Avery's "First" Maker: The TA-800

I was measuring on the bottom side (inverted position) across what represents the sides of the chassis proper -- not including the space between the front of the chassis proper, and the rear of the face plate. That's why I used the term "chassis surface".

OK. thats the 11 3/4" measurement I ended up with. I only asked because 3 people measured the same amp and got different numbers, so either Fisher changed the size of the chassis or we were not all measuring to the same points. I suspected the latter was more likely the case.
 
Thanks Gadget. With your measurements then and the ones I took, it appears that the chassis foot print of the two TA's is identical then, which would certainly be a worthwhile goal to minimize retooling costs. Thanks for checking!

Dave
 
So a lot has happened with the TA since my last entry in terms of things accomplished, observations indicating more investigation/work required, and basic information gathered. Some is good, some is not so good, and some the realities of life with an early TA. In a very loose order of events then:

1. AKer rufleruf made a heroic effort to check if a couple of the switches in his on/off switch stash would work in Rob's TA. Ultimately neither did, but one important fact was gleaned from the effort: The general understanding is that Fisher used two types of power switches: the kind that snaps on over the rear volume control cover, and the kind that mounts via three tabs into slots in the rear volume control cover. The control in this unit used the kind that mounted via the three tabs. But apparently there are switches with three tabs, and then there are other switches with three tabs. Both look nearly identical, with four terminals, containing two separate SPST switches that both make when rotated clockwise. But a careful side by side examination shows that the mount tabs are in a different location with some switches, and that the activating lever is somewhat different as well. So close, but not close enough. The two types are not interchangeable, as the following pic shows. The original switch is shown on the right:
SAM_2017.JPG
With to date no response from Mark (I've always used his email -- anybody use anything different?), the only approach left then was to try and fix the original. When it was opened up, I found that one connecting arm (that served the rear AC plugs) had simply fallen out of place, which was strange since it otherwise appeared as new. What's more, it had lodged itself between the power terminal of the companion switch, and the load terminal of its own side to effectively make the rear AC plugs always on -- which is why initially I though that side had its terminals fused together. Whether it was a manufacturing defect or not is unknown. But it had plenty of tension, and the disk it fits into looked perfectly normal, so other than refitting the connecting arm back in place, that side of the switch was otherwise undamaged. The receiver side of the switch was just filthy. Caked on grease that had hardened over the years, making that side of the switch intermittent: sometimes it would work, sometimes not. By my count, mostly not. But the connecting arm was still very serviceable, so cleaning up that side of the switch returned it to normal operation as well.

In one of those extremely rare cases of good luck then, the switch was able to be repaired, reinstalled, and functions normally now. With the one connecting arm back in place, switch action now feels normal as well, whereas before, you never felt a good snap when advancing it to the on or off positions. With that, the volume control/switch assembly could be reassembled, and installed back into the unit to finish up that portion of the work:
SAM_2030.JPG

2. The B+ power supply has been completely recapped now, and new diodes installed. Cans are all new from CE Distribution. The cardboard cover over the old doubler high side cap was able to be removed and and secured over the new can cap in that position, and appropriately relabeled. The single discrete cap mounted behind the level controls is required to replace the 40 uF section in the old voltage doubler low side can cap. One section of the can cap in the bottom left corner of the four can caps is not needed at this point. It was intentionally ordered as an additional cap to accommodate a potential installation of EFB:
SAM_2019.JPG

SAM_2020.JPG

3. With the volume control finally reinstalled, I could finally make some assessment of channel balance with regards to amplitude and frequency response for the active circuits, and for tracking of the same with regards to the volume, balance, and tone controls. In listening to it (still on life support through an external monitor amp), one channel sounded brighter and fuller than the other. In stereo mode (both channels operating), it might be missed easily enough, but with a mono signal applied to both channels, and listening to one channel at a time (switching back and forth), it was rather obvious right off -- as if one channel had the loudness switch turned on, and the other didn't. Positioning a 2 kHz square wave presentation from each channel over each other on a scope, the difference is readily apparent, and with a 2 kHz fundamental frequency, the differences all appear within the audio bandwidth. A similar disparity was found using the same technique using a 200 Hz square wave. Because of the characteristics of square waves, using these two frequencies then allows for a quick and easy check for uniform response over the entire 20 Hz - 20 kHz audio band between the two channels. The discrepancy at 2 kHz is shown on top, 200 Hz on the bottom:
SAM_2018.JPG

SAM_2027.JPG

The disparity on the HF end hardly surprised me. The build of the unit all but ensured such a disparity would exist to some extent. With the tone control amplifiers spread widely apart on the chassis, yet the tone controls for both channels located to one side of the chassis, it means that there will be a couple of feet of shielded cable for Channel A to send its signal to and receive it from the actual controls themselves. Since these operate at a relatively high impedance, the capacitance of the shielded cable will act to attenuate HF response, which it does. In future models, Fisher addressed this issue (when needed) by specifying different values of trim capacitance between the two channels, to ensure that both channels had an identical flat HF response out to 20 kHz. In the TA-800, such was not the case, with identical trim capacitance used in both channels, with the value used being a compromise between the two channels. That left Channel B over compensated, and Channel A under compensated, leading to the disparity in timbre. The caps requiring adjustment -- C104 in Channel A and C105 in Channel B -- were both originally 47 pF, and almost precisely split the difference between the values actually required in each channel. Adjustment of C104 up to 56 pF, and C105 down to 39 pF produced an identical flat response in each channel up to 20 kHz, with the direction of adjustment in each channel in agreement with the original response errors caused by the layout of the set. The pic shows the improvement this made, and the two caps that were adjusted. C105 is on top, C104 is on the bottom. It is the small cap bent over and buried between the volume and balance controls:
SAM_2026.JPG

SAM_2023.JPG

SAM_2029.JPG

On the low end, the response between the two channels was identical when the bass controls were either at the full max or full minimum position. This let the tone control PECs off the hook, and meant that the bass controls themselves were not tracking properly at the 12:00 setting. As Fisher often did however, the tone controls included an (unused) center tap terminal, which can be used to add an appropriate trim resistance to effect identical flat response between the channels at the 12:00 noon setting. In this case, once again, it was channel B that was deficient in bass response when the control was centered, so adding (in this case) a 2.2M resistor between the high side of the Channel B (front) bass control and the center tap terminal enforced flat performance between the two channels. It should be noted that flat response only extends down to ~ 30 Hz, due to the presence of the permanent LF filters installed. It is not recommended to remove these since increasing LF response below 30 Hz only serves to accentuate the LF thump heard as the FM tuner section is tuned in and out of a carrier frequency. The pic shows the improvement made. Since the adjustment resistor is hidden beneath the bass control, no pic of it is given:
SAM_2025.JPG

4. With the changes performed as described in #3 above, both channels now displayed good tone control tracking, frequency, and amplitude balance up to the balance control, but that control's own balance was less than ideal when set the the 12:00 noon position. It was found that a 33K resistor from the high side of the Channel B balance control to its wiper terminal ensured a channel balance within .20 db for the signals presented to the volume control when the balance control is centered. It can be seen in the second pic as the small resistor added between the two terminals of the front balance control.

5. With all of the balance issues addressed up to the point of the volume control, the volume control's tracking could finally be assessed, which turned out to be quite good over the full range of the control. Finally! Some light at the end of the tunnel now!

Next up however, issues with the 7199 tube rear its ugly head, and the results of some basic performance measurements are discussed.

Dave
 
Dave; Please tell me you WERE a FISHER Engineer in a previous life!! Talk about :bowdown:Detective work. You come up with something new on every piece of gear you work on. ;)

Now I know where all the Tech manuals and cheat sheet's went when SANYO bought Fisher from Emerson :bigok:. The Northridge quake didn't get them!
 
Wow Dave,

Very glad I sent this unit to you. I can't imagine anyone else being able to bring her back in the state she was in. Have you ever seen a Fisher in need of so much work?
 
With both tuners, the power supply, and now the complete audio control sections operating properly, I could finally turn my attention to testing the power amplifier sections. The restoration work in both the driver and output stage areas had already been done, but up until this time, the unit has been operated without any of the power amplifier section tubes installed -- this to keep the heat down, and prevent needless wear on the tubes while the other sections of the receiver were serviced. During that time, an external amp and monitor speakers let me hear all I needed to know. But now, the new pair of TS output tubes along with the new pair Rob had recently purchased, as well as the last two 7199s that Jim McShane had were installed, so that the set could finally operate under its own steam for the first time since work on the unit began.

The output tubes were initially biased to a current draw of 32 mA each, with the new pair showing a decent match to Rob's original pair (thanks to Jim's efforts) -- although this is much less important now that individual bias controls have been added. However, in terms of the bigger goal being to have the two channels of any stereo equipment perform as identically as possible, having the output tubes of both channels represent a good match to each other certainly aids towards that end. With this, some basic checks were made, with problems right off the bat.

A. In this unit, Channel B seems to be the stepchild of the unit, since it seems to have had most of the problems to date in terms of working to achieve identical performance from both channels. In this case however, it was the luck of the draw, as the problem turned out to be one of the brand new JAN 7199 tubes.

The 7199 was the ultimate tube developed to implement Laurent's driver configuration (of Dynaco/Hafler fame), wherein a high performance pentode/triode tube is employed with the high gain pentode direct coupled into the triode section, which acts as the phase inverter. When all is right, it is a very high performance configuration indeed. Fisher, with being first an ever present goal, gave the tubes a whirl in both of the TAs. But the tubes that are appropriate for this application are prone to various problems that can make them unacceptable to use in this application. When supply is plentiful however, that's not particularly a problem. But with availability of good NOS examples all but gone now, we're scraping the bottom of the barrel even with the best tubes to be found. The problems these tubes can present include:

1. They are prone to developing heater cathode leaks in the pentode section, leading to the introduction of hum into the audio signal from an AC powered heater.

2. With two high performance sections in one small envelope, they tend to run very hot, making their life span notably shorter than that of a conventional double triode tube.

3. The high heat the tubes operate at tends to cause migration of cathode material to the control grid, causing grid current issues in the pentode section. When this happens, and a tube so afflicted is installed into a circuit with the topology that Fisher used, the immediate effect is that the volume control becomes "noisy" with rotation. No amount of cleaning the control will solve the issue, because the noise is caused by grid current from the tube passing through the volume control, causing the noise to be heard whenever the wiper is moved.

4. The plate voltage of the pentode section can be highly variable if the precise manufacturing tolerances required for the screen grid varies at all. If that happens, the pentode's plate voltage can vary widely from one example to the next in a given application. Because of the direct connection between between the pentode and triode sections, any significant variance in the pentode's characteristics then can (and will) upset the bias in the triode section, where upon the whole house of cards collapses, with performance going right out the window.

In the case at hand, gain was notably reduced in the Channel B power amplifier, distortion was well over double that of Channel A at all power levels, and upon inspection, it was found that the pentode plate voltage in the 7199 installed in Channel B was just 29.5 volts -- less than a third of the 100 volts specified to appear there. Since all the parts had been checked/cleared/replaced as needed previously in this section, the problem had to be the tube, and of course it was. Neither of the original Fisher tubes installed suffered from this problem as outlined in #4 above, but both suffered greatly from that of #1, 2, & 3, besides testing poorly.

In my experience with these types of designs using the 7199, as well as all such tubes like it, while once were a very viable design option, they are in more and more danger today of not being able to maintain proper operation because the bottom of the barrel with these tubes is in sight. Scott widely used this type of design in nearly all of their products, but wisely did not place a shield over these tubes, in an effort to have them operate as coolly as possible. That would be a very good suggestion for those with a TA to adopt, which will help to minimize the problems noted in 1, 2, & 3 above. But the problem with #4 above is becoming more and more prevalent, simply because the tubes originally culled as having the tightest screen grid tolerance are all but gone now, leaving the crumbs that display wide variation. Since the 7199 was one of the last tubes developed, it of course would be one of the first to run out due to lower overall manufacturing numbers, and less demand from application versus that of other similar tube types.

Fisher (imo) wisely stepped away from this tube rather quickly, never returning to it again in any of their products to my understanding -- and I believe largely due to the potential issues they noted when marrying this tube with direct coupled operation, even during their short stint using it. Besides keeping the covers off, there are circuit modifications that can be done to allow those tubes with wider screen grid tolerances to be used effectively in a direct coupled configuration such as is the case at hand. It amounts to doing much the same thing as is done today to accommodate modern manufactured output tubes whose tolerances are not as tight as those achieved by the original American manufacturers of such tubes. For this project however, in trying to minimize the cost damage to Rob, I decided to have another go at locating a tube that would work properly in the circuit as designed.

Finally, understand that the problem detailed in #4 above is NOT something that will be caught even with the best tube testers, as the problem does not manifest itself as a deviation in Gm in either section -- and particular to this discussion, the pentode section -- as the very nature of a pentode tube causes it's Gm (the parameter that the better tube testers test for) to remain rather constant regardless of the actual plate voltage it operates with. Rather, it is a performance issue that ONLY crops up when the two sections of the tube are tied together with direct coupling -- a condition that NO tube tester emulates. Being ever true to the caliber that Jim McShane represents as a tube vendor however, he credited the cost of the "defective" tube back to me for Rob, and even went so far as to locate and suggest another new example he found for me from a competitive vendor that he recommended. That is simply service without equal, and why I cannot recommend him highly enough.

So, yet another tube is on the way in the hopes that it will fill the bill, with one of my own tubes filling in in the mean time. Understanding that the channel balance was virtually perfect up to the volume control, and with that control turned up full (to eliminate any tracking issues it might contribute), the pic shows just how significant the channel balance is due to the one new 7199 that doesn't work well when direct coupled, but otherwise tests very strong in both sections. The pic shows a 1 kHz signal at 5 watts RMS power output in Channel A, lower in Channel B:
SAM_2032.JPG


B. Initial data gathered:

1. With a CL-80 installed to protect the power switch, and otherwise operating directly from a 121 vac line, the AC heater voltage measured directly at the output tube socket furthest from the power transformer is 6.35 vac. Therefore, this unit may be operated directly from most residential AC power lines, with external adjustment not required until AC power exceeds 126 vac.

2. Best output stage performance coupled with safe operation is obtained with at least 40 mA quiescent current draw per tube. This unit will display significant cross-over distortion when the tubes are biased to less than 30 mA each.

3. Measured mid-band (1 kHz) power output:

Individually driven: 26.1 watts RMS in each channel at just <1% THD
Both channels driven: 20.48 watts RMS per channel at nearly 2% THD in each channel.


C. Suitability for EFB:

Measurements indicate that the screen grid voltage drops some 75 volts from a quiescent condition (375 vdc) to 300 vdc when full power is produced in both channels. However, the main B+ voltage drops only 20 vdc under the same conditions. Even accounting for the 20 volt drop in the main B+ voltage, this represents a screen grid regulation of >14% which is rather poor, and contributes significantly to reduced power output produced when both channels are driven. Also, with mid-band THD approaching 2% with both channels driven, the impact on distortion is significant as well. This represents a perfect environment for EFB to thrive in, so work will now begin on developing those circuits, with the initial work aimed at determining what the potential gains can/will be.


For now however, a quick pic of the underside as it currently stands. The only work unaddressed at this point is the DC Heater/Bias filter cap installation by a previous tech, which has been left un-touch at this point, as they will be addressed when the EFB board is constructed and installed. In the mean time, while waiting for the new tube to arrive, the unit moves over to the listening room for a little critical listening and burn in time. I'll grab a pic or two once its over there.
SAM_2033.JPG

Dave
 
Rob -- There have been a couple others, if only because I seem to attract a fair number of these poor, dare I say pathetic pieces. But yours ranks right up there with some of the worst I've gotten. A rather dubious award to achieve....... :confused:

Dave
 
Dave,

What is your opinion of using a 6U8A in place of a 7199 -with wiring changes of course. I have a bag of really nice 6U8A's and, like everyone else, only a few 7199s. I have also seen 6BL8 and 6GH8 discussed as replacements, but haven't given them much thought - not having any to play with.
 
Nice work.

Would indirectly coupling the phase inverter be of any help in mitigating variables in the pentode section or would that have enough negatives to not justify it?

Also, would this be worth considering a re-wire for a 6U8 or similar or using adapters since the supply of good 7199's is pretty well shot at this point?
 
Dave,

What is your opinion of using a 6U8A in place of a 7199 -with wiring changes of course. I have a bag of really nice 6U8A's and, like everyone else, only a few 7199s. I have also seen 6BL8 and 6GH8 discussed as replacements, but haven't given them much thought - not having any to play with.
I remember when Dave was re-restoring my TA-600, he stated that the replacements sony6060 used (6BL8) were not very compatible for the TA-600. I see sony6060 used the same adapters in my TA-800 and have to worry that there is a problem by him doing so. I'm curious to see the solution.

http://audiokarma.org/forums/index.php?posts/7952136/
 
If you look at the design spec's for the 6u8a and the 7199 most of the spec's are pretty much the same. The 6BL8 and the 6gh8 have substantially lower design spec's, and you'd have to lower the B+ to their grids and screens to get them to operate in their regimes Also I noted on the 7199 this little ditty
In direct-coupled voltage-amplifier phase-splitter circuits, the pentode unit should drive the triode unit.
Anyone want to take bets on guys setting up similar tubes are using the triode side to drive the pentode side???

This is on the N7JP tube site.
http://www.nj7p.org/Tubes/SQL/Tube_query.php?Type=7199
http://www.nj7p.org/Tubes/SQL/Tube_query.php?Type=6U8A
http://www.nj7p.org/Tubes/SQL/Tube_query.php?Type=6bl8
http://www.nj7p.org/Tubes/SQL/Tube_query.php?Type=6gh8
http://www.nj7p.org/Tubes/SQL/Tube_query.php?Type=6gh8a
 
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I received a very fast response from Mark on March 26 via his email address. He lives in Plymouth MI, close to where I lived my first 20 years, and we may arrange a visit sometime when I'm in the area. Of course, the fast response may have been because I was not requesting a switch or pot.
 
Anyone want to take bets on guys setting up similar tubes are using the triode side to drive the pentode side???

I'd hope not. Pentodes don't make good phase inverters if you want it split load. Plate output and cathode output won't be equal unless you triode strap it, and honestly whats the point of using a pentode/triode just to wire it up as a dual triode tube? You'd also lose the high gain pentode voltage amp.

I suppose if you were clever about it you could make an adapter to run a 12ax7 or similar in place of a 7199, though it probably wouldn't work anywhere near close to proper.
 
Generally, all of the popular tri-pents have adequate static and dynamic specifications to do the job in the classic Laurent circuit. The problem comes in at supersonic frequencies and with AC gain, where the pentode section of each tube acts somewhat differently with these characteristics. As a result, while rewiring the socket will allow for the correct base connections between the different tubes, it doesn't necessarily guarantee the same FB level, or HF stability characteristics, which is where the problems can come in. Therefore, to properly change between a 7199 and (for example) a 6U8A, not only will the socket need to be rewired, but FB level and stability testing with appropriate adjustments would also need to be made as required.

As for the problem with the 7199 I had, Gadget's suggestion of using AC coupling between the pentode and triode section certainly works to resolve the problem. But it also potentially introduces another problem in terms of LF phase shift, which can then lead to LF stability problems. The best answer usually comes down to powering the screen grid dropping resistor from a variable power source, and adjusting it for each example as needed to produce the required pentode plate voltage. But of course, that's just one more complication to add to a unit. But with the tube crumbs we have left, that may become a more and more common modification. That, and keeping the tube shields off......


Had a chance to listen to the unit late this afternoon. Playing a mono signal through both channels now produces a very definite point of center presentation, whereas before, with the frequency response discrepancy between the channels, the center point was vague and undefined. Stereo performance was excellent, with very good on-center balance of the tone and balance controls, and the volume control tracked extremely well at virtually all volume settings. The sound was sharp and clear, and as others have noted, with plenty of bass. In fact, for my tastes, the loudness switch provides too much bass boost -- in keeping with what I have found with other early stereo models from Fisher. On the other hand, the tone controls are excellent, providing plenty of controlled boost and cut -- much more than I perceive in the later receivers. With the extra gain of the pentode AF Amplifier tube in the power amplifier section, it may be that it allowed Fisher to build in more boost and cut into the early TA's tone controls producing the effect noted. I plan to check the range of boost and cut when its back on the bench to see if there really is more. The high level sensitivity of the unit is .24 vac to produce full power output -- much in keeping with that of the later receivers.

After an hour of operation, the power transformer reached just 113F in my open basement setting, but the output tubes were all running around 315F. That's an awful lot of heat to concentrate in one area, so if a case is in the works, then a fan should be as well.

Finally, the new LEDs are a definite improvement over the old blue ones it came installed with. However, the new "white" LEDs also have a distinct blue tinge to them as well. By contrast, the original lamps gave a soft/warm white look. This will be seen in the pics -- which the camera tends to accentuate. For comparison however, look at the warm white glow of the CFL lamps behind the receiver:
SAM_2034.JPG

The picture is blurry, but that's unimportant in this case. The point is that the pic is a time exposure taken in a darkened room to verify that there is no color to the output tube plates:
SAM_2038.JPG

A top view:
SAM_2039.JPG

This virtually finishes the basic restoration of the unit. I plan to put it through its paces over the next few days, and then it will be on to the EFB development/installation, which will finish out the project.

Dave
 
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Generally, all of the popular tri-pents have adequate static and dynamic specifications to do the job in the classic Laurent circuit. The problem comes in at supersonic frequencies and with AC gain, where the pentode section of each tube acts somewhat differently with these characteristics. As a result, while rewiring the socket will allow for the correct base connections between the different tubes, it doesn't necessarily guarantee the same FB level, or HF stability characteristics, which is where the problems can come in. Therefore, to properly change between a 7199 and (for example) a 6U8A, not only will the socket need to be rewired, but FB level and stability testing with appropriate adjustments would also need to be made as required.

As for the problem with the 7199 I had, Gadget's suggestion of using AC coupling between the pentode and triode section certainly works to resolve the problem. But it also potentially introduces another problem in terms of LF phase shift, which can then lead to LF stability problems. The best answer usually comes down to powering the screen grid dropping resistor from a variable power source, and adjusting it for each example as needed to produce the required pentode plate voltage. But of course, that's just one more complication to add to a unit. But with the tube crumbs we have left, that may become a more and more common modification. That, and keeping the tube shields off......


Had a chance to listen to the unit late this afternoon. Playing a mono signal through both channels now produces a very definite point of center presentation, whereas before, with the frequency response discrepancy between the channels, the center point was vague and undefined. Stereo performance was excellent, with very good on-center balance of the tone and balance controls, and the volume control tracked extremely well at virtually all volume settings. The sound was sharp and clear, and as others have noted, with plenty of bass. In fact, for my tastes, the loudness switch provides too much bass boost -- in keeping with what I have found with other early stereo models from Fisher. On the other hand, the tone controls are excellent, providing plenty of controlled boost and cut -- much more than I perceive in the later receivers. With the extra gain of the pentode AF Amplifier tube in the power amplifier section, it may be that it allowed Fisher to build in more boost and cut into the early TA's tone controls producing the effect noted. I plan to check the range of boost and cut when its back on the bench to see if there really is more. The high level sensitivity of the unit is .24 vac to produce full power output -- much in keeping with that of the later receivers.

After an hour of operation, the power transformer reached just 113F in my open basement setting, but the output tubes well all running around 315F. That's an awful lot of heat to concentrate in one area, so if a case is in the works, then a fan should be as well.

Finally, the new LEDs are a definite improvement over the old blue ones it came installed with. However, the new "white" LEDs also have a distinct blue tinge to them as well. By contrast, the original lamps gave a soft/warm white look. This will be seen in the pics -- which the camera tends to accentuate. For comparison however, look at the warm white glow of the CFL lamps behind the receiver:
View attachment 949527

The picture is blurry, but that's unimportant in this case. The point is that the pic is a time exposure taken in a darkened room to verify that there is no color to the plates:
View attachment 949528

A top view:
View attachment 949531

This virtually finishes the basic restoration of the unit. I plan to put it through its paces over the next few days, and then it will be on to the EFB development/installation, which will finish out the project.

Dave
Thanks Dave,
As for the heat concern, I am planning on putting this unit in a cabinet. Will the the EFB modification help cool it down much? I would also consider a fan. Do you have any recommendations?
Again thanks for all of your hard work on this. Can't wait to see what EFB does to improve this old girl!
Rob
 
Dave,
Riding the 7199 track a bit off topic a bit more, i know way back during the st70 thread someone asked about the 6gh8 adaptors. Would that also fall into the previous discussion and the possible FB low freq stability issues. I know the adapters are pretty available and that boards are made for 70s and sca35s to run them out of the box. That sort of says that its a known but at this point I'd prefer proven rather than assumptions.
 
Are those warm white or cool white LEDs? I changed the dash lighting in my car to 2800K white and its extremely close to the stock light color. The 4000-5000K bulbs have a distinct blue tint to them.
 
Dave,
Riding the 7199 track a bit off topic a bit more, i know way back during the st70 thread someone asked about the 6gh8 adaptors. Would that also fall into the previous discussion and the possible FB low freq stability issues. I know the adapters are pretty available and that boards are made for 70s and sca35s to run them out of the box. That sort of says that its a known but at this point I'd prefer proven rather than assumptions.
I know Dave used adaptors in my ST-70, but said in the TA-600, the circuit needed the 7199 to function properly.
 
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