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 Post subject: How do you set bias on tube amps?
PostPosted: Oct Fri 06, 2017 7:08 pm 
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Joined: Mar Tue 03, 2009 11:12 pm
Posts: 1358
Location: Hutchinson KS
Probably a stupid question. But am working on an old Harman Kardon Citation 2 that uses push pull KT88 tubes. The plate on one of the tubes gets slightly red, indicating an over current. The Sam's service manual shows a bias voltage of -50 volts on the grids. The voltage were all set at -30 on this unit. Each tube has its own bias adjustment, so I set them all to read at -50. Is this the proper way? This unit has a built in meter to adjust the bias and balance controls but it is inoperative.


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 Post subject: Re: How do you set bias on tube amps?
PostPosted: Oct Sat 07, 2017 1:58 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 4874
Location: Rochester NY USA
Equal grid voltages will get you in the ballpark. Equal plate currents will eliminate current imbalance in the output transformer, giving the lowest distortion and best low-frequency response. Each tube has a 15 Ohm resistor in the cathode, whis used to sample current. The Sams folder shows 1.5V across each, for a current of 100 mA per tube, about 44 watts dissipation. This is on the high side, will give short tube life, but it's the value originally used to give the lowest possible distortion. 75-85mA is safer, will prolong the life of the tubes (more recent 6550s and KT88s may not even survive 44 watts for a few minutes!)

First, check those 15 Ohm resistors! If they're carbon, replace them with 1% metal film. Set bias for 1.10 to 1.20V on each cathode, go for equal readings vs exact number- you may have to repeat, as turning one down will make the rest go up a little... (BTW, if you can get the meter to work, changing the resistors to 20.0 Ohms will put 75 ma at the SET mark)


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 Post subject: Re: How do you set bias on tube amps?
PostPosted: Oct Mon 09, 2017 6:36 pm 
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Joined: Mar Tue 03, 2009 11:12 pm
Posts: 1358
Location: Hutchinson KS
Thank you very much for the information! Two of the cathode resistors were burnt, one in each channel, and one channel had the screen resistor burnt to a crisp, possibly a tube shorted. The original KT88's were replaced in the past with 6550's and before I bought new KT88's I wanted to make sure the new tubes aren't going to be damaged! I noticed the -55 volt power supply has a high ripple so I better check that out too!


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 Post subject: Re: How do you set bias on tube amps?
PostPosted: Oct Fri 13, 2017 11:22 pm 
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Joined: Apr Tue 30, 2013 11:03 pm
Posts: 528
Location: Tioga, TX
Steve;

On high current push-pull output tubes it is important to also make sure that the coupling capacitors from the driver/phase inverter stage are good. Leaky coupling capacitors can cause red plating even if the bias has been set correctly when the unit is first warmed up from turn on. They can cause gradual change in bias the longer the set is on. If the parts are old, or original equipment, change them to new parts. A protection against shorting output tubes is also to add 100 ohm 1/4 watt resistors in series with the B+ to the screen grids of the output tubes. That way if the tube shorts it opens the resistor and shuts down the tube thus preventing damage to the output transformer.

Joe


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 Post subject: Re: How do you set bias on tube amps?
PostPosted: Nov Tue 07, 2017 10:43 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 7463
Location: Beaver Falls, PA. USA
Amplifiers which are direct-coupled must have good tubes in the previous stages, or the bias will be wrong. I had that problem in a McIntosh MC75, where a gassy driver tube caused the outputs to cherry up. If you find any leaky caps feeding the output tubes, better "bite the bullet" and change all of them.

Many, if not most, of the classic amps used a selenium rectifier in the bias supply. This is trouble waiting to happen, and should be replaced.

_________________
Tim KA3JRT


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