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JL Sargent

Hunting for tube bias adjustment 101.

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My Sansui AU-70 plays great, but I wonder, how do I measure and adjust the bias on this amp?

While my Sansui is push/pull, what about the single ended amps. Do they need bias adjustments too?

Lastly, can a person tell by tube type and arrangement what the bias should be?

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The bias procedures for tube amps is very dangerous and can kill you, even when the amp is unplugged - not something a novice at the "101" level should consider attempting themselves. Find someone to do it for you that is well experienced.

Some single ended designs use a self bias circuit that does not need adjustment.

In general, you cannot predict the correct bias for a circuit until the measurements are done, and those measurements vary among tubes of the same design.

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Pauln, you have absolutely no knowledge of my elec. experience or previous exposure to high voltages. Just because I have not Biased a tube amp does not make me inadequate to do so. Since I have worked with high voltage (CRTs for one) for 25yrs with first the USAF/ANG as s sensor systems specialist and then as a high voltage electrical service provider, I am certainly no stranger to electricity or high voltage.

"even when the amp is unplugged" on a tube amp? I don't thinks so, as vacuum tubes bleed themselves off immediately when power is removed, right pauln? Maybe I should be warning you sir about the dangers of high voltage.[^o)]

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Hey Mark, thanks for your help. I have two amps here. The Sansui AU-70 which is EL84s in a push pull and a little Magnavox with EL84s single ended. I realized the little Magnavox output impedance was only 4 ohms with help here. My answer to that was paralleling speakers for now, which works fine. Just wondering about checking the bias on these two amps.

Here is a picture of it.

2382958amp1.jpg

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I cannot find my copy of the AU-70 service manual. I would suspect the tube bias is similar to the AU-111. If I'm not mistaken it's ~-43 or ~-45. Some of the Sansui tube amps were not adjustable, but I do not recall which models to which that would apply. I will try to find out for you today and respond to the thread if you have not found the answer

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PS: tech called me back. There are three pins on each channel's bias pot; one should test at 18, one at 51, and the third when adjusted should be -44 +1

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Groomlake: Mighty nice of you to go to that trouble for me. Thank you very much!

Mark: I will check that. Thank you also.

Common sense points to my tubes being happy. When at idle no heat, when getting a workout, heat not excessive and tubes "look" good.

Still its nice to have the numbers to verify.

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Groomlake: Mighty nice of you to go to that trouble for me. Thank you very much!

Mark: I will check that. Thank you also.

Common sense points to my tubes being happy. When at idle no heat, when getting a workout, heat not excessive and tubes "look" good.

Still its nice to have the numbers to verify.

The little Magnavox runs in class A pretty much. The output tubes are always fully "on", if there is a input signal or not. The console amplifier should have both output tubes sharing the cathode resistor. It's a 100 ohm 2 watt carbon composition I think. So you have to determine the plate/bias current and dissipation rating for both tubes from the one resistor. You could probably measure across the plate and cathode of the 6BQ5, and determine current to figure up the dissipation for each tube.

Duncan's tube amp's website has pretty extensive tube data, you can check over 6BQ5/EL84 data for 6BQ5/EL84 in class A1 pentode connected.

Raising the 100 ohm cathode resistor to 120 or 150 ohm would probably help with today's higher line voltages from the wall, where the 100 ohm R might run the 6BQ5's a bit hard.

Magnavox had a thing with shared cathode resistors. Most the push-pull amplifiers have the quasi-bias, with the four output tubes sharing one cathode resistor.

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"Raising the 100 ohm cathode resistor to 120 or 150 ohm would probably help with today's higher line voltages from the wall, where the 100 ohm R might run the 6BQ5's a bit hard."

This certainly makes sense. I have 125v at the outlets right now. I'm guessing the circuit may have been designed for what, 115v?

I'm thinking I'll install a headphone jack on the little console amp next. Should work quite well for that I suspect.

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I was looking online for AU-70 info, and found the following on a forum. Says the AU-70 uses 7189 tubes (four of them, 3 12AX7, 2 6AN8...Are you sure that's an AU-70? Here is the text:

The spec for the AB1 PP fixed bias operation of 7189A is 400V plate, 300V SG, bias is -15V. The AU-70 operates a little above of this (410V and 315V), so I would say you will be safe to set at -16V to -17V. But, this is a case for a pair of matched tubes. If your tube is not matched, you need to insert a small resistor, such as 16Ohms 1/2 +-1% between plate and OPT primary for each 7189A. Then, you need to make the voltage on these registers even with the bias level of -16V or -17V. In essence, by doing that, you are make the 7189A plate current same at no signal. This procedure is possible only AU-70's with 4 bias pots (later versions).

The bias needs to be checked between control grid (pin 2 of 7189A) and ground with no input signal. I always connect a dummy load when I power up tube amps. As transistor amps don't like shorted output, tube amps don't like opened output.

post-7149-1381943805588_thumb.jpg

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Yes, of course your right. 7189 it is. Just looked at em.[:D] Thanks.

Let me ask this then. Doesn't EL84 = 7189?

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Let me ask this then. Doesn't EL84 = 7189?

They don't have the same ratings. The max anode voltage for the 7189 is 400volts instead of the 300 for the EL84. It might need to be adjusted if you were to put in EL84 tubes.

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Oh, that seems significant. It might need to be adjusted and it might just be less WPC too I'M guessing.

I've really been enjoying my Sansui AU-70 driving a pair of Heresys.

I think a pair of Cornwalls would do really well with it too though.[;)]

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"Pauln, you have absolutely no knowledge of my elec. experience or
previous exposure to high voltages. Just because I have not Biased a
tube amp does not make me inadequate to do so. Since I have worked with
high voltage (CRTs for one) for 25yrs with first the USAF/ANG as s
sensor systems specialist and then as a high voltage electrical
service provider, I am certainly no stranger to electricity or high
voltage.

"even when the amp is unplugged" on a tube amp? I
don't thinks so, as vacuum tubes bleed themselves off immediately when
power is removed, right pauln? Maybe I should be warning you sir about
the dangers of high voltage."

I beg your pardon. You must agree that "101" is the designation for the very first beginning instruction class and using it in thread titles is the common way of indicating one is not knowledgable on a subject and requesting basic information. I have no knowledge of your experience because you did not introduce any, you used the label '101" and I told you the most important knowledge - the danger. I posted because a "101" level inquirer might not be aware of the
danger of the caps (even when the amp is unplugged). People have died
from not knowing the dangers of an unplugged amp.

Since you
demonstrated that you didn't know about that, my post served its
purpose.

Since you were so kind to ask me directly; no, you are incorrect, it is not the vacuum tubes that bleed anything (except heat for a few minutes), it is the power supply caps, and they may continue to hold a lethal charge for weeks if not discharged using a shop procedure. They certainly do not immediately do so when the power is removed unless the manufactuer has deliberately installed a resistor to do that. The discharge procedure is used even when the resistor is present to avoid the risk of it having failed and the charge remain.

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The spec for the AB1 PP fixed bias operation of 7189A is 400V plate, 300V SG, bias is -15V. The AU-70 operates a little above of this (410V and 315V), so I would say you will be safe to set at -16V to -17V. But, this is a case for a pair of matched tubes. If your tube is not matched, you need to insert a small resistor, such as 16Ohms 1/2 +-1% between plate and OPT primary for each 7189A. Then, you need to make the voltage on these registers even with the bias level of -16V or -17V. In essence, by doing that, you are make the 7189A plate current same at no signal. This procedure is possible only AU-70's with 4 bias pots (later versions).

Patience pays off......

The design of the AU-70 is apparently quite different than the 111. I also need to find a schematic for the AU-70 as I have a line on one that supposedly works for $50...... Isao's post on the other forum in 2006 would indicate that he had one...

I sent an email to Isao to see what he's got as there seemed to be nothing available showing on the "googler".

Here's what he said: http://www.obsincst.com/servlet/Detail?no=96 for the later one with four bias points. and...

http://www.obsincst.com/servlet/Detail?no=94 for the earlier one.

They are $11.95 for a CD. Very high quality printed out.

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For those wishing to know the procedure for discharging the power supply caps before working inside a tube amp (rewiring, mods, putting bias measuring circuits in place)...

Make an insulated lead wire that has an alligator clip on one end and a 10K resistor on the other.

Make it so that the leg of the resistor at the end connected to the wire is insulated either with shrink wrap or electrical tape, and leave the terminal free leg bare.

----[ 10Kohm resistor ]================================[alligator clip]

After pulling the power plug, remove the power tubes and if there is a stand-by switch, set it to "On".

Connect the alligator clip to the chassis, then hold the other end of the wire by the insulated part, and use the resistor's bare terminal leg to make contact with all the pins in one of the power tube sockets.

Maintain contact for about 30 seconds on each pin. Don't touch anything else conductive.

Only one of the pins is the one that will discharge the PS caps, but doing them all will insure that you get the right one. This only has to be done for one of the power tube sockets.

Be cautious. Use one hand.

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