Solid state/class d vs tube amps

[Drgonzonm]

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.



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.

regarding grid resistors, I would look at the upstream amplifier to determine what the loadline is, then verify the wattage is adequate on the grid resistor. The common 12ax7 is capable of about a watt output.
Regarding the OT, Imo, the pp amps have too large of caps in the b rail, and you see voltage spikes on the primary windings that cause insulation failure, the OTs typically have class A insulation, sets temperature limits, could set voltage limits. Old fenders had undersized OTs, bass frequencies are hard on transformers. Five string bass is harder than std 4 string. I would hard at the unit operation at the OT and determine the robustness of the design assumptions. This includes looking at speaker curves.
Remember the builder has to balance what the designer wants versus what the MBA in accounting says.

The availibilty of aftermarket OTs is also driven by the clone market, there only so many baseman out there, and caps do die.

 

[agedhorse]

regarding grid resistors, I would look at the upstream amplifier to determine what the loadline is, then verify the wattage is adequate on the grid resistor. The common 12ax7 is capable of about a watt output.
Regarding the OT, Imo, the pp amps have too large of caps in the b rail, and you see voltage spikes on the primary windings that cause insulation failure, the OTs typically have class A insulation, sets temperature limits, could set voltage limits. Old fenders had undersized OTs, bass frequencies are hard on transformers. Five string bass is harder than std 4 string. I would hard at the unit operation at the OT and determine the robustness of the design assumptions. This includes looking at speaker curves.
Remember the builder has to balance what the designer wants versus what the MBA in accounting says.

Upstream isn't generally the problem, as the grid stop resistor's primary purpose is to set the HF corner on the response which (combined with the global feedback) can otherwise cause the amp to oscillate. A nice byproduct is that under heavy drive conditions when the grid may conduct, again dual purposes are served... limiting the grid current which also reduces the distortion (changes the distortion components too) of the driving stage. The currents are still pretty small, so a failure is very uncommon without a catastrophic failure such as a grid to plate or screen short.

The value of the caps on the B+ have nothing to do with the voltage spikes (and larger caps would REDUCE any conducted back-emf spikes anyway, much the way larger caps can limit bus supply pumping effects on a non-bridged class D amp).

 

[Chucky Stiletti]

[

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.

Hmmm, not sure about that. Agree with violating the maximum rated Ic/Id on the ss amp. This will surely cause device breakdown/failure.

With the tube amp, an unloaded secondary reflects no Z back to the primary so that excessive current will flow through the primary opening the winding at some point.

The mechanism of EMF “kickback” you describe is more likely to damage an output tube before it damages the transformer.



Plate to grid shorts can happen with most designs. Even mechanical shock can responsible for this failure mode. I was wondering if you observed this on any particular design repeatedly.

 

[agedhorse]

[



Hmmm, not sure about that. Agree with violating the maximum rated Ic/Id on the ss amp. This will surely cause device breakdown/failure.

With the tube amp, an unloaded secondary reflects no Z back to the primary so that excessive current will flow through the primary opening the winding at some point.

The mechanism of EMF “kickback” you describe is more likely to damage an output tube before it damages the transformer.



Plate to grid shorts can happen with most designs. Even mechanical shock can responsible for this failure mode. I was wondering if you observed this on any particular design repeatedly.

Back EMF can cause breakdown in any area where this voltage exceeds the insulation breakdown limits. This can occur within a transformer as well (where would depend on how the transformer was constructed as well as the insulating materials used). On commercial designs, OT's are often evaluated for various faults (such as these) to insure end user safety.

Generally the open circuit of the primary is do to collateral damage from a voltage arc-over. With an open primary, the current flow (except reactive currents) are pretty low, the primary's inductance at AF governs this.

These are general statements because there are plenty of possible exceptions if you look hard enough. I'm just trying to discuss some of the more commonly seen failures rather than stretching for the weeds.

 

[beans-on-toast]

Old fenders had undersized OTs, bass frequencies are hard on transformers.

Yes. By adding a high pass filter in the amp, they reduced the low frequencies. Now the transformer doesn't need to be capable of reproducing the lowest frequencies, they save on the cost of the output transformer. Installing an enhanced transformer can help but it never hurts to also examine how the low end is being limited in the amp design. Of course, then you might need to beef up the power supply so it will have the reserves avaiable to reproduce those low notes.

Always a lot to consider.

 

[Drgonzonm]

Yes. By adding a high pass filter in the amp, they reduced the low frequencies. Now the transformer doesn't need to be capable of reproducing the lowest frequencies, they save on the cost of the output transformer. Installing an enhanced transformer can help but it never hurts to also examine how the low end is being limited in the amp design. Of course, then you might need to beef up the power supply so it will have the reserves avaiable to reproduce those low notes.

Always a lot to consider.

compare the fender baseman schematics to similar schematics of other models, many times you will find the baseman output transformer to be physically larger. If you look at the replacement. Transformers, for bassman typically have lower frequency rating.
It is my understanding, that power sag is part of the flavor of the 5f6a bassman, so reducing it too much might change amp quality.

 

[beans-on-toast]

compare the fender baseman schematics to similar schematics of other models, many times you will find the baseman output transformer to be physically larger. If you look at the replacement. Transformers, for bassman typically have lower frequency rating.
It is my understanding, that power sag is part of the flavor of the 5f6a bassman, so reducing it too much might change amp quality.

That’s very true, sag is part of the 5F6a character. It is a heavy amp as well.

My 80’s reissue Bassman came with a solid state rectifier module plugged into the socket intended for the tube. Not sure if it was a money saving tactic or if they wanted people to try the amp with less sag. Maybe a little of both. I have a Marshall Bluesbreaker as well and they are different performing amps. The small changes in component values make a big difference.

There is an interesting technical book that provides an in depth analysis to the 5F6A circuit as well as the Marshall Bluesbreaker and 50W plexi amps based on the Fender circuit.

Fender Bassman 5F6A Book

 

[Drgonzonm]

That’s very true, sag is part of the 5F6a character. It is a heavy amp as well.

My 80’s reissue Bassman came with a solid state rectifier module plugged into the socket intended for the tube. Not sure if it was a money saving tactic or if they wanted people to try the amp with less sag. Maybe a little of both. I have a Marshall Bluesbreaker as well and they are different performing amps. The small changes in component values make a big difference.

There is an interesting technical book that provides an in depth analysis to the 5F6A circuit as well as the Marshall Bluesbreaker and 50W plexi amps based on the Fender circuit.

Fender Bassman 5F6A Book

there is web site that references the book. On that site includes some work from a german university showing an analysis of the impact of the b-rail filter design. I have pdf copies of those short articles. For now I will rely on the articles. I have book on my wish list.

 

[beans-on-toast]

there is web site that references the book. On that site includes some work from a german university showing an analysis of the impact of the b-rail filter design. I have pdf copies of those short articles. For now I will rely on the articles. I have book on my wish list.

I like the english translation of Ohms original article where he presented what we call Ohm’s Law.

 

[agedhorse]

Note that size of an output transformer does not have to correlate with the low frequency limits... there are different core materials and geometries that can be used to reduce the size while maintaining the low frequency response. This typically costs more, but is another tool in a designer's handbook.

 

[J-Bassomatic]

I don't understand any of this in a practical way but it's fascinating to read about the knowledge behind the amplification we take for granted.
AgedHorse said (I don't know how to do the @agedhorse thing)(Oh wait, I did it):"On commercial designs, OT's are often evaluated for various faults (such as these) to insure end user safety." That's kind of spooky really, under certain situations these things can actually kill you. Hats off to you guys and women who design these amps for us.

 

[Drgonzonm]

Note that size of an output transformer does not have to correlate with the low frequency limits... there are different core materials and geometries that can be used to reduce the size while maintaining the low frequency response. This typically costs more, but is another tool in a designer's handbook.

if you take a given core material, a given laminate thickness, a given winding pattern, the larger transformer will have a lower frequency respounce. I am not disagreeing with you. I don't know if any one is winding with square wire, but in electric motors, it provides a greater wire density.

 

[agedhorse]

if you take a given core material, a given laminate thickness, a given winding pattern, the larger transformer will have a lower frequency respounce. I am not disagreeing with you. I don't know if any one is winding with square wire, but in electric motors, it provides a greater wire density.

I was comparing 2 transformers with identical performance, the cores (and wire length) can be smaller with different materials. You can't always judge a transformer by its size unless you know that the materials are the same. The constant arguments that "the bigger transformer is always the better transformer, any idiot know that" simply shows who the real idiot is. There's a lot more to transformers than size.

 

[Drgonzonm]

I was comparing 2 transformers with identical performance, the cores (and wire length) can be smaller with different materials. You can't always judge a transformer by its size unless you know that the materials are the same. The constant arguments that "the bigger transformer is always the better transformer, any idiot know that" simply shows who the real idiot is. There's a lot more to transformers than size.

no argument.

 

[AE1]

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?

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





Fig 1.2 - see attached file



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

 

[agedhorse]

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.

 

[DavesnothereCA]

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.

~
I was thinking about the failure of power tubes taking them out too, though some of the other ideas could happen, and in certain cases even draw a chuckle, in addition to the excess current.

Now to be clear, have we been referring to resistors in series with the control grid, or the screen grid (or both) ?

EDIT : As I read more posts, I see that they look to be the ones on the control grid - though I have run into individual screen grid resistors failing after a tube failure too.

BTW, frequently the screen resistors were mounted directly on each tube socket in the case of some output tube types (NOT that way for 7027s, where the extra socket pins are live from the tube itself).
~

 

[DavesnothereCA]

....Remember the builder has to balance what the designer wants versus what the MBA in accounting says....

~
Oh, THAT again !
~

 

[DavesnothereCA]

....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....

~
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.
~

 

[agedhorse]

I am talking specifically about the control grid.