Does anyone have a good article of a DIY tube amplifier designed to run off 12vdc?
I've considered building a 4ch tube amplifier for my car. There aren't many affordable options for the HV supply but I had a thought. Couldn't a 220v power inverter work? Internally they step 12vdc to around 220vdc before it's turned into AC by another stage. Couldn't the first stage be used for the HV supply in a mobile tube amp?
If so it would be a nice small low-cost solution. Anyone done it?
I've considered building a 4ch tube amplifier for my car. There aren't many affordable options for the HV supply but I had a thought. Couldn't a 220v power inverter work? Internally they step 12vdc to around 220vdc before it's turned into AC by another stage. Couldn't the first stage be used for the HV supply in a mobile tube amp?
If so it would be a nice small low-cost solution. Anyone done it?
Inverters work by turning 12vdc into 110/220/240vac. They do not first convert the 12vdc to some higher voltage dc. They use a transformer-based oscillator to create the alternating current and also to step the voltage up to whatever line voltage that is appropriate for the region.
You would need to rectify and filter the 220vac to get your B+ voltage.
You would need to rectify and filter the 220vac to get your B+ voltage.
That is not accurate. Most inexpensive inverters are modified sine-wave and have 2 stages to create "ac". The first stage is a high frequency push-pull SMPS step-up convertor. The second stage is 4 (or more) MOSFETs that alternates the DC polarity (at 60hz) to the AC receptacle. That much I know.
I'm wondering if anyone's ever tried it.
Here's a schematic. The top half is the DC-DC step-up converter. There are various topologies. Most common is a push-pull which looks a lot like the output stage of a pp tube amp or the power supply of an automotive amplifier. There will be high voltage on C3. The bottom half is the the DC-AC converter. If you tap the power before the second stage you will have a nice DC supply.
I'm wondering if anyone's ever tried it.
Here's a schematic. The top half is the DC-DC step-up converter. There are various topologies. Most common is a push-pull which looks a lot like the output stage of a pp tube amp or the power supply of an automotive amplifier. There will be high voltage on C3. The bottom half is the the DC-AC converter. If you tap the power before the second stage you will have a nice DC supply.
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This 120vac inverter was rated at 300w and has 147vdc coming from the DC-DC portion. Using that math, a 220v inverter might have about 270vdc. Rough schematic showing below of the main components.
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there is another-simpler solution :
you can use the valve as a buffer, without gain.
If you decide to do that, you dont need an extra high voltage winding from your trafo, a +30-40VDC (the positive rail voltage of your power amp stage) is usually sufficient, and you have all the warmth of the tube.
Of course a gain stage is still mandatory.
you can use the valve as a buffer, without gain.
If you decide to do that, you dont need an extra high voltage winding from your trafo, a +30-40VDC (the positive rail voltage of your power amp stage) is usually sufficient, and you have all the warmth of the tube.
Of course a gain stage is still mandatory.
After hearing my Magnavox in the car I'm bent on building a 4ch automotive amp.
I decided on building a 400v SMPS supply and likely running 6v6 PP output. It's simple enough. A power inverter would be fine if you didn't need a B+ higher then 150-250v.
I decided on building a 400v SMPS supply and likely running 6v6 PP output. It's simple enough. A power inverter would be fine if you didn't need a B+ higher then 150-250v.
They think driving and text is dangerous, wait till everyone gets distracted by beautiful glowing tubes coming out of the dash.
not to mention the major drawbacks of valve equipment and vibration in automotive environments. Yea its a great sound, maybe even a great idea. But not practical because when hitting road bumps, it can knock tubes loose, cause them to crack, and even worse, microphonics pickup causing loud noises in the speakers when vibrating/bouncing around.
Sure, if its designed well enough it could overcome alot of those problems, but they will always still be there even if its in their slightest form. Makes me wonder how the old tube radios in the early cars ever worked....
Sure, if its designed well enough it could overcome alot of those problems, but they will always still be there even if its in their slightest form. Makes me wonder how the old tube radios in the early cars ever worked....
Tube are pretty robust. Sure it's glass but it takes a pretty good smack to break it. The military used tubes in countless portable and mobile devices. They were a higher quality then consumer tubes but I'm not flying my car or firing artillery from it, although the thought has crossed my mind. 🙂
Last night I had my Motorola chassis running off a car battery on my bench. B+ was only 210v due to my miscalculation on the secondary winding. It sounded good but a little thin and overall gain was low. Zero hum and zero noise. Heaters ran directly off the battery. Configuration of the Motorola amp is 2x 12AX7 preamp/pi and 4x 6BQ5. 6BQ5 heater wired in series pairs. Total heater current measured at 1.6a.
I just finished rewiring a new transformer. This should give me 400vdc with a little sag under load. The final supply will have a heater tap and aux winding that could be used for negative grid bias.
Last night I had my Motorola chassis running off a car battery on my bench. B+ was only 210v due to my miscalculation on the secondary winding. It sounded good but a little thin and overall gain was low. Zero hum and zero noise. Heaters ran directly off the battery. Configuration of the Motorola amp is 2x 12AX7 preamp/pi and 4x 6BQ5. 6BQ5 heater wired in series pairs. Total heater current measured at 1.6a.
I just finished rewiring a new transformer. This should give me 400vdc with a little sag under load. The final supply will have a heater tap and aux winding that could be used for negative grid bias.
my gut tells me they wouldn't have used used military spec valves in cars either, I'd say all the pop and crackle of am radio would have blended nicely road noise microphonics, not to mention old cars typically being ALLOT louder over all (engine, tyres, exhaust, engine mounts, etc) I don't think sound quality was noticeable to be a priority as it is today. I could be wrong..
You reminded me of something I saw on an old Cadillac. There was a spring in the dust cap of the wheel bearing that had a point in the middle to keep in contact with the center of the spindle. I figured this was to keep the hub/drum/wheel grounded to discharge static caused by the tires spinning.
I don't have any worries about vibration or microphonic tubes. A car is already an awful listening environment and anything would be an improvement.
Having heard a tube amp in the car raised the bar and now my Alpine amp sounds like crap.
What is it, electrically speaking, with tubes and stringed instruments? Why do they make strings sound so darn good?
As for my project... I got my mock-up SMPS to output 400v now (rewound secondary) and, combined with the heaters, draws 6.4a (77w) for 4x 6BQ5 and 2x 12AX7. It's a lot less then I was expecting. 🙂
I don't have any worries about vibration or microphonic tubes. A car is already an awful listening environment and anything would be an improvement.
Having heard a tube amp in the car raised the bar and now my Alpine amp sounds like crap.
What is it, electrically speaking, with tubes and stringed instruments? Why do they make strings sound so darn good?
As for my project... I got my mock-up SMPS to output 400v now (rewound secondary) and, combined with the heaters, draws 6.4a (77w) for 4x 6BQ5 and 2x 12AX7. It's a lot less then I was expecting. 🙂
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my preamp goes to (what I consider) a midrange class D power amp. I've always been told class D are only good for bass and do not sound as good as a true class Or even class AB,(I put this down to people who made up their mind based on some of the early primitive class D amps) but I once i threw that valve pre amp in there, the difference is not only completely obvious to me, but to anyone else who jumps in for a listen, even those who know nothing about audio. I have my system configured so if I want I can plug my phone directly to the valve preamp, or via the aux jack off the head unit so a comparison is very easy; the head unit is fairly pathetic. have faith in your amp, try plugging a high quality source directly into it and see how it sounds remembering that if the sound is coming from a headphone output there may be some impedance mismatch, but it will give you an idea of what that amp is capable of, clarity wise.
regarding strings sounding so good, I think there are many layers to strings, particularly gut strings. I've noticed my head unit, as well as sounding muddy and dull, sounds almost like the amp processes an expander and compressor effect on the sound, then amplifies the remaining signal to what ever you choose, so you loose all dynamics, it's all just the same volume. just loud or quiet. i had a class D amp that did a similar thing too. I don't know if this is what it actually does, just what my ears tell me. in this way, most of the sound info is lost in all the other sound, especially those of which are quite delicate, like strings, piano, cymbals etc
regarding strings sounding so good, I think there are many layers to strings, particularly gut strings. I've noticed my head unit, as well as sounding muddy and dull, sounds almost like the amp processes an expander and compressor effect on the sound, then amplifies the remaining signal to what ever you choose, so you loose all dynamics, it's all just the same volume. just loud or quiet. i had a class D amp that did a similar thing too. I don't know if this is what it actually does, just what my ears tell me. in this way, most of the sound info is lost in all the other sound, especially those of which are quite delicate, like strings, piano, cymbals etc
Today is my lucky day. I found another Maganavox 9302 based console stereo for $15 bucks and it works. Now I have a matched quad of output transformers but I bet they're 8-ohm.
I did try feeding an input directly into the tube amp and it sounded better then the head unit but not by much. A few guys over on the RX-8 forum have been analyzing this car's Bose system and even plotted the frequency response which was surprisingly flat. Turns out Bose did all the (awful) shaping in the amps. The radio is very integrated into the dash so I'm trying to make the most of it. It does have a balanced line output. Maybe I could use that to my advantage and eliminate the phase inverter and simply use a 12AX7 for PP drivers.
I've heard some class-d "full range" amps and it's hit or miss. Some sounded good, some sounded bad. Nothing amazed me. I'll stick with class-d for my little 8" subs and tube the rest. The Magnavox was plenty loud for my taste and that was on old tubes, tube rectification, and only playing on the door speakers which are rated at 96db. I just added some 93db 6x9s in the deck but haven't had a change to try the tube amps on them. Now that I have two stereo amps I'm dying to try it.
About that phase inverter. Are there any quality transformers suitable for audio use? I'm thinking along the lines of something that would accept a speaker level input and output 2-phase capable of driving grids. I think it's called an interstage transformer.
I did try feeding an input directly into the tube amp and it sounded better then the head unit but not by much. A few guys over on the RX-8 forum have been analyzing this car's Bose system and even plotted the frequency response which was surprisingly flat. Turns out Bose did all the (awful) shaping in the amps. The radio is very integrated into the dash so I'm trying to make the most of it. It does have a balanced line output. Maybe I could use that to my advantage and eliminate the phase inverter and simply use a 12AX7 for PP drivers.
I've heard some class-d "full range" amps and it's hit or miss. Some sounded good, some sounded bad. Nothing amazed me. I'll stick with class-d for my little 8" subs and tube the rest. The Magnavox was plenty loud for my taste and that was on old tubes, tube rectification, and only playing on the door speakers which are rated at 96db. I just added some 93db 6x9s in the deck but haven't had a change to try the tube amps on them. Now that I have two stereo amps I'm dying to try it.
About that phase inverter. Are there any quality transformers suitable for audio use? I'm thinking along the lines of something that would accept a speaker level input and output 2-phase capable of driving grids. I think it's called an interstage transformer.
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If you guys were going to build a mobile tube amp, what tubes/topology would you recommend?
I'm thinking about using the 6V6 in push-pull. It's heater current is lower then a 6BQ5 and it offers a little more power. Though the new EH 6V6s I had on hand to test were a bit microphonic in my bench test.
6BQ5 = 0.76a (4.79w). 8 tubes = 6.08a (55.19w)
6V6 = 0.45a (2.84w). 8 tubes = 3.6a (22.68w)
The current will be half that since I'll be connecting them in series pairs for 12v operation.
Next would be bias. I could do a fixed-bias (more power) but cathode bias might be a better option in the conditions the tubes will be used. Opinions on that? Is there a self-adjusting fixed-bias circuit? LED bias?
I do like the size of the 6BQ5 and the output was fairly sufficient. At what I consider loud, the 6BQ5 was right at the onset of distortion.
I'm going to swap out the sockets in my Motorola chassis and give them a try.
Is there another small tube like the 6BQ5 that might be a better option?
I'm thinking about using the 6V6 in push-pull. It's heater current is lower then a 6BQ5 and it offers a little more power. Though the new EH 6V6s I had on hand to test were a bit microphonic in my bench test.
6BQ5 = 0.76a (4.79w). 8 tubes = 6.08a (55.19w)
6V6 = 0.45a (2.84w). 8 tubes = 3.6a (22.68w)
The current will be half that since I'll be connecting them in series pairs for 12v operation.
Next would be bias. I could do a fixed-bias (more power) but cathode bias might be a better option in the conditions the tubes will be used. Opinions on that? Is there a self-adjusting fixed-bias circuit? LED bias?
I do like the size of the 6BQ5 and the output was fairly sufficient. At what I consider loud, the 6BQ5 was right at the onset of distortion.
I'm going to swap out the sockets in my Motorola chassis and give them a try.
Is there another small tube like the 6BQ5 that might be a better option?
Here's a drawing of the Motorola schematic.
It looks like they placed a resistor divider across both phases outputs rather then use a resistor divider to ground. How well does that work? It seems to me that it may have a more symmetrical output then some other paraphrase PI circuits I've seen. Wouldn't an asymmetrical output change the gain of the second triode?
What is the purpose of C4?
There seems to be a problem with the site at the moment. As soon as the lag goes away I'll upload the schematic.
It looks like they placed a resistor divider across both phases outputs rather then use a resistor divider to ground. How well does that work? It seems to me that it may have a more symmetrical output then some other paraphrase PI circuits I've seen. Wouldn't an asymmetrical output change the gain of the second triode?
What is the purpose of C4?
There seems to be a problem with the site at the moment. As soon as the lag goes away I'll upload the schematic.
I wish there wasn't a time limit on editing posts.
For a single 12AX7 as a preamp, what has more gain? A paraphase (as shown in the schematic) or a cathodyne with 1/2 of the 12AX7 in front of it? I should have a 2-4v signal coming in.
For a single 12AX7 as a preamp, what has more gain? A paraphase (as shown in the schematic) or a cathodyne with 1/2 of the 12AX7 in front of it? I should have a 2-4v signal coming in.
As for the power amp...
1. 400v B+ with 2x EL34 PP. Can it manage 50w AB? What OPT? (think cheap).
2. Can a 12ax7 cathodyne drive two EL34s?
3. Is there a method of bias that's a mix between fixed (for power) and cathode bias (for low maintenance)? Or a low maintenance fixed bias?
1. 400v B+ with 2x EL34 PP. Can it manage 50w AB? What OPT? (think cheap).
2. Can a 12ax7 cathodyne drive two EL34s?
3. Is there a method of bias that's a mix between fixed (for power) and cathode bias (for low maintenance)? Or a low maintenance fixed bias?
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