Kuei Yang Wang said:
if you like their sound.....
It is my experience that a 300b lies to be driven with low impedance, remember it is 80pF to drive, and in A2 up to 30 mA peak can be needed.
A driver should have at least 10 times lower distortion imo.
Given above you won't make it with a single stage
cheers
Konnichiwa Tent San,
On the 6SL7....
I personally don't, particulary. I merely commented that it was possible to "make it work" after a fashion.
You can drive a 300B from a fairly high source impedance IF you do it right. Using a current starved triode is arguably not "the right way".
Yes, I found limiting myself to Class A1 allowed me to avoid having to supply grid current and thus allows a single stage driver to manage fine.
I agree.
In Class A1 I WILL, not sure about others though... ;-)
Sayonara
On the 6SL7....
Guido Tent said:
I personally don't, particulary. I merely commented that it was possible to "make it work" after a fashion.
Guido Tent said:
You can drive a 300B from a fairly high source impedance IF you do it right. Using a current starved triode is arguably not "the right way".
Guido Tent said:
Yes, I found limiting myself to Class A1 allowed me to avoid having to supply grid current and thus allows a single stage driver to manage fine.
Guido Tent said:
I agree.
Guido Tent said:
In Class A1 I WILL, not sure about others though... ;-)
Sayonara
Konnichiwa,
True, but someone here insists he wants to start with a 6SL7 so let's make sure he has at least a minimal chance?
Well, I normally suggest that we need 100KHz/-3db for Audio. So with a 300B this suggests 20K maximum source impedance. However, the "trick" is that we require a sufficient current capability behind that 20K Source to avoid slewing. An arbitarily lower source impedance is not neccesarily "better". If we go not require Class A2 operation that 20K backed by enough current suffices.
In fact, there is a tricks that allow even a 310A loaded by 91K to drive a 300B well at HF, as long as we stay in Class A1.
Now if we do require Class A2 Operation (like we use lousy speakers) we must reduce the driving impedance as much as can be acommodated together with making sure any gridcurrent flowing does not cause any long term operating point shift (eg no RC coupling to the Grid please).
An excellent option for a single stage driver to 300B grid would be a high transconductance triode combined with a high inductance, bifilar interstage transformer additionally bridged across by a significant value coupling capacitor to swamp the resonances and to better couple high frequencies across. In that case a 1K source with some 10 - 20mA standing current goes a long way for that extra headroom in Class A2.
A more extreme solution is a DC coupled follower, possibly in a DC coupled circuit. Using this I got around 15 Watt pretty cleanly from a single 300B. Re-building that Amplifier to use driver pentodes (C3m) dropped available power under similar conditions to around 7 Watt, so 3db maximum output where lost. As at the time however my speakers went from around 90db/W/m to > 97db/W/m I still had more SPL available cleanly.
My final conclusion was that Amplifiers are best optimised for Class A1 with good overload recovery and to concentrate at the real problem, speakers.... ;-)
Sayonara
Guido Tent said:
True, but someone here insists he wants to start with a 6SL7 so let's make sure he has at least a minimal chance?
Guido Tent said:
Well, I normally suggest that we need 100KHz/-3db for Audio. So with a 300B this suggests 20K maximum source impedance. However, the "trick" is that we require a sufficient current capability behind that 20K Source to avoid slewing. An arbitarily lower source impedance is not neccesarily "better". If we go not require Class A2 operation that 20K backed by enough current suffices.
In fact, there is a tricks that allow even a 310A loaded by 91K to drive a 300B well at HF, as long as we stay in Class A1.
Now if we do require Class A2 Operation (like we use lousy speakers) we must reduce the driving impedance as much as can be acommodated together with making sure any gridcurrent flowing does not cause any long term operating point shift (eg no RC coupling to the Grid please).
An excellent option for a single stage driver to 300B grid would be a high transconductance triode combined with a high inductance, bifilar interstage transformer additionally bridged across by a significant value coupling capacitor to swamp the resonances and to better couple high frequencies across. In that case a 1K source with some 10 - 20mA standing current goes a long way for that extra headroom in Class A2.
A more extreme solution is a DC coupled follower, possibly in a DC coupled circuit. Using this I got around 15 Watt pretty cleanly from a single 300B. Re-building that Amplifier to use driver pentodes (C3m) dropped available power under similar conditions to around 7 Watt, so 3db maximum output where lost. As at the time however my speakers went from around 90db/W/m to > 97db/W/m I still had more SPL available cleanly.
My final conclusion was that Amplifiers are best optimised for Class A1 with good overload recovery and to concentrate at the real problem, speakers.... ;-)
Sayonara
Dear Colleques.
Thank yu for the numerous informations. Given the various alternatives , I wuld at current proceed (as the first step - and from there on enhance it..).
1. 300B on AC heater supply , bypassed with around 30R/2Watt each to the centrepin of a humpot , further grounded via a paralleled 150uF/250V cap + 1K/20w resistor. I hope that will give me a tolerable yum level with speakers of around 87dB sensitivity.
2. 6SL7 - since I dont have a centertapped heater supply, I may have to install a rectifier diode from the 6.3-0V/3A taps and install a 1000uF plus o.1uF . Hope that will give a decent DC heater supply to both 6SL7 at this stage.
Input signal into grid via a 100K pot and 2K7 resistor. Grounding will use 1M resistor from the pot center.
3. The 1st half of the 6SL7 :
will hv a cathode resistor of 1K8 +paralel 100uF cap to ground.
the plate via a 8K2 resistor + 1K resistor to the 2nd half cathode of 6SL7. Bypass uses 0.05uF + 1M + 2K7 into 2nd half grid of 6SL7
4. 2nd half of 6SL7 will be fed a HF of around 403VDC from the HT supply.
5. Input into 300B grid is via 0.22uF and 2k7 from the 2nd half of 6SL7
Summary:
this probably satisfy suggestions using cathode resistor of 1K.
also on driving the 6SL7 high.
I aim first to get initial result, following w customization / enhancings using better line/drivers types.
I will keep yu advised, gentlemen.
Million thanks , H a nice weekend
Paul
Thank yu for the numerous informations. Given the various alternatives , I wuld at current proceed (as the first step - and from there on enhance it..).
1. 300B on AC heater supply , bypassed with around 30R/2Watt each to the centrepin of a humpot , further grounded via a paralleled 150uF/250V cap + 1K/20w resistor. I hope that will give me a tolerable yum level with speakers of around 87dB sensitivity.
2. 6SL7 - since I dont have a centertapped heater supply, I may have to install a rectifier diode from the 6.3-0V/3A taps and install a 1000uF plus o.1uF . Hope that will give a decent DC heater supply to both 6SL7 at this stage.
Input signal into grid via a 100K pot and 2K7 resistor. Grounding will use 1M resistor from the pot center.
3. The 1st half of the 6SL7 :
will hv a cathode resistor of 1K8 +paralel 100uF cap to ground.
the plate via a 8K2 resistor + 1K resistor to the 2nd half cathode of 6SL7. Bypass uses 0.05uF + 1M + 2K7 into 2nd half grid of 6SL7
4. 2nd half of 6SL7 will be fed a HF of around 403VDC from the HT supply.
5. Input into 300B grid is via 0.22uF and 2k7 from the 2nd half of 6SL7
Summary:
this probably satisfy suggestions using cathode resistor of 1K.
also on driving the 6SL7 high.
I aim first to get initial result, following w customization / enhancings using better line/drivers types.
I will keep yu advised, gentlemen.
Million thanks , H a nice weekend
Paul
Konnichiwa,
From experience, this will likely be okay.
So? Make a virtual centertap from a pair of 100 Ohm Resistors, preferably paralleled with a pair of 100nF Ceramic capacitors (small size) at the Valves heater pins.
DC is not neccesary for the Driver.
Mu follower setup. Should be okay. It will be ESSENTIAL to "bias up" the Heater Voltage. As you seem to have only one 6.3V winding and as you intend to use the two systems within one valve envelope to cover both functions of the Mu-Follower you MUST lift the heaters to 100V.
For example you can use a 330K->100K Voltage divider from +B (430V?), the 100K resistor byypassed with 10uF/160V and with the junction connected to the center point of the two 100 Ohm resistors in the heater section (or a centertap if present) will answer the purpose well.
Sayonara
Kriskom said:
From experience, this will likely be okay.
Kriskom said:
So? Make a virtual centertap from a pair of 100 Ohm Resistors, preferably paralleled with a pair of 100nF Ceramic capacitors (small size) at the Valves heater pins.
DC is not neccesary for the Driver.
Kriskom said:
Mu follower setup. Should be okay. It will be ESSENTIAL to "bias up" the Heater Voltage. As you seem to have only one 6.3V winding and as you intend to use the two systems within one valve envelope to cover both functions of the Mu-Follower you MUST lift the heaters to 100V.
For example you can use a 330K->100K Voltage divider from +B (430V?), the 100K resistor byypassed with 10uF/160V and with the junction connected to the center point of the two 100 Ohm resistors in the heater section (or a centertap if present) will answer the purpose well.
Sayonara
Konnichiwa
I was just surfing around and I came across these three very nice, well researched and usefull articles on the topic of AC Heaters by Dmitry Nizhegorodov:
On Correlation Between Residual DHT Filament Hum and AC Frequency. Distortion-induced hum in directly-heated triodes
On Correlation Between Residual DHT Filament Hum and AC Frequency
DHT Filament Hum Cancellation
Most excellent work on the mechanisms and even better, finally a methode to fairly reliably model the DHT Hum and the cancellation.
Sayonara
I was just surfing around and I came across these three very nice, well researched and usefull articles on the topic of AC Heaters by Dmitry Nizhegorodov:
On Correlation Between Residual DHT Filament Hum and AC Frequency. Distortion-induced hum in directly-heated triodes
On Correlation Between Residual DHT Filament Hum and AC Frequency
DHT Filament Hum Cancellation
Most excellent work on the mechanisms and even better, finally a methode to fairly reliably model the DHT Hum and the cancellation.
Sayonara
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