IBAM board - What are the connections again?

Speaking of "slip" I got a 395 vdc hit two times from the tube rectifier. My hand slipped onto the terminal that feeds the power resistors. Wow, what a wakeup !

LOL..... wish I were there, a fly on the wall!
Hit 2 times! It's always funnier when its someone else lol.
But seriously... glad your ok and seriously..... wish I was there!
Jef
 
Jef - It wasn't much to watch. I withdrew my hand very quickly and chastised (and laughed at) myself for being such a careless dweeb. But I went right back in and continued working. It really wasn't any more eventful than a 117 vac shock. But it would have been fun seeing YOUR reaction. Haha.
 
My IBAM board was a fail. I have no idea why. Turning the pots does nothing.

I added wiring to each pin 3 (cathode) of the output tubes and 1 wire to pin 7 (plate) of 1 tube. I ran these wires externally so I don't have to keep opening the chassis to test BIAS and DC Balance. Haven't yet decided on how to mount the leads.
 
Thorne. You should have 4 wires on the IBAM
1.) B- power from the board to the cap
2.) Chassis Ground
3. & 4.) leads to center tap on the Bias BALANCE POT's.

Schematic from page 1 with Red lines of Bias circuit have been altered to show the following

a.) Disconnect the 5K BIAS POT (Back Wall) center tap wire from the cap.
b.) Disconnect and remove old BIAS WIRING from Cap to BOTH BALANCE POT Center Taps.
c.) Install IBAM BOARD as close as possible to the BALANCE POTS.
d.)Ground wire to closest convenient chassis ground. Ensure ZERO (0) ohms from board end of wire (lead).
e.) The wire disconnected in a.) is connected at the Junction of the 5K pot and the 10K resistor to it's right (I can't make out the part #'s). This wire will become your B- power lead to the IBAM.
f.) The 2 remaining wires on the IBAM (3 & 4 above) each go to the center wiper on the BALANCE POTS (1 wire for 1 pair of tubes.) Make sure the old wiring fom the cap to the balance pots is removed.

NOTHING GOES TO PIN 7!!!!

299 ibam.jpg
 
Thanks for the details, Larry. But how do I calculate watts if I don't know plate which is pin 7 on a 7189 tube? My post was about 2 different tasks - IBAM and setting BIAS based on wattage.
 
Sorry, mis-read your post. If the lead from 7 is a test jack for plate voltage, that's ok but make damned sure it's got some substantial insulation on it where it goes thru the chassis. But nothing from the IBAM connects there.
 
Last edited:
Larry - I have SIX wires coming from the IBAM: one for B-, one for ground, and 4 for each 7189 tube (my assuming the wiring would be like the 500-C). If I'm to have only FOUR wires, then what do I do with the 4 pots on the IBAM and the 4 caps and resistors on the IBAM ? Is a completely different IBAM board required?
 
Because you are tying in to the BALANCE POTS and NOT directly to the tubes , you only need 1 IBAM wire for each pair of tubes. You can cut off the extra 2 wires and just not use the extra pots on the board (Mark the two that you WILL USE) or build a new board with only 2 adjustment setups. And you'll probably have to change the resistors and or the pots (to what I don't know, I'd ask Dave on that). You'll have what amounts to a IBBA board. Connect it as shown in the last picture.

On my LK-72, I added 2-10K bias pots, and ran them to the balance pots. This is off the 299D receiver (later one with Bias pots on the top of the chassis (one per channel. See Dave 451's LK-72 resto here........ http://audiokarma.org/forums/index.php?threads/scott-lk-72-rehab.784404/ in which he details the circuit (picture) of the bias pots. http://audiokarma.org/forums/index.php?threads/scott-lk-72-rehab.784404/page-4

299D Bias adapted to LK-72.jpg
 
First of all, apologies for hi-jacking this discussion. I am hoping Scott grurs on this forum can help with a puzzling problem I am having.

I was given an LK-72 back in the '80s and it was working but needed the 0.1uf caps feeding the 7591s replaced. It sit idle for many years because of other work/life priorities. However eventually I did manage to replace all the electrolytics and film caps plus matched resistors where required. The last time I used it after the changes I noted one channel was playing at lower (considerable?) volume as well as having the occasional loud pops when volume knob was turned. In trying to debug it, I seem to have made matters worse. This lists what I have done to date:
1. I cleaned all pots/switches and tube sockets with DeoxIt D5.
2. Checked voltages on all tubes and noted voltages on V6 pins were very different than that on V5 (Pin 6 70.1 vs 59.9, Pin 8 73.1 vs 63, Pin 9 69.9 vs 59.9). So I rewired and replaced all caps and resistors for V6.
3. I adjusted the bias voltages for the output tubes to around -18v as mentioned in past Scott LK-xx threads.
4. I should have stopped there and check if the problem were solved. but I went on to add DC bias as well as cathode resistors (10 ohm, 1/4 w).

Once done with all that I checked the voltage on the newly added DC Bias pots and was puzzled that it was reading around -70 volts (instead of -44 volts before). The DC voltage at the replaced silicon rectifier now reads something like -70.5 volts. All B+ voltages are within spec. (Even if no current was flowing to the pre-amp tube filaments, should the voltage be so high?)

I suspected one of the 12ax7s was bad but after checking each individually (by connecting 12v to the filament pins) I think that was ruled out (?) (they each read around 140 ma).

I am now wondering if the rectifier or power transformer could be bad but don't think that makes sense either.

Would really appreciate any and all suggestions on how to troubleshoot this problem.
Thanks greatly in advance!

Fred I
 
If you replaced a selenium rectifier with a Silicon, the output voltage will be higher. You'll need to adjust the downstream resistor to bring the bias input voltage back into range. Dave recomended a 30Ω 2W or so. This is what I installed in mine and brought it down to within 2volts of nominal at the heaters. Suggest reading this thread and making some if not all of Dave's improvements/mods. I would also ensure that a the V6 Tube pin sockets are TIGHT against the pins.

Could also be that V6 is pretty much shot. If it's a 6GH8, go ahead and replace both. abcvacuumtubes has NOS 6gh8/A for $3.00 ea.

UNLOADED circuits will always have a higher voltage on them. Something is open or partially crapped out.. You'll have to meter it out.

Any further questions should be addressed in a NEW Thread in the TUBE FORUM. ADMIN can merge your post into the new threadalong with this post.
 
Thanks very much for your reply and comment Larry!
I think I just solved the mystery (with the high voltage at the output of the silicon rectifier anyway). It turned out the wire that connects V2 filament to V1 filament was pinched by the pot I added for DC balance adjustment!! :confused: After replacing that lead the voltage is in the right range (-44 volts, I did increase the resistor value after the change to silicon rectifier). Now I can continue debugging other problems :).

Will post new questions in new thread as suggested.

Thanks again,

Fred I.
 
Back
Top Bottom