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

Don, I like that idea. Dave also had an old article about his 'grid-stopper' resistors in a magazine of some years ago. When I read through it back then I didn't have the depth of understanding that I do now...which is not to say I'm all that much better. But, at least I understand the concept better to some degree. And, with 'only' 5 volts difference between plate and first grid......I think those 'stopper' resistors are a good idea as well. Perhaps someone else will chime in and confirm or suggest a different approach. WC
 
Yeah, I don't know enough, but I'd read that article about maximizing tube life a few years back and it made no sense and now I can follow the whole thing. Progress.
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The more I look at this amp, the more I think it was designed to be a dual mono amp and that they pulled back from a full dual mono topology into what it is now, which defies explanation.
Just take a quick look at the power supply.
I went to put resistors on the screens last night and stopped because I realized I was leaving some flying leads.

Looking at the power supply on the schematic again after a good night's rest, I see that there's:
*one 410 volt source
*two separate 395 volt sources
*two separate 290 volt sources.
*One pair of screens goes back to one 395 volt source and the other pair of screens goes back to the other 395 volt source.

Why would the company support that additional expense of having dual voltage sources other than the highest B+ unless they had something in mind?

Why would that be the case in a console amp unless they were thinking of other uses?
 
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Hey Don,

I've seen this kind of power distribution in 'stereo' amps in the past. So, I'm not so sure it supports and prior consideration of being 'dual-mono'. But, it IS an interesting design. That 410 volts is also taken to the other side of the input grids.......at C43 and C22. And, don't forget the 350 volts going to the AC balance pots. That's an awful lot of 'tapping'......and you're right.....a very complex design for a 'console' amp. However, I'm wondering if this amp was 'reverse' engineered in that, if you recall, Fisher did have a stand-alone power amp much along the lines of the Dynaco, HK and other competitors. Perhaps this amp is the result of those designs....and just re-purposed into console use? I see a lot of the Citation 5 amp in this thing...... but not enough to make a direct comparison. Likewise the ST-70 similarities....but again, not enough to draw any straight lines. I think Fisher was just trying to build a very reliable and low maintenance unit that, once up and running in the console, would require little if any further attention from it's owner.....and last for years based on over-design and high quality components. Let's face it....these are some VERY beefy transformers.....and the EL34s don't seem to be being pushed all that hard at 410 volts of plate current. Even the 12AX is being run in parallel.....so how hard is it working as compared to use in other amps? Probably the most highly taxed component would the the 12AU.....and maybe the PS filters. But, my PS filters appear to still be all good in my amp......so...what's that say about 'em?

Anyway......if you can shoehorn in those 100 ohm resistors......great. I'm not going to worry too much about that now.......as I don't see those affecting the 'sound' of the amp at this point. And, that's what I'm trying to focus on right now....just getting the amp to sound better. Once I'm satisfied with the way it's sounding, then I'll work on improvements to long-term use....like the addition of grid-stoppers.

TSD
 
My plan is to simply get this running for now and assess it again later. After I figure out how to get a signal generator in house, I'll assess its square wave performance, copying what Dave has documented.
I'm really glad that pesky work for money thing keeps me from having the time and energy to just do this right now.
 
LOL.....Don, I paid the household bills today......and I envy the 'work for money' life! I also envy a well-planned and well-executed 'retirement'......of which I have neither! But, I DO have some short-term work-for-pay scheduled beginning next week.....and hopefully that will help bridge the gap between now and 'full retirement' next March. 11 months and counting!! TSD
 
I just did another 'listening' test on my amp. I noted that there was a good 'hum' as the primary filter caps charged....which I don't take as a good sign. However, it was about a 1-second event...then gone. I also noted that my 'level' pots act more like on/off controls. At full 'on'...... I get signal. But, with just a slight counter-clockwise motion to the knob, and I lose signal completely. Answer: Do away with 'em!! I'm don't care for 'gain' or 'level' controls on power amps anyway!!!! That's what volume controls are for on pre-amps!!! So..... I'm slowly developing a plan for this amp. Now it's moving toward a signal re-cap, removal of the input level controls, installation of the 10 ohm resistors and test points so I can adjust the DC balance properly......and perhaps a split of the bias control itself so as to be able to adjust either channel separately from the other. A good matched quad 'should' make this unnecessary....but still nice to be able to do. Finally, I may end up replacing the power supply filter caps as well......or at least the first few stages. I think one of the multi-caps covers the first 3 stages......so that would be a good start!! WC
 
Regarding the input level control situation, I posted a separate question on the Fisher Forum about removing them, along with the 'filtered' input. Got response from Mr. Gillespie as follows:

http://www.audiokarma.org/forums/showthread.php?p=7702438#post7702438

Since my level pots are bad....and my need for 'filtered' inputs is ZERO, I'll be making this mod to my amp as well. Anything to 'remove' unwanted and unneeded 'stuff' under that chassis will help!! TSD
 
Well, I continue hammering away on mine and yammering on about it too.
So far I've:
1. Put in screen stability resistors. This involved pulling out the old screen wiring, putting in a terminal strip and setting up two pairs of screens with 100 ohm resistors on each output tube's screen.

2. Drilled out the AC outlet, put the metal nipper to work and fit in a toggle switch. I'll have to play with it a bit because I bought a lighted switch for it to remind me to shut the unit off if the cage is on. I'd left it on before, so I know that's humanly possible.

3. Removed the pair of CL-90 thermisters. After Dave Gillespie pulled one out of my X-1000 because it wouldn't pass enough current, I downloaded the spec sheet and realized that the two that I have in there have nice high resistance, but only pass two amps of current. That's a limiting factor in this build. Someday I'll order some CL-60s, which will pass 5 amps of current. This unit has a 5 amp fuse.

4. Untied pins 1&8 on each output tube and put a 10 ohm resistor between each pair of pin 8s and pin 1s, with the pin 1s going to the RC network for cathode bias. I hope to be able to read a dissipation rating across the 10 ohm resistor. On the X-1000, though fixed bias, it says to set it up to read .86 volts, which in the ideal world would be 43ma dissipation per tube of a matched pair. I'll be targeting .70 volts.

5. Put in a new polarized AC cable with a death cap (incomplete - still installing).

These are painted kind of like HK Citation II transformers. It seems like they didn't prime before putting the paint on. I always thought people must be rough on these, but the truth is that the brown paint comes off very easily.

After that, I might put some test EL-34s in and actually bring it up on the variac to see how it's behaving now.

Once it's working and nothing goofy is happening, I"ll try to learn to characterize baseline stock behavior, but that's probably a task for later. Right now I just want it working correctly.
 
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Don..... I think there's an error in how you've done the 10 ohm resistors. Pins 1 and 8 have to remain tied together. Right now, the combination of those two pins on BOTH sets of output tubes is what's going to the ONE R/C network for each channel. According to DSG (and I believe I have this correctly) you need to keep pins 1 and 8 tied together, but break the connection to the R/C network, and put the 10 ohm resistor in between there. So, you'll have the two 10 ohm resistors terminating at the top (the 'ungrounded' end) of the R/C network......and THATs where you'll take your current reading from....there, and the other end of the 10 ohm resistor. I"m going to draw it out now and attempt to post it here after I scan the drawing. Stand by for a few........ TSD
 
If I pop the resistors, I'll be the first to know.
I'll probably try it both ways. I do have a variac and perhaps I'll start with the tubes I was using.
 
OK......sorry if this drawing doesn't make it.....or if it doesn't make sense. It may also require you to download it and spin it 90 degrees. But, hopefully you'll get it and be able to make sense of it. NOTE: Pins 1 and 8 stay tied together. The resistors go between both pins and and the 'top' of the R/C network which is the 50 ohm resistor and the 150 uf cap. In the original design, both tubes from each channel go to the one network....so that doesn't change. And, the network is 'grounded' on one end...and that stays the same. The ONLY change is that the connection between each tube and it's corresponding network...currently just a wire, now that is replaced by the 10 ohm resistor. And, instead of reading 'bias' between ground and the pin 8 of each tube, you'll read it between pin 8 (still connected to pin 1) and the 'top' or ungrounded side of the R/C network. According to DG, that reading should be about 70ma. However, you can also connect between each pin 8 of the given pair....and use the DC balance control to get a 'zero' reading on a DVM. This will simplify the DC balance procedure. You can't do that now and they are essentially 'hard-wired' together in going to the R/C network. But, with the 10 ohm resistors in place, that will permit this measurement to be done like in the SA-100. Hope this helps....... TSD
 

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Wharfcreek said:
OK......sorry if this drawing doesn't make it.....or if it doesn't make sense. It may also require you to download it and spin it 90 degrees. But, hopefully you'll get it and be able to make sense of it. NOTE: Pins 1 and 8 stay tied together. The resistors go between both pins and and the 'top' of the R/C network which is the 50 ohm resistor and the 150 uf cap. In the original design, both tubes from each channel go to the one network....so that doesn't change. And, the network is 'grounded' on one end...and that stays the same. The ONLY change is that the connection between each tube and it's corresponding network...currently just a wire, now that is replaced by the 10 ohm resistor. And, instead of reading 'bias' between ground and the pin 8 of each tube, you'll read it between pin 8 (still connected to pin 1) and the 'top' or ungrounded side of the R/C network. According to DG, that reading should be about 70ma. However, you can also connect between each pin 8 of the given pair....and use the DC balance control to get a 'zero' reading on a DVM. This will simplify the DC balance procedure. You can't do that now and they are essentially 'hard-wired' together in going to the R/C network. But, with the 10 ohm resistors in place, that will permit this measurement to be done like in the SA-100. Hope this helps....... TSD
Click to expand...

Ya mean like dis??
attachment.php


Copy the shortcut after uploading the pic. Then copy to the post with
after. Now you have the full size pic showing.
 

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I plan to edit this post later to add the output tube schematic from the Fisher X-1000. The way the outputs are terminated are different. I'd love to know WHY it's different.
 
My unglued socket savers came in yesterday. To measure current through the cathode to ground I plan on breaking the connection from the top (socket) side of the saver to the bottom (pin) side and run two wires out to (stackable) banana plugs. I can either measure current directly through the DMM or across a precision 10 ohm resistor (why I'm using stackable plugs).
If you want to make your own, this is what you need:
http://www.tubesandmore.com/products/P-ST8-209MIPPCL
and
http://www.tubesandmore.com/products/P-SP8-476

The extended pins of the top will fit right into the hollow base for soldering. I'll put all but the cathode through. Two wires will come out of the unit from the now open cathode , one connected to the top, the other connected to the bottom.

Using two of these devices and two DMMS, I can measure the current of a pair of tubes and balance with the DC balance switch.

Really couldn't be simpler and I don't have to install test points on the SA300. Plus-I can use these on other amps.

I can also run wires out from the plate and grids if I want to measure voltage or current.

Any thoughts?
 
Heyraz,

I like your plan.......and actually have already purchased such devices in the past. As I think I mentioned, I have both single and dual versions of these 'bias' probes.....and have used them often on past projects. My only reason for wanting test points on the amp is that they will stay with the amp! The 'bias probe' is like a tool......a device that stays in the shop. By adding test points to the amp, it's kind of like Dynaco did by putting them into the octal connectors in the front. W/ DVMs being all of $4 or $5 these days...... it's not unreasonable to figure someone buying an amp might also have a volt meter. So, with test points, certain reasonable adjustments can be made in order to maintain that amp! But, if you didn't have them, you'd either have to get more involved and start probing the actual circuits, or have 'adapters' like yours......and a lot of people aren't that competent or savvy to be able to do that kind of thing. Hell, I couldn't a few years back!! Now I'm not much better! But, my personal preference is to build the test sites onto the amp, and minimize the amount of special considerations that have to be made....tools, knowledge, or otherwise. The 'K.I.S.S.' method if you will....... I think your bias probes are an essential tool to the 'working man's shop'. But, the test points are 'for the next guy'........ so to speak. WC
 
I would have put test points on the amp (like the SA-100) but couldn't figure out a safe way to do it.
With the SA100, all output tube cathodes are tied to a common ground (no cathode resistor and bypass as with the SA300). On the back of the stock amp are 3 screws jumpered together representing the cathode pairs from channel A on the right, the cathode pairs from channel B on the left and a center ground screw. Inside of the amp, each cathode pair is tied to the center screw through a 10 ohm resistor. But outside of the amp with the shorting straps in place, the cathode pairs go straight to ground. It's no big deal if the shorting straps are on or off because one way or another, it all goes to ground. My amps sit on a metal shelf where the metal can short any external screws. With the straps on or off, no big deal if a short occurred on the shelf.

The SA300 presented me with a problem...the cathode resistor and bypass. I needed a way to insert a 10ohm resistor between each tube's cathode and the cathode resistor (with bypass) and bring it out to 2 test points for each tube in order to measure voltage drop across each 10 ohm resistor. That represented 8 test points just for the cathodes. That's a lot of test points.

To me, the test socket seemed like a better all around solution. I can use it with this and other amps, I don't even need a resistor to measure voltage drop across as I can insert it directly into the circuit, and if I want, I can monitor plate voltage, screen voltage etc on this and any other amp using an octal socket.

It seemed like one of those one time investments that could address any issue that might possibly arise.

And if I sell the amp, no special instructions need to be included. Any future user can test the amp according the original instruction manual.
 
I'll have to make some changes to make the output look like the revised diagram#2 of Tom's in post #52.

What was confusing me is that the X-1000 does it differently.



In other news, I'll have to change the wiring back to jumper pin 1 & 8 on each tube and add in a 10 ohm resistor.

Here's the new switch from the outside and the inside.
The inside is incomplete. I've got to play a bit to see if adding the AC neutral lights the switch.

I'm not finding a lot of time to work on this during the week . . . a few stolen moments here and there.

I'll have to append this post with graphics. For some reason permissions on this machine won't allow me to upload photos.
 
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