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Author
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Topic: Silverface Bassman Bias Adjustment?
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Garth Highsmith
Member From:
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posted 01 May 2004 03:48 PM
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.[This message was edited by Garth Highsmith on 09 January 2006 at 08:50 PM.] |
Mark Herrick
Member From: Los Angeles, CA
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posted 01 May 2004 05:10 PM
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If you really want to bias your amp you should do some THOROUGH research beforehand. And I will preface the following by saying, I AM NO EXPERT.The "hum balance" is not a bias adjustment. It minimizes the amount hum you hear from the speaker(s). On my Custom Vibrasonic the procedure is: "Adjust for minimum hum from speakers with an open plug in J3 (third input jack from left), Vibrato channel Volume, Mid and Bass controls full CW (clockwise), Normal channel controls full CCW (counterclockwise), Reverb and Vibrato OFF." (I used the same procedure on a '74 Twin and it worked...) The bias pot is accessible under the chassis near the power transformer. I looked up in the Gerald Weber book the circuits that are referenced in your link. The AA270 and AA371 appear to have the "bias balance" setup; I'm not sure what the AB165 bias setup is, but it looks like it is possibly a "balance" setup as well; I couldn't find the AB568 at all...but it's probably a "balance" as well. The "bias balance" doesn't really allow a true "adjustment" as you cannot vary the bias voltage without changing the circuit itself. You can only "balance" it between the two power tubes. (Or the pairs of power tubes if there are 4.) To actually check and adjust the "balance" you would need an instrument like the Weber Bias Rite or the Bias King (the safest way to go), or a Digital Multimeter and a couple of 1 Ohm resistors that have to replace to the ground connections on the power tubes. It sounds more complicated than it actually is, and it's kind of fun and satisfying to do it yourself, if you are so inclined. But again, I must recommend that you do THOROUGH RESEARCH on the procedure before actually trying it; there are dangerous voltages inside that chassis and EVERYONE will tell you so... Before I did mine I was going to take it to a tech. The charge wasn't so bad, $35 as I recall, but I would have to leave the amp there for two weeks, so I bought the Bias Rite and did it myself. Now I can swap tubes and experiment with the bias as much as I want... (Dang I talk a lot for someone who doesn't know that much...) Here's a starting point:
http://www.duncanamps.com/technical/lvbias.html (NOTE THE BIG DISCLAIMER AT THE BEGINNING. THIS GOES FOR ME, TOO...) [This message was edited by Mark Herrick on 01 May 2004 at 05:11 PM.] [This message was edited by Mark Herrick on 01 May 2004 at 05:55 PM.]
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Garth Highsmith
Member From:
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posted 02 May 2004 12:05 PM
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.[This message was edited by Garth Highsmith on 09 January 2006 at 08:50 PM.] |
Ken Fox
Member From: Ray City, GA USA
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posted 02 May 2004 01:03 PM
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Even the AB165 is not a true bias adjust. It is allows bias adjust of one tube only. |
Larry Robinson
Member From: Peachtree City, Georgia, USA
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posted 02 May 2004 02:03 PM
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Solder a 1 ohm resistor in the cathode circuit of each 6L6 tube. Adjust the balance control for equal currents thru each 1 ohm resistor. Use a digital volt meter and read the drop accross each resistor. i.e. 30 mv = 30 ma, (ohms law). You can leave the resistor in the circuit as it will not degrade performance. This procedure will allow balance of the current thru each leg of the push-pull. It will also show how much current is flowing in each tube.Regards, Larry Robinson |
C Dixon
Member From: Duluth, GA USA
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posted 02 May 2004 02:12 PM
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Larry,How much is correct? 30ma? carl |
Larry Robinson
Member From: Peachtree City, Georgia, USA
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posted 02 May 2004 03:21 PM
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I just picked 30 ma as an example. You would adjust for equal currents thru each tube by varying the bal pot. I don't know the actual value. My twin uses 6L6GC tubes and they are set for approx 36 ma per tube. I don't believe you can adjust the bias on the circuit as it is except to balance the currents. |
C Dixon
Member From: Duluth, GA USA
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posted 02 May 2004 03:32 PM
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Thanks Larry,And this confirms my thoughts. Let me ask you another question. If you adjust the bias pot so the currents are equal in the tubes, would that then not be the same point where minimum hum is obtained? If so, would it not be easier to simply adjust the pot for minimum hum with no signal present? carl |
Larry Robinson
Member From: Peachtree City, Georgia, USA
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posted 02 May 2004 06:10 PM
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Carl, that is correct. Minimum hum would be when curents are equal. Bias adjustments should be made with no signal input. My twin reverb is a SF I bought new in '69 and it had the balance pot as shown on AC-568 schematic. However, I modified the circuit in order to adjust the bias to all tubes. I did this on a recommendation when I bought new tubes for the amp. Presently, I have no bal control in the twin, although I can add it later which I intend to do. I bought a matched set of tubes. However, after being in operation a few hours, they will tend to change. Once the bias is adjusted, it is rare to re-adjust unless one of the tubes goes sour. What you want is balanced push-pull on each side of the output xfmr.
Each amp is different. For example, my twin reverb schematic calls for 470V B+ voltage +/- 20 % at the plates of 6L6's. B+ in my twin is 410V, which is low in the 20% tolerance. I had to set the static current for approx 38 ma for each tube to get the sweet spot on my twin. Someone else might have 490 v on the plates and therefore would reduce the static current in each tube.
Did I answer your question? Hope so.
Larry |
Mark Herrick
Member From: Los Angeles, CA
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posted 02 May 2004 06:18 PM
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Again, I am no expert...But I think the idea is to strike a balance between the best tone and the "static dissipation" of the tube. The link I posted shows a maximum dissipation for a 6L6 tube of 23 watts. The info in that link also states that PLATE VOLTAGE x CATHODE CURRENT = STATIC DISSIPATION WATTAGE. So if the plate voltage is 450, multiplied by a cathode current of 30mA (.030) equals 13.5 Watts static dissipation. Ken Fox was kind enough to send me an Excel spreadsheet that will calculate these things as well. One thing I don't quite understand is that Ken's calculation for 450 volts shows a high end of 33.21ma for 23.5 Watts. But if I use those same numbers in the other equation I only get 14.9 Watts, which is well below the maximum. Maybe Ken can tell me what I'm doing wrong.  Anyway, today I readjusted the bias on my Custom Vibrasonic with 7581A power tubes to about 41mA (it was previously at about 35mA). The plate voltage is about 453. It seems to sound much nicer and the plates aren't glowing. I'm not sure this is safe or not. I'm only playing in the apartment, so I'm not really cranking the volume that much. Maybe if I was playing louder I might overload the plates? The truth is out there... |
Ken Fox
Member From: Ray City, GA USA
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posted 03 May 2004 06:54 AM
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For a 6L6GC tube you would have a max plate dissipation of 30 watts. I think the 23.5 watts refers to a 6L6GA tube.Also on a hum balance adjustment I have found hum at the lowest levels when voltage is equal, not current. For a set of matched tubes the current would be near equal for minimum hum. Also after setting bias (based on plate voltage) take another plate voltage reading. As the power supply in non-regulated, plate voltage decreases with increase in bias current.[This message was edited by Ken Fox on 03 May 2004 at 06:56 AM.] |
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