Is All Tube Worth It?

[Count Bassie]

Or you can model it in ltspice
Voltage drop for a silicon diode is .7V but they start to break over at lower voltages. That's pretty low for a power amplifier input.
Surprising that no other amp manufacturer came up with such a circuit for "protection"

You guys're getting a little arcane for some of us here! But hey, carry on...

 

[fdeck]

Speaking from a service standpoint, pure tube amps are less likely to spontaneously combust under stress. The flip side to that is that the tubes themselves aren't what they used to be, and THEY are more prone to spontaneous failure.

That seems reasonable. After all, tubes are intended to get hot, and are also designed to be self sufficient for their cooling needs if provided with a modest amount of forced air when used in enclosed spaces.

When you get tube amps in, what typically fails? Is it the tubes, or something else? The only time I wasn't able to fix a tube amp by repairing a broken connection, the failed component was the power transformer.

 

[projectMalamute]

Measurements should be unnecessary. It's a cathode feedback circuit. What's the gain equation? You only need to look at the front end of the circuit to figure this out.

Seriously. If you are such an expert on this matter run the numbers and tell us.

Here's a schematic for the SVT power amp: Invalid Link Removed

Here's another if that is hard to read: Invalid Link Removed

 

[fdeck]

Or you can model it in ltspice.

And the answer is? You're the one making the claim about the diodes.

 

[seamonkey]

Solid State makes for an easier life... Plus people who aren't musicians hardly notice... A great player is a great player regardless of amp.

+1 - I have no doubt that a great player can play any amp or bass and within seconds figure out how to get the most out of it.

Acoustic Piano's are drastically different and it only takes a few bars for most Pianist to figure it out.

 

[chadds]

Another consideration. Besides your touch, tonally timid basses need tubes to fill in their stuff.

 

[seamonkey]

And the answer is? You're the one making the claim about the diodes.

They "start" to conduct at a little over .6 on average.

Measurement will verify the answer.

Aged Horse Answered two different ways. So it must not be as cut an dry as you seemed to indicate.

 

[fdeck]

Seriously. If you are such an expert on this matter run the numbers and tell us.

Here's a schematic for the SVT power amp: Invalid Link Removed

Here's another if that is hard to read: Invalid Link Removed

Okay, fair enough.

I get a gain of around 210. On back of envelope, I suspected that the gain is equal to R46 / R5 = 47000/220 = 214. Admittedly, I already have one or two tube circuits "built" in LTSpice, and modified one to simulate just the first stage of the SVT power amp. The difference in gain between the input and the output is 46.6 dB = 214. So, unless I'm really dense about this, I'm going to say that this is the gain of the SVT power amp.

So the 0.7 V forward drop of a silicon diode is amplified up to around 150 V. That would correspond to about 2800 W into 4 Ohms.

Now for my own speculation: The diodes are to protect the user from an unpleasant surprise if the input triode develops a short.

Note that I am not a real electrical engineer.

 

[Count Bassie]

Another consideration. Besides your touch, tonally timid basses need tubes to fill in their stuff.

There are also dirt-boxes, compressors, eq, various kinds of 'boost'... tubes are the luxury item, but there are other ways of 'getting there' that are pretty dang impressive.

 

[projectMalamute]

Okay, fair enough.

I get a gain of around 210. On back of envelope, I suspected that the gain is equal to R46 / R5 = 47000/220 = 214. Admittedly, I already have one or two tube circuits "built" in LTSpice, and modified one to simulate just the first stage of the SVT power amp. The difference in gain between the input and the output is 46.6 dB = 214. So, unless I'm really dense about this, I'm going to say that this is the gain of the SVT power amp.

So the 0.7 V forward drop of a silicon diode is amplified up to around 150 V. That would correspond to about 2800 W into 4 Ohms.

Now for my own speculation: The diodes are to protect the user from an unpleasant surprise if the input triode develops a short.

Note that I am not a real electrical engineer.

I actually meant that other fella.

Thanks though.

 

[agedhorse]

You've seemed to have changed your position. No matter. Measurements will tell.

I hadn't offered an opinion on this before so how did I change?

I don't think these diodes are sturdy enough to handle something as stupid as that. With too much current they would blow and leave the amp unprotected. Other tube amp schematics don't have any protection circuits like this. All the other manufacturers don't have this and don't seem to suffer from stupid user failure.

1k source impedance and 1N400X diodes at 50% duty cycle could handle over 100 volts peak input without breaking even a hint of sweat. Depending on the size of the 1k resistor, eventually it would burn open after a significant period of time. I looked in the manual and couldn't find the power rating of that part. I have seen this protection scheme before, both on tube and solid state amps. We obviously have a difference of opinion on stupid user error frequency.

UncleFluffy was going to do some measurements to see when the diodes kick in. They may still kick in after the power amp clips, but that is still SS in the signal path for those who claim the real sound comes out when you crank an SVT.

They are not in the signal path until the forward voltage is reached. As a shunt element, they are an open circuit until Vf is exceeded.

With this configuration, the voltage levels into the amp seem very low without the diodes tripping.

I think there's enough closed loop gain to allow the amp to clip at around 0.25VRMS input (this is a rough estimate based on the topology, I may be a bit rusty since working with all that class D stuff)

If you bought home amplifier that claimed all tube path, but found these diodes on the circuit board and on the schematic, you would have a valid complaint for the FTC.

I don't think you quite grasp how they are being used here. In fact, there are solid state diodes used in many tube power amps for handling things like bias voltage steering, HV supply, heater supply, backswing protection, etc. If you follow the complete signal path, you will find that the power supply diodes are in series with the audio current through the power supply. It's a loop, the current through the speaker starts at the power transformer, goes through the dioded through the tube(s), through the transformer matching and onto ground. Each electron passes throug the silicon rectifier diodes in the HV supply on every power amp that has silicon HV rectifiers.

In this case, there is zero current through the diodes under normal operation and only where the threshold is exceeded do they conduct and EXCESS current flows through them and onto ground. This current doesn't even flow to the speaker (just for the sake of argument though, because in reality what doesn't flow through the audio path can have exactly the inverse effects)

 

[coreyfyfe]

One trained listener out of many was able to detect artifacts that had no productive contribution to the sound. Like rumble and crackle on LP's. If you fixed the tube amp, he wouldn't be able to tell.

You should re-read the article. He was untrained, as were the rest of them, and picked it up as they did the tests.

Not really the point. You said no one noticed. You were wrong, as you continue to be. You don't like tubes, fine. But it's the same as with the clipping diodes business in the face of people much more qualified than yourself commenting and telling you otherwise - don't spread your ignorance around as fact.

 

[agedhorse]

Okay, fair enough.

I get a gain of around 210. On back of envelope, I suspected that the gain is equal to R46 / R5 = 47000/220 = 214. Admittedly, I already have one or two tube circuits "built" in LTSpice, and modified one to simulate just the first stage of the SVT power amp. The difference in gain between the input and the output is 46.6 dB = 214. So, unless I'm really dense about this, I'm going to say that this is the gain of the SVT power amp.

So the 0.7 V forward drop of a silicon diode is amplified up to around 150 V. That would correspond to about 2800 W into 4 Ohms.

Pretty much what I expected too.

Note that the 2400 watts/4 ohms would be based on the peak voltage measurement, not RMS.

 

[seamonkey]

I hadn't offered an opinion on this before so how did I change?

You said it here, post 76
http://www.talkbass.com/forum/f15/bugera-all-tube-head-618353/index4.html#post8593370

1k source impedance and 1N400X diodes at 50% duty cycle could handle over 100 volts peak input without breaking even a hint of sweat. Depending on the size of the 1k resistor, eventually it would burn open after a significant period of time. I looked in the manual and couldn't find the power rating of that part. I have seen this protection scheme before, both on tube and solid state amps. We obviously have a difference of opinion on stupid user error frequency.



They are not in the signal path until the forward voltage is reached. As a shunt element, they are an open circuit until Vf is exceeded.



I think there's enough closed loop gain to allow the amp to clip at around 0.25VRMS input (this is a rough estimate based on the topology, I may be a bit rusty since working with all that class D stuff)



I don't think you quite grasp how they are being used here. In fact, there are solid state diodes used in many tube power amps for handling things like bias voltage steering, HV supply, heater supply, backswing protection, etc. If you follow the complete signal path, you will find that the power supply diodes are in series with the audio current through the power supply. It's a loop, the current through the speaker starts at the power transformer, goes through the dioded through the tube(s), through the transformer matching and onto ground. Each electron passes throug the silicon rectifier diodes in the HV supply on every power amp that has silicon HV rectifiers.

In this case, there is zero current through the diodes under normal operation and only where the threshold is exceeded do they conduct and EXCESS current flows through them and onto ground. This current doesn't even flow to the speaker (just for the sake of argument though, because in reality what doesn't flow through the audio path can have exactly the inverse effects)

The other diodes in the amp are not in the signal path.

Diodes have a soft break over. In this configuration they cause soft clipping. Threshold is more of a break over. Measurements will confirm when/if they kick in for one amp.

Diodes with similar configuration may be used in some op-amp input circuits, very infrequently , but the - input is at virtual ground. It's not the same.

I doubt any Genz Benz amplifiers use any diodes like this.

 

[seamonkey]

You should re-read the article. He was untrained, as were the rest of them, and picked it up as they did the tests.

Not really the point. You said no one noticed. You were wrong, as you continue to be. You don't like tubes, fine. But it's the same as with the clipping diodes business in the face of people much more qualified than yourself commenting and telling you otherwise - don't spread your ignorance around as fact.

There's no statistic evidence that shows that anybody can tell. Not enough sample size. Run a few more double blind tests and let me know.

I like tube sound fine. I use it all the time.

I think those that know more than me can carry on a decent exchange of information. Measurements would help.

 

[metron]

I know to the get tone i wanted out of my svt i had to turn it up to 10. Thats when it starts to get really good.

Just call me sneakers. Sneakers O'Toole.

 

[cheezewiz]

Seamonkey...tell us about your ACTUAL EXPERIENCE playing through tube amps and your ACTUAL EXPERIENCE a/b'ing them against ss amps.

 

[LiquidMidnight]

Considering you said 30 years ago I'm assuming you mean this paper from April 1981, which says:

On tube vs solid state power amps: "A total of 12 subjects participated, resulting in a total of 54 trials. Four subjects did not report a difference in any trial. While eight of the remaining subjects reported hearing differences, seven were unable to identify the differences reliably. Reliable detection requires that the subject correctly identify the difference in amplifiers more than 50% of the time and not indicate a false difference. A false difference is reporting a difference even though A and B are assigned to the same amplifier. The remaining subject (subject #1) correctly identified the difference on two of three trials with no false differences."

On tube vs solid state power amps driven to distortion: "A total of six subjects participated, resulting in 23 trials. Three subjects did not report a difference on any trial. One of the remaining three subjects reported only false differences. The other two subjects were able to reliably detect a difference in sound. One of them was able to identify a "buzzing" sound which was more pronounced in the tube amplifier. He was able to detect this difference 100% of the time. It is interesting to note that this was subject #1 that detected a difference in test one. The other subject (#2) described the tube amp as sounding "fuller" two out of three times."

On tube vs solid state pre amps: "Five subjects participated, resulting in 24 trials. Two of the subjects did not report a difference on any trial. One subject reported only a false difference. The other two subjects could not reliably detect a difference."

On a full system, frequency and gain matched, power amps driven: "A total of 9 subjects participated, resulting in 49 trials. Three subjects did not report a difference on any trial. The remaining six subjects were unable to reliably detect a difference, including test subject #2 from the previous test. However, test subject #1 from the previous trials was unavailable."

Their final conclusion was: "Although ripple intermodulation distortion is easily detected by trained listeners, only one of seventeen test subjects was able to detect it without tranining. The change in frequency response caused by the high output impedance of the tube amp is by far the predominant cause for differences."

Or in other words, people who know what they're listening for, can hear the difference.

FWIW the test equipment was a twin reverb for the tube amp and a custom built pre-amp using 4558 op amps into a commercially available power amp.

That's a very interesting article. Unfortunately, I can't access it to see the methodology section. I'm not being automatically skeptical, but there could be some HUGE issues with internal validity depending on experimental design. The difference in amplifiers - and speakers if using different speakers - since they won't necessarily sound the same when played flat, may have implications for type I error (which actually would go against the hypothesis that there's a difference between the percieved sound quality of tubes and SS).

Again, I'm not saying there are issues; I just would need to read the methology section before making any conclusions.

 

[nysbob]

Seamonkey...tell us about your ACTUAL EXPERIENCE playing through tube amps and your ACTUAL EXPERIENCE a/b'ing them against ss amps.

I'd be interested in that as well.

Seamonkey continues to blather about these diodes. I've been playing through an SVT head for about 30 years. I don't run it into distortion or have headroom problems. When I sit in on other cat's class D rigs (and I've played some very nice ones) I can tell a significant difference in the dynamic response.

Everybody plays differently and asks different things from their gear. Seamonkey, you don't have to like my amp, but don't try to tell me that your modeled flavor of the day is just the same. I've been playing and recording bass for 40 years - I know better.

 

[agedhorse]

You said it here, post 76
http://www.talkbass.com/forum/f15/bugera-all-tube-head-618353/index4.html#post8593370

I thought you were talking about this current thread, but good point, and after looking at the circuiit closer, I think there may be enough gain so that the diodes do not conduct until after the power amp begins to clip. BUT, without doing more work than I care to do, I'm not positive either way. As I also stated, there's more to the circuit than meets the eye.

The other diodes in the amp are not in the signal path.

But they are, take a highlighter and follow the path of an electron as it travels from the power supply through the diodes, through the tube and through the load to ground. The electrons pass through the diodes on the way to the speaker. It's like the difference between lookig at the circuit in the Thevenin versus the Norton equivelant. The perspective is different but the circuit is the same.

Diodes have a soft break over. In this configuration they cause soft clipping. Threshold is more of a break over. Measurements will confirm when/if they kick in for one amp.

All diodes have a finite knee slope, the slope is the impedance that's added to the junction drop. Ifthe slope is non-linear, then there is a third added term called dynamic impedance. The threshold is the junction conduction voltage based on the material and is the corner of the knee. The impedance is reflected in the slope of the knee, if there was zero impedance then it would be an ideal diode with a hard voltage drop Vf and no slope. This is difficult to achieve without gain, sometimes called an amplified junction, or amplified reference junction, or amplified diode or an amplified zener diode, etc. The higher the impedance the shallower the slope and softer the clamping (clipping) action. Dynamic impedance can be used to modify the shape of the slope and cliiping response.

Diodes with similar configuration may be used in some op-amp input circuits, very infrequently , but the - input is at virtual ground. It's not the same.

It's almost EXACTLY the same. Virtual ground or actual ground doesn't matter. The method of operation of the circuit is different but the action is identical. And, it's also used for protection (generally zeners) from running the feedback open loop and sticking, and on the input (just like the SVT topology) to protect the input from overvoltage and reverse voltage breakdown. Clipping the signal for tonal reasons is not the only reason why diodes are used in audio circuits.

I think some of these design points are taken way out of context. I don't know the history of this particular discussion amongst some of you guys, but the SVT's design is just one designer's perspective on how to build an amp. I hesitate to criticize it because it's one of the most successful bass amp designs spanning a lot of years with good reliability and a good following.