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In Reply to: RE: Adjusting Plate Voltage on Preamp Output Tubes posted by Maxamillion on June 30, 2009 at 18:23:59
Maxamillion, I have no experience with 2C22 tubes, however the RCA data sheet recommended operating point is -10.5V grid (or + 10.5 V cathode bias) at 300V plate voltage and 11 mA plate current.
You will find it here:
http://www.mif.pg.gda.pl/homepages/frank/sheets/049/2/2C22.pdf
Take a look at the -10.5 line on the last graph.
Most 6SN7s run around 9V cathode bias and 180-250 V plate voltage, so you are correct in thinking that a different op point may be better, however, it is dangerous to look for the best op point around 0 Volts grid. The proper way to "ballpark" a good op point is to look for equally spaced grid lines around a SAFE op point. Equal spacing shows linearity, meaning that for a given grid input signal, the plate signal will swing equally in the positive and negative portion of the waveform.
By SAFE I mean: most triodes will go into thermal runoff (lack of control, maximum plate current, red plates, kaput) near 0 Volts, so a good starting point is the manufacturer's recommended op point.
You may work up or down from there, but anything lower than 3 V bias is asking for trouble. You want to stay away from 0 volts even at maximum volume with a highly modulated CD (2V RMS).
Also do not exceed the maximum power of 3.3 Watts.
300V * 11 mA = 3.3 Watts, so you may want to reduce the plate voltage or plate current by 20% to be on the safe side.
I suggest that you post the power supply (B+) voltage, a simple schematic and ask for favorite operating points from those that have experimented with 2C22 tubes.
A change in plate resistor will certainly require a change in cathode resistor to bring the tube to your chosen operating point.
Good luck
I have used 6sn7's at 210V, and 15mA with a CCS for about 4V on the grid. They sounded great. The pre I was using at the time put out about 10V max. You also need to consider the amp's input requirement. There is no definite setting for voltages without a necessary input / output voltage requirement. You can even drive a 300B with a 6sn7 with the right approach.
I see your point on plate dissipation, Casouza; it looks like 150V is too much at a grid voltage of 0. Somewhere around 125-130V keeps the dissipation at ~ 70% of max.
Thanks!
Maxamillion, your grid voltage may be zero Volts, however tubes are biased by the difference between grid potential and cathode potential. Usually, the cathode is run at a positive voltage by means of a cathode resistor.
I suggest that you locate the cathode resistor and measure the voltage across it.
That's your actual grid-to-cathode bias voltage.
Divide the voltage by the cathode resistor value in ohms, you will find out the current in miliamperes.
For example on a typical 6SN7 circuit you may find 9 volts across a 1500 ohm cathode resistor:
9V/1500 ohms= 0.006A = 6 miliAmps
I suggest that you read this to clarify my point:
http://www.bonavolta.ch/hobby/en/audio/t_bas9.htm
Yes, that's what I was saying. The grid voltage is 0V, not the bias. I don't see any magic about 0 or any other absolute voltage, it's all relative, so as long as the grid is negative compared to the cathode, and both are negative relative to the plate, what's the difference where the grid is set? Am I off base here?
Tubes are simple analog devices. All curves, etc, are informaion about events from the tube's perspective.
Tubes do not realize anything is grounded.
Measuring relative to ground is only done to try to simplify documentation.
It might ... or might not ;)
Happy Ears!
Al
I have the datasheets for both the 2C22 and 6SN7. The operating points are exactly where Supratek set them for the stock tubes (one 6SN7 per side) - the first half of the 6SN7 is run at 0V grid and 100V plate and the plate is directly coupled to the grid of the second half of the 6SN7, which is run with the plate at 230V. This is typical of what is shown in the 6SN7 datasheet for a two stage amplifier. What I did was to substitute the single triode 2C22 for the first half of the 6SN7, and just go without a second gain stage, so the anode and cathode of the 2C22 are connected to the output tranformers through a DC blocking cap. The 2C22 thus sees the same voltages that the first half of the 6SN7 did
It sounds excellent like this, believe it or not! Looking at the 2C22 plate curves on the datasheet, however, it looks like going up to 150V or so will put me in a more linear part of the curve, so I'd like to try that, hence the post.
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