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Old 10th Dec 2018, 5:41 pm   #1
kipperdog
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Default Powering up valve amplifier

Hi all
I am certain this subject has been discussed in depth but I can't find where on the forum.
I am looking to power up my valve amp for the first time in years and I obviously want to do so in stages so as not to blow anything up. I don't own a variac and I can't find a source of incandescent lamps to act as a surge limiter. Has anyone got suggestions as to what I can use as an alternative? Maybe a wirewound resistor?

thanks in advance
Chris
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Old 10th Dec 2018, 5:58 pm   #2
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Default Re: Powering up valve amplifier

Hi Chris,

I think perhaps a more sophisticated approach may be required. It would probably be helpful to know what amplifier it is and one of our audio experts would be likely to be able to help you.
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Old 10th Dec 2018, 6:01 pm   #3
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Default Re: Powering up valve amplifier

Sadly a lamp limiter won't stop things blowing up. It's designed to limit the fault current and the clue is in the name - it only does any good after you've blown something up i.e. caused a fault.

In principle a Variac might help, but you need to wind it up very slowly, keeping an eye on capacitor reforming currents. If you have a valve rectifier then the situation is even more complex because when the Variac is at a low setting the rectifier's heater will be cold and it won't pass any current. By the time it starts to pass current you'll have wound the Variac up to the point where fragile capacitors can start to draw more current than is good for them.

Capacitors are reformed most effectively using constant (low) current sources. If you search for capacitor reformer you'll find designs for quite a few, some of them consisting of not much more than a DC supply and a resistor (doesn't even need to be wirewound).

Cheers,

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Old 10th Dec 2018, 7:01 pm   #4
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Default Re: Powering up valve amplifier

Thanks for the replies. The Amp is an Armstrong 222 which has a solid state rectifier http://www.audiomisc.co.uk/Armstrong/200/200page2.html
I am going to replace the filter caps and maybe the coupling caps. Is there anything else I should keep an eye on when I power it up?
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Old 10th Dec 2018, 7:22 pm   #5
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Default Re: Powering up valve amplifier

Quote:
I am going to replace the filter caps and maybe the coupling caps
I ask why, do you know they are suspect?
 
Old 10th Dec 2018, 8:10 pm   #6
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Default Re: Powering up valve amplifier

I have done two old valve amplifiers in the last year. I had the advantage of a variac to control the supply. Both amps used a valve rectifier. I removed all the valves and tacked a silicon diode from anode to cathode across the rectifier holder in series with a couple of test leads so I could insert a (milli) ammeter and resistor to limit the current.
I could then check for reforming current and also prod round the grid pins of the holders looking for any positive voltages.
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Old 10th Dec 2018, 8:36 pm   #7
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Default Re: Powering up valve amplifier

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Originally Posted by merlinmaxwell View Post
I ask why, do you know they are suspect?
I thought they may have dried out over time, it must be 10 years since I powered it up last
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Old 10th Dec 2018, 8:49 pm   #8
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Default Re: Powering up valve amplifier

There is also the London Sound method.
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Old 10th Dec 2018, 9:46 pm   #9
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Default Re: Powering up valve amplifier

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Originally Posted by kipperdog View Post
Is there anything else I should keep an eye on when I power it up?
Hi there, welcome to the forum.

It looks pretty straightforward but I wouldn't power it up yet.

What equipment do you have available?
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Old 10th Dec 2018, 10:28 pm   #10
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Default Re: Powering up valve amplifier

Quote:
Originally Posted by kipperdog View Post
Thanks for the replies. The Amp is an Armstrong 222 which has a solid state rectifier http://www.audiomisc.co.uk/Armstrong/200/200page2.html
I am going to replace the filter caps and maybe the coupling caps. Is there anything else I should keep an eye on when I power it up?
The 222 uses a voltage doubler for the HT. It's quite often possible to reform the elelctrolytic capacitors in that part of the circuit, but they have quite a hard life and you may find that they're electrically more leaky than you would like even once they've been reformed. If that's the case then the only solution is to replace them. Just make sure that the replacements can handle the ripple current.

Cheers,

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Old 11th Dec 2018, 12:42 am   #11
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Default Re: Powering up valve amplifier

I usually apply power to the rectifier cathode from an isolating transformer with a silicon diode and 100k 2W resistor and leave it over night and then next day measure the grids of all the valves to see if there is any leakage in the coupling capacitors.
If you have a valve radio with an isolated mains transformer in regular use a little bit of HT from it for a few hours via the 100K resistor with a carefully added tap.
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Old 11th Dec 2018, 1:07 am   #12
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Default Re: Powering up valve amplifier

Of course if we power up the HT rail with the valves cold then all the HT volts will end up dropped across the coupling capacitors. Often they are rated to cope with that. But sometimes they aren't, in which case they may be destroyed by the test when they might have been OK if the amp had just been brought up more gently.

Cheers,

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Old 11th Dec 2018, 1:15 am   #13
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Default Re: Powering up valve amplifier

I would never use a capacitor in the coupling position that is rated at less than the HT voltage.
That would be asking for trouble.
Leaving half the HT voltage on in this way for a good few hours normally reforms the main power supply capacitors quite well and shows up any coupling capacitors easily.
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Old 11th Dec 2018, 9:59 am   #14
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Default Re: Powering up valve amplifier

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Originally Posted by Refugee View Post
I would never use a capacitor in the coupling position that is rated at less than the HT voltage ...
I wouldn't either. But neither of us built the Armstrong 222. I stopped assuming that everyone was as risk-averse as me decades ago. I've got a 222 somewhere. I should dig it out and find out what the coupling cap ratings after the phase splitter are. According to this circuit http://jcgl.orpheusweb.co.uk/Armstro...RTDiagram1.gif they operate with only 135V across them. The normal HT is 265V, so nearly twice that, and in the absence of any load at all I'd expect it to end up a long way north of 300V. There's a 47k/100u smoother on the phase splitter HT supply so the stage is largely protected against switch-on surge even with a solid-state rectifier. I can see the bean counters pushing the designer to cut a corner ...

Cheers,

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Old 11th Dec 2018, 10:24 am   #15
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Default Re: Powering up valve amplifier

That is right.
Full HT on the coupling capacitors until the valves are warmed up.
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Old 11th Dec 2018, 11:24 am   #16
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Default Re: Powering up valve amplifier

post#6 (buggies) brilliant idea !! logged in mind for future ref... could be used on other situations also (series strings aside)
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Old 11th Dec 2018, 11:34 am   #17
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Default Re: Powering up valve amplifier

Going back to the original question, on this amp, it looks like you could remove the HT fuse and connect a WW resistor, 10k upwards, across the fuseholder.

If you also remove all the valves, you should be able to see the HT rail coming up, and check the coupling caps by looking at the voltage on the signal grid pin of each valve holder. Measuring the (AC) voltage across the WW resistor will give a clue about leakage in the big electrolytic (unless there is a resistive load across the HT with the valves removed).

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Old 11th Dec 2018, 3:42 pm   #18
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Default Re: Powering up valve amplifier

Quote:
Originally Posted by astral highway View Post
Hi there, welcome to the forum.

It looks pretty straightforward but I wouldn't power it up yet.

What equipment do you have available?
I don't have any special equipment but thought I may be able to limit the switch on surge to a safe level by using a WW resistor. Can't afford a variac just to test one item
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Old 11th Dec 2018, 11:29 pm   #19
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Default Re: Powering up valve amplifier

Do you have a voltmeter and the experience to check voltages around a valve amp circuit? That is really the basic starting point - if not, then do you have any friends with that experience? Otherwise you run the risk of damaging parts in the amp.

And there is always the concern that you are competent enough to assess the electrical safety of the equipment, given that you are going to plug it in to the mains and we have no idea about the history of the amp.
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Old 12th Dec 2018, 3:18 pm   #20
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Default Re: Powering up valve amplifier

You should still be able to find incandescent lamps. Most shops which sell lamps include halogens in their range; these will work OK in a lamp limiter. Some smaller shops may still sell 'rough service' lamps, which are probably mostly relabelled ordinary incandescents from Eastern Europe.

Unfortunately these will probably all be clear bulb lamps. I find that judging how bright the lamp is is much easier with a pearl bulb.
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