5E3 Bias

[dpm309]

My 5E3 build (1st amp I built several years ago) was experiencing crackling on the volume pots. Took it apart and cleaned the pots and is working. While I had it on my bench, I checked the voltages and bias.  A few years ago I performed the Simplified Dumble 5E3 Tweedle Mods on it. The amp sounds great with this mod. I replaced the 5Y3 rectifier with a GZ34 along with other mods. My plate voltage on the power tubes is 372VDC which gives me a bias of 56ma. This seems high for a 6V6 but the amps sounds great and have not had any issues with the power tubes.  Is 56ma too high? Will this affect the life of the power tubes?

[passaloutre]

What is the cathode voltage? Assuming 250r bias resistor?

Is 56mA per tube, or both combined?

[pdf64]

To provide a valid response, more measurements are required.

Anode voltage
Screen grid voltage
Cathode voltage
Cathode resistor value
Screen grid current

And an accurate schematic, as it's not a standard 5E3 anymore.

Does reverting to the 5Y3 cause the amp to sound worse?

[dpm309]

I have attached the schematic for the Dumble 5E3 mods. The only difference is that I am using a choke instead of the 4.7K 2Watt resistor.  I am getting 374 VDC on pin 3, 379VDC on pin 4 and 25VDC on pin 8. B+:  382VDC, B+1 : 380VDC and B+2: 300VDC. Haven't tried swapping rectifier back to the 5Y3. I'll try swapping the rectifier after I can locate one.

[shooter]

you could just make the 250 cathode R a 470, 500, 530, whatever you have close to double the 250. then re-measure VDC at pin 3 n pin 8


do the ball park math  ((plate VDC-cathode VDC) / Cathode R) / 2 (tubes) will yield total tube current including a few mA for G2
anything in the 35-45mA range is good for starts

[stratomaster]

That's not a schematic.  Also, by replacing the 4.7k dropping resistor with a choke you've stripped the circuit of any screen protection.  If you're going to keep the choke, please add 1k screen resistors to each socket.  This will also buy a bit of compression to offset the increased voltage.

25V at pin 8 with a 250Ω resistor and approx 350V plate to cathode comes out to about 125% idle dissipation.  On a 1957 5E9-A Tremolux I recently restored, I had a similar situation.  Increasing the cathode resistor to 300Ω and using a 5Y3 brought the idle dissipation down to about 105% and the amp was very happy with that.

Also: move your standby switch to after the reservoir node.  This layout illustrates a hot switching arrangement that will lead to premature rectifier tube failure.