The Stereo 400, Stereo 416, and Stereo 410

Stereo 400 with meters lit up by blue light kits
Stereo 400, meters lit by blue light kits

About the Stereo 400 and its Cousins

Phase Linear had the 700. Dynaco answered the challenge with the Stereo 400 power amplifier. It was conservatively rated at 200 WPC into eight Ohms. Massive heat-sinks even allowed it to pass the old FTC pre-conditioning tests. When it came out, that much power in an amplifier seemed scary to many people, particularly Dynaco. That's why they included the Dynaguard power limiting system...to keep speakers from going up in flames.

After its introduction, various options and variations came to be available, though they were based on the same basic design. (Some of this information is courtesy of posters on Audiokarma.org).

Stereo 400 Documentation

The first link is a very clean scan of the Stereo 400 Assembly manual.

The next links have three pictures of the large fold-out wiring diagram. It's too big to scan, and pretty tough to find online. Each picture is a large file, about 4 Mb each. If you zoom in, you can see all the detail you need to see.

Stereo 416

The Stereo 416 followed the 400. It was a beast with 8 output transistors per channel, or 16 output transistors in total, thus the "416". You could also order the C-100 capacitor bank (assembled only) to add 100,000 uF to the power supply. It had a two speed fan, and was rated down to 2 Ohm loads (450 Watts at 2 Ohms, single channel driven, 1% distortion). It had LED power metering with a meter range switch, and you could bypass both the Dynaguard and level controls. It was the no-holds barred power amp.

Tom P of California was kind enough to share a scan from his collection of a document that covers both the Stereo 416 and the PAT-5 Bifet.

Here's a scan of the Stereo 416 assembly manual, and of the large fold-out.

Stereo 410

The Stereo 410 was the basic 200 WPC amp. No Dynaguard, level controls, or meters, and a price advertised at "under a dollar per watt".

Once again, Tom P of California was kind enough to share a scan from his collection of a document that covers the Stereo 410.

This copy of the Stereo 410 Assembly and User's manual was formerly posted on Tubes4hifi's website.

Here's another version of the Stereo 410 manual (77 MB file size), from my collection.Stereo 410 Assembly and User's manual, DMJ collection.

Stereo 400 Secrets

There are two rather different versions of the Dynaguard board, but maddeningly, both are called PC-29. I found this out recently when I realized a boyhood ambition to own a Stereo 400. This one has the metering kit. The left channel Dynaguard light was permanently on. I redrew the Dynaguard schematic in LTSpice to better understand its working. Upon looking at the PC-29 in my Stereo 400, I quickly found that there were significant differences from the manual I had. That manual was purchased in a pilgrammage taken to Dynaco in Blackwood, NJ in the mid 1970's.

A quick trip to Greg Dunne's Dynaco Site site showed me a different PC-29 schematic. Contrast this with the PC-29 schematic in the manual that I have. Note: It's a large, high res file, at about 5 Megs, so will take a little while to download.

Speaker Relay

The speaker relay had grown intermittement. It's a large, beefy relay with a 48 Volt coil and big contacts rated at 15 Amps. Still, with age, I guess some oxidation had corrupted the contacts. The most blameless suggestion I could find was to clean it with laser printer paper soaked with 90% isopropyl alcohol. I pulled the alcohol soaked paper through the contacts on December 29, 2012. We'll see how long it lasts.

Replacement Speaker Relay Kit

You can now get a replacement speaker relay kit for the Stereo 400. It uses a pair of sealed relays with contacts rated at 16 Amps. This exceeds the original relay's current carrying capacity. We've named this the RY301 kit after its designation in Stereo 400 schematics. Here's the Stereo 400 Relay Kit Manual. Order the Stereo 400 Relay Kit.

C-100 Capacitor Bank

Large filter caps in the power supply hold up the power supply rails even when the going gets rough. The Stereo 400 and 416 were there at the birth of the "Super Amps" and the start of the microfarad wars. Manufacturers started throwing 10's of thousands of microfarads into their power supplies, all in the name of better bass. The C-100 capacitor bank was the ultimate answer to the microfarad wars. You could add a C-100 to a Stereo 400 or a 416.

The schematic shown below contains revisions and corrections pointed out to me by Jim Switzer in an email on 12/17/2016. I added a few more annotations that make the schematic a bit easier to analyze and understand. In the course of that, I've written a circuit description that contains a mystery, so the schematic might still have an error.

C-100 Capacitor Bank Schematic, corrected

Dynaguard Problem

One channel had continuous Dynaguard indication, although it wasn't actually limiting. Q3 on the PC-29 had gone bad. I replaced it with a 2N3904, and it worked fine. A quick check showed reasonable indication, but I think that Dynaguard is probably not much needed.

Meter Lights Are Out

The beautiful blue VU meters are illuminated by small incandescent bulbs powered by AC from low voltage taps on the transformer. Each meter has two bulbs. The right meter had one bulb that still worked. Both left meter bulbs were blown out. The low voltage transformer winding puts out 15 volts RMS (open circuit) between the grounded center-tap and the winding that drives the meter bulbs.

Closeups of Various Parts of a Stereo 400

Here we've just posted a number of pictures of a partially disassembled Stereo 400. This is one that I'm doing a bit of restoration on. The main cosmetic thing is that the meter lamps are pretty well gone. I think I might be able to put some white LED's in there. Or, I wonder...is this the place for a Blue Light Kit?

Stereo 400 with front panel folded down
Front Panel Folded Down

Stereo 400 with escutcheon plate removed
Front view, escutcheon plate removed

closeup of escutcheon plate
closeup of escutcheon plate

meter closeup, showing incandescent illumination lamps
Closeup of meter with bezel removed, showing illuminating lamps

Replacing the Incandescent Lamps with Blue Light Kits

We replaced the original incandescent lamps with blue light kits. The assembly method is a bit modified from the classic way we've done it in the case of Neon lamp replacement. We used a single 470 Ohm 1/4 Watt resistor with each blue light kit. We then used 1 blue light board to replace each of the incandescent lamps. This is shown in the next picture.

Stereo 400 with blue light kits
Stereo 400 Meter with blue light kits replacing incandescent lamps

The next picture shows the amp with the meters replaced, and the blue light kits installed in the meters. The room has normal daylight illumination.

Stereo 400 with meters lit up by blue light kits
Stereo 400, meters lit by blue light kits

The next picture shows the amp with the meters replaced, and the blue light kits installed in the meters in a dark room.

blue lights in dark room
Blue Light illuminated meters in a dark room

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