Smalles tube amp

[jeremy]

Re: Smalles tube amp

trim pots for what? cathode bias shouldnt really need adjustment.

i dont really have a practice amp, the smallest amp i have is a custom build 12w cathode biased 1x12" class a/b pushpull with two 6v6. it has one preamp tube and a driver. ive run the gamut of preamp tubes, brands and types, and in my experience changing the tube didnt significantly effect the maximum volume from the amp. it did significantly alter the amount of distortion in the signal. with a 12ax7 i start getting dirty at 3, with a 5751 it was more like 5, with a 12au7 there was only a little dirt when the amp was full up, much cleaner but the maximum volume was about the same with all the tubes. the tubes that produced significantly more distortion might seem a little louder but a sound level meter showed virtually no difference. the power tranny in this amp is really beefy so the extra current draw wont have any effect since there is lots of reserve and filtering so the bias point wont really change on this amp.

if you changed the driver tube from a 12ax7 to a 12au7 you might be able to reduce the volume a little but this might not be a great idea in some amps and it may not sound all that great. in some amps it may work relatively effectively though

 

[Myles]

Re: Smalles tube amp

Myles this will in fact reduce the rms output if you go to a low enough value pre amp tube.

If an amp is designed around 12ax7's, as most are, then of course it does work.
It won't reduce the gain in the ratio of the gain percentage table, perhaps I should have elaborated, but it will and does reduce the oputput of an amp.

If a power tube needs an input of say X mv to produce full output and you only feed it 20 percent of Xmv then the power tube can't produce it's maximum power rating it may only produce 50 percent.

Turn the output of the pre amp via the gain control to say 1 and crank the master volume to 10.
The amp isn't very loud because it's not recieving enough gain from the pre amp for the output tubes to produce full output.
You are "starving " the power tubes of the required input voltage.

Substituting 12ax7's for 12au7's.... same deal.
There is not enough output from a 12au7 ( gain factor of 19 compared to 100) to supply the output tubes with enough voltage to produce full rated output.
The amp has maybe 50 percent of it's normal rms output.
There are some negatives to this.
Perhaps you could do some spdbl measurents on an amp using 12ax7's and substitute 12au7's then post the reduction in output rms results here.

You are not taking many other factors into account .... plate resistance (which effects true gain in circuit more than any other factor and varies WIDELY in todays tubes) or plate current. Most 12AX7s today fall into the 80-85% actual gain area as a general rule.

An AU7 is never a satisfactory tone/gain stage tube from my point of view. It is not just a matter of gain ... they are great current pushers (phase inverters) in some amps but never in my mind a tone stage tube.

The difference between a 12AY7 and 12AX7 is less than two points on a volume knob and one does want the output section in an amp to work in the range where it is touch sensitive and dynamic or you may as well get some high gain front end amp with a master volume control and forget about output tube distortion.

I am not sure if this post is talking about levels or tone. If all one wants to do is drop level using tubes, sure ... stick low gain and low current output tubes in every preamp position and leave your volume knob in the same place as where it was when it was too loud using the proper tubes. It will sound awful and not be fun to play at all but it will not be as loud.

But .... saying that a 70% gain tube vs a 100% gain tube will drop the output or level or loudness or power of the amp 30% is not correct at all. Remember, these are cathode biased tubes and what about 1.2mA of a 12AX7 vs 10.0mA of a 12AT7 as an example? You also need to figure plate current into the picture. 12AX7=1600 mU vs 12AT7= 5500 mU. mU is .... for a given amount of input you get a given output and thus a 12AT7 in this formula would give you more gain? Well, not really (and it breaks the rules here) because it depends how a tube is used in a given circuit.

What I am saying here is there are many factors and a general rule or the concept that was stated here is incorrect and misleading to a lot of folks that may be reading this post earlier on.

I am not trying to be a jerk or hard ass ... I just like to help folks and keep them on the right track when I can.

On my website there is a downloadable tube primer, part II that may explain more at http://www.guitaramplifierblueprinting.com/Tubeprimerandselection320-2.pdf

 

[wanmei1]

Re: Smalles tube amp

so the more speakers you add. does it increase for each speaker added. i know if you go from 2 speakers to 4 you gain about 2 or 3 more db's. so up the sensitivity of the speaker's to say 103 and it gains even more. so what your saying is my 15 watt amp ran into a 4x12 cab with 103db's speakers will make it sound like it's a 60 what amp?

Yeah that's right and so are your calculations.

Doubling the number of speakers gives double the volume.
In fact doubling the speaker cones surface area doubles the volume.
So if you have a 15 watt amp with 1 speaker, and you add another speaker, you have doubled the surface area of the original speaker cone and increased the volume by 3db which equates to being twice as loud as a 30 watt amp into your original single speaker.
By adding another 2 speakers you have again doubled the surface area of the speaker cones and this means yet another 3 db increase in volume. Now your 15 watt amp is as loud as a 60 watt amp using your original single speaker.
....Providing of course the impedance of the four speakers when wired up together remains the same as that of your original single speaker.

 

[bancika]

Re: Smalles tube amp

that's just not true, if you put 1000 speakers on 1W amp will it be as loud as 1000w amp?

 

[Rich_S]

Re: Smalles tube amp

Seriously. "Tubes" and "quiet" just don't go together, no matter how low the wattage, unless you're using an attenuator. I have a Weber MiniMASS, and I think it's great for adjusting gig volume, but it's not great at squashing a tube amp's outpout down to bedroom levels.

I have the Classic Rock (Marshall) version of the Amp Plug, and I love it. Cheap, too.

 

[wanmei1]

Re: Smalles tube amp

An AU7 is never a satisfactory tone/gain stage tube from my point of view. It is not just a matter of gain ... they are great current pushers (phase inverters) in some amps but never in my mind a tone stage tube.


I am not sure if this post is talking about levels or tone. If all one wants to do is drop level using tubes, sure ... stick low gain and low current output tubes in every preamp position and leave your volume knob in the same place as where it was when it was too loud using the proper tubes. It will sound awful and not be fun to play at all but it will not be as loud.

But .... saying that a 70% gain tube vs a 100% gain tube will drop the output or level or loudness or power of the amp 30% is not correct at all. this formula would give you more gain? Well, not really (and it breaks the rules here) because it depends how a tube is used in a given circuit.

Myles, thankyou for clarrifying that substituting a 12ax7 for a 12au7 does in fact reduce the output volume of an amplifier.

Your say that an amp will sound non toneful with a 12au7 ?
Have you tried it?
In my opinion they sounds superb.
If you really believe that a 12au7 sounds awfull then perhaps you should contact many of the worlds leading high fi stereo amp designers and tell them.
A large number of these more expensive amps only use 12au7's and we are talking $5000 plus amps. Interesting to note that most of the less expensive designs use 12ax7's.
I am not saying that the gain table I posted earlier is the reduction in rms produced by the amp, it is as you know simply the gain supplied to the power tubes, I included the reduction in gain percentage referenced to a 12ax7. I was illustrating the gain factor of each tube designation in descending order of output, to show which tubes can be subbed and in what order there output decreases and by how much in relation to a 12ax7. So that if a 12au7 produces too much of a reduction in power output of an amp you can look at that table and try the next incremental tube on the list and see if that ones more suitable etc.
See my follow up posts for clarification.
In one of these I think I stated an estimated 50 - 60 percent reduction in gain using a combination of selected 12au7's and selected (20 percent less) low gain power tubes.

 

[jeremy]

Re: Smalles tube amp

I need a really small tube amp for bedroom use. I had a go on an Epi Valve Jnr combo today, which was good, but still too loud if I want to make the tubes sing. Does anyone know of any smaller/quieter tube amps than this?

If I want to be properly loud I'll use the Fender amp downstairs.

wanmei1,
this is the original post. by changing preamp tubes he is not going to get the amp to sing at a lower volume. he can change the tone and gain structure so he has to crank the volume up higher to get it to sing but that isnt helping anything.

 

[wanmei1]

Re: Smalles tube amp

that's just not true, if you put 1000 speakers on 1W amp will it be as loud as 1000w amp?

In a word.......

Yes !

1024 speakers connected to a 1 watt amp is the same as a 1024 watt amp connected to just one of these speakers. ( in theory )

In theory because in practice you would lose a bit of power via the matching transformers needed to keep the original impedance rating.

Better hook up some more speakers just to be safe !

 

[bancika]

Re: Smalles tube amp

but that's against elementary school physic mate, more volume means more sound wave energy and that energy is supplied by a constant power source (let's say a 1w amp). Adding one more speaker (or 1023 more) will not be the same as having two (or 1024) 1w amps, but only one, so that 1w must be distributed over to both (all 1024) speakers.
Why would anyone bother with high power amps when they can just add more speakers.
Any clearer?

 

[bancika]

Re: Smalles tube amp

but maybe that's how it works in "The Universe", I'm talking about laws on the Earth

by the way, you wouldn't need matching transformers as long as speaker count is a power of two. Take two speakers in series paralleled to another two speakers in series and impedance doesn't change. Take those four speakers, connect them to another four in series and then parallel to another 8 connected the same way...you can go on like that forever...

 

[jeremy]

Re: Smalles tube amp

more speakers move more air which will give you a fuller sound but not necessarily louder. i have a mojo 30w speaker, a g12h clone in a deluxe amp and a 2x12 loaded with a v30 and g12h. the cab sounds much fatter and wider but doesnt really sound much louder with the 2x12 which surprised me. not emperical evidence since i havent checked it with a sound level meter but one experience in the real world

 

[bancika]

Re: Smalles tube amp

jeremi, that's what it should be like. It will sound different but not too much louder.
It's like switching from 2 wheel drive to 4 wheel drive, you won't go any faster but it will be different

 

[ImmortalSix]

Re: Smalles tube amp

jeremi, that's what it should be like. It will sound different but not too much louder.
It's like switching from 2 wheel drive to 4 wheel drive, you won't go any faster but it will be different

That's a great analogy man

 

[Myles]

Re: Smalles tube amp

Myles, thankyou for clarrifying that substituting a 12ax7 for a 12au7 does in fact reduce the output volume of an amplifier.

Your say that an amp will sound non toneful with a 12au7 ?
Have you tried it?
In my opinion they sounds superb.
If you really believe that a 12au7 sounds awfull then perhaps you should contact many of the worlds leading high fi stereo amp designers and tell them.
A large number of these more expensive amps only use 12au7's and we are talking $5000 plus amps. Interesting to note that most of the less expensive designs use 12ax7's.
I am not saying that the gain table I posted earlier is the reduction in rms produced by the amp, it is as you know simply the gain supplied to the power tubes, I included the reduction in gain percentage referenced to a 12ax7. I was illustrating the gain factor of each tube designation in descending order of output, to show which tubes can be subbed and in what order there output decreases and by how much in relation to a 12ax7. So that if a 12au7 produces too much of a reduction in power output of an amp you can look at that table and try the next incremental tube on the list and see if that ones more suitable etc.
See my follow up posts for clarification.
In one of these I think I stated an estimated 50 - 60 percent reduction in gain using a combination of selected 12au7's and selected (20 percent less) low gain power tubes.

Well ... in the end it is personal taste. I have never heard an amp that uses a 12AU7 in the first tone and gain stage that I liked.

As far as amps that do use a 12AU7 in the first gain stage can you give me an example as I cannot think of any myself which are guitar amps. Perhaps in hifi amp it is done but I cannot think of any standard classic amps that use an AU7.

 

[wanmei1]

Re: Smalles tube amp

wanmei1,
this is the original post. by changing preamp tubes he is not going to get the amp to sing at a lower volume. he can change the tone and gain structure so he has to crank the volume up higher to get it to sing but that isnt helping anything.

Have a look at my post number 24, the third last line, which says:
"There are some negatives to this. "
You have just pointed out one of these negatives.
If the op wanted to play clean then no problem as he has just gone from a 5 watt amp to a 2 watt amp simply and economically by swapping tubes.
As the op wants to drive the output tube ( I assume the Epi Jnr uses an EL84 ) into distortion then this presents a minor but not insurmountable problem for reasons you mentioned.

The solution is to order a power tube that is below spec rated output and has an early onset of distortion.
There are now a few tube sellers that test and rate tubes for just about any parameter you like, including onset of distortion . Some use a number system to grade tubes by the onset of distortion, where as others use a color code. By ordering a tube with early distortion onset, say 25% of maximum output then now the power amp will go into break up at 500 mv.
If however half a watt is too loud or doesn't provide enough power tube breakup and the thought of an overdrive pedal isn't appealing and or too expensive then it's the end of the road and time to choose a lower powered amp or another speaker, whichever is the most cost effective.

 

[Myles]

Re: Smalles tube amp

Have a look at my post number 24, the third last line, which says:
"There are some negatives to this. "
You have just pointed out one of these negatives.
If the op wanted to play clean then no problem as he has just gone from a 5 watt amp to a 2 watt amp simply and economically by swapping tubes.
As the op wants to drive the output tube ( I assume the Epi Jnr uses an EL84 ) into distortion then this presents a minor but not insurmountable problem for reasons you mentioned.

The solution is to order a power tube that is below spec rated output and has an early onset of distortion.
There are now a few tube sellers that test and rate tubes for just about any parameter you like, including onset of distortion . Some use a number system to grade tubes by the onset of distortion, where as others use a color code. By ordering a tube with early distortion onset, say 25% of maximum output then now the power amp will go into break up at 500 mv.
If however half a watt is too loud or doesn't provide enough power tube breakup and the thought of an overdrive pedal isn't appealing and or too expensive then it's the end of the road and time to choose a lower powered amp or another speaker, whichever is the most cost effective.

There is a bit of a problem with using tubes at "lower rating" numbers. After 1/2 a dozen years at Groove Tube it became very clear to me that tubes with high or low numbers are tubes that are not close to design spec, as in, something is "wrong" with the tubes basically. These high and low rating numbers are tubes that have problems in assembly, improper workmanship, low emissions or other problems. In the end the high rating tubes will generally fail earlier than expected and the lower rating tubes will not sound proper or at times develop full output. I proved these things to myself and others countless times in my role as tech support for GT and was the only one on site at GT that could program the tube testing machines.

Then again, these are just my own feelings, thoughts and observations.

In a case of the Epi, going to a lower rating will reduce output but the amp was designed around a spec tube and will run colder and sound grainy.

But, if you want less wattage think about lowering the B+ voltage and running higher current as tubes prefer current over voltage in many amp designs.

Or ... get the proper amp for the task at hand.

If you want a tube amp that goes from about 2 watts to 12+ take a look at the 65amps Lil' Elvis. It has a feature called "master voltage" which lowers the voltages across the entire circuit but is the first time I have seen all the voltages stay in proper relationship. All that have played the prototypes have fallen in love with the amp.

 

[wanmei1]

Re: Smalles tube amp

jeremi, that's what it should be like. It will sound different but not too much louder.
It's like switching from 2 wheel drive to 4 wheel drive, you won't go any faster but it will be different

I lost the link that spoke specifically about hooking up 1000 speakers to a 100 watt amp, however here's something I pulled of the net that throws the fly of phase cancellation in the ointment.

Number of Speakers is a factor that effects volume. This concept is not as simple as it first appears. If we double the number of speakers we will increase the SPL by 3dB. Here is an example. If you have a 50 watt amp with one 12" speaker and you add another 12" speaker you will get the magic 3dB increase. You would have the same SPL as a 100 watt amp with one 12" speaker. To get the next 3dB increase we need to double the speakers again so we would need four 12" speakers. Having 4 speakers will give us a 6dB increase in SPL compared to 1 speaker. Sounds like the same system as the power ratio above doesn't it. Here is were the complex part comes in. If we double the 4 speakers to 8 speakers you would think that there would be a 9dB increase in SPL compared to one speaker, right? Nope. What we get is only a 6dB increase compared to one speaker. Huh? We have now introduced a new factor to this equation...Phase Cancellation. (fig C) In short the distance between the speakers causes the sound to reach your ears, from some of the speakers, at a different time . This has the effect of canceling some of the sound. So...More speakers are better up to a point.

 

[wanmei1]

Re: Smalles tube amp

Well ... in the end it is personal taste. I have never heard an amp that uses a 12AU7 in the first tone and gain stage that I liked.

As far as amps that do use a 12AU7 in the first gain stage can you give me an example as I cannot think of any myself which are guitar amps. Perhaps in hifi amp it is done but I cannot think of any standard classic amps that use an AU7.

Vox AC50 ..The Beatles used these for early recordings.
Vox AC 100
Ampeg SVT
Traynor YBA 200
Ashton Blue Tongue 100
Gibson Gibsonette.

 

[karpathion]

Re: Smalles tube amp

I lost the link that spoke specifically about hooking up 1000 speakers to a 100 watt amp, however here's something I pulled of the net that throws the fly of phase cancellation in the ointment.

Number of Speakers is a factor that effects volume. This concept is not as simple as it first appears. If we double the number of speakers we will increase the SPL by 3dB. Here is an example. If you have a 50 watt amp with one 12" speaker and you add another 12" speaker you will get the magic 3dB increase. You would have the same SPL as a 100 watt amp with one 12" speaker. To get the next 3dB increase we need to double the speakers again so we would need four 12" speakers. Having 4 speakers will give us a 6dB increase in SPL compared to 1 speaker. Sounds like the same system as the power ratio above doesn't it. Here is were the complex part comes in. If we double the 4 speakers to 8 speakers you would think that there would be a 9dB increase in SPL compared to one speaker, right? Nope. What we get is only a 6dB increase compared to one speaker. Huh? We have now introduced a new factor to this equation...Phase Cancellation. (fig C) In short the distance between the speakers causes the sound to reach your ears, from some of the speakers, at a different time . This has the effect of canceling some of the sound. So...More speakers are better up to a point.

thats closer to what iv'e read. it works to a certian point (amount of speakers). then after that point it decay's. but your only gaining 3 db when you double the speakers up to that point.

 

[Myles]

Re: Smalles tube amp

Vox AC50 ..The Beatles used these for early recordings.
Vox AC 100
Ampeg SVT
Traynor YBA 200
Ashton Blue Tongue 100
Gibson Gibsonette.

Yes .... those are a few and the AC50 is one of the most amazing amps of all time but ..... those amps were designed around that tube rather than one putting an AU7 in an AX7 intended spot.