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

Hello Fisher Fans!

I'm attempting to bring back to life a couple of SA-300s I'd picked up some years back. One is waiting in the wings while the other is 'on the bench'. So, dealing with this first one, I'm curious as to what (if any) instructions Fisher provided to 'adjust' this amp for proper use? It seems to be pretty well loaded with adjustable pots.......2 for AC balance, 2 for DC balance, what appear to be 2 'hum' pots, and a single bias pot? I'm surprised that it didn't contain a bias and balance for both channels.....but I guess when this thing was popular the idea of getting 4 'matched' tubes wasn't all that far fetched.

Anyway...... I've 'cooked' it for about 24 hours on the 2 rectifiers without driver tubes.....slowly bringing it up to about 320 volts DC at the B+ to the OTs. The filter caps stayed bone cold.....so I decided to load it with tubes, install the dummy loads and input-shorting plugs......and start the slow-cook process again. I did this formerly by measuring my AC input to the PT, but now I monitor the HV after the rectifier...and particularly watch to make sure I see a small 'jump' as I crank the variac up a notch or two. I found that bad filter caps resist the immediate voltage increase.......so even if they're not getting warm.... I can at least get an idea that they 'might' be heading south!

It appears that though Fisher has provided all these adjustment capabilities, they didn't include and convenient 'test points' on the chassis. So, I'd like to be sure about not only 'what' the proper adjustment level is, but 'where' the measurement should be taken.

My good pal AudioDon has indicated he has some material he's going to copy and mail to me on this amp. So, as I find need for 'repair' or 'refurbishing'......I'll certainly be using his information with much appreciation. But, being the 'type A' personality that I am....my compulsion to want to just keep rolling is getting the best of me here. Hopefully I won't blow anything up.......as 'patience' is NOT one of my virtues! However, I do take more than average time on the 'slow-cook' process......and where some people will complete this in a matter of a few hours, I generally take at least a day, if not 2 or 3......so give a guy some credit!

If anyone can help me with just the Bias Adjust portion of this, I'd really appreciate it. I'll worry about hum balance, AD and DC balance later. But, once I get to some higher B+ levels, I'd like to make sure I'm not 'overly' biased and perhaps jeopardizing my new tubes!!

Thanks..... WC

Just a quick update: It made some music! I got it up to full line voltage, had it hooked to my 'mains'.....and it sounded pretty good. At least NO HUM....very quiet, and nothing appeared to be out of line. Filter Cap Cans were 'cool' to the touch, and PT was not overly hot. The smell of the paint on the new tubes was a bit annoying, but I didn't see anything wrong going on inside them.......so....a 'successful' first test run. I did shut it down after a few minutes of run time. Not knowing where the bias is set, I decided not to risk anything until I get a better handle on getting that properly adjusted. My be a good time to consider adding some 'test-points' to the unit. I think I still have a set of 4 kicking around here some place. But, any help w/ Fisher's procedure would be great! Tom D.

I am looking forward to your rebuild thread. I am pretty sure I have every thing needed with exception of some polyester caps to do mine. I want to try the polys opposed to micas or ceramics for the smaller values.

I have not seen the 300B schematic either and hoping it comes up in the other thread.

Perch,

I've got a copy of the schematic......and I'll attach it to this post. Problem is, the Schemo does nothing to address the 'how to' of making the adjustments. Also, as I look at the amp, I see it as being a kind of 'hybrid'.......or a combination of both 'cathode bias' and 'adjustable' bias. The output cathodes aren't tied straight to ground, but rather they go through a 50 ohm / 5 watt resistor w/ a 150uf bypass cap. This is pretty similar to most any 'cathode bias' amp I've seen in the EL84 world. BUT, this amp also has a negative -25 volt supply tied into the output tube 'input grid resistors'........with a DC balance control and a bias supply pot. So, clearly there are some 'adjustments' to be made.....but the 'how to' is not revealed on the schematic. Further, the amp has no 'test points'....which again, means that Fisher had some intentions in their Service Procedures.....but didn't show them on the schematic.

I've refurbished several Scott 7591 amps.......299Cs, 299Ds, and 233s. In those amps, I've added 'test points' to the cathodes of the output tubes by just detaching the grounds and adding a 10 ohm resistor and then wiring in a chassis mounted test point directly to the cathode pin on the tube socket. From there I could measure bias, as well as adjust DC balance by just placing the two leads of my DVM across both cathodes on one output pair....and adjusting the balance pot until I read 'zero' volts. I'm hoping to do the same thing with this amp.....but with the 50 ohm cathode resistor in the cathode circuit......I'm concerned about how 10 more ohms from each tube would affect the circuit? Would I need to 'adjust' the 50 ohm resistor to perhaps 40 ohms? Also, I used 1/2 watt resistors in the Scott amps. With this being a 5 watt cathode resistor....would I need to use 5 watt 10 ohm resistors in fabricating test points?

Sorry to say.......what little I know about this has me in a position of not wanting to make another move until I get some advice from 'smart people'!! Wish I had the ability myself....but I guess this is how you learn! Anyway..... Schematic will be attached....and you can take a look and see what I mean. And, hopefully someone else will have some knowledge about this and will post a response. Oh, and one more thing: assuming I get some test points 'installed' ..... just what should the ma draw be adjusted to in order to accommodate this kind of bias design?

WC

that is not the 'b' schematic, I would love to see that version...

lets see if I can attach the rest of that Sam's manual. hope it helps

Hey Perch,

First, sorry about it being the wrong Schematic. I guess I just compared it to the amp, and everything I looked at 'appeared' to be consistent with that schematic. I suppose if there's a '300'....then there's and "A" and a "B" version as well? Do you have a clue as to the differences?

BTW, I able to use your Sams info and 'adjust' the bias pot as per their instructions. Took me a while to find where the 'A' and 'B' locations were. I finally found them on the second 'under-carriage' picture on the Rt side. In attempting to get mine in line with the recommended level, I had to turn the pot all the way 'clockwise'..... That gave me like 7.75 volts on one side and 8.3 on the other. I couldn't do much better than that. I did try to go from cathode to cathode on the output pairs......and got nuthin'!! No voltage reading at all. So, I guess without those 10 ohm resistor......there's no discrepancy that can be measured between the two sides on the output pairs.

Guess I'll just play 'wait & see' if anyone posts about the 'hybrid' aspect of this..... and my hope to be able to put test points in on a 'per tube' basis. I'd still like to do that.....and be able to do a DC balance using those test point.

Again, thanks for the additional info! Big help! Tom D.

Perch,

Have you confirmed that 'The 300' and a SA-300B are, in fact, two different animals? I'm only asking as I see that my unit says exactly this on the side. There is a 'label' that says: Fisher SA-300B Chassis, and yet the amp itself has 'The 300' written in gold on the front. So, I'm just wondering if the two could be one in the same???

My understanding was that the only differences between the original and the B version was that the B version also had a line level center channel output on it.

Dave

For the Record-
The Fisher Service Manual and Schematic for the SA300B Amplifier can be found on The Fisher Console Website under the 1963 President VIII 8000 Console tab.
http://www.fisherconsoles.com/service manuals/FISHER PRESIDENT VIII 8000 SM.pdf

The Fisher SA300 (non "B") was used in the President VI 6000 console-a service manual or schematic is not on that website.

Dave,

Looking at the schematic of the SA300B, I suspect the SA300 (non "B") may have used different OPT's and possibly some different component values because of the way the secondary taps are used to derive the center channel output. I don't think the "center channel output" was line level either. The configuration of my SA300B seems to be the same as my SA100. (I suppose I could put this issue to bed if I hooked up a center channel speaker to the SA300B).

Rich

Rich, I'll check tomorrow and see what I can find by looking at each of mine. One has the center channel, and the other doesn't. So....guess I'll check the transformers and see if they have the same numbers on them. I'll let you know.

DCG, if you're still looking......I'll try to remember to get that info for you on the ST-70 6GH8 conversion.

TSD

Rich -- Thanks for providing the Fisher schematic! After review of it and the Sams schematic, I would expect the OPTs to be very much the same between the B and non B models.

The center channel output of the B model is presented at an RCA jack, which is derived from the main speaker outputs by way of an 8.2K resistor in each channel. Therefore, the signal presented to this jack is incapable of driving a speaker directly, but requires the use of a third amplifier driven by the signal presented at this jack to produce a center channel element in a system -- which is the same exact scenario required with the center channel output jack on an SA-100 amplifier as well. As a result, the signal level, impedance, and jack type used are very much in keeping with that of a line level "type" signal, and with center channel thinking at the time these amplifiers were produced.

As for the remainder of the output connections, they appear to be identical between the B and non B units, with both employing the same, but somewhat goofy reduced damping connections (the "X" and "Com" terminals), that were available on the SA-100, X-202, and SA-300 amplifiers.

I hope this helps!

Dave

I really wish I had time to mess with mine, but calculus class is kicking my butt :tears:

Here's another thread on this subject from 2012:
http://www.audiokarma.org/forums/showthread.php?t=488753

In my folder, I have the schematics for the President VIII console with an SA-300-B as well as the SAMS schematic also mentioned in this thread. I also have the HiFi engine version also linked in this thread.

So, to add completeness, here's a link to Wharfcreek's Tube forum thread:
http://www.audiokarma.org/forums/showthread.php?t=584019

My SA-300 redplated an output tube a year ago and it has been a shelf queen since. I just haven't bothered to look into it.

What I have learned in the meantime is that the two 5AR4 rectifiers are used to increase the available current, if I understand correctly.
At one time I wondered about splitting them out as one rectifier per channel and doing some other rearrangement to . . . in effect . . . to create a dual mono amp, but I never researched that beyond a few random thoughts.
I think I never went any further because it's tight enough under the chassis as it is and anything bigger than small added would probably have to be outboard of the amp. Mine's too clean to mess up.

Where that leaves us Wharfcreek is that you're on tap to take us all forward by exploring the possibilities and we'll hang onto your coat tails.

Sorry to double-post but . . .
The President VIII console documentation does have a power amplifier service notes page.

I've scanned and enclosed it twice and it's not showing up, so I made the PDF a jpeg and posted it as a graphic. Everything's there. Nothing's cut off.

I recall doing it, since my values are written in, but my SA-300 redplated anyway. It was running baseline Shugang EL-34s though. The sounded pretty good until it didn't.

Hey Don,

Just sent you a PM on this.....and DCG is in the loop as well. But, here's where I'm at on this amp:

I'm trying to put some test points in so as to be able to 'measure' some things without having to pull the bottom off and invert the amp again in the future. To that end I've noted that this amp is a combination of both a 'cathode biased' amp AND an 'adjustable' biased amp. So, the typical grounded cathode of an adjustable biased amp isn't on this unit. Thus, 'test points' are best installed ahead of the Cathode R/C. I've not done them yet......but I've also been advised that a 3rd test point on each channel will be necessary....connected to the 'top' of the Cathode R/C network (or the un-grounded side). I'm still unclear about 1) if the 10 ohm resistor I'd planned on using needed to be of a higher value......like 2 watt vs. 1/2 watt. and 2) if the pin 1 connection that is currently attached to pin 8 just needs to be made directly to the top of the R/C network.

I'll note that in re-tubing this unit, I used some new JJ rectifiers. I'm not sure if they're the problem or not, but I've noted that my B+ is pretty low.....like about 50 or 60 volts off. Schematic calls for about 410 volts.....and I'm getting 350 after the rectifiers. That's a pretty substantial discrepancy. As I 'listened' to this amp for a while yesterday, I felt as if it was just 'off' somewhat. I didn't get too involved as I'd not yet confirmed any 'bias' adjustment.....so I wanted to shut it down relatively quickly. But, when I found the voltage as low as it was.....I figured that was part of the problem. Today I'm going to 'swap' in some 'good' rectifiers and see what happens. Hopefully that will increase the B+ .......

Last comment: This amp has some other design aspects I've not seen in the past. If I'm not mistaken, the first stage AF amp..... the 12AX7 tube.....is running both sections in parallel? Like the rectifiers.......I'm 'guessing' that this is to run more current through that stage of the amp. With a 12AX7 being a high gain tube....but also a higher internal resistance.....I'm just guessing that by paralleling both sections, it's producing a higher, stronger current than just one section alone? I need to learn more about this....and perhaps someone reading this can comment.......

Fun stuff........... Onward! WC

Don,

One quick comment on your red-plating: I've noted that the coupling caps on one of my 2 units are 'different'.....meaning someone already changed out one or two of them on the one amp. Likewise, the other amp had one 'cut', or disconnected.... which I don't understand at all....but one end was just 'snipped' on one of them. I soldered it back in place....but have since considered that I should perhaps just change all 4 of them. My red-plating experience has been 98% caused by failed coupling caps......so.....I'm now just replacing the main 4 in most everything as I get it out and 'tinker' with it. But, I guess I'm just suggesting that perhaps these amps are 'tough' on coupling caps? Not sure if that's possible....but as both mine show evidence of it being a problem.....that gives me cause to wonder.......... Tom D.

Tom,
Dave G is an excellent teacher and taught me much while I was rebuilding my SA100. One of the things about the SA100 that I like is the ability to swap output tubes and adjust them for bias and balance without having to remove the bottom. Dave's approach was to add test points by using some rarely used terminals on the speaker terminal strips. My approach was to replace the 3 terminal bias test strip with a 5 terminal strip, making it possible to measure the current draw from each individual tube rather than channel A or B.




The SA100 has individual bias controls for each channel, the dc balance is then used to adjust each pair for equal current draw. Very precise. I believe the same is possible for the SA300 by separating the single bias pot into two (one for each channel) along with the addition of external test points. The addition of external test points may be considered blasphemous by purists, but I believe it is an enhancement and really doesn't have to be seen externally. Separating the single bias control into two (one for each channel) shouldn't be too much of a problem if space permits. Accessing the individual bias pots and test points can be done by removing the side panel under the Mains Transformer (did you noticed that it is designed to be removed while leaving the bottom on?) In this manner the amp could be adjusted in place rather than on the bench. I believe it is better to adjust the bias and balance after the amp has played and is really warmed up, rather than on the bench after barely warming up. You know how the amp sounds better after about 30 minutes, that's when I think bias and balance should be "tuned in". Leaving the side panel open and having access to individual bias controls and test points for each tube along with the existing externally accessible dc balance controls would make this very simple to monitor and adjust the amp as it breaks in and ages. Much easier and more accurate I think then doing it "barely warmed up" on a test bench.
Just my 2 cents.

Dave-thanks for the center channel explanation. I never had the need to try it out and never paid much attention to it.

Rich

One quick comment on your red-plating: I've noted that the coupling caps on one of my 2 units are 'different'.....meaning someone already changed out one or two of them on the one amp.

Click to expand...

On my Fisher 481-A Console pull amps, two different types of coupling caps were used to the EL84 tubes, one Erofoil MKP(?) and one "Brown Turd" (can't remember what was used from the plate but I think it was the Erofoil MKP(?)).

This was done by design, presumably to take advantage of each type's sonic characteristics resulting in a very nice sound. The 481-A is the amp that got me started with EL84 tubes-very impressive sound, especially for the price I paid ($125).

Thanks heyraz.

I changed the coupling caps to Jensen Copper Foil in oil. The couplers aren't the problem in mine.





I may follow along vicariously and do some updates to get it back up to snuff now that you've made me blow the dust off mine.


This is about Tom's amp though. He's the one pulling the sled right now.

Don, Au Contrare'....... this is about 'Fisher 300s'...... So, chime in frequently!

Rich, Take a look at that 'underside' pic that Don posted. Aside from his very pretty Jensen caps.....note the location of the one bias control.......and the 'hum' pots next to it. There 'may' be room to move the bias pot closer to the side and then add a second pot.....but wouldn't it be nice if you could re-locate the AC pots, then put both bias and balance pots into that current 'AC/DC Balance' location? That would be 'tricky'....but perhaps do-able. Probably over my head.....and I need to just get the test points in. BTW, 1/2 watt resistor for this is OK.....but seems it's appropriate to keep both pin 1 and pin 8 strapped together and put the 10 ohm resister between the both of them then to the top of the 50 ohm/150uf network. Then add the 3rd test point at the top of the R/C....and look for about 70ma on the two tubes, and you can then 'null' across both for DC balance. If a closely matched quad is used, then the one bias pot should get you in the neighborhood for both pairs.....at which point I think having the DC balance set is probably more critical than being a ma or two apart on the pairs. Your thoughts (or Don's)? WC