Hello All,
I'm bulding my first amp and need advice on how to heat the 300B's
(and the input/driver tube as well)
For the 300B, should I:
1. use 5vac with the pot and Resistor/Cap on the fillement
2. use 5vac with the Resistor/Cap on the center tap of the 5vac transformer
3. use a solid-state 5DC -wlth slow startup (and the pot/Res/Cap setup)
same general question for the 6sn7
1. use6.3 vac
2. use 6.3 DC solid-state -wlth slow startup
I'm bulding my first amp and need advice on how to heat the 300B's
(and the input/driver tube as well)
For the 300B, should I:
1. use 5vac with the pot and Resistor/Cap on the fillement
2. use 5vac with the Resistor/Cap on the center tap of the 5vac transformer
3. use a solid-state 5DC -wlth slow startup (and the pot/Res/Cap setup)
same general question for the 6sn7
1. use6.3 vac
2. use 6.3 DC solid-state -wlth slow startup
I guess the question to ask is how efficient are your speakers, how far down do they go and how much hum can you tolerate on the output of the amplifier as a consequence.
My speaker system efficiency is over 100dBsplw/m and the low frequency cut off is around 25Hz, (reflex loaded JBL D130's) and 2mV of hum on the output of an amplifier is quite audible.
I use constant current dc heating in my 300B SE amplifier and residual broadband noise is less than 400uVrms, and there is no discernable hum or ripple component in the output.
My 45 SE OTOH used AC heating, and had roughly 1.5mVrms broadband noise which was dominated by 60Hz hum, which was audible, but not annoyingly so from a meter away.
Some people have had good success with AC heating 300B's, but the best I have been able to do was about 5mVrms hum on the 8 ohm tap with a 300B, this is not acceptable with my speaker system, but might be with yours.
There is a raging argument about what sounds better, and I can't draw any specific conclusions - they all seem to work fine if well implemented. I have used both ac heating, constant current dc, and constant voltage dc with chokes - all sounded good to me.
My speaker system efficiency is over 100dBsplw/m and the low frequency cut off is around 25Hz, (reflex loaded JBL D130's) and 2mV of hum on the output of an amplifier is quite audible.
I use constant current dc heating in my 300B SE amplifier and residual broadband noise is less than 400uVrms, and there is no discernable hum or ripple component in the output.
My 45 SE OTOH used AC heating, and had roughly 1.5mVrms broadband noise which was dominated by 60Hz hum, which was audible, but not annoyingly so from a meter away.
Some people have had good success with AC heating 300B's, but the best I have been able to do was about 5mVrms hum on the 8 ohm tap with a 300B, this is not acceptable with my speaker system, but might be with yours.
There is a raging argument about what sounds better, and I can't draw any specific conclusions - they all seem to work fine if well implemented. I have used both ac heating, constant current dc, and constant voltage dc with chokes - all sounded good to me.
Hi Tweeker,
Interesting that you mention that, yes in a sense it should greatly reduce the warm-up stress on the filament as the current is limited to the normal operating current. A cold filament depending on the source impedance of the supply might momentarily draw far more current, in 300B I have measured cold inrush currents >5A which was the limit of the supply doing the heating.
Another very important consideration which is problematic with constant current heating is possible cathode stripping and arcing if the plate voltage comes up to operating levels before the 300B filaments are warm.
I use CC heating in my 300B and it takes more than 30 seconds for the filaments to heat, which is also about the time that the power supply pass regulator tubes are warm enough to start passing current (they're 6550's) - it's kind of a guess as to whether the dht filaments are actually hot enough when plate current starts to flow and I am seriously considering a 45 second thermal delay relay on the B+ to the outputs. So far I have not had any reliability issues with the JJ 300B's I use.
Unrelated perhaps: I really like the JJ 300B's sonics, and they are rugged, well matched, and pretty consistent in my experience. They don't look as good as a WE300B, OTOH they cost me a 25% of what the WE cost.
Interesting that you mention that, yes in a sense it should greatly reduce the warm-up stress on the filament as the current is limited to the normal operating current. A cold filament depending on the source impedance of the supply might momentarily draw far more current, in 300B I have measured cold inrush currents >5A which was the limit of the supply doing the heating.
Another very important consideration which is problematic with constant current heating is possible cathode stripping and arcing if the plate voltage comes up to operating levels before the 300B filaments are warm.
I use CC heating in my 300B and it takes more than 30 seconds for the filaments to heat, which is also about the time that the power supply pass regulator tubes are warm enough to start passing current (they're 6550's) - it's kind of a guess as to whether the dht filaments are actually hot enough when plate current starts to flow and I am seriously considering a 45 second thermal delay relay on the B+ to the outputs. So far I have not had any reliability issues with the JJ 300B's I use.
Unrelated perhaps: I really like the JJ 300B's sonics, and they are rugged, well matched, and pretty consistent in my experience. They don't look as good as a WE300B, OTOH they cost me a 25% of what the WE cost.
One other consideration when using constant current heating with 300B, the filament currents from one brand to another aren't that consistent.
For example to properly heat a JJ 300B requires ~1.5A as evidenced by the voltage across the pins when fully warm, and a slightly annoyed gander at their data sheet confirmed this.
OTOH the WE300B really needs about 1.2A as do some of the Chinese Valve Arts types I have tried.
Boutique brands often require even more current than the JJ so I recommend making the constant current source adjustable - its quite easy and does not require the pot to handle any current. (I will eventually post the circuit to my website.) To set it t properly you must patiently measure the filament voltage as well as the current (measure current too - initially anyway) when the tube is fully warmed up.
edit to fix typo and minor additional clarification added
For example to properly heat a JJ 300B requires ~1.5A as evidenced by the voltage across the pins when fully warm, and a slightly annoyed gander at their data sheet confirmed this.
OTOH the WE300B really needs about 1.2A as do some of the Chinese Valve Arts types I have tried.
Boutique brands often require even more current than the JJ so I recommend making the constant current source adjustable - its quite easy and does not require the pot to handle any current. (I will eventually post the circuit to my website.) To set it t properly you must patiently measure the filament voltage as well as the current (measure current too - initially anyway) when the tube is fully warmed up.
edit to fix typo and minor additional clarification added
Thank you to all that have replied so far.
My speakers are 98dB@1watt/1meter
Here is what I'm thinking:
Start with ac... with the current setting restistor on the center tap of the 5vac transformer.
If that results in too much hum...
Use 100R pot and 2 22R bypass resistors
If that still not good enough... DC
My speakers are 98dB@1watt/1meter
Here is what I'm thinking:
Start with ac... with the current setting restistor on the center tap of the 5vac transformer.
If that results in too much hum...
Use 100R pot and 2 22R bypass resistors
If that still not good enough... DC
300b filament
Kevin, does your implementation still use a humpot, perhaps to equalise bias current? I built a pair of 300b amps using dc on filaments. One amp uses a pair of bias resistors and caps (one on each side of the filament, the other uses a single cathode resistor and cap connected via a humpot used to balance the bias. I cannot hear any difference between the two arrangements, nor can I measure any difference in hum (both are less than 1 mv)
Jim
Kevin, does your implementation still use a humpot, perhaps to equalise bias current? I built a pair of 300b amps using dc on filaments. One amp uses a pair of bias resistors and caps (one on each side of the filament, the other uses a single cathode resistor and cap connected via a humpot used to balance the bias. I cannot hear any difference between the two arrangements, nor can I measure any difference in hum (both are less than 1 mv)
Jim
Segerus,
I agree with your decision to put the bias network on the 5 VAC winding's CT. There's minimal fuss that way and you may just be fine.
Your speakers are efficient enough to work WELL with a 2A3 based amp, which would cost you LESS to build. What made you choose the 300B?
An easy way to lower overall hum is to put DC on the heater of the 'SN7. If the idea appeals to you, I suggest you use either a 12SN7 or a 14N7 for voltage amplification. They are electrically equivalent to the 6SN7 and a 12 VDC heater supply is easier to execute than a 6 VDC supply is.
I agree with your decision to put the bias network on the 5 VAC winding's CT. There's minimal fuss that way and you may just be fine.
Your speakers are efficient enough to work WELL with a 2A3 based amp, which would cost you LESS to build. What made you choose the 300B?
An easy way to lower overall hum is to put DC on the heater of the 'SN7. If the idea appeals to you, I suggest you use either a 12SN7 or a 14N7 for voltage amplification. They are electrically equivalent to the 6SN7 and a 12 VDC heater supply is easier to execute than a 6 VDC supply is.
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.
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.
humpots on dc filaments
Kevin,
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.
Jim
Kevin,
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.
Jim
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.
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.
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.
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.
9 out of ten documents I try to attach are too large, heres a text version of the pdf.
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.
9 out of ten documents I try to attach are too large, heres a text version of the pdf.
Sgerus,
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.
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.
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
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
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.
Rick
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.
Rick
Rick,
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
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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