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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment.

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Old 28th Feb 2019, 3:56 pm   #21
Trigon.
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Default Re: 18-0-18v transformer question.

Just for info, that looks rather like the kit here:-

https://www.tindie.com/products/ICSt...m1875t12008-2/

Which includes the circuit diagram below:-

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2018-03-01T10 49 05.380Z-12008_.pdf

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Old 28th Feb 2019, 4:52 pm   #22
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Default Re: 18-0-18v transformer question.

Ahh, it is. Thanks for finding that. I had searched around a bit and not found it.

It turns out mine had been supplied with a heater resistor for running on 12-0-12 even though the board was marked 18-0-18.
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Old 28th Feb 2019, 5:08 pm   #23
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Default Re: 18-0-18v transformer question.

Just out of curiosity, what if I replaced the 47 ohm 3 watt dropper with a big wirewound pot and a permanent wired voltmeter or two even (I have both) so the heater voltage is adjustable. The location of the resistor on the board is getting the adjacent capacitor unhealthily warm after a few hours. Any drawbacks to doing this?
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Old 28th Feb 2019, 5:32 pm   #24
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Default Re: 18-0-18v transformer question.

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Any drawbacks to doing this?
Apart from cost, no.
 
Old 28th Feb 2019, 5:44 pm   #25
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Default Re: 18-0-18v transformer question.

The danger in having an adjustable heater voltage is that you can get it wrong and shorten valve life. Big wirewound pots may be much more expensive than big wirewound resistors.

Why these Chinese audio circuits choose to use a VHF pentode designed for radar IF strips is unclear. Maybe there are thousands of them sitting in warehouses and someone needed to find a purpose for them?
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Old 28th Feb 2019, 5:58 pm   #26
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Default Re: 18-0-18v transformer question.

Well.....


That seems to confirm that the heater supply voltage is the reason for the tight spec on the 18V ac. It suggests you got the resistor for 12V supply too, though if they're 6.3V heaters I would have thought a link would be more appropriate. Maybe they're being deliberately underrun?


The highest HT you can get from voltage doubled 18Vac or bridged 36Vac is around 48Vdc on light load, so higher ac in might push your luck with 50V capacitors! Cheap enough to replace with 63V types if you're bothered.


A quick reverse engineer of the power supply would be interesting just to see what they're up to with the valve and silicon HT rails.


It ought to be possible to kill any audible hum, though if the PC layout is poor this could involve cuts and links

Edit: Posted this after reading post#20 (Bottom of "page"1) before I saw the rest.
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Old 28th Feb 2019, 6:22 pm   #27
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Default Re: 18-0-18v transformer question.

Having looked at that circuit, the PSU is interesting. The heaters are running via 4R7 from the positive dc silicon rail- nominally about 21V-ish.

The silicon chip power amps (LM1875) are running on around +/- 21V unregulated rails, so on a 4R speaker you might actually get about 20W according to the amp spec sheet

http://www.ti.com/lit/ds/symlink/lm1875.pdf

The valves are running from an odd looking active(?) voltage multiplier putting -? on the cathodes and +? on the anodes. The signal grounding as a result looks to be a bit weird- may explain the hum?

Anyone got an explanation for the odd HT supply system? Simulation suggests it's just a slow startup circuit
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Old 28th Feb 2019, 6:26 pm   #28
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Default Re: 18-0-18v transformer question.

Quote:
Originally Posted by G8HQP Dave View Post
The danger in having an adjustable heater voltage is that you can get it wrong and shorten valve life. Big wirewound pots may be much more expensive than big wirewound resistors.

Why these Chinese audio circuits choose to use a VHF pentode designed for radar IF strips is unclear. Maybe there are thousands of them sitting in warehouses and someone needed to find a purpose for them?
I have the meters, military spec wirewounds and boxes full of 6J1 equivalent valves of various brands out of old RAF kit. I hadn't got a 47 ohm resistor here anywhere, only a couple of 24 ohm ones so the heaters are under running a bit at the moment. I'll fiddle with it some more tomorrow....
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Old 28th Feb 2019, 6:40 pm   #29
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Default Re: 18-0-18v transformer question.

As you have loads of valves to fit how about an experiment running them at a full 18V (or whatever it turns out to be) to see if they do degrade?
 
Old 28th Feb 2019, 8:40 pm   #30
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Default Re: 18-0-18v transformer question.

From what I have read elsewhere it works but produces lots of weird distortion more in line with guitar type amplification. I did also read that its possible to convert these to run on proper HT voltage after changing to higher voltage capacitors. All stuff to play with without messing up "proper" vintage gear....
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Old 28th Feb 2019, 9:09 pm   #31
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Default Re: 18-0-18v transformer question.

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Originally Posted by Bomber county View Post
Frequency response: 14Hz-100kHz (by RC high pass circuit is considered as the maximum range 3.29Hz-482.29kHz)
Now that's just a silly claim for them to make...

Is it designed for bats to listen to? Even 'just' the 100KHz! As for 500-ish KHz, that is way, way, way into RF territory, and just because there's a high pass filter doesn't mean it's going to come through the speaker!

With such nonsense about frequency response, it's hard to know what to take seriously. I'd be surprised, though, if that 19V is strict, and your experience with 10% over seems to bear this out.
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Old 2nd Mar 2019, 8:45 am   #32
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Default Re: 18-0-18v transformer question.

I'm wondering if the hum problem could be caused by a lack of decoupling.

I could be wrong, but it looks like there's a kind of virtual earth created by using the -ve line of the dual power regulator as a signal ground while the rest of the circuit uses the 0v line as ground.

Even though the diagram suggests that the junction between resistors R11,12, 14 and 15 is almost a single point on the board, judging by the photo, it actually goes all around the place. The only capacitance between -20v line and 0v is cap C8 at the regulator.

Perhaps a cap between the junction of R11 and R12 and the actual 0v ground, and the same from junction of R14 and R15 to 0v.

As I said, 'I could be wrong'


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Old 3rd Mar 2019, 3:33 pm   #33
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Default Re: 18-0-18v transformer question.

I'll give it a go. I found a bit on another page that suggests a 470uF cap between the junction of R7 and R8 and the audio output ground may help too. Also beefing up all of the "ground" tracks on the board with a layer of solder. Weirdly the hum appears to be on one channel only and comes on the other channel if I touch the glass part of the valve on that channel.
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Old 4th Mar 2019, 2:16 pm   #34
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Default Re: 18-0-18v transformer question.

Made a bit of progress with it. removed the two 23 ohm resistors I had fitted vertically in place of the wrongly sent heater dropper. Replaced them with a pot on a short lead and the unpleasant hum on one channel is gone. I have the heater voltage set to bang on 12.6 volts but to be honest a volt over or under seems to make no difference. I still have a bit of low level hum on both channels but not really any worse than any of the other valve stuff I have. Next thing I might do if I can get hold of one is try a proper 18-0-18 torroidal transformer on it too see what difference it makes.
What I did notice though is if I crank the volume up the heater voltage fluctuates a little, is this too be expected or do I have a power shortage?
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Old 4th Mar 2019, 4:54 pm   #35
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Default Re: 18-0-18v transformer question.

Wrong heater voltage can affect valve life more than it affects valve performance. Hence you will not hear much difference. 10% either way is tolerable, 5% would be better.
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Old 4th Mar 2019, 7:04 pm   #36
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Default Re: 18-0-18v transformer question.

Since the heater supply comes from the same unregulated rail as the silicon power amps, I would expect the the voltage to vary a bit with the output level of the amplifier. It's unlikely to be a problem in itself. If you want to "fix" it, you could replace the series resistor by a series regulator circuit, but you may just be into polishing the t**d* at that point.

Did you say you have a bench psu? Try using that to feed the heaters with a stable 12.6 V and see if it makes any difference.

* An old technical expression for wasting your time......
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Old 4th Mar 2019, 10:21 pm   #37
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Default Re: 18-0-18v transformer question.

Just found a bit on youtube about the same amp, that guy has put a 12 v regulator in place of the 3 watt resistor. Somewhere I have a box full of strange adjustable ones with a pot glued to them that can be preset anywhere from 5v to 30v so I might explore that out of curiosity. Data sheet with them is in French as they came from the old Philips factory years ago so not got a clue what sort of power they can handle.
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