Alternate Individual Bias Adjustment Modification

larryderouin said:
Tom; 1/4watt are sufficient. Right off the top of my head, 22ga single strand wire will handle the load, but 20ga is a bit better as it doesn't have the tendency to break as easily. And the board I use is mainly the Radioshack dual 2"x4" board with perforations between them so you can snap them apart to 2"x2" board. Home Depot should have 20ga stranded wire in the electrical section (or try lowes). Mouser is ridiculous with regard to wire.


RadioShack Dual Mini Board
catalog #: 2760148 $2.49
Closest Ratshack with sprint store is in Las Vegas. 4 of them there.

Mouser
Keystone breadboard
534-3396
4.5" x 2" (No perforation for splitting in 1/2). $2.52
Click to expand...

Thanks, Larry. I'd ordered some of the parts for the IBBA before but chose the wrong capacitors. I haven't been able to locate any perforated board anywhere locally and the ones I ordered from Amazon aren't scheduled to arrive until the end of the month. I can get started on the IBBA built before my other parts arrive.
 
What value caps did you order? Its not super critical as long as the voltage is sufficient to make it not blow up and the value is high enough to ensure no bleed-through from the other side of the pot.
 
gadget73 said:
What value caps did you order? Its not super critical as long as the voltage is sufficient to make it not blow up and the value is high enough to ensure no bleed-through from the other side of the pot.
Click to expand...
gadget73 said:
What value caps did you order? Its not super critical as long as the voltage is sufficient to make it not blow up and the value is high enough to ensure no bleed-through from the other side of the pot.
Click to expand...

Hi Gadget, I ordered 10uF 35v ( Mouser #: 647-UKL1V100KDDANA ). I think that's what Larry had on another post about the IBBA.
I do have a question about the connections to the board. I see the test points connect @ Pin 5. On the four connections @ A. B. C & D in Dave's drawing do I disconnect R121-124 from the terminal strip they are now and connect those ends to the wires leading to A - D? I remember seeing some photos of actual installations on another post. I will look there to see if they show those connections.
 
larryderouin said:
Tom; You might want to read Dave's article on keeping output tubes healthy. A fair amount of it may go over your head, but it falls right in with what you are doing. Some more light reading from the master and his pals.
http://tronola.com/html/7591a_tubes.html
http://tronola.com/html/maximize_tube_life.html
http://tronola.com/html/better_hookup_wire.html
Click to expand...

Hi Larry, I just saw this post. I will read each of those articles. Much of what I read in this forum is over my head but some things are getting clearer.
Tom
 
dcgillespie said:
More Pics -- 800C DC Bias/Balance Modification

First, for you late nighters or early risers, I went back and edited my initial post to include more detail about the pics provided that will help to answer some basic questions that might arise, so I wanted to alert you to that.

Beyond that, the underside shot is really quite uneventful, as one of my goals is to leave a unit looking as much Fisher produced as possible after I'm done, as it did before I started.

The usual work has been performed by way of installing matched 10 ohm cathode resistors and 100 ohm Screen Stability resistors at each output tube socket, so that the new tubes can operate worry free. I always use pin #1 of the 7591 sockets as the new daisy chain screen buss terminal for the Screen Stability resistors to tie into at each socket.

For now, the bias leads from the new DC Bias/Balance board tie into the existing grid return resistors with very secure and safe air connections. This unit is a work in progress with more work ultimately planned for the area, so they will be more properly addressed at that time. For now however, the connections represent no concerns even over the long term.

The original bias wiring has been left in the unit, as have been the original -17 volt bias tap components (5.6K, 15K, and .1 uF) -- although the end of the 5.6K resistor that attaches to the heater feed terminal of the heater/bias supply has been lifted from that terminal, so that the old network is no longer powered. It is this same heater feed terminal that the supply lead to the new DC Bias/Balance board attaches to.

Pics include:

1. The finished look on the back side. No, it hardly looks like it was part of the original build. But for the ease of accessing the new controls and test points the installation provides, the neatness and practicality of it all trumps maintaining original external appearances in my book.

2. The under side view hopefully shows how well the modification's wiring just nestled right in with the original wiring, drawing little attention to itself or the work done. Here you can see the new 10 ohm cathode resistors, the 100 ohm Screen Stability resistors, and the new connections to the grid return resistors.

After it enters the chassis, the pig tail lead is first secured by a cable clamp that is sandwiched between two nuts installed on a longer screw installed that secures the back left mounting tag of the left OPT when viewed from the rear. To effect a good ground connection, the shield of the pig tail is then soldered to the nearby mounting lug of the T-strip servicing the grid resistors for the left channel output tubes. The yellow lead from the pig tail snakes its way over to the DC heater/bias supply to apply power to the new DC Bias/Balance board.

3. The new (to me) Westinghouse output tubes are quite happy with the new arrangements. They are used, but still have a considerable amount of life left in them.

4. I drew the circuit out on a note pad and took a pic of it. If the resolution is not adequate, I'll scan and re-post it later, but I think this will suffice. I notice I left an arrow off the wiper of the right channel DC Balance control, but no doubt you get the gist of the circuit.

5. Finally, I now have a wonderful 800C that performs as good as it looks -- and it performs exceedingly well in all aspects now. The original volume control even still tracks quite nicely at all volume levels. When it all works the way its supposed to, it really is unmatched when the total package is considered. It was an amazing unit to produce in its day, and remains just as amazing today.

BTW -- Even though the 800C has a powered center channel output feature, unlike so many units with that feature, this unit does not change absolute polarity for any internal signals or external signals provided to selector switch inputs. Just some extra info I meant to pass along earlier.

With this unit really coming into its own now, are there any recommendations as to who might build a quality aftermarket copy of a Fisher cabinet to properly dress up this looker? The few real ones that are currently available on the auction site look pretty rough!

Dave, I am about to build a board with your design and I have a few questions regarding the connections to the 500c.
1. Do the Test Points connect to Pin 5 of each output tube? Do I ground both the Gnd TP and the IBBA circuit to chassis ground?
2. You say the bias leads from the new DC Bias/Balance board tie into the existing grid return resistors with very secure and safe air connections. Apparently I do not know what a grid return resistor is. I only see 2 resistors connected to V8 - 11. Can you tell me what the number is for that resistor? Also, do I lift one end and connect it to its respective IBBA lead.?
I can't quite make out the connections in your photos. When I increase the size I lose the detail. I see a wire from Pin 1 going towards the IBBA (I think) but I lose it.
I thought I understood the cathode sampling resistors from Pin 5 to ground and the screen sampling resistors from Pin 1 to 4 but the grid return resistor has me stumped. Could that be those 220ohm resistors I substituted for the original 330ohm resistors? If so, do I list the end from the terminal strip and connect it to the respective IBBA lead?
I apologize for all the questions. I've been reading all I can but some of the terms used in this forum seem to be beyond this old man's grasp. You AKer's are my lifeline.
Tom

Dave
Click to expand...
 
I tried to post this to Dave's earlier post but it doesn't look like it worked.

I am about to build a board with Dave G's design and I have a few questions regarding the connections to the 500c.

1. Do the Test Points connect to Pin 5 of each output tube?
2. Do I ground both the Gnd TP and the IBBA circuit to chassis ground?
3. Dave says, "the bias leads from the new DC Bias/Balance board tie into the existing grid return resistors with very secure and safe air connections". Apparently I do not know what a grid return resistor is. I only see 2 resistors connected to V8 - 11. (I haven't installed the 100ohm screen stability resistors yet) Can someone tell me what the numbers are for those resistors? Also, do I lift one end and connect it to its respective IBBA lead?
I can't quite make out the connections in the photos. When I increase the size I lose the detail. I see a wire from Pin 1 going towards the IBBA (I think) but I lose it.
I thought I understood the cathode sampling resistors from Pin 5 to ground and the screen sampling resistors from Pin 1 to 4 but the grid return resistor has me stumped. Could that be those 220ohm resistors I substituted for the original 330ohm resistors? If so, do I lift the end from the terminal strip and connect it to the respective IBBA lead?
I apologize for all the questions. I've been reading all I can but some of the terms used in this forum seem to be beyond this old man's grasp. You AKer's are my lifeline.
Tom
 
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1.) Yes to the TP's connecting to Pin 5 on each tube. If the GND TP is wired back to the IBBA, NO.
2.) Grid returns resistors are the 220K resistors that you used to replace the 330K. 4 of them and the A,B,C, D points from the IBBA (1ea.) connect to each resistor, replacing the original bias wiring from the bias dual cap. The end away from each tube is the end lifted. You'll see a wire connected to that terminal that daisy chains to each of the resistors. That is the original bias system wiring. When you connect the IBBA, you'll disconnect that wire at both ends and at each terminal with the grid return resistors as the IBBA is now supplying the bias voltage.
 
Thanks Larry.
1. I got.
2. When I connect the IBBA I will be pulling C86 & R131 & R132. Correct? That should disconnect the original bias system, won't it? Where those 3 terminate there is a white wire heading to a terminal strip with R121 & R122 attached. That wire is daisy chained (?) to a 2 tab terminal strip which is connected via 2 resistors (R129 & 130 ?), to the terminal strip where R123 & R124 are attached. Do I need to disconnect the 3 points of the white wire? I'm thinking that will eliminate those 2 resistors connecting those 2 terminal strips?
Tom
 
DELETE C86, R131, R132. Remove the White wire leading over to the Grid Resistors (R121, 122, 123,124) and remove all vestiges of the white wire at the grid resistors. R121 and R122 were paired as were 123 and 124 at the terminal strips to the white wire. You need to separate the R121-122 and R123-R124 pairs. Facing the rear of the 500C the R121 will be on the RIGHT hand tube. The resistors are right to left 121-122-123-124. This lines up with the bias leads from the IBBA of A-B-C-D. If you can keep them separate with another 2 terminal strips to connect the bias wires it's neater, otherwise use lengths of heatshrink to Insulate the Wire leads with the resistors.

STOCK OUTPUT SECTION
500 stock.JPG

500 output section modified for IBBA.
500 modified.JPG

NOTE that I only copied one channel. A & B will be on the upper channel on schematic, and C & D will be on lower.
 
So does this mean the end of the IBAM for everyone?
I don't understand the DC bias and balance part of things, just that just mean you are doing the adjustment now in pairs versus one at the time?
What's being balanced?
Is IBAM inferior results wise if the adjustments are done carefully and stay put? Or is this new approach just more user friendly with less back and forth?
I read the part about wipers lifting, how often does that happen?
Or are you adjusting two separate things?
What is it that is being zeroed?
Thank
Joe
 
No it doesn't mean the end of the IBAM. When you use the IBAM you balance the current output of all 4 tubes individually, and the tubes can drift off that set point individually so you get one tube drawing more or less than the other 3. (or any combination of the 4). With the IBBA you actually break the 4 individual adjustments to 2 paired adjustments, with the pairs balanced for tube current output differential of zero volts. This way the tubes are equally handling the loads. The bias voltage will be different for each tube in both the IBAM and IBBA, but the CURRENT LOAD is the most important thing here. The current load in MV measured at the cathode resistor(converted to ma) is what is being ZEROED in all cases ( the two tubes will have exactly the same current load. The IBBA does a better job of it, but it can be done with the IBAM with some patience. Multi turn pots in this case is a definate plus as you can get more accurate adjustments with them. I use both IBAM and IBBA in my gear. The only plus actually I see with the IBBA is convenience in adjusting and that the pot is bypassed with a resistor to keep you from getting zero volts of bias and blown tubes. I use both in my gear, and I really don't have a preference for operation. The IBAM is a bit easier for me to assenble however.

Wipers lifting isn't all that common, but it's possible for it to happen, especially with old or cheap pots. Using a quality pot like a Bourn's is good insurance. The additional resistor is good insurance too.

The IBAM and IBBA accomphish the same thing, just a matter of convenience and choice. Kind of like cars. Ford, Chevy, Dodge, Studebaker. Your choice. they all accomplish the same thing, but are different in their own ways.
 
Thank you for the nice detailed post
I asked as it still seemed to me that it still all comes down to how healthy your tubes are to begin with and adjusting in pairs wouldn't be as flexible as the IBAM as I understand it, meaning if you had a quad and one was really far off from the other in a pair or the other pair for the other channel
It just seems as if the IBBA alternative would only be more effective if you have reasonably matched tubes to begin with?
Because isn't the condition of the tube going to have an effect on how much it draws regardless of adjustment scheme you choose?
Thanks larryderouin!
Joe
 
As Larry has explained, both approaches accomplish the same end in terms of of being able to balance the current draw so that all of the output tubes draw the same amount of current. However, for those with lab equipment at their disposal, the IBBA approach offers a huge benefit.

It is important that the output tubes of an amplifier are balanced for a number of reasons, be it (1) The health of the tubes, (2) minimizing saturation of the OPT at low frequencies, and (3) minimizing distortion at all frequencies. However, what is ALSO important is the level of current flow that the tubes are balanced at. In most circuits, distortion will hit a null at a precise point, which is then the ideal operating point for the tubes to operate at under quiescent conditions. In well equipped labs then, it is quite easy to connect the output of an amplifier to a Total Harmonic Distortion Analyzer, and adjust the bias for a null in the reading.

Except that if each pot only adjusts one tube, it is not easily possible to raise or lower the current draw of both tubes together at once in a push-pull amplifier, as the stage will likely become unbalanced in that process, which in and of itself will raise distortion, making the null point very elusive to find.

With the IBBA modification, one control raises and lowers the current level of BOTH tubes IN MIRROR FASHION (one goes up while the other goes down and visa-versa), so that a balance point is easily found, while the OTHER control then raises or lowers the current draw of BOTH tubes at once in unison -- doing so WITHOUT disturbing the balance set by the first control.

In this way then, the low distortion operating point can easily be found with the IBBA modification, where as it would be quite hard to find that point with the IBAM modification. From a point of simply balancing the tubes however -- and to that only end -- both approaches will produce the same outcome, with personal preference ruling the day.

I hope this helps!

Dave
 
dcgillespie said:
As Larry has explained, both approaches accomplish the same end in terms of of being able to balance the current draw so that all of the output tubes draw the same amount of current. However, for those with lab equipment at their disposal, the IBBA approach offers a huge benefit.

It is important that the output tubes of an amplifier are balanced for a number of reasons, be it (1) The health of the tubes, (2) minimizing saturation of the OPT at low frequencies, and (3) minimizing distortion at all frequencies. However, what is ALSO important is the level of current flow that the tubes are balanced at. In most circuits, distortion will hit a null at a precise point, which is then the ideal operating point for the tubes to operate at under quiescent conditions. In well equipped labs then, it is quite easy to connect the output of an amplifier to a Total Harmonic Distortion Analyzer, and adjust the bias for a null in the reading.

Except that if each pot only adjusts one tube, it is not easily possible to raise or lower the current draw of both tubes together at once in a push-pull amplifier, as the stage will likely become unbalanced in that process, which in and of itself will raise distortion, making the null point very elusive to find.

With the IBBA modification, one control raises and lowers the current level of BOTH tubes IN MIRROR FASHION (one goes up while the other goes down and visa-versa), so that a balance point is easily found, while the OTHER control then raises or lowers the current draw of BOTH tubes at once in unison -- doing so WITHOUT disturbing the balance set by the first control.

In this way then, the low distortion operating point can easily be found with the IBBA modification, where as it would be quite hard to find that point with the IBAM modification. From a point of simply balancing the tubes however -- and to that only end -- both approaches will produce the same outcome, with personal preference ruling the day.

I hope this helps!

Dave
Click to expand...
Thank you for that thorough yet concise explanation!
Makes perfect sense to me
So, with the IBBA one could conceivably wind up with two very different operating pairs though, as far as draw is concerned, correct? That assuming that the two pairs don't enjoy the same optimal adjustment points due to the tubes themselves.
And, if a user doesn't happen to have a distortion analyzer
More precision to the adjustment within the pair but not necessarily mirroring the other pair, as in winding up at the same optimal spot, left compared to right.
It would just appear to me that for this IBBA to realize it's fullest potential for precision over the IBAM one would have to have a pretty well matched quad to begin with, which I understand is the ideal and what we all want but not always the case
I was under the impression that adjusting the bias based on plate voltage x draw and keeping dissipation within safe rated specs was all I needed to worry about
Or do I still not get it?

Also interested in asking, how much of a difference between the two implementations can you hear?
 
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Anything you would hear different between the two implementations would be solely how the controls are adjusted -- not due to the differences in approach. If they are set to produce the same bias conditions for the output stage, then there is no difference in performance.

The ideal is to in fact start with a well matched quad of tubes to begin with, and then let the bias and balance controls tweak the tubes to a very tight operating match. This will deliver the very best overall performance. The problem comes in when folks assume that the controls are to allow for the use of even widely mismatched tubes, which would be a poor way to go. That is why the range of the controls is rather intentionally limited (which eases their adjustment), rather than trying to cram in so much range to them that you end up requiring multi-turn pots to make the adjustment, allowing such mismatched tubes to be used. The very best performance will always be had when starting from a well matched set of tubes, and then tweaking them for the tightest operation. This will also ensure the match performance wise between the channels as well.

Dave
 
I've used both methods, with and without matched tubes. For grossly mismatched stuff, frankly the IBBA doesn't always fare so well. You end up doing a lot of back and forth jockeying to try and make things work right. Works absolutely wonderfully with matched tubes though. The real fix for mismatched tubes is a different set of tubes honestly. Even if you do get them to idle evenly, they aren't going to perform quite right and you will be able to measure that should you feel the need to.
 
Great information Dave, thanks! I've pretty much gone to the IBAM when I use old stock 7591/7868s as it's pretty hard for me to find reasonably matched quads or pairs. I would find that I couldn't get the pair to balance, or if balanced couldn't get the bias range right. The IBBA is great for matched new production tubes however.
 
larryderouin said:
Tom; You might want to read Dave's article on keeping output tubes healthy. A fair amount of it may go over your head, but it falls right in with what you are doing. Some more light reading from the master and his pals.
http://tronola.com/html/7591a_tubes.html
http://tronola.com/html/maximize_tube_life.html
http://tronola.com/html/better_hookup_wire.html
Click to expand...
Hi Larry & Dave,
Larry, I've read and re-read Dave's article regarding maximizing tube life and have installed 10 ohm resistors on my output tubes from Pin 5 to Ground and will be installing the 100ohm screen resistors between Pin #'s 1 & 4 when they arrive today. My question is in regard to the dual delay timers and relays in the last part of the article. Is this something I should be adding to my 500c?
If so, Dave you refer to RadioShack's relay (Item # 275-217). I cannot locate that part # of the RadioShack website. Do you know if there's a current relay available from RadioShack or another source for this relay? Also, can you provide a make/model # of an appropriate 120v AC timer? Lastly, can you provide a schematic/instructions for my install. Your sketch #6 in the article reflects 2 delay switches I don't think I have. I think I understand the concept but don't know how to implement the solution.
Tom
 
Tom -- It is really only extreme circuit conditions or those who just "want" to delay the B+ that should consider that option. For a while, delaying the B+ to the tubes was all the rage, and the article was written so as not to fan the flames of that debate. But in truth, it is hardly necessary for tubes of the "receiving" type classification (which all the tubes in your 500C are), to operate with the B+ delayed, and the circuit conditions hardly dictate it be done, either. For 99% of the Fisher owners out there, the take away from the article is the addition of Screen Stability resistors, while the remainder of the information really applies to those who "roll their own" or design their own circuits (advanced experimenters), so as to keep their tubes healthy as they work out their designs.

I hope this helps!

Dave
 
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