Tube Cube - attenuator

[mshilarious]

i'm afraid you have a bit of a chicken or the egg problem here.... without heater voltage to the tubes you cant get an output to provide the heater voltage.... and the single largest current draw is the heaters in most designs...

Nah, not really chicken and egg. Tubes are so massively inefficient that you will always need much more power for the heaters, so it's not an all or nothing proposition. You would still obviously need a heater supply. But it's possible to recover some of the output power, if only to provide a handy internal pad for the output. It would be somewhat like switching between battery and external power like portable devices do.

I don't think there is any danger of me going into business modding tube amps. They are heavy and expensive to ship, and that sounds like far too much liability.

 

[ggunn]

Nah, not really chicken and egg. Tubes are so massively inefficient that you will always need much more power for the heaters, so it's not an all or nothing proposition. You would still obviously need a heater supply. But it's possible to recover some of the output power, if only to provide a handy internal pad for the output. It would be somewhat like switching between battery and external power like portable devices do.

I don't think there is any danger of me going into business modding tube amps. They are heavy and expensive to ship, and that sounds like far too much liability.

But the output from the power tubes is highly variable. When you're not playing it's virtually zero, and it's always AC, while the heaters use DC. I guess you could rectify it and send it to a battery... Maybe you could return it to the power grid to lower your electricity bill. ;^)

 

[mshilarious]

But the output from the power tubes is highly variable. When you're not playing it's virtually zero, and it's always AC, while the heaters use DC. I guess you could rectify it and send it to a battery...

The power the heaters normally use starts out as AC too. Heaters can use AC though, they don't particularly care which way the electrons move, but an AC supply has an effect on audio. Actually it could be an interesting form of feedback, either positive or negative. Or you could filter (and store) it with a cap and stick with DC. Yes, it would have to switch with the normal heater supply. I thought about doing a DC bias from the heater supply, but then the attenuation is occuring in the heater supply circuit, which somehow doesn't seem as romantic

 

[ggunn]

The power the heaters normally use starts out as AC too. Heaters can use AC though, they don't particularly care which way the electrons move, but an AC supply has an effect on audio. Actually it could be an interesting form of feedback, either positive or negative. Or you could filter (and store) it with a cap and stick with DC. Yes, it would have to switch with the normal heater supply. I thought about doing a DC bias from the heater supply, but then the attenuation is occuring in the heater supply circuit, which somehow doesn't seem as romantic

Yes, but it starts with 60Hz AC at a constant voltage. The AC voltage from the amp's output stage is highly variable in both frequency and amplitude. As to saving money on power consumption, I think you'd get more benefit from turning off some lights and raising the thermostat a bit.

Just running the tubes "cold" (less heater voltage) will burn out the tubes very quickly.

As to "interesting" feedback, to me the best sort of attenuation is that which does not change the sound of the amp any more than it has to.

 

[cusebassman]

As to "interesting" feedback, to me the best sort of attenuation is that which does not change the sound of the amp any more than it has to.

Are there any reasonably priced attenuators out there that actually work, and aren't highly suspect then (a few hundred, perhaps)? I get a beautiful clean tone from my Blues DeVille when its cranked on the gain and the master is set reasonably low, but that still blows my face off...

 

[mshilarious]

Yes, but it starts with 60Hz AC at a constant voltage. The AC voltage from the amp's output stage is highly variable in both frequency and amplitude. As to saving money on power consumption, I think you'd get more benefit from turning off some lights and raising the thermostat a bit.

Just running the tubes "cold" (less heater voltage) will burn out the tubes very quickly.

As to "interesting" feedback, to me the best sort of attenuation is that which does not change the sound of the amp any more than it has to.

No, the tubes wouldn't run cold, the power supply would switch. And the feedback would be optional. Tonal options can be interesting, or they can suck, I dunno, I haven't built it.

Anyway, it's been fun to play with mentally, but I think I came to the conclusion that the circuit would need another transformer. The original idea was to get free attenuation by using parts that were already in the amp. Not sure about that now.

Ultimately, I don't have the time or money to build a power tube circuit from scratch just to play around with something that nobody else is going to build. Output transformers are REALLY expensive.

Also I am still supposed to be working on my balanced guitar idea

 

[cusebassman]

Also I am still supposed to be working on my balanced guitar idea

Your idea???? Every halfwit with a guitar who has experienced the inherent noise problems of long, unbalanced runs has thought "Man - I wish my guitar were like a microphone - balanced!"

Okay, not really, but I have thought that myself - so get on it, so I can rewire my amp and guitar to fully utilize having XLR jacks

 

[WERNER 1]

+1 on the Weber Standard Mass (100W)...I've been using two of them for over three years now and I love them....I use one for live applications and one for home jamming' and recording.

good stuff!

Rick

 

[ggunn]

Your idea???? Every halfwit with a guitar who has experienced the inherent noise problems of long, unbalanced runs has thought "Man - I wish my guitar were like a microphone - balanced!"

Okay, not really, but I have thought that myself - so get on it, so I can rewire my amp and guitar to fully utilize having XLR jacks

One, the noise usually comes from the guitar itself, not the cable, and two, it's not a new idea - there was a Les Paul model sold back in the early 70's with a low Z balanced output.

A ten or even a twenty foot run is not long enough for a balanced signal line to make any difference, at least from noise picked up by the cable, IMO.

 

[mshilarious]

One, the noise usually comes from the guitar itself, not the cable, and two, it's not a new idea - there was a Les Paul model sold back in the early 70's with a low Z balanced output.

A ten or even a twenty foot run is not long enough for a balanced signal line to make any difference, at least from noise picked up by the cable, IMO.

Boy, you really hate all my ideas, don't you?

OK, first I never said low impedance. That requires a buffer circuit, which isn't a terrible idea, but requires power and that is not always desirable. Plus some people like the inductive source-tube load interaction. So my idea was to have a balanced high-impedance output that would also work as a regular unbalanced output when a TS cable was plugged into it.

And yes, the noise comes from the guitar, not the cable. Thus, my idea is to reconfigure the internal wiring of the guitar so the entire signal path is balanced and floating, like a dynamic microphone.

 

[ggunn]

Boy, you really hate all my ideas, don't you?

OK, first I never said low impedance. That requires a buffer circuit, which isn't a terrible idea, but requires power and that is not always desirable. Plus some people like the inductive source-tube load interaction. So my idea was to have a balanced high-impedance output that would also work as a regular unbalanced output when a TS cable was plugged into it.

And yes, the noise comes from the guitar, not the cable. Thus, my idea is to reconfigure the internal wiring of the guitar so the entire signal path is balanced and floating, like a dynamic microphone.

Hmmm. No, I don't necessarily hate your ideas, but the last two seem a little, well, I dunno, screwy. It's just my opinion, though; don't let that stop you. ;^)

Low impedance does not require a buffer, BTW; you can easily do it with a transformer. As a matter of fact, I don't see how you are going to create a balanced line (where one conductor carries the inverse signal of the other) without a transformer. Guitar pickups (most of them, anyway) are high Z unbalanced devices; good luck on changing that.

And the whole signal path of a dynamic microphone isn't balanced; only the part downstream from the internal transformer is. Many dynamic mics have a wiring option at the connector which bypasses the transformer for an unbalanced high Z output. I just last weekend converted two Shure 548's from high Z unbalanced to low Z balanced operation.

I don't hate your ideas, necessarily; it's just that they make it look to me like you don't really understand how this stuff works. Not that I claim to know everything about it, myself, you understand, but I do have an EE degree and have studied tube circuits a bit.

 

[mshilarious]

Hmmm. No, I don't necessarily hate your ideas, but the last two seem a little, well, I dunno, screwy. It's just my opinion, though; don't let that stop you. ;^)

Low impedance does not require a buffer, BTW; you can easily do it with a transformer. As a matter of fact, I don't see how you are going to create a balanced line (where one conductor carries the inverse signal of the other) without a transformer. Guitar pickups (most of them, anyway) are high Z unbalanced devices; good luck on changing that.

And the whole signal path of a dynamic microphone isn't balanced; only the part downstream from the internal transformer is. Many dynamic mics have a wiring option at the connector which bypasses the transformer for an unbalanced high Z output. I just last weekend converted two Shure 548's from high Z unbalanced to low Z balanced operation.

No, pickups and dynamic coils are unbalanced only if they are ground referenced (which pickups normally are). Adding a transformer to a guitar would be expensive and unnecessary--the signal loss for low impedance operation is significant. Not exactly what you want if the destination is a guitar amp.

It would be helpful to have an input transformer in a tube amp given a balanced guitar output, but it would have to be a custom job, I don't know of an appropriate off-the-shelf transformer. It would be much simpler for a solid-state amp; all you'd need would be an extra opamp stage, and maybe not even that, depending on how the input stage is configured (and can be reconfigured).

Also I don't believe the 548 high impedance option bypasses the transformer; it just taps different pins and ground-references one of the pins on the secondary, but the capsule leads are still floating.

 

[ggunn]

Also I don't believe the 548 high impedance option bypasses the transformer; it just taps different pins and ground-references one of the pins on the secondary, but the capsule leads are still floating.

I'm pretty sure that the red wire which is disconnected for the low Z option and which is connected to the hot pin in the high Z option is the hot side of the primary, but I'll check the wiring diagram to make sure.

This is how I remember it:
Red - Hot side of the capsule to hot side of primary of the transformer and extended to output option
Black - Ground side of capsule to ground side of primary and pin 1 of output
White and Blue - Secondary terminals of the transformer

Low Z option: white and blue to pins 2 and 3 on output XLR

High Z option: replace white with red

That's from memory and I can't find it on line, so my references to pins 2 and 3 could be backwards, and it could be blue replaced with red instead of white, but that's essentially how it works.

 

[mshilarious]

I'm pretty sure that the red wire which is disconnected for the low Z option and which is connected to the hot pin in the high Z option is the hot side of the primary, but I'll check the wiring diagram to make sure.

This is how I remember it:
Red - Hot side of the capsule to hot side of primary of the transformer and extended to output option
Black - Ground side of capsule to ground side of primary and pin 1 of output
White and Blue - Secondary terminals of the transformer

Low Z option: white and blue to pins 2 and 3 on output XLR

High Z option: replace white with red

That's from memory and I can't find it on line, so my references to pins 2 and 3 could be backwards, and it could be blue replaced with red instead of white, but that's essentially how it works.

http://www.shure.com/stellent/group..._UG/documents/web_resource/us_pro_548s_ug.pdf

All those wires are off the secondary; the primary wires are green and yellow.

 

[ggunn]

http://www.shure.com/stellent/group..._UG/documents/web_resource/us_pro_548s_ug.pdf

All those wires are off the secondary; the primary wires are green and yellow.

I can't see the graphics here, but I'll check it out from home. Thanks