Tube biasing advice needed

audiojerry · « **on:** 16 Mar 2003, 03:21 am »

I'm having a problem trying to set the bias on an Audio Research CA50 integrated. The optimal setting is 65ma, but the highest setting I can get using the bias adjustment is 50-52. The output tubes are 6550, and the input and drivers are 6922. I swapped out the 6550's with newer ones, but it made no difference. What would be the reasons for not getting a higher bias setting? Thanks in advance

JohnR · « **Reply #1 on:** 16 Mar 2003, 03:55 am »

All kinds of reasons... how is this being measured exactly?

audiojerry · « **Reply #2 on:** 16 Mar 2003, 07:33 pm »

Hey John.
There is a resistor that is designated for checking bias; probes from a multimeter are placed on each side of the resistor and measurements are taken in milliamps. The same procedure is used iwth my other Aidio Research amp, and using the same tubes in that circuit, the bias reading is the correct 65ma.

JohnR · « **Reply #3 on:** 16 Mar 2003, 08:11 pm »

OK, well I assume that you're setting the meter to read millivolts, not milliamps

But if it works on another amp, that rules out the tubes, a flat battery in your meter, and a whacked-out wall voltage... leaving a problem inside the amp! Which seems odd, but it's possible that someone modified it or something (I'm assuming you bought it used).

Is there one resistor for all output tubes, or one for each?

If you wanted to go further, I would think the next step would be to open the amp up and check the voltages on the grids of the output tubes (pin 5). They should all be identical.

audiojerry · « **Reply #4 on:** 16 Mar 2003, 09:13 pm »

I will elaborate. I have two Audio Research amps. Both use 6550's. I measure both the same way. I set the multimeter to read mA, for milliamps. When I read the meter on the good amp, it reads 65-66 mA, which is what Audio Research specifies. I took those 6550's out of the good amp, and put them into the problematic amp. Measuring there in the same way yields 52, with the bias adjustment screw turned as far up as possible.

The resistor that is labeled for reading bias is for two output tubes.

Would it make a difference if I removed the tubes, turned the bias adjustment all the way down, reinserted the tubes, and then checked bias? Would the bias remain the same as before, but now I'd have leeway to turn the bias screw up?

Someone else suggested that I check/replace my driver tubes. Would this make any difference?

Thanks for offering to help, btw

JohnR · « **Reply #5 on:** 16 Mar 2003, 11:11 pm »

Ok, sorry to be a pedant, but can you quote from the manual where it says how to set your meter?

I ask because, if the bias measurement is being done across a resistor (as you also said) then it does not make sense to set the meter to mA...

J

audiojerry · « **Reply #6 on:** 17 Mar 2003, 02:48 am »

Well, since you are offering your help, I'll play along. The manual says verbatum:

Quote

As shipped from the factory, the output "bias" adjustments are set for a nominal 65mA per 6550C tube. Under these idle conditions the tubes are each dissipating approximately 27 watts of their 41 watt rating (35 watt plate, 6 watt screen). This point of operation provides "enriched" Class AB, and will satisfy the most critical listener.

For best results, operate and adjust the CA50 at the normal rated line voltage listed on the rear panel. Adjustment must be made under zero-signal conditions after at least 15 minutes of uninterrupted stabilization time.

A digital voltmeter capable of accurate measurements with lmVDC resolution is preferred for accurate adjustment (must have 3 1/2 digit display). Use plastic alignment tool provided to make the adjustment. The test points are accessible from the top of the circuit boards above the output tubes. Adjust the "bias" on each channel for an average reading of 65mVDC (0.065 Volt DC) between test points (across 0.5 ohm resistor.) Caution: resistor is 420V above ground.

The resistor is labeled: "Measure bias here".

As I wrote this, I realized that it said measure for 65mVDC, not mA.

JohnR · « **Reply #7 on:** 17 Mar 2003, 02:59 am »

Cool

How does the measurement look if you set the meter to mV?

audiojerry · « **Reply #8 on:** 17 Mar 2003, 04:29 am »

Quote from: JohnR

Cool

How does the measurement look if you set the meter to mV?

uh, er, um....using the mV setting, I was able to adjust bias from 79 down to optimal 65.5...

My bad, my very very bad.

Thanks for forcing me to read the manual correctly.

It's a great sounding integrated, btw.

JohnR · « **Reply #9 on:** 17 Mar 2003, 05:04 am »

Great, glad I was able to help

Remote debugging is kinda fun!

Just in case... you may want to check the other amp, if it uses the same procedure