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hybrid amp question
hybrid amp question
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Old 15th September 2014, 09:53 PM   #1
Illinest is offline Illinest
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Default hybrid amp question

Hello again.

Background:
I completed my amp - different thread - it's got a single 12ax7 that covers the input gain stage and the phase inversion, and a pair of 6l6 tubes in push-pull for the output. 3 valves in total. It works properly, it's loud enough, but it's a completely clean tone and as one person here put it to me - it's not entirely fair to call it an amp. I was planning to add another 12ax7 valve to the pre-amp as had been suggested but...

Question:
I saw a schematic for a pedal equalizer that also provides a clean boost (tonemender pedal at runoffgroove) and I was wondering if I could substitute solid-state circuitry instead?
Specifically I was planning to build this tonemender pedal in a small housing and then permanently attach it to the side of the amp. The guitar input would go into the amp, straight to the tonemender housing, through the tone-stack and boost circuit, then on to the volume knob and the pre-amp tube.

I'm asking about this because I'm not sure about impedance matching between ICs and tubes, and I'm also not really aware of where the distortion comes from. I thought that I could just clean-boost the guitar signal and eventually start to get some nice tube distortion out of the 3 tubes. Am I misunderstanding something? Does the pre-amp NEED to have more tubes, or is that single triode enough? Assuming a boosted signal of course...


Bonus question: how does guitar volume relate to the gain stages? How would one go about designing an amp that allows the guitarist to change the distortion levels with the guitar volume pot, but not also have an accompanying huge change in amp volume.
Is it something like having a low gain input stage that feeds into higher gain stages in the preamp?
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Old 15th September 2014, 10:54 PM   #2
sreten is offline sreten  United Kingdom
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Hi,

Your amplifier has the tubes for a power amplifier, but not enough
tubes to accept a guitar input and work well. You don't normally
overdrive the input stage of a power amplifier, its distortion
profile comes from driving the valve output stage hard,
i.e. nothing to do with high gain overdriven preamps.

Rejig your amplifier into a power amplifier. Add a preamp,
it could be a multieffects pedal. Point is it should have
a Master volume on its output, and lots of preamp
distortion options available, simple or modelled.

FWIW most guitarists don't use the guitars volume knobs.*
A channel switching or boosting stomp box is preferred.

You need to get your head round guitar preamps.

rgds, sreten.

* Except Fender Strats rewired for the volume at
at the front and the little finger used to control it.
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Old 15th September 2014, 11:19 PM   #3
famousmockingbird is offline famousmockingbird  United States
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Quote:
Originally Posted by sreten View Post
FWIW most guitarists don't use the guitars volume knobs.
Maybe the ones that don't know anything about how to use or play a guitar.
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Old 15th September 2014, 11:57 PM   #4
Illinest is offline Illinest
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Quote:
Originally Posted by famousmockingbird View Post
Maybe the ones that don't know anything about how to use or play a guitar.
I appreciate it but I don't think it matters if i play the guitar like pros do. I like it when the volume knob on the guitar functions as a sort of distortion control. Some amps work better than others and i don't have a clue how that works.

As for the pre-amp - i truly do need help.

I don't understand what it means to "re-jig my amp into a power amplifier". What's the difference?

I thought that the basics were like this:

Guitar ouput signal of maybe 1v peak to peak
First triode amplifies the signal.
Second triode is phase inversion - but no amplification.
Then the paired pentodes amplified the signal again - and the OT turns a high voltage, low current waveform into a low voltage, high current waveform.

I thought that if you increased the guitar output enough (via pedal or whatever) the first triode and the paired pentodes would eventually start to distort.
Do they both distort at the same time? Is the goal to balance them so that they distort together? What happens if the triode distorts but the pentodes are still amplifying cleanly? What about the opposite - where the triode amplifies clean signals but the pentodes distort?
Why would you need a tube pre-amp at all?
What does the master volume control have to do with it?
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Old 16th September 2014, 12:20 AM   #5
nigelwright7557 is offline nigelwright7557  United Kingdom
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I picked up a SS circuit in Wireless World in the 1980's for producing valve type distortion.
You can tweak the pot to get the sound you want.
It needs a pre amp before it to raise the signal enough.
I use it with my electric guitar.
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Old 16th September 2014, 12:29 AM   #6
famousmockingbird is offline famousmockingbird  United States
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Quote:
Originally Posted by Illinest View Post
I appreciate it but I don't think it matters if i play the guitar like pros do. I like it when the volume knob on the guitar functions as a sort of distortion control. Some amps work better than others and i don't have a clue how that works.

I am agreeing with you, I wasn't inferring anything about your guitar skills, sreten's statement that most guitar players don't use the volume control is bogus.

I too use my volume control quite a bit, and yes with some of my amps as a distortion control.

And yes if you use a boost in front of your amp it will distort how you want it too. All guitar amps don't distort the same way, it depends on how many gain stages and how they are biased, and if you want to distort the output tubes etc.......This is what gives different amps their own characteristics. I use a 5E3 and turn the volume control all the way up and use my guitar's volume as a distortion control like you want to do. You will get a similar effect with your amp if you use a boost pedal.

I have your amp you built in a simulation file and will do a block diagram of your amps characteristics and compare it to a typical fender amp after I eat some dinner. Basically all your amp is missing is one gain stage at the beginning which a boost pedal will take the place of.
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Old 16th September 2014, 12:58 AM   #7
Illinest is offline Illinest
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Quote:
Originally Posted by famousmockingbird View Post
I am agreeing with you, I wasn't inferring anything about your guitar skills, sreten's statement that most guitar players don't use the volume control is bogus.

I too use my volume control quite a bit, and yes with some of my amps as a distortion control.

And yes if you use a boost in front of your amp it will distort how you want it too. All guitar amps don't distort the same way, it depends on how many gain stages and how they are biased, and if you want to distort the output tubes etc.......This is what gives different amps their own characteristics. I use a 5E3 and turn the volume control all the way up and use my guitar's volume as a distortion control like you want to do. You will get a similar effect with your amp if you use a boost pedal.

I have your amp you built in a simulation file and will do a block diagram of your amps characteristics and compare it to a typical fender amp after I eat some dinner. Basically all your amp is missing is one gain stage at the beginning which a boost pedal will take the place of.
I have heard that even order harmonics are the most pleasing to the ear and that the distortion of a valve amplifier produces more of the even order harmonics, while solid state produces more odd harmonics.
I have also heard that a pentode sounds better in distortion than a triode.

I have a feeling that i should've gotten opinions about those statements first.

If those are true, then my attempt to substitute a solid state clean boost in place of a preamp valve ought to make better sense. I'm also trying to get a handle on the minimum number of valves that will produce good distorted guitar tones.

If you told me that 16 pre-amp valves in a row feeding a quadruple parallel push-pull output stage would create the best tone then i'd accept it but i'd still want to know why.
But if you only need a pre-amp valve and a pair of output valves (ignoring single-ended circuits of course) then it seems to me that solid state is a more efficient way to get the signals up to the needed levels - as long as the solid state portions are staying clean that is.


@nigelwright7557
I've heard some good solid state models of distortion before, but i'm generally not too keen on the way they sound. I also don't understand how the designers have attempted to model valve distortion. The transistors couldn't possibly clip the same so the process doesn't make sense to me yet.

Last edited by Illinest; 16th September 2014 at 01:01 AM.
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Old 16th September 2014, 07:10 AM   #8
famousmockingbird is offline famousmockingbird  United States
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Ok lets compare it to the Fender 57 Deluxe (5E3) circuit. For simplicity I omitted the tone control.

Block diagram is as follows:

Deluxe
Input--->gain stage--->volume control--->gain stage--->phase splitter--->output

Your amp
Input--->volume control--->gain stage--->phase splitter--->output

lets use a 1kHz 20mV input signal because that's what Fender uses for testing.

First lets look at the Deluxe. With a 20mV input signal we get 1V out of the plate on the first gain stage which gives us a 34db gain. Your amp with the volume control on max and 20mV input we also get 1V out at the plate of the first gain stage giving us 34db gain. This is where things differ. The Deluxe goes to another gain stage through volume control. To get max clean output the volume control is set to -12db attenuation bringing the 1V at the plate from the first gain stage down to 250mV. When this passes through the next gain stage we get 15.5V out of the plate (35db gain) going to the phase splitter. We now get an inverted 15.5V out of the phase inverter's plate and 15.5V of the same phase seen at the grid out of the cathode. This 15.5V is plenty to drive the 6V6's into giving us 10V into an 8 ohm load on the secondary of the OPT, 12watts of power. Back to your amp, the 1V out of the first gain stage goes to the phase splitter giving us an inverted 1V at the plate and 1V at the cathode matching the grids phase. This 1V is sent to the 6L6's and is not even close to driving the power tubes to full power.

As you can see with a small input signal you can drive the Deluxe into full power with the volume control about 1/4 of the way up. When the amp's volume control is about 1/2 the way up (-6db of attenuation giving 500mV) you start to overdrive the phase splitter, anymore volume after that you start to overdrive the power tubes.

Back to your amp. To get max clean output power of 30 watts you will need an input voltage of around 320mV. Remember the to get max clean output on the Deluxe we were putting about 250mV into the second gain stage. If you increase the input of your amp to 400mV you start to overdrive the power tubes.

Your amp is somewhat close to the Deluxe. Just picture your first gain stage as the Deluxe's second. The simulations I did was with a 12AX7 in the Deluxe and a 12AT7 in your amp. Your amp's phase splitter is operating in grid current with a 12AX7, Vgk .8V. With the 12AT7 the phase splitter Vgk is -1V.

Last edited by famousmockingbird; 16th September 2014 at 07:28 AM.
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Old 16th September 2014, 02:37 PM   #9
Illinest is offline Illinest
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Quote:
Originally Posted by famousmockingbird View Post
Ok lets compare it to the Fender 57 Deluxe (5E3) circuit. For simplicity I omitted the tone control.

Block diagram is as follows:

Deluxe
Input--->gain stage--->volume control--->gain stage--->phase splitter--->output

Your amp
Input--->volume control--->gain stage--->phase splitter--->output

lets use a 1kHz 20mV input signal because that's what Fender uses for testing.

First lets look at the Deluxe. With a 20mV input signal we get 1V out of the plate on the first gain stage which gives us a 34db gain. Your amp with the volume control on max and 20mV input we also get 1V out at the plate of the first gain stage giving us 34db gain. This is where things differ. The Deluxe goes to another gain stage through volume control. To get max clean output the volume control is set to -12db attenuation bringing the 1V at the plate from the first gain stage down to 250mV. When this passes through the next gain stage we get 15.5V out of the plate (35db gain) going to the phase splitter. We now get an inverted 15.5V out of the phase inverter's plate and 15.5V of the same phase seen at the grid out of the cathode. This 15.5V is plenty to drive the 6V6's into giving us 10V into an 8 ohm load on the secondary of the OPT, 12watts of power. Back to your amp, the 1V out of the first gain stage goes to the phase splitter giving us an inverted 1V at the plate and 1V at the cathode matching the grids phase. This 1V is sent to the 6L6's and is not even close to driving the power tubes to full power.

As you can see with a small input signal you can drive the Deluxe into full power with the volume control about 1/4 of the way up. When the amp's volume control is about 1/2 the way up (-6db of attenuation giving 500mV) you start to overdrive the phase splitter, anymore volume after that you start to overdrive the power tubes.

Back to your amp. To get max clean output power of 30 watts you will need an input voltage of around 320mV. Remember the to get max clean output on the Deluxe we were putting about 250mV into the second gain stage. If you increase the input of your amp to 400mV you start to overdrive the power tubes.

Your amp is somewhat close to the Deluxe. Just picture your first gain stage as the Deluxe's second. The simulations I did was with a 12AX7 in the Deluxe and a 12AT7 in your amp. Your amp's phase splitter is operating in grid current with a 12AX7, Vgk .8V. With the 12AT7 the phase splitter Vgk is -1V.
There's a lot to digest there. I think I need to re-examine some of my assumptions.
I've been using guitar output voltages that are significantly higher than any of these figures. I know that there is a pretty significant difference between peak and rms wattage on car stereos, and I suppose that the output of guitar pickups must be similarly exaggerated. If Fender assumes 20mV then I guess it might be a good idea for me to do the same.
I guess I need to come up with a rule of thumb for guitar voltages. Web says Strat pickups vary from 100mV up to 1V. My active pickup is supposedly 3V to 4.5V. I guess that I could safely assume that I could use proportional figures - so I'll assume that my actives are pushing 60-80mV in order to better align with your example.

Now here is where I'm clueless about the math. I don't know how dbs and amplification factors are figured. I know that I'll be in good shape if I can raise that 60-80mV (from the actives) and 20mV (from passives) up into the 400mV to 1V range.
I can guess that another tube gain stage (1 triode of a 12ax7 for example) would get me there.
But I don't know anything about solid state amplification yet. I've got an idea about how much voltage I can put into the Tonemender circuit but I don't know how to figure out how much voltage I can expect to come out of it. Now I have a direction and I'm going to go try to learn something about it.

I really appreciate the help that you've given me. Thank you.
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Old 16th September 2014, 06:28 PM   #10
famousmockingbird is offline famousmockingbird  United States
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I think it's like amplifiers, everybody wants to market their amps as "puts out this many watts", and speaker "these are 1000 watts speaker", it's just marketing. Can those pickups put out those large transient voltages? Of course they can , but those peak voltages happen during hard strumming when you attack the strings hard, but for most of the time you are playing the rms voltage can be quite low so they design their amps with lots of gain.

Quote:
Originally Posted by Illinest View Post
Now here is where I'm clueless about the math. I don't know how dbs and amplification factors are figured.

It's simple algebra. Let's use the 20mV to 1V for example.

Amplification factor would be 1/.02 = 50 So it has a gain or amplification factor of 50x, the output is 50 times larger than the input.

We like to usually express things in decibels which is a ratio of two numbers, again simple algebra.

1/.02 = 50

log50 = 1.69897

1.69897 x 20 = 33.97db
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