• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Digital volume/tone control in Tube amps

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Hi everyone!

I'd like to control my guitar tube amp using a Microcontroller. This should include all potentiometers like gain, master, tone, etc.

Digital potentiometers are not applicable due to the high voltages.

Any ideas to solve this would be WELCOME :D

Thanks in advance....
 
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Digital potentiometers most definitely can be used in the signal path, signal levels are generally small in most stages, except the output stage and drivers. 12AX7 in most common cathode implementations clip with more than a volt of so of audio at their grids. Use diode clamps to the chip's supply rails to to prevent damage if the audio exceeds the supply rail voltages during overload. You'll have to design to make sure that the digital pots used can't be seriously overdriven in normal usage scenarios.

Doing a tone stack might be tricky, but is possible with some careful selection of devices.

Not having done this I can't vouch for the sound quality, and it will be a significant design challenge.
 
Digital pot + Tube amp = Blasphemy!

I understand this position if you are just sitting back in your easy chair listening to your stereo, but there are places where these items go well together.

Lets say that your are a guitar player playing live on stage. You just finish playing an ear bending heavy metal song with lots of distortion, now you want to play a soft tune without distortion, how do make the change. The traditional way is to use a tray full of stomp boxes, and remember which ones to stomp on and when. The new tech way is to have a midi controlled effects rack and a midi pedal board. OK, now how do we build our own and do it with tubes instead of DSP chips.

I have been tinkering with this on and off over the past 5 years and can offer the following:

The usual pigital pot that operates from a 5 volt supply just doesn't cut it. There is not enough headroom and they sound really nasty when they clip. Analog Devices to the rescue! The AD5290 chip will work on a 30 volt supply while its SPI port works from a 5 volt PIC chip. You can get these in a 100K ohm flavor which can be used in a tone stack, just don't put too much gain in front of it.

Build your favorite tube preamp circuit that has the usual overdose of gain, and use mosfets to switch the cathode bypass caps in and out. This technique can be used to adjust the overall gain by about 30 db.

Reed relays can be used to switch circuits in and out of the signal path. Careful circuit layout is needed to avoid a pop when you turn the relay off.

I am slowly developing these circuits to be installed right inside the guitar using subminiature tubes. It will all be posted on my web site when it is done, but I just don't have the time to work on it right now. I want to use a PIC chip and an EEPROM memory so that your favorite settings can be saved and then called up by slapping a button on the guitar.
 
Another Thought

Tubeater / All,
Something I have thought about for this was modiying John Swenson's BDT (6AR8 Beam Triode) preamp - use a digital pot to adjust the negative bias voltage - see attached. Modification of this design would enable a digitally controlled completely tube signal path.
Food for thought, cheers -ALBQ

My current preamp
http://mywebpages.comcast.net/gillespie147/12AU7-Preamp.html
 

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tubelab.com said:

The usual pigital pot that operates from a 5 volt supply just doesn't cut it. There is not enough headroom and they sound really nasty when they clip. Analog Devices to the rescue! The AD5290 chip will work on a 30 volt supply while its SPI port works from a 5 volt PIC chip. You can get these in a 100K ohm flavor which can be used in a tone stack, just don't put too much gain in front of it.

Build your favorite tube preamp circuit that has the usual overdose of gain, and use mosfets to switch the cathode bypass caps in and out. This technique can be used to adjust the overall gain by about 30 db.

Thanx, I have to try this.....
My doubts are that the AD5290 won't survive the poweron, since at this moment the caps that filter the HV are not charged and the whole HV hits the chip, but I will do some simulations.... I will post when done.

Any ideas of using a small DigiPot in combination with some JFET or MOSFET to do the control work?

aah, and I don't want to use motorized pots since for my application they are too slow....
 
Wavebourn said:
8 FETs and 8 resistors for one 8 bit variable resistor - what may be simpler? You may add 8 D-triggers to lock up the state.
Also, it does not harm the sound unlike ICs with active VCAs.

Thanks, I will try that also.... I just have to get the parts :)
I will try both the chip and the FET / resistor version and compare it...

By the way: my simulations show that I have some signal level of +- 40V something... I will have to stack 3 of the chips :( but it seems to be possible tough...
 
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