transistor/tube biasing law??

[Learner]

Hi guys,
I am in the process of building a tube amp and was wondering if the concept of biasing is the same as BJT? or if there is any distinctive difference?

I am under the assumption that the resistor placed at the collector/plate will determine the DC current from the power supply and the base/grid resistor to the gnd will appear as the load to determining the input voltage level.
I guess the difference for biasing the BJT is that at the base you would need a resistor from the PS to determine the base current, where as in tubes you would need a -ve DC voltage at the grid to bias the plate current.

Can someone confirm that I am on the right track so far? and also what other factors I would need to consider in biasing these components?

I am trying to understand the concept of biasing and aiming to find a most effictive/quickest/simplest method to do so.

Thanks in advance!

 

[Nigel Goodwin]

Hi guys,
I am in the process of building a tube amp and was wondering if the concept of biasing is the same as BJT? or if there is any distinctive difference?

I am under the assumption that the resistor placed at the collector/plate will determine the DC current from the power supply and the base/grid resistor to the gnd will appear as the load to determining the input voltage level.
I guess the difference for biasing the BJT is that at the base you would need a resistor from the PS to determine the base current, where as in tubes you would need a -ve DC voltage at the grid to bias the plate current.

Can someone confirm that I am on the right track so far? and also what other factors I would need to consider in biasing these components?

I am trying to understand the concept of biasing and aiming to find a most effictive/quickest/simplest method to do so.

A valve (tube) is fully ON by default, and you need to bias it's grid negative to set it's operation at a suitable point.

There are commonly two ways of doing this, one is to have a negative supply feeding the grid - this is mostly done in high power output stages. The other method is called auto-bias, you connect the grid to chassis with a high value resistor and place a resistor in the cathode to chassis. The voltage dropped across the cathode resistor (by the current through it) is the bias voltage. This provides negative feedback and helps to keeo the bias constant, you should bypass the cathode resistor with a capacitor to prevent AC negative feedback.

But if you're having to ask such basic questions as these, I don't hold much hope of you designing a quality valve amplifier?.

There are hundreds of suitable circuits already available, why not use one of those?.

 

[Learner]

Hi Nigel,
Thanks for coming to the rescue, the main the reason I asked is because I am trying to implement my own gimmics to the traditional way of building tube amps and have had some major set backs, which made me question about my basic foundation of biasing.........

I have tried placing a jfet at the cathode of a triode and it seem to work ok and simulated ok as well, however when I tried the same trick with a pentode it seem to clip the top peak of the signal. I have then tried using an opamp with a transistor as a voltage controlled current source and got the same result, in the simulator as well.

Therefore, with some more hair pulling I have incorporated the following circuit at the cathode, I have yet to build this circuit but looking to do so. The simulator do not have a pentode model so I used a tetrode instead, it seems to simulate ok but I've found flaws from this simulator previously. Such as running a DC voltage into a step up transformer at the primary and got step up DC at the secondary :shock: , this has made me lost confidence in the simulator and was wondering if you guys could check it out and advise me if this would actually work with a pentode? or even a tetrode as shown in the simulation?

I would probably wire the supressor straight to the cathode and screen as shown in the tetrode below....

I donno what I'll gain by adding all these extra bits at the cathode, I was hoping this would condition the current and may be increase the life expenctancy of the cathode by using transistor as the current source. It could be just a pile of BS since I have absolutely no proof of any benefits and have yet to find a way to proof it, however it does provide some satisefaction for myself knowing that I can make things work by applying what I have learnt so far from college and by overcoming these challenges it makes me learn more.

I can't use any of the available tube circuits because of the tube I want to use are not the readily available 12AX7s etc.... and by using existing circuits just defeats the purpose of design which is what I am trying to practice doing. I would like to build things that has not been built based on my own ideas/imagination, aiming to practice my skill of problem solving....

 

[Nigel Goodwin]

I donno what I'll gain by adding all these extra bits at the cathode, I was hoping this would condition the current and may be increase the life expenctancy of the cathode by using transistor as the current source. It could be just a pile of BS since I have absolutely no proof of any benefits and have yet to find a way to proof it, however it does provide some satisefaction for myself knowing that I can make things work by applying what I have learnt so far from college and by overcoming these challenges it makes me learn more.

Seems a total waste of time, particularly considering you're usually aiming at a constant grid voltage, this is setting a constant cathode current, which will change the grid voltage accordingly.

I can't use any of the available tube circuits because of the tube I want to use are not the readily available 12AX7s etc.... and by using existing circuits just defeats the purpose of design which is what I am trying to practice doing. I would like to build things that has not been built based on my own ideas/imagination, aiming to practice my skill of problem solving....

What an earth is R6 doing!!!.

You certainly SHOULDN'T have a resistor from anode to gate, the valve is going to seriously melt down!.

You're also using an output tetrode as a low signal gain stage, which seems rather bizzare to say the least?.

Valves are very tried and tested items, there are many successful circuits about which use them - try studying some, you will see the types of designs that work - you can't just start randomly connecting components together.

What sort of amplifier are you wanting to build?, and what valves/tubes do you have available?. Your biggest problems are likely to be transformers - VERY expensive!.

BTW, I built my first valve amplifer back in about 1968, it used an EF80 preamp, and a 6BW6 output valve (giving about 3 watts), I used a metal rectifer for the HT supply.

I happen to have a book that I found in a second hand shop a number of years ago, "Quality Amlifiers for A.C. mains", it's dated 1958 - it gives full details of five valve amplifiers!.

 

[Learner]

What an earth is R6 doing!!!.

hehe, yes I know its bizzare. I have been reading on a lot of tube schematics and the Radiotron Designers Handbook 3 & 4 and have never seen that been used ever, however I thought I would try that in the simulation since I have seen it been used to bias BJTs. I was under the assumption that by using this as a feedback may be it would decrease some distortion at the cost of gain?

You certainly SHOULDN'T have a resistor from anode to gate, the valve is going to seriously melt down!.

Is it because instead of working like a negative feedback it is positive feedback and it would draw too much current? but doesnt the plate resistor limit the current? I am very confused, I know I must be missing something....... Soorry about my newbieness again, since I have not been taught anything on tubes except some minimal reading on my own.....

You're also using an output tetrode as a low signal gain stage, which seems rather bizzare to say the least?.

I am actually aiming to use this as a second stage of my amp, with the first stage consisting of a 6SN7 differential amplifier. I can not have the source signal bigger than 1V p-p or I'll get clipping in the simulation, thats why I've only used 1V p-p as the source.

Valves are very tried and tested items, there are many successful circuits about which use them - try studying some, you will see the types of designs that work - you can't just start randomly connecting components together.

That's why I am trying to grasp the basics of biasing but its the gimmics that I am trying to add stops it from working properly, otherwise I have got things to work fine without it. And I would like to find out why my gimmic don't work.......

What sort of amplifier are you wanting to build?, and what valves/tubes do you have available?. Your biggest problems are likely to be transformers - VERY expensive!.

I am actually building this amp as the output of my audio compressor, which I have completed building using the THAT CORP vcas and RMS detector based on their application note. **broken link removed**

Currently, I am trying to build an amp with 3 stages. The first one consist of 6SN7 differential amp follwed by 2 x 6BA6 remote cutoff pentode differential, then 2 x 6SK7 remote cutoff push-pull but I am having trouble finding a cheap center tap audio transformer..... I am think about getting these.... any thoughts?

**broken link removed**


Since I still haven't got the transformer yet, I've decided to just couple the 6SN7 to 6BA6 and the 6SK7 in series and try to turn this into a stereo output pre amp.(with the gimmics)

Seems a total waste of time, particularly considering you're usually aiming at a constant grid voltage, this is setting a constant cathode current, which will change the grid voltage accordingly.

What gave me the gimmics idea was that I wanted to build a tube VCA, so at first I stuck a fet at the cathode of a triode and trying to change the voltage at the gate to control the gain and it seemed to work OK. I then wanted to use a RMS detector such as the THAT 2252 and feedback the output into the detector to get the DC to control the fet, aiming to turn it into a simple compressr which I have yet to do.

Which lead me to more bizzare fantasy about using transistors at the cathode, since I bought a batch of 100 off ebay and have no use for them at the moment and wanted to find a way to use them up..... I found the voltage controlled current source circuit in my book collection called Converter and Filter Circuit Encyclopedia by Rudolf Graf, so I thought about using it to replace the fet and may be I can rectify the output and invert the DC to the op amp terminal to attenuate the signal. But I want to make sure the circuit is working first before I go and build the rectifying stage for the voltage controlled current source.

BTW, I built my first valve amplifer back in about 1968, it used an EF80 preamp, and a 6BW6 output valve (giving about 3 watts), I used a metal rectifer for the HT supply.

I happen to have a book that I found in a second hand shop a number of years ago, "Quality Amlifiers for A.C. mains", it's dated 1958 - it gives full details of five valve amplifiers!.

Its nice to know I have someone that I can count on whenever I am going off track!

 

[Nigel Goodwin]

What an earth is R6 doing!!!.

hehe, yes I know its bizzare. I have been reading on a lot of tube schematics and the Radiotron Designers Handbook 3 & 4 and have never seen that been used ever, however I thought I would try that in the simulation since I have seen it been used to bias BJTs. I was under the assumption that by using this as a feedback may be it would decrease some distortion at the cost of gain?

A valve isn't a BJT, you won't have seen it used because it wouldn't work, a valve requires negative bias (because by default it's permanently ON), applying massive positive bias is making really sure the valve is absolutely hard ON!.

You certainly SHOULDN'T have a resistor from anode to gate, the valve is going to seriously melt down!.

Is it because instead of working like a negative feedback it is positive feedback and it would draw too much current? but doesnt the plate resistor limit the current? I am very confused, I know I must be missing something....... Soorry about my newbieness again, since I have not been taught anything on tubes except some minimal reading on my own.....

It's simply biasing, the grid must not be positive - in fact a common fault with valve stages is the coupling capacitor from the previous anode leaking, causing the grid to go positive.

That's why I am trying to grasp the basics of biasing but its the gimmics that I am trying to add stops it from working properly, otherwise I have got things to work fine without it. And I would like to find out why my gimmic don't work.......

The actual operation of a valve is EXTREMELY simple, based purely on the attraction of electrons by a positive voltage, and the repulsion of them by a negative one.

What sort of amplifier are you wanting to build?, and what valves/tubes do you have available?. Your biggest problems are likely to be transformers - VERY expensive!.

I am actually building this amp as the output of my audio compressor, which I have completed building using the THAT CORP vcas and RMS detector based on their application note. **broken link removed**

Currently, I am trying to build an amp with 3 stages. The first one consist of 6SN7 differential amp follwed by 2 x 6BA6 remote cutoff pentode differential, then 2 x 6SK7 remote cutoff push-pull but I am having trouble finding a cheap center tap audio transformer..... I am think about getting these.... any thoughts?

**broken link removed**

Sorry, no idea, it all depends on the impedances you require.

Since I still haven't got the transformer yet, I've decided to just couple the 6SN7 to 6BA6 and the 6SK7 in series and try to turn this into a stereo output pre amp.(with the gimmics)

What gave me the gimmics idea was that I wanted to build a tube VCA, so at first I stuck a fet at the cathode of a triode and trying to change the voltage at the gate to control the gain and it seemed to work OK. I then wanted to use a RMS detector such as the THAT 2252 and feedback the output into the detector to get the DC to control the fet, aiming to turn it into a simple compressr which I have yet to do.

I must admit, this seems a really bizarre idea?, why on earth would you want to build such a device using valves?.

 

[Learner]

Currently, I am trying to build an amp with 3 stages. The first one consist of 6SN7 differential amp follwed by 2 x 6BA6 remote cutoff pentode differential, then 2 x 6SK7 remote cutoff push-pull but I am having trouble finding a cheap center tap audio transformer..... I am think about getting these.... any thoughts?

**broken link removed**

Sorry, no idea, it all depends on the impedances you require.

Hi Nigel,
According the Radiotron's designers manual, it suggests that the plate resistor should be at least several times the plate resistance. Would I be on the right track to apply that rule here? Using a center tapped transformer with primary impedance serveral times higher than the plate resistance?

Since I still haven't got the transformer yet, I've decided to just couple the 6SN7 to 6BA6 and the 6SK7 in series and try to turn this into a stereo output pre amp.(with the gimmics)

What gave me the gimmics idea was that I wanted to build a tube VCA, so at first I stuck a fet at the cathode of a triode and trying to change the voltage at the gate to control the gain and it seemed to work OK. I then wanted to use a RMS detector such as the THAT 2252 and feedback the output into the detector to get the DC to control the fet, aiming to turn it into a simple compressr which I have yet to do.

I must admit, this seems a really bizarre idea?, why on earth would you want to build such a device using valves?.

I have previously built a simple resistance coupled 2 stage tube amp using 6SN7 in series with 6SK7, simply because there has been so much hype in revival of tube amps believing that it sounds "better" than transistor amps.

I wanted to find out for myself and built an instrumental pre amp with LM833 at the input followed by the tube amp, surpisingly I found that the amp does sound "better" with the tube amp at the output as oppose to without the tubes.

I am not totally convinced that any tube circuit will sound better, that's how I start my journey on exploring tube circuits and comparing them to monolithic op amps.

To describe my impression of sounding "better" in an observer's term, it simply sounds like it's coming from a deeper source eg. the impression of the stereo imaging has more depth and appears to be more clearer or sharper or defined but yet not in a disturbing fashion. I am not saying that the ICs do not sound clear as amplifiers but I believe its something to do with the stereo imaging that fools my impression thinking that it sounds "better"and I would like to find out why and what causes this???? high voltage?? spaced vaccum??

In the world of EE, it might be considered as a bunch of non sense where different active device would sound different unless they have different frequency response within the audible frequency range. Also the design of the circuit has some sort of audible effect, eventhough all circuits might have freq response will beyond 20khz....

 

[Nigel Goodwin]

According the Radiotron's designers manual, it suggests that the plate resistor should be at least several times the plate resistance. Would I be on the right track to apply that rule here? Using a center tapped transformer with primary impedance serveral times higher than the plate resistance?

Sorry, I've no idea, I've never been involved in transformer design - why do you want a transformer anyway?.

I wanted to find out for myself and built an instrumental pre amp with LM833 at the input followed by the tube amp, surpisingly I found that the amp does sound "better" with the tube amp at the output as oppose to without the tubes.

I'm glad you put "better" in quotes :lol:

I suspect that the general liking, by a small number of people, for the 'valve sound' is due to their increased distortion (but more even harmonics than odd), and limited frequency response.

But if you like the sound, then go for it - personally I wouldn't entertain a valve amp, except perhaps as a guitar amp for lead guitar.

 

[Learner]

Sorry, I've no idea, I've never been involved in transformer design - why do you want a transformer anyway?.

I don't but I have not seen a tube push pull schematic to learn how to make a push pull tube circuit without a transformer, frankly I am not a fan of transformers either they are bulky, heavy, expensive not to mention generates electromagnitic field which either cost more money for isolation of more space in a rack to be isolated.

Honestly, I am a IC kinda guy negative feedback and shunting resistor ratio for gain simple and cost effective is what I love to be staying with. Not to mention active filters, cheap and effective with great performance.

Making a tube circuit is a real pain, needing high voltages and high voltage transformer, coupling transformer not to mention heat in the circuit. :roll:
I can see all the reason why it was abandoned in the first place and I am all for it, until I got infected with this tube audio bug.

I'm glad you put "better" in quotes :lol:

I suspect that the general liking, by a small number of people, for the 'valve sound' is due to their increased distortion (but more even harmonics than odd), and limited frequency response.

Actually, I think there has been quite a following of the tube audio... or revival I should say.
I have also read the same thing also about the second harmonics, I think it was one of the AES paper? Would love to read more on it but there donest seem to be much info about it, I wonder what makes the circuit generate harmonics? tube grid?

But if you like the sound, then go for it - personally I wouldn't entertain a valve amp, except perhaps as a guitar amp for lead guitar.

Yea, tube amps has been the only preference for guitar players especially when it comes to distortion since it does not clip like a transistor and adds that second harmonics to the sound.........