Desperatly Need Help AES/Cary Amp Bias Process
 

I am a new poster in need of help and I am hoping that some folks here that are familiar with Cary products can help.

I recently purchased Audio Electronics Superamp Mk II and I could not be more excited about getting it up and running. However, I am novice when it comes to tube gear, unfamiliar with the bias process and want to make sure I set the amp up properly without damaging the amp, associated equipment and possibly myself in the process (if that’s even possible)!

So I will cut to the chase and hopefully my questions make sense are not viewed as being too amateurish.

1. When using a multi-meter (DC Current setting) with options for 200 ma and 10A, which setting do I use if the bias is to be set at 200ma? What happens if you exceed 200 ma while in the 200 ma setting while adjusting the bias?

2. Should I connect or disconnect the speakers when setting the bias?

3. Should I disconnected the pre-amp when setting the bias?

4. For bias accuracy, do I need to use the electrical outlet where the amp will be setup or can I use an outlet/circuit in another area of the house, set the bias and move the amp to its permanent location?

5. According to the manual, you insert the supplied bias phono/cable plug, attach the meter and power up the amp, bring the bias up slowly using the setscrew on the rear of the amp and set the bias to 200 ma when the amp has warmed, approx. 5 minutes. When ending the process in what order do you do the following; disconnect the meter, unplug the bias phono plug and turn off the amp?

6. If you are rolling input tubes, do you need to adjust the bias the amp again?

7. How often should you check the bias? When checking (vs setting) the bias what do you need to disconnect (speakers or pre-amp)?
Any other tips for a new guy? Anything you that you can think of would be greatly appreciated.

Thank you very much!

I think I can be of some assistance here...

1. In general when setting bias current (amps) you're usually measuring the voltage across a fixed resistance, giving you a voltage reading which corresponds to the current flowing through the bias resistor. So someone else here may be more familiar with your specific amp but if that's the case you would set your meter to measure volts and not amps. Whether you're measuring volts or amps you should choose a range that exceeds the desired setting so that an overload condition doesn't occur, although if it's a digital meter no damage would occur anyway.

2. Tube amplifiers should never be powered without a proper load on its outputs, so the speakers should be connected when setting the bias. Output transformers can be damaged if a tube amp is operated with no load on its outputs.

3. You can leave the preamp connected when setting the amp's bias, but allow the electronics sufficient time to stabilize (~20 minutes is usually fine) and then either mute the preamp or set its volume to zero before setting the bias of the power amp.

4. You should set bias with the amp connected to the outlet where it will be used. Line voltages can vary in different locations and they will affect the bias settings.

5. You can power down the amp first and then safely unplug the test cables, but if you plan on listening you can unplug the meter and then the test cable leaving the amp powered.

6. Changing input tubes should not significantly affect the bias of output tubes.

7. When an amp and tubes are new there will be some drift in bias setting as components settle in, so you can check them again in a couple of weeks after the initial setting. If the settings are relatively stable (didn't need to be reset) then you can likely run the amp for several months without the need to recheck it. You can likely go to a six month interval for checking it again, but if it is found to be drifting then you should recheck it again sooner. No difference in checking or setting bias, you keep the speakers connected and make measurements with the amps warmed up and preamp volume muted or at zero.

Hope this helps and welcome to Audio Aficionado! :music:

If you are truly measuring milliamps, I would use the 200mA setting and adjust for 199mA. Using the 10A setting will give you very poor resolution.

Here's an excerpt from the manual:

Initial bias check: Make sure the meter is in the DC CURRENT range.
This is very important… if the meter is set in the DC voltage position the reading will be
completely wrong. Please check and double check to make sure you are in DC current mode.
Turn on the Super Amp MKII with the front panel AC power switch. As the amplifier warms up
you will notice the DC current readings. The basic operation range should be 200 ma. DC current
after five minutes of operation. This value will vary plus or minus 10 to 15 ma. with changing AC
wall socket voltages. This is normal and does not change the basic performance of your Super
Amp MKII

So I guess using the dc current setting on the meter for ma is not as common as dc voltage?

I might just look for a new auto range meter that extends to 400ma or so ......

Once again I appreciate all the help!

I didn't know you had the full manual, some would say that's cheating! :eek: Yes, given their explicit instructions you should have the meter set to measure DC current. As long as it's a digital meter Cohibaman's suggestion is a good one for the best accuracy. If you measure beyond the set limit with a digital display meter it will just indicate an overload condition without causing any damage. So you could adjust it to 199mA or even right up to 200mA in the 200mA setting which will be much more accurate than trying for 200mA in the 10A setting.

I need all the help I can get. LOL even with the cheat sheet it still left me with a ton of questions!

Thanks guys. You’ve been a big help! Sometimes it’s nice to have reassurance that you’re heading down the right path.

Ok....so I set the amp up, day 1 I’m in tube heaven. Next day turn the amp on and there’s a nasty buzz from the left channel. Audible from a good 12’ + away. Right channel pretty clean, just a really faint buzz only noticeable when you put your ear right up to the speaker.

Volume has zero impact but pulled the pre amp cables anyway, and the sounds still there, the amp’s definitely the source. Buzz decreases substantially when the bias jack is inserted or the 6sn7 input tube is pulled. Also swapped in another 6sn7 input, still buzzing. Tried a cheater, not a ground loop.

Any suggestions? I’m so frustrated! The amp was new old stock, factory sealed.

Have you cleaned all the tube pins and sockets?

Sockets and pins all look good. It’s a brand new amp and tubes.