This is by far the most ambitious project I have attempted.  My goal is to build a surround sound home theater system using all tube amps, and to do it without bankrupting myself in the process.  To pull this off I had to set a few guidelines that would keep this project under control:

• This project must cost no more than a respectable 5.1 receiver ($300), speakers ($500) and subwoofer ($200).  $1,000 total price tag.
• This must be fully compatible with a modern home theater experience: digital audio inputs, remote volume control and input switching, Dolby AC3, DTS and ideally ProLogic decoding.
• Using new iron would break the bank, so salvaged amps are needed.  There are plenty of cheap Chinese-made tube amps out there, but where's the mojo, and how long are those crappy components going to last?
• The amp designs should require as little modification as possible to suit the purpose.  Vintage amps should remain virtually stock to retain their character.
• The end product must be a visually-appealing showpiece without distracting from the home theater experience.

So there it is.  Not just tube hi-fi on a beer budget, but SIX channels of home theater goodness, including speakers, tubes and all, for under a grand?!?  Rest assured there won't be any Jensen gold-wired 300B monoblocks in this system!

The first thing I had to figure out was how to decode digital audio and provide analog preamp outs on the cheap.  There are a number of Chinese-manufactured DTS decoding units with full remote-controlled configurability sold under the brand names MOCHA and HLLY.  The MOCHA X-3B is the latest in the company's product line, and it is available with a US-compatible power transformer.  The HLLY / LEMING unit looks superb, but I have only been able to find it with a 220v/50hz input.  The X-3B is the winner, at $170 including shipping.

Second - what sort of vintage amps can I adapt as simple hifi power amps?  Thankfully this answer was very easy - Leslie speaker amps!  Many people gut the amps from Leslies in order to use them with other amp systems, such as guitar amps.  There's also a salvage industry for Hammond organ amps, oftentimes turning them into homebrew guitar amps.  Most Leslie salvages are looking for big dual-octal designs like the 122 and 147, but there are some overlooked gems out there.  It turns out Leslie developed their own standard monoblock design using a pair of 7189s (ruggedized EL84s) and a 12AU7 pre/PI.  This monoblock was used as a reverb channel in 122RV, 145RV, 147 and 251 (hereafter called "145"), as well as in a 4-channel configuration in the 100GK, 101, 205 and 610 Leslie speakers (hereafter called "610").  I picked up one of each, plus a full set of vintage RCA tubes, for under $200 including shipping!

So let's take a closer look at the amplifier's implementation.  It consists of a solid-state power supply section and each amp channel printed on its own PCB.  On some of the 4-channel models (100GK), each channel has its own input, and on others (205, 610) two of the channels' inputs are bridged together.  The difference is one 10K pot and the location of the input wire.  With that quick modification done we have four fully independent channels.  The only other thing that's different is channel 2's output transformer - it's bigger than the other three.  Channels 1, 3, 4, and the single-channel 145's OTs have a primary impedance of 6800 ohms, and channel 2's OT has a primary impedance of 8000 ohms.  Since it's the only oddball, it will drive the center channel.  The actual circuit, identical in every channel, is perfect for what I need - it has a low-impedance input with channel volume adjustment, a simple 10-mu-ish gain stage followed by a DC-coupled cathodyne PI, then a high-efficiency push-pull output section with negative feedback.  The result is a low-distortion amplifier that accepts line-level output voltages!  The only downside is that they're all designed for 16 ohm output impedance, and basically all home theater speakers are 8 ohm impedance.  It's a hurdle, but definitely not a showstopper.

The physical layout allows the 145 and 610 chassis to sit side-by-side with their tubes facing in the same direction with no plugs or extrusions along the front.  All of the 145's wiring and the power & input for the 610 sit inbetween the two amps.  The 610 uses a unique 9-pin Amphenol male plug for its input.  The speaker output is the same as a standard 7-pin tube socket, located on the rear of the 610 chassis.  A separate chassis box will be built to house the power cord, RCA input jacks and output binding posts, which will connect to the amps using adapter cables.  This allows me to keep the original amps intact, unmodified and on display, while the wiring is hidden elsewhere.

The 145 amp has a fixed AC cord, which will be plugged into the "Tremulant Motor Outlet" on the 610 to get its power.  It has a Leslie 2-pin for input and another one for output.  Below are the pinouts as I've wired them:

145 OUTPUT:

1. Output CH5 +
2. Output CH5 -

145 INPUT:

1. Input CH5
2. Sig GND

610 INPUT:

1. Sig GND
2. Input CH3 (was 125vDC, rewired to match 100GK/101 schematic)
3. Input CH4
4. Input CH2
5. Input CH1
6. Passthru (Output CH5+)
7. Passthru (Output CH5-)
8. AC
9. AC

610 OUTPUT:

1. GND
2. Output CH1
3. Output CH4
4. Output CH2
5. Output CH3
6. Passthru (Output CH5+)
7. Passthru (Output CH5-), tied to pin1 GND

To dress up the chassis I plan to overlay a single veneer board of wood across the top surface of the two chassis, making them look like a single amp.  Holes will be drilled for the tubes and volume controls, and dimples will be drilled in the bottom to hide the chassis mounting screws.  A glass wall with 100% mirror tint will hide the transformers and emphasize the glow of the tubes, kind of like this sweet-looking amp I found online: