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Old 5th Mar 2017, 5:12 pm   #61
bikerhifinut
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Default Re: Rectifier Flashover and open circuit anode resistors

Just a quick one.
When I measured the AC conditions across the 100R anode resistors on the GZ34 I got a 9V voltage drop. This equates to a total current draw of 180mA by ohms law. Which is a bit high for a 5-20 based output stage etc. I measured the DC voltages including the PSU cap bleed resistor and it all added up to 146 mA, pretty much bang on the calculated circuit conditions.
All I can think is that my DVM which isn't a very expensive one doesnt measure AC RMS voltages very well off a sinewave.
I havent used my analogue meter yet but I'm wondering if I am missing something in the theory as surely the current draw should be the same at either end of the PSU choke as there are no divisions of supply until then. (RC filters/droppers for the 2 preceding stages).
i did a quick calculation and if the voltage drop is nearer to 10V across the 100R (quite possible as the meter only measures whole volts on the 750VAC range) then the current would calculate as 100mA each leg for total 200. Multiply my DC current of 145 by 1.4 and it comes out as more or less 200mA. So i am wondering if my meter is measuring peak not RMS volts. And if so its a caveat to people looking for a DVM to buy something perhaps more "professional".
But the meter measures the mains voltage accurately so whats going on?

A.
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Old 5th Mar 2017, 10:32 pm   #62
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Default Re: Rectifier Flashover and open circuit anode resistors

Yes RS are very efficient with orders. I have used them for years in various jobs and also privately. I have never known them to get an order wrong. I used to drop in to their trade counter in the TVTE but now just order stuff on line. It's delivered pretty quick too. The only downside is sometimes there is a minimum standard supply multiple on components but I find that if you build a lot of stuff it isn't really a problem. I have always considered wirewounds to be the most robust of resistors and understood they could take a lot of stick but maybe things have changed.
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Old 5th Mar 2017, 10:48 pm   #63
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Default Re: Rectifier Flashover and open circuit anode resistors

I think it's just those white cement coffin jobs from maplins Biggles. I have heard other anecdotal evidence suggesting they aren't robust.

A>
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Old 6th Mar 2017, 1:35 am   #64
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Default Re: Rectifier Flashover and open circuit anode resistors

Thanks all for the offers!!
I used to have both RS and Farnell accounts, but due to the fact I dont operate a business anymore, and dont have an ABN ( Australian Business Number) I cant use them. Some vegetable in the govt has come up with the idea that unless you turn over MORE than $100K/year, you cant have an ABN.
And yes, Mouser and Digikey do ship here but I have never used them.
Err hmm the last NTC inrush limiting resistor I fitted was a black carbon rod without markings, with crimped on end caps. Hence my dumb question. Thanks for your tolerance

Bikerhifinut !!! you DONT use an analogue meter Sheesh I would say you deserve everything you get. If ever you use ANY of my circus, I mean circuits, ALL voltages are measured with an Avo8, and is shown in "notes" at the bottom of the circuit. IF, for some reason I need to accurately measure grid volts I use either a HP400, or a HP 410B, as will also be noted.

If you read ANY valve manual from ANY valve manufacturer, there is a note that says, and I quote:
Voltages differing by +/- 20 can be considered normal.

So a valve amp requiring 400 volts is totally unusable when you measure 401.14571548715415451487518754 volts.
THAT is what you measure with a digital disaster.
BUT I do measure components with a DVM, AND I measure power with a DVM. An ancient Fluke 8050A. Its also very good for dB measurements.
The LCD has gone black from disuse.

Sorry all, I do like my analogue stuff. NOT to mention, if my avo is set for 100 volts and the needle swings 3/4's of the way, its close enough to 75 volts
WHICH I can see from the corner of my eye, while watching where I have the probes inside this amp which uses 750 volt HT. Hmm err, IS there a DVM that can measure 750 volts ? My fluke is 750 volts AC and 1000 volts DC.
Its also over 30 years old.

Joe
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Old 6th Mar 2017, 1:51 am   #65
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Default Re: Rectifier Flashover and open circuit anode resistors

Dissipation in the resistors is higher than DC ohms law would suggest due to Ripple Current.
Had this discussion last week on another thread.
You need to multiply the worked wattage by around 3 at least, its not a steady current and DMMs will be inaccurate.
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Old 6th Mar 2017, 4:24 pm   #66
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Default Re: Rectifier Flashover and open circuit anode resistors

Thanks Sam, It is what I was suspecting. Just me on the learning curve again. You think you know whats needed and then you find out you didnt!
Joe I know I should have dug out my cheerful little 20k/volt analogue meter but it was more convenient to use the desktop DVM. My Avo9 needs a proper service and calibrate as it's gone inaccurate, under reading very badly on the higher voltage ranges. And I know i should get it fixed cos its a blinking useful bit of tough kit.
As an aside with these big resistors in, the amp seems to behaving very well and I am sure the transformers are not as buzzy as before either so somethings happening right.

A.
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Old 7th Mar 2017, 6:18 am   #67
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Default Re: Rectifier Flashover and open circuit anode resistors

RS are ok but a shadow of what they were, 3/4's of the components I've looked for are out of stock or have a 6 weeks lead time.

Ok why doesn't a DMM measure RMS voltage accurately? I've measured output of my big amp with my Fluke 25, a B&K AC voltmeter and a HP and Marconi distortion meter, and got roughly the same reading. I know the last three arn't the same as an AVO but still.

The Fluke 25 can measure over 1000v AC, with a DMM don't need to see the meter, you use the "hold" function. There's no dicking around with a DMM trying to figure out where the needle is, and then doing the math's in your head to convert the meter scale reading to the range your on. No faffing about with a DMM with your hands inside a potentially lethal power supply switching ranges either. Guess it's what you're used to.

Andy, some of those cheap coffin R's don't come with a data sheet and don't tell you what their working voltage is. If they have one I reckon it'd be suspect at the top end. Finding decent R's that are safe with the high voltages present in a valve amp ain't easy or cheap.

Nice one for getting this sorted. A.
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Old 7th Mar 2017, 6:30 am   #68
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Default Re: Rectifier Flashover and open circuit anode resistors

Lawrence ms66o gave me the heads up on ripple current, learn something every day. Thanks Lawrence.
No doubt he will drop in here. I'll leave it to him.
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Old 7th Mar 2017, 9:12 am   #69
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Default Re: Rectifier Flashover and open circuit anode resistors

True RMS measurement is hard to do. You need a very fast A-D converter so you can sample the waveform properly. But fast A-D converters are either less precise or more expensive.
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Old 7th Mar 2017, 9:29 am   #70
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Default Re: Rectifier Flashover and open circuit anode resistors

Quote:
Originally Posted by Boater Sam View Post
Lawrence ms66o gave me the heads up on ripple current, learn something every day. Thanks Lawrence.
No doubt he will drop in here. I'll leave it to him.
It's easy to imagine if you remember that the load never stops drawing current and that the reservoir needs topping up.

Lawrence.
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Old 11th Mar 2017, 4:51 pm   #71
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Default Re: Rectifier Flashover and open circuit anode resistors

Quote:
Originally Posted by Radio Wrangler View Post
If the rectifier arced over A to A, then the resistors most probably were over voltaged, over currented and over powered all at the same time. It looks like they opened up and maybe saved your mains transformer. If so, they did a good job.
Looks like that could be the case David. Out of curiosity I decided to examine the one surviving 100R 7W resistor of the 4, It didnt even get to the point of me cutting the ceramic box open as one of the leadouts simply detached itself somewhere inside the resistor and literally fell out. So I reckon that one was about to go too.
I think it has been mainly a case of resistors run close to their "limit" and in the case of these nondescript white coffin jobs they look to be flimsy things that are constructed to be only just "good enough", if that. And then I checked some data sheets (thank you RS for providing links to manufacturers data.) and found out something that surprised me, in that some wirewounds have rather low voltage ratings. This made me much more wary when ordering replacements, it hasn't just been about power ratings this time.
Ok so things seem to be running much nicer now, but I am baffled as to why the buzzy transformers run so much cooler now and although still a bit "thrummy" are a lot quieter to the point where they are acceptable enough given that the amps live directly behind each speaker. So distance from my lug oles is maximised.
I even think I may have an inkling to the rattly metal top shrouds. I suspect a shorted turn effect as they really rattle when touching the tops of the mounting bolts and thus are completing a circuit to earth. The wet dog shaking when simply held clear of the mounting bolts seems to have receded beyond my ability to feel it. Again since I reworked and double checked the connections from Ht secondary via surge limiters to the rectifier anodes.
I was also wondering if the GZ34 really had failed or was it a function of a resistor going open circuit causing an overload on one anode as the full load went through one side? But as it seemed to happen within a second or 2 of switch on I suspect not as the valve hadn't time to warm up and start conducting to any great extent. But I don't feel brave enough to try it out, I might see if someone would run a quick test on it sometime though.
Sorry if folks think I am obsessing a bit, I do get a bit like a dog with a bone when I have problems that maybe others would just walk away from so to speak. But I like to try and understand what is going on with my hobby builds. I could save an awful lot of money by trawling fairs and auction sites and buying commercial amps but wheres the satisfaction in that?
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Old 11th Mar 2017, 5:19 pm   #72
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Default Re: Rectifier Flashover and open circuit anode resistors

Let's do a thought-experiment:

Suppose one of the rectifier anode-resistors was open-circuit then one half of the centre-tapped transformer winding and one of the 2 diodes in the bottle would not be doing anything.

The other halves would have to work twice as hard to keep the volts up (meaning higher peak currents and so greater DC loss in the half-winding, as well as causing one significantly overworked diode), and also it would mean that there was no "DC Component cancellation" going on across the 2 halves of the tranny secondary so the effective (and excessive) DC in the half of the secondary that _was_ being used would push the core towards saturation, meaning both DC magnetisation of the shrouds [causing hum...] and more heat!

Half-wave rectification would mean more ripple on the smoothed output too.

Maybe the cause of your hum has finally revealed itself - in a somewhat catastrophic way?

[I had a similar issue in an AR88 receiver: one of the two 'tape' filaments in the directly-heated 5Y3GT rectifier went open-circuit. The radio continued to work, but with increased hum and the VR150 voltage-regulator glow-tube would sometimes flicker and go out because there were insufficient HT volts. Tapping the rectifier would bring the broken ends of the tape-filament back in contact where they would micro-weld themselves together with the filament current - until they warmed up and thermal expansion broke the micro-weld]
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Old 11th Mar 2017, 5:50 pm   #73
bikerhifinut
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Default Re: Rectifier Flashover and open circuit anode resistors

That is kind of what i was thinking as I'd had issues previously before i tackled the over voltage on the primaries and i was using 220R resistors, You kind of knew one had gone and it was half wave rectifying without measuring as the noise got really loud. And of course i was merrily replacing resistors and wondering why the blessed things seemed to fail so easily. This was before I was made aware of the stresses ripple current can place on PSU components and then i could see that a 7W resistor was in fact only just inside the actual power dissipation. (For the output stage I chose, Mullard specified 6watt resistors and now I think that was just a bit too close for comfort). Combine that with poor quality resistors or maybe they were just not rated for the voltages used? and i was on a hiding to nothing.
Ok the transformers are still far from perfect, the windings still chatter on their former and I doubt there's anything I can do about that after Mike Barker gave the verdict on them. But it's kind of nice to find out I can get things a whole lot better.
Just to bear in mind, the issue wan't electrical hum rather mechanical noise from the transformers, any hum I had would generally be after a resistor failed and it was half wave rectifying.
I have also worked out that the steel chassis seems to be very effective at transmitting and amplifying the vibrations and this gives me ideas to completely rebuild the amps on a different chassis. The sort I have in mind would be a sturdy hardwood outer framework with a thick aluminium top and bottom plate which I think would have less tendency to "ring like a bell". I'd have to conduct an experiment first with some scrap wood and some ally plate which i do have lying about in the shed. I digress sorry folks.

Andy.
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