Baby Plexi Build

Yeti

New project time!
(Don’t worry, the Splawn isn’t dead… it’s just big and scary).

I recently picked up this old unit.

It was originally the power amp for a radiogram, probably manufactured in the 50s. I spent some time looking into the company that made it (SMD) and they (or their founder) have a very interesting history - real pioneers in the radio field in SA and contributed extensively in WWII. Here’s a link to an obituary for anyone who is interested.

It sounds like SMD wasn’t building its own home electronics at this time (they were focused mostly on radio equipment) and this is likely to have done the heavy lifting for a Super Sonic radiogram, although I haven’t been able to find the exact model. I’ve also seen photos of an almost identical chassis (with very different circuit) from a GEC/Osram 912 from 1955.
Incidentally, SMD lives on today (although there have been a number of buy outs, splits and mergers as one might expect).
There seem to be two main companies, SMD ComNav who specialise in marine navigation and communications and SMD Technologies who brand and distribute consumer electronics.

… but you didn’t come here for a history lesson, let’s look at the amp.

First things first… this thing is solid, like really solid. You can see the military heritage coming through. The steel is at least 1mm thick and the transformer boxes are reinforced. It weight a ton.

The transformers themselves have been rubberised, all components are solidly fixed in place and the wires are well routed and neatly bundled.

The circuit itself is a relatively straight forward power amp. It has a single triode gain stage, followed by a cathodine phase inverter, into a pair of EL84s as an ultra-linear, class AB, push-pull output stage.
The power transformer is nice and beefy, able to supply plenty more than the EL84s call for (no sag here…). It has an enormous 10mH inductor for smoothing and a very interesting output transformer (I’ll come back to this in another post).
It also has 5Y3 a tube rectifier.

There are a few other interesting items.

An octal plug on the front that probably fed to the radio unit. It has pins for AC power, DC power, heater power and a return pin for the output signal.
A hum neutralising knob on the back, which balances the grounding of the heater supply.
A damping knob on the back, which appears to put a resistance in series with the speaker. At some point an inductor has been soldered across this pot, probably to mimic the reactive load of the speaker… This would have been a hidden control so I’m guessing it was used to tune the sound of the unit rather than control the volume.

So that’s what I’ve got… the next question is what to do with it (hint in the thread title 😛).

Yeti

There are two big advantages to building an amp on an existing chassis rather than doing a completely new build. The first is that it can be a lot cheaper than buying parts individually (especially the transformers… they’re expensive). The second is that you can often use significant portions of the old amp exactly as it is (transformers, power supply, power tubes, heater wiring, etc.).
… of course the downside of this approach is that you are limited to building something that works with the original components.

When this amp came up for sale and I noticed the EL84 output stage I immediately thought of a schematic for a 18W plexi with power scaling that I’ve been sitting on for a while.

Other than the power scaling it’s a nice, simple circuit: 1 pre-amp tube (two triodes), long tail phase inverter, cathode bias class AB power amp, solid state rectifier, no negative feedback, and the only controls are volume and tone.

The power scaling is a variable voltage regulator that allows you to adjust the voltage on the phase inverter and power tubes without changing the voltage on the pre-amp tube. In principle this should function as a master volume control with almost no effect on the tone and, from what I hear, they are pretty good.
I used this VVR to drop a little excess voltage on my blackface build and it worked well there.

So, I guess the big question is how well does this amp fit with this circuit?
To start with, the chassis is more than large enough for the job. It’s been designed around the same power amp, so the tube sockets are in place and the transformers are all correctly specced... and frankly that alone is enough.

So what’s different?

Well, the amp has a tube rectifier, I’ll need to decide whether to just use that as is or remove it and install silicone (it’s a lot easier to go this way than the inverse).
It doesn’t have a socket for the pre-amp tube but there is an RCA plug in the perfect place that I’ll be removing anyway, so that’s an easy change.
This amp was in the guts of the radiogram so I’ll need to create my own faceplate but there is a large open surface on the back and there aren’t many controls.
The amp has an inductor in the power supply, which isn’t called for in the circuit. Again, this is easy to bypass but since it’s there I might as well use it, in principle the only effect will be to reduce noise on the power supply, which is no bad thing.
… and then of course you get the rest of the circuit, which will have to come out… but that’s only to be expected.

There is a degree of unknown in the power transformer at this stage, since I’m not 100% sure what voltage it will produce. I still need to plug it in and measure.
… actually, on that note I’m making quite a big assumption that everything in the box works. Frankly, at this point I’m just assuming it does and holding thumbs.
One final note on the subject of compatibility. The output transformer is WAAAYYY more than this circuit calls for. From the sounds of things these amps had a reputation for good bass and this transformer is supposed to go as low as 30Hz without significant roll off. For context, most guitar amps go down to 80Hz. This is why it’s so big!
It also has taps on its primary windings to allow ultra-linear connection. Without going to too much detail here, the power to the screen grid of the power tubes is taken through these windings instead of directly through a separate resistor. This provides a type of negative feedback within the tube itself, which is great for keeping things nice and tidy and providing a large clean headroom… but isn’t exactly what you want in a plexi!

I briefly considered turning this into a bass amp instead to take advantage of the bass… or possibly try to design the loudest, cleanest amp I could but in the end I decided to go with the plexi because that is the amp that I actually want right now. 😛

modulator

Interesting. I once hauled an old radio onto the bench, planning to change it into a low-Watt guitar amplifier, but resisted the temptation. With this unit, I would in all probability "restore" it and use as intended.
That said, it should be an interesting project.

Yeti

modulator I did consider that but it's just a power amp, so would need an source that had already gone through a pre amp stage and I don't really have any use for that.

Yeti

The tubes have arrived so I guess we're on our way... I have to say though, EL84s are disappointingly small...

Yeti

Took some measurements last night. The transformer has three primary taps (200/210V, 220/230V & 240/250V). I'm not too interested in switching primary voltages, but it does give me some control over the secondary. At the 230V primary I measured last night I got secondary voltages of 710V, 650V and 600V and B+ DC voltages or 437V, 415V and 390V.

The design I'm looking at calls for a secondary voltage of 550V and DC voltage of 335V, so even on the lowest setting this transformer is a bit hot for the design. I'm not too worried about it though, for two reasons. 1) This SMD amp was designed for EL84s so the voltage shouldn't be too high. 2) The design includes a power scaling circuit and it is easy enough to include an additional resistor to fix the maximum value.

I'm only expecting to receive the bulk electrical components next week but I'll probably start stripping the old components this weekend. I also need to take some measurements on the output transformer so that I know which terminals relate to what speaker impedances.