Did you know you can use a 10 watt 70.7 volt to 8 ohm or (4 ohm with tap on some) for a tube single ended audio transformer. I see these for $5 on ebay. 10 watt rated frequency response is likely 100Hz @ 10 watts, 50Hz @ 5 watts and 30Hz @ 3 watts, All I seen have a gapped lamination core. Here is the resistance chart.

0.625 watt tap= 8000 ohm
1.25 watt tap= 4000 ohm
2.5 watt tap= 2000 ohm
5 watt tap= 1000 ohm
10 watt tap= 500 ohm.

I would imagine they have a frequency response like a wet noodle.

DM

Have you tested any of the transformers in a single-ended tube amplifier? If so, what makes/models of transformers were tested and what is their max operating DC plate voltage and max operating DC plate current? What is the inductance of the primary and secondary?

I only tested one branded Universal. I do not own a lab. I recommend for a radio, not a high-end audiophile application. I assume the transformer would hold up to 300 volts. The one I owned was wound on a plastic bobbin. I use a choke with a plastic bobbin at 700 volts. Enamel wire itself is rated at 200 volts and if built with fish paper, one layer is likely rated over 100 volts. Transformer should be ok at 300 volts. I know of no radios with a single-ended tube that operates over 300 volts.

Transformers frequency response vs power output is related to lamination core size. The 10 watt 70.7 volt transformer is likely rated at 100Hz. At the power levels used in radios, I believe the low frequency response is sufficient as posted. If the same size core as any other audio transformer, the 70.7 volt transformer would not have less of a frequency response. There is no magic involved. The core is gapped for more inductance like any other single-ended audio transformer.
,
It is not a interleaved audio transformer, but I doubt 99% of radio audio transformers are interleaved. Interleaved transformers have superior frequency response above 8kHz. Most radios audio transformers are not audiophile quality and are lacking a tweeter. You can always buy a high-end audio transformer yourself. Some cost $500. Your choice. I will stay with the $5 audio transformer myself.

I've used 70.7 volt transformers as outputs for years and they work fine.

RRM

Thanks. I get a lot of 'it is no good' replies when I post tips.

Well, they sound pretty good to me at the grocery store where I have 10 of them installed playing music. Seems like they would work just fine in a radio. I have used a couple that way as well.

As I said I've used them lots of times with good results. The only concern is that they aren't intended to have dc in the primary and if it's too large the core may saturate. I haven't made any measurements so I don't have any numbers for this but haven't had any problems so far.

RRM

Primary winding DCR measures about the same as a standard audio transformer. I recommend 10 watt rated for no more than 5 watts audio. Most tubes are 3 watts output. All 70.7 volt transformers I seen were gapped to have enough inductance for the DC current.

A common suggestion "in the old days"was to use a surplus 400Hz power transformer for modulating a transformer. They were plentiful but cheap since they weren't useful as power gransformers on the 60Hz line.

I once measured the current corresponding to core saturation on the Bogen T725 and the figure was around 15-20mA.
So the 10W audio transformer will be able to serve as a Class-A single-ended output transformer to around 1-2 Watts. Purpose built single ended output transformers are air-gapped and have a much larger core size for a given power level. Any comments?

Yes, designed for Muzak and voice announcement, elevator music

Bogen transformers used in homebrew radios:

http://makearadio.com/misc-stuff/t-725.php

As to "wet noodle".... AM radios aren't exactly "hi-fi."

Rich

This exact page make me aware of the Bogen T725 a while ago, however the article only mentions the use of this type of transformer as a hypothetical possibility, without ever mentioning the problem of core saturation. I think the author wasn't aware of this problem.

I'm sure this transformer will produce sound in an average AA5 as long as the plate current is kept below 10mA, which limits the output power to around 0.5W, even though the transformer could handle way more power in an AC setting.

EDIT: the push-pull arrangement is interesting (although not too much radios use a P-P audio), the two currents cancel out each other. The transformer is not symmetrical though, but I guess a decent PP amplifier could be made with a strong negative feedback.