The End of an Era: The Fisher 55-A

Not really Gadget -- the other half of the 12AU7 that makes up the regulator circuit is the input stage preamplifier.......

Dave
 
Dave,
Have you ever considered doing a similar project on an MC60 ( I believe a peer of the 55A doing a different take on the same niche). I've seen the Unity article. I don't think I missed it if you did. Sorry for the side track.
 
Modifications Complete

Parts arrived today, so the work could be finished. With that, it 's time to start putting a wrap on this project. To summarize the work done:

1. The Z-matic circuits were removed. This removed a .7 Ohm resistance in series with the speaker on 8 Ohm (only) connections (even when the Z-matic feature was turned off), and also removed two separate switch contacts that the speaker connections went through before connecting with the secondary winding of the OPT. The practical use of the Z-matic feature in today's audio environment is less than nil.

2. The pre-driver stage was re-biased for more linear operation over the entirety of it's load line. This also produced an 11.5% increase in the open loop gain of the amplifier.

3. The NFB loop was adjusted to account for removal of the Z-matic components, and the increase in open loop gain. This restored the feedback factor to that of the original design when the Z-matic feature was turned off, which then necessarily increased the sensitivity of the amplifier slightly. Whereas before, it took .65 vac rms to drive the amplifier to full power output, now it only requires about .58 vac.

4. The regulator tube (V1-B) has been removed from the circuit, and had its heater connection severed as well. As a result, that section of the tube just resides within the tube unused now, not even lighting up during operation. In its place, a plastic power Mosfet (STF10NK50Z) and 1000 ohm gate stopper resistor have been installed (bolted to the chassis via one of the mounting holes for the Auxillary Power Socket that had previously been removed), to become the basis of the new bias regulator. Where as the original regulator circuit displayed a regulation level of just over 40% when sinking 20 mA of grid current at full power output, the modified regulator displays a regulation level of just 0.11%, when passing an even greater 25 ma of grid current now.

Side Bar: I was contacted (second hand) by a gentleman questioning if the original regulator actually displayed an internal impedance of 2950 ohms as I stated earlier. He was suggesting that the regulator was basically a cathode follower configuration, with his Spice calculations indicating that internal impedance was more on the order of 350 Ohms. I responded back suggesting that the regulator more resembles a grounded grid amplifier, but his comment about the internal impedance being lower than I indicated got me thinking. Bottom line -- I'm glad he questioned it!

In my haste, the figure I posted amounts to the static resistance within the tube with a specified plate voltage and current flowing through it -- but that is not the same thing as the dynamic internal impedance of the regulator circuit proper. With the original design, the output of the regulator rises from -42 vdc to -59 vdc (both measured) at the onset of clipping, which is a change of 17 volts. When this happens, 20 Ma of (measured) grid current is being impressed upon the regulator circuit. This indicates that the internal impedance of the original regulator then is more correctly 850 Ohms -- a considerable improvement over the 2950 Ohm figure I originally posted, but still extremely poor when compared to the 2 Ohms of the modified regulator (.05 volt rise with 25 mA impressed). So, while the regulation performance and real world improvement represented between the stock and modified regulators is still very real and accurate, the characterization of the original regulator's internal impedance was in error, and I thank the gentleman (Chuck) for his input!

5. The filament circuit of the rectifier tubes now includes a relay whose dual 15A (resistance rating) contacts are paralleled to form a 30 Amp rated switch to delay the heating of these tubes when power is applied. The relay is controlled by a 30 second delay on make SS timer module. This allows all of the audio tubes to fully warm before the filaments of the rectifier tubes are activated. When they are, the B+ ramps up smoothly and uneventfully, with no surge or stress on the filter caps at all. This will extend the life of the filter caps immeasurably.

Pics of the completed work include:

1. With the Z-matic circuits removed, the tapped power resistor can be removed, presenting a much cleaner, neater look. On the impedance switch, both halves were supposed to be paralleled to double the effective current rating of the switch, but you can see that they missed doubling the terminals in the 8 Ohm position -- the very position that passed the most current. All switch contacts are now removed from the load circuit, eliminating the effects of any possible resistance or deterioration.

2. The wiring to the Z-matic control was also removed, except for the ground connections to it. At the pre-driver tube, sharp eyes will also notice that the cathode resistor is now 2200 Ohms instead of the original 4700 Ohms. Also note that the connection to pin #5 of V1 has been removed to prevent the heater from lighting in the unused section of this tube now. The piece of lint landed just as the pic was taken.

3. The feedback divider network was modified to account for removal of the Z-matic circuit components, and the increased gain of the pre-driver stage. The new values retain the original FB factor in spite of the modifications.

4. The new rectifier filament control relay and timing module are mounted to the side wall over in the power supply area. Using a good silicon adhesive for both devices, the chassis was not altered in any way for any of the modifications.

5. The new Mosfet bias regulator is mounted to the rear of the chassis, close to where it connects into the circuit. The device used is way overrated for the application, but does the job very well, and should last multiple lifetimes. Dissipating just .21 watts under class AB1 conditions, and slightly over 1 watt under sustained full power (Class AB2) conditions, this 9A 500 volt device just loafs along, not even requiring any thermal grease for installation.

6. A complete view of the finished amplifier. Leads that are now unused within the laced cabling are neatly folded back and tie wrapped to the cable trunk. As always, while the new components installed are obvious, they have been installed in keeping with the original construction, to minimize the impact on the look of the original build.

In my final post, I'll give some final performance results, and final thoughts on the design of these great amps.

Dave
 

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NJ -- The MC-60 is not a class AB2 design, and those versions that use 1/2 of the input tube as a regulator use it (as I recall) to regulate the B+ to the input stage. So while there is some similarity in using 1/2 of the input tube for regulator duty, I think all similarity stops there.

Dave
 
Epilog

What a wonderful journey this has been! Class AB2 designs are rare indeed, but to be able to work with one under the Fisher Flag has been a real treat. It's been fascinating to unravel all the design hoops that Fisher had to jump through to produce a practical Class AB2 design for commercial hifi use.

In a way though, it was already a relic before the first one rolled off the production line: Class AB2 designs were primarily a tool for high noise level industrial applications where high power and high efficiency were required, but high fidelity definitely was not. Think large industrial plants, stadium, or ship-at-sea applications. Simple in concept, but not so simple to pull off in reality, they are really an amplifier driving an amplifier when the afterburners of turbo-Class AB2 operation kicks in. They are heavy and associated with high distortion levels -- but ingenuity at its best none the less.

Because it is difficult to produce a high quality AB2 amplifier, they were more than rare for home use -- with the Fisher 50-A platform being the ONLY AB2 design I am aware of for home high fidelity use. But at the time it was on the drawing board, it was the only practical way to get any significant amount of power out of triodes, which at the time, was still the best way to achieve a low output impedance for driving a loudspeaker. The idea of 20+ db of NFB that would allow pentodes to achieve such low impedance levels was in its infancy, with the early designs employing that much FB generally being more unstable than stable. UL was brand spankin' new, still unproven -- and a patented concept. Against that backdrop, Fisher went with tried and true technology of the 40's: Triode operation through out, and Class AB2 operation to do an end run around everybody else power wise. They dressed it up with a good power supply, phase inverter, OPT, and a better driver stage/transformer than any industrial designed ever hoped to have, to coax some good performance from it for its day. It was a gutsy move to say the least at a time when AB2 operation was widely considered as producing low brow performance. By the end of it's run however, the seeds of UL and higher power pentodes planted at the time of the 50-A's introduction were in full bloom: UL operation was thoroughly vetted as capable of producing triode performance with pentode power, and double the power output with greater quality could be achieved from a single pair of tubes -- without the heavier weight and complications of Class AB2 designs. Fisher made a good run with it, but by the 2nd half of the 50's, the 50-A's platform had run its course. Today however, these amplifiers continue to enjoy a huge following -- but beyond simply being a Fisher, why?

These amplifiers are not the kind you sit down and discuss performance specifications with over a tall cold one. If that were the criteria, there are many other designs that can easily beat it in any performance category you could imagine. In fact, I dare say most owners of these amplifiers could care less about anything but the most general of specifications like, how much does it weigh, and will it operate safely on today's line voltages! As far as any other specifications, shut up and listen to the music! When you listen to these amplifiers, its hard to argue with that logic. There are reasons for that.

From a design standpoint, the biggest sonic contributor to this series of amplifiers is that they are based on an open loop design that targets good response, low output impedance, and minimum distortion without the need for large amounts of corrective overall (global) NFB. To that point:

1. Plate load resistors are keep low in value to maximize good HF response.

2. Maximum use of local NFB is employed via unbypassed cathode resistors.

3. Direct coupling is used where practical.

4. A triode output stage is employed.

5. A (relatively) high quality driver and high quality OPT are used.

6. A stiff choke input power supply is employed.

To this recipe, Fisher added a meager amount of global NFB -- just 8 db in this example! (with more frequency poles than you can keep track of in the design, it really can't employ much more and still remain so stable)

This approach produces a high degree of stability (square waves don't ring, and pulses settle very quickly), a very flat frequency response (within .1 db across the audio spectrum), reasonable distortion (<2% at 1 kHz at 50 watts), and reasonable output impedance (~1.5 ohms) -- but also, it produces a very large, open, and and full sounding sound stage. Realizing that ANY electronically reproduced sound will sound smaller than real life, then any amplifier that causes the sound to appear larger than others do will present a more realistic and favorable listening experience for many people.

Global NFB improves measured performance significantly, but unchecked, can also make for a more clinical or sterile sound that can be un-engaging. This design however is anything but that. It is in fact extremely musical and listenable for long periods of time without fatigue. Fisher thread the needle very well with this design -- even if they were forced to. The specifications are good enough to ensure a reasonable degree of accuracy, while not sucking the life out of the presentation.

So what do the modifications do to the sound? Well, nothing -- until you light the fuse. Low and medium power presentations are unchanged from before: Big open presentation with plenty of detail, but without any sense of restraint. However, high power levels are now produced with an improved level of effortlessness. Is that because of the knowledge that the driver stage can now drive the output tubes right out of their sockets before giving up the ghost? Maybe -- but anytime effective driver impedance can be lowered during class AB2 operation, performance improves: power goes up, and distortion is reduced, which is exactly what happened.

In this case, the amplifier can now deliver greater than 55 watts RMS at ~ .70% THD at 1 kHz, which is a significant improvement over the stock performance (1.8% at 50 watts) -- and this is actually produced with a slight decrease in NFB. Since the lowest distortion operating point of the output stage is just 120 mA, it can now operate at that level without fear of outstripping the driver stage capabilities (lower quiescent current = higher grid bias voltage for the driver stage to overcome). The stock quiescent current level of 180 mA (requiring less grid bias voltage) was required to keep the drive requirements of the 6550 output stage within the limits of the driver stage.

But in lowering the quiescent current level of the output stage, the effective gain of the stage is also reduced, meaning open loop gain is reduced, meaning that the global NFB level is also reduced when the loop is closed. The amplifier can still be operated at a quiescent level of 180 mA of course. But because of the improved linearity of the pre-driver stage, and the lower effective drive impedance presented to the output stage during Class AB2 conditions, distortion actually increases now above a 120 mA setting, due to moving the output stage away from its low distortion operating point. In this case, the driver stage impedance itself wasn't lowered, but because the bias regulator -- whose impedance WAS lowered -- is in series with the driver stage as far as the output stage is concerned -- the effect is just the same: Effective driver impedance is lower, leading to improved high power performance.

Pics include:

1. A 1 kHz sine wave at ~ 47 watts RMS with the stock design, and output stage quiescent current set at 180 mA. Maximum power output is 50 watts RMS. But at 47 watts, you can see crossover distortion beginning to set in from the driver section already crying uncle before rated power output is reached. This is with the unit operating from a 121 vac line.

2. The same waveform under the same conditions at 56 watts RMS with the new bias regulator and pre-driver cathode resistor in place, and the output stage set for 120 mA. Even though the driver is delivering more power into the output stage under these conditions to produce greater power output, there is no sign of driver distress at the onset of output stage clipping. Before, power output was limited by the power the driver could deliver into the output stage. Now, the output stage is the limiting factor, which makes for improved Class AB2 performance.

3. The lower quiescent current of 120 mA does upset accurate power indications, but heck, even that could be dealt with if an addition bias current of 60 ma from a separate internal source is applied to the meter to fool it into submission. But that's another discussion for another time.

4. In good company with a modified (but naked) Fisher 30C driving this bad boy. It's been running flawlessly for about 8 hours in this pic.

Turning the unit on now no longer leaves me cringing until the circuit starts to draw current to draw down the previously very high turn on voltage surge. In fact, start up is boringly unspectacular, even with the new lower quiescent current level: Turn it on, 32 seconds later, the relay kicks in, the rectifiers heat, and the output tube B+ ramps up to 445 vdc -- all quite noiselessly and uneventfully. The same 3A fast blow fuse holds up well at the new higher power output levels that the unit is now capable of, while also powering the delay relay and timer as well. If the power burps, the rectifiers immediately cool, and then automatically reengage after 32 seconds.

So, this project comes to an end. I should have a rough schematic of the modifications installed posted by tomorrow morning. If one of you computer capable folks want to work it into the original schematic and re-post it, so much the better.

It's been quite a ride for me -- and what a bonus to listen to it as well!!

Dave
 

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A couple of quick mop-up points to pass along:

1. After removing the tapped power resistor that was part of the Z-matic circuit, that resistor is more correctly identified as a 2 Ohm resistor tapped at 1 Ohm -- NOT "1 & 2 Ohms" as described in Fisher's parts list. The effect of this is that for 8 Ohm (only) connections, the Z-Matic circuit presented a .5 Ohm resistance in series with the speaker connections even with the Z-Matic feature turned off -- not the nearly .7 Ohm resistance I calculated from Fisher's description of this resistor. Still, this is like having a really really lousy set of speaker cables in place when the 8 Ohm tap is used with the stock design.

2. Understand that with the stock circuit, using any output tube that typically requires a greater bias voltage than the 6550 does to achieve the same quiescent current flow will just aggravate the already stressed out driver section under conditions requiring high power output. In that scenario, for the same performance then, it will require an even higher quiescent current level in the output stage to bring the drive demands back in line with driver capability. This would be the case for example when substituting KT88 output tubes, which typically require a slightly higher negative bias voltage to produce the same quiescent current flow as 6550s do.

With the modified unit, driver coupling into the output stage is improved significantly, eliminating this point of concern.

Dave
 
Dave thanks for a good read on this, still learning but I envy the owner of this fine example of Fisher History. Your a genius on all things Fisher. Al
 
Followup Observations

THE METER -- Earlier I indicated the possibility of re-calibrating the meter to the new lower quiescent setting by supplying a supplemental bias current to the meter to accomplish that. Well, as is so true in life, some ideas work, and others, not so much. In this case, it did recalibrate the meter pretty accurately -- up to about 25 watts. But after that, the meter read increasingly high, due to the extra current from the supplemental source. So scratch that one.

On the other hand, one neat little observation I did make, is that with the amplifier adjusted to 120 mA (lowest distortion operating point), and the meter indicating down low in the red area, then the remainder of the red portion equates quite accurately to all Class AB1 operation (~ 23.5 watts RMS maximum), with Class AB2 operation commencing at the 0 Power Output indication point (end of the red zone). So that was a nifty little observation. Of course, the meter still accurately indicates power up around the 50 watt mark, with frankly, a 120 mA quiescent current providing more accurate maximum power readings than the stock 180 mA setting does.

So, take your pick. At 120 mA, the red zone indicates Class A/AB1 operation above which is Class AB2, or 180 mA produces a fairly accurate power output indication over the entirety of the meter face. Both settings result in fairly accurate high power indications, with the 120 mA setting being most accurate, and providing the lowest distortion.

Of course, remember that in either case, the meter indications are AVERAGE power output levels, and that the peak output at any given time is much greater. That might just make the 120 mA setting the more useful setting: Peak power output is double that of average power output, so keeping the meter in the red zone under all signal conditions will ensure that the amplifier is kept out of over load conditions.

ALTERNATE OUTPUT TUBES -- GE 6550A, early production KT90s, and Gold Lion KT88 reissue tubes all produce virtually the same power output and distortion levels in this amplifier. On the other hand, the EL34 produces slightly more power and slightly less distortion than the other tubes do.

This would be expected since the EL34 is actually a higher transconductance tube than the 6550 family of tubes are. The 6550 and EL34 are both rated for a transconductance of 11,000 micromhos, but this rating was determined with the 6550 operating with 150 mA of quiescent plate current, while the EL34 was determined with only 100 mA. If both tubes are tested for transconductance at the same plate current level, the EL34 is clearly the transconductance winner. This is born out in practical circuits, where the EL34 always requires less grid bias voltage than a 6550 class tube will in the same circuit for the same current flow.

This translates into the driver stage having an even greater reserve of driver power, resulting in the slight increase in power output. It also results in slightly higher open loop gain, so that the NFB level is increased as well, and distortion therefore reduced.

None of this amounts to an earth shaking change, with power increasing by about only 2 watts, and distortion being reduced by about .1%. Still, the effect is noted.

Against this picture however, is the durability of the EL34 tube. All things considered, the 6550 family of tubes is a much more hearty tube than the EL34 family is. Ask anyone whose ever played in a band. 6550 class tubes are basically the proverbial Timex watch: They can take a lickin' and keep on tickin'. The EL34? By comparison, not so much. That becomes important, as Class AB2 operation is not the friendliest environment for a control grid to find itself in.

Now, to keep this all in perspective, for use with home music reproduction -- even with demanding speakers --, EL34 tubes will be just fine. On the other hand, if the 55-A was being used in an application that required sustained operation deep into Class AB2 mode, then they'd be the first I'd cross off the list.

The range of the existing bias control can easily accommodate any of these tube choices.

I also meant to mention that later pentode output stage versions of this platform include an AC balance control in the phase inverter stage. While this will have some effect in altering the drive presented to each output tube in the output stage (due to the presence of T1), it will primarily help to optimize driver stage operation. This is a modification that can also be considered for the triode output stage versions of this platform, but becomes much less significant with the reduced driver power required as a result of lowering the impedance of the bias regulator. It was almost certainly an effort to coax as much undistorted power out of the driver stage as possible, in view of the needs of the output stage and the losses through the bias regulator, and an ever increasing spotlight on performance specifications.

Dave
 
The scary thing to me is that some of this is starting to make sense, though extrapolating about getting to the updates remains elusive.
That said, it seems like you could drive some speakers from the driver stage if properly set up.
 
Hiya,

The scary thing to me is that some of this is starting to make sense, though extrapolating about getting to the updates remains elusive.
That said, it seems like you could drive some speakers from the driver stage if properly set up.

I am not sure Merlin should ever be understood. Some of what Dave does is actual magic.

Frannie
 
Schematic

As promised, I'm posting a schematic of the modifications made during this project. It integrates with the original Fisher schematic of the 55-A (6550 version), so it should not be too difficult to decipher.

Only new component values are shown. If a component is identified with an original schematic designation but no value is given, then it is unchanged from the stock value of the component.

These modifications should be relevant to any design relating directly to the 50-A platform.

Dave
 

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Dave--one question. How did you end up wiring the filament of the 12AU7 tube that contains the (now) unused triode section? You said the unused side does not even light up now, so I assume you took 6V from either pin 4 or 5 to 9? I don't have the original schematic to reference, so I assume it was easy to get 6V from somewhere.

Great thread, even though I will probably never have one of these amps, learning about how AB2 works in a high fidelity application was great.
 
The section of V1 that acted as the bias regulator was comprised of pins 6, 7, and 8, with the heater of that section appearing between pins 5 and 9. The original heater wiring was configured for 6.3 volt operation, with pins 4&5 tied together, and the 6.3 volt power applied between pins 5 and 9, allowing both sections to heat. I simply removed the connection between pins 4&5, and then moved the power lead from pin 5 to pin 4, so that power was now only applied to the preamplifier section's heater, appearing between pins 4 and 9. The other section now lies completely dormant, saving it from unnecessary deterioration were the heater allowed to operate without any current flowing though the elements of that section.

It was a neat project for me as well. I've always known of Class AB2 operation, the theory behind it and how it functions, but have never had an example of it up close and personal to work with, so it was a first for me as well. It's been a very neat time capsule giving a look back into the thinking of the day, and how such a design could be effectively pulled off by Fisher in a product destined for home high fidelity use.

Dave
 
Hi Andrew -- With the 200 employing GZ34 rectifier tubes, the timer module and relay would not be needed, unless you just wanted to give the output tubes a little extra time to warm before the B+ is applied from the GZ34s. GE produced 6550 tubes -- as used in this 55A -- require a full 35 seconds of heating time for them to really come up to speed, so I set the timer for 32 seconds, which with the heating time of the 5AW4 rectifiers, then allows the tubes to fully heat. On the other hand, EL34 tubes typically warm quicker than 6550 tubes do, so you should really be fine as far as the voltage surge issue is concerned.

The modification to the bias regulator would be completely appropriate for the 200, with the Mosfet and 1K resistor being a drop in fit -- much as it is in the 55A.

It appears that Fisher too noted the issue with the Z-matic circuit still allowing resistance in series with 8 Ohm speaker connections when that feature was turned off, and as a result, employed a more complex impedance switch in the 200 to eliminate the problem. Therefore, that fix would not be needed either.

As for the changes to the pre-driver and NFB network, those would only be appropriate for the 55A amplifier, as significant changes were made to the front end of the 200 design, disqualifying those modifications.

I hope this helps!

Dave
 
Un-modification

It occurred to me that in my efforts to be minimally invasive into the original design of (specifically the 6550 version of) the 55A, the individual modifications installed have collectively left the unit somewhere between the performance of the stock design, and that which the modified unit is ultimately capable of. Specifically, this is with regards to the global NFB factor now in place, and the impact it has on perceived and measured performance. Consider that:

1. In re-biasing the pre-driver stage for more linear operation, that produced an 11.5% increase in the gain of that stage, which produced an attendant increase in the open loop gain (OLG) of the amplifier.

2. As a result, the FB resistor (R31) was adjusted upwards to account for the increased OLG, to return the FB factor to its original value.

3. However, with the modified bias regulator now providing greatly improved coupling between the driver and output stages, it meant the output stage was no longer required to operate at 180 ma of quiescent current, which was used to accommodate the marginal driver swing available due to the compromised coupling produced by the original bias regulator in the stock design.

4. As a result, the output stage can now operate at its lowest distortion operating point, which occurs at a much lower 120-125 mA of quiescent current. Even though this requires a greater negative grid bias voltage to be applied to the output tubes, the driver stage still has plenty of low distortion reserve voltage swing left at full power output, because of the improved coupling afforded by the improved bias regulator.

5. However, the greater bias voltage applied to the output stage reduces the gain of this stage by 10.6%, with an attendant drop in OLG by the same amount.

6. For all intents and purposes then, the loss in output stage gain due to its re-biasing almost perfectly compensates for the increase in pre-driver stage gain due to re-biasing that stage as well.

However, with the new value for FB resistor (R31) installed, it means that the reduction in output stage gain from operation at 125 mA has not been accounted for. In effect then, as is, you either set the output stage to 125 mA for it to operate at the lowest distortion operating point, but the unit overall to operate at a lower NFB factor -- OR -- you operate the output stage at 180 mA to achieve the original NFB factor, but elevate distortion in the output stage.

Since one of the huge benefits of the modification has been to not only improve performance but also extend output tube life from operation at 125 mA, it was felt that this will be the quiescent current setting of choice. As a result, the NFB loop then needs to be full compensated to account for both the change in pre-driver, AND output stage characteristics. Doing so then ensures that all the benefits of the original FB factor remain in place:

1. The original damping factor, frequency response, transient response, and distortion reduction capability is maintained.

2. The original sensitivity of the unit is restored as well.

As a result, R31 has now been returned to its original stock value of 2200 Ohms, with the actual FB factor of the modified unit now being within 2% of the original stock value. This difference is so small as to be well within the tolerance of the components used between any two examples of this amplifier. This then allows the unit to retain all of its sonic characteristics as it relates to the original FB factor used, while still enjoying all the reductions in distortion derived from re-biasing the pre-driver and output tube stages to their optimum operating points. This, coupled with the improved bias regulator, then allows for all of the modifications to produce maximum performance benefit. With the original FB factor restored now, the new operating points in place, as well as the improved bias regulator, distortion of the amplifier now approaches just .5% at 50 watts RMS, for a greater than 70% reduction from that produced when first received.

I will update and re-post the modification page I posted earlier to reflect this change. Time to pack this baby back up!

Dave
 
Updated Modification Page

It's a wrap. Here is the updated modification page showing all modifications installed. As before, if a component is identified, but no value given, then the stock value is maintained.

I am sorry to see this one go!

Dave
 

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Hi Dave,

Regarding the 470k resistor that joins pin 4 to pin 5 of the SSAC make-on-delay timer, what minimum wattage value should be used?

Cheers!
 
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