IBAM won't adjust below 35.5mA... what to do?

thornev

Super Member
I've got IBAM installed. Infrequently I test to verify the tubes are working (outputting) within 10% tolerances of each other. Today I discovered one of the adjusters adjusts upward fine but will not adjust below 35.5mA. Does this mean the adjustment pot has gone bad and needs to be replaced? (not an easy fix as soldering a new pot is going to be challenging for me) 35.5 is still within the 70%-80% range of max wattage. Thanks.
 
Try re-sitting tubes first. Switch affected tube with another one in same channel and attempt to re-adjust to see if the problem follows the tube.
 
This means you're not able to get enough negative voltage to this tube via the circuit to bring down the tube current. Are the other pots near the max end of the range? If you're using new production tubes, they often need more negative bias voltage to bring down the current. This tube may have started to draw more as it broke in. New tubes? If all the pots are near max, you may need to adjust the whole circuit for more negative voltage and this can mean fiddling around with the resistors instead of the pots.
 
I had a very hot quad of new issue Tung-Sols 7591s that wouldn't adjust like that. Had to change out the 4 - 12k fixed resistors in the IBAM down to 10k to get more negative voltage. Me, I think I would adjust them all to 35ma.
 
Thanks for the responses, All. They are not new tubes. They are new Tung-Sol 7591A's, but I've been using them for over 1/2 year.

I'll try the tube swap trick and report back.

Yeah, 35.5mA all around sounds good, but what worries me is that the tube used to be around 34.5mA and it adjusted itself (?) upward. I don't like tubes or circuitry that has a mind of its own. Too Terminator-ish. =:) I may have to open it up and poke around.
 
If your line voltage shifted upward a bit for whatever reason, that could explain it. Cooler weather with less AC usage since last check?
 
On my 800c's the bias floats btwn 33 and 34 depending on season due to changes in Line Voltage. If it's just one tube, clean the pins, clean the socket contacts and tighten them up. Same with the 400. When the voltage goes up I put them on variac's in the winter.
 
If your line voltage shifted upward a bit for whatever reason, that could explain it. Cooler weather with less AC usage since last check?
Exactly why I have a power conditioner that keeps wall voltage at 120 V, just a couple volts added can get the bias to climb quite a bit.
 
Steve, whats the model no on that conditioner if I may ask? Might look into one as our volts are high.... Thanks!
 
I switched the tube pair where the one tube was the one that had the adjustment issue. After the switch I was able to get all 4 tubes very close to 35.5mA. However, some pots are very close to their peak adjustment. Also, I moved the AC source to a new $22 power strip that claims to be a surge protector and has 5V 2.1A USB ports on it as well as IN and OUT coax connectors for what devices I don't know (cable modem?). I'm getting a power conditioner that keeps voltage stable though I'm also curious about the device that Steve uses.

So it would seem all is well though I doubt it. Something caused the pot adjustment problem and I don't believe I fixed it. Thorne

PS - I looked up Steve's conditioner and it has been discontinued by the manufacturer and seems out of stock everywhere I looked. I bought "Furman SS-6B 6-outlet Power Strip 3-pack" from Sweetwater.
 
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I switched the tube pair where the one tube was the one that had the adjustment issue. After the switch I was able to get all 4 tubes very close to 35.5mA. However, some pots are very close to their peak adjustment. Also, I moved the AC source to a new $22 power strip that claims to be a surge protector and has 5V 2.1A USB ports on it as well as IN and OUT coax connectors for what devices I don't know (cable modem?). I'm getting a power conditioner that keeps voltage stable though I'm also curious about the device that Steve uses.

So it would seem all is well though I doubt it. Something caused the pot adjustment problem and I don't believe I fixed it. Thorne

PS - I looked up Steve's conditioner and it has been discontinued by the manufacturer and seems out of stock everywhere I looked. I bought "Furman SS-6B 6-outlet Power Strip 3-pack" from Sweetwater.
Yeah, they don't make this beast anymore but they do show up on the auction site from time to time. I found mine in a thrift store for 100 bucks years back, quite the deal! Parts Express sells a couple conditioners that should do the job.
 
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