Voltage/Bias & Amp Performance Questions

[grinder965]

Hey gang.....a few questions regarding voltage/tube biasing and amp performance. I pretty technically challenged when it comes to understanding amp electronics. Anyway, I have two (2) great amps (Maven Peal Ganesha & Mojave Peacemaker) that are both 100 watters and have the capabiltiy of adjusting the wattage from 1-100w on the Ganesha and 6-100w on the Peacemaker. According to the manufactuers, these are not built in attenuators or master volumes.

With the Ganesha, I can individually bias my own power tubes (great feature) and have recently done so. Maven Peals suggest to bias the EL34's at 30ma (the amp has a high 530 plate voltage) while the wattage control is maxed out (100 watts). A couple things I noticed that I would like to understand better. After putting new tubes in and biasing per MP's instructions after playing the amp for a few hours, I test again (with wattage maxed at 100w) and I get readings lower than 30ma (25ma or so). Is this normal...should I test and bias when amp is first turned on or after playing through it for several hours?

Also, when the wattage control is lowered, I get lower readings on the tubes....pretty significantly lower readings if I'm running the amp at say somewhere around 10 watts, I might get a reading of 12ma's or so. How might this, in a theorectical world, affect tube performance, i.e. - longevity,tone, dynamics, touch sensitivity, feel etc? Amazingly enough, I'm still able to get good power tube distortion at much lower volume levels with both these amps and want to better understand how they work. Obviously there's more at work here than just the tube bias reading i can measure.

What kind of changes could I expect, if I biased the tubes hotter than normal but never maxed out the volume levels on these amps?

Any thought on this would be much appreciated....Mike

o

[UR12]

Hey gang.....a few questions regarding voltage/tube biasing and amp performance. I pretty technically challenged when it comes to understanding amp electronics. Anyway, I have two (2) great amps (Maven Peal Ganesha & Mojave Peacemaker) that are both 100 watters and have the capabiltiy of adjusting the wattage from 1-100w on the Ganesha and 6-100w on the Peacemaker. According to the manufactuers, these are not built in attenuators or master volumes.

With the Ganesha, I can individually bias my own power tubes (great feature) and have recently done so. Maven Peals suggest to bias the EL34's at 30ma (the amp has a high 530 plate voltage) while the wattage control is maxed out (100 watts). A couple things I noticed that I would like to understand better. After putting new tubes in and biasing per MP's instructions after playing the amp for a few hours, I test again (with wattage maxed at 100w) and I get readings lower than 30ma (25ma or so). Is this normal...should I test and bias when amp is first turned on or after playing through it for several hours?

Also, when the wattage control is lowered, I get lower readings on the tubes....pretty significantly lower readings if I'm running the amp at say somewhere around 10 watts, I might get a reading of 12ma's or so. How might this, in a theorectical world, affect tube performance, i.e. - longevity,tone, dynamics, touch sensitivity, feel etc? Amazingly enough, I'm still able to get good power tube distortion at much lower volume levels with both these amps and want to better understand how they work. Obviously there's more at work here than just the tube bias reading i can measure.

What kind of changes could I expect, if I biased the tubes hotter than normal but never maxed out the volume levels on these amps?

Any thought on this would be much appreciated....Mike

o

Mike

Those are some nice amps you got there. Both use a form of what has been termed as power scaling. Maven Peal's take on "Power Scaling" is a little different than the conventional Power Scaling that Kevin O'Conor licenses. Maven's circuit is so different that it is patented. Anyway, Power Scaling gives you a knob on your front panel that allows you to change the B+ voltage on your tubes. As you turn down the wattage you are actually reducing the voltage on your tubes. Anytime you change the voltage on your tubes you need to rebias your amp. So as you turn down your voltage, the amp has additional circutry that also changes the bias. As you lower the voltage your bias current also needs to be lowered to keep the tube in it's operating range.

This is definately not like an attenuator like you would hook between your amp and speakers. An attenuator just dumps off some of the wattage going to your speakers so that your speakers sees less signal but the amp is still putting out the same amount of power. This can be hard on tubes and your OT. Powers Scaling does just the opposite. By turning down your voltages you are putting out less watts but your also running your tubes under powered so they are not stressed. Your still getting the distortion characteristics and sound of a 100 watt amp, but since your are only asking the amp to produce 10 watts your not hurting your tubes and if you played the amp at 10 watts all the time they would last a lot longer.

I don't think I would want to bias the amp hotter at lower volumes as if by accident someone cranked the amp back up to max levels you would probably run the chance of melting down your tubes (red plating).

I am not sure why your bias would change after playing a few hours unless something changed with one of your output tubes. In other words you may have a flakey tube in there.

We could carry the theory and power scaling circuit discussion out for days, and there are people out there who love it with a passion and others who hate it. Once you get the amp down to a low enough level that you aren't putting out enough power to move the speaker efficently you will loose some of the tone and response of the amp. They same thing can be said of an attenuator.

Hope this helps!

Dana

[grinder965]

Dana,

Thanks for taking the time to answer with such a comprehensive, knowledgeable and well written response. I now have a better understanding of whats happening. I think the thing that really threw me for a loop was understanding how and why the bias current on the tubes would go down so dramatically when reducing the wattage. What I hadn't factored in was that I'm still running 4 tubes at a much lower wattage as opposed to 1 or 2 tubes. Nevertheless, I'm still confused with why or how the tubes can generate power tube distortion at the lower wattage levels given that individually, the tubes (and I would think the tranformer) are being pushed at a much lower level. With respect to the tubes, it perhaps it has to do with still pushing a greater current through the tubes than the wattage level would normally require to run the amp at a clean non distorted level.

Anyway, these are two great amps that have taken the best of the old and combined it with new features that make the whole experience so much more user friendly. Like you said, you still need to push enough air through the speakers to get great tone at the lower volume levels so they are not bedroom amps. But if you can play reasonably loud at home, they are capable of providing great overdriven/distorted output tube performance at levels that will not destroy what's left of your hearing.

I'll test the reduction in readings I get after playing through it for while by putting the old tubes back in and seeing if I get the same outcome. If I do, I contact MP and see what they have to say.

Thanks again....Mike

[UR12]

Nevertheless, I'm still confused with why or how the tubes can generate power tube distortion at the lower wattage levels given that individually, the tubes (and I would think the tranformer) are being pushed at a much lower level. With respect to the tubes, it perhaps it has to do with still pushing a greater current through the tubes than the wattage level would normally require to run the amp at a clean non distorted level.



Actually, as you lower the voltage on the tubes, they distort eaisier or sooner and less signal is needed to drive them into distortion. This is why the amp gives you those cool tones at a lower volume. There is also less current flow through the power tubes at lower volume. The more watts you produce the more current. Less watts / less current. Some people who have installed Power Scaling Kits in their amps have scaled just the power section of their amp and not the preamp. This usually leaves you with the preamp running at normal levels and since you reduce the voltage to the power tubes you will end up overdriving them. Kevin O'Conor uses a Drive compensation Control (like a master vol) to reduce the drive to the power tubes. I perfer to scale the whole amp and this control is not needed.

I would guess that one of the reasons they have individual bias controls on each tube is so they can match the tubes independently. This would allow each tube's bias to track with the power control and reduce the bias as you turn the voltage down. If you aren't using matched tubes and you had a single bias setup, one tube's bias may not track linearly with the others and reach a point where it could redplate and the other 3 be fine.

Regarding the output transformer, I would guess that the amp is designed with an OT that is not adding any saturation to the signal going to the speaker at full power. If it was designed with a undersized tranny then as you turn down the voltage the OT saturation would go away and the tone of the amp would change. If you designed the amp with an OT that didn't saturate then the tone of the amp would remain the same at reduced power.

Dana

[grinder965]

Thanks Dana....yes that's a logical explation re: the trannys. It's interesting you mention the pre-amp section of the amp cause I find that the more I use the wattage/power damping features on the amps, I have to compensate by hitting the pre-amp side of the circuit with a stronger signal. Fortunately both amps take to pedals really well.

[epluribus]

Regarding the output transformer, I would guess that the amp is designed with an OT that is not adding any saturation to the signal going to the speaker at full power. If it was designed with a undersized tranny then as you turn down the voltage the OT saturation would go away and the tone of the amp would change. If you designed the amp with an OT that didn't saturate then the tone of the amp would remain the same at reduced power.

Dana

Great discussion here, thanks to all.

Somewhat OT: What does an OT do at saturation? My limited understanding is that it doesn't clip like a tube does, but that the frequency response tails off at various points in the band, changing the EQ but not the grit. Er no?

--Ray

[UR12]

Hi Ray

Somewhat OT: What does an OT do at saturation? My limited understanding is that it doesn't clip like a tube does, but that the frequency response tails off at various points in the band, changing the EQ but not the grit. Er no?

--Ray


There are a few ways to saturate an OT. The most common that we talk about is saturation caused by an over voltage condition at the Primary. You reach a point where it doesn’t matter how much more signal you feed to the primary, you won’t get any more magnetic flux built up in the core. This results in a signal coming off the secondary that no longer resembles the signal applied to the primary, or in a word distortion. This distortion really isn’t the same thing as tube distortion. Once the saturation point has been reached, there are harmonics generated by the distortion produced. These added harmonics are what makes it sound differently, (Changing the EQ) but there is definitely some grit added to the signal also in the form of square waves. It’s the square waves that help to add the harmonics and they can sound great or horrible depending on the tranny.

The other two ways that I know of saturating a tranny are feeding it a signal outside it’s normal frequency response range, usually lower (this rarely happens in a guitar amp) or providing the primary with a DC offset voltage.

Small doses of saturation for short periods can be great but I would say that you are shortening the life of your OT if it is being saturated for sustained periods.

Dana

[epluribus]

Hi Ray




There are a few ways to saturate an OT. The most common that we talk about is saturation caused by an over voltage condition at the Primary. You reach a point where it doesn’t matter how much more signal you feed to the primary, you won’t get any more magnetic flux built up in the core. This results in a signal coming off the secondary that no longer resembles the signal applied to the primary, or in a word distortion. This distortion really isn’t the same thing as tube distortion. Once the saturation point has been reached, there are harmonics generated by the distortion produced. These added harmonics are what makes it sound differently, (Changing the EQ) but there is definitely some grit added to the signal also in the form of square waves. It’s the square waves that help to add the harmonics and they can sound great or horrible depending on the tranny.

Dana

Hi Dana, thanks for this terrific reply. Hadda mull this for a few days to let the concepts sink in and do a bit more reading.

If I'm getting this correctly, tube saturation and sag are not produced by or affected by the OT spec. So if I used a bigger OT in an amp, it theoretically would have little effect on saturation and sag, but rather it would deliver flatter freq response throughout the volume range. If true, it sounds to me like the OT might be a key to getting rid of the low end "farting-out" issue in a Blackface-style amp like my DR II. Plz don't hesitate to sharpshoot my provisional conclusion here, I like knowing I'm oversimplifying.

Thanks again to all for this great thread.

--Ray

[UR12]

If I'm getting this correctly, tube saturation and sag are not produced by or affected by the OT spec. So if I used a bigger OT in an amp, it theoretically would have little effect on saturation and sag, but rather it would deliver flatter freq response throughout the volume range. If true, it sounds to me like the OT might be a key to getting rid of the low end "farting-out" issue in a Blackface-style amp like my DR II. Plz don't hesitate to sharpshoot my provisional conclusion here, I like knowing I'm oversimplifying.

Thanks again to all for this great thread.

--Ray

Hi Ray
You are correct. Tube saturation occurs when the input signal on the grid of the ouput tube becomes so high that the tube can't amplify any more. (She's givin me all she's got captain!) The plate of the tube has been saturated with electrons and can take no more. This has a compression effect meaning no matter how much more you turn up the volume you will get no more amplification. The wattage of your OT has nothing to do with the the tube's saturation. It is the load for the tube and supplies the B+ for the tube and couples as much power as it can from the tube to the speaker.

Sag can occur before a tube reaches saturation. It occurs when you crank the volume and the tube is pulling so much current that the power supply can't deliver any more current and the voltage starts to drop or sag. So basically it occurs because your power supply is undersized for the particular amp. It also causes compression but it's not the same as saturation. Compression is usually thought of as more sustain.

As far as "farting out" is concerned, that's usually caused by the latter to an extent. Bass notes require a lot more power to produce than high notes. If you know anything about a PA system, you know that you run a BIG amp and a lot of power on your bass cabs. If you bi-amp you only need to run say a tenth of the power you use on your bass cabs to run your tweeters/highs for them to sound as loud as your woofers. This is because of the way your ears work. Same thing in an amp. When you play a low note, you need to be able to supply enough power to produce that note. If your power supply can't suppy enough power (or sags) then the bass note comes out mushy or distorted.

To fix the problem with farting out, you can try a few things. If your using a tube recto then you can switch to solid state diodes as they don't sag like a tube recto. You can also try changing your filter caps to a larger value. Not only do they filter hum out of your power supply they are also like little batteries that supply current during short demands. If all of this fails it might be that your power tranny needs to be bigger. If it is rated at 200ma try changing to one that is 300ma. A power tranny that is being ask to deliver more power than it is designed for will run hot and probably fail over a long period of time depending on how much it is under sized.

Changing the OT to a model that will handle more watts will only affect OT saturation. Your OT may be fine and not saturating but your bass will still fart out because your power supply is lacking the curent to produce the bass note. All trannys are designed to give you a particular freq responce. In a guitar amp you usually only consider the range of the guitar and this is usually about 50hz - 6khz. If you go to something like a HI-FI tranny that has a range of 20hz-20khz then you MIGHT get a flatter response in the guitar range but it will probably do very little to take care of the farting out.

In a nutshell all of these things work together and when you design an amp you must look at the current required by all of the tubes you intend to use. Then you design a power supply that can supply at least this amount of current and add a little in for fudge factor. Based on these calculations, you need to look at how many watts the amp will be capable of producing to get the correct size OT so it doesnt add any OT saturation. This will give you a good clean amp. Of course there is no reason that you could'nt design the amp with an undersized OT to add a little saturation if you want or add a tube recto that will sag if that is the tone your looking for.

Sorry for being so long winded!:)

[epluribus]

Not long-winded at all, great stuff, terrific job of making the subject understandable. I wish all tube amp writers did that so well. (I'm wading through a mid-70's copy of Jack Darr right now.) Sure do appreciate the info and the tech detail--makes it so much easier to understand the tradeoffs. Much more to read and think about here.

If all of this fails it might be that your power tranny needs to be bigger.

Let's see, seems to me saturation is a result of the PT running out of power and causing the waveforms to clip. (As opposed to clipping being caused by the tube itself.) In light of the above discussion of bass tones requiring significantly more power, let me guess that when the PT runs out of power, clipping sets in with the low notes saturating the most and becoming porportionately quieter, not to mention more distorted than the mids and highs--the farting out.

So if we stuck a bigger PT in there, the lows come back focused and tight, but the amp doesn't go into saturation like it used to either--it becomes a cleaner amp with more headroom. Is this what people mean by a "stiffer" amp?

So the trick is to get the amp to saturate, but at the same time maintaining a "better" proportion of low and high tones. The cap values, if I get this, won't change that tonal proportion, they'll simply delay the onset of clipping by temporarily supplying extra voltage till they run out of stored power. Let's see...that wouldn't get you a ton more clean and loud low end, but it would clean up and focus the attack a bit more by delaying saturation till later in the note, making the attack more detailed and the lows "tighter." Reminds me of an attack control on a comp except this would be an attack control for the onset of saturation.

Heavens! It's possible this is beginning to make sense.

Thanks again Dana, I got more reading to do here...

--Ray

Oh yeah, one mo' thing...

If you go to something like a HI-FI tranny that has a range of 20hz-20khz then you MIGHT get a flatter response in the guitar range but it will probably do very little to take care of the farting out.


In other words, the output tranny right now may be delivering sufficiently flat response in the guitar range, so using a bigger one, while it might run cooler, won't get any appreciably flatter response in the amp's range. Great for a home stereo, but barring square-wave distortion, it may not very useful for a guitar amp, other than tranny life.