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Old 2nd May 2005, 09:48 AM   #1
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Default Minimum filament voltage for 300B

Hi,

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?

Thanks.
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Old 2nd May 2005, 01:50 PM   #2
Giaime is offline Giaime  Italy
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Mmmm.... you shouldn't exceed the 5% rating... and you're over 10%... not counting line fluctuations... underpowered filaments tend to worn out the tube rapidly... since a 300b is expensive I would not suggest it to you...

Try convert them to DC and use a voltage regulator to get 5V.
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Old 2nd May 2005, 04:56 PM   #3
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For normal 300B specification, the filament voltage is 5V +/-0.3V.
But for many application and experiment, the voltage below 5v and above 4v will not be a big problem.
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Old 2nd May 2005, 09:29 PM   #4
Giaime is offline Giaime  Italy
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I still suggest you to rectify the 4.5V, you'll get about 6 volts that you can regulate down to 5. Better and noiseless.
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Old 3rd May 2005, 03:43 AM   #5
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I agree with Giaime, DC is better for the 300b filament
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Old 3rd May 2005, 11:48 AM   #6
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Hi,

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.
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Old 3rd May 2005, 03:14 PM   #7
Giaime is offline Giaime  Italy
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Quote:
Originally posted by kusojiji

If this doesn't work out, then I'll go buy all the extra stuff and make a DC filament supply.
That seems to be a good idea
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Old 3rd May 2005, 03:33 PM   #8
dhaen is offline dhaen  Europe
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Hello Kusojiji,

When you test for heater voltage, I recommend that you measure actually on the tube pins. There is always some loss through the wiring, and sometimes some loss through the socket.
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Old 3rd May 2005, 08:11 PM   #9
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dhaen,

That's where I measure at. Thanks! I also use a true-rms meter.
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Old 3rd May 2005, 08:19 PM   #10
dhaen is offline dhaen  Europe
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Quote:
Originally posted by kusojiji
dhaen,

....... I also use a true-rms meter.
Good point
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