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Topic: More biasing questions....sorry! |
Mark Herrick
From:
Bakersfield, CA
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Posted 2 Feb 2004 10:40 am |
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I was successful in adjusting my "Custom" Vibrasonic using a multimeter and the Bias Rite device. Woo Hoo! (I'm using the "Lord Valve" biasing instructions from Kerry and the current calculation info from Ken - thanks guys!)
However, when I went on to check out a '74 Twin Reverb I encountered the following:
An initial check of the cathode current with the Bias Rite showed the tubes on one side of the output at about 15.5 mA and the tubes on the other side at about 5.5 mA.
I pulled the chassis and without the power tubes in the amp checked the voltage on pin 5 (grid 1 voltage according to Lord Valve and the 6L6 tube data sheet I have) and found the voltage to be -53VDC. I then tried to adjust that voltage to maximum and found that when it went up (to about -57) on the two sockets on one side of the output, it went DOWN on the other side. Is this normal?
The only place I could get the voltages to "balance" on both sides was at -53VDC.
Now, I do know that the tubes in that amp are not well matched, but it seems that I should be able to adjust the grid voltage to something other than -53VDC.
Help!! |
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Jay Fagerlie
From:
Lotus, California, USA
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Posted 2 Feb 2004 10:44 am
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Mark,
It sounds like you have a "balance" adjustment there, not a true bias adjustment. This adjustment is used to balance out the tubes so each one conducts equally. There are mods to convert a balance pot to an actual bias adjustment, and it's not that big of a deal to do. Another option is to balance the voltages with the adjustment and then find tubes that end up having the correct current draw using the bias tool you have. Either solution is usable.
Jay |
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Mark Herrick
From:
Bakersfield, CA
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Posted 2 Feb 2004 11:46 am
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I think I found the answer on the Fender forum. Here are two posts I saw:
“Some amps have a pot that changes the bias voltage to all the tubes equally so this type control doesn't balance the current it just sets the amount. Showman AB763 is an example.
Other amps have a balance pot that changes the bias voltage in one tube but not the other which allows the idle current to be balanced even if the tubes are not matched. Bassman AB165 is an example.”
“Yep. The upside of the bias balance control is that you can compensate for mismatched tubes. The downside is that the average bias is fixed, so you can't run the pair of tubes hotter or colder. With a slight mod to the bias circuit in the amp (adding a pot and maybe a resistor), you can have both. Also in an amp with a bias balance control, you can tweak a resistor to adjust the average bias.”
I don't really want to be buying and trying multiple sets of power tubes to find the ones that match the bias setting of this amp.
Can anyone point me to instructions for the mods that are mentioned above?
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Kerry Wood
From:
Tucson, Arizona, USA
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Posted 2 Feb 2004 3:49 pm
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Mark, this is more info from NBS Electronics.
"I might also add that if you want to mod the BIAS BALANCE control,
the best thing to do is leave it in place and replace the pull-down
resistor (this is the resistor that goes from the center lug of the
BIAS BALANCE pot to ground; usually soldered to the pot's body)
with a lower-value resistor in series with a trimpot. Use a resistor
that's around half the value of the existing one, in series with a
trimmer that's around the same value as the existing resistor.
This will give you a supply which is not only adjustable up and down,
but tweakable side-to-side; it's the best bias circuit you can have
in a Fender."[This message was edited by Kerry Wood on 02 February 2004 at 04:01 PM.] |
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Ken Fox
From:
Nashville GA USA
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Posted 2 Feb 2004 6:08 pm
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Be sure to use a cermet pot. I have done the above mod to quite a few Twins and Vibrasonics, it works well. Lately I just convert the balance pot back to a bias pot, Blakface the inverter and power section, and also the change the resistors that feed the grid stop resistors (1500 ohm) back to 220K. With power matched tubes available, it is not to bad a deal to go that way.
Both a bias and balance is the best of both worlds, for sure. [This message was edited by Ken Fox on 02 February 2004 at 07:27 PM.] |
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Mark Herrick
From:
Bakersfield, CA
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Posted 3 Feb 2004 10:26 am
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Thanks for all the help!
I think I get this now. So, the smaller trim pot kind of hangs between the center lug of the larger balance pot and the resistor (which will be changed to half the original value) that's soldered to the balance pot's body. Does the trim pot have to be secured in any other way?
I've been searching for "cermet pots" and found examples of several. Does it matter if the pot is "single turn" or "multi-turn"? Sounds like the resolution (?) of the multi-turn is greater than the single turn? |
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C Dixon
From:
Duluth, GA USA
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Posted 3 Feb 2004 11:30 am
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The multiturn simply makes the single turn more of a vernier control. Not sure you need this in setting it; since you are not dealing with microvolts or microamps in this case.
carl |
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Mark Herrick
From:
Bakersfield, CA
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Posted 5 Feb 2004 4:24 pm
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I found the cermet trim pots at my local electronics supply. But the closest values they have to the 15K resistor that is currently attached to the balance pot are 5K, 10K, 20K and 25K.
Can I use one of these? Should the value of the replacement resistor still be 7.5K, or something else?
And can anyone give me a specific description of how to add this trim pot into the circuit? From the previous info it sounds like it goes between the lug that the existing "pull down" resistor is attached to on the balance pot and ground (the case of the balance pot), but there are three connections on the cermet trim pot and I'm not sure which ones to connect where.
One final question (yeah, right...) why couldn't I use a full sized pot like the one that is already there? Seems like I could drill a hole in the chassis near the balance pot and have access to it from under the chassis.
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Ray Minich
From:
Bradford, Pa. Frozen Tundra
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Posted 5 Feb 2004 4:47 pm
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If you really wanna get sexy you can install one of them full size Bourns 10 turn 10 watt pots that are half the size of a can of beer.
Then you can put a "counter" dial on the output shaft that counts from 0-999 with a friction lock.
It all depends on how much room you have and how far ya gotta run them grid lines and how much you wanna spend :>)[This message was edited by Ray Minich on 05 February 2004 at 04:49 PM.] [This message was edited by Ray Minich on 05 February 2004 at 04:50 PM.] |
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Mark Herrick
From:
Bakersfield, CA
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Posted 5 Feb 2004 5:29 pm
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Oh, a wise guy, eh?
Nyuk, nyuk, nyuk!
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Ken Fox
From:
Nashville GA USA
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Posted 5 Feb 2004 7:15 pm
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| Put a 10K fixed resistor in series with the 10K pot. Range from 10K to 20K can be achieved. |
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