Hakan Ericsson wrote:
> I called a friend that runs an AL1200 amp, here is the story.
> Line volt is 228VAC no load, drops to 219VAC full load = abt 4%
The supply drop can not be measured with a RMS meter.
Since the current load is only during peaks, he must use a peak reading
meter with the same time constant as the power supply when under full load.
So your friend has no idea what the mains voltage drop really is. Neither
does QROking.
> This is a very typical situation in this country.
> The amps transformer primary is tapped to 230V
> Anod volt is 3400 at no load and drops to 2800 full load = abt 18 %
> (Readings from amps panel meter)
> Full load in this case is 650mA anod current, drive pwr is 100w
> Tom is saying 10 % drop is typical.
> Now, I let you guess if this amp gives the owner 1500w out?
I have the identical power supply amplifier (AL-1500) with an 8877 tube,
with one ampere of dc plate current HV is 3350 volts. With no load 3650
volts.
8.2% here.
The theoretical sag of the supply is 7.1% with every component at the
best end of tolerance, it is specified as 10% typical since that is
worse case tolerance plus allowance for power main sag.
> I have to my PT2500A amplifier an stand alone Swedish made HV
> transformer
> rated 3000VA CCS. The secondary side does not drop alot abt 3%.
> The HV DC drops from 3900 to 3000 !! (Fluke + Fluke HV probe).
If you measure the secondary with a peak meter, you would see the real sag.
I bet if you measure the primary with a peak meter, you will see the same sag.
It sounds like you are power mains limited. In that case a choke might help
correct some of the problem. Here in the USA, that is rare since the smallest
houses are 100 amperes mains.
> I took out the voltage doubler and used a fullwave bridge and 34uF
> oilfilled C. (same transformer). Ha ha,, abt the same drop.
That makes sense, a full wave voltage doubler is no worse than a bridge
supply for regulation if the components are sized correctly. There is a
rumor a bridge is better, but pound for pound they are essentially the same.
> It's going to be very interesting to hear "The No Sag Guys" explanation to the
>QROKINGS setup, but Carl's point abt. 6kW vs. 10kw
VA rating is the volts times amperes of the load times a power factor
rating. A 6kVA transformer is a 6000 watt transformer. That is assumed
to be the CCS rating rating (24 hrs a day uninterrupted service) with
unity power factor unless listed otherwise.
If the duty cycle is 50% the VA of the load is square root of the load
VA squared over 2. So if you have a 2.5kW input amplifer (2.5 kVA) the
load is sqrt of 2500^2/2 or around 1.768 kVA. A 50% duty cycle does not
double the transformer's kVA rating.
If the short term duty cycle is 25% the effective load kVA is 1.250. A
1.3 kVA CCS transformer would handle a 25% duty cycle amateur
transmission at the 2.5 kW input level (assuming unity power factor).
kVA rating has little to do with this sag problem, as you found out.
A 1.8 kVA transformer might work better than a crummy design 10 kVA
transformer in a capacitor supply. ESR is the all important parameter
for voltage stability with load change.
The voltage sag is caused by I*R voltage drop during the conduction
period of the rectifiers. RMS voltage can change very little while peak
voltage changes a lot. That is why you need to measure the power line
with a peak volt meter. Voltage sag is a resistance or impedance
problem, NOT a transformer kVA rating problem.
Since Hakan changed all kinds of things and got the same results, I
would say his power mains are the source of unwanted ESR. If the ESR
is low a capacitor supply will be just as stable as a choke supply, just
as Carl has said.
If I lived in SM, I'd use three phase or a 400 volt primary single phase
and forget about all the chokes!!! The money would be better spent on three
small transformers than one big transformer and a big choke.
With a full wave three phase supply, the voltage output difference is only
5% between peak (cap input) and average (choke input). Using a choke
in a three phase supply is a good idea if the amplifier is too light and
you are afraid it might float away. The unfiltered ripple is under 5%
with no capacitor at all right from the rectifier, so why use a choke? Even a
few mFd of capacitance would make hum disappear.
73 Tom
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