Sony SS receiver's amp converted to tube amp

Someone gave me a broken Sony STR-D2020 receiver. All sorts of bells and whistles, but the SS audio power amp portion became too flakey to try to keep fixing. And I'm a tube fan anyway, so....

This receiver already has a big honkin' plus 65V and a minus 65V power supply, from a big centertapped power transformer. Easy enough to tie the negative supply rail to ground, and the positive side now becomes 130V. other parts of this receiver has their own transformer secondaries and thus independent power supplies, aside that the florescent display wants the +65V. So I moved that from the positive supply to the CT, which now looks like +65V. Changed the big filter caps, as the OEM ones didn't look so great (bulges on their tops). Now they are 1000uF for each portion of the bridge to center tap. Drop the new 130VDC thru a 130 ohm resistor and another filter cap to filter out hum. So what tubes to use? Well, there's no high current low voltage secondary on this transformer, and no real room for a filament transformer. But if I go for low audio power, around 1-1/4 WPC, some options open up. I have lots of 50C5s from my AM tube radio sub-hobby, and combined with a 12AT7 twin triode, makes for 112V worth of heaters, in series. All these tubes have the same heater current, 150ma. So that matches up well with the 130V (add a dropping resistor to drop the 130V to 112V). Okay, I need a pair of output transformers. Check the web, Most only single ended OTs are either too big to physically fit or have no bass. I did find good ones that would fit and have bass, though at lower but enough power, on that auction site everyone knows. A pair of OPTs made by Transcendar Transformer co, model TT-10-OT.

They spec out as (copied from the auction listing):

3k Audio Output Transformers for SE amp, 8 ohm
output transformers for a Single Ended Tube Amp.
"These are high quality, excellent sounding Audio Transformers". Made in USA
These transformers may be used for single ended amplifiers with a variety of tubes including
but not limited to 300B, 2A3, 6A3, 6B4G, 6L6, 807, 5881, 6550, 6V6, EL84, and 6AQ5
The smaller size of these transformers along with the single 8 ohm secondary tap allows for
extended high frequency range.
Frequency response is 20 Hz to 100 kHz 1 db at 1 watt, tested on Hewlett Packard audio analyzer.
Each transformer has the insulation Hipot tested to 3000 Volts DC.
Constructed with high grade M6 grain oriented silicon steel laminations for excellent 
low frequency response and low distortion.
Primary and secondary interleaved for excellent high frequency response.
Core has an air gap for single ended operation.
Primary impedance is 3000 Ohm (3K) , Primary current 100 mA, DC resistance is 300 ohms, 20 Henrys
Secondary is 8 ohms and is conservatively rated for 10 watts continuous.
Lead wires are 18 gauge Teflon insulated, type E, with silver plated copper conductor.
Overall dimensions for each transformer is 3-7/8" x 3" x 2-1/2" tall
Each transformer weighs 4 pounds.

Below is the schematic diagram:

The 50C5 is a tube commonly used in old AM tube table radios. Its load resistance is rated as 2.5K. The 3K OPT if the speaker load is 8 ohm. The "dummy load" 47 ohm resistor (connected inside the amp, to reduce the possibility of damage due to no load on the amp) would reduce that a little. The left-over mismatch is not at all critical, and can be ignored. The 12AT7 driver is just a typical circuit pulled from the tables in the back of the RCA tube manual. No feedback other than that you'd get from an unbypassed cathode resistor on the output tube. The amp has about -42dB of 2nd harmonic, and the 3rd is down at about 72dB, as you can see in this below plot:

Plotting the frequency response was a little difficult in that I had to "fake it" with one computer with cooledit playing a linear frequency chirp feeding the amp, and a 2nd computer capturing with cooledit the speaker output. Ended up with linear/linear plots, where I'd prefer log/log plots. But I did it with 3 passes, all starting at 20Hz, and one ending at 200Hz, another at 2KHz, and the 3rd ending at 20KHz. The X axis is a linear plot of output voltage seen across the speakers. The speakers are 3 ways, made by "Audio Electronics Systems" model 310. Yeah, not one of your famous makes. Mentioned as speakers are not usually even with respect to frequency loading, so some of that will show up in these plots. And on the off chance that some of you might know these speakers, but I doubt it. As it is, the plots (via eyeball estimate) show that it deviates about 2.5 dB from flatness from 20Hz to about 17KHz.

Now into just an 8 ohm resistor (no strange bumps, just smooth ones, now about 1.7dB from flatness. This is a little higher than the claimed flatness of the transformers themselves, but this is probably due to the above mentioned 2.5K - 3K plate load mismatch.

Using a pair of 50EH5s would be an exact match, as they are designed for a 3K load. These tubes need cathode resistors of 62Ω instead of 150Ω the 50C6s needed. I paralleled 120Ω resistors on the existing 150Ω resistors to get 66.6Ω, close enough. I then tried the 50EH5s, they sound fine.

What it looks like inside (I used an old thick piece of "glass-phonolic" board as the amp "chassis", usual point-to-point wiring used underneath):

In this receiver I also installed some tube cathode followers

One other thing that would need to be modified: The speaker switch. SS amplifiers don't mind no-load condition, but hate shorted outputs. Tube amps don't mind shorted outputs, but hate no-load conditions. The speaker switch on this Sony receiver can select speaker set A, another B, none "off", and both "A+B". When both are selected, both sets are connected in series. I need to change this to place both sets of speakers in parallel, and to place a pair of dummy loads of 8Ω power resistors when none "off" is selected. There's a headphone jack that is always active, so one might want to listen via headphones without speakers. I also added a pair of 39Ω resistors that are always across the outputs to protect against missing speakers being selected. You'll have to do some rewiring of this switch, which doesn't require any special handling beyond good soldering work.

Some snake oil that might have some basis in fact: To reduce hum in the enviroment of your hi-fi setup, have an electrician install "BX" housewire to feed the system. "BX" cable looks like microphone cable on steroids, twisted pair inside a conductive shield. This would keep the hum inside the cable, like mic cable keeps outside hum from getting in. Okay...

Another AA5 radio mod, to run off low voltage AC

Yet another radio mod, this one I doubt anyone would replicate. At my mom's house there is this 8/16VAC @ 5A power bus, using the old 1/4 inch Jones plugs. Why these voltages? It dates back from the time we had a model railroad setup, and the lights in it used 8V. I have a few 4BE6's and 4AV6's, and a small 17V centertap transformer I could wire backwards to get the plate voltage. The rectifier became made of sand, and the 50C5 became a submini 5902. And the 12BA6 changed to a 6BA6, and a small 2 ohm heater dropping resistor. The B+ came out a little low, around 90VDC, but the radio still works well.