Hood Amp Bridge

[Strelok]

Hi all!


I am plannng to build the Hood Amp, it is in Tonnes of Tone.

Is it necessary to use the hefty diode bridge or can discrete diodes be used?



Warm regards,

Strelok

[K O'Connor]

Hi Guys

An integrated bridge is always easier to mount than individual diodes. Package types that bolt to the chassis provide secure attachment points for related wiring when mounted, plus, the chassis provides heat sinking. For a given current rating, the leads of modern bridge packages are thinner than for equal-rated single diodes.

For the Hood amp, you would want to use 3A diodes if singles, and these always have fat leads - a lot of terminal space when using solder-lug terminal strips. The integrated bridge is usually smaller.

Have fun

[Strelok]

Thanks for all the info!

So it is the current that does it.

I have that bridge, but not the version with the fancy terminals, just straight leads.
It looks safer to not bend the leads and hope it will hang on, but to use terminal strips hanging over it.
And so I did.








With a bit of luck I can wire the complete amp this way.

I am very curious about the sound. I have a very old transistor combo here and it sounds very nice clean.

Plus I think power tubes are pretty expensive and I guess they will be for a long time.

[Sherlok Ohms]

Hi Mr. Strelok

That is a very nice clean chassis.

I wonder why you would bury the bridge under the turret strips? I believe some would refer to this as a "service nightmare".

I have used that type of bridge in the past and you can solder wires to its lugs. Of course, the intended connection method uses 1/4" fast-on terminals that one crimps to insulated wire. These lugs are very common, widely available and very inexpensive. You can get them at any electronics parts store, or from the usual large distributors, or from places like Aliexpress.

To crimp the lugs onto wires, you can use pliers, or those cheapo wire cutter-crimper tools, or a dedicated crimper. All of these are again super affordable - a few Euros - what regular folk like us can afford

Such a clean chassis demands not starting on the wrong foot.

Cheerio

[Strelok]

No it is a stupid chassis.
It is just for testing.

Under because the heatsink is then bolted.
It is easy to service. If anything should be done with the bridge you can just unscrew the whole thing.
All the rest is reachable from above like it should be.

Never ask why something is done. People have different views.
How big is the chance other folk would approach it just the way you would?

This is the most stable solution.
Leads can always bend or break if they are hanging in the air.

I don't like your judgement "starting off at the wrong foot".
Everybody has their own way of doing things, let's celebrate that.

[makinrose]

No it is a stupid chassis.
It is just for testing.

Under because the heatsink is then bolted.
It is easy to service. If anything should be done with the bridge you can just unscrew the whole thing.
All the rest is reachable from above like it should be.

Never ask why something is done. People have different views.
How big is the chance other folk would approach it just the way you would?

This is the most stable solution.
Leads can always bend or break if they are hanging in the air.

I don't like your judgement "starting off at the wrong foot".
Everybody has their own way of doing things, let's celebrate that.

I know the reply that criticized the placement of the bridge could be read as untactful, but I think the point is that placing a component under a turret board that presumably will have more parts and leads running it may make it hard to access in the future.  I'm not sure it is important because solid-state rectifiers so rarely fail.  I will however say that having flying leads when done well is not problematic.  I have 1941 Gibson amp in my shop right now that was built entirely point to point. It's a pain to service but the build is very robust.

[nauta]

Hey Dudes

Can you describe how the bridge is attached tro the turret board, or show a different pic? It looks like there is bare wire coming up through the turret and this must be soldered to the bridge first.
I'm probably wrong thinking that soldering the turret could heat the bridge solder joint ??

I agree with sherlok that the chassis looks cool 8)
I don't think its stupid at all.

[Strelok]

Hell all you good people,

Thanks for the replies!

Nice things have been said about the chassis.
But it is too low, I cannot cover it.
And I cannot place the Power Transistors heatsink.

Plus there is a big hole on the side where the jacks are.


Ah well, I ordered a brand new chassis.

Here is another pick of the bridge placement:




Yes, maybe the thing will get too hot when soldering a turret.
I am using a TX giving 2 x 18 volts and the amp is running at 53 Volts, which is nice.



I did manage to build it on only two terminal strips:





So that was fun.


It is not working well though.
Low volume and when the input signal is lower you can hear a strange noise, af if you are tearing apart paper.

It could be a wiring issue.
And corrosion comes to mind like in another post. As if the signal has to struggle to come through.

It could also be that there was a slight meltdown in the diode bridge.

Or maybe soldered the socket of the Power Transistors for just a little too long.
Should have kept them out while soldering.

All in all easy things to check.

Bye all!

[K O'Connor]

Hi Guys

In the 1969 Wireless World article John Linsley-Hood laid out his circuit on perf-board, actually two channels as a stereo amp. The circuit is simple and has many flaws that make it less than desirable as a hifi amp, yet it still measured better than most tube amps of the day and that was the goal.

In a modern view of circuitry with regard to voltage feedback amplifiers versus current feedback amplifiers, one could argue this is the latter. The input stage is a single BJT with the input signal applied to the base - a high-impedance input. The feedback signal is applied to the emitter which is a low-impedance input. For some people, this defines the circuit as current feedback.

Another aspect of the circuit that could give the same definition is that the overall circuit can be thought of having two complementary devices connected as a loop. The driver Q2 and the outputs Q3,4 can be thought of as one device and effectively as NPN since Q2 dominates. The input Q1 is PNP, so it is a convoluted complementary-feedback pair (CFP), which is inherently a current feedback amplifier unto itself.

Why this talk of types of feedback? because CFPs and current feedback amplifiers in general tend to have very wide bandwidth. As such, they need to be laid out very carefully and usually much tighter than a turret strip or terminal strip allows. TOT (Tonnes of Tone) follows Hood's use of perf-board for the front end.

A month or so after the article appeared, John wrote an update, as many hobbyists had built the project and it turned out to be a bit unstable. He added a few RF traps to limit the bandwidth and this fixed things for tight layouts. The TOT project includes these.

In high school we built this amp and were given a standard Hammond chassis which was HUGE for a stereo version. I wanted to fit mine into a smaller decorative metal box that radio Shack (RIP) offered, and I knew also that real heat sinks would be needed, just as Hood used and were plainly visible in the photos of his prototype. The heats sinks also came from Radio Shack and I bent my own chassis to hold all the electronics.

The rest of the class followed the plan of spreading things out on the large chassis, usually with the power transformer centered. Some even glued the wire to the chassis after dressing the wires into neat parallel and squared paths! That is a complete mistake for an amp that runs as hot as this one does. Only a few students got their amps working who built it that way and the amps did not stay working for too long as the insulation on the wire quickly melted.

My amp worked fine and the heat sinks kept the output transistors within their heat limit. The teacher said "this amp looks like a GE (General Electric) amplifier", which to me was complimentary, but he meant it sarcastically. It was not as "pretty" as the other amps, but there is beauty in engineering especially when the result is functional.

So, regarding your results, Strelok, the basic layout on turrets is flawed from the beginning. Laying out a solid-state amp is nothing like laying out a tube amp inasmuch as "airy" is contrary to stability. For solid-state, you have to be careful about how the input and output signals are oriented to each other to avoid oscillation. Many individual sections within the circuit must be more tightly spaced than you have. Were you to scope the output of the amp, you might see very-high-frequency oscillation that you could likely cure with more RF traps.

As always, oscillation means the amp has gain - good - but its layout is less than optimal - bad.

I doubt any harm came to the power devices when you soldered the wiring to the sockets - although I have no idea what your soldering technique is like? The connections look good on the turret strip, so I assume this aspect of construction is "good".

As far as trying to stuff a heat sink inside the enclosure goes, remember that the class-A amp runs super hot. The PT is nominally rated at 72VA, so think of the heat as being 72W. Imagine having a 72W incandescent bulb inside the box. The larger the area for this heat to interface with the air the better.

I did not end up using the decorative enclosure for too long (even though I drilled a lot of air holes), and the amp ended up in a much more airy box where it was happy for a decade.

Yes, you see a lot of hifi and home theater amps (two separate things) built with internal heat sinks. These amps run class-AB at a very low idle heat. Most use does not create too much heat and the heat sinks are quite an efficient design. As long as the top and bottom of the amp is not obstructed AND there is air flow through the shelving or equipment cabinet the amp is placed in, then there will be no issue.

Have fun