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Old 16th August 2012, 07:41 PM   #21
TheGimp is offline TheGimp  United States
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Any other progress on the tube tester Michael?
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Old 17th August 2012, 01:56 PM   #22
MelB is offline MelB  Canada
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PIC 16C773 using it's 12 bit A/D converter with 8x oversampling and two INA122 opamps as a custom two channel ammeter. The INA122's are at +400v as I have a common filament circuit for the 300B's and it's the only place I could measure the separate current through each tube. Been running for 5 years without a hitch. Could add a relay and shut down if one of the tubes runs away, which would be an incredibly good idea as one always does after about two years.
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Old 17th August 2012, 08:03 PM   #23
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Originally Posted by TheGimp View Post
Any other progress on the tube tester Michael?
Still playing with the breadboard and tweaking the programmable regulator design to accomodate a range of devices including low voltage, high current I/O for solid state and hybrid device tracing.

I'm pretty sure I'm going to package this up as an open hardware + open source design and kit, using an Android device for local controls and internet connection. The Arduino is just going to set the measurement ranges and run the measurement cycles under control of the Android, which is connected to and powered by the tube tester over USB.

For a wireless control, the tester could use a WiFi interface to a web app

Last edited by Michael Koster; 17th August 2012 at 08:06 PM.
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Old 17th August 2012, 09:16 PM   #24
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could you post a simple schematic of how you hooked everything up? Did you use any protection diodes on your high voltage inputs into the MCU (after the voltage divider)?
I noticed that Circuit Cellar no longer has my dsPIC controlled tube amp on their web site, so I will post it or put it on my site.

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Here's an 8 channel 10 bit DAC Arduino Shield I built to control my Internet of Things tube tester.
Digilent has a PIC chip board that is pin AND software compatible with the Arduino Mega. It has many more additional I/O's and runs a 32 bit chip at 80 MHz for blazing speed. You can use the built in bootloader to program it in Arduino scetch, or you can blow away the bootloader and use the regular Microchip tools (MPLAB and C). It is available from Digilent, Microchip, and Digikey for about $50. Most Arduino shields fit. I am using one at work, and it has enough power to drive my test radio hardware making GSM ramp by repeated SPI loads to a DAC chip.

Digilent Inc. - Digital Design Engineer's Source

Yes, there is a ChipKit controlled guitar amp being tested on my workbench....far from being ready for prime time yet.
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Old 17th August 2012, 09:20 PM   #25
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I used a small PIC microcontroller in my hybrid amp to control the output relay.
It holds the relay off for 4 seconds on power up.
The PIC also monitors the output voltage and turns off the relay if a DC event occurrs.

I also use one on an amp test jig and it has saved me a fortune in speakers.
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Old 17th August 2012, 10:44 PM   #26
ChrisA is offline ChrisA  United States
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Originally Posted by 807tubenut View Post
The arduino mega (based on the atmega1280) is a much more "useful" uC, though the 128/328's found on the smaller unit's have there place to I suppose.


C# is my forte, But I really don't see the use of a uC in a tube amp, the highest logic level you could ever need would be a "re-purposed" 555 timer to generate PWM for cooling fans
..
I agree if we are talking about HiFi power amps. But if you include a preamp then the uP can handle things like switching between sources, volume, tone and balance controls and to interface with a remote control or even run a web interface.

If we are talking abuot musical instruent amps then there are lots more settings and yo might want to implement "presets" like on a car radio where you can dial-in the sound you want then press a memory button.

But for HiFi power amps there are no controls, not much to be done.
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Old 17th August 2012, 10:58 PM   #27
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I pick up PIC16F506 for about 60p so they are cheaper than using lots of op amp or logic to do functions.
The PIC16F506 has A2D on board.
I also use the PIC for a bargraph display.
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Old 17th August 2012, 11:02 PM   #28
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But if you include a preamp then the uP can handle things like switching between sources, volume, tone and balance controls and to interface with a remote control or even run a web interface.
.
I designed a mixer with digital volume and tone controls so it could be controlled from a PC by USB. It means I dont have to leave my PC to alter the tone or volume of the mixer !
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Old 18th August 2012, 12:25 AM   #29
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But for HiFi power amps there are no controls, not much to be done.
I managed to extract a very clean sounding 20 watts out of a single 6AS7GA with the sections paralleled in SE mode. I can tell you that the tube will melt if you try this without "digital augmentation". How does it work?

The tube amplifier itself looks pretty normal (board on the right). It isn't quite what the average SE amp builder makes though. The smaller tube is a 6EM7 in a two stage SE triode amp. Its main purpose id to develop about 300 volts P-P of drive for the output stage. The output stage is a cathode follower using both sections of a 6AS7GA, or 5998, or even a 6336A in parallel. Why a cathode follower? The cathode follower is relatively immune to ripple on its plate (high PSRR). The amp was tested with a normal bench power supply, and makes about 8 watts in SE mode with a 175 volt power supply and a 600 ohm OPT. This is limited by the dissipation in the output tube.

Now what if we created a variable power supply and modulated it with the audio signal. Set it up such that the output tube sees a constant voltage across it for all signal voltages. It turns out that the voltage dependent nonlinearities inherent in the triode, goes away and distortion nearly vanishes. This is not new, in fact it was patented by Ross MacDonald in 1957, and was the basis for several succesful small signal designs like the SLCF and the front end of many Tektronix scopes. A thread about it is here:

augmented cathode follower?

I succesfully applied the modulated cathode follower for output stage use, in that thread. The top tube can be a mosfet, but you are still dissipating a lot of energy as heat. What if the top tube is replaced by a modulated SMPS that tracks the audio signal and holds the voltage across the cathode follower output stage constant, and low...like 75 volts. Agian, this is not new, it is done all the time today in cellular base stations and some handsets to double (or more) the RF power amps efficiency.

I was working on the RF stuff nearly 10 years ago. Why do we care about cellular base station efficiency when they have line power? Because 10 years ago Nextel's electric bill was 1 MILLION dollars a MONTH! What does this have to do with HiFi audio amps? Everything. The power output from an SE amp is limited by the output tube's dissipation. SE amps are usually about 10% efficient. If we double the efficiency, we double the power output for the same dissipation.

The technique is simple, you just need some DSP power to sample the incomming audio signal (I tapped the plate of the driver tube) and control an SMPS such that it's output closely follows the audio signal. Microchip makes some dsPIC's for SMPS use that do all the hard stuff in hardware inside the chip. The software just sets everything up and loops forever. There is ample processing power left for houskeeping like bias control, current monitoring, and even tube condition monitoring.

When the magazine article was published I got a call from the owner of a major guitar amp company. Sadly I explained that it didn't sound too good for guitar since the distortion is very low until all headroom is exhausted and then clipping comes on fast and furious. About a year later I got an email from him showing me a patent application in England that was lifted directly from my original project submission.

Circuit Cellar has removed the original project submission, and they want money to see the published article. I can dig up something and post it if there is any interest.
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Old 20th August 2012, 04:45 PM   #30
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Originally Posted by tubelab.com View Post
I can dig up something and post it if there is any interest.
I'd love to see the details of this! I'm specifically interested in grounding and isolation between the audio amp and the micro-controller. Did you use a voltage divider, or transformers (or some other witchcraft) to get the voltages within measurement range?

Thanks!

p.s. My interest in this is for a Tublab SE (which sounds so good it brings forth tears) so it's awesome that you are on this thread!
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