bias question

[g(f(e))]

Hello,

I have recently finished my power supply for a breadboard amp, and I am now working on the input stage for a 6SN7, and have a question that I cannot find an answer for.

I am only going to be using a cd player for the input. The cd player has a 2V output, so do I bias the grid for the 6SN7 for only 2 volts? Or do I use the minus 12.5 volts shown in the characteristics charts.

Thanks

Gary

[Vinylsavor]

Hi!

2V RMS output means an amplitude of 2.8V or 5.6V peak to peak.

You want to bias your tube at least such that it won't get positive at full input signal.
In addition you want some headroom. I consider 6dB minimum. This would require 5.6V bias. But the more headroom the better.

The bias point also depends on your circuit. All needs to be matched. You cannot just select a sensible bias point in isolation

Best regards

Thomas

[g(f(e))]

Thanks.

I assume if I do not use the full swing of the chart voltage that there is no harm. Is there a penalty? Such as lower gain?

[Vinylsavor]

Hi!

Sorry, but I don't understand your question. Without any sensible details about the circuit it is diffcult to give any recommendation

Thomas

[Eli Duttman]

Use the 12.5 V. shown on the data sheet. Remember, 2 VRMS is 2.83 V. PEAK. The bias voltage on the grid has to handle the peak swing and a bit more. Yes, you could use a -3 V. bias, but you have to figure out the operating point. The tube makers have already figured things out for -12.5 V.

[Sheldon]

Originally Posted by Eli Duttman

Use the 12.5 V. shown on the data sheet. Remember, 2 VRMS is 2.83 V. PEAK. The bias voltage on the grid has to handle the peak swing and a bit more. Yes, you could use a -3 V. bias, but you have to figure out the operating point. The tube makers have already figured things out for -12.5 V.

Eli, I don't know where that 12.5v number comes from, but it doesn't make a very practical operating point for a 6SN7, other than for a choke loaded plate.

Sheldon

[g(f(e))]

Thank you - I think I understand now.

Thomas - the circuit is going to be a single channel 6SN7 into a 6V6. It is just a test/learning amp.

[Sheldon]

Originally Posted by g(f(e))

Thank you - I think I understand now.

Thomas - the circuit is going to be a single channel 6SN7 into a 6V6. It is just a test/learning amp.

Then go simple. That means cathode bias and resistor loaded. Here's an example: Assuming 400V B+, and 100k plate resistor and 120v on the plate, to get a bias of -4V (I don't think you necessarily need 6dB headroom, because your CD player will never clip the input, as it has a hard limit on the output.), you will have about 2.8mA plate current. This will require a cathode resistor of about 1.4k, and a cathode bypass cap of about 20uF.

Sheldon

[keaster]

I didn't want to start a new thread on something I'm sure as been talked about before so I found this and though I'd post here. I'm a noob in the tube amp world and have built a EL-34 PP amp and I'm trying to understand biasing, I get the basics of it, it a negative idle voltage on the plate or I'm wrong?
Well my amp is built around the Dynaco ST-70 but I didn't want a a ST-70 I only used it because it seemed like a simple and popular amp to build and finding help would be easy
I cant seem to get it to bias correctly, the only differences are that my power transformer has a 60v bias tap and not a 55v like the ST-70 replacement transformers and also I read that in the bias power supply circuit some people replace the 10k resistor to a 5k along with the 10k pot per channel, but when I check the bias the max I can get is about .5v on each channel. I want to know is that bad or did I make a mistake somewhere?
also should the bias make the B+ change, the higher the bias or closer to the 1.5v that the ST-70 calls for the less B+ I have.

The last thing I noticed (now I don't know if this is normal or still apart my the problem) is that If I increase the bias on one channel it reduces it on the other.

[boywonder]

Keaster: Can you post a schematic? The bias voltage (typically around -35V or so for EL34's idling at 50-60 ma or so) is applied to the grid of the output tube, so it is negative with respect to the cathode in a fixed bias design like the ST-70.

The more negative the bias voltage, the lower the idle current will be in the tubes.

Your 60V bias tap on your power transformer should be fine, since once rectified, it should produce about 60*sqrt2=-84V

If you do not have a handy schematic to post, a sketch of your bias circuit from the transformer to the EL34 grids would help.

What is the value of your cathode resistor? Measuring the voltage drop across this R gives you the idle current.

Changing the bias of one tube affects the bias settings of the others since you are pulling more (or less) current from the power supply, that is normal.