Bias Voltage Question

[bbaug14]

I'm nearly done with my first Express build (waits for applause....). I'm building from an RJ 'Kit', which is fantastic.

With no tubes installed, 60W light bulb in the current limiter, I was getting a max voltage of around -28V. I swapped the 220k bias feed resistor for a 150k. Now my measurements top out at -37V.

I believe this is OK, but thought it'd be in the -40s, especially with the 150k. Any thoughts here? Appreciate any help.

[bbaug14]

Okay, put tubes in it and 416V on the plates and 62mA static (stand by switch to pin 3). That seems WAY too high. Suggestions welcome.

[martin manning]

Simple calc for determining the idle current:

Typical bias setting for a Class AB amp is 70% of Pa max, and for EL34 Pa max is 25W. At 416V that would be 0.7*25W/416V = 0.042A. If you are measuring current at the cathode (with a 1R resistor, say) you will be conservative since screen current is included. Using 75% would compensate for that.

Take the current limiter out when you measure voltages for biasing. Measuring with tubes out (which you should do) is just to see if you have enough negative voltage range to set the idle current and to get a plate voltage to start with. The voltages will vary as you change the bias setting, so you'll have to iterate to get it dialed in.

Nice looking build, BTW!

[bbaug14]

I'm taking my measurements with tubes in, no current limiter, utilizing the OT shunt method. 416V on the plates, bias pot set to max negative and taking the idle current. The current is 62mA. This should be a lot lower. 416V * 0.062mA = 25.79. I'm at max dissipation for EL34s at max negative bias.

[martin manning]

Ok good, the shunt method gives actual plate current. Looks like you will have to further reduce the resistor in front of the bias diode.

[bbaug14]

Doesn't that seem odd though? I mean 150k is already low by the Express schematic. Do builders usually have to go even lower for this amp?

[martin manning]

My Express is using 150k before the diode, with plate voltage a bit under 400. Are the other resistor values in your bias circuit correct? What voltage are you getting on pin 5's?

[bbaug14]

The other resistors are checked and correct. I have to check the pin 5 later as I'm out right now.

[pdf64]

I think that the current shunt method stinks - it's pointlessly hazardous and inaccurate.
eg maybe hanging a meter off the plate is sending the amp into oscillation (Trainwrecks have been known to operate at the margin of stability), what's the resistance of the DMM current mode compared to the OT primary legs?

[bbaug14]

I think that the current shunt method stinks - it's pointlessly hazardous and inaccurate.
eg maybe hanging a meter off the plate is sending the amp into oscillation (Trainwrecks have been known to operate at the margin of stability), what's the resistance of the DMM current mode compared to the OT primary legs?

I don't understand what you mean. Tagging a resistor in and converting amps to volts is "easier" but it's not as accurate as measuring the actual current draw. Why does it "stink"?

In any event, the max negative bias setting is -37V no tubes which is border line too low. 62mA draw at the lowest bias setting is just way too high, no matter what your preferred method of measurement.

Is there something I'm missing?

[bbaug14]

Nevermind. Now I understand what you were getting at. The internal resistance of the meter.

[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?

[brewdude]

Is there a reason that amps aren't built with a 1 ohm, 1% resistor on the anode of each tube such that an easy (presumably accurate) measurement plate current could be observed?

[Littlewyan]

If the anode current is too low then you need a lower bias voltage. Sounds like you need to put the 220k resistor back so you can bring the anode current up.

What is your bias voltage range with the 150K resistor? Measure this without valves installed

[bbaug14]

No tubes installed with the 150k the bias pot puts out roughly -28 to -37. It was lower than that with the 220k which it maxed out at around -28.