Phil’s Old Radios

Phil’s Old Radios


CLICK THE TITLE: One subject of interest I have not written about here is vintage audio and radio restoration, but I’ve got some article ideas in the works. In the meantime, I’ll link you to one of my favorite reference sites. Phil’s Old Radios is entertaining to read and handy to get reference material on radio restoration projects. Phil tells the whole story so you can learn from his experience.

Push-Pull 6B4G Amplifier


This is a design I’ve built five times! I don’t own one myself, as I had other desires. This particular one I built for my father, and he wanted it set up so any rectifier tube, including a solid-state plugin, could be used. Again we have the standard 17″ x 10″ x 3″ hammertone-painted steel chassis, this time with Hammond 1645 output transformers and a surplus power transformer. My father insisted on painting the transformers I would have kept as black. The solid-state rectifier is the painted cylinder in the middle. The completed unit puts out 15 watts per channel.


This, unlike the single-ended 6B4G amp, is not a simple affair. This is a three stage circuit, using one section of the input 6SL7 as a paraphase phase inverter. The second stage is the driver, using the 6SN7, to achieve the high voltage swing the 6B4Gs require. The output stage is very straightforward, except for the connection of the cathode-bias circuitry. In order to set up each channel’s output tubes, two 6.3 volt center-tapped windings are required. The three stages are enclosed in a negative feedback loop, without any step circuits to prevent oscillation! This is possible because the 6B4G has a high input capacitance, acting as it’s own step circuit or filter. The circuit can still be unstable unless a sufficiently wideband output transformer is used. This design is a close copy of a design in the Acrosound catalog, and in this case, the Hammond transformers have a somewhat lower bandwidth. This made it necessary to carefully tune the feedback loop for stability. The end result is a solid performer with decent power, sounding unlike push-pull pentode designs. It does sound louder than the watts indicate. Anyone building this circuit should spare no expense on the output transformers.

ATMega8-Powered Peltier Controller


In this project a set of four Peltier coolers, also called TE Coolers, are to be controlled via USB. The power requirements for the coolers are high, +/- 12 volts at up to 7.5 amps each. The ATMega8 on this MPU schematic page is responsible for controlling duty cycle and current flow direction through an H-bridge circuit, which is on another schematic sheet. This control is accomplished via a set of commands issued to the ATMega8 from an application running on the host PC. There is also a thermistor and A/D converter connected to the ATMega8, which will report the temperature the TE coolers have achieved at the target. The completed unit will require firmware that is not too complex, but could grow to include a full PID (Proportional-Integral-Derivative) algorithm. This will be programmed in C via the AVRStudio IDE.

Series Regulated all-Tube Power Supply


The Kay model 397 power supply, a surplus store find. Full of some good parts, this actually works, and I’m considering how to use it in a  project as-is. It was originally used in a television-related test instrument, but was found all alone. I’ve replaced the power cord, built a new interconnect cable, and replaced some small capacitors underneath. This would have been part of a very expensive piece of equipment back in 1952 when it was built. I have a freshly drawn schematic and brief description for you:

Kay Power Supply_Bill

Here is the schematic of the Kay  Electric Company power supply. This circuit is typical of vacuum tube era regulated power supplies, with 6Y6 pass tubes, a 6SL7 “long-tail-pair” or current mirror voltage amplifier, 6J5 error amplifier, and VR150 150-volt reference tube. Note how this circuit is supplied by a floating 6.3VAC filament supply, while a separate transformer supplies 6.3VAC to the off-chassis circuitry. This second supply could be biased to eliminate hum in that circuit without exceeding heater-cathode voltage limits in this circuit. The output is adjustable via the potentiometer next to the 6J5, and with the components shown, the range of +250 to +300 is accurate. Changing the 82K resistors in series with that potentiometer would increase and/or alter the adjustment range. This power supply might be useful as a bench test instrument for trying amplifier circuit ideas, or to allow construction of finished items without each having an included power supply. What it does not include as-is is any sort of negative bias supply, although one could be added very easily.

Single-Ended 6B4G Amplifier

DSCF0473 - Best

This is my current favorite and latest build. This is the cure for any single-ended amplifier addiction. It is built on a 17″ x 10″ hammertone-painted steel chassis, and features Transcendar output transformers and an Edcor power transformer. This is the first directly-heated triode amplifier I built for myself, and had the time to wrap my ears around. It’s an amazing synergistic match to my sealed-enclosure speakers, and sounds outstanding at moderate listening levels.


This is as simple as such an amplifier can be, and I always believed shorter is sweeter when it comes to audio signal paths. The two 6SN7 triode sections are directly coupled, with one capacitor coupling to the output tube. All stages are cathode-biased and bypassed to ground. A worthy experiment might be to connect the output tube cathode bypass cap from the cathode up to B+, a connection often referred to as ‘ultrapath’. The power supply is very ordinary, but has a high inductance high current choke to keep hum down. The heaters and filaments are all supplied a rectified and filtered 6.3 VDC. Hum balance potentiometers are used across the output tube filaments so any hum there can be adjusted out at that point. See the global feedback? Nor do I! There isn’t any! This runs open-loop. The large output triode should help to keep output impedance down, but it will still be somewhat high, so damping will be poor. This requires the right sort of speakers to use effectively. When it does work out, it has punch, bounce, and puts you right there with those musicians. The bad news? This amplifier will reveal all of your poorly recorded music, and change your musical tastes to well recorded music.

Single-Ended 1626 “Darling” Amplifier


This is an amplifier type often referred to as the “Darling”. It uses a WWII-era oscillator triode, the 1626, as an output tube. This is all about finesse, as the power output of this tube is only 3/4 of a watt! This is built on a hammertone-finished steel chassis, and features volume control and headphone jack. This was built as a gift for my wife, and she uses it in her office with mini speakers, or headphones, and her IPod as source.


Here is the schematic for the 1626 SE amp. A three-stage CLCRC power supply filter was needed to reduce hum to inaudible for headphone use. A voltage doubler supplying DC to all filaments was used for the same reason. Otherwise a very normal audio signal path. The 6J4/8532 is an interesting tube to use here. It is one triode per tube, and often microphonic, meaning it picks up mechanical vibration and amplifies it. This means I had to try several before finding two good ones. The 8532 version of the 6J4 is usually better. Another feature of this design is that the two channels share the cathode resistors & bypass capacitors of each stage. Although output is an amazing 3/4 of a watt, it sounds very nice on headphones and speakers that are sufficiently efficient.

1626 6J4

Low Voltage Single-Ended Madness!


This little amplifier was inspired by a simple design found on the ‘net. I had found some Philco output transformers from a stereo record player that had tapped secondaries, which measured to be 2500:8 ohms or 4000:8 ohms. This opened up some interesting possibilities for using different output tubes in the same circuit but maintaining good impedance matching. If a 6Y6 is used, the cathode bias settles out at a current level that pulls B+ to 205V. Connecting the speakers to the 2500:8 taps gives the best impedance match. If a 6W6 is used, the current draw will settle out lower, pulling B+ down to 230V. This is where 4000:8 is the best match. The amp was built on a hammertone finish-painted steel chassis, with the power transformer under the diecast aluminum cover, and the output transformers under the chassis. This will output 3.5 watts per channel with 6Y6, and 3 watts per channel with 6W6.


Here is the schematic of the Madness, which is actually my first SE design. Overall similar to the previously posted 6Y6 amplifier, there are several important differences. This is connected as single-ended pentode, which will deliver more output but requires some sort of global feedback. It is also cathode-biased, which lowers output but simplifies design. The power supply is CRC filtered solid state, with a large R (680 ohms). This gives the power supply a lot of what is termed as “sag”, where the output voltage varies greatly with load. The interesting effect of this power supply is that the amplifier never ‘clips’ when overdriven, it simply compresses! This, like several of my projects, was a budget build that almost shouldn’t work out, but with a little tuning, it worked out beautifully.

Single-Ended Loktal amplifier


This interesting little amplifier started life intending to be a loktal version of the 6Y6 amplifier already posted. I did not get the results I was hoping for with it, so it was redesigned. Here you see the amplifier built on a Hammond powder coated black chassis. In front are two 7B4 tubes as voltage amplifiers, and behind are two 7A5 tubes as power output. The design will take 7B5 and 7C5 just as well. The transformer cover is actually a pre-painted diecast aluminum box also made by Hammond.


Here you see the design of the finished amplifier. The power supply is solid state with heavy filtering to keep hum from the b+ supply. The voltage supply to the 7B4 tubes and the output tube screen grid voltage supply are not stacked, allowing the 7B4 plate voltage to be higher. This gives some improvement in gain and linearity. The output tubes can be any of 7A5, 7B5, or 7C5. All three have the same base pinout. Although the three tubes have different operating characteristics, this circuit will allow their use. The output tube cathode bypass capacitors are not routed to ground, but rather to B+. This is a connection called “ultra-path”, and in theory should help isolate signal AC from the power supply, basically removing the power supply from the signal path. The output transformers were originally radio-grade 5000:8 ohm parts, but were replaced by higher-quality transformers from a vintage reel-to-reel tape deck. Because this is a single-ended pentode design, inverse feedback is unavoidable or the output impedance will be very high. This makes an amplifier sound funny, and that “funny” will vary depending on the speakers used! The feedback here was carefully tuned on my speakers by playing music and turning a potentiometer connected in the loop. The human ear is still the best test instrument for audio! As long as you haven’t played it too loud….

Push-Pull 6V6 Amplifier


This amplifier was a gift to my father-in-law for Christmas a few years back. It is a distributed load push-pull output topology called “Ultra-Linear” back in the day. That is driven by a direct-coupled split-load phase inverter, preceded by a single gain stage, all using the common 12AU7. With this circuit no preamplifier would be needed, as my father-in-law has a turntable with line-level output.


This schematic represents possibly the best bang-for-your-buck project I’ve done. Although parts were purchased new to ensure a “shiny & new” appearance, they were not expensive. This circuit contains all of the features that make tube amps what they are while minimizing inherent drawbacks.