300b lovers

Hello 300B audio gurus,

I have a technical question regarding the AC 300B filament in my SET amplifier. Upon measuring the filament voltage, I found it to be 4.85 volts.

Regarding this measurement, I have a few specific questions:

  - Accuracy: How critical is filament voltage accuracy for this specific tube?
  - Line Fluctuations: Should I account for the fact that the AC network voltage in Ottawa fluctuates between 118V and 123V?
  - Voltage Preference: In terms of tube longevity and performance, is a slight overvoltage or a slight undervoltage preferable?

Upon measuring the filament voltage, I found it to be 4.85 volts.

Regarding this measurement, I have a few specific questions:

  - Accuracy: How critical is filament voltage accuracy for this specific tube?
  - Line Fluctuations: Should I account for the fact that the AC network voltage in Ottawa fluctuates between 118V and 123V?
  - Voltage Preference: In terms of tube longevity and performance, is a slight overvoltage or a slight undervoltage preferable?

@alexberger If the correct filament Voltage cannot be obtained I find it preferable to err on the low side rather than the high side as that will likely promote tube life.

Filament Voltage is important. As the filament cools off, the output impedance of the circuit will rise, output power will fall and distortion will go up. The opposite is true with the filament Voltage increased.

But the filament has a life span independent of the rest of the elements of the tube. Running it at a high Voltage than spec will shorten its life. 

AC line Voltage variation is why you hear the system sound great one day and maybe not so good at other times. 

There are DC to DC converters available these days that allow you to run a DC Voltage. They can keep the filament Voltage spot on regardless of the AC line Voltage. I recently used some in a push pull amp using 45s, which have a lower filament Voltage than can be pretty tricky to set up otherwise. The DC to DC converters were quite compact and allowed me to float the supplies for the tube- no hum problems, no noise problems so I could wire the cathode circuit as needed. They worked a treat!

Hi @atmasphere 

Thanks for your input.

Can modern DC-DC switch-mode converters inject HF/RF noise into a SET amplifier system? Given the high sensitivity and purist nature of SET topologies, do these modern converters pose a risk of radiating EMI/RFI or polluting the ground plane?

@alexberger Its all in how you bypass the device so it doesn’t make noise, and if you are using one that is quality built.

If you’re not careful, you can certainly have noise problems. If you are careful its no worries at all. Usually a small capacitance is enough to shut them up. 

I’ve not seen an SET circuit that was particularly high sensitivity. A phono preamp or the like would be more sensitive! I recently installed a 48V supply in a multichannel microphone preamp that had 6 tube channels. The supply, made by Meanwell, injected quite a lot of noise but a small bypass capacitance shut it up entirely. I placed the capacitance at the other end of the wiring harness that fed the 48V to the phantom power connections of the microphone inputs, so not directly at the output of the supply. Once that was set up the preamp was silent and I could not find any noise when using the oscilloscope. 

The biggest problem IMO is not noise, but the longevity of the SMPS or DC to DC converter- one day it might not run anymore. So I like to have a spare on hand if I install one.  

If you go on some of the DIY sites that have SET construction threads, you’ll see that people are using such supplies for filament circuits. 

Hello Don,Lynn and Ralph,

Don&Lynn . In the past, you've posting, you've made a passing reference to Thomas Meyer of vinyl savior fame, which kind of got me remembering this little tidbit about power transformers. I was curious, do you spec them out like Thomas Meyer does? If so, how much of a difference does this dual screen windings make over a single screen winding or a one without it?  this from Thomas' Blog  

This is the portfolio of power transfromers which I currently use in all of my projects. They are all quite versatile and allow different configurations. All of them have several features in common:

Grain oriented core laminations

This is a similar core material as is commonly used in output transfromers. It has higher permeability and produces less stray fields. It is also less prone to mechanical buzz compared to conventional cores of the same size. After assembly the complete transformers are vacuum varnished.

Dual screen windings

This is a very important feature of power transformers and it is surprising that it is not more commonly used.
Each power transfromer has parasitic capacitance between primary and secondary windings. This capacitance couples high frequency noise from the mains to the secondary side and thus into the circuit. A screen acts as a shield. With a screen there are parastic capacitances between primary and screen and secondary and screen, but not directly between the windings any more. This effectively shunts noise from the mains to earth. I even have my transformers made with two independent screens. The first one,which is closer to the primary is directy connected to the core, which should have an electrical connection to the chassis and thus to mains earth. The second screen is meant to be connected to signal ground. This provides an even improved isolation from mains noise. With this feature, amps which use such power transformers don't react much to polluted mains. No expensive external mains filters and isolators needed any more!

External flux band

The outer copper foil further reduces stray fields.