Building Fisher SA-100 Clone

[Dave451]

I got the itch to build again and, with than in mind, have been watching for something that caught my eye. Well, my eye found Dave Gillespie's threads on re-constituting the legendary Fisher SA-100 EL-84-based power amplifier with EFB (tm), so I took the plunge and acquired a pair of Heathkit/Stancor AA-100 output transformers and started around those. Most of my gear is 7591 based, but I particularly enjoy my Scott 299B using 7189's and was impressed with Dave's EFB mods for my Dynaco SCA-35, so EL-84 topology it is!

I decided to build on a Hammond black powder-coated steel chassis and have been busy planning, measuring, and drilling and cutting (metal bits everywhere!). The parts have been attached now and wiring has commenced.

I'm basically building using Dave's schematics and photos from his threads and very little creativity is involved on my part. I was so impressed with the look and performance of the clone that I basically want to duplicate it, with just a couple of minor departures so far (I'm installing a muting switch on the inputs and may ultimately add an input level control in its place. Used an old-timey looking pilot lamp because I liked the look).

I tested the output transformers before beginning, of course. DC resistance measurements showed that all the windings were intact. Measured primary impedance was virtually identical for both at 7600 ohms at the 16 ohm tap at 1 kHz. The wiring is, of course, stiff but the insulation is in good shape on the wires. A few have to be extended for the build.

Dave recycled a power transformer from a Fisher X-101ST and, as it had no lower-voltage tap he adapted EFB accordingly. Having no extra PT's around, I opted for a new Hammond transformer (273BX) and decided just to duplicate Dave's circuit because the results were so good and I didn't want to get too far afield from it, even though this Hammond has a lower-voltage tap that could be used for bias. I'm not as intrepid at this point as kward and others in my builds yet. Discretion is the better part of valor at this point, I think.

With the 700V CT PT, I'm using a 5H 300 mA Hammond choke (193L). PSUD II shows that this should work out fine for the voltages needed using the same filter cap set and the 5AR4/GZ-34, so I did drift off a little there.

So, forwarding a couple shots of the build so far. Initial impression: this thing is heavy! Lots of iron mounted on the steel chassis. I worried a little about eddy currents in the steel chassis (Dave used aluminum, I believe), but I wanted the stiffness of a steel chassis. I did not raise the PT off the chassis as a preventive, but if this turns out to be an issue I'll use phenolic to offset the PT from the chassis.

Couple of pictures:

1. Two veterans of the audio world: AA-100 output transformers before testing:


Chassis populated with hardware underneath:



3. Chassis--top view front



4. Chassis--top view, rear:



I'll forward more as wiring progresses and will no doubt ask for advice in the (inevitable) event of problems.
Dave

 

[RanDibble]

Following with GREAT interest !

 

[bhamham]

Thank you Dave!!!

Looking forward to your build.
Bruce

 

[RS Steve]

Looks great!

 

[AlTinkster92]

Looking good! Watching as you make progress!

 

[Dswankey]

Nice, clean work so far looking forward to the completed build.

 

[Dave451]

Here's after the first full day of wiring. It won't be a masterpiece like Dave's build, but I'm shooting for 'workmanlike' on this one. My good wire (600/1000V) is stranded and it doesn't behave quite as well as solid core. The heaters are all wired, as is the AC wiring, the 5AR4 circuit, most of the ground bus, and nearly all of the output tube wiring. Following along with Dave's work is an education itself in good layout, particularly the ground system which is basically a bus bar system with the main nexus being the #14 wire between the filter cap negatives with the PT 700V center tap at one end. That's enough for today !
Dave

 

[dcgillespie]

Nice work Dave! I continue to use mine near daily, so I think as a new 6BQ5 family addition to your line up, you'll enjoy it immensely!

Dave

 

[fred soop]

.....
My good wire (600/1000V) is stranded and it doesn't behave quite as well as solid core.
.....

Strip the insulation from the end, twist the strands tightly, then hit it lightly with a bit of solder before making a connection. No big blobs, just enough that you can see it wick into the wire strands. Then the end will behave much like solid wire. I will often strip a bit of extra insulation, then cut the end of the ire where the solder tends to blob.

 

[Dave451]

Thanks, Fred. I do strip and tin every pretty much every connection with stranded wire. What I meant was the way stranded wire can't be precisely bent like solid conductor wire, so it tends to go more where it wants to instead of being amenable to nice straight runs.

Appreciate all the encouragement, everyone. Dave, can't wait to hear what this one sounds like once I get it going. I'll do some measurement as well and see if this one's up to your original builds standards.

I finished wiring the output transformers and NFB loops today (other activities took away some bench time, as they always do). I'll hit it some more tomorrow!
Dave

 

[Dave451]

More progress on the wiring. Getting well along now and finished up around the output tubes, the 6FQ7 phase inverter stages, and the input wiring around the 12AX7/ECC83 input stage. Keeping the left and right (A&B) channels straight requires some thought to be sure you're using the correct sides of the 12AX7 and 6FQ7 tubes, since each tube serves both channels. For example, you wouldn't want to cross the NFB circuits from the 4 ohm tap of the output transformer to the wrong input side! I ended up reversing L and R in my unit compared to Dave's original build, so the 12AX7 is a little 'cross-wired' and I reversed the 6FQ7 socket to avoid same for the phase inverter.

Here is a pic of the wiring around the phase inverter/power amp stages with the output transformer secondary wiring complete. My couplers were significantly larger than those Dave used, but of course I was able to fit in the IC MPW caps. I have always liked these, particularly in push-pull service, so we'll see how those sound in this amplifier.

DAVE G (if you're looking): I used the same values as you (from the schematic) for the R-C step networks, noting that the values are different for each channel. The schematic says these values were determined by testing, so I suspect achieving proper HF stability was the objective. These values may be specific to the circuit components and the output transformers (especially), but I didn't know any reason to make them different until I test the amp. What should I be looking for to finalize these values--minimize ringing on 10 kHz square wave output? Will these values work with the transformers I have, or can you know until you get it on the scope and do some square wave testing? Any thoughts appreciated by Dave or anyone else who wants to weigh in.

Here's a shot of the output and PI wiring:



Here's the input wiring and circuitry around the 12AX7 first stage. It's pretty busy, and you'll see that I wired in a muting switch to the left of the input RCA jacks in this view. When switched in, it grounds the inputs through a 1K resistor installed in line with the inputs. Hopefully this will work OK without being a hum magnet, but we'll see. Any thoughts or feedback on this welcome. (PS I did solder the 215V plate supply input through the 220K resistors to the 12AX7, though it's not done in this shot).



Last wiring is to complete the EFB II circuits, then it's time for checkout and initial power-up. Still need to install the bias regulator and MOSFET screen regulator and associated components.

Dave

 

[dcgillespie]

Dave -- More than anything else, it's the specific OPTs used that determine the values of the step networks. For want of anything better, I'd take an average of the Cs and Rs used and install that in both channels until testing can anything different for your particular transformers.

Looks good!

Dave

 

[Dave451]

Thanks, Dave, for your response and the support. I'll ponder the step network component value approach and ultimately I'll let the measurement process guide me on values to ultimately choose for these output transformers. I'm excited to get this unit going and have thoroughly enjoyed the build so far. Putting something coherent together from a pile of parts is pretty satisfying (fingers crossed everything works!).

One thing I've noticed is that the signal grounds are pretty much isolated from the chassis, with the grounds running separately back to the filter cap bus bar from around the chassis--sort of a 'bus bar/star' system. At no point I can discern is the signal ground run to or taken from the chassis. This being the case, would there be an issue using a three-prong AC power inlet with the green wire grounded/bonded to the chassis for Class I service rating for the unit without the issue of ground loops when connected to other equipment (eg. pre-amp)?

After completing the EFB II wiring, I'll police up the wire runs a bit and then go about a triple check of connections and consistency with the schematic and proceed with a slow power-up.

 

[Dave451]

Well, the wiring's finished. If there was any ambiguity between what I saw on Dave's originally posted pictures and the schematic, the schematic won. Hopefully I didn't zap the MOSFET and 337 regulator installing them.

I checked each run against the schematics from the 2012 thread by marking off the wiring on a hard copy of the schematic for the power supply and separate sheets for the right and left channel audio. Then, I went over every connection with a strong light looking for bridges, shorts, errors, and poor/no soldering. Found three joints without solder and fixed those. So, feeling pretty good about how things look, but I'm going to wait until tomorrow to do initial smoke/voltage checks after a pretty long day finishing up the wiring.

Dave

 

[thornev]

Wow, Dave. You seem to have found a way to minimize the wiring. Good luck. Thorne

 

[Dave451]

Thanks, Thorne. Bit of an optical illusion, maybe: I'm building on a large chassis at 17" x 10" x 3", so maybe it doesn't look so crowded. Also, Dave G simplified the circuit somewhat compared to the original SA-100 by eliminating the center audio channel and associated circuitry. On the other hand, Dave built in adjustment potentiometers for screen voltage and EFB range that I duplicated, but wouldn't necessarily be added in new builds where these values would be developed and fixed in the design. All in all, though, it's a fairly simple circuit with not much in the signal path beyond the essential.

 

[thornev]

Is there some way you can explain how you keep the signal path and power (DC) path separated? And what components do you use to send the signal down one path and the power down another path? Obviously they originate from different places, but I imagine at some point they merge into the same circuits and then are separated again somewhere. This is the design factor that fascinates me.

 

[Dave451]

Not sure I'm following your question exactly, but know that I'm just following Dave G's basic layout for the amp which adheres to good practice for tube amplifier layout. A basic need is to keep the power supply components located away from the signal processing as much as possible (especially the input circuits). As you can see on the picture, the power supply components are on the right side and the signal components are on the left side of the chassis. This is done to isolate electrical noise of AC power and rectified, but unfinished (unfiltered or partly filtered) DC, the noise being hum and buzz. The laminates of the power and output transformers are oriented at 90 degree angles to prevent inductive coupling and noise transmission between these components.

The purpose of the power supply is to generate proper voltages to run the heaters of the tubes, bias the output tubes, and to make different levels of filtered B+ DC voltage to power the amplifier circuits (generally at the plates of the tubes). Dave's design here also provides regulation for the output tube bias and screen supplies in the form of EFB II (tm) circuitry to manage the bias and B+ for improved operation (see Dave's Tronola articles on EFB). Those are managed by the two solid state components you see (the MOSFET and LM-337 regulator) seen mounted in the center and on the back panel of the chassis).

The power circuits necessarily connect to the amplifier circuits for each of these functions to power and operate the amp by wires (generally red; yellow for the output tube screen supply) running from take off points in the power supply to where power is needed throughout the amp at differing voltages, depending on the circuit/tube. Another critical connection is the signal ground system which provides a return for the DC voltages, generally the black wires running through the amp to grounding points for the circuits. The ground system layout is particularly important to keep noise out of the signal path and I think Dave's layout here is excellent IMHO. There really aren't any 'special compoents' making these connections, just the kind of practice that has evolved over 100 years for tube circuits.

For more on how all this is done properly, I highly recommend Morgan Jones' books, "Valve Amplifiers" and "Building Valve Amplifiers."

 

[Dave451]

[Now we'll see if I've done all that properly and the magic smoke stays in and sound comes out ]

 

[thornev]

Dave - You sort of answered my question. If I follow you, the DC voltage circuitry is kept separate from the signal circuitry. But looking at my schematics, I don't see that. Or at least I can't follow it. From studying my 299 schematic, after the power comes through some resistors, it goes eventually to the 12AX7 tubes and the signal that comes in through, say a PHONO input, also goes to the same 12AX7 tube. Am I reading the schematic incorrectly? I know 12AX7's have 2 sides. Does one side support the signal and the other side the DC power? This is what I mean about trying to follow the signal and DC circuits. They seem to converge and at some point, they must be diverging, I don't know where. Am I completely off base with this analysis?