Solid state/class d vs tube amps

[Cuzzie]

@seamonkey I think you prob have more experience than I reading your post, and I think you are probable right, but to my ears, it’s not quite nailed on, as close as damnedest and we are probably splitting hairs. It’s just from what I have heard from these floor units and kempers.
Studios I think they are about nailed, live, to be fair to it it’s not quite as good as it is in the studio, but it’s not bad by any means

 

[AE1]

Except that on a properly designed amp, load boxes, power soaks, and such devices are used all the time without any of the issues that you are postulation.

You are trying WAY too hard.

Is a fine line between pleasure and pain

 

[mesaplayer83]

Is a fine line between pleasure and pain

I'd still like to see some of the amps you've built...

 

[AE1]

~
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.~

I responded by saying it is risky to consistently excessively overdrive a tube amp.

"Aged Horse" responded to that with:

Except that on a properly designed amp, load boxes, power soaks, and such devices are used all the time without any of the issues that you are postulation.

That confirms the original conclusion from "DavesnothereCA" - i.e. the expert opinion says it's OK to use that method to create distortion.

This thread is about Class D SS v tube amps, so the following questions now arise:-

1:"does a properly designed Class D solid state amp need a load box, power soak, or such device to replicate the kind of distortion experienced with tube amps using such devices ?"

2."what is the ideal Class D SS amp wattage for use with a load box, power soak, or such device ?"

 

[mesaplayer83]

I responded by saying it is risky to consistently excessively overdrive a tube amp.

"Aged Horse" responded to that with:



That confirms the original conclusion from "DavesnothereCA" - i.e. the expert opinion says it's OK to use that method to create distortion.

This thread is about Class D SS v tube amps, so the following questions now arise:-

1:"does a properly designed Class D solid state amp need a load box, power soak, or such device to replicate the kind of distortion experienced with tube amps using such devices ?"

2."what is the ideal Class D SS amp wattage for use with a load box, power soak, or such device ?"

I'd still like to see some of the amps you've built...

 

[BadExample]

I responded by saying it is risky to consistently excessively overdrive a tube amp.

"Aged Horse" responded to that with:



That confirms the original conclusion from "DavesnothereCA" - i.e. the expert opinion says it's OK to use that method to create distortion.

This thread is about Class D SS v tube amps, so the following questions now arise:-

1:"does a properly designed Class D solid state amp need a load box, power soak, or such device to replicate the kind of distortion experienced with tube amps using such devices ?"

2."what is the ideal Class D SS amp wattage for use with a load box, power soak, or such device ?"

I think no universal answer to that... Mostly no. There may be an exception, but if you add another load in parallel with a given cab, the cab will see more or less the same wattage as without the load and all you'll do is have a small space heater with the dummy load. Same goes with adding a 2nd cab. More a matter of selecting components that meet your needs.

 

[BadExample]

I'd still like to see some of the amps you've built...

+1 out of curiosity. You could post some pics in the amp gut shots thread. DIY stuff is always interesting to see.

 

[Bad azz tone]

I love the di signal I get out of my tonehammer for studio use. Nice and full and gets close in a studio mix. The ag500 not so much, too tight and articulate to fill the studio mix.

Now I would love to get a similar vibe going on live

I have of course turned every knob there is hehe. The problem is usually with the meaty low mids. I either get a nice round bottom on low notes but lose on the upper register resulting in thin upper notes, or I get a nice and beefy g and d string but get overwhelmed on the e string. Could a hpf filter do the trick perhaps?

Somebody mentioned the tube preamps, which is also something I might try out.

Do you use any kind of eq?

 

[AE1]

+1 out of curiosity. You could post some pics in the amp gut shots thread. DIY stuff is always interesting to see.

Will pass on that. They are out there in the wide world somewhere and have no pics.

My personal 100 watt all tube bass amp with 6 x 12 inch drivers ran out of puff trying to match the 150 watt lead heads as far back as the surf music era in the 60's. Have been personally using SS for the past twenty five years for live bass because it is way more powerful than any tube amp I could ever build or buy for the same weight and functionality - eg a 500 watt rms tube amp will weigh in at about 60-80 lbs or more - well beyond my lifting capacity.

e.g. the Hammond 1650W transformer weighs in at 28 lbs and is rated 280 watts rms. Need two of those for a quality DIY 500 watts tube mono or stereo plus an equivalent power transformer and choke. Custom made transformers for DIY are very expensive. The situation gets worse with higher power levels

The same phenomena is now occuring with Class D displacing Class AB in solid state amps

I still do the occasional upgrade of a commercial tube amp because there are some people out there who are not satisfied with what they have. Is much easier than designing and building a custom amp from nothing but an idea.

In my case it would be a waste of time and resources to try to build a tube amp with built in preamp which would match some of the hybrid solid state preamps built with custom chips.

Check out this preamp from 25 years ago - has 9 x 12AX7 tubes with a pile of IC's - is not something the typical DIY'er would want to tackle but is still readily available at a fraction of original cost. Anything less would not be competitive.

As the posts in these pages show, many players now prefer Class D for all the reasons they state, but tube amps will still be around for a long time because of their unique qualities - but for my needs in bass I use SS exclusively

I have posted a personal mod at Amp Guts Photos - Official Thread - Lets see those pics

My posts have been intended to inform - not to generate angst among the disbelievers. Stay with what works for you.

 

[ThisBass]

That confirms the original conclusion from "DavesnothereCA" - i.e. the expert opinion says it's OK to use that method to create distortion.

No its not cause (for example) 1/3 duty cycle means plenty of distortion which is NOT the same as full power 100% duty cycle

edit. may it be that duty cycle number still remains as an incrombehenible property regarding audio program material?

 

[xk49w]

In the case of typical power tubes as used in guitar and bass amps, when the grid voltage approaches zero in a push-pull power tube stage the grids commence to lose control over plate current, causing the tubes to move towards the Class B region of operation. This change rapidly increases distortion because the positive signal plate current is no longer directly proportional to the signal - ie linearity is lost.

That's a bit of a glossing over what really happens. In fact, plate current is controlled by the grid for even positive grid voltages, though there will be considerable grid current when this happens. Maybe it's best to say that in a heavily driven AC-coupled tube output stage a bias shift can occur which reduces the tube conduction angle, sometimes drastically. There are circuit tricks that address this (see the Paul Ruby Zener diode mod).

1:"does a properly designed Class D solid state amp need a load box, power soak, or such device to replicate the kind of distortion experienced with tube amps using such devices ?"

Not really. The Quilter guitar amplifiers have tube amp overdrive emulation circuitry followed by a linear class D output stage with an output level control. Does his bass amp? I dunno. Pat Quilter holds a patent on solid-state techniques that duplicate over-driven tube amp characteristics, e.g., sag, crossover distortion, etc.

This article discusses his overdrive shaping, even some basic class D theory. There is also a link to the patent.

What's the Magic Behind Quilter Amps?

 

[DavesnothereCA]

Except that on a properly designed amp, load boxes, power soaks, and such devices are used all the time without any of the issues that you are postulation.

You are trying WAY too hard.

~
Happy New Year ! - to you and everyone else reading

In my post to which AE1 was replying, I did qualify my comment by stating "....while pushing a power stage just the right amount....".

I suspect and hope that this sweet spot would be reached somewhat before anything destructive (such as what AE1 suggested) would/could occur, at least in a well-designed amp.
~

 

[DavesnothereCA]

Here is an attempt at one explanation option

IMHO, might have something to do with the control grids (G1) being driven positive in a Class AB tube amp fixed bias system

All power tubes draw low levels of control grid (G1) current due to tiny levels of residual gas in the tube - i.e. an imperfect vacuum

In the case of typical power tubes as used in guitar and bass amps, when the grid voltage approaches zero in a push-pull power tube stage the grids commence to lose control over plate current, causing the tubes to move towards the Class B region of operation. This change rapidly increases distortion because the positive signal plate current is no longer directly proportional to the signal - ie linearity is lost.

At that amplitude of signal swing drive voltage the opposite negative swing signal will most likely cutoff the tube altogether, resulting in true Class B

When progressively driven further positive above zero, the positive grid voltage increases to a more positive potential than the real cathode, causing the metal grid electrodes to become a "virtual cathode" alongside the real cathode, thereby also supplying the plates with DC grid current drawn directly from the fixed bias supply. This causes a positive voltage drop across the grid resistors, offsetting the negative bias, and further increasing plate current

At the same time, the limited AC signal current from the preceding driver stage, which normally controls the bias voltage, diminishes its capacity to control drive bias, also increasing distortion.

A sign of this malfunction is burned out grid resistors - common in some brands - resulting in complete loss of bias and potential self-destruction of the associated power tube

At the same time as all the above, because plate current is now excessive, the power supply is likely to lose its capacity to maintain plate voltage and/or current (sag). Tube rectifier power supplies typically sag earlier than solid state. As the power supply voltage progressively decreases, the capacity of the power tubes to deliver power also decreases, causing the whole output system to collapse in a heap.

Most guitar and bass tube amp fixed bias supplies are half-wave with limited low current capacity so when overloaded they quickly lose their capability to maintain bias voltage at the source

So what you have is the supply end of the bias resistors at less negative potential caused by bias power supply sag and the other end (G1) at less negative potential caused by grid current passing through the bias resistor - both interactively causing an increase in plate current

Add all the above in the mix with an undersized power transformer and output transformer and you get a wide range of design options for manipulating the behaviour of the tube amp power stage

Amp designers have no control over individual tube characteristics so it would be unreasonable to expect them to design for unlimited grid current needs in a conventional guitar or bass amp. Even if they did, the player could just crank up the volume a bit further to replicate the same overload condition

So players, the power is in your hands

~
Interesting series of events !

But it would seem to require a 'perfect storm', as some folks call it, in order for all of that stuff to snowball and cause an amp failure, yes ?
~

 

[DavesnothereCA]

....This thread is about Class D SS v tube amps, so the following questions now arise:-

1:"does a properly designed Class D solid state amp need a load box, power soak, or such device to replicate the kind of distortion experienced with tube amps using such devices ?"

2."what is the ideal Class D SS amp wattage for use with a load box, power soak, or such device ?"

~
I had not considered what you just asked, but now that you have, I too would be curious as to what would happen to the character of the output signal, if a user deliberately power soaked and pushed a Class D solid state amp, and for that matter, to compare if they did the same with a Class AB solid state amp, and whether various makes and models of each class might behave differently.

We pretty well know how tube amps behave under such operation, but not much I have heard about ANY class of s/s amp run that way.
~

 

[agedhorse]

~
I had not considered what you just asked, but now that you have, I too would be curious as to what would happen to the character of the output signal, if a user deliberately power soaked and pushed a Class D solid state amp, and for that matter, to compare if they did the same with a Class AB solid state amp, and whether various makes and models of each class might behave differently.

We pretty well know how tube amps behave under such operation, but not much I have heard about ANY class of s/s amp run that way.
~

It depends on the amp designer's approach to overload management (not unlike with tube amps, because all tube amps and not all class AB solid state amps are not alike in this regard).

 

[AE1]

That's a bit of a glossing over what really happens. In fact, plate current is controlled by the grid for even positive grid voltages, though there will be considerable grid current when this happens. Maybe it's best to say that in a heavily driven AC-coupled tube output stage a bias shift can occur which reduces the tube conduction angle, sometimes drastically. There are circuit tricks that address this (see the Paul Ruby Zener diode mod).

Does not explain why grid stopper resistors and output transformers tend to burn out in some popular tube amps (designed by professionals). Just look at the range of aftermarket replacement transformers available. If they were not needed they would not be made.

Not really. The Quilter guitar amplifiers have tube amp overdrive emulation circuitry followed by a linear class D output stage with an output level control. Does his bass amp? I dunno. Pat Quilter holds a patent on solid-state techniques that duplicate over-driven tube amp characteristics, e.g., sag, crossover distortion, etc.

You have nicely verified my earlier post that it is smarter to do the distorting in the preamp/driver stages than to overload the power stage. The preamp method delivers the required distortion at all levels - not just when overloaded.

The key term in the above post is "emulation".

Thank you.

 

[Drgonzonm]

Pure speculation on my part regarding grid stopper, too much current from upstream sources. Even the lowly 12ax7 has a max output of 1w, and what is the watt rating of the grid stopper.
Burnt out OT's, a couple of guesses, playing bass on a guitar amp, particularly 5 string. Too large of caps in b rail filtering system. , too small of OT. Spilt beer. Wall voltage?

 

[agedhorse]

A more common cause of OT failure is driving the amp at high output with an intermittent speaker cable, or one that goes open.

Very rarely have I seen failed grid stopper resistors. Sometimes with a catastrophic tube failure depending on the circuit design.

 

[Chucky Stiletti]

A more common cause of OT failure is driving the amp at high output with an intermittent speaker cable, or one that goes open.

Very rarely have I seen failed grid stopper resistors. Sometimes with a catastrophic tube failure depending on the circuit design.

Funny how a tube amp can almost run into a short circuited load and sustain no damage while a ss (regardless of class) would cook. Conversely, a SS amp can run very lightly loaded all day long. I guess this brings me to what in my opinion is the crux when comparing conventional tube bass amplifiers to ss ones and that would be their AC coupling. As a bass player, “AC” intuitively implies a high pass function, something I would want to steer clear from when trying to faithfully reproduce 45 Hz or less at high SPL levels. Here the concept of “a wire with gain” is paramount. Understand that when the SVT was designed a ss option with the same characteristics would have been unfeasible and unrealistic. Increasing Watt to weight ratios has proven the way forward commercially. Why people continue to beat the dead horse of tube bass amplification is beyond me..


On another note, can you elaborate on the designs that you observed catastrophic tube/grid stopper resistor failures on? Were they topology (and I ask because in grid leak bias transmitters this is common) issues? Misuse? Both?

 

[agedhorse]

Funny how a tube amp can almost run into a short circuited load and sustain no damage while a ss (regardless of class) would cook. Conversely, a SS amp can run very lightly loaded all day long. I guess this brings me to what in my opinion is the crux when comparing conventional tube bass amplifiers to ss ones and that would be their AC coupling. As a bass player, “AC” intuitively implies a high pass function, something I would want to steer clear from when trying to faithfully reproduce 45 Hz or less at high SPL levels. Here the concept of “a wire with gain” is paramount. Understand that when the SVT was designed a ss option with the same characteristics would have been unfeasible and unrealistic. Increasing Watt to weight ratios has proven the way forward commercially. Why people continue to beat the dead horse of tube bass amplification is beyond me..


On another note, can you elaborate on the designs that you observed catastrophic tube/grid stopper resistor failures on? Were they topology (and I ask because in grid leak bias transmitters this is common) issues? Misuse? Both?

AC coupling has little to do with an amp's ability to survive either almost shorted or open loads. It has to do with the different mechanisms involved. A solid state amp (almost always) has a much lower output impedance than a tube amp, so under shorted conditions it will try to deliver all the current available. This means that the devices may be overloaded (short term or longer term) unless good protection mechanisms are employed. On the other hand, tube amps have much higher source impedances, so a short circuit will draw whatever current it can but the source impedances tend to limit this short term, though the tubes will age more quickly and it's possible to overheat the output transformer (long term). Open circuits on a tube amp are a different story. Since there is significant inductance in the OT primary, when the amp is driven (especially into clipping), there can be large back EMF generated which exceeds the safe operating voltages of the tubes and transformer insulating systems, breaking down and causing flashover.

So, there are different mechanisms responsible for the failures observed in the different types of amps.

As far as grid stopper resistors, a short from plate to grid can be one fault that can occur from a catastrophic output tube failure. This may also take out components in the PI stage, and even bypass caps in cathode followers as the fault's damage propagates backwards.