Yes, those coupling caps have been sitting in my Mouser shopping cart for a week. I haven't finished adding everything I need, so I haven't checked out yet. The ones that come with the kit now are generic yellow caps that are relatively small. I won't be using those. I did find a miniature rocker power switch at Mouser that looks like it will fit on the front panel, so that would be great.
As for which resistors actually generate heat in this amp (see schematic in post #1) I have no idea. Anyone who does know, please share your knowledge.
The ones that are going to get absolutely roasted by the output tubes are R8 and R9, situated directly between the output tubes. Those two are 470k Ohm 1/2 watt, so I can't imagine that they will generate much heat on their own? What is a proper technical name for those resistors in this circuit?
R7 is sitting right against one of the output tubes, and R6 is right against the driver tube. Since these are rated 1 watt, I assume they will be generating some heat? They are the only ones rated 1 watt.
R5, R10, and R1 sit right against the driver tube, and the pins of that socket are soldered to the PCB so that whole area is going to be hot.
C6 is mica, so it may not matter, but it's easily tucked underneath. C3 is going to get roasted too, sticking way up between the output tubes and the driver tube. I don't see why it can't be safely tucked underneath. I looked up the ones that came with the kit online, and they are a polypropylene cap from Illinois Capacitor.
Here is what the PCB looks like (the stuffed board is not mine):
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Why not mount all components of concern underneath? You can space from heat sources by leaving leads lil long and bending away.
If chassis not open underneath, then offset PCB w/ spacers
Yes, everything will be fine whatever I do. The amplifier has a proven track record. The kit has a proven track record. However, I always think that very few things in life leave no room for improvement. Many of those things cost little or nothing. So, it's worth looking at all options in my opinion. BTW, I consider time free since this is a hobby.
I don't know if anyone has ever considered that. I'll have to check the spacing to see if there is any room between the top of the output tubes and the tube cage, which I won't use, but which future owners may. I suspect there is very little room, but I'll check.
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R8 and R9 are often called "grid leak" resistors (kinda funny, really) and ideally have no DC voltage across themselves, so dissipate no power. In general all the resistors on the board will be absorbing more heat than they generate, but the overall plan has worked for 60+ years, so probably not an angsty issue.
C6 and C7 are micas, perfectly appropriate, but if you wanted to change them to polystyrene film, temperatures could become an issue. Solder leads only with heatsinks between the solder joint and the body, and consider ambient temperature and radiant heat in operation.
All good fortune,
Chris
C6 and C7 are micas, perfectly appropriate, but if you wanted to change them to polystyrene film, temperatures could become an issue. Solder leads only with heatsinks between the solder joint and the body, and consider ambient temperature and radiant heat in operation.
All good fortune,
Chris
'An Ultra-Linear Amplifier', the original article in Audio magazine-
https://worldradiohistory.com/Archive-All-Audio/Archive-Audio/50s/Audio-1951-Nov.pdf
on Page 15, Hafler & Keroes were in the transformer business so this was a natural path for theim to follow.
Hafler attached the name 'Ultra Linear' to his amplifier much as Chrysler managed to hijack 'Hemi' for many of their engines.
Hafler continued on & was very successful. Keroes disappeared. The advent of high 'G' power pentodes made
amplifiers of high power but one less stage possible. 👍
https://worldradiohistory.com/Archive-All-Audio/Archive-Audio/50s/Audio-1951-Nov.pdf
on Page 15, Hafler & Keroes were in the transformer business so this was a natural path for theim to follow.
Hafler attached the name 'Ultra Linear' to his amplifier much as Chrysler managed to hijack 'Hemi' for many of their engines.
Hafler continued on & was very successful. Keroes disappeared. The advent of high 'G' power pentodes made
amplifiers of high power but one less stage possible. 👍
If your manual is like the one below, then looks like PCB mounts underneath the open chassis (pgs 12, 13)
So you can locate components top or bottom of PCB
JIm
So you can locate components top or bottom of PCB
JIm
Yes, it does. For some reason I thought the PCBs mounted from the top, but they are mounted from below.
This is important, as some traces could short to the chassis if the board is mounted on top instead of underneath.
I am almost finished with the chassis modifications (cutting and a lot of filing), triple binding post fabrication, adding a front panel rocker power switch, and stuffing my filter cap can with bigger caps (120 uF x 3) - finally.
I'll move on to wiring soon. As I go through the directions and the wiring diagram, I notice multiple different ground points (four I think) to the chassis. Is it worth the little extra time and effort to implement "star" grounding instead? The bias control upgrade adds yet another ground point. I am using insulated input jacks, so those will require their own ground wires (not pictured).
I'll move on to wiring soon. As I go through the directions and the wiring diagram, I notice multiple different ground points (four I think) to the chassis. Is it worth the little extra time and effort to implement "star" grounding instead? The bias control upgrade adds yet another ground point. I am using insulated input jacks, so those will require their own ground wires (not pictured).
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There are the two added filament "virtual center tap" resistor grounds,
the original several wires and parts grounding on the quad cap, and the jumper from the quad cap
to the terminal strip ground lug (to ensure a good chassis connection).
Don't change any of these.
the original several wires and parts grounding on the quad cap, and the jumper from the quad cap
to the terminal strip ground lug (to ensure a good chassis connection).
Don't change any of these.
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I guess I'm curious as to whether there are any benefits to "star" grounding everything to a single point vs. multiple ground points, or is it just snake oil?
There isn't really such a thing as star grounding, since you can't connect a bunch of wires or traces to one exact point.
Each sub-circuit should have a local common reference "ground" point, and then those points are interconnected
in a way that minimizes noise coupling from the"noisy" grounds, like the filter capacitor just after the rectifier,
to the more sensitive points of the circuitry, like the first voltage amplifier stage. See your ST-35 layout for an example.
The inductance of the common return connections should also be low, not just the resistance.
Each sub-circuit should have a local common reference "ground" point, and then those points are interconnected
in a way that minimizes noise coupling from the"noisy" grounds, like the filter capacitor just after the rectifier,
to the more sensitive points of the circuitry, like the first voltage amplifier stage. See your ST-35 layout for an example.
The inductance of the common return connections should also be low, not just the resistance.
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Unlikely you can make it better, easy to introduce hum and stability problems by changing grounding. As long as the largest noise sources (power supply ripple current and heater current) are kept out of the chassis and away from low-level circuits, other circuits can be grounded by the shortest path.
The original ST-35 only had a 2-wire power cord. How was that ever safe?
Seems to me like modern "best practice" would be for every ground point for the amplifier to be run to one place on the chassis, namely the power supply filter cap, not 4 or 5 different spots. Grounding has always been sort of a mystery to me.
Like I said, grounding has always been sort of a mystery to me. This list of 11 items leaves me scratching my head. 🤔
Seems to me like modern "best practice" would be for every ground point for the amplifier to be run to one place on the chassis, namely the power supply filter cap, not 4 or 5 different spots. Grounding has always been sort of a mystery to me.
- Left input jack shield
- Right input jack shield
- Left PCB ground wire
- Right PCB ground wire
- Left "virtual center tap" resistors (filaments)
- Right "virtual center tap" resistors (filaments)
- Power supply filter cap (a triple cap with one shared ground wire in my case, not grounded to the chassis or cap can)
- Bias PCB ground wire
- Left speaker common
- Right speaker common
- AC power cord Earth ground
Like I said, grounding has always been sort of a mystery to me. This list of 11 items leaves me scratching my head. 🤔
https://www.xppower.com/resources/blog/iec-protection-classes-for-power-supplies
Actually a single point "ground" is the worst case practice. The very highest performance electronic systems use a
ground plane (and power planes), when each component "ground" lead goes directly to the plane, and is very short.
The return image currents flow through the plane, each seeking their own path of lowest impedance.
Power and ground planes have a low impedance independent of their absolute size, similar to a transmission line
having a characteristic impedance independent of its absolute length.
It's not such a mystery, if you avoid using the word "ground", instead it's a 0V reference with a low impedance.
Actually a single point "ground" is the worst case practice. The very highest performance electronic systems use a
ground plane (and power planes), when each component "ground" lead goes directly to the plane, and is very short.
The return image currents flow through the plane, each seeking their own path of lowest impedance.
Power and ground planes have a low impedance independent of their absolute size, similar to a transmission line
having a characteristic impedance independent of its absolute length.
It's not such a mystery, if you avoid using the word "ground", instead it's a 0V reference with a low impedance.
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If it ain't broke, don't fix it. I try to keep that in mind when building proven kits like the ST-35, or building from scratch using a proven design. You go changing things and you could wind up opening a whole can of worms. There are usually a few simple tweaks that can help. Like adding a choke to the ST-35. It helps to reduce that last little bit of AC ripple.
But there can be pro's and con's to any modification.
But there can be pro's and con's to any modification.