Alternate Individual Bias Adjustment Modification

[Tom Boley]

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

Dave, thanks for clearing that up for me. I've read your article 3 times and think I understand the benefit of the adding that 10 ohm resistor between Pin 5 & Ground and adding the screen stability resistors. When you said that after adding the screen stability resistors you had no more instances of arc's that was enough for me. I'm waiting for the cans from Hayseed so I'll build but not install the IBBA. I'll wait until my initial issues are resolved before plowing forward. I still have 2 sections of the MetalBone update kit to complete, as well. Hopefully by next week I'll be ready for an inspection before the next power up. Tom

 

[Tom Boley]

Now that I've restored proper operation in the AM and FM tuners, MPX decoder, and FM Automatic matrix switching circuits in my 800C (there were problems in all four of these sections), I've now turned my attention to the audio section.

When received, it still had the "original" Fisher 7591 output tubes installed, which were advertised as having tested fine, but in reality were so bad that although "functional", they would only allow a small fraction of the unit's performance to be produced. But, what the hey, everything else was advertised as operating properly in this unit as well, so it fit right in with the Grand Canyon of difference that existed between truth and reality by the seller. At any rate, with the unit really starting to come together, a decent set of low mileage Westinghouse 7591's was purchased as the next order of business to attend to, in the quest to make this unit be all that Fisher intended it to be.

As is so often the case today however, while the new tubes were a great improvement over the old ones (almost anything would have been!), one was out in left field with it's grid bias voltage requirements. This was aggravated by the fact that the tech who serviced the unit in readying it for sale put in a bias modification to cool down the output tubes. Boy did it ever cool them down! Good tubes were only drawing 9, 11 and 13 ma of total cathode current each, with the odd-ball pulling 22 ma, and collectively producing an abundance of crossover distortion at even modest power levels. Throttling back the bias voltage a bit brought the three tubes up into the 16-18 ma territory, and the odd-ball up to about 28 ma. Clearly, this would hardly do for the otherwise great tubes this receiver now had, and the condition it was rising to. The ability to adjust each output tube individually then became top of the list important.

When it comes to installing individual output tube bias adjustments, the approach incorporating DC Bias and Balance controls in each channel offers a number of advantages over having four separate bias controls:

1. The overall adjustment process is easier to execute, with only two bias controls requiring adjustment to match channel performance rather than four.

2. It offers greater fail-safe protection when properly designed, where if the wiper of either the DC Bias or Balance control in a given channel lifts away from its carbon track, both output tubes in that channel immediate go to a maximum negative grid bias condition.

3. It is the only configuration that allows for the complete output stage quiescent current draw to be easily adjusted up or down (both tubes in unison) to set the stage for the lowest distortion operating point.

When Fisher included a FULL complement of output tube adjustment controls, the DC Bias and Balance approach is always the approach they used. Think SA-100, X-202/X-202B, X-1000 (EL34) and K(X)-200 integrated amplifiers as some examples of this type of design. If I was going to install adjustment controls to optimize output stage operation, this is the type I wanted to employ.

Also, many of the examples of individual biasing schemes I could find had the controls and/or test points located underneath the receiver chassis, which I try to avoid at all cost. As I get older, these things get heavier and more cumbersome. Fixed umbilical cords to separate external boards or monitoring boxes can address that, but make removal of the unit a hassle from a cabinet when necessary. Therefore, anything I can do to either eliminate removal, or minimize the maneuvering of these things when removal is required is a blessing for sure!

With these design and installation goals then framing the project, I thought I'd share the approach I used to achieve them, as but one more offering to include in the stable of available options when these types of circuits are installed in Fisher receivers.

The circuit containing the 4 pots, 6 resistors, and 4 caps, along with the 5 test points (one being ground) for use when making adjustments are all contained on a small piece of perf board measuring 1" tall X 2&13/16" wide. The board requires 10 leads to connect it into the 800C, which was conveniently handled by a 9 conductor shielded cable, with the shield acting as the 10th (ground) lead. The leads do not need to be shielded, but it provided a convenient way to neatly bundle the 10 leads required to install the board.

The test points are nothing but insulated closed-end wire connectors bent into an "L" shape to form a convenient point for mounting the board to the chassis as well as a test point for the circuit, with similar connectors bent into the same shape but mounted on the back side of the board behind the test points to allow for a connection to the test point. The insulated barrel of the wire connectors forming the test points are then affixed to the chassis with hot glue. I know. Cheezy. But in practice, it works quite well, and is in keeping with my effort of being only minimally invasive when necessary, and only when necessary. This time, it wasn't necessary, and all work can easily be reversed without a trace of the installation remaining. The pics show all of this rather clearly.

The final results turned out very well indeed. Located just behind the Reverb connection jacks on the top of the chassis, the bias can now be measured and adjusted quite easily at will, even when installed in a cabinet. The wiring from the board enters the chassis through one of the ventilation holes drilled around one of the can caps, and is secured underneath by a wire restraint.

Pics include:

1. The front side of the board during construction. You can see how compact it is, yet contains everything needed to properly adjust the output tubes for optimum operation.

2. The rear side of the board during construction. The wiring is close quarters, but hardly the tightest quarters.

3. The assembly was finished by attaching the flying pigtail lead that will connect the board into the circuits of the 800C. Each twisted pair represents the wiring to a specific output tube: one lead connects to the cathode terminal to establish test point operation, while the other is the adjustable grid bias supply source from the board for that same tube. Individual 10 ohm cathode sampling resistors are installed at each output tube socket, as are individual 100 ohm Screen Stability resistors as well.

4. A close up of the board mounted in place. It was my first hot glue experience so the job is not the neatest, but does the job of mounting the board down quite admirably none the less. The center test point is a ground point for the negative meter lead, while the test points on either side of the ground test point represent a channel, with the test points electrically connected/arranged in the same order as the output tubes themselves appear on the chassis. For long term memory of the test point/output tube relationship, the two most left terminals are color coded green, with a green dot then marked on the chassis between the two most left (left channel) output tubes. Red indicates the same information for the right channel tubes and test points.

The control centered above a channel's pair of test points is that channel's DC balance control. It is first adjusted for 0.0 vdc between the two test points below it. The control next to each channel's DC Balance control is that channel's Bias control, which is then adjusted for the target cathode voltage from either of that channel's test points to ground after the DC Balance control is adjusted. The range of the controls are such that the least negative bias voltage that can be applied to either tube in each channel is -13.5 vdc, preventing any catastrophic tube damage from setting the grid bias voltage too low.

5. A rear view of the finished installation, showing how the flying pigtail lead snakes around the Reverb jacks, making them still fully usable if needed, before entering the chassis through a ventilation hole.

The design allows for a 3 volt spread over the range of the DC Balance control, and another 3 volt spread over the range of the Bias control. This nicely accommodates the rather wide spread of the tubes installed in one channel, while still allowing an easy enough adjustment of the two more closely matched tubes in the other channel. The output tubes are now all happily idling at 30 ma each.

In the final post, I'll show some underside shots of how the board's pigtail lead is connected into the wiring of the receiver, and a quick scribble of the circuit I developed for this project.

Dave

Dave, I've just about got my IBBA finished but have one more question. My board looks very similar to your drawing with the top control for the balance with the 2 (across) connections going to A, B, C or D. The second control (bias) is connected (at the across connections) to the empty tab on the upper control and to a 10k resistor on the bottom. What do I do with that orphan tab on the bottom control?
Tom

 

[dcgillespie]

To make it simple, the "wiper" connection of the DC Balance Control connects to the wiper connection of the Bias Control. One of the outside connectors of the Bias Control connects to the 10K resistor. Which outside connector you chose to use will determine whether turning the Bias control clockwise increases current draw, or decreases it. I like to wire the control so that turning the control clockwise increases current draw. That will mean that the 10K resistor should connect to the outside terminal that decreases in resistance when the control is turned clockwise. Once that determination is made, the other (unused) outside terminal of the Bias Control can be either left with no connection, or (ideally), shorted to the wiper terminal (of the Bias Control).

In reality, once the two terminals of the Bias Control are identified that you're going to use (wiper and particular outside terminal) for connection into the circuit, it matters not which goes to the wiper of the Balance Control, and which goes to the 10K resistor. In either case however, the unused outside terminal of the Bias Control should be shorted to its own wiper.

I hope this helps!

Dave

 

[Tom Boley]

To make it simple, the "wiper" connection of the DC Balance Control connects to the wiper connection of the Bias Control. One of the outside connectors of the Bias Control connects to the 10K resistor. Which outside connector you chose to use will determine whether turning the Bias control clockwise increases current draw, or decreases it. I like to wire the control so that turning the control clockwise increases current draw. That will mean that the 10K resistor should connect to the outside terminal that decreases in resistance when the control is turned clockwise. Once that determination is made, the other (unused) outside terminal of the Bias Control can be either left with no connection, or (ideally), shorted to the wiper terminal (of the Bias Control).

In reality, once the two terminals of the Bias Control are identified that you're going to use (wiper and particular outside terminal) for connection into the circuit, it matters not which goes to the wiper of the Balance Control, and which goes to the 10K resistor. In either case however, the unused outside terminal of the Bias Control should be shorted to its own wiper.

I hope this helps!

Dave

Thanks, Dave. I did have it wired incorrectly. Will digest above and reconnect. Tom

 

[Tom Boley]

Thanks, Dave. I did have it wired incorrectly. Will digest above and reconnect. Tom

Dave, If I understood you correctly I removed the bias control, rotated in 90 degrees, connecting the two controls via the wiper terminals. I then shorted the unused terminal to the wiper terminal of the bias control. When I can see it operate the bias control should make more sense to me. I've come up with a pretty cool mounting for the board. I'll post photos when I get the cans installed. Thanks again, Tom.

 

[dcgillespie]

Sounds good! I'll look forward to the photos!

Dave

 

[Tom Boley]

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

Hi Dave,
I apologize for asking so many questions but here are 2 more. I'm ready to install the screen stability resistors. Looking at your 2nd photo:
1) Are the 100ohm screen stability resistors connected to Pin #'s 1 & 4? I thought I saw another post where Larry said to connect Pins 3 & 8. Maybe that was for a different model.
2) I think I see a second wire on Pin #1 of each tube. Do I need to run a wire connecting the output tubes? To anywhere else? I seem to recall a post where you discussed using Pin #1 for new connections since it was originally unused but I cannot find that post.
Tom

 

[dcgillespie]

The 100 Ohm resistors are connected between pins 1 and 4, with the jumper remaining in place between pins 4 and 8. Then, all of the pin 1 terminals are daisy chained together and form the "screen buss", where anything that WAS connected to pins 4 or 8 (other than the resistor or jumper as mentioned) NOW connects to.

I hope this helps!

Dave

 

[larryderouin]

Tom; It was Pins 1 & 7 on 7868 tubes (for the 400) on the FISHER COMMON PARTS Sticky.

 

[Tom Boley]

Larry, I thought that was the case but I've learned to ask if there's any doubt.
Dave, that answers my question perfectly. It looked like they were 'daisy-chained' but I wanted to make sure it wasn't 2 pairs of two sockets. Now all I need is those blasted caps and 1 tube. Both should arrive this week.
Thank you both,
Tom

 

[larryderouin]

Tom; No problem. We're all getting more and more of those.......what?,,,, Huh, I can't find my (insert keys, glasses, wallet, etc here) moments. Go ahead and ask the questions, as it helps keep the rest of us on our.....??????? what was I trying to say.......

 

[Tom Boley]

To make it simple, the "wiper" connection of the DC Balance Control connects to the wiper connection of the Bias Control. One of the outside connectors of the Bias Control connects to the 10K resistor. Which outside connector you chose to use will determine whether turning the Bias control clockwise increases current draw, or decreases it. I like to wire the control so that turning the control clockwise increases current draw. That will mean that the 10K resistor should connect to the outside terminal that decreases in resistance when the control is turned clockwise. Once that determination is made, the other (unused) outside terminal of the Bias Control can be either left with no connection, or (ideally), shorted to the wiper terminal (of the Bias Control).

In reality, once the two terminals of the Bias Control are identified that you're going to use (wiper and particular outside terminal) for connection into the circuit, it matters not which goes to the wiper of the Balance Control, and which goes to the 10K resistor. In either case however, the unused outside terminal of the Bias Control should be shorted to its own wiper.

I hope this helps!

Dave

Hi Dave, I'm not sure about the connection of the 2 pots on each side. Above I think you are saying the wiper connection of both pot on each side are connected together. That's how I think I built my IBBA but something is wrong. I know my wires running from the IBBA to the outputs is correct but the actual board wiring seems like it's wrong.
One more time, can you tell me how to connect those pots? If my understanding about the pots is correct the outside pins of the balance pot go to the A/B & C/D leads to the 220k resistors. It looks like you have the center pin of the balance pot connected to one of the side pins of the bias pot not the wiper.
The way I had it wired (center pins of both pots on each side wired together) I get a reading of 10.5 - 11 ohms when I touch any test point to ground. Turning the pot doesn't seem to make any difference. I get some over 20 ohms when I touch 2 test points together. Turning any of the pots doesn't seem to change the readings.
I have removed the bias pots from the board. I've attached photos of the front and back of the board.
Tom

 

[larryderouin]

TOM; These pics should help you understand the connections and how to test for balance and bias once it's powered up.

 

[dcgillespie]

Thanks-you Larry!

Dave

 

[Tom Boley]

TOM; These pics should help you understand the connections and how to test for balance and bias once it's powered up.

View attachment 749767

View attachment 749766

Larry,
To be sure I'm wiring the board correctly:
Am I correct in connecting the wiper (center pin) of the balance pot to Pin 1 of the bias pot? Do the center pin and Pin 3 connect to each other? Then, is Pin 3 also connected to one side of the 10k resistor?
After reading your post I think I know what I supposed to connect but it doesn't seem like, when wired with no power, I don't get any change from turning either pot.
this may against the rules but would it be possible for me to talk to you about my connections? I'm missing something.
Tom

 

[larryderouin]

Connect the BALANCE POT CENTER PIN (wiper) to either pin 1 or 3 of the BIAS POT, WHICH EVER IS CLOSEST to the BALANCE POT. The bias pot can go in either way (it's ambidexterous!!!) so it works the same way no matter which way you place it. Just as long as the pins are in a VERTICAL orientation for the bias pot, and a HORIZONTAL orientation for the BALANCE POT.

Dave would be the one to talk to as he designed it. I've put it in 1 400, and that was over a year ago.

 

[Tom Boley]

Connect the BALANCE POT CENTER PIN (wiper) to either pin 1 or 3 of the BIAS POT, WHICH EVER IS CLOSEST to the BALANCE POT. The bias pot can go in either way (it's ambidexterous!!!) so it works the same way no matter which way you place it. Just as long as the pins are in a VERTICAL orientation for the bias pot, and a HORIZONTAL orientation for the BALANCE POT.

Dave would be the one to talk to as he designed it. I've put it in 1 400, and that was over a year ago.

Larry,
I've got the wiper of the balance pot connected to Pin 1 of the bias pot and Pin 3 of the bias pot connected to one end of the 10k resistor. Where do I connect the wiper of the bias pot? I've only installed the bias pot for V8 & 9 right now. The balance pot is horizontal and the bias pot is now vertical.
I'm not getting a reading connecting Pin1 to Pin 3 of the bias pot. When I connect TP 8 to ground I get 10.8 ohms. When I touch TP 9 to ground I get 13.4.
At this time the wiper of the bias pot is not connected to anything. Is that correct?
When testing TP's 10 & 11 I get readings of 10.6 & 10.7. No bias pot installed yet for 10 & 11.
Am I not getting any ohm reading when touching Pin 1 & Pin 3 of the bias pot because the wiper is not connected to anything? Turning the pot doesn't seem to change anything. When I touch Pin 1 and the loose Pin 2 I'm seeing a reading of 0.2. If I turn the dial all the way in the opposite direction I get nothing. But, if I then touch Pin 2 & Pin 3 I get 0.2 ohms.
I'm sorry to be bothering you guys on Fathers' Day, by the way, Happy Father's Day to all you Dads. I can wait until sometime next week if that would be more convenient.
Tom

 

[Tom Boley]

TOM; These pics should help you understand the connections and how to test for balance and bias once it's powered up.

View attachment 749767

View attachment 749766

Dave & Larry,

I am very confused. Dave, in your post #84, above, you say the wipers of both pots are connected but in Larry's post #93, above, he says to connect the wiper of the balance pot to Pin 1 of the bias pot.

I now have the wiper of the balance pot (for V8 & 9) connected to Pin 1 of the bias pot for V8 & 9 and the wiper of the balance pot (for V10 & 11) connected to Pin 3 of the bias pot for V10 & 11. The wipers of the bias pots are not connected to anything.

I'm thinking the way I have them connected will result in having to turn one pot one direction to reduce the bias and the other direction to reduce the bias on the other pot.

When I touch the wiper and one of the outside pins of the bias pot I get 0.2 ohms with the pot turned in one direction and 0 when turned in the other. When I turn the pot the all the way the other direction I get 0.2 on the opposite pin and 0 on the one I previously read 0.2 ohms. Does this help you to tell you what I've done?

One last thing. In Dave's drawing he shows a chassis ground connection along the bottom line and a ground connection to the center test point. Should there be 2 ground connections?

Would it be possible for me to talk to one of you about this? I promise I will not abuse the privilege. I can message you with my # or if you would rather I call you, please message me with your number. I'm thinking this will result with less of your time wasted.

I really appreciate all of the help you guys have given me. To be honest, if I knew of a nearby repair facility I would throw in the towel and take it there. I'm about 2.5 hours east of Las Vegas and 4.5 hours south of Salt Lake City.

Tom

 

[streetwise]

Dave what are in your experience the difference in sound quality after balancing tubes? my outputs are currently at 19ma, 21ma, 21ma, 30ma. Also is there anything different about this installation with your efb, power amp, control section & SS Buffer installed? It would seem like the 100 ohm screen stability resistors arent used since the efb is still involved.

 

[dcgillespie]

1. Under-biased tubes will produce a restricted, thin sound that lacks any sense of responsiveness. Scientifically (in the lab that is), under-biased tubes will be shown to produce reduced power output, and copious amounts crossover distortion. While 30 Ma will produce reasonably good performance, there is no doubt that your other tubes are clearly not operating on the linear portion of their characteristic curve during the hand-off period between the tubes once Class AB1 operation commences with such a low bias setting.

The control section and buffer modifications address completely different issues than that addressed by balancing output tube current flow and the installation of EFB(tm).

The need for the installation of Screen Stability resistors is determined by the type of output tubes used and how they are operated. Recapping the power supply with modern low ESR caps and the installation of EFB are certainly influencing factors relative to the need for SS resistors, but the main criteria starts with the design of the circuit to begin with.

Dave