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Fisher SA-300B Set-up and Bias Adj. Question

Kit and Kaboodle. Loosely everything you own in a bag (ie: old time tramps)

wikipedia:
Kaboodle is a spelling derivation of the English word "caboodle", meaning a group, bunch, lot, pack, or collection of things or people.[1] Caboodle is further derived from boodle or booty.[2]

This spelling of "kaboodle" has come into popular usage in culture and commerce, often found as part of the idiomatic expression "the whole kit and caboodle", which can be loosely interpreted to mean "everything and more".[3]

In popular culture[edit]

1.)In the original Star Trek episode "The Trouble with Tribbles", Mr. Scott says: "Before they went into warp, I transported the whole kit and kaboodle into their engine room, where they'll be no tribble at all."

2.) Kitten Kaboodle was a character in an anti-cat story written by Charles Schulz's Snoopy in June of 1974. "Secretly, Kitten Kaboodle wished she were a dog. She was aware of the natural superiority of a dog, and it bothered her." Linus to Snoopy: "I think your anti-cat stories show too much prejudice.... I think you're going to make a lot of enemies.... Not everybody hates cats, you know!" Snoopy, in thought balloon: "I find that hard to believe."[4]
 
Larry,
Very good. Without resorting to Wikipedia, Google or any other reference, do you know how the vehicle made by Willy became know as a "Jeep"?

Back on the subject of the SA300B...my unit came with two (huge), bullet shaped 220pf 500 volt bumblebee capacitors, one of which I noticed has an open seam. Does anyone have a source for them? I'm going to have to use the next best available thing, Silver Mica's and I'm not sure if this will impact the sound in any way. Probably not, but I'd rather stick to the (original) plan if possible. Certainly other caps were available at that time in that value, why bumble bee ceramics?
Rich
 
During WW II, the Willis vehicle was technically known as a General Purpose vehicle, or a GP vehicle. Pronounced as two letters together with the G pronounced not by its long or short sound, but by its name, it came off as "Geep". After the war, the surplus GP vehicles found such favor on the surplus market, that Willis continued production to help fill the demand. Later, AMC and then Chrysler bought the rights to the vehicle, where at some point along the way, a standardized spelling format came into play, with the name of the new vehicle being spelled as Jeep. Same pronunciation, just different spelling.

Dave
 
Pretty Good. But do you remember Eugene the Jeep, the fictional character from the Popeye cartoon, an alien seemingly able to move anywhere by inter-dimension means?
Many, including R. Lee Ermey, suggest that soldiers of World War II were so impressed with the new Jeep vehicle that they informally named it after Eugene the Jeep as "small, able to move anywhere while seemingly able to solve impossible problems."



Rich
 
On railroads, the "general purpose" diesel was designated by EMD as models GP7, GP9, ... up to GP60. These were all called "geeps" rather than "jeeps" (pronounced the same).
 
Not to go too far off topic here.....but I recently departed from Chrysler as my employer for over 33 years. When Chrysler 'acquired' Jeep from AMC, it was about the same time the brand was really coming into it's own. As Fred points out, the 'Geep' nomenclature was one of many terms....most notable the term SUV....which all came into being because the term 'Jeep' was trade-marked' and being used by other manufactures to support their vehicle's off-road capabilities. This became quite a battleground itself, but eventually the SUV term won out.......and perhaps rightfully so. But, Dave is 100% correct about the origin of the term. And, I am proud owner of 2 of them....and consider them to be exactly as advertised: The World's Finest 4-WD Off-Road Sport Utility Vehicle!!! Now, back to the SA300.......... WC
 
Done!

No hum or noise! Caps and new carbon comps have to break in (still a little crunchy) but all of the voltages were in range and it was a breeze to set the bias, AC Balance and DC Balance. Once I make the individual bias controls externally accessible, I won't have to open the bottom to adjust the bias.

The sound isn't bad, even with my "burner" tubes. I'm going to let it "shake out" for a few days while I clean up my workspace. The next day my wife isn't home I'll let the monster out of the cage for some air.
Second song and it's already getting clearer. I love the break in period! Can't wait to hear this thing with quality tubes.

IMG_4237_zps1b62a86c.jpg
 
Dave,
Thank you so much. It really means a lot coming from you and I don't think I could have done this without your guidance.
I still have a few small things to do, I'm waiting for it to break in a little before I address some minor issues I have. I'll shoot some more pictures later and provide details for future reference.

Richj
 
Now that I have it working, I'm wondering how it works and I have a question.
Basically, all I did was split the bias supply circuit into separate channels, replace the FP caps and the 0.22uf coupling caps. The only other thing I did was to increase the wattage of the 1meg resistors that are parallel to the 0.22uf coupling caps. I increased the wattage to (theoretically) reduce the contact noise generated by the resistor.

But what I don't understand is the 410 v supply through a 220pf bumblebee capacitor attached to the supply side of the 1meg.0.22uf output stage coupling capacitor. My concern is the health of the 220pf bumblebees, each of which have a visible crack along the seam. One bumblebee appears slightly bubbled, suggesting it was exposed to excessive heat at some point in its life. I don't know what the mode of failure is for this type of capacitor (open or short). what that would mean in circuit, or if a better capacitor is available now and should be used.

Sorry for the strange questions, I'm puzzled and would appreciate some insight.
Rich
 
Rich -- The 220 pF cap is part of the scheme used to achieve HF stability in the amplifier. It acts to roll of the HF gain at supersonic frequencies so the amplifier will remain stable under a variety of different load conditions it may encounter.

Dave
 
Dave,
Thanks. I apparently have to read up on the design philosophy of this circuit. The 220pf caps (C22 and C43 on the Sams Schematic) are not part of a feedback loop, at least none that I can discern. I see them blocking a lot of DC voltage, 410 volts right from the smoothing capacitor, yet they are only rated 600V and are the only bumblebee capacitors in the amp. Every other cap is ceramic or mylar, some rated much more than double the voltage they would be seeing. As those bumblebees are coming right from the 410v source of the smoothing capacitor, I expected them to be rated at least for 1000 volts. As I apparently do not understand them yet, I'm guessing their mode of failure would be to fail open rather than short, otherwise a lot of voltage would flow into that part of the circuit. If that is correct and I wanted to replace them, I would need caps that fail open. Correct?

Also- I followed the instructions in the service notes regarding AC balance adjustment and found the voltage read between speaker ground and 16 ohms remained the same regardless of where the AC balance pot was moved-always zero or a few millivolts. This caught me off guard. I'm not sure if I used a proper alternate procedure to set the AC balance, but I used 2 DMMs and adjusted the pot till the readings were the same at both plates (1 and 6) of the dual triode 12au7 AF Amp/phase inverter. Did I do that right or did I miss-interpret the concept entirely?

Thanks again and I hope my questions are making sense.

Rich
 
Rich,

This AC balance thing has me a bit confused as well; not on this amp particularly, but on all amps in general. I think I 'get' the idea....that the phase splitter output should be equal to either of the final output tubes? Seems to me that this adjustment should be set with a signal generator on the input, then as you mentioned in using 2 dvms, it just seems to me that there should be a point where you simply attach both dvms and set the AC balance pot so both dvms read the same. I suppose the actual reading wouldn't matter since the input voltage might vary, as well as the output from any prior gain stages (AF amp?). I'm also making what is perhaps another 'rookie' assumption, and that is that swapping PI tubes might affect this adjustment.....or perhaps not? I guess that might be a function of PI design. The more I think about it, the more confused I get in my attempt to understand this adjustment factor. But, seems to me it's really pretty critical, though I 'think' I understand that a lot of any PI in-balance might be compensated for by the function of the OT. Also, to some degree, I seem to recall that a bit of in-balance was actually designed in to some amps for stability purposes? I see this more on the OT connection to the output tube itself....where I see some amps with like a 10 ohm resistor on one side of the plate connections to the OTs...... Again, only guessing, but seems to me that this somewhat does the same thing as a slight in-balance in PI output? Anyway, please give me your thoughts on this as well.....as this is an area of amp 'design' that I want to understand better. Thanks....... WC
 
PS, for the record....... after watching what you and Don have been going through on this amp, I'm having some thoughts about simply pulling the iron off the chassis, and starting from scratch! The way you guys are adding these adjustment devices is simply amazing....and the workmanship is really first class! I don't honestly know that I'm 'up' to the task of doing it the way you're approaching it. And, part of me thinks that simply laying that amp out on a chassis about half again as big might be just a simpler way to go in the long run. Nothing like having some real estate to work in. And, it would facilitate making sure all the passive components were checked and 'at spec'. I could stick to the OE design, but add the improvements that you and Don (w/ Dave's help) have so successfully added in. Still not sure....but I do have to hand it to you guys! You're doing some great work!! Tom D.
 
Tom,
Sorry to respond so late, but I see you are a night owl yourself. I've read many different posts from people with different amps about setting AC balance. Some suggest unless you have the proper equipment, just set the control to the middle and it'll be good enough. Other's say measure the actual value of a particular resistor, and then set the balance control until a particular pin on the phase inverter tube is the same as the measured value.

I can't tell you if what I did is the correct method or not. In fact, I don't know why the momentary switch on my amp didn't function as I expected, perhaps I wired something wrong even though I made every effort to keep all of the Channel A and B circuits from becoming mixed. Maybe I had a bad component, which is why I wondered so much about those 220pf capacitors.

So when the procedure recommended by Fisher failed to work as described, I looked at the schematic and thought as you...the voltage at the top and bottom should be the same. Adjusting the AC balance control seemed to confirm this as each of the DVMs could were affected by the control and could be adjusted till they were equal.

I agree with you that the AC balance should be checked whenever the phase inverter tube is changed, and in fact when I swapped the tube in one channel , I had to adjust the balance in that channel, meaning that the two different 12au7s in the same socket were behaving differently.

I read in one article that said the hardest working tube in a push pull amplifier is the phase inverter tube. When the amp began to distort, it was because the PI tube was breaking up.

So like you, I'm not sure about the best way to adjust AC balance and it may in fact require a signal generator and 2 channel scope (neither of which I own or know how to operate).
Here's what I think:

AC Balance: looking at a sine wave, when the amplitudes of the top and bottom of the sine wave are equal relative to the center "zero" -you are in AC balance. It may not be as critical as DC balance because the complete sine wave is reassembled in the OT. The signal may be slightly distorted and possibly cause the OT to get warm if the AC balance is off. The AC balance control would be very useful for PI tubes if the two sides of the dual triode tube were unmatched and had different gain.

DC balance however refers to applying the proper amount of DC Bias to the control grid of each output tube in a complementary push pull pair so that each tube dissipates equal power and draws equal current during operation. Put another way, they should each swing the same amount of voltage as each tube turns alternatively on and off in their operating range. You don't want one tube to turn completely on while the other tube turns completely off, which will cause a failure. Bias, as defined pushes each tube slightly on, thus never allowing either to turn completely off, because if one tube in the complementary pair is completely off, the other one is completely on.

So my impression is that DC bias is most important for the health an operation of the output tubes while AC balance is most important for the quality of the signal. I'm not sure if the method I used to adjust AC balance was correct (equal voltage output from top and bottom of PI stage), but it was the best I could come up with and seemed to make sense.

Update on the sound and performance of the amp: It's getting better as the new caps and resistors break in. Initially, the amp sounded like crap, honestly offensive to my ears. My plan has been to run it at low volumes for short intervals and then shut it down to cool off. At first it ran very hot, tranny's were all hot to the touch, especially the mains. After the amp cooled down I played it again, for a little longer and a little louder. After gradually increasing the operating time and volume over a couple of days the amp seemed to smooth out and better handle high power output. And the tranny's aren't getting so hot anymore. Yesterday a loud song came on and really shook the windows and floor with no loss of control. This amp is like a muscle car along the lines of a GTO, it's just built for power. What Iron!

Finally, I would recommend not removing the iron and rebuilding the amp on another chassis to gain real estate. There are other factors in play such as RF layout and magnetic flux that we couldn't begin to predict, much less understand. You want a transformer far enough away from another component so that its flux doesn't have an effect, but too far away and inductance becomes a factor. Engineering is the art of compromise and like plumbing or HVAC, you can't simply move a duct or pipe without affecting and perhaps unbalancing the operation of the entire system.

It's a ballsy amp, separating the bias into individual channels is no big deal, trust me, it just takes time and planning. Hopefully the photo's I uploaded can help you develop your own plan. Just be careful if you use the Sam's schematic-it's not at all accurate and should not be used as a guide. As you take your amp apart, take notes and detailed photos. Verify everything against the schematic and you'll see what I mean about the inaccuracies. From what I've seen in the photos between the SA300 and the SA300B, they are not the same amp, many of the pf capacitor values are different. Be wary of the schematics. You can do it if you take your time. The amp is a keeper and is very, very musical.

Relax and enjoy.. it's a hobby not a job.

Rich
 
Rich,

Thanks for that detailed response. It's kind of funny when you think about the PP process in general. You start with a signal.....split it in two...then put it back together again. And this is progress? Then you add the UL aspect of things...... WOW! Get's kinda complicated. I can see why some of the SET guys get frustrated w/ PP amps.

Actually, I'm a huge fan of PP......and have really very little experience with any SET amps other than some little SE EL84 amps......and a pair of mono-block monsters I acquired some time ago from an estate sale. I own some Antique Sound Labs DT805 Explorer amps....which weigh about 60 pounds each.....and use a 12AX7 to drive a KT88 which drives an 805......for about 45 watts. I've heard both good and bad about them.......where the bad referred to some distortion numbers.....and the good referred to just general sound quality. My own impression is that they sound 'different' than the 50 watts or so that my old Dynaco MkIIs produce....and I greatly prefer the MkIIs sound. So, the ASLs are on a shelf. I also have a very nice sounding pair of old Heathkit W-7s.....which are also EL34 PP at about 45 watts. All these amps are 'brethren' to this SA300(A)...... so, I"m going to be interested to hear the 300 when it all gets done.

I will mention something to you that I found ironic; that being that you indicated that when you first turned your amp 'on' and listened to you, you found it almost 'offensive'. I had almost exactly the same reaction to my amp! However, I have only gone so far as to 'adjust' mine as per factory spec w/ the currently available controls. Perhaps you'll recall when I started this thread...... I was just seeking a way to properly set up the amp 'as-is'. Well.....now that we're a dozen pages in, and some 178 posts on the subject........ I clearly have some work to do on my amp.

BTW, I plan to PM you regarding the parts we discussed earlier. Looks like I'll finally have some time in another week or so.....and perhaps be able to get back to my amp and start some of the mods. Now that you've 'completed' yours..... I'd be interested in your appraisal of a 'step 1, step 2, step 3' process suggestion...vs just having at it with no specific plan or 'order'. Maybe Don could also provide his input there as well. I still like the idea of starting over on a bigger chassis...though I understand your thoughts about the issue. However, I'm somewhat of the opinion that problems occur when things are close together......so if I go 'larger' with more space, I should also be somewhat eliminating that potential. I can take layout ques from other amps, and probably not encounter the kinds of problems you so correctly refer to. My thoughts are that back in the day...they simply built these things on smaller chassis to simply save space and weight....and possibly even 'cost'. Smaller chassis cost less to make, and can 'fit' better inside a wooden cabinet that's also loaded with other 'stuff'. But..........who knows........ guess we can't go asking the manufacturer...lol.

Anyway........thanks again for the comments. BTW, signal generators are pretty cheap these days. I have a few of them (don't know why)..... and didn't pay too much for any of them. The 'Leader' company makes a nice one.....I think 'new' it was only about $100 or so.....and they sold the crap out of 'em to schools. I see them come up on ebay as inventory clear out items....where one guy will have 10 or 20 of them to sell all at once.....and have bought them for as little as $25. My preference is the old 'Heathkit' Sig-Gens. They had a pretty beefy output section.... I believe upwards of 10V....and they have a volt meter on the front.....so when measuring power output with a 1K / 1V 'in' signal...the meter makes it easier to see that input information. Not that it's necessary....but would be nice to have a good 'frequency counter' along with one of these units. I've often wondered just how accurate any of them are when reading CPS.

Anyway.......thanks again. I'm off to my Girlfriend's Son's commencement exercise at University of Maryland. I take some credit for getting the kid to this point, since he was only 15 when I met him.....no one in my GF's family has ever gone to college, and I was the one who dragged this kid down to the community college after High School, then after 2 years of that, dragged him up to University of Maryland to finish his degree. Kid's smart!!......we're talking 4th level Calculus and Quantum Physics here! I think NASA is his next stop!

Later,

TSD
 
I've spent some time playing my SA-300 while working on a Fisher 400 for a friend last weekend and cleaning up my electronics room.
I will say that I'm surprised how good it now sounds. Everything seems to have settled in nicely. The bass weight and smoothness without mid-bass hump is just right. The high end? We'll see. What I once would have attacked with fancy coupling caps and tube rolling may require some feedback or minor cap tuning instead.
TSD,
If you choose to go with a new chassis, you could put a nice choke in the power supply and use the rest of the design, decide on an entirely new design based on what the transformers do, or some combination of the two.
That being said, these units behave Ok and with the bias control split, you would have full control over the output tubes.

I found the combination of putting cathode resistors on each tube, splitting the bias control into two and putting on screen stability resistors made this quite a nice amp.
I'd even gotten some premium output tubes to play with, but my test Sovteks are sounding so good and are behaving with such stability that I don't really feel any compulsion to change them.

The output iron on my X-1000, with a date stamp of November 1961, as opposed to the SA-300's likely 1959 build date, is considerably smaller than the SA-300 iron. Dave Gillespie tested the heck out of that X-1000 and it is well behaved and powerful, yet Fisher didn't do it's frequent trick of reusing the same transformers from the SA-300 in the X-1000. That makes me wonder about whether the SA-300 iron was too expensive or had some other type of compromise not required in the strictly pentode mode X-1000, or if the transformers just would have added too much mass to the X-1000 so alternatives were used. Both of these are pretty hefty pieces.

This amp is one of Fisher's last efforts in the old '50s style of amps. In that sense, some of the resistors are suspect and some of the components are less than top shelf. But that in itself should not damn the amp. The small footprint and compromises made to make the small footprint happen are probably more to blame for design compromises and behavior problems than anything else.
I wonder if the cathode bias scheme and some of the funky design stuff was done because the iron was just a bit too large to do high frequencies well.

I did just get a signal generator (Wavetek) but I lent my 'scope to AK'er Prelius. When it comes back, I plan to bench this unit and run some square waves ala dcgillespie and see how the HF and LF response is.

I don't consider it done, but it's behaving well and sounding good.
Remember, if you pull the iron and redo it, you'll never recover the money you could get for a restored one if you were forced to sell. This is probably a case of the whole being worth more than the sum of the parts.
 
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If I may, here's a couple of thoughts in on the 300s.

Don -- From an volume standpoint (as in cubic inches), I would have thought that the X-1000 transformers had more lamination mass to them than the 300's transformers do. I know the 300's transformers are quite tall, but I also thought much thinner than the X-1000's are. A dimension of height was not too much of a concern with the SA-300, but with the X-1000 being an integrated, component height is always a concern. Therefore, it is customary to lay the larger transformers down on their sides in an integrated settings, trim the length and width down, but make for a thicker stack. The 500C and 800C transformers are a variation on this theme. Bottom line is, no matter how they get it from a dimension standpoint, you've got to judge the "size" of a transformer on its total lamination mass. That's why transformer weight is one of the best judgements to use as opposed to just a quick dimension check. If you do go by dimension, be sure to use all three dimensions.

Rich -- regarding the voltage rating of the 220 pF cap. It only has to withstand the voltage across it, which is the 410 volt B+ value, minus the value of the plate voltage, or about 150 volts or so, PLUS the value of the peak AC voltage developed at that plate. For EL34s, this would be no more tan about 35 volts, so in other words, the cap never "sees" more than 200 volts across it -- even under full power conditions.

This cap -- and the one on the opposite plate -- are not part of the NFB loop per se, but really are. The NFB connection forms a circle, with the active amplification element being a part of the circle, and the NFB loop being the other part. The stability of "circle" needs to be controlled at high frequencies, which is what these caps do. If there were no NFB, then these caps would not be required. In that vane then, they are a required part of the NFB network for it to operate properly.

However, these caps would have nothing to do with the proper operation of the AC Bal adjustment. The feature provided to effect AC balance in this amplifier is really only a semi accurate method, as it does not take into account the active Gm of the phase inverter tube sections. It is only taking into account Gm variations between the two output tubes employed.

It operates based on injecting a common mode AC signal into the B+ supply of the phase inverter stage. The signal then travels through the AC bal control, up the two plate resistors, to the plates of the phase inverter tube. Because the tube sections are conducting current, there is current flow along both of these paths that not only reduces the DC voltage at the plates, but also the AC voltage applied to the B+ connection when the AC balance adjustment is being made. The AC Bal control is adjusted so that the AC voltage drop across these two paths is such that the resulting test signal available at each phase inverter tube plate, when then applied to their respective output tubes, amplified, and presented to the output transformer primary winding, cancels the signal out in the OPT secondary for no, or very little AC signal present. In that condition, the output stage is operating in a balanced AC (dynamic) state.

The problem with this approach, is that the dynamic characteristics of the phase inverter tube are not being accounted for -- only static characteristics are. Therefore, the adjustment is only accounting for the dynamic characteristics of the output stage. No doubt that this is the place where the most deviation can occur in the push-pull signal, but a thoroughly accurate adjustment would be one which includes ALL push-pull stages of the design.

There is very little that can go wrong with the AC balance function. Check that the 30 vac is being applied to the wipers of the AC Bal controls via the coupling cap, the value of the 3300 ohm resistors supplying the B+ to the wipers, and that the switch, and coupling cap are all operational. There's really little that can go wrong with the circuit.

I hope this helps!

Dave
 
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