Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: kagliostro on April 20, 2015, 05:08:46 am
-
I recovered a 40v-0-40v Power Transformer from a dismanted SS 80W amp
the label on the amp says 235W consumption
Is correct if I assume an efficiency of near 80% and an available current of 2.3A at 80v or 4.6A at 40v ?
Thanks
K
-
Sounds reasonable.
Let me issue you one caution: Very often, such transformers for SS gear will have TWO secondaries, or at least they LOOK like they have 2 secondaries. For example, 40-0-40 and 15-0-15. That would not do you much good in the tube world. I have several such transformers obviously from SS gear with a 80 volt secondary and a 160 volt secondary. Although powering heaters will always be a problem (need a separate heater transformer) the mental wheels start turning and I start thinking "Hmm, if I placed those secondaries in series and ran a voltage doubler I should be able to get 425-450 volts."
In every case, however, the secondaries are "nested". In other words, they look like this:
(http://i172.photobucket.com/albums/w32/ttm4/nested_secondaries_zpsv10op21z.png) (http://s172.photobucket.com/user/ttm4/media/nested_secondaries_zpsv10op21z.png.html)
And so the secondaries cannot be placed in series. (without sparks and smoke)
-
Thanks Eleventeen
This particular transformer has only a 0-220v-240v primary and a 40v-0-40v secondary
I was thinking to something like 80v AC (CT disconnected) and a quadrupler
to obtain near 450v @ ~500mA (of which I think to use only 1/2)
as you say the problem is I must use a separated heater transformer
Franco
-
You've got tons of amperage on the secondary side. With a voltage quadrupler, you'd net say 300V or so, and still have 1/4 the amperage, or over 1A available. That could power, say, a quad or more of power tubes. As you go from voltage doubling to tripling, quadrupling, etc., noise & hum go up. But what the heck depending on the application.
-
> dismanted SS 80W amp
> the label on the amp says 235W consumption
80W total? Or 80 watts per side, twice for stereo?
80+80= 160W total output. 235W input and 160W sine-wave audio rating is fairly close to right.
Since the PT is clearly all or most of the 235W input, you can work out the rating.
> 40v-0-40v
So 80 Volts AC total. 235W = 235 VA. 235VA/80V= 2.9 Amperes AC.
Efficiency may be better than 80%, but I will agree with your 2.3 Amp AC rating.
Now try some sanity checks.
The 80V into a rectifier capacitor DC supply will give about 113V DC (40-0-40V implies +/-56VDC). A perfect amplifier, fed 113V, can give up to 40V Sine RMS output. In 8 Ohms this is *200* Watts sine output.
Are you sure of the "40v-0-40v"?
Was the amp maybe wired with the 2-rectifier plan, to give a single 56V supply? This would give 49W in 8 Ohms, which might be 80W in 4 Ohms. That does make sense.
As a Quadrupler, it may not quite make 450V 500mA; but that would be a BIG tube amp (over 100W) and you can make plenty of sound even if it comes out a little shy of 450V.
-
Thanks PRR
The amp was the Roland DAC 80D 80W guitar amp I recovered, so, no stereo
The transformer has a primary with 0-220v-240v and the secondary is only 40v-0-40v
if it is useful I can weigh and measure
Attached, in the .pdf file, the spech of the amp (EDIT: the speaker is an 8ohm)
Franco