[TNTrawlerman]

Hello to all,  I am bringing back to life a battery operated model 41-90 Philco "Home radio" and am tantalizing near to success, but have one extremely irritating issue left.  The radio now plays, it has decent sensitivity, I have tested all the tubes and they seem fine as far as the tester is concerned.  However, on every station (all two of them) that are reasonably local, the sound is strange.  I would best describe it as if the singer in the music (and to some extent the music) sounds like they are "gargling" at the same time they are singing.  The voltages all seem to be fine, and when I couple a 1000 Hz modulated RF signal at 1000 kHz, the demodulate signal looks smooth up to the 1A5G output tube, but the plate signal looks distorted.  I have tried a second tested good 1A5G but with no difference.  The output transformer seems to perhaps be the culprit, as there is not much else, as the 4 ohm speaker is brand new, however the transformer obviously works to some extent.  The lower the volume is, proportionately less the "gargle" effect becomes.  I see the 1A5-G tube wants a load resistance of 25,000 ohms, but when I do a back calculation on the transformer (inject to secondary, read primary output, calc turns ratio, square the ratio and multiply by the 4 ohm speaker) the load looks like about 7000 ohms.  Would this be likely the issue?  I hate to spend $65 and find out that I have solved nothing.  The original output xfrmr "looks" good, but I guess that does not mean much.  Thanks for your thoughts.    

[radiorich]

Hello TNTrawlerman,
Welcome, it could be a bad speaker !
Sincerely Richard

[TNTrawlerman]

Thanks Richard,
I have tried another speaker (4 ohm) and it does not seem to make any difference. When the volume is low, the sound seems to be pretty well clear, so that leads me to believe that the problem is after the volume control. That leaves the 1H5G detector/pre-amp circuit and/or the 1A5G final output circuits as one or the other of the "culprits". Reading the specs for the 1A5G tube, it seems that it wants a 25,000 ohm impedance. I temporarily removed the output transformer and injected 0.1 volts @ 1000Hz into the speaker side secondary, and measured the primary/secondary ratio, then squared the ratio and multiplied by 4 (speaker impedance) and the result only came out as about 4800 ohms impedance. It looks like the output transformer is original, but hard to tell for sure. I just wondered if anyone had run into the situation where the wrong reflected impedance had caused this kind of "gargling" effect to the sound at any volume. Also, while I realize that the 1A5G tube can only output about 100 milliwatts, the volume at max seems very "puny" to me. Any thoughts from anyone are appreciated.

[radiorich]

Hello TNTrawlerman,
If you can you might inject Audio into the audio amp and see how it sounds.
Sincerely Richard

[TNTrawlerman]

Hello Richard,
Your suggestion was a good one to narrow down the issue, and I wish I had posted/asked much earlier, as I had suspected the output transformer, and after spending $60 found out it made no difference. Yes, a nice sine wave into the final stage became distorted at the plate at any reasonable level. I bought some 1A5G tubes, but they made no difference, i.e. low volume and the sound was clear but at any reasonable listening level distortion ("gargle"). Looking for a substitute, I noticed that a 3Q5G could be used with pins 2 and 7 jumpered and filament voltage applied between them and pin 8. I happened to have one of these, and lo and behold the radio plays nice and reasonably loud without distortion!
So again, thanks for the suggestion, (which I should have known anyway) and all is well.
Ken

[MrFixr55]

Hi Ken,

Sorry for joining the conversation late but the impedance mismatch was likely your problem. BTW if you want to keep the wiring the same, a 1Q5 is a drop-in substitution for a 1A5, transformer matching notwithstanding. Although the 1A5 is a pentode and the 1Q5 is a beam power tube, the pinouts are the same. The 1A5 has a load impedance of about 25 KOhms. The 1Q5 and 3Q5 beam power tubes have a load impedance of about 8 KOhms, which is much closer to the transformer that you have. Using the 1Q5 or 3Q5 has the added bonus of a much greater power output. The 1Q5 and 3Q5 have an output of more than 250 mW, vs 110 mW for the 1A5.

Do verify your grid bias, as it is based on a resistor between B- and chassis ground and is current dependent.

[TNTrawlerman]

Good info Mr. Fixr55,
I had ordered some 1A5 tubes, and they finally came in, so I "unmodified" the filament wiring, and tried them, and they were much better than the original, but still not nearly as good as the 3Q5, so I re-modified the unit. It really runs surprisingly well, despite the copious RF noise that our house provides, especially from the under counter halogen lights in the kitchen. I will tuck this away for future benefit.

Many thanks for this info.
Ken

[MrFixr55]

Hi Ken,

To get the 1A5s to work well, you need the 25K transformer. If you can get 1Q5s, consider getting them and undo the 3Q5 mod. The reason is that the original tubes for this set are likely the longer, skinner "G" versions as opposed tom the shorter, fatter GT versions. 1Q5s are available in the G version, if you are looking for an original look. This way, you can switch to the 1A5 for display or the 1Q5 for performance.

These tubes were developed by Sylvania , who was the OEM supplier for both Philco and Zenith

[TNTrawlerman]

Thanks for the tips MrFixr55. Will take the pertinent advise.
Ken

[Arran]

I suspect it's more of a bias issue, that is assuming that the usual step of replacing the paper caps has been taken, whilst I am recapping a radio I always check the resistors as well, many overlook this step because the internet tells them that caps fix everything. I have never had an output transformer partially fail, they either have continuity, or they do not. Part 16, likely an electrolytic capacitor should be replaced, as should part #15, the tone correction cap across the output transformer primary. The resistors parts #24 and #25 should also be checked, actually I would check all of them, if any are more then 20% out, replace. If you also have a spare 1H5 tube, I would substitute another, but I suspect a bias issue in the audio output stages.
Regards
Arran