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AC or DC for 300b Heaters
AC or DC for 300b Heaters
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Old 21st December 2005, 03:56 AM   #11
kevinkr is offline kevinkr  United States
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AC or DC for 300b Heaters
Hi Jim,
I don't use hum pots at all with dc supply.. I float the supply off of a pair of 22 ohm resistors typical through which the cathode current flows to ground or a small resistor to measure the cathode current and then to ground.. All of my designs use fixed bias..

I used hum pots on an ac heated SE amplifier and found that they worked fine, but in general did not seem to do the job better than the center tap on the filament transformer - which is generally what I do with 2A3 and 45's.
"To argue with a person who has renounced the use of reason is like administering medicine to the dead." - Thomas Paine
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Old 21st December 2005, 11:35 AM   #12
JimW is offline JimW  United States
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Default humpots on dc filaments

I use pots on my 2a3's with ac. I built a variation of TL's Legacy, but instead of cathode r on one leg of the filament and cap on the other, I used an old Verdier idea of one r and one cap on each leg. Thorsten suggested a humpot arrangement to balance the bias current through the filament due to the 5 volt difference between sides. I tried this on one of the amps, and I honestly don't know/can't tell if it makes any difference to anything. I was just curious with your experience. Thanks for your input.
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Old 21st December 2005, 04:38 PM   #13
sgerus is offline sgerus  United States
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Hello Eli,

You must have read my mind... I actuall bulit a slow start 6.3 vdc power supply and a time delay for the HT.

Good timing on the 2A3 comment, I have purhased everything but the tubes... so I can still build a 2A3 amp with the parts I have.

My logic for 300B vrs 2A3 was if I'm spending $700-$800 for the pair of
2A3 mono blocks, the extra $120 for the pair of 300b's was no big deal.

So far I spent about $800, for everything except the 6sn7 & 300b

You given me something to thing about...

I'm looking for the best sound, not power.
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Old 21st December 2005, 05:13 PM   #14
dhaen is offline dhaen  Europe
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Beware most 2A3's have 2.5V heaters. However, the hum will be 6dB lower than with a 300B for this reason.
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Old 21st December 2005, 05:55 PM   #15
Tweeker is offline Tweeker  United States
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Location: Cincinnati, Ohio
An article on extending tube life for rather bigger tubes. By Walter Johnson, who was at Voice of America, Bethany, OH. The township has ripped out most of the newer transmitter and thrown it right it in the trash. It really pains me to think about all the stuff that went to Mt Rumpkee. Only the last skip was salvaged from, its being abused in the Tesla Coil pictured in PP transformer for SE use.

When a cold filament is turned on, damage is
caused by two effects: (1) The current inrush into a
cold filament can be up to 10 times the operating
current if the filament supply is of very low impedance;
(2) Grain reorientation occurs at about 600 to 700
degrees centigrade; this is called the Miller-Larson
Effect. The grain reorientation will result in a
momentary plastic state that can cause the wire to
grow in length and therefore become thinner.
Assuming you dont hammer it with B+ CCS heaters seems like it might be pretty good as it eases it through cold the most gently and by the time it gets to the 600-700C region resistance is going up and it shouldnt stay long there.

At some sites most of thier tube wear was coming from turn on. Cant say how turn on wear in 4CV100,000Cs scales to 300Bs though.

In Greenville, North Carolina, VOA has four
500 kW transmitters with vapor cooled tubes. The
transmitters are used for 12 to 15 hours per day.
Initially, the filaments were turned off and on two or
three times per day during gaps in the schedules.
Three years ago, VOA began leaving the filaments on
at all times except for major maintenance. As a result,
the expenditures for tubes for these transmitters has
dropped from about $420,000 per year to under
$100,000. This was done at a cost of $15,000 per
year in added electrical power cost. For every extra
dollar spent on added filament power, we had a return
of more than 20 dollars in reduced tube costs eat your
heart out Wall Street!

Calculations for both of the above situations
indicate that each Off/On cycle of the filament was
reducing the life expectancies by over 75 hours. An
unexpected result was that it appeared to be
independent of the filament construction - straight
wire, hairpin, and mesh filaments all benefited nearly
equally by leaving the filaments on continuously.
9 out of ten documents I try to attach are too large, heres a text version of the pdf.
Be sure your foil hat has a good low impedance ground.
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Old 22nd December 2005, 01:59 AM   #16
Eli Duttman is offline Eli Duttman  United States
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Location: Monroe Township, NJ

IIRC, the Full Music brand "mesh" plate 2A3 is really a 300B with a 2.5 V. filament. While I wouldn't push it to 8 W., 6 W. ought to be safe.

Another option for more power and less DHT hum is the Sovtek monoplate 2A3. The Russian tube can take a pounding that will destroy NOS and Valve Art brand 2A3s. 4 W. seems safe enough.
Eli D.
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Old 8th January 2006, 03:19 AM   #17
sgerus is offline sgerus  United States
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Hello Eli,

You where right on the money with the idea to build a 2a3 amp
instead of the 300b.

I built the 6sn7/300b amp.... got the hum down to 6.8mV.

Three lessons learned.
This much hum is a big problem with my 98db/w/m speakers.
You don't need this much power...
Driving this amp fron my receivers pre-amp output you don't
need .25V input sen.

My plan for now is to return the tubes and build a 6sn7- 2A3.
With the differance in price from the 300B to the 2A3's I can get
the better version of a 6sl7 ( the red base 56??)

I'm thinking of:
1. the je labs simple 45
2. a 6sl7 SRPP design
3. a cathode follower 6sl7-2A3
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Old 8th January 2006, 03:46 AM   #18
fragman56 is offline fragman56
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Location: Denver, CO
Default 300b hum

Did you try reducing the ac heater voltage on the 300b down to say something like 4.5v ac? I did this and got the hum down below 4mv ac--made a world of difference. I use Cornwalls (98db/m) and Altec 605As (103 db/m).

I have a 45 set but this 300b set rocks! It's just another amp--maybe not any better but different.

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Old 8th January 2006, 11:38 PM   #19
sgerus is offline sgerus  United States
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No, I didn't try 4.5 vac.... but I did test with 5VDC.
( no better with 5VDC)
still the best I can get down to is 6.8mV.

I started out around 11mV. I got down to 6.8 by re-aranging my
grounding and moving the B+ wire away from the speaker output.

The 300b heater is heated my the 6.3 vac output from T1, I use two
.75R to drop it down to 5V.

Here are some of the facts in my hum fight so far.
1. Best I can get is 6.8mV
2. Same with R-C on 6.3 V center tap or with a 100R hum pot setup
3. Same hum with ac or DC
4. Grounding the ct of the 6sn7 heater transformer causes hum to
jump to .35 Volts!

Some things I think I did wrong (this is my first amp)
1. useing a cheap Cap on the 300B heater cathode
2. useing the main transformer 6.3 vac with resistors to heat the 300b (vrs a stand alone 5vac transformer)
3. having my GZ34 too close to the input/driver tube
(the tubes are in line... GZ34, 6sn7, 300B.... about 2.5" c/c)

Starting to wonder if my hum problem is related to something other
than the 300b heater
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Old 9th January 2006, 12:36 AM   #20
ThorstenL is offline ThorstenL  Germany
Previously: Kuei Yang Wang
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Originally posted by sgerus
No, I didn't try 4.5 vac.... but I did test with 5VDC.
( no better with 5VDC)
That tells you that the hum source is from either magnetic or capacitive coupling or from the High Tension supply being poorely filtered.

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