70s SVT Bench Test Results

[Mark Reccord]

Also talking about how much power it puts out really is just numbers because anything in this power range is definitely loud enough for most situations.

As a quick comparison that I have a lot of experience with. A Mesa/boogie bass 400+ only tests out power wise the way I do it at about 240 watts clean before any waveform clipping/distortion but nobody is ever going to say it isn't as loud as an svt!

It has a lot more to do with how that clipping/distortion happens and what it does after that point, that's one of the points of a tube amp.

Oh, fully agreed. As @ThisBass has mentioned, power at some percentage THD probably isn't a great metric for any MI amp (unless the goal is super high fidelity) and especially not for tube amps . And besides, there isn't much difference between 250W and 360W in the real world anyway. I really only did the output power measurement because I had everything set up. It will be interesting to see what new tubes and a rebias will do in my amp.

 

[Mark Reccord]

The cathodes on the SVT power tubes are grounded through a pair of one ohm resistors which are used for setting the bias. I think that you are referring to the 6.3 heater voltage which Andy addressed.

It is important to note that variation in the wall voltage will affect the performance of most amps. It will affect the bias, the plate voltage and power supply voltages in general. The amps are designed to work that way. Some touring bands have power supplies that keep their line voltage stable. Others will rebias the amps at each venue to keep the performance consistent. Then again, not everyone is overly concerned. There are other factors that can affect the sound on any given night.

My assessment of this over many years of playing and being involved with audio production is that variations in the line voltage within an amp's design tolerances (i.e. +/-10%) don't make enough difference to justify the added cost dragging around special power supplies or the extra shagging around required to rebias at every gig. YMMV, of course.

 

[beans-on-toast]

Oh, fully agreed. As @ThisBass has mentioned, power at some percentage THD probably isn't a great metric for any MI amp (unless the goal is super high fidelity) and especially not for tube amps . And besides, there isn't much difference between 250W and 360W in the real world anyway. I really only did the output power measurement because I had everything set up. It will be interesting to see what new tubes and a rebias will do in my amp.

I agree, for these amps it doesn't make a lot of difference to the player.

On the other hand, as you know, in R&D, if you want to quantify the performance of an amp, the best technical way to do it is to look at all the metrics you can. You want to have the data to back up your claim. A tech would perform sweeps to validate that the amp is performing to spec. Performance based on a specified frequency range at less that a THD is the standard of the industry.

The problem with many vintage amps is that they may not meet these specifications and it would cost too much to bring these amps up to spec. Also, companies take a lot of liberties, especially back then. Who know if the original specs were real or creative writing by the marketing department.

I like to believe that less of that goes on today. At the same time, companies have different interpretations of where the onset of distortion lies. This isn't being dishonest, just different. Hold yourself to too high a standard and you shoot yourself in the foot.

 

[beans-on-toast]

My assessment of this over many years of playing and being involved with audio production is that variations in the line voltage within an amp's design tolerances (i.e. +/-10%) don't make enough difference to justify the added cost dragging around special power supplies or the extra shagging around required to rebias at every gig. YMMV, of course.

It depends on how much money the band has and how picky they are. AC/DC as shown in the video I posted above, can do whatever they want. They carry a very expensive power stabilization unit.

They find it so important, they highlight this by calling themselves AC/DC.

But too low line power can make an amp suck on any given night.

 

[agedhorse]

The cathodes on the SVT power tubes are grounded through a pair of one ohm resistors which are used for setting the bias. I think that you are referring to the 6.3 heater voltage which Andy addressed.

It is important to note that variation in the wall voltage will affect the performance of most amps. It will affect the bias, the plate voltage and power supply voltages in general. The amps are designed to work that way. Some touring bands have power supplies that keep their line voltage stable. Others will rebias the amps at each venue to keep the performance consistent. Then again, not everyone is overly concerned. There are other factors that can affect the sound on any given night.

Wait a minute, 6.3V across 1ohm is 6.3 amps which is a bit high for cathode current isn't it???

 

[beans-on-toast]

Wait a minute, 6.3V across 1ohm is 6.3 amps which is a bit high for cathode current isn't it???

Since the cathodes are grounded, I assumed that he was refering to the heater voltage, not the cathode voltage.

 

[agedhorse]

Since the cathodes are grounded, I assumed that he was refering to the heater voltage, not the cathode voltage.

Me too

 

[chaosMK]

I understand this is science but what is the function of knowing this information?

 

[timv]

I think we are just geeking out a bit with techie info.

 

[Mark Reccord]

I understand this is science but what is the function of knowing this information?

Probably not much other than knowledge for knowledge's sake. I can totally understand people not finding it useful or even interesting.

 

[chaosMK]

Probably not much other than knowledge for knowledge's sake. I can totally understand people not finding it useful or even interesting.

I was trying to follow the diagrams but all I saw were green lines. Are the Blue Lines on a separate chart?

 

[beans-on-toast]

Just hop on the orange line bus, it'll get you there.

 

[rufus.K]

Looks like india

 

[beans-on-toast]

Looks like india

Yes, one place that you don't want to drive. I like how they have a tree on the roof of the bus.

 

[cerrem]

Just hop on the orange line bus, it'll get you there.



View attachment 1051675

I believe that Tour Bus looks familair..
Roll up, roll up for the Magical Mystery Tour!
Step right this way!... LOL

 

[12bass]

Just hop on the orange line bus, it'll get you there.

Color coding the subway system made it extremely easy to get around Barcelona!

 

[beans-on-toast]

Color coding the subway system made it extremely easy to get around Barcelona!

Yes colors are a lot easier than names. Although if a bull got into the red line, he might start going after everyone in sight.

 

[beans-on-toast]

I believe that Tour Bus looks familair..
Roll up, roll up for the Magical Mystery Tour!
Step right this way!... LOL

That bus does look familiar, doesn't it. With detours you really would get to go on a magical mystery tour.

 

[tubedude]

The cathodes on the SVT power tubes are grounded through a pair of one ohm resistors which are used for setting the bias. I think that you are referring to the 6.3 heater voltage which Andy addressed.

It is important to note that variation in the wall voltage will affect the performance of most amps. It will affect the bias, the plate voltage and power supply voltages in general. The amps are designed to work that way. Some touring bands have power supplies that keep their line voltage stable. Others will rebias the amps at each venue to keep the performance consistent. Then again, not everyone is overly concerned. There are other factors that can affect the sound on any given night.

I misspoke, I meant the 6.3 volts on the heaters, not the cathodes. Regardless of the biasing method, the emission of the cathode is dependent upon the temperature of the heater. The specs for tubes show a marked performance decrease with slightly low heater voltage and premature wear and heater burnout with higher voltage. Both conditions affect cathode emission, in preamp tubes and driver/splitters in addition to power tubes. In my 60's.tube amps, designed to run on 117V but frequently at 125V, I have almost 7 Volts on the heater string. As this is a reality of my local utility, I have installed resistors and diodes in my vintage amps to drop the heater voltage back into range. Those having a similar situation might be able to afford the cost and weight of a well regulated UPS, or are using commie tubes that are easily and cheaply replacable, but I use only US and western euro vintage tubes that are too expensive to sacrifice. YMMV.

 

[cerrem]

I misspoke, I meant the 6.3 volts on the heaters, not the cathodes. Regardless of the biasing method, the emission of the cathode is dependent upon the temperature of the heater. The specs for tubes show a marked performance decrease with slightly low heater voltage and premature wear and heater burnout with higher voltage. Both conditions affect cathode emission, in preamp tubes and driver/splitters in addition to power tubes. In my 60's.tube amps, designed to run on 117V but frequently at 125V, I have almost 7 Volts on the heater string. As this is a reality of my local utility, I have installed resistors and diodes in my vintage amps to drop the heater voltage back into range. Those having a similar situation might be able to afford the cost and weight of a well regulated UPS, or are using commie tubes that are easily and cheaply replacable, but I use only US and western euro vintage tubes that are too expensive to sacrifice. YMMV.

Yes... You have a valid point...
The 6146B big claim to fame was the "Dark Heater" ...this allowed the heater to maintain an adequate space charge durring lower heater voltage operation...
If you take a look at the tiny wire gauge used in the SVT for the heater wires, it's ridiculous.... The drop across the heater line is insane... starting at output tube 1 you may be at 6.3V but your are 5.7V by the time your at output tube #6 .. And thats at IDLE !! LOL..
At full power output the heaters will dip down a lot more....
Even though the heaters are on a different transformer...The 120V at you Mains or Variac will droop significantly from the long AC cord then the small wires that feed the 120V to FUSE holder then to the MOLEX connector... up to the pre-amp..through more tiny wires and ON switch.. then back down through the MOLEX then through some more tiny wires, then to the transformer primary leads...LOL if you think there is 120V entering the transformers at this point...
I have previously mentioned I use a relay for the 120V ...and do not pass all that current up through the MOLEX and switches... I install a relay in the socket location of the where the delay tube would have been..
I also use HUGE wire on the heaters across the tube sockets.....also re-wire so that the heater feed is at the center of the 6 sockets..ie between socket #3 and socket#4 ...