Hi, just finished an amplifier with the OR120 preamp and a 12AU7 PP power stage:
The output stage uses a line transformer, with the 0, 2.5W and 0.625W taps and a 8 ohms speaker connected to the 4 ohm tap to get a primary impedance of 32k.
Plate voltage is at 240V, biased at roughly -12V, which should give roughly 0.9W in output power.
I added the 6dB feedback, as in the original amplifier.
The enclosure is a 1590N1 box with the tubes aligned.
PTP style build, since it is a prototype.
It uses a SMPS I got some time ago from ebay:
https://www.ebay.com/itm/133882043567
Much easier to install (and silent) than my own designs.
I think it proves the point, that one can use tubes and get 1W of power without a heavy transformer. The SMPS could be cheaper though...on the other hand, the parts are also quite expensive (mosfet, inductor and IC)
The final thing:
The FAC control is the switch at the back of the amp. It only has 3 settings: 4.7nF, 1nF, 470pF or, as I call it: too much bass, ok, lots of treble.
Cheers
The output stage uses a line transformer, with the 0, 2.5W and 0.625W taps and a 8 ohms speaker connected to the 4 ohm tap to get a primary impedance of 32k.
Plate voltage is at 240V, biased at roughly -12V, which should give roughly 0.9W in output power.
I added the 6dB feedback, as in the original amplifier.
The enclosure is a 1590N1 box with the tubes aligned.
PTP style build, since it is a prototype.
It uses a SMPS I got some time ago from ebay:
https://www.ebay.com/itm/133882043567
Much easier to install (and silent) than my own designs.
I think it proves the point, that one can use tubes and get 1W of power without a heavy transformer. The SMPS could be cheaper though...on the other hand, the parts are also quite expensive (mosfet, inductor and IC)
The final thing:
The FAC control is the switch at the back of the amp. It only has 3 settings: 4.7nF, 1nF, 470pF or, as I call it: too much bass, ok, lots of treble.
Cheers
You need a perforated metal screen to put over it so you can throw it in your pocket and take it places.
I wonder if it is really a good idea to put the volume control(s) within the NFB loop?
Best regards!
Best regards!
Uhm, haven't thought about that, but the Matamp with master volume also uses one.
It sounds OK at low volumes, and I haven't played it full volume yet (not even when clean).
It is loud for only 1W, and I have neighbours (I live in an apartment).
With the master as shown, turning it down will reduce the loop gain. At low master settings you will have very little negative feedback. I suppose this could actually be desirable to some extent since decreasing NFB will result in boosted highs and lows with a typical loudspeaker load. Sort of like a loudness control. With the master really low it might sound a little fizzy.
Just realized that I was thinking of pentodes when I wrote this. Triodes have a fairly low output impedance due to internal negative feedback, so there won't be as much upper treble as you'd get with pentodes with the master low.
My intention with my question in #7 surely was not to disregard this idea of putting the volume control. I've already seen the same decades ago in some Marshall modification advice (= to add a master volume). 'Cause I never tried it, I'm just curious how it might sound 🤔?
Best regards!
Best regards!
Finished a short video where I try to show different settings:
Fantastic! How would it sing if you replace the 12AU7 cathode bias by fixed bias, using the already existing heater voltage supply?
Best regards!
Best regards!
That is definitely something I can try, good idea!
It is even closer to the original OR120 circuit, which also uses fixed bias.
It is even closer to the original OR120 circuit, which also uses fixed bias.
Well, tried it, had some problems though.
First, I initially grounded the cathodes and connected the bias both to the grid-leak resistors (100k) and the master volume (1M), and it sounded weird when I changed the volume.
I then added two coupling caps, so that the master volume was still grounded, and only the grid-leak resistors where connected to the bias. This improved things, but I still had some weird fizziness, even when playing clean. It sounded as if the stage was not correctly biased. There was a considerable increase in output volume when I turned the thing off.
I used the same feedback resistor as was already installed. Since I adjusted it by checking the change in amplitude of a 800Hz sine wave, the value might be way off now. I have not tried changing it, since I only thought about it now.
The worst thing, however, was that the B+ dropped with the gain/volume settings, from 242v to 219v and the SMPS was really hot after just some minutes playing, as if the current sourced by the triodes was way higher than it should be.
So, I definitely did something wrong there.
I will do some simulations to check what I should expect in terms of output and currents. Maybe the simulation shows me something I was missing.
First, I initially grounded the cathodes and connected the bias both to the grid-leak resistors (100k) and the master volume (1M), and it sounded weird when I changed the volume.
I then added two coupling caps, so that the master volume was still grounded, and only the grid-leak resistors where connected to the bias. This improved things, but I still had some weird fizziness, even when playing clean. It sounded as if the stage was not correctly biased. There was a considerable increase in output volume when I turned the thing off.
I used the same feedback resistor as was already installed. Since I adjusted it by checking the change in amplitude of a 800Hz sine wave, the value might be way off now. I have not tried changing it, since I only thought about it now.
The worst thing, however, was that the B+ dropped with the gain/volume settings, from 242v to 219v and the SMPS was really hot after just some minutes playing, as if the current sourced by the triodes was way higher than it should be.
So, I definitely did something wrong there.
I will do some simulations to check what I should expect in terms of output and currents. Maybe the simulation shows me something I was missing.
Ok, that was dumb.
No, I used the 12v supply, so the bias was actually +12v. I should have lifted the cathode, and not the grid. That explains the high current...
Should have simulated it first, that one would be really easy to spot.
Now if I lift the cathode, and use it at +12v, it is not necessary to add a cathode capacitor, right?
The 12V switched wall wart probably already has a large cap to ground to smooth the output voltage. The signal would have to take the same path in this case, right?
No, I used the 12v supply, so the bias was actually +12v. I should have lifted the cathode, and not the grid. That explains the high current...
Should have simulated it first, that one would be really easy to spot.
Now if I lift the cathode, and use it at +12v, it is not necessary to add a cathode capacitor, right?
The 12V switched wall wart probably already has a large cap to ground to smooth the output voltage. The signal would have to take the same path in this case, right?
No, don't lift the cathodes. It would just be another kind of cathode biasing, with increased power waste. Reverse bias voltage polarity instead.
Best regards!
Best regards!
Well, after a long time I uploaded a step-by-step of the build>
https://www.instructables.com/Push-pull-Portable-Tube-Amplifier/
Thank you all for the valuable suggestions!
https://www.instructables.com/Push-pull-Portable-Tube-Amplifier/
Thank you all for the valuable suggestions!
Very cute! Nice build! 
(A trumpet is so loud, that a popular book of religious myths states that a trumpet-blast will be used to wake the dead!)
I found that a 1-watt tube guitar amp is okay for playing clean guitar in my apartment. But if I want to overdrive the amplifier output stage, 0.1 watt (i.e. 100 mW) is already quite loud - too loud for night-time use.
In technical terms, if you have a guitar speaker with, say, 93 dB/W sensitivity, then driving it with 0.1 watt of power will still create an SPL of 83 dB.
83 dB is very loud, really. A typical household vacuum cleaner generates about 70 dB SPL. 83 dB is equivalent to twenty vacuum cleaners all running at the same time.
Not many apartment neighbours will put up with the sound of twenty vacuum cleaners running simultaneously next door!
-Gnobuddy
Same story here. One watt into an efficient loudspeaker is actually very loud. At least as loud as a trumpet-blast, and very few people want to hear a trumpet-blast from the next door apartment!
(A trumpet is so loud, that a popular book of religious myths states that a trumpet-blast will be used to wake the dead!)
I found that a 1-watt tube guitar amp is okay for playing clean guitar in my apartment. But if I want to overdrive the amplifier output stage, 0.1 watt (i.e. 100 mW) is already quite loud - too loud for night-time use.
In technical terms, if you have a guitar speaker with, say, 93 dB/W sensitivity, then driving it with 0.1 watt of power will still create an SPL of 83 dB.
83 dB is very loud, really. A typical household vacuum cleaner generates about 70 dB SPL. 83 dB is equivalent to twenty vacuum cleaners all running at the same time.
Not many apartment neighbours will put up with the sound of twenty vacuum cleaners running simultaneously next door!
-Gnobuddy