Bias Voltage Question

[Littlewyan]

Ok I see but you can only get 38mA Anode current with -28V Bias Voltage?

[bbaug14]

Ok I see but you can only get 38mA Anode current with -28V Bias Voltage?

I'm not sure I follow. I set the bias pot to max negative voltage with no tubes installed. That was -37V. After installing tubes and setting bias, the reading was about 5mA. I turned the bias pot up to get to 35mA (38mA was at the max pot setting). I didn't check the bias voltage again after that. I just checked the plate voltage, which at that point was at about 400V.

The bias voltage would be pin 5 correct?

[Littlewyan]

You can check the bias voltage at pin 5 yes.

So just to confirm, you've installed 1Ohm resistors on the cathodes of the EL34s to measure the bias?

[martin manning]

Okay, so I think my meter was causing an issue, so I went to the resistor method. I set the bias to 35mA and the plate voltage dropped to 400V. This is still a pretty cold bias. The max the bias will put out is about 38mA, so my bias range isn't even high enough to set 70% dissipation, which takes me back to the max settings no tubes of -37V being too low.

My Pin 5 voltage was 36V.

So what are your guy's thoughts on this?

You have 400V Va (pin 3), -36V Vg1 (pin 5), and 35mA. At 400V, you want 0.75*25W/400V = 47mA. here I used 75% to get somewhere close to 70% Pa max measuring cathode current. To get up to 47mA you need to increase the Vg1 voltage from -36 to -32 or -30 or something, i.e. make it less negative. The tubes-out range you measured goes from -38 to -27, so I'm wondering why you can't get the idle current up higher than 38mA??

[bbaug14]

Okay, so I think my meter was causing an issue, so I went to the resistor method. I set the bias to 35mA and the plate voltage dropped to 400V. This is still a pretty cold bias. The max the bias will put out is about 38mA, so my bias range isn't even high enough to set 70% dissipation, which takes me back to the max settings no tubes of -37V being too low.

My Pin 5 voltage was 36V.

So what are your guy's thoughts on this?

You have 400V Va (pin 3), -36V Vg1 (pin 5), and 35mA. At 400V, you want 0.75*25W/400V = 47mA. here I used 75% to get somewhere close to 70% Pa max measuring cathode current. To get up to 47mA you need to increase the Vg1 voltage from -36 to -32 or -30 or something, i.e. make it less negative. The tubes-out range you measured goes from -38 to -27, so I'm wondering why you can't get the idle current up higher than 38mA??

Okay, I think I might be confusing things. Sorry.

I added a 1 ohm resistor to make things simpler. With the amp on, tubes installed, all controls at noon except volume (which is off), I measured 38mV across the 1 ohm. I also get about 407V at pin 3 (plate). On pin 5, I get -29V. A good amount of hum through my 2x12 too.

The problem is if I max the bias pot, the highest reading I can get across the 1 ohm resistor is about 38mV - 40mV, which isn't high enough to set 70% dissipation. This is with the 150k bias feed resistor.

It appear s to me that my bias circuit doesn't have enough range to set the correct bias. Am I missing something?

[dorrisant]

Put the 220k back in and try the measure the bias across the 1R resistors...

[bbaug14]

Put the 220k back in and try the measure the bias across the 1R resistors...

With the 220k in there, I get 412V at the plates, -28 on pin 5, and 42mV on the 1 ohm resistor. This is at the lowest setting on the bias pot.

So, if I'm understanding this correctly, it looks like I need a bias feed resistor value between 150k and 220k to get me into the correct bias range?

[Littlewyan]

You want the bias range so you can go from too cold to too hot.

I'm slightly concerned that you're only getting 42mA with -28V Bias. Have you measured the 1Ohm Resistors to ensure they actually are 1Ohm? Also can you try different EL34s?

In my amp the bias voltage is on -34V to get 43mA.

[RJ Guitars]

Great looking build and a typically great response from the "TAG Team"...

I am watching here with some expectation of learning something that is not immediately obvious. I know I have had to diddle with the resistor values in my bias supply when experimenting with various tubes but I can't recall if I ever needed to change anything when using EL34s.

[martin manning]

Some numbers from my Express with EH new production EL34's installed:
Line 120VAC
VB+ 389
Va 386 (pin 3)
Vg2 367 (pin 4)
Vg1 -30.0 (pin 5)
Vk 0.042, 0.047 (pin 8, with 1R to ground)

Bias circuit has 150k before the rectifier, 15k between the filters, then a 33k into a 50k pot with its wiper shorted to one end.

[bbaug14]

Would a lower wall voltage cause this? My wall voltage is about 112-115V.

My bias supply is the same as the schematic/layout. No variation other than the bias feed, which is 220k right now.

I could try different tubes, but I don't think it'll make a difference.

The 1 ohm measures correctly.

[martin manning]

If your B+ is around 400V then the rest of the secondary voltages should be pretty much as expected. If you want more idle current, increase the 220k, or decrease the 47k in front of the bias pot. Either way, you need to make the pin 5 voltage higher, meaning a bit less negative.

[bbaug14]

If your B+ is around 400V then the rest of the secondary voltages should be pretty much as expected. If you want more idle current, increase the 220k, or decrease the 47k in front of the bias pot. Either way, you need to make the pin 5 voltage higher, meaning a bit less negative.

This doesn't make any sense to me. When the bias feed resistor increased from 150k to 220k my effective bias range shrunk such that the lowest bias I could achieve was 42mV. If I increase further, the bias will continue to get hotter.

I can't seem to get both the bias current and the pin 5 voltage in line with what should be expected.

What am i missing here?

[bbaug14]

Here is a picture of my board. It shows I've got the correct values at least. Ha ha.

[martin manning]

I can't seem to get both the bias current and the pin 5 voltage in line with what should be expected.

The exact voltage on pin 5 doesn't matter. The voltages on pins 3, 4, and 5, and the current you measure will all be varying as you change the bias pot, but all that matters is that you hit the dissipation figure (Ia x Va) that you are shooting for.