• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Trouble in a new DIY amp.

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A

andRookie

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Joined 2006
I've built a PP EL34 following the Lundal/Byrith document, but used hashimoto 40-5 transformer and bulit the of PSUs different.

I can turn it on with dummy load attached and measure the DC - levels thrugh both channels. They are fine, and so are the heather voltages. Tubes also seemes fine. Bias is -28 at grid, 460mV over the catode res. Some hum in one channel, the other is very silent.

Connected a CD-player and a pre to it yesterday. One channel played fine a while, the other was still silent. Only a faint hum was audible. Tried switching input connections, but when I turned it on again the secondary fuse (500mA, just before B+ tap in the HT-psu) was blown.

I have some pdf and powerpoint schematics I`ve drawn, but I do not know how to upload them. I do also get a lot of help on a norwegian forum. I`ll post if we find some faults.
 
A

andRookie

Member
Joined 2006
Found a wire break to the grid of one EF86, so that explained the lack of signal. For the fuse, C filter takes a while to reach max, so when the cathodes are hot (fuse popped when I turned on - of - and on again) the fuse pops. I'll rebulid a delay circuit so it does not encompass the C filter, only B filter.
 
Dave Cigna

Dave Cigna

Member
Joined 2003
What are you using for power transformers? Are the toroids by any chance? Toroids, by nature of the remanence of the core, will sometimes blow a fuse when powered up. It has to do with the phase of the AC when turned off, then the phase when turned on again. I once had it figured out what set of conditions result in trouble, but my memory... It doesn't matter...

I have also had the problem with seriously oversized EI transformers, though I have not figured out why.

-- Dave
 
A

andRookie

Member
Joined 2006
157_5751.jpg


With a delay on B+ no fuses blows. The bias - level pot has 90kOhm to ground to make sure bias cannot be turned off too much.

With a passive pot between a cd player and input there is almost no hum or noise at all. So far I am very happy with the sound.
 
EC8010

EC8010

Ex-Moderator
Joined 2003
Mr Chris said:
Counter-intuitive but switch big transformers at mains peak to minimise inrush current!
Click to expand...

But the switch-on is a transient, not a steady-state condition. The initial current into the HT reservoir capacitor will be:

I = C dV/dt

Imagine that you switch at exactly 0V and look at the voltage a microsecond later. Granted, the dV/dt of a sine wave is greatest as it passes through 0V, but wouldn't you agree that the dV/dt at that instant is rather lower than the dV/dt imposed by stepping from 0V to full peak voltage (which is what happens if you switch at the peak)?
 
kevinkr

kevinkr

Administrator
Joined 2004
kta-hifi.net
Paid Member
EC8010 said:


But the switch-on is a transient, not a steady-state condition. The initial current into the HT reservoir capacitor will be:

I = C dV/dt

Imagine that you switch at exactly 0V and look at the voltage a microsecond later. Granted, the dV/dt of a sine wave is greatest as it passes through 0V, but wouldn't you agree that the dV/dt at that instant is rather lower than the dV/dt imposed by stepping from 0V to full peak voltage (which is what happens if you switch at the peak)?
Click to expand...


Hi EC8010,
While I don't remember the exact reasons why any longer, Mr. Chris is actually correct. I had an audio amplifier application where I switched a triac at the zero crossing, and had lots of exploded triacs, and when that didn't happen blown fuses. In fact the solution was to make sure that the triac was initially switched at the crest, and this is recommended for large inductive loads and transformers in general. (App notes for triacs and opto triac drivers often mention this.) Something to do with the behavior of the core during the initial application of power, iirc during that first half cycle it actually looked like something close to a dead short when switched at the 0 crossing point, whilst applying power at the crest elicited an entirely different behavior, and much lower inrush current during the first couple of cycles. It's totally counter intuitive, and I honestly don't remember the explanation, but I experienced it whilst working for a large audio company debugging someone elses design and later on my own.
 
EC8010

EC8010

Ex-Moderator
Joined 2003
Hello kevinkr,

but the problem referred to is fuse-blowing if the HT supply and heater supplies are switched on simultaneously. At such a cold switch-on, the valve heaters are cold, making them nearly a short circuit, and the HT reservoir capacitor is at 0V, making it nearly a short circuit. A transformer with a short circuit across its secondary shows only its leakage inductance, so I don't think we can consider these two examples to be inductive loads. The transient current is due to those cold heaters and a discharged HT reservoir. Switching at 0V changes the voltage across such loads rather sharply, but not as sharply as stepping instantaneously to the full voltage.

Nevertheless, I will go away and look up switching into inductive loads (and have a think about it). If either you or Mr Chris can remember exactly why switching at the peak of the waveform is more appropriate for an inductive load, I'd be delighted to know.
 
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