Solid state/class d vs tube amps

[DavesnothereCA]

All I can currently recall are Class AB amps with 2 or 4 x EL34 tubes - a tube rated for Class B operation. No grid current values are provided in the data sheets but GEC (UK) cover this subject in their Design Handbook "An Approach to AF Amplifier Design" - see attached file extracts from pages 5 and 7

View attachment 2870528

View attachment 2870529

View attachment 2870530....

~
I see that this design uses a cathode follower topology for the driver stage on the left side of the drawing, and that it does not itself split the phase to serve the power stage (the driver stage has two inputs rather than one), therefive also needing a 'pre-driver' stage (not shown) to do that.

I should check again to see what Garnet did in their implementation.
~

 

[Rodslinger]

~
I do not recall which amp models, but I recently examined a compilation of old schematics from a Canadian company called Garnet, and they for the most part did NOT copy Fender's driver stage such as Marshall and Traynor and many others were known for doing.

In fact, some of the Garnets actually used a transformer to couple the driver stage to the output stage.

(BTW, other models of theirs used a split output single triode driver, RC coupled from the plate and cathode to get the phase relationship correct to feed the push-pull o/p.)

I did not know any of these details at the time that those amps were in production (I lived in Traynor territory, and Garnet was less common here), nor how these designs affected their performance and sound.

Oh, and back in the day, Garnet had a big bass amp head or two, something like an SVT, as did Traynor.
~

garnetamps.com/specs.htm

 

[Drgonzonm]

~
I do not recall which amp models, but I recently examined a compilation of old schematics from a Canadian company called Garnet, and they for the most part did NOT copy Fender's driver stage such as Marshall and Traynor and many others were known for doing.

In fact, some of the Garnets actually used a transformer to couple the driver stage to the output stage.

(BTW, other models of theirs used a split output single triode driver, RC coupled from the plate and cathode to get the phase relationship correct to feed the push-pull o/p.)

I did not know any of these details at the time that those amps were in production (I lived in Traynor territory, and Garnet was less common here), nor how these designs affected their performance and sound.

Oh, and back in the day, Garnet had a big bass amp head or two, something like an SVT, as did Traynor.
~

class B is a tough application, as the grid is suppose to be a high current drawer,. Some circuits use an additional stage of amplification to drive the power amp grid. The transformer as a PI, can do the job. Gibson used a transformer as a PI in some of its old amps.
The 400ps, may be an aB2 amp, looking at the PI on that amp.
It tough to get a distortion free amp in class B, some where in my files is a class B amp that was powered by a 6v car battery, the 6tubes of choice were 6f6s,

 

[xk49w]

class B is a tough application, as the grid is suppose to be a high current drawer,. Some circuits use an additional stage of amplification to drive the power amp grid. The transformer as a PI, can do the job. Gibson used a transformer as a PI in some of its old amps.
The 400ps, may be an aB2 amp, looking at the PI on that amp.
It tough to get a distortion free amp in class B, some where in my files is a class B amp that was powered by a 6v car battery, the 6tubes of choice were 6f6s,

The Music Man amplifier output stage was (is?) a grounded-grid push-pull design. It looks to me like that topology is able to run close to class-B. The cathode current is determined by a current source (bipolor collector) rather than by the tube's transconductance function, which changes quite a bit near tube cutoff. So, it should be more efficient while maintaining linearity at the crossover region. It can also supply grid current and operate with a positive grid voltage, and can take advantage of the increase in available plate current thereby. The plate current can approach double that available from a conventional AC coupled grounded-cathode output stage that does not operate with positive grid voltage. (See attached image)

I'm not seeing any power output advantage from running with positive grid voltage. Even though more plate current is available, the power output is still limited by the tube dissipation ratings regardless of topology. Maybe the higher efficiency helps there. It should be immune from overdrive bias shift and a resulting increase in crossover distortion though. Guitar players probably would not be amused.

6L6-GC Ip curves showning positive grid voltage and grid current.

 

[agedhorse]

There's a lot being made of grid current, but in practice it's very small, it's intermittent, and on guitar amps it only becomes a factor under very heavy overdrive conditions. Rarely (with typical drive methods) is is an issue, and G1 power dissipation due to grid current when heavily overdriven is small because the G1 voltage (with respect to the cathode) is also small.

Since I do much of my design around 6L6 tubes, that's a very forgiving tube for this on top of it all. Even so, it's not nearly as big of a deal as it's being made to be IMO & IME.

 

[xk49w]

Since I do much of my design around 6L6 tubes, that's a very forgiving tube for this on top of it all. Even so, it's not nearly as big of a deal as it's being made to be IMO & IME.

Horse, I see no big deals, at least from me. I was just trying to have a conversation (evidently one-sided ) about what I find to be an interesting technical aspect of tube operation. I grant it is waay off topic, but that seems to be the norm around here after a dozen or so pages.

That said, I have never seen burned grid stopper resistors. But I have seen amplifiers oscillate without them and will not rebuild a Leslie amplifier without them being added. Some tubes ain't what they used to be.

 

[agedhorse]

Amps can certainly oscillate without grid stoppers, that's probably the most valuable use for them! My comments were specifically around burned grid resistors, which I can't recall the last time I have seen one, but perhaps they really meant SCREEN resistors, which I have of course seen burned open.

 

[ThisBass]

Of course I well know about the meaning of the grid-stopper.
At the other hand side (especially with electric 6-string preamps) they are often not there for sound goal reasons. The absence can help to get a distinct "pithy" sound at overload range when the positive half period saturates more gently while negative half period of the signal saturates more "flattened".

 

[DavesnothereCA]

....SCREEN resistors, which I have of course seen burned open.

~
Those I certainly have seen, but the control grid resistors burning up, I honestly cannot say for sure that I have ever run into that happening, though I do understand the circumstances being stated under which it could occur.
~

 

[DavesnothereCA]

....That said, I have never seen burned grid stopper resistors. But I have seen amplifiers oscillate without them and will not rebuild a Leslie amplifier without them being added....

~
....and for what little they cost to add, if you're already inside.
~

 

[AE1]

Horse, I see no big deals, at least from me. I was just trying to have a conversation (evidently one-sided ) about what I find to be an interesting technical aspect of tube operation. I grant it is waay off topic, but that seems to be the norm around here after a dozen or so pages.

That said, I have never seen burned grid stopper resistors. But I have seen amplifiers oscillate without them and will not rebuild a Leslie amplifier without them being added. Some tubes ain't what they used to be.

Thanks for the graph - provides a missing link and tells its own story

As for "waay off topic", the thread is SS Class D v Tube Amps. Tube guitar/bass amps mostly operate in Class AB so the comparison must inevitably be between Class D SS and Class AB tube

The discussion was about deliberate overload distortion in the power stage and how that might be achieved in a Class D amp - compared with using dummy load devices with tube amps. Aged Horse quoted a figure of 10% at rated max power so we are talking about the range of progressive distortion between and 10%

DavesnothereCA said: ↑
~
OK, it could have been some other factor which the lead players like about the way in which tube amps broke up when pushed, but that was what anecdotally reached me at the time.

And as I am sure that you recall, companies came out with dummy load boxes (such as from Altair and Tom Scholz), to let a player variably dial down the final volume to the speaker while pushing a power stage just the right amount, whatever THAT was. ~

Seems everyone is forgetting that the "B" in "AB" means Class B

In my understanding that the transition into Class B operation has the effect that grid current will flow during that part of the cycle when the grid is driven positive - i.e. above zero

That current, which is audio signal current - must be supplied from somewhere - normally the driver stage

So to clarify - In a fixed bias system at zero signal the grid bias is determined by the DC bias supply. MI tube amps typically include a DC bias adjustment mechanism to one or both power tubes to optimise performance and limit the current flowing through the tubes to a safe level within dissipation limits. At zero signal audio power output to the speaker would normally be zero (residual hum and noise excepted).

To cause a change in plate current and therefore power output it is necessary to change the grid bias positively or negatively as applicable in response to the signal amplitude. That task is performed by the driver stage immediately preceding the power tubes

The advantage of transformer drive is that phase splitting is automatic, accurate, reliable and can be driven by a single power tube (the transformer does the phase inversion)

The advantage of RC coupling is that it saves cost and weight but introduces significant resistances into the grid bias supply circuit. It is also challenging to achieve exactly equal signal drives to each tube. Many of the vintage tube amps use half-wave rectifiers, further reducing the capability of the power supply to maintain a stable DC bias under grid current conditions. Application of negative feedback from the speaker further complicates the system as it tries to self-correct.

Taking into account the "duty cycle", which thanks to various contributors we now know is about 12.5 % for a bass amp, we can say that Class B operation will only typically occur in the asymmetrical peak of the first half-cycle in a normal note - and when it does it will only be for an extremely brief time period

So while the 6L6 curves show substantial grid current for all positive grid voltages, in practice it will not be for long. The average plate current over the full cycle and subsequent cycles will be significantly less and it must therefore be assumed that this is the reason Class AB bass amps are not self-destructing on a massive scale

But the graph also shows that if a 6L6 amp is overdriven continuously - such as with sustained echo - then the ball game changes

Also if the duty cycle equivalent is sufficiently compressed it will no longer be 12.5% but something greater. If the dynamic transient peak at the start of the waveform is removed by compression then the average wave must be of greater area under the curve - ie have a higher average power value

The conclusion then is that if a bass amp is played within its design limits it will do a great job reliably but if pushed too far anything can happen along the path to self-destruction

The discussion on grid resistors has drifted into a distraction from the main thread. When one is faced with an amp which has shorted power tubes, shorted output transformer, shorted power transformer and fused grid stoppers then one can only speculate as to a first cause. I tried to present an explanation for one possibility.

For me, seeing is believing, so if you have not experienced these events then it is difficult to convince you. The doubters are also prepared to abandon proven textbook theory as to how tubes work.

Despite many posters suggesting tube amps are a historical novelty, there are still many bass players out there who are not only using them but prefer to buy them new instead of cheaper SS alternatives. To each his own.

They are the folks I have been talking to.

 

[Drgonzonm]

Amps can certainly oscillate without grid stoppers, that's probably the most valuable use for them! My comments were specifically around burned grid resistors, which I can't recall the last time I have seen one, but perhaps they really meant SCREEN resistors, which I have of course seen burned open.

I was able to purchase a 67 fender head about 25 years ago because one of the 6l6's 470 ohm resistors had burnt. Bought the head for $25 and replaced the resistor, amp worked well for another ten years. It now needs a cap job.
I too was confused on the grid resistor discussion, when I read this portion of the thread, I thought the discussion was about grid stoppers on triodes, so I made my comments regarding the failure of the grid stoppers., not the grid resistor on power tubes.
This discussion added to my limited knowledge of tube amps and class D amp design.

As followup, isn't that 68k resistor at the input Jack on 50s fenders, put there to as a low pass filters

 

[xk49w]

Thanks for the graph - provides a missing link and tells its own story

As for "waay off topic", the thread is SS Class D v Tube Amps. Tube guitar/bass amps mostly operate in Class AB so the comparison must inevitably be between Class D SS and Class AB tube

The discussion was about deliberate overload distortion in the power stage and how that might be achieved in a Class D amp - compared with using dummy load devices with tube amps. Aged Horse quoted a figure of 10% at rated max power so we are talking about the range of progressive distortion between and 10%



Seems everyone is forgetting that the "B" in "AB" means Class B

Class B by definition requires grid current to flow during all or part of the cycle

That current, which is audio signal current - must be supplied from somewhere - normally the driver stage

So to clarify - In a fixed bias system at zero signal the grid bias is determined by the DC bias supply. MI tube amps typically include a DC bias adjustment mechanism to one or both power tubes to optimise performance and limit the current flowing through the tubes to a safe level within dissipation limits. At zero signal audio power output to the speaker would normally be zero (residual hum and noise excepted).

To cause a change in plate current and therefore power output it is necessary to change the grid bias positively or negatively as applicable in response to the signal amplitude. That task is performed by the driver stage immediately preceding the power tubes

The advantage of transformer drive is that phase splitting is automatic, accurate, reliable and can be driven by a single power tube (the transformer does the phase inversion)

The advantage of RC coupling is that it saves cost and weight but introduces significant resistances into the grid bias supply circuit. It is also challenging to achieve exactly equal signal drives to each tube. Many of the vintage tube amps use half-wave rectifiers, further reducing the capability of the power supply to maintain a stable DC bias under grid current conditions. Application of negative feedback from the speaker further complicates the system as it tries to self-correct.

Taking into account the "duty cycle", which thanks to various contributors we now know is about 12.5 % for a bass amp, we can say that Class B operation will only typically occur in the asymmetrical peak of the first half-cycle in a normal note - and when it does it will only be for an extremely brief time period

So while the 6L6 curves show substantial grid current for all positive grid voltages, in practice it will not be for long. The average plate current over the full cycle and subsequent cycles will be significantly less and it must therefore be assumed that this is the reason Class AB bass amps are not self-destructing on a massive scale

But the graph also shows that if a 6L6 amp is overdriven continuously - such as with sustained echo - then the ball game changes

Also if the duty cycle equivalent is sufficiently compressed it will no longer be 12.5% but something greater. If the dynamic transient peak at the start of the waveform is removed by compression then the average wave must be of greater area under the curve - ie have a higher average power value

The conclusion then is that if a bass amp is played within its design limits it will do a great job reliably but if pushed too far anything can happen along the path to self-destruction

The discussion on grid resistors has drifted into a distraction from the main thread. When one is faced with an amp which has shorted power tubes, shorted output transformer, shorted power transformer and fused grid stoppers then one can only speculate as to a first cause. I tried to present an explanation for one possibility.

For me, seeing is believing, so if you have not experienced these events then it is difficult to convince you. The doubters are also prepared to abandon proven textbook theory as to how tubes work.

Despite many posters suggesting tube amps are a historical novelty, there are still many bass players out there who are not only using them but prefer to buy them new instead of cheaper SS alternatives. To each his own.

They are the folks I have been talking to.

AE1, please take no offence but there is so much incorrect about this I'm not even sure where to begin. I will say this though, the definition of class B has nothing to do with drive, tubes or grid current.

 

[AE1]

~
I see that this design uses a cathode follower topology for the driver stage on the left side of the drawing, and that it does not itself split the phase to serve the power stage (the driver stage has two inputs rather than one), therefive also needing a 'pre-driver' stage (not shown) to do that. ~

Sorry DavesnothereCA, the pic was intended to show the transformer drive configuration only

The GEC book from which this came is available on the net and discusses Class B in great detail. Also checkout the Radiotron Designers Handbook 3rd and 4th editions They are old texts but still valid for current production tube amps.

The attached schematic shows a 10W Class B amp from Thordarson in about 1934. Transformer interstage coupling was standard then. Class B was also a common mode for PA until the solid state era.

You may recall the first generation of SS amps had transformer drive for the same reasons - ie to minimise source impedance to the power stage.

The use of a low-impedance cathode follower permits a lower ratio transformer to be used to deliver better frequency response and lower distortion, but conventional plate circuit drivers are usually used for Class AB, which is the most common mode for guitar and bass amps

Cathode-follower drivers are commonly used in hi-fi and some guitar amps but require an extra stage - which adds to cost

Is all a matter of how far into Class B territory the designer or user wants to go.

The attached PA amp schematic from the 1970's shows 4 x EL34 in Class B mode. Plate volts = 790. Grid 2 volts = 410, Grid 1 bias = -39. Zero signal plate current = 18 mA per tube. Rated power out = 130-160 watts. Interstage coupling is RC. It was designed to run cool and be reliable.

 

[agedhorse]

AE1, please take no offence but there is so much incorrect about this I'm not even sure where to begin. I will say this though, the definition of class B has nothing to do with drive, tubes or grid current.

Correct, it has to do with quiescent conduction angle and of course there is no grid current for class B until the grid is driven positive with respect to the cathode. Since the grid on a typical class B output stage is biased ~30 - 40V negative with respect to the cathode, no grid current flows, and remains this way until the signal on the grid increases high enough to swing positive relative to the cathode.

IF it was a problem, there are ways to avoid it or protect against it, but since it's not an issue in practice, nobody bothers.

 

[AE1]

AE1, please take no offence but there is so much incorrect about this I'm not even sure where to begin. I will say this though, the definition of class B has nothing to do with drive, tubes or grid current.

I am not and cannot be offended because all I am expressing is my understanding. This is a "forum" is it not?

I agree the scientific definition of Class B is not how I presented it but then there are several definitions about.

Have attached an extract from the Radiotron Designers Handbook 4th edition page 587 which covers much of what I said. This text has been around since 1953. In this definition I am referring to Class B2 in relation to Class AB as used in bass amps.

The cause/effect relationship is the same insofaras when the grid is driven positive grid current flows. G rid current was the focus of my contribution.

Also the attached extract of Chapter 2 of the 3rd edition (1940) gives us a different take.

Obviously Class B in solid state applications will be worded differently because transistors do not have grids or plates.

If your understanding is different then feel free to express it - then we all learn.

I can handle it.

 

[Passinwind]

I was able to purchase a 67 fender head about 25 years ago because one of the 6l6's 470 ohm resistors had burnt.

Those are screen grid resistors, not grid stoppers on the control grid. The former are often designed as a fusible element and when they go it may well be for good reason. I have worked on a few thousand MI tube amps and can't recall a grid stopper on a power tube ever failing without a catastrophic failure elsewhere being responsible.

 

[Chucky Stiletti]

AE1, I think you are misunderstanding what's being presented by that publication, which also doesn't represent any guitar amp output circuit that I am aware of.

I have designed 2 transformer driven output stage amps, commercial products, and the grid currents are not anywhere near 15-20mA with EL84 tubes.

But he's talking about EL34s not 84s...

 

[Chucky Stiletti]

All I can currently recall are Class AB amps with 2 or 4 x EL34 tubes - a tube rated for Class B operation. No grid current values are provided in the data sheets but GEC (UK) cover this subject in their Design Handbook "An Approach to AF Amplifier Design" - see attached file extracts from pages 5 and 7

View attachment 2870528

View attachment 2870529

Fig 1.2 - see attached file

View attachment 2870530

What this publication tells us is that grid current in full Class B mode is around 15-20 mA - enough to offset much of the grid bias in an RC coupled driver stage

The data sheets for EL34 state a grid 1 circuit resistance of 700k Ohms per tube for Class AB is permissible. At -40VDC only 0.6 mA is required through each grid resistor to offset the bias completely.

This is why transformer drive is desirable - ie to supply sufficient AC voltage and current to the power tubes to prevent reduction of bias but transformers are not normally seen in guitar amps. But transformer drive is not without its challenges either.

Musicman produced a series of successful amps using solid state driver stages - a novel solution

As to "misuse", well I have already said enough on that subject - maybe someone else has a view on this

Yes, I remember those old MM amps. They drove the output tubes in a "quasi" tube/FET cascode configuration where the tube was being driven in a common grid topology by the signal being driven into the tube via the cathode from the drain of the FET . This stage also provided Z transformation as the input of the tube is low Z and it's plate output high Z. Very clever and novel as far as guitar amp output stages were concerned at the time. I believe the got a patent for that. I like how those amps sound.

 

[Drgonzonm]

Those are screen grid resistors, not grid stoppers on the control grid. The former are often designed as a fusible element and when they go it may well be for good reason. I have worked on a few thousand MI tube amps and can't recall a grid stopper on a power tube ever failing without a catastrophic failure elsewhere being responsible.

the grid stoppers are typically found on the g of a triode, its confusing when the nomuclare (bad spelling) we use has multiple names for the parts on the tubes. The grid and the screen, I believe is same component on a triode, and on some pentodes, you have three grids, others a beam deflector, and two grids. And you can have a grid stopper and a grid resistor on the same tube, but not the same grid.
Now that I have been very verbose, the 470 ohm resistor I replaced was a grid resistor, and not a grid stopper.