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Old 12th May 2007, 01:55 PM   #1
horace is offline horace  United Kingdom
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Default Help - need some info on this valve amp, please

Hi

I wonder if any of you recognise this amp. To the best of my knowledge it originates from China and was built in the 1990s.

It's branded 'KONES TA-801' and weighs about 25k. It seems pretty well built.

I bought it used, so I don't have any documentation at all. It developed a fault not long after I acquired it, but from memory it sounded pretty good.

Photo attached. I will post a photo of the innards as a follow up and would be grateful for any comments re design, quality etc (as much as you can tell from the pictures.

Basically, I'm trying to decide whether it's worth fixing......

Cheers

Martin
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Old 12th May 2007, 02:03 PM   #2
horace is offline horace  United Kingdom
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And a photo of the internals.....
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Old 12th May 2007, 02:05 PM   #3
horace is offline horace  United Kingdom
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Second attempt.....
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Old 12th May 2007, 03:46 PM   #4
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what was the fault?


Fran
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Old 12th May 2007, 04:37 PM   #5
horace is offline horace  United Kingdom
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Amp works fine for about 15 or 20 mins after switch on, then I get gradually increasing noise in one channel. Tried swapping valves, but the fault stays on the same channel. Someone suggested a dodgy capacitor or dry joint.

Cheers

Martin
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Old 12th May 2007, 09:00 PM   #6
xaviery is offline xaviery  France
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Check the electrolic filtering capacitors in the two power supplies. Maybe on electrolic capacitor changes when heated by its losses.
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Old 12th May 2007, 11:18 PM   #7
kevinkr is offline kevinkr  United States
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Sounds more like a resistor to me. Most likely candidates are ones with large voltage across them and/or dissipating a significant amount of power.
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Old 19th August 2007, 01:44 PM   #8
horace is offline horace  United Kingdom
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Hi

Thought I'd resurrect this thread in order to ask for some more advice.

I'd like to get this amp fixed - can anyone recommend a valve amp service place somewhere near me? I'm in Northumberland (not far from Morpeth).

Cheers

Martin
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Old 19th August 2007, 10:32 PM   #9
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Heres a really simple idea to try. Sometimes the valves become loose in their holders - try taking each tube out and then use a small pair of pliers to crimp the tags of the valve base a little bit tighter.

Its possible that as the valve heats up the grip on the pins slackens off as the contacting metal expands. It has been the source of some crackle issues in one of my amps. The crackle is the sound of tiny amounts of arcing over at the pins.

Alternatively, upend the amp and use a wooden chopstick to prod the big resistors and see if it effects the noise, try this on the cap pins as well - this should identify dodgy resistors or dry solder joints. This should be relatively safe to try if you have the amp well supported before you start.

DC on the wipers of those pots could also be a source of trouble, is the noise effected by the wiper positions ??

Good luck.

Shoog
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Old 14th October 2007, 01:25 PM   #10
horace is offline horace  United Kingdom
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Shoog - Belated thanks for your suggestion. You were right! At least, I think you were....problem is now sorted.

Here's what I did:

Just to make doubly sure that it wasn't a valve on the blink, I swapped them all over (left to right). This didn't fix the problem, but did improve sound quality quite a bit. I don't understand why, but it occurred to me that I had swapped the valves over when the problem first appeared, so I had just swapped them back to their original positions. Could it have something to do with the fact that the amp is fixed bias and the power tube bias was upset when I swapped the tubes over first time around?

I then tried removing and replacing each tube several times to see if I could clean off any dirt that might have found its way onto the pins/valve bases. Just for good measure, I gently bent the pins a little to make them a tighter fit in the bases. This didn't completely solve the problem, but things did improve.

Then I realised that the valves were not sitting absolutley vertically in the bases, so I straightened them until they did. The problem became worse again. It transpires that some of the valve bases are mounted a little squint, so the valves need to be slightly off vertical to sit in them properly.
Job done, problem solved, amp working again!!!

Hooray!

Many thanks to all of you for your advice.

Cheers

Martin
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