Author Topic: Retro fitting DSP to a valve amp?  (Read 2982 times)

RobB

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Retro fitting DSP to a valve amp?
« on: March 19, 2006, 05:01:07 AM »
When talk of DSP first came up (last year some time?), it occurred to me that a reverb kit could become popular with valve amp builders/modders. 

There are pros and cons.  Reverb tanks are bulky, and expensive.  DSP reverbs (or chorus etc) require much lower voltage rails, a separate power supply. 

Today I stumbled across a simple circuit to obtain 9VCD  from a 6.3VAC heater winding. 

http://aga.rru.com/TechTips/disp.cgi?file=BobFlint/dc_fan.tt

I had always assumed something like this would cause problems by inducing diode switching noise into the tubes via their heater filaments. 

Am I wrong?  Would it be feasible to power a DSP circuit in a valve amp this way?  Please tell me my assumption is incorrect. 

Perhaps I should experiment?  How much current would a digital reverb draw? 

Peter Snowberg

Re: Retro fitting DSP to a valve amp?
« Reply #1 on: March 19, 2006, 10:10:57 AM »
Quote
Please tell me my assumption is incorrect.

Sorry, uhhh, you are incorrect. :)

Digital reverb circuits draw much less current than a tube heater and it's quite easy to just tap a little power off for one without any kind of noise entering the supply. The heater supply is fairly removed from the audio path so even if there were a little noise you would still be fine.

If you full wave rectify a 6.3VAC source you get 6.3*1.414141-(Vf*2), or roughly 8VDC out. You can also us the voltage doubler shown in the second illustration to get just shy of 17VDC.

Digital reverbs all want logic voltage which means +5VDC. Simply full wave rectifying the heater supply gives a perfect source for a 7805 regulator. If you need more voltage for the analog signal conditioning in front or behind the verb you can get that via the voltage doubling with some good filtering after (no need for another regulator).

Now for the fly in the ointment.... many tube amps run the heaters with a connection to the cathode of the output tubes. Putting a DC bias on the heater raises their potential above the grid and that stops a potent source of hum from entering the signal path. The "ground" connection of your new heater-derived supply will be sitting at maybe 30 volts above the signal ground the tubes are using. Now you have to couple everything (including your dual grounds) with capacitors and you can get some nasty variation between grounds as the tubes heat and cool. The other popular heater powering method is to ground the center tap of the heater supply and give the 6V heaters +/- 3.15 volts for power. Now your rectified ground if at roughly -3VDC in comparison to signal ground.

It's much less trouble IMHO to just use a 2nd little transformer to get your reverb supply. A split secondary 6.3V transformer is perfect. Remove the connections to the primary and you can use one of the two secondaries as a new low voltage primary. Rectify the secondary like a normal supply and there you go.  :icon_smile:
Eschew paradigm obfuscation

R.G.

Re: Retro fitting DSP to a valve amp?
« Reply #2 on: March 19, 2006, 11:00:02 AM »
The power supply is the least of your troubles if you're putting a digital reverb into a tube amp.
R.G.

Quick IQ Test: If anyone in a governmental position suspected that YOU had top-secret information on YOUR computer, how many minutes would you remain outside a jail cell?

RobB

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Re: Retro fitting DSP to a valve amp?
« Reply #3 on: March 20, 2006, 07:40:47 AM »
Ok, so there are still plenty of difficulties to be overcome in running an audio stage this way. 

Iíll put this idea back to bed for a while. 

Thanks.