Power consumption of amp

[Twang46]

Something interesting has arisen after the failure of the mains fuse (500ma) on my "champ" style amp using 1 6v6 output valve.

I measured the current draw as 178ma (after stabilising) by removing the fuse & fitting the ammeter in series to get a current reading.

Now this equates to around 43w power consumption on what is nominally a 5w amp, although I would think a 6v6 can go a fair bit higher depending on the output transformer's capability.

The meter I'm using can read ac current & seems to be accurate (although as I sold my pro gear last year when I retired from testing/inspections it has been about a year since I was able to check it properly)

I have heard of a "rule of thumb" with valve amps that is a 3-1 ratio input-output power. Although the 2 commercial amps I have declare a ratio of more like 6-1 on the specification plate.

Checked around adverts & similar amps seem to declare broadly similar power consumptions as I am getting.
Since replacing the fuse the amp has run for about 6 hours without any problems

It's an Ampmaker kit with a couple of "mods" to the ht & smoothing that has run for around 250/300 hours without any problems
This arose after I mailed Barry at Ampmaker to see if there had been other instances of the mains fuse failing, he seems to think however that the current draw should be around 15/16w although he seemed surprised that I chose to measure the current across the fuse terminals, he however is going to try for himself when he has time (xmas rush)

Thanks if you have managed to read this far & would be interested to hear what you think.
Ideally if you have a 5w amp by looking at the spec plate, or even better by measurement if you have access to an ac ammeter

Cheers

Dick.

[ecca]

What type of fuse is it ?
Fast blow or anti surge ?

[Twang46]

Hi Ecca

It's a Siba fuse described as a "Time lag" that I presume means slow blow as we would know it

I'm using a "pro" digi multimeter that looks like a copy of a Fluke & is of decent quality, however doesn't have peak hold on the display. Checked for unusual surges when powering up but seems to be what I would call normal at around 250/260 ma surge then fall back to 125ma rising to 178ma as the heaters draw full load.

As you know fuses can fail for a number of reasons apart from excess current, unfortunately the casing of the fuse is opaque ( would have to break it open to see inside which normally causes damage itself) so can't see what has happened to the conductor inside to get a clue as to any current "surge" if that's what has been going on

Amp ran for around 6 hours yesterday without any signs of distress, all looks good inside (turret board construction)

Is 178ma to high for a 5w 2 valve single ended amp ?
My Super Champ XD has a declared power consumption of 96w for a 15w 2x 6v6 "push pull" valve amp so doesn't seem to me to be that far away from norm, that why I was hoping for other peoples readings or info off the spec plates

Cheers

Dick

[peter roest]

Dick,

The power consumpsion at stabile situation is not important. As you see the current in this situation of the amp is about 180 mA. This is much lower than the amp fuse of 500 mA. The problem is the inrush current of the power transformer. The inrush current of such a transformer is more than 20 times the nominal current for a couple of periods. In the case of 50 Hz one period is 20 milliseconds. The current decreases in about 0,1 sec to normal value. Every time you switch on the amp the fuse get's an short blow. After anough blows it's gone knock out. Important is that the fuse is slow (in Dutch) to avoid those problems.

Peter

[Twang46]

Thanks for the reply Peter.

My problem is not with the fuse which is correctly rated, but is if the current draw of around 180ma is normal for this type of amplifier.

I do not feel there is a problem with the amp running at around 35% of the nominal fuse value but was wondering if any other member had a similar style of amp that they could check to compare readings.

Cheers

Dick.

[ecca]

If you're just measuring the mains AC input current then does not power factor come into it at some point because of the inductive load ?

[Didier]

Early production Vox AC15-CC1 were fitted with a mains fuse of a too low value which kept failing. Replacement with a fuse of the proper value solved the problem.

Didier

[Twang46]

If you're just measuring the mains AC input current then does not power factor come into it at some point because of the inductive load ?

Spot on Ecca

Of course,..... the power factor is nowhere near unity with a significant phase lag due to a combination of the inductive & reactive load

Using 0.6 as a R.O.T figure puts the "true" power consumption closer to 25w (around 95ma at my mains voltage) which looks better

Cheers

Dick

ps......................... R.O.T = rule of thumb, an accepted basic average amount

[Twang46]

Hi Darryl & Didier

Thanks for the reply's please see my response to Ecca

Cheers

Dick.

[ecca]

See, I told you I was clever !