Solid state/class d vs tube amps

[AE1]

~

The octal driver tube was likely a carryover from radio and TV days, and a 6SN7 was pretty common in many older B&W TVs (vertical oscillator ?), I think being superceded by the 12AU7 when the industry went to miniatures.

The 6CG7 and 6FQ7 are a 6SN7GT in a miniature 9 pin bottle

Octal tubes tend to run cooler than their miniature cousins because of a higher bottle surface area for heat dissipation, so there is real benefit in retaining octal drivers where natural ventilation applies

Octals were replaced with miniatures because of lower manufacturing and shipping costs, lower socket cost and chassis space reduction. The greatest benefit of miniatures is a reduction of internal capacitances, which enables a very high operating frequency range

The 6SN7 is available in a wide range of builds, with ruggedised "W" versions preferable for portable amplifiers for stage use

The 12AU7 has significantly more gain than a 6SN7

 

[AE1]

IMO, the Garnet designs are great examples of poor engineering. There is a reason most of these circuit elements were never used by the main stream and died in obscurity.

Thank God today's amps are designed by real engineers who do it right every time. No more pioneering experimentation, no more trial and error - just pure science

What a joy it is to live in the Class D digital era

 

[somebrains]

Many of the guys came out of military and commercial experience and were wary of coloring outside the lines too much. Those that did color outside the lines made some terrible amps, horribly unreliable. Not all of them but there are many that lost family fortunes in the quest of amp manufacture.

Architects work this way too.
Stay within the lines and you are safe.
Get too artistic and everyone will finger point you if there is an incident.

Those that go too avant garde get branded and work becomes difficult to secure.
Reliability of your design affects your bottom line.

 

[beans-on-toast]

Speaking of amps that were based on design elements from radios, there were musical instrument amps that didn’t have a power transformer.

One example:
http://harmony.demont.net/documents/schematics/amps/harmony_400a.pdf

Here is another one:

———————-

A Neumann tube mic called the U47 (and U48) used a tube called the VF14. A military radio tube whose heater was designed to run on a line voltage.

The Neumann U 47 Large Diaphragm Tube Condenser Microphone - Vintage King Pro Audio Outfitter

Telefunken in the USA has reissued the mic.
Invalid Link Removed

 

[Drgonzonm]

The 6CG7 and 6FQ7 are a 6SN7GT in a miniature 9 pin bottle

Octal tubes tend to run cooler than their miniature cousins because of a higher bottle surface area for heat dissipation, so there is real benefit in retaining octal drivers where natural ventilation applies

Octals were replaced with miniatures because of lower manufacturing and shipping costs, lower socket cost and chassis space reduction. The greatest benefit of miniatures is a reduction of internal capacitances, which enables a very high operating frequency range

The 6SN7 is available in a wide range of builds, with ruggedised "W" versions preferable for portable amplifiers for stage use

The 12AU7 has significantly more gain than a 6SN7

A single triode on the sn7, can handle more wattage, than the combined output of the two triodes from an au7. Input voltage to screen better than au7.
I believe there are better tubes for PI applications that au7s

 

[agedhorse]

Thank God today's amps are designed by real engineers who do it right every time. No more pioneering experimentation, no more trial and error - just pure science

What a joy it is to live in the Class D digital era

Come on, you are professing all of this crap and then making a joke of those who have designed quality reliable amps of all types.

Good amps were designed by good engineers throughout history, and those amps tended to be safer and more reliable as well.

I know you don't think much of class D amps, and those of us who design them. I have also designed plenty of linear solid state amps as well as all tube amps, all on a commercial basis.

Who wants to live those good old days where warranties were 30 - 90 days, not transferable with a high failure rate?

High level engineering is responsible for some manufacturers offering 5 year warranties on all types of amps including tube amps. These are not built like in the good old days. Thank God.

There's plenty of innovation, experimentation, and creativity in new products. For you to insinuated that there isn't is a flat out lie, and insulting to every player who has chosen something new, something innovative and something different that your holy grail.

 

[seamonkey]

Thank God today's amps are designed by real engineers who do it right every time. No more pioneering experimentation, no more trial and error - just pure science

What a joy it is to live in the Class D digital era

That is still done, but mostly all on computers. Design, built, tested all in the virtual world today.
Small one man shops can use computers and spice. They mix it in with CPU or DSP and wow ... there are sure a lot of makers popping up everywhere.

 

[DavesnothereCA]

....A more modern (2009) IC is the TDA8953, which delivers up to 420 watts (PMPO) into 8 ohms - https://www.nxp.com/docs/en/data-sheet/TDA8953.pdf

Put a bass preamp in front to it and you have a low cost bass amp.

In times to come, when the preamp becomes integrated into a single chip, a bass amp will comprise a preamp chip, a power amp chip and a SMPS chip - housed in a small molded plastic case.

The future career prospects for bass amp designers and service techs look bleak

People might then moan about the loss of point to point wiring, tubes and tube sockets and repairable amps

It's called "progress"

~
Hmmm, I wonder what the RMS output of that IC would be ?

Of course that would also depend on the chosen supply voltage, which is allowed a wide latitude.

And I see on page 27 that the external components such as the HPF output filter can be chosen to suit the intended user's needs.

Also, on the applications page they do not specifically mention musical instrument amps, though do say that driving subwoofers is OK.

And yes, I think I have already noticed mention (in other TB posts) of some recent brands/models of bass amps using power amp and/or power supply which are pre-manufactured modules.

In some ways, it's kind of odd that having a complete preamp in a single module has become the 'final frontier, as one would think it to have been the easiest to do, given the low power requirements.

OTOH, to many of us, the preamp is the 'personality' of an instrument amp, so prob'ly it has been verboten to shrink it that far so far.

I for one have tinkered with parts values in tone circuits over the years to get desired results, and even still have a couple of mods in the back of my mind for my Acoustic 150 bass head from the 70s, which admittedly has only high and low shelving EQ and all discrete transistors.
~

 

[DavesnothereCA]

The 6CG7 and 6FQ7 are a 6SN7GT in a miniature 9 pin bottle

Octal tubes tend to run cooler than their miniature cousins because of a higher bottle surface area for heat dissipation, so there is real benefit in retaining octal drivers where natural ventilation applies

Octals were replaced with miniatures because of lower manufacturing and shipping costs, lower socket cost and chassis space reduction. The greatest benefit of miniatures is a reduction of internal capacitances, which enables a very high operating frequency range

The 6SN7 is available in a wide range of builds, with ruggedised "W" versions preferable for portable amplifiers for stage use

The 12AU7 has significantly more gain than a 6SN7

~
Yes, I admit winging it on that example.

I do also recall the 6CG7 and 6FQ7 in TVs, and that they are taller than a 12AU7 and its immediate siblings, thus preserving the bigger bottle of the octal design somewhat.

And of course extra gain (such as of a 12AU7) can be managed by external parts choices.
~

 

[DavesnothereCA]

Speaking of amps that were based on design elements from radios, there were musical instrument amps that didn’t have a power transformer.

One example:
http://harmony.demont.net/documents/schematics/amps/harmony_400a.pdf

Here is another one:
View attachment 2876037....

~
Add one more tube and a few other parts, and I still have an old AM radio tucked away which shares the other 4.

But did these designs not also present an increased shock hazard for the users ?

No, wait - I now notice the 100 or 150K resistor and .03 cap between the 2 grounds.

Also, the Harmony H-400A in the PDF seems to be going one step further on user safety, by using a small isolation transformer to feed the heater of the 12AU6.
~

 

[beans-on-toast]

~
Add one more tube and a few other parts, and I still have an old AM radio tucked away which shares the other 4.

But did these designs not also present an increased shock hazard for the users ?

No, wait - I now notice the 100 or 150K resistor and .03 cap between the 2 grounds - also doubling as a bias circuit for the 12AU6 ?

Also, the Harmony in the PDF seems to be going one step further on user safety, by using a small isolation transformer to feed the heater of the 12AU6.
~

Yes these designs were unsafe. They would never be allowed today. But people back then were smarter when it came to tube appliances. They’ve since made natural selection illegal.

The Harmony heaters droppd different voltages: 35v for the 35W4, 50v for the 50C5, 12v for the 12AU6. The small transformer provided the 12V.

 

[DavesnothereCA]

Yes these designs were unsafe. They would never be allowed today. But people back then were smarter when it came to tube appliances. They’ve since made natural selection illegal.

The Harmony heaters droppd different voltages: 35v for the 35W4, 50v for the 50C5, 12v for the 12AU6. The small transformer provided the 12V.

~
The other diagram did not use such a transformer.

Was that because it had one more tube and was closer to delivering the correct filament voltage to all tubes without a transformer ?

The Harmony PDF also had a series resistor (R10) in the filament string, so I am suggesting that the more expensive transformer was not necessary ONLY to achieve the proper filament voltage for the 12AU6 - instead just the appropriate value of the series resistor would have done that.

Also, how is the 12AU6 biased ? - There must be some point of theory which I have forgotten.
~

 

[beans-on-toast]

~
The other diagram did not use such a transformer.

Was that because it had one more tube and was closer to delivering the correct filament voltage to all tubes without a transformer ?

The Harmony PDF also had a series resistor (R10) in the filament string, so I am suggesting that the more expensive transformer was not necessary ONLY to achieve the proper filament voltage for the 12AU6 - instead just the appropriate value of the series resistor would have done that.

Also, how is the 12AU6 biased ? - There must be some point of theory which I have forgotten.
~

The 10 ohm resistor is selected to drop the line voltage as required to make it work. Back thn the line voltage could have been 110 or 115. So the amp would be stressed more running on a higher line voltage. One of the problems with the design.

The transformer would have helped with hum in the first stage where it could make a difference. A dc heater is ideal and this is implemented in many modern designs.

The 12AU6 is grid leak biased. The high valued resistor, 10M, and the 0.03uF capacitor serve to deliver a very small leakage current to the grid which makes the grid voltage negative relative to the cathode. This is not a very reliable way of biasing the stage. They save pennies by cutting corners. Here is a reference: Invalid Link Removed.

The moral of the story is that just because an amp is marketed, it doesn’t mean that it is well designed. Higher end amps cost more for a reason, you are usually getting what you pay for.

 

[DrummerwStrings]

Architects work this was too.
Stay within the lines and you are safe.
Get too artistic and everyone will finger point you if there is an incident.

Those that go too avant garde get branded and work becomes difficult to secure.
Reliability of your design affects your bottom line.

Form over Function.

 

[beans-on-toast]

Years ago people used to say that you could never go wrong with IBM. That has since been disproven by people who think outside the box.

 

[DavesnothereCA]

The 10 ohm resistor is selected to drop the line voltage as required to make it work. Back thn the line voltage could have been 110 or 115. So the amp would be stressed more running on a higher line voltage. One of the problems with the design.

The transformer would have helped with hum in the first stage where it could make a difference. A dc heater is ideal and this is implemented in many modern designs.

The 12AU6 is grid leak biased. The high valued resistor, 10M, and the 0.03uF capacitor serve to deliver a very small leakage current to the grid which makes the grid voltage negative relative to the cathode. This is not a very reliable way of biasing the stage. They save pennies by cutting corners. Here is a reference: Invalid Link Removed.

The moral of the story is that just because an amp is marketed, it doesn’t mean that it is well designed. Higher end amps cost more for a reason, you are usually getting what you pay for.

~
The ubiquitous 5-tube superhet radios back then had yet an extra 12v tube more than the 2nd schematic (and 2 tubes more than the 1st PDF one), and I'm sure that the heater voltages were designed/chosen to make radios not need anything to match the series total of the heaters to the line voltage, where these related guitar amps had to do something for the heaters as they were not using as many tubes, though actually, the designer of the 2nd amp with the 4 tubes did not bother, and took the chance to just let the extra 12 volts be distributed among the other 4 tubes, where the first amp maker needed the 100 ohm 5W resistor (and maybe the transformer - we do not know its ratio) to take up some slack.

I had earlier considered the chance of the transformer being there to reduce hum in the 12AU6 of the H-400A, yet the other amp designer did not bother, so go figure....

But I still feel that the transformer was implemented more for safety, as it would not allow the AC line voltage to reach the cathode in the event of a heater to cathode short in the 12AU6.

As for the 'grid-leak' biasing, I do recall this now , but that said, I do not understand why a maker who would spring for the cost of a transformer to feed a preamp stage's filament would welch on the cost of a 1/2 watt cathode resistor to bias that same stage in a more stable manner, like is done with most triode preamp stages of MI amps.

Possibly there was a TEAM of designers.
~

 

[Drgonzonm]

Years ago people used to say that you could never go wrong with IBM. That has since been disproven by people who think outside the box.

Not so loud, Watson might be listening.

 

[beans-on-toast]

~
The ubiquitous 5-tube superhet radios back then had yet an extra 12v tube more than the 2nd schematic (and 2 tubes more than the 1st PDF one), and I'm sure that the heater voltages were designed/chosen to make radios not need anything to match the series total of the heaters to the line voltage, where these related guitar amps had to do something for the heaters as they were not using as many tubes, though actually, the designer of the 2nd amp with the 4 tubes did not bother, and took the chance to just let the extra 12 volts be distributed among the other 4 tubes, where the first amp maker needed the 100 ohm 5W resistor (and maybe the transformer - we do not know its ratio) to take up some slack.

I had earlier considered the chance of the transformer being there to reduce hum in the 12AU6 of the H-400A, yet the other amp designer did not bother, so go figure....

But I still feel that the transformer was implemented more for safety, as it would not allow the AC line voltage to reach the cathode in the event of a heater to cathode short in the 12AU6.

As for the 'grid-leak' biasing, I do recall this now , but that said, I do not understand why a maker who would spring for the cost of a transformer to feed a preamp stage's filament would welch on the cost of a 1/2 watt cathode resistor to bias that same stage in a more stable manner, like is done with most triode preamp stages of MI amps.

Possibly there was a TEAM of designers.
~

It isn‘t unusual for some companies to provide a list of acceptable parts. Designers can use those and only those parts.

There is a maximum heater positive to cathode voltage spec of 100V for the 12AU6. It is possible that the wall plug polarity could factor into why the transformer was used. I’d have to work that one out.

 

[DavesnothereCA]

It isn't unusual for some companies to provide a list of acceptable parts. Designers can use those and only those parts.

There is a maximum heater positive to cathode voltage spec of 100V for the 12AU6. It is possible that the wall plug polarity could factor into why the transformer was used. I’d have to work that one out.

~
And a transformer was on that list but a pennies-even-back-then cathode bias resistor was not. - Hmmmm....

Thanks for looking up that 12AU6 spec. - It supports what I was thinking/guessing/saying.

But it does not explain why the designer of the OTHER maker's amp took a chance and did not use a transformer.

Except that it could have been for the same reason that they also hooked the 4 tubes' heaters in series directly across the 117 volt AC line without a resistor in series.
~

 

[AE1]

~

But I still feel that the transformer was implemented more for safety, as it would not allow the AC line voltage to reach the cathode in the event of a heater to cathode short in the 12AU6. ~

What happens if the mains leads are reversed by mistake and the active mains wire is connected to the chassis - and therefore the guitarist ?