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zappazapper

Is there some way I can measure how my power amp performs?

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I run my LT in 4CM with my Mesa Boogie .50 Caliber Plus. Because I've only had the Helix for a year, and because the pandemic has offered me little opportunity to play (I took it as an opportunity to take a break from music after gigging almost every weekend for a decade), I've almost exclusively used the .50 Cal preamp on all my patches instead of amp models. Now that things are picking up and I'm playing more, I'm starting to become more interested in experimenting with the amp models. 

 

Just to get it out of the way, I want to state for the record that I know that using 4CM makes cabinet models basically unusable, at least if accuracy is any factor at all. No cabinet model is going to sound accurate coming from any of my speaker options, and at any rate, what's being modeled is a MIKED cabinet, and that's a whole other thing entirely from an amp in the room. So at least in terms of using my Helix as a live-performance device, I have no plans at all to use cabinet modeling. 

 

What I'm wondering, however, is whether or not I should stick with Preamp models for the same reason (power amp modeling is unlikely to sound accurate through a tube power amp...  maybe?), or whether I can use the full Amp models with their power amp modeling. 

 

Just before I got my Helix I had my .50 Cal serviced - I was getting serious sparking inside the amp, and I believe the amp tech said it was the bias cap, and while it was in there I had all the filter caps replaced, and I replaced the power tubes. 

 

https://www.thetubestore.com/tung-sol-7581a

 

I ended up getting these. At the end of the day, I'm really not a roots/country/blues player, and the .50 Cal is not the ideal amp for those styles anyway (it's sister amp, the Studio .22, with its lower wattage power amp using EL84s is much better suited). It's a rock amp, dare I say a heavy metal amp; the kind of amp where you get your dirt from the preamp. And so tubes described as having more clean headroom and a balanced frequency response seemed to be appropriate for the amp's intended purpose, and also how I would be using it: with an amp modeling multi-fx. 

 

And so, since I have tubes that are, in theory, giving me all this clean headroom and a relatively flat frequency response, I'm thinking that maybe I can use the power amp modeling in the Helix to some degree. But instead of just asking for anecdotal opinions on the matter, what I'm hoping is that someone can point me in the direction of being able to actually run some tests on my power amp to see how it performs. If my power amp is already providing the characteristics that are modeled with the Hum, Ripple, Bias and Bias X controls, then I might have to live with just using the Preamp models. On the other hand, if my power amp is still pretty linear(?) at full stage volume, then maybe I can get some use out of these controls.

 

I have an pretty good audio interface for testing (MOTU 896HD) and Room EQ Wizard (REW) for software. I'm assuming that I should be able to send test signals into the FX Loop Return and measure what comes out of the Direct Out, as it is an attenuated line-level signal that is tapped off the power amp. Or maybe I need a load box for this? 

 

So, a few questions I would like answered...

 

Is REW the right software for this? 

What am I looking for in REW, assuming it is?

How would the power amp controls modeled in the Helix manifest themselves in testing software?

How do I measure how much "Hum", "Ripple", "Bias", and "Bias X" my .50 Cal power amp has?

 

Any advise is most appreciated. I've received some very good, detailed technical advice on this forum and I look forward to another interesting and informative conversation. On the other hand, if all you have to offer is that I shouldn't bother thinking about this kind of stuff, thank you but I'm not interested in a discussion about why I'm interested in the things I'm interested in. 

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I run my Helix in 4CM as well.  

I had a .50 Calibre for about 10 years and then moved to a Mark V-35.

I was having heat issues with the .50 Calibre so after I had it repaired the second time I sold it and bought the Mark V-35.

It has a very quiet built-in cooling fan and does not have the same issues.

The .50 Cal was a great sounding amp and a wonderful platform for effects but the design of having the tube sockets mounted directly to the board instead of to the chassis was the reason it heated up and had issues.  (4 x EL84 in my case as it was the first generation .50 Calibre.  Yours will likely have 2 x 6L6)

You may want to consider opening up your amp and examining the board.

If you see burns on the board it may be an indication of trouble coming at some point.

 

I use amp models in many of my presets and I also use cab blocks in some of them and IRs in others.  Quite often I find them to be shortcuts to the sound I'm after.  Some presets just don't work well with cabs or IR blocks but many presets do work well.  I suggest you experiment with them.

 

 


51682538212_605d458e5c_h.jpg

 

 

 

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I usually set my sag relatively low, maybe about 2 or 2.5 because I'm going into an amp and I don't see the need to do that twice.  As for hum and ripple I dial those way down even lower. Like 1 or lower.   Bias and biasX I tend to keep in the neighbourhood of 4 to 6, so around middle.

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On the subject of preamps vs full amp models. I know you asked for some solid scientific amp stuff, but bottom line is, how does it SOUND?

Try it both ways. Some of the preamps sound kind of thin by themselves, and might need eq/gain blocks.

Some of the full amps sound better than the preamps. Probably depends a lot on the RW amp you're putting them through. Your .50 cal is not a whole lot like my Tweaker15.

The advantage (disadvantage?) of full amp models is that there's a baked in speaker impedance curve based on the speaker it was modeled with (the default speaker if you choose amp+cab). TBH I'm not sure I understand all that impedance curve stuff. I know that you're also over on TGP, so you've probably read James Freeman's thoughts on the subject. I tried his eq settings for faux impedance curve and thought...huh? Speaking of James Freeman, you should definitely ask him about the scientific amp stuff. He probably even has graphs and charts :-)

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1 hour ago, zappazapper said:

I'm assuming that I should be able to send test signals into the FX Loop Return and measure what comes out of the Direct Out, as it is an attenuated line-level signal that is tapped off the power amp.

 

That's what I'd try at first. In case you have doubts about the validity of the direct out, you could as well insert a DI box between speaker out and cab (the latter saving you a load box). On my MkIV, the signal coming from the DI out was pretty much identical to what I got from a DI box.

 

1 hour ago, zappazapper said:

 

Is REW the right software for this? 

 

Likely. Can't tell you about the proper procedure as I've always been using the Impuls Response Utility coming with Logic, but people all over the world use REW to generate IRs, so I'm sure there's several tutorials about the how-to.

You will however miss out on any dynamic information about the power amp behaviour - and from my experience, even the cleanest power amps with the largest headroom still add some bits of compression. You could check that by sending a dynamic signal through your power amp, rerecording it and comparing the two.

No idea how you'd measure "ripple" and such ("hum" is just static hum - and fwiw, I turn both of them all the way to zero in all my HX patches anyway, as IMO they serve no purpose others than indicating the amp needs some servicing - at least that's what I'd do in case my real amps would hum around).

 

Apart from all that, I'd simply compare how things sound when A/B-ing preamp vs. full amp models. Personally, I'd likely go for the full amp models, simply because using their master volume as an additional gain stage is a pretty neat tool here and there.
Yeah, I know, there's impedance curves baked into the power amp models of the Helix, so you'd double those, but then, in the end the most they're doing is some EQ-ing, which can be dialed out in case it's getting in the way.

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4 hours ago, MGW-Alberta said:

I run my Helix in 4CM as well.  

I had a .50 Calibre for about 10 years and then moved to a Mark V-35.

I was having heat issues with the .50 Calibre so after I had it repaired the second time I sold it and bought the Mark V-35.

It has a very quiet built-in cooling fan and does not have the same issues.

The .50 Cal was a great sounding amp and a wonderful platform for effects but the design of having the tube sockets mounted directly to the board instead of to the chassis was the reason it heated up and had issues.  (4 x EL84 in my case as it was the first generation .50 Calibre.  Yours will likely have 2 x 6L6)

You may want to consider opening up your amp and examining the board.

If you see burns on the board it may be an indication of trouble coming at some point.

Thanks. Ya, you had the .50 Caliber, I have the .50 Caliber Plus. Two 6L6s, sockets on the chassis. When I started having problems I deep dove the Caliber series. Although the EL84 versions might have more "personality", it seems like mine has a more dependable design. The amp tech said that nothing that was wrong with it was unusual for a 30 year old amp. 

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5 hours ago, MGW-Alberta said:

I usually set my sag relatively low, maybe about 2 or 2.5 because I'm going into an amp and I don't see the need to do that twice.

 

It's my understanding that Sag is a phenomenon that occurs more in the preamp than the power amp. I only say that because the parameter is available in the Preamp models while Ripple, Bias and Bias X are not. 

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8 minutes ago, zappazapper said:

It's my understanding that Sag is a phenomenon that occurs more in the preamp than the power amp.

 

Well, then Line 6 got that sort of wrong (or interpretes things differently or has simply transfered that modeling part to the preamp section). Usually "sag" is refering to the effect when a power amp is hit hard.

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5 hours ago, rd2rk said:

 bottom line is, how does it SOUND?

 

Well, I think it's part of the same conversation. I mean, I get that it's entirely possible to get useful tones by doing things that are completely wrong from a technical point of view, but let's not forget that Helix is a modeling processor, with the supposed goal of accurate representation of the amps being modeled. IIRC, you were part of the discussion I started about impedance of the loops and why my amp sounded "screamy" in 4CM. If I hadn't had that discussion, I would have never figured out that the issue was actually that by placing a buffered device between my guitar and my amp, I wasn't loading my pickups in the same way. In that case, "how does it sound" was being affected by a technical issue that I didn't properly understand, and now that I do, it sounds exactly like it does when I plug straight into the amp.

 

5 hours ago, rd2rk said:

Speaking of James Freeman, you should definitely ask him about the scientific amp stuff. He probably even has graphs and charts :-)

 

Y'know, I saw a YouTube video the other day where someone was measuring the effect of the Bias and Bias X controls on an oscilloscope and I think it may have been him. And I didn't put two and two together, but that's the guy that's always on about the 5150 III, IIRC. Thanks for the tip. I'll check out his stuff and maybe contact him with some questions. 

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45 minutes ago, SaschaFranck said:

 

Well, then Line 6 got that sort of wrong (or interpretes things differently or has simply transfered that modeling part to the preamp section). Usually "sag" is refering to the effect when a power amp is hit hard.

Is it possible that both preamp tubes and power amp tubes experience sag, and that the preamp models are modeling only preamp sag and full models are modeling both? 

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5 hours ago, SaschaFranck said:

You will however miss out on any dynamic information about the power amp behaviour - and from my experience, even the cleanest power amps with the largest headroom still add some bits of compression. You could check that by sending a dynamic signal through your power amp, rerecording it and comparing the two.

 

That is essentially what an impulse response is, isn't it? 

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25 minutes ago, zappazapper said:

Is it possible that both preamp tubes and power amp tubes experience sag, and that the preamp models are modeling only preamp sag and full models are modeling both? 

 

No idea, but typically, "sag" is refering to a power amp "function".

 

24 minutes ago, zappazapper said:

That is essentially what an impulse response is, isn't it? 

 

No, an IR isn't representing anything dynamic, it can only capture linear processes.

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Sag is caused by the power tubes loading the power supply and pulling the B+ voltage down.

Preamp tubes don't draw any noticeable current and so don't cause sag, they also have additional filtering between the B+ supply and themselves and so are not affected by the B+ sag.

 

Craig

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^^^  This.
Craig has it correct.

 

I personally don't care for a lot of sag.  Some is good but more isn't better.  One thing I almost always do with an amp block is to stiffen it up.

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6 minutes ago, MGW-Alberta said:

I personally don't care for a lot of sag.  Some is good but more isn't better.  One thing I almost always do with an amp block is to stiffen it up.

 

Same here. If anything, I often find the amps a bit too squishy. Hence, the first thing I'm doing is to turn down Hum/Ripple (which aren't exactly doing much but "masking" some things), Sag and most often Master as well (sometimes even on those amps that don't have a master volume originally). I may raise them back up a little later on, but in the first place I usually want a very solid sound, especially when slamming drive pedals in front.

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https://www.aikenamps.com/index.php/what-is-sag

 

I'm not suggesting that Aiken Amps is a particularly credible source of information, but it was the first thing that popped up when I googled "tube sag". It mentions rectifiers, transformers, and filter caps. I know the transformer is part of the power amp, but what about rectifiers and filter caps? 

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5 hours ago, SaschaFranck said:

Apart from all that, I'd simply compare how things sound when A/B-ing preamp vs. full amp models.

 

I was thinking about doing something where I record a guitar track in Reaper direct, reamping it through my Helix in 4CM with my amp, recording the direct out, recording one pass with a preamp model and another with the same full amp model, flipping the polarity on one and comparing the difference. Like, try to find which settings null out as much as possible, and maybe that's as close of a representation of what the power amp in my own amp is doing, in the same terms as the Helix parameters. But there's a lot of variables there. I would have to select an amp model that had a power amp that is similar to my amp, and I don't really know enough about power amps to know which one that would be. The .50 Cal is essentially a poor man's Mark III but what's essentially different between it and a Mark series amp is the straight Class A/B power amp instead of the Simul-Class thing. So I don't know how useful that kind of approach would be. 

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21 minutes ago, zappazapper said:

but what about rectifiers and filter caps?

 

The rectifiers main purpose is driving your power amp tubes.

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Some (not all, I think) Fender amps that use the 5AR4 rectifier tube (also known as GZ34) can be re-biased for use with a 5U4, which will stiffen them up a little bit.  On my old vintage Vibrolux Reverb the point of the start of breakup changed from 2.5 to 3.5 on the volume which made the amp more useful for my purposes.  Possibly all in my head but I always felt the amp was also slightly more articulate and responsive/sensitive with the 5U4 as well.  I say I think not all because I don't know if I've ever heard of anyone using a 5U4 in a 5E3.  I've never investigated that though.

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1 hour ago, SaschaFranck said:

 

The rectifiers main purpose is driving your power amp tubes.

Not quite, the rectifier's main purpose is to convert (rectify) AC current to DC current for use in your amplifier. all of your amp, not just the power tubes, the capacitors are for smoothing AC ripple in the power supply.

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1 hour ago, Rattlehed said:

Not quite, the rectifier's main purpose is to convert (rectify) AC current to DC current for use in your amplifier. all of your amp, not just the power tubes, the capacitors are for smoothing AC ripple in the power supply.

So if is rectifying power for both the power amp and the preamp, then sag also affects the preamp, and theoretically when using a Preamp model, the Sag control will emulate only those phenomenon that exist in a preamp, and when using a full Amp, it would emulate the phenomena that exist in both sections of an amp?

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My understanding is that sag is mainly a power amp phenomenon, the filter capacitors should store enough energy on the preamp nodes that the voltages should stay fairly steady in the preamp stages. as they draw very minimal current,  I could be wrong on this however

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5 minutes ago, Rattlehed said:

My understanding is that sag is mainly a power amp phenomenon, the filter capacitors should store enough energy on the preamp nodes that the voltages should stay fairly steady in the preamp stages. as they draw very minimal current,  I could be wrong on this however

 

You're not wrong, I explained that earlier.

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2 hours ago, Rattlehed said:

Not quite, the rectifier's main purpose is to convert (rectify) AC current to DC current for use in your amplifier.

 

Sure. But as you said yourself, the preamp tubes likely won't be affected as much.

Whatever, again as you said, sag is pretty much known as a power amp thing.

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42 minutes ago, CraigGT said:

 

I explained that earlier.

Derp... yes you did.

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10 hours ago, SaschaFranck said:

 

Well, then Line 6 got that sort of wrong (or interpretes things differently or has simply transfered that modeling part to the preamp section). Usually "sag" is refering to the effect when a power amp is hit hard.

 

So, it's just a mistake then? That seems hard to believe. They must think it has enough of an effect to put it there. I'll have to check tonight and see what it does when it's a preamp vs a full amp. Weird. 

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Wow... sometimes it can be very humbling reading these posts.  It lets you realize you didn't know what you didn't know :)

 

Thanks!

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I've been thinking about this a lot. Hear me out. 

 

On 11/16/2021 at 8:36 AM, CraigGT said:

Sag is caused by the power tubes loading the power supply and pulling the B+ voltage down.

Preamp tubes don't draw any noticeable current and so don't cause sag, they also have additional filtering between the B+ supply and themselves and so are not affected by the B+ sag.

 

A real-world analogy, because it's all I got - when my air conditioner turns on, my lights go dim for a split second as my air conditioner also struggles to ramp up to full power. My light bulbs don't draw a significant amount of power, but the effect on them is noticeable all the same.

 

Is it possible that the effect of sag on the preamp is being understated here?

 

Is it possible that when L6 was testing amps, they noticed that sag caused by the power amp was also affecting the preamp?

 

Is it possible that when they decided to include Preamp models, they considered that the target audience for Preamp models is either 4CM users or those who use standalone tube power amps; basically anyone who's using a power amp that would not only experience its own sag but also cause sag in a preamp if one existed and was being powered by the same power supply?

 

Is it possible that L6 figured that, in the interest of accuracy, they had better include a Sag parameter in their Preamp models that simulates the phenomenon as it occurs in a preamp sharing a power supply with a power amp being hit hard?

 

I have had many very interesting discussion with the people on this forum and have learned SO MUCH from those discussions, but I'm having a hard time accepting that the Sag parameter in the Preamp models is just a mistake. I'm hoping someone can chime in here and help me understand why a preamp isn't the same as my light bulbs. 

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1 hour ago, zappazapper said:

Is it possible that when they decided to include Preamp models, they considered that the target audience for Preamp models is either 4CM users or those who use standalone tube power amps; basically anyone who's using a power amp that would not only experience its own sag but also cause sag in a preamp if one existed and was being powered by the same power supply?

 

Is it possible that L6 figured that, in the interest of accuracy, they had better include a Sag parameter in their Preamp models that simulates the phenomenon as it occurs in a preamp sharing a power supply with a power amp being hit hard?

I've done some googling and stared at some schematics and have come to the same conclusion as you, and I believe you are correct here.  the sag control in the preamp models makes sense to me as you described above.

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On 11/16/2021 at 1:08 AM, rd2rk said:

Speaking of James Freeman...

 

So this is the video I was talking about: 

 

 

I'm pretty sure I understand what the Bias parameter does. Basically one side is Class AB and the other Class A, with varying degrees of crossover distortion. It seems to me rather impossible to simulate Class A behavior with a power amp that almost certainly will behave like a Class AB amp (because my amp IS Class AB), and probably more likely that you could simulate Class AB behavior with a Class A amplifier. Like, if it were that easy to make a Class AB amp without crossover distortion, somebody would be very rich, I think. 

 

I'm not sure I understand what Bias X is doing. I understand that it stands for Bias Excursion, but I don't know what that means. 

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8 hours ago, zappazapper said:

'm not sure I understand what Bias X is doing. I understand that it stands for Bias Excursion, but I don't know what that means.

 

Could as well represent Bias Crossover Distortion - the two seem to be related. Here's Fractals Cliff Chase on both subjects:
bias-excursion.99698

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