Hi all. Tube neophyte here with a potentially stupid question. Please bear with me.
I already own a tube preamp which is currently feeding a solid state amplifier. I am interested in adding a <5 watt valve amplifier to my listening room. I see many of the designs I'm looking at have tubes on the input stage like a 12ax7 or 6j1, as well as their output stage tubes. Is this going to be redundant or unnecessary since I already have the tube preamp? Should I just be looking for plans to build a model that is valve output only, to simplify the design and reduce cost? Is a tube input signal chain "too many" tubes?
Thanks for your patience.
I already own a tube preamp which is currently feeding a solid state amplifier. I am interested in adding a <5 watt valve amplifier to my listening room. I see many of the designs I'm looking at have tubes on the input stage like a 12ax7 or 6j1, as well as their output stage tubes. Is this going to be redundant or unnecessary since I already have the tube preamp? Should I just be looking for plans to build a model that is valve output only, to simplify the design and reduce cost? Is a tube input signal chain "too many" tubes?
Thanks for your patience.
A power amp will usually need appreciable voltage gain (as well as current gain).
Most output stages in power amps have little or no voltage gain, so the input stage
must supply that. Also gain is needed for overall negative feedback to be applied
and leave enough gain for use.
This gain is in addition to the typical gain in a line stage preamp, although this can be low
with digital sources. Many low gain phono stages will need a line stage with some voltage gain.
There are a few "power buffers" such as some from First Watt, but then you need a preamp line stage
with unusually large output voltage swing (about equal to the voltage needed at the speaker).
You could design a preamp/amp combination that would give just enough volume at full gain
for your sources and speakers, but that could change if you use different speakers.
Most output stages in power amps have little or no voltage gain, so the input stage
must supply that. Also gain is needed for overall negative feedback to be applied
and leave enough gain for use.
This gain is in addition to the typical gain in a line stage preamp, although this can be low
with digital sources. Many low gain phono stages will need a line stage with some voltage gain.
There are a few "power buffers" such as some from First Watt, but then you need a preamp line stage
with unusually large output voltage swing (about equal to the voltage needed at the speaker).
You could design a preamp/amp combination that would give just enough volume at full gain
for your sources and speakers, but that could change if you use different speakers.
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You have asked a very good question. I think the place to start is to question what is an "amp" and what is a "preamp". These are conventional expectations of what constitutes a hi-fi system, but if you reduce the question to its very basics, what you have is just a number of "stages" or building blocks. You can put a tube stage anywhere - in a DAC, in a preamp, in an amp or whatever. As long as it's not direct coupled or linked in other ways (e.g. feedback loops for pentode designs), these can be considered as separate building blocks concatenated together to give you the amount of gain you need. This is done most easily with SE triode stages with no global feedback.
So your question appears to be saying "if I already have the gain of e.g. a 12AX7 in a preamp stage, what do I need in the amp?" And the answer may be, as you are guessing, that you just need the output stage because that will give the whole system enough gain. So as long as your preamp stage is a good driver for an output stage (may not be the case with the 12AX7) and has enough gain, you could indeed just build an output stage. And bear in mind that output tubes vary in gain between low (300b) and high (EL84 in triode) But it would be better to optimise the design so you select the right driver and the right output tube.
I myself have several amps like this because I build SE triode stages in smaller size enclosures to keep weight down. What boxes to put the various stages in is a choice anyone can make. I have several SE 2a3 and 300b output stages and a few preamp stages with medium mu triodes which are good drivers for the outputs. You have in effect a 2 stage amplifier. The preamp stage has its own power supply and this has some advantages, like in my case being able to use a mesh plate rectifier.
So yes, building a preamp stage which drives a single output stage can be done and has been done. But what you do need to do is make sure the two stages will talk to each other properly in a well thought out design. You may have enough gain - and this is conditional on the sensitivity of your speakers - but that doesn't in itself guarantee a good enough result.
So your question appears to be saying "if I already have the gain of e.g. a 12AX7 in a preamp stage, what do I need in the amp?" And the answer may be, as you are guessing, that you just need the output stage because that will give the whole system enough gain. So as long as your preamp stage is a good driver for an output stage (may not be the case with the 12AX7) and has enough gain, you could indeed just build an output stage. And bear in mind that output tubes vary in gain between low (300b) and high (EL84 in triode) But it would be better to optimise the design so you select the right driver and the right output tube.
I myself have several amps like this because I build SE triode stages in smaller size enclosures to keep weight down. What boxes to put the various stages in is a choice anyone can make. I have several SE 2a3 and 300b output stages and a few preamp stages with medium mu triodes which are good drivers for the outputs. You have in effect a 2 stage amplifier. The preamp stage has its own power supply and this has some advantages, like in my case being able to use a mesh plate rectifier.
So yes, building a preamp stage which drives a single output stage can be done and has been done. But what you do need to do is make sure the two stages will talk to each other properly in a well thought out design. You may have enough gain - and this is conditional on the sensitivity of your speakers - but that doesn't in itself guarantee a good enough result.
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Thanks for this. I am a budget builder and tinkerer. I have the DAC and tube pre. Both are just FX Audio units but they play well together and both have been upgraded. The pre has matched NOS GE 5654w tubes, Muses 02 opamps and Wima caps. I have measured max output of 4.7v with zero clipping on my scope. I also have a DSPB-K that I use to handle active crossover, EQ and time delay duties. So I feel like I am pretty well sorted on the input side to adapt to some entry-level tube life. Running a 2.1 system with the DSP allows me to free up some headroom on a smaller amp as well. I would much rather invest my limited in better caps, tubes, transformers, pots, etc. in an output-only build if it's not going to be to the detriment of the final product.
4,7V is high , but not enough for driving directly a usual power tube like EL84 , so you still need a complete amplifier with input stage . Input tube could have much lower gain , not like 12AX7 . Simple single ended schematics are plenty , with different input sensitivity .
And you can change the amplifier sensitity by adjusting the negative feedback .
And you can change the amplifier sensitity by adjusting the negative feedback .
Thank you. I'll keep this in mind. Do you have insight or recommendations on the range of input sensitivity I should look for or build for, given my preamp output?
NegativeGhostrid,
You said you wanted a tube amplifier with less than 5 Watts (< 5 Watts).
Questions:
What loudspeaker model will the 1 Watt to 4 Watt amplifier be driving?
What is the loudspeaker impedance (rated; and actual varying impedance versus frequency)?
Be sure that any low power amplifier you build will perform well at the loudspeaker's actual minimum impedance.
What is the loudspeaker sensitivity rating?
. . . Caution:
A true 8 Ohm load at 2.83Vrms dissipates 1 Watt.
A true 4 Ohm load at 2.83Vrms dissipates 2 Watts.
So . . .
All speaker ratings being true (they often are not):
An 8 Ohm loudspeaker rated at '90dB at 2.83Vrms @ 1 meter' that is connected to the amplifier 8 Ohm tap, Sounds exactly as loud as . . .
A 4 Ohm loudspeaker rated at '93dB at 2.83Vrms @ 1 meter' that is connected to the amplifier 4 Ohm tap.
That 3 dB "improvement" of the 4 Ohm loudspeaker is a marketing lie (more accurately, not a lie, just a plan to deceive).
Early tube amplifiers typically had multiple output impedance taps; 2 or more of the following: 4, 8, 16.
When solid state amplifiers started coming out, most did not have "taps". The power rating was often the highest on the lower impedance speakers; such as more power in 4 ohms than in 8 Ohms (perhaps some loudspeaker marketeers took advantage of this, and used the 2.83Vrms as the standard for sensitivity ratings).
How big is your room?
How loud do you like to listen?
What kind of music will you listen too? Jimmy Hendrix played full bore, or a string quartet?
Think of this as a complete system:
From music playback source (CD player for example); then the electronic path (processor(s), preamp, 4 Watt power amp; then the loudspeaker; and finally the room.
You said you wanted a tube amplifier with less than 5 Watts (< 5 Watts).
Questions:
What loudspeaker model will the 1 Watt to 4 Watt amplifier be driving?
What is the loudspeaker impedance (rated; and actual varying impedance versus frequency)?
Be sure that any low power amplifier you build will perform well at the loudspeaker's actual minimum impedance.
What is the loudspeaker sensitivity rating?
. . . Caution:
A true 8 Ohm load at 2.83Vrms dissipates 1 Watt.
A true 4 Ohm load at 2.83Vrms dissipates 2 Watts.
So . . .
All speaker ratings being true (they often are not):
An 8 Ohm loudspeaker rated at '90dB at 2.83Vrms @ 1 meter' that is connected to the amplifier 8 Ohm tap, Sounds exactly as loud as . . .
A 4 Ohm loudspeaker rated at '93dB at 2.83Vrms @ 1 meter' that is connected to the amplifier 4 Ohm tap.
That 3 dB "improvement" of the 4 Ohm loudspeaker is a marketing lie (more accurately, not a lie, just a plan to deceive).
Early tube amplifiers typically had multiple output impedance taps; 2 or more of the following: 4, 8, 16.
When solid state amplifiers started coming out, most did not have "taps". The power rating was often the highest on the lower impedance speakers; such as more power in 4 ohms than in 8 Ohms (perhaps some loudspeaker marketeers took advantage of this, and used the 2.83Vrms as the standard for sensitivity ratings).
How big is your room?
How loud do you like to listen?
What kind of music will you listen too? Jimmy Hendrix played full bore, or a string quartet?
Think of this as a complete system:
From music playback source (CD player for example); then the electronic path (processor(s), preamp, 4 Watt power amp; then the loudspeaker; and finally the room.
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Is that 4.7V peak, peak-to-peak, or RMS?
I'm going to assume you meant peak volts. If peak-to-peak, then the peak voltage would be half, so 2.35V peak.
If you have 4.7V peak out available, and a KT88-triode output stage needs 47V peak to be driven to full output, that means you will still need a driver stage with at least 10X gain (20dB gain).
I could see a 2-stage SE amp, with something like a 6N6P or 6SN7 driver stage (there are many other choices as well), with that driver tube's grid bias set to something like -5VDC. 6N6P with fully bypassed cathode has gain of about 15X, so you'd have a little bit of extra gain available for getting loud from quiet recordings.
Normally a KT88-triode SE amp would need either a high-mu tube as its driver or a 2-stage driver circuit. Having 4.7V peak signal available means you can lose a stage.
Even though a KT88 or EL34 triode has about 10X gain itself, the output transformer is a step-down device, which means voltage gain is reduced to the speaker. That's why the output stage ends up with something close to unity gain. Then there's negative feedback reducing gain more, if you use that.
Sounds like a good project!
Probably also worth mentioning that not all tubes in a power amp actually amplify. The phase splitter in PP amps is typically a low-gain stage, but can be accomplished with “traditional” preamp tubes for economy. If you only need three halves of a 12A*7 tube, then you have a spare triode with which to build a phase splitter.
I'm looking at projects in that wattage range because that is my responsible working budget. Leaning heavily towards the Skunkie 6bm8 design with 15w output transformers at the moment. I have a couple of sets of DMLs that I think will potentially play well with that power. If not, I have plans for a few different open baffle speakers with sensitivity in the high 90's on standby. (I have far more experience with speaker design and crossover building than electronics).
My room is about 450 square feet and pretty well treated (it's also my theater room). I have larger class AB amps and quite a few speakers for louder play or more dynamic music. The tube amp would be for more delicate/specific types of music. It might also find its way into other, smaller rooms of the house. Regarding impedance I can build the speakers to the appropriate rating and check the impedance curves and design passive crossovers, if needed, to play nice with the amp.
That would be 4.7 VRMS. Sorry. I should have been more specific.
Attachments
Are you measuring 4.7VRMS at 40Hz? Did you try other frequencies?
If your preamp is starting to roll off in the lower Hz then you would get more voltage in the center of its range; typically you might try 1kHz as a nice, round number.
If your preamp is starting to roll off in the lower Hz then you would get more voltage in the center of its range; typically you might try 1kHz as a nice, round number.
Looks like I was at 40Hz at the time, yeah. I can retest at 1khz at some point.
Note:
If a Hi Fi/Stereo preamp is down more than 1 or 2dB at 40Hz versus at 1kHz, the preamp needs to be fixed/re-designed.
A single-pole roll off that is -1dB at 40Hz versus 1kHz will be -3dB at 20Hz.
A multi-pole roll off that is -1dB at 40Hz versus 1kHz will be -6, -9, -12dB or more depending on the number of poles, and their individual -3dB frequencies.
There are the Orchestral Thunder Drums, 32 foot organ pipes, and other low frequency music. They will be missing their power.
-3dB at 40Hz is not too bad for a guitar amp that is for Lead Guitar (it will only be -1dB at the lead guitar's lowest note of ~ 80Hz).
For a Bass Guitar, -3dB at 40Hz will take away 1/2 of the power and vibration of the bass note.
If a Hi Fi/Stereo preamp is down more than 1 or 2dB at 40Hz versus at 1kHz, the preamp needs to be fixed/re-designed.
A single-pole roll off that is -1dB at 40Hz versus 1kHz will be -3dB at 20Hz.
A multi-pole roll off that is -1dB at 40Hz versus 1kHz will be -6, -9, -12dB or more depending on the number of poles, and their individual -3dB frequencies.
There are the Orchestral Thunder Drums, 32 foot organ pipes, and other low frequency music. They will be missing their power.
-3dB at 40Hz is not too bad for a guitar amp that is for Lead Guitar (it will only be -1dB at the lead guitar's lowest note of ~ 80Hz).
For a Bass Guitar, -3dB at 40Hz will take away 1/2 of the power and vibration of the bass note.
A couple of quick points. The tubes used in the preamp have almost nothing to do with the output voltage swing. A 12AX7 with a mu of 100 would give a gain of at least 50 and no preamp I have ever heard of has nearly that much gain. Some of that gain will be used for either losses in things like balance and/or tone circuit or will be reduced with local feedback to improve linearity and consistency of gain. As others have pointed out virtually all power amps will require a voltage gain stage.
For a nice power amp SE Ultralinear KT-88s are very nice. I built one using plate to plate feedback and really like the results. Should be about 5-8WPC. I am not currently using mine as the current domestic layout is not appropriate. If I can get speaker and preamp issues addressed I may set it up in the music room.
For a nice power amp SE Ultralinear KT-88s are very nice. I built one using plate to plate feedback and really like the results. Should be about 5-8WPC. I am not currently using mine as the current domestic layout is not appropriate. If I can get speaker and preamp issues addressed I may set it up in the music room.