• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Minimum filament voltage for 300B

This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.

I just finished a 300B SET and had some serious hum. Found out that my Hammond 166L5's were putting out 5.5vac since I was giving it 120VAC from the wall. My house normally supplies 100VAC (Japan), so I plugged the amp directly into the house. The filaments are now getting 4.5vac and the hum is gone.

Is there a problem with running the tubes with low filament voltage?


Thank you all for your inputs. Appreciate it all.

I did not go DC because I didn't want to have to buy all that extra stuff! You should see the bottom of my amp! No room anywhere! It's a stereo unit.

I've talked to some of my buddies at work that have centuries of experience in tube stuff (don't tell them I said that!!). They all agree that 4.5 volts is too low (out of spec).

What I plan to do is buy a Hammond 185C12 filament transformer. It is rated for 115 vac 50/60 hz and has two 6.3 vac 2 amp secondaries. With an input of only 100 vac, I should get about 5.5 vac out of each secondary. All I have to do, is knock the input voltage down to about 90 vac and the outputs should be good at 5.0 (4.9 actually). This is a good transformer for three reasons: it has two outputs and it is the same size as the 166L5's that it will replace. I don't want to have to drill more holes in my chassis. The last reason is that it doesn't have leads. The units that have the leads have 6 inch leads. Too short. I'd rather solder on my own leads rather than have a splice somewhere in the mess.

If this doesn't work out, then I'll go buy all the extra stuff and make a DC filament supply.
To reach 5 V you must have minimum a secondary of 5,5 Vac- 4A (for two tubes), connected to a bridge at min. 10A and 10.000 uF or more.
If you prefer a LM7805T (if I remember well), 1,5A good for one 300B you need minimum 8Vac with 10,000 uF in input and 100uF on outuput, in this way you will haven't noise.
In every case between 4,5 to 5 Vdc the 300B runs good without any type of problems regarding performances and life.

TJ300B limits

I think I found the limits of my tubes.

I upgraded (?) my driver resistors to Vishay bulk metal foil (1 watt) and also changed my main voltage drop resistor to 100 ohm, 100 watt (originally 200 ohm, 50 watt to get my B+ down to around 390 vdc). With the reduction in resistance, B+ went up to 415 vdc with about 78 ma current flow through the cathode resistor. My driver circuit (6SN7GTB) was operating at 403 vdc (3.3K drop resistor from B+), 4.6 ma on the half that is connected to the output tube.

The sound was harsh, shrill, and gave me a headache. I was worried that the resistors was showing me the limits of my amp.

I reduced my voltage drop resistor back to 200 ohms and kept the driver voltage drop resistor at 3.3K. The 300B is now operating at 390 vdc @ 72 ma, 6SN7GTB at 380 vdc @ 4.2 ma. Everything is now sweet again. The grid and grid stop resistors are carbon comp.

I'm contemplating changing the driver voltage drop resistor back to 1.8K to get the voltage up to ~390 vdc.

I hope this makes sense (newbie learning the terminology).
Hum balance


There is a good article on Steve Bench'e website about hum reduction in DHT. I have implemented this kind of circuit in my 6C45+300B parafeed amp. Astonished it is very quiet. I use no DC filament supply for 300B; stabilised 6.0V only for russian 6C45 (lat. 6s45pi-ev). I read somwhere, that DC is more destructive to the heater (some physics of course behind it), so I do not take risk of losing 300B too quickly.

Steve Bench's Hum Balance in DHT

Thank you very much for the link to the article. It was very interesting and I might use it in the future.

I was able to reduce my hum at the speaker terminals down to 3.6 ma in both channels. I wired the filaments out of phase with each other, so while standing 1 foot away from each speaker, I can hear the hum, but once I move away, the hum becomes inaudible and as I move between the speakers, whatever hum was there is cancelled out.

Yeah, I've also heard some bad things about DC filament. While it is the quieter of the two methods to heat the tube, you have to switch the polarity every so often to keep the filament from going bad. I don't know what properties causes this, though.
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.