If you have a valve that only needs a small amount of -ve grid bias, then the differential dc voltage across the heater can be a significant fraction of the bias, meaning that the emission across the filament varies as the effective heater-to-grid voltage will vary along the filament.
As most DHT triodes require a high -ve bias voltage, the small dc change in effective bias voltage resulting from a 6V (say) heater supply won't result in too much variation in emission across the heater.
It will still have some effect though, so it might be audible or measurable.
As most DHT triodes require a high -ve bias voltage, the small dc change in effective bias voltage resulting from a 6V (say) heater supply won't result in too much variation in emission across the heater.
It will still have some effect though, so it might be audible or measurable.
Konnichiwa,
AC will cause a mudulation of the signal which represents itself as hum and intermodulation. That gives AC it's charateristic sound. Most DHT's sound by far to opaque and hum by far too much with AC. AC heated Amp's often are also replete with parts having a forward/bright sound signature, cutting through the Fog created by AC heating.
DC causes it's own problems and needs to be implemented with great care, or Amplifiers will sound strident and unpleasant, moreso if an Amp designed and voiced for AC heaters is converted to DC.
So, the answer is obvious.
If you do not wish to tolerate too much noise the choice is DC. Then design and voice the Amp to work well with DC. You can get as low as <0.3mV noise levels out of a sensibly implemented DC heated 300B Amp, or > 80db below 1 Watt.
If a hum level around 50-60db below 1 Watt (which is the best achievable with AC Heating and non-extreme hum cancellation circuits) is acceptable to you (the average for AC Heated SE Amp's with 2A3 and 300B is around 3-5mV) then go AC and voice the Amplifier to work well with AC.
Sayonara
Kriskom said:
AC will cause a mudulation of the signal which represents itself as hum and intermodulation. That gives AC it's charateristic sound. Most DHT's sound by far to opaque and hum by far too much with AC. AC heated Amp's often are also replete with parts having a forward/bright sound signature, cutting through the Fog created by AC heating.
DC causes it's own problems and needs to be implemented with great care, or Amplifiers will sound strident and unpleasant, moreso if an Amp designed and voiced for AC heaters is converted to DC.
So, the answer is obvious.
If you do not wish to tolerate too much noise the choice is DC. Then design and voice the Amp to work well with DC. You can get as low as <0.3mV noise levels out of a sensibly implemented DC heated 300B Amp, or > 80db below 1 Watt.
If a hum level around 50-60db below 1 Watt (which is the best achievable with AC Heating and non-extreme hum cancellation circuits) is acceptable to you (the average for AC Heated SE Amp's with 2A3 and 300B is around 3-5mV) then go AC and voice the Amplifier to work well with AC.
Sayonara
Arigato Kuei -san for yr explanation.
Very interesting. I am still a rooky.
But I still need to make a decision based on the following progressing 300B project technical specs :
Power supply trafo :
Primary 0-220-230VAC
Secundary :
2.5-0-2.5V / 3A for the filaments of two Russian Flying C 5U4GTB rectifiers.
380-0-380V/ 300mA for the high current
two 5V/2A for the two 300B heater filaments . There is no centertap on this winding
two 6.3V/3A for the two 6SL7 drivers
For both channel high voltage I am using two chokes of 10H/200mA , two 20uF/600V oilcaps , two 40uF/600V oilcaps and to come to a B+ of 403VDC (6SL7) and via two 51K/0.5W resistors to B++ 455VDC for the 300B
While the 6SN7 heaters will go on the 6.3V standard converted to DC via a rectifier diode and some caps.
I need to decide whether using AC or DC for the 300B heaters.
Basedupon this scantily information, what wuld yu suggest ?
I have provisionally provided a humpot of 100R/4W pAralelled pin 1 & 4 of 300B with 39R/2watts to the pot centerpin , and via a 1K/20W resistor to ground.
Ideally null hum is preferred , but I am prepared for little hum as ong as it is not much audible during silent passages.
Thanks for yr attention
Konbanwa
Paul
Very interesting. I am still a rooky.
But I still need to make a decision based on the following progressing 300B project technical specs :
Power supply trafo :
Primary 0-220-230VAC
Secundary :
2.5-0-2.5V / 3A for the filaments of two Russian Flying C 5U4GTB rectifiers.
380-0-380V/ 300mA for the high current
two 5V/2A for the two 300B heater filaments . There is no centertap on this winding
two 6.3V/3A for the two 6SL7 drivers
For both channel high voltage I am using two chokes of 10H/200mA , two 20uF/600V oilcaps , two 40uF/600V oilcaps and to come to a B+ of 403VDC (6SL7) and via two 51K/0.5W resistors to B++ 455VDC for the 300B
While the 6SN7 heaters will go on the 6.3V standard converted to DC via a rectifier diode and some caps.
I need to decide whether using AC or DC for the 300B heaters.
Basedupon this scantily information, what wuld yu suggest ?
I have provisionally provided a humpot of 100R/4W pAralelled pin 1 & 4 of 300B with 39R/2watts to the pot centerpin , and via a 1K/20W resistor to ground.
Ideally null hum is preferred , but I am prepared for little hum as ong as it is not much audible during silent passages.
Thanks for yr attention
Konbanwa
Paul
Konnichiwa,
Why? You are introducing unneccary rectification noise back into the mains transformer and the 6SN7 does not need AC, just make sure the 6SN7 heater is biased suitably to avoid parasitic rectification effects.
That said, why on earth a 6SN7? It would be at the very bottom of my list of Valves to use in a 300B Amp (actually, it would be OFF THE LIST).
You don't need to commit youself. Leave space for a decent DC supply and start with AC. The 5V will rectify to around 6V DC using schottky diodes and a 10,000uF Capacitor, add a nice filter choke with around 0.75 Ohm DCR and a second capacitor and you have a pretty decent DC Heater supply. Then use what sounds better to you.
That is fine and will work for both AC and DC.
Why 1K? I feel 880R is more apropriate at the HT you will be ending up with. Parallel 7K5/5W with the 1K resistor and you have pretty close to 880R.
Okay, lets see.
If your speakers are 90db/2.83V sensitive you will get between 30db and 40db absolute SLP of 100Hz hum in 1m Distance. A pair of speakers in stereo and at around 3m listening distance will result in 27-37db SPL of hum at the listening position. I personally find the 27db tollerable but not the 37db and in late night "quiet" listening even the 27db absolute can cause irritation.
Now if your speaker are more sensitive than 90db/2.83V/1m you will likely experience significant audible hum.
Sayonara
Kriskom said:
Why? You are introducing unneccary rectification noise back into the mains transformer and the 6SN7 does not need AC, just make sure the 6SN7 heater is biased suitably to avoid parasitic rectification effects.
That said, why on earth a 6SN7? It would be at the very bottom of my list of Valves to use in a 300B Amp (actually, it would be OFF THE LIST).
Kriskom said:
You don't need to commit youself. Leave space for a decent DC supply and start with AC. The 5V will rectify to around 6V DC using schottky diodes and a 10,000uF Capacitor, add a nice filter choke with around 0.75 Ohm DCR and a second capacitor and you have a pretty decent DC Heater supply. Then use what sounds better to you.
Kriskom said:
That is fine and will work for both AC and DC.
Kriskom said:
Why 1K? I feel 880R is more apropriate at the HT you will be ending up with. Parallel 7K5/5W with the 1K resistor and you have pretty close to 880R.
Kriskom said:
Okay, lets see.
If your speakers are 90db/2.83V sensitive you will get between 30db and 40db absolute SLP of 100Hz hum in 1m Distance. A pair of speakers in stereo and at around 3m listening distance will result in 27-37db SPL of hum at the listening position. I personally find the 27db tollerable but not the 37db and in late night "quiet" listening even the 27db absolute can cause irritation.
Now if your speaker are more sensitive than 90db/2.83V/1m you will likely experience significant audible hum.
Sayonara
Kriskom said:
In addition there's the 2nd dimension of voltage versus current
Roughly:
AC increases intermod. You can increase the A frequency to shift that outside the band, but it still does not sound clean
DC Does not increase intermod, but a DC voltage source (reg, lytic) shorts the cathode which is not desired.
read more at
http://www.tentlabs.com/Info/Articles/Heatingmethods.pdf
But just try it yourself, and arange for a mains transformer with more taps for the 300b heater......
regards
Re: Re: Re: Re: Re: Re: which driver?
hi KYW
did not know i can do this to get twice the voltage.
unfortunately my main trans has heater windings for 1 x 3.15-0-3.15vac(driver), 1 x 5vac(rectifier) and 1 x 7vac(300b, to be rectified to 5vdc).
doubt i can squeeze in a c3m.
the specs of c3m and c3g looks close. just wonder if they would be ok as drivers.
thanks.
rgds
garbage
Kuei Yang Wang said:
hi KYW
did not know i can do this to get twice the voltage.
unfortunately my main trans has heater windings for 1 x 3.15-0-3.15vac(driver), 1 x 5vac(rectifier) and 1 x 7vac(300b, to be rectified to 5vdc).
doubt i can squeeze in a c3m.
the specs of c3m and c3g looks close. just wonder if they would be ok as drivers.
thanks.
rgds
garbage
Re: Re: Re: Re: Re: Re: Re: which driver?
Otherwise, I basically agree with most things Thorsten has said in the thread, as my experience parallels it.
Get a small second mains transformer to supply the 20Vac to the C3m filaments. They're usually very cheap, readily available and small enough to be tucked away in most chassis'.garbage said:did not know i can do this to get twice the voltage.
unfortunately my main trans has heater windings for 1 x 3.15-0-3.15vac(driver), 1 x 5vac(rectifier) and 1 x 7vac(300b, to be rectified to 5vdc).
Otherwise, I basically agree with most things Thorsten has said in the thread, as my experience parallels it.
Re: Re: Re: Re: Re: Re: Re: which driver?
Hi,
Well, you do now.
Well, that's perfect.
I don't.
The C3m has a rated 120V HK maximum voltage. So, if you rectify your 5V for the 300B and you voltage double the 6.3V you should have just the right voltage to stack on top of the rectified and filtered 7V AC from the 300B and running the series connected DC voltages will be just dandy, as you shoudl "bias" the heater supply of the Driver valve a little anyway to reverse bias the heater cathode diode.
Sure. But the 6.3V C3m is the C3o IIRC....
Sayonara
Hi,
garbage said:did not know i can do this to get twice the voltage.
Well, you do now.
garbage said:
Well, that's perfect.
garbage said:doubt i can squeeze in a c3m.
I don't.
The C3m has a rated 120V HK maximum voltage. So, if you rectify your 5V for the 300B and you voltage double the 6.3V you should have just the right voltage to stack on top of the rectified and filtered 7V AC from the 300B and running the series connected DC voltages will be just dandy, as you shoudl "bias" the heater supply of the Driver valve a little anyway to reverse bias the heater cathode diode.
garbage said:
Sure. But the 6.3V C3m is the C3o IIRC....
Sayonara
Dear Kuei-san and other colleques
Thanks for yr previous info. Just to add :
this is my first project building a tube amp and it happened to be a 300B ( How insane I am using a 300B as first step !)
So, for the drivers I am using the Brimar CV1985 a equiv of 6SL7. Having said 6SN7 (and yes I am not using this) wuld be off-list , what wuld be then THE driver tubes in tradition of the 300B - I wuld certainly go for it after I succesfully complete this first one.
I probably will try the AC heater supply for the 300B first as the initial simple step , (and brace myself for the hum , thou a humpot is ready but I am aware it will not eliminate all the hum. )
Paralelly I will try build a "decent DC" supply for the 300B heaters . I hv to figure out the direction yu gave me. Allow me to go back if I lose my way then.
The CV1985 heaters will be on DC , or do yu forsee it can go on AC mode ?
More help from all is much appreciated.
God bless yu all helping a rooky !
Paul
Thanks for yr previous info. Just to add :
this is my first project building a tube amp and it happened to be a 300B ( How insane I am using a 300B as first step !)
So, for the drivers I am using the Brimar CV1985 a equiv of 6SL7. Having said 6SN7 (and yes I am not using this) wuld be off-list , what wuld be then THE driver tubes in tradition of the 300B - I wuld certainly go for it after I succesfully complete this first one.
I probably will try the AC heater supply for the 300B first as the initial simple step , (and brace myself for the hum , thou a humpot is ready but I am aware it will not eliminate all the hum. )
Paralelly I will try build a "decent DC" supply for the 300B heaters . I hv to figure out the direction yu gave me. Allow me to go back if I lose my way then.
The CV1985 heaters will be on DC , or do yu forsee it can go on AC mode ?
More help from all is much appreciated.
God bless yu all helping a rooky !
Paul
what a coincidence...
i'm also a first time tube amp builder.
also using 300b as the output tube, because of my inefficient speakers.
initially i had the 6SL7GT in mind as the driver. now am thinking i should heed the advice of experienced members here.
my 6SL7 was to be heated via AC. you can place 2 matched resistors across the filaments of 3.15 - 0 - 3.15 to balance the AC. this should reduce the hum. 6SL7 is known to be a microphonic tube. see if you are able to catch hold of the RCA red base 5691. these are mil spec version of the 6SL7 and has a real solid build.
try 300b heaters with AC first, then at least you have an option to change to DC at a later stage. maybe you might want a higher voltage for the heater, otherwise you'd have to use low dropout rectifiers when wanting to use DC.
rgds
garbage
i'm also a first time tube amp builder.
also using 300b as the output tube, because of my inefficient speakers.
initially i had the 6SL7GT in mind as the driver. now am thinking i should heed the advice of experienced members here.
my 6SL7 was to be heated via AC. you can place 2 matched resistors across the filaments of 3.15 - 0 - 3.15 to balance the AC. this should reduce the hum. 6SL7 is known to be a microphonic tube. see if you are able to catch hold of the RCA red base 5691. these are mil spec version of the 6SL7 and has a real solid build.
try 300b heaters with AC first, then at least you have an option to change to DC at a later stage. maybe you might want a higher voltage for the heater, otherwise you'd have to use low dropout rectifiers when wanting to use DC.
rgds
garbage
Kriskom said:
Hi Paul
If you like some 20dB extra intermod, go for AC, see also page 4 of
http://www.tentlabs.com/Info/Articles/Heatingmethods.pdf
If you want to get rid of that, go for DC current source. Better avoid voltage sources, be it AC or DC.
DC current source also has an advantage, you finally get rid of the humbucking potmeter that is straight in the middle of your signal.
cheers
<my 6SL7 was to be heated via AC. you can place 2 matched resistors across the filaments of 3.15 - 0 - 3.15 to balance the AC. this should reduce the hum. 6SL7 is known to be a microphonic tube. see if you are able to catch hold of the RCA red base 5691. these are mil spec version of the 6SL7 and has a real solid build.>
Thanks for the tip, I will start looking for thge milspecs.
What is the resistors value yu put across the 6SL7 ?
Mine secudaire winding offers me only two 6.3-0 for both 6SL7 , no central tap.
Thanks for the tip, I will start looking for thge milspecs.
What is the resistors value yu put across the 6SL7 ?
Mine secudaire winding offers me only two 6.3-0 for both 6SL7 , no central tap.
Konnichiwa,
Sorry, this really belongs into the other thread....
This CAN work fairly if you use the 6SL7 as SRPP. It will swing the required around 140VPP (50VRMS) even though the output impedance tends to be on the high side. The way you get this is by running the 6SL7 very much at the limits of a number of parameters, but it tends to work okay for Cary and Wavelength, so I'd be not too worried.
Set up the SRPP with 1K cathode R for each section, a cathode bypass capacitor of at least 33uF is needed. The +B on the top anode should be around 400V and the heater needs to be biased up to 100V.
The Gain will be around 50 and the output impedance in the 10K region. The input section will allow up to around 90V swing with reasonable linearity, the SRPP will kill off a lot of the 2nd harmonics so the linearity at 50V RMS is okay, if not stunning, it certainly beats the commonly seen 6SN7 circuits to drive a 300B by a good degree, though it is more of the "get you going" type than of the "great sound" type.
In my experience, either good Pentode connected Pentodes like EL84, C3m, WE717A 6E5P and the like (WE310/6C6, 6J7, 6SJ7 work well with a few tricks, not as per WE 91A) or high transconductance, medium Mu triodes (or triode wired Pentodes) like 6S45PE, 417A, 437A, 3A/167M, EC8010, EC8020 and for Pentodes D3a, E81F, E280F, E180F, C3g (not tested but looks good on paper).
I do hope your speakers are "average" sensitivity and no higher.
The DIY HIFisupply Billie/Lady Day+ and Cary as well Wavelength used AC on their 6SL7 SRPP Drivers and generally with no noise issues, so AC should be fine.
Sayonara
Sorry, this really belongs into the other thread....
Kriskom said:
This CAN work fairly if you use the 6SL7 as SRPP. It will swing the required around 140VPP (50VRMS) even though the output impedance tends to be on the high side. The way you get this is by running the 6SL7 very much at the limits of a number of parameters, but it tends to work okay for Cary and Wavelength, so I'd be not too worried.
Set up the SRPP with 1K cathode R for each section, a cathode bypass capacitor of at least 33uF is needed. The +B on the top anode should be around 400V and the heater needs to be biased up to 100V.
The Gain will be around 50 and the output impedance in the 10K region. The input section will allow up to around 90V swing with reasonable linearity, the SRPP will kill off a lot of the 2nd harmonics so the linearity at 50V RMS is okay, if not stunning, it certainly beats the commonly seen 6SN7 circuits to drive a 300B by a good degree, though it is more of the "get you going" type than of the "great sound" type.
Kriskom said:
In my experience, either good Pentode connected Pentodes like EL84, C3m, WE717A 6E5P and the like (WE310/6C6, 6J7, 6SJ7 work well with a few tricks, not as per WE 91A) or high transconductance, medium Mu triodes (or triode wired Pentodes) like 6S45PE, 417A, 437A, 3A/167M, EC8010, EC8020 and for Pentodes D3a, E81F, E280F, E180F, C3g (not tested but looks good on paper).
Kriskom said:
I do hope your speakers are "average" sensitivity and no higher.
Kriskom said:
The DIY HIFisupply Billie/Lady Day+ and Cary as well Wavelength used AC on their 6SL7 SRPP Drivers and generally with no noise issues, so AC should be fine.
Sayonara
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