AM radio performance vs power supply - Page 1
Forum section: electronics / projects

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: May 13, 2025, 03:31:45 pm
content: Hello!This has been a burning question that I have had for a while. My brother is into working on late 1920s battery radios, they are incredibly simple but powering them is a bit of a pain because they were designed to run on a stack of 22.5V batteries, totaling 90V. To get around this he bought an ARBE-III battery eliminator, which provides the 90V, filament current, and a -4.5V bias. I have built a couple of other equivalent power supplies in the past and he swears that his radios perform better on the ARBE. He said that my power supplies would even sometimes cause the radio to "scream" or oscillate.So my question is why is this happening? My power supplies used a basic linear design, some discrete some integrated. I used one big transformer with a bunch of taps for the 3 different parts. I imagine that the ARBE is similar, but I have not opened it up since it has tamper proof screws on it.

author: floobydust
author_rank: Super Contributor
timestamp: May 13, 2025, 04:56:27 pm
content: TheARBE-III_REV3I don't see a schematic for but it looks plain and the usual dozen zener diodes there. Chassis PE grounded, so there is another big coupling not seen with a stand alone using just batteries.You might have oddball stray capacitance between the outputs (causing oscillations) or need better capacitance for RF decoupling.Lost knowledge is that rectifier diodes make switching hash in the AM band (slow diode recovery+ transformer leakage inductance), so you need snubbers for DX.Kinda need to see your schematic to offer solid advice.Another thread about a battery eliminatorhttps://www.eevblog.com/forum/beginners/question-about-zener-diodes-in-series

author: Benta
author_rank: Super Contributor
timestamp: May 13, 2025, 11:52:59 pm
content: As long as you don't show your "equivalent power supplies" (schematics), it's impossible to answer your question.

author: NiHaoMike
author_rank: Super Contributor
timestamp: May 14, 2025, 04:23:51 am
content: Perhaps some radios expect the high voltage battery to have a relatively high internal resistance while most power supply designs try to get the output impedance as low as possible?

author: floobydust
author_rank: Super Contributor
timestamp: May 14, 2025, 04:50:56 am
content: One factor is antique radios got "cute" with the circuits, and they were stone simple.They (rightly) assumed the three battery supplies were completely independent and electrically quiet - no hum, ripple, RF or stray capacitance between them etc.But these mains "battery eliminators" have capacitive-coupling and common-mode noise between channels. They lack bypass caps, as well as the power transformer has inter-winding capacitance which no old radio wants lol. These details are not really considered in their design.edit: OP's brother likely running regenerative radios? They of course squawk if dialed in with too much regen.

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: May 16, 2025, 01:46:56 am
content: I found an old incomplete schematic, I couldn't find the original, but this should have all the right stuff in question. Note the different "GND" flags, each supply has its own floating reference, probably not best practice, but it is what I did.I have never actually used any of his radios so I cannot first hand compare the performance of my power supply vs the ARBE. I believe floobydust is correct, most of his radios are regenerative type with five tubes. They all have three RF transformers and two AF transformers (if you could call them that).

author: ftg
author_rank: Frequent Contributor
timestamp: May 16, 2025, 07:27:33 am
content: Are you sure they are regens?Five tubes sounds a lot ofr a simple regenerative receiver. While five tubes sounds more like a battery powered variant of an All American Five.https://en.wikipedia.org/wiki/All_American_Five

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: May 16, 2025, 06:09:45 pm
content: I don't really know what is regenerative and what is not.I attached a schematic for a radio that my brother was working on recently, the majority of what he works on is very similar to this, most have 5 tubes some have 6.

author: radiolistener
author_rank: Super Contributor
timestamp: May 16, 2025, 06:10:44 pm
content: The oscillation issue you're describing is quite common in vintage radio circuits and is likely related to the dynamic interaction between the radio's internal circuitry and the characteristics of the power supply - especially with respect to impedance, regulation stability, and decoupling.One key factor is output impedance and decoupling behavior of your power supply. Radios from the 1920 were designed with battery supplies in mind, which inherently have very low output noise and zero ripple, but more importantly, they have a relatively high and resistive source impedance. When using a modern regulated supply, especially one based on linear regulators or switching converters, the low output impedance and fast transient response can create a mismatch, especially if the power supply lacks proper high-frequency decoupling or exhibits capacitive loading characteristics that are unsuitable for the radio's design.This mismatch can cause regenerative feedback through the power rail, especially in high-gain stages like RF and AF amplifiers, resulting in oscillations or "screaming". The ARBE-III likely avoids this by carefully replicating not just the voltage levels but also the impedance and filtering behavior of the original battery setup. It may include series resistance, extensive filtering, and intentional damping to suppress parasitic oscillations.Your supplies might be over-regulated or too "stiff" (i.e., low output impedance with little filtering), which could unintentionally couple stages together via the shared power rail, especially the 90V B+ line. Also, incorrect or absent negative bias (e.g., -4.5V C-bias) can drastically affect tube operation, making them too sensitive or unstable.To address this:- Add series resistors (e.g., 1kΩ to 10kΩ) followed by high-voltage electrolytics (10 - 47 µF) on the B+ line to emulate the RC filtering of battery chains.- Ensure proper decoupling at each stage (using 0.01 - 0.1 µF ceramics in parallel with larger electrolytics).- Check and stabilize the C-bias line with appropriate load resistors and filtering.- Consider the supply's grounding and whether it introduces ground loops or noise coupling.Essentially, it's not just about voltage levels - it’s also about emulating the electrical behavior of a battery.

author: floobydust
author_rank: Super Contributor
timestamp: May 16, 2025, 08:17:07 pm
content: Do you have an earth ground connection via the line cord? To what - PSU floating, grounded chassis like the ARBE-III seems to use.Is C9, C10, C11 populated?C5 0.1uF is large and makes the A regulator quite slow. You could check if it's what oscillates, the TL-431 circuit perhaps. That would explain the one complaint.I would worry about rectifier switching hash, and the RF output impedance like C9-C11 but also across the rail outputs.

author: TimFox
author_rank: Super Contributor
timestamp: May 16, 2025, 09:01:00 pm
content: Quote from: Alex_Baker on May 16, 2025, 06:09:45 pmThat is a “TRF” receiver (tuned radio frequency) with a “plate detector” in the third position.The three variable capacitors should be on a common shaft, but I have seen photos with separate knobs.There is no regeneration, nor frequency shifting to an intermediate frequency (as in a superheterodyne).

author: floobydust
author_rank: Super Contributor
timestamp: May 16, 2025, 10:26:12 pm
content: Model 300 is circa 1925 and I haven't read the manual. Sometimes they had (inductive) feedback paths.Volume control seems to be on the potentiometer from filaments to output transformer low-side return, I guess a rheostat.The "A" battery would see a heavy load, so this PSU I wonder if it has enough current there.UX112Atube is 5V 250mA filament and 5 of them I think for 1.25A total.Stewart Warner 300's and later model's schematics1925-1928 including AC PSU.Stewart Warner schematic collection