Hi everyone,
I'm in need of a new preamp, and Pete Millett's Low-Mu Triode Preamp looks like it will fit the bill for my system:
Low-mu preamp
I'm looking for feedback on a few mods to the design, I know there's a few people here who have built or heard this design.
1. I want to use the Hammond 270FX power transformer, but its 6.3V winding can't supply enough current for all the tube heaters. Would it make sense to use a separate 6A 6.3V transformer to power the EL34 heaters, while using the main transformer 6.3V winding to power the 6AS7 heater? The EL34 heaters need to be floated above ground to avoid exceeding the heater-cathode voltage difference, so this way the 6AS7 heater wouldn't need to be elevated too. Might be possible to rectify and filter the 6.3V heater supply for the 6AS7 as well.
2. Instead of a 5U4GB rectifier, I'm thinking a 5AR4 might be a better choice. The 5U4GB heater current would max out the 5V winding on the 270FX at 3A, whereas the 5AR4 only draws 1.9A. It doesn't look like the power supply would exceed any limits of the 5AR4.
3. I think for the preamp tube itself, I'm stuck with the 6AS7 and 6H13C variants. The 7236 and 5998 tubes seem to be in short supply. Would either the 6080 or 6H13C supply more gain than the 6AS7?
Thanks in advance!
I'm in need of a new preamp, and Pete Millett's Low-Mu Triode Preamp looks like it will fit the bill for my system:
Low-mu preamp
I'm looking for feedback on a few mods to the design, I know there's a few people here who have built or heard this design.
1. I want to use the Hammond 270FX power transformer, but its 6.3V winding can't supply enough current for all the tube heaters. Would it make sense to use a separate 6A 6.3V transformer to power the EL34 heaters, while using the main transformer 6.3V winding to power the 6AS7 heater? The EL34 heaters need to be floated above ground to avoid exceeding the heater-cathode voltage difference, so this way the 6AS7 heater wouldn't need to be elevated too. Might be possible to rectify and filter the 6.3V heater supply for the 6AS7 as well.
2. Instead of a 5U4GB rectifier, I'm thinking a 5AR4 might be a better choice. The 5U4GB heater current would max out the 5V winding on the 270FX at 3A, whereas the 5AR4 only draws 1.9A. It doesn't look like the power supply would exceed any limits of the 5AR4.
3. I think for the preamp tube itself, I'm stuck with the 6AS7 and 6H13C variants. The 7236 and 5998 tubes seem to be in short supply. Would either the 6080 or 6H13C supply more gain than the 6AS7?
Thanks in advance!
6080 and 6AS7G and 6N13S are all electrically the same. 6N5S is close enough. All have a mu of 2. They all have a max Vhk of 300V so no elevation is required.
Since the required B+ is 320V, I would use a Triad VPT230 of a large enough VA rating as the PT (I do this in all of my amplifier designs by using a doubler from 120V to get 320VDC) and ditch the choke and ditch the vacuum diode for HER208. I'd use a 12V SMPS and run heaters in series pairs, too. This would make it cheaper, lighter, and more efficient.
The 5U4GB would max out the heater current, but that's what it's designed for. If you only load the 5V winding to 65%, you'll end up with more than 5V but it should still be within spec. By the look of the schematic, you could also use 5Y3, too
Since the required B+ is 320V, I would use a Triad VPT230 of a large enough VA rating as the PT (I do this in all of my amplifier designs by using a doubler from 120V to get 320VDC) and ditch the choke and ditch the vacuum diode for HER208. I'd use a 12V SMPS and run heaters in series pairs, too. This would make it cheaper, lighter, and more efficient.
The 5U4GB would max out the heater current, but that's what it's designed for. If you only load the 5V winding to 65%, you'll end up with more than 5V but it should still be within spec. By the look of the schematic, you could also use 5Y3, too
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I've tried a 12V SMPS on a previous build, I had trouble with the over current protection tripping when the heaters were cold. I ended up using a lead acid 12V battery!
The 5V full load makes sense, I had read that it's best to not run the 6.3V winding at full load.
That looks like a good one. I looked at the big 300 series Hammond transformers in the 550 VCT range, but oh man they're expensive. The 272JX should have enough 6.3V current to run a rectified and filtered heater supply.
Oversize it.
This one is my "go to" for heaters in power amps etc. AC DC Converter LED Driver 110v 220v to DC 12V 13A 150w Switching Power Supply Board power source Module|led driver 110v|led driver220v to dc 12v - AliExpress
This one is my "go to" for heaters in power amps etc. AC DC Converter LED Driver 110v 220v to DC 12V 13A 150w Switching Power Supply Board power source Module|led driver 110v|led driver220v to dc 12v - AliExpress
I've built this preamp with some small changes in the power supply, not as much capacitance and with a 5V4G rectifier. I've elevated the heater of the 7236/6AS7 too, but it shouldn't be any problem with just elevating the EL34 heaters.
You won't use much more than 100mA with this circuit so almost any rectifier tube will do. 5AR4 has very low voltage drop so that may be an advantage, and also slow warmup which is nice.
6080 and 6AS7 are identical gain wise, the 7236 has noticeably more gain in this circuit.
The Cetron 7236 is available if you look at eBay etc., but prices have gone up quite a bit lately. It's a great sounding tube and pretty much the same as the old Tung-Sol 7236, I can't hear any difference between them. The 6H13C is plentiful, cheap and sounds nice, and old RCA 6AS7 is also very very good for not much money. The 6080s that I've tried sound a bit lifeless and dull in comparison.
You won't use much more than 100mA with this circuit so almost any rectifier tube will do. 5AR4 has very low voltage drop so that may be an advantage, and also slow warmup which is nice.
6080 and 6AS7 are identical gain wise, the 7236 has noticeably more gain in this circuit.
The Cetron 7236 is available if you look at eBay etc., but prices have gone up quite a bit lately. It's a great sounding tube and pretty much the same as the old Tung-Sol 7236, I can't hear any difference between them. The 6H13C is plentiful, cheap and sounds nice, and old RCA 6AS7 is also very very good for not much money. The 6080s that I've tried sound a bit lifeless and dull in comparison.
Good to know! I was thinking the big 750uF cap in the power supply might stress the rectifier, even though it's after the choke. I'm still trying to decide if a small 550 VCT transformer with a separate 6.3V transformer is more economical than a big 600 VCT transformer that can supply enough heater current on its own.
Did you have to adjust any values in the preamp circuit when switching between the 6AS7 and 7236 tubes?
I asked Pete Millett about switching between the 6AS7 and 7236, and he replied that he didn't change anything in the circuit. I measured that the 6AS7 gets a little lower voltage and more current (100V/60mA) than the 7236 (125V/50mA) when the same resistor values are used, but that's no big deal with these tubes. It 's a great sounding preamp with either tube, so I recommend it 
I asked Pete Millett about switching between the 6AS7 and 7236, and he replied that he didn't change anything in the circuit. I measured that the 6AS7 gets a little lower voltage and more current (100V/60mA) than the 7236 (125V/50mA) when the same resistor values are used, but that's no big deal with these tubes. It 's a great sounding preamp with either tube, so I recommend it
Awesome, thanks for your feedback! I'll let you guys know how the build goes, and maybe post some pics if it doesn't turn out too badly
Hey everyone, I'll revive this old thread now that I've built this beast of pre-amp. Check out my pics!
Seems to work fine as it is, and surprisingly quiet for the most part. It does pick up some hum on the volume control. Dead quiet at minimum and maximum volume, but it picks up 60Hz noise at around the 3 o'clock point. I'm thinking the wiper in the pot might be picking up noise from the heater wiring at certain points, what do you guys think?
I also want to make absolutely sure it isn't oscillating and putting out high voltage, high frequency AC on the outputs. I'll use my VTVM to check it, my DMM didn't pick up anything.
Let me know if you have any comments/criticisms!
Seems to work fine as it is, and surprisingly quiet for the most part. It does pick up some hum on the volume control. Dead quiet at minimum and maximum volume, but it picks up 60Hz noise at around the 3 o'clock point. I'm thinking the wiper in the pot might be picking up noise from the heater wiring at certain points, what do you guys think?
I also want to make absolutely sure it isn't oscillating and putting out high voltage, high frequency AC on the outputs. I'll use my VTVM to check it, my DMM didn't pick up anything.
Let me know if you have any comments/criticisms!
Hi Ian, there is a connection between the top plate and mains earth in the bottom left corner of my pic. The power supply ground is also tied to mains earth. I neglected to earth the front plate where the controls are. Grounding it should help reduce the hum!
That is good. I was going to suggest grounding the front panel if you had not already done so. Should help the hum for sure.
Cheers
IAn
Nice clean build! I'ld look at the zone near the input switch where the heater wires are close to the (unshielded part) of the input wires. You could increase the distance by inserting a spacer between the input wires and the top plate, or bend the heater wires more inward. Twisting them tighter could help as well.
Apparently it isn't a ground loop problem if the hum is worst halfway: https://www.diyaudio.com/community/...olume-knob-at-max-or-zero.266579/post-5042729
Apparently it isn't a ground loop problem if the hum is worst halfway: https://www.diyaudio.com/community/...olume-knob-at-max-or-zero.266579/post-5042729
Is it worse in one channel? Red?
Those unsheilded signal lines run right over heater lines. Twisted, but the twist-length is comparable to the distance between the two circuits, so the cancellation is imperfect.
I don't have a really happy answer. Pushing the heater line around may give somewhat less hum. Routing it the other way around the caps may give less hum at part-volume but more hum right into the very large caps. A complete redesign to avoid trouble is more complex than the internet would have you think.
Well I've now properly grounded the front panel, that has taken out a lot of the hum. The hum level no longer varies with volume position, but it is slightly noisier in the right (red) channel. There is a 60Hz hum and some microphonic noise, fortunately I can't hear it at all from the listening position. I'm actually quite pleased with how quiet the circuit is now, several others who have built this pre-amp had trouble with excessive hum.
