is being charged.
All this is often brought together on the various published rectifier data sheets, always a useful set of data to refer too.
The peak current can be easily monitored thru the insertion of a 10R resister in the PT CT lead. Then measured with a scope.
Or a meter that has a peak hold mode, Some of the old analogue meters did this well. Check the HP 410 Series.
The Hashimoto PT looks like overkill but with care it would work. The PS regulation does not need to be good,
This is a rather olde Class A amplifier. So using the 420 wdg & a say 10 microF input cap & some adjustment of the cathode
resistor would get you to the voltage indicated on the schematic. These voltages are not recommended for Newbies!
But things are never as they seem, the final voltage under load will be somewhat less. There are significant voltage drops
in both the PT winding resistance & the rectifier tube itself. Worse still, the voltage drop does not depend on the average
DC current but rather the peak during the time the filter cap is being charged.
All this is often brought together on the various published rectifier data sheets, always a useful set of data to refer too.
The peak current can be easily monitored thru the insertion of a 10R resister in the PT CT lead. Then measured with a scope.
Or a meter that has a peak hold mode, Some of the old analogue meters did this well. Check the HP 410 Series.
The Hashimoto PT looks like overkill but with care it would work. The PS regulation does not need to be good,
This is a rather olde Class A amplifier. So using the 420 wdg & a say 10 microF input cap & some adjustment of the cathode
resistor would get you to the voltage indicated on the schematic. These voltages are not recommended for Newbies!
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I love the sound of Loftin White circuit topology amplifiers, but . . .
There are two reasons why I am not a fan of Loftin White amplifiers.
1. It requires a lot to make all the voltages and currents correct (including any change in B+ voltage).
2. It "wastes" DC voltage; many other amplifiers with the same input and output tubes need less B+ voltage.
Just my opinions.
I remember the nice 2A3 amplifier designed by Gordon Rankin.
It was built by and written up by Frank Reps.
Not the highest performing 2A3 amplifier ever, but close in performance, an elegant and straightforward design, and just good fun.
It appeared in "Sound Practices" as the "Baby Ongaku".
The link to the Baby Ongaku article, page 5 of: Sound-Practices-1995-09-Fall.pdf
There are two reasons why I am not a fan of Loftin White amplifiers.
1. It requires a lot to make all the voltages and currents correct (including any change in B+ voltage).
2. It "wastes" DC voltage; many other amplifiers with the same input and output tubes need less B+ voltage.
Just my opinions.
I remember the nice 2A3 amplifier designed by Gordon Rankin.
It was built by and written up by Frank Reps.
Not the highest performing 2A3 amplifier ever, but close in performance, an elegant and straightforward design, and just good fun.
It appeared in "Sound Practices" as the "Baby Ongaku".
The link to the Baby Ongaku article, page 5 of: Sound-Practices-1995-09-Fall.pdf
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Ye... I'm gonna bag the smaller (cheaper) PT and move forward with the larger Hashimoto or this one I just found. I'll be in Tokyo this summer buying parts
...and will order a 120V PT before hand.PMC-4023M - https://www.gtrans.co.jp/SHOP/PMC-4023M.html
P.100V
S.400-375-350-90-70-0-350-375-400V AC230mA
0-2.5(CT)-5-6.3V 3A 2 circuits
0-6.3-10V 3A
0-5V 3A
This looks super cool and anything approximating/approaching an Ongaku peaks my interest. Thank you for this! I can see a stereo version of this and have a couple 12AT7s lying around... And yes @jhstewart9, it's an old one... an Asano design from the 70s(?).