isolation transformer install

[J M Fahey]

I have an 8 ohm and a 5 ohm resistor, and here is what I see with 120mV input, 0.5v/div

8 ohm, a little over 2.5 v p-p
[ATTACH=CONFIG]49880[/ATTACH]

2.4 ohm, about 2 v p-p
[ATTACH=CONFIG]49881[/ATTACH]

I don't know what to make of this?

Well, we are getting results and a trend, which points to possible improvement:

1) "8 ohm, a little over 2.5 v p-p"=0.1W RMS

2) "2.4 ohm, about 2 v p-p"=0.21W RMS

So by going down in impedance we have more than doubled available power, a valuable find, donŽt you think?

3) I would *love* to find how much it puts out into, say, 2 ohm, and even 1.5 ohm.

Since power is so low (so far), I bet you have more usable resistors in your junk drawer.

No need for 5W ones (so far), probably a couple 1W ones in parallel will do for testing.
Worst case one might smoke if overheated ... weŽll worry about that when/if we reach that point.

To make my position clear, I would be more than happy to reach near 1W RMS, go figure. Even surpassing 0.6W or so would be an achievement.

4) and keep posting Vpp values if you wish, but please also measure "RMS Volts" across load with a multimeter.

Preferably a meter which has available 200mVAC or at least 2VAC scales ... even if we donŽt use them, the point being that such meters have a real AC metering system inside, while $10 multimeters, sporting 200VAC and 750VAC scales *only*, have not, as simple as that, just a crude 1N4007 in series with the standard VDC scale ... something we can NOT trust.

5) so in a nutshell: measure similar waveforms but into 2 ohm and if possible 1.5 ohm or so and measure and post "multimeter" AC volts across the load.

Once we find the load, no matter what, which provides maximum power output, then me might try to mod bias and improve that further.

My suspicion, which of course will need confirmation or dismissal, is that the "mystery transformer" or whatever you are using now is very mismatched, IŽm trying to find a better operating point.
Nothing more, nothing less.

[Enzo]

The 50C5 is only rated for 1.9 watts max signal out anyway. And a 9% distortion rating. Remember this tube was intended for table radios.

http://www.r-type.org/pdfs/50c5.pdf

[Helmholtz]

The 50C5 is only rated for 1.9 watts max signal out anyway. And a 9% distortion rating. Remember this tube was intended for table radios.

http://www.r-type.org/pdfs/50c5.pdf

....and the datasheet calls for an unusually low load of 2.5k. Supposing someone used a replacement OT having 10k/8 ratio, the output would be max. around a 2 Ohm speaker load.

[J M Fahey]

Transformers do not have "own impedance" by themselves but just "transform" (duh) secondary impedance into a primary one,
So we should not worry (for now) about experimenting with "crazy" secondary impedances, be it 3.9 ohm , 1.5 ohm , 1 ohm or whatever, and worrying "but .... no such a speaker exists" because that is not the point.
All we are doing is finding what *primary* impedance does that tube like most.
Once we optimize that, then weŽll know what OT do we need to couple that preferred impedance to a real world speaker.

I guess in its long history, somebody replaced dead OT with anything they had nearby, fwiw it might not even be an OT at all , but, say, a 110:12V or 110:6V wall wart transformer or anything, really.

IF datasheet suggests 2k5 optimum load impedance and we have been testing so far with a 15/17k one, very poor power performance is the norm, not the exception.
But we may solve that, although I guess you will have to get a real and matching OT from somewhere.

Which might be as near as an old radio/tape recorder/TV/record player OT which can be cannibalized.

[Randall]

I think this has reached the "it is what it is" stage. I can't keep spending time I wont get paid for on an anemic little table radio type amp. The speaker is ratty and the wrong impedance for the champ style transformer, and since I already have re-capped it and put in an iso, I doubt the owner will want to go any further with costs on this. But, I'm waiting to hear back.

I will check in when I find out. As always, I appreciate the help and guidance.

[eschertron]

I think this has reached the "it is what it is" stage. I can't keep spending time I wont get paid for on an anemic little table radio type amp. The speaker is ratty and the wrong impedance for the champ style transformer, and since I already have re-capped it and put in an iso, I doubt the owner will want to go any further with costs on this. But, I'm waiting to hear back.

I will check in when I find out. As always, I appreciate the help and guidance.

Next time a customer brings you a rare relic like this and says "see what you can do with it" make sure they also bring you a blank check. Otherwise their level of commitment is not enough.