Velleman K4040 is Always Blowing Output Tubes - diyAudio
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Old 22nd August 2006, 03:50 AM   #1
GeorgeK is offline GeorgeK  Australia
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Default Velleman K4040 is Always Blowing Output Tubes

Hi Everyone,

I have a Velleman K4040 valve amp which I have had for a couple of years. The unit works fine most of the time BUT does have an unusual problem which has been present more or less from day one.

The amplifier tends to blow “lots” of EL34 output valves destroying the corresponding 10ohm bias resistor. To date, I have lost around “10” output valves. Some were new valves which did not last more than a few days. I’ve tried many different new valve types including KT-77’s etc and several NOS valves with the same result.

The time period between blown valves varies from several months to only a few days. I seem to loose more valves from the left channel than the right. The amplifier in between valve failures works and sounds great. Any ideas how I can eliminate the problem? Constantly replacing valves is becoming a very expensive exercise. HELP!

Cheers
George
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Old 22nd August 2006, 03:56 AM   #2
SY is offline SY  United States
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Replace the sockets.
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And while they may not be as strong as apes, don't lock eyes with 'em, don't do it. Puts 'em on edge. They might go into berzerker mode; come at you like a whirling dervish, all fists and elbows.
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Old 22nd August 2006, 04:35 AM   #3
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This is bad. If I had a design that zapped my NOS then there's real trouble somewhere. What'about if one had a car which behaved like this ? A good design should have tubes that last years not months.
Perhaps we need to know what happens onset tube failure.....glowin' tubes, screen grid glowing inside, or loose bias connections. Does it seriously crackle when one fists the chassis ? I often do this test before I go on-stage. A well designed tube amp should take the physical rigours....tubes are pretty robust.
It isn't unknown for poor soldering on tube pins. check these.

richj
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Old 22nd August 2006, 02:04 PM   #4
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there are other issues, apparantly, some el34s do short occasionally, it may be screen grids, and it may also be bias.

the best el34s are jj, EH, and valve arts, also svets.

make sure the amp is run always with speaker connections, it shoudln't matter with no signal, but still valves/tx's don't like open circuits.

so cool the output stage down, I suspect this fella is a class AB, tho, and if that doesn't work, use 5881s, they are quite cheap direct from russia.

if that doesn't work, check for any shorts, and output tx's, its poss there may be a winding short, then get new ones, or throw it out.
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Old 22nd August 2006, 03:12 PM   #5
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I'm unfortunate enough to own a 4040 also even though its been used as a fancy door stop the last few years. I had the same problem with it eating EL34 tubes. It seems they really run them hot. Ironicly the only EL34s that seemed to last were also the worst sounding ones, the original Sovteks that came with it. I ended up converting it to run JJ KT88 tubes. All that involved was putting in higher wattage same value bias resistors and using 1" spacers on the sockets so they are now up in the holes in the chassis. Otherwise the KT88s won't physically fit because they have bigger bases that won't fit through the holes when the sockets are recessed. Never had a KT88 fail in it.

Dave
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Old 22nd August 2006, 05:19 PM   #6
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Almost forgot as its been a while, if you convert it to use KT88s you also have to change R5 and R10 to 10k to bias the KT88s properly and make the led bias setting work right..This sets the plate current at 40ma.

Dave
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Old 22nd August 2006, 07:01 PM   #7
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DAve...40mA plate quies that's a bit low for KT88's ? What 's the B+ on the Velleman 4040 ?
I've been using 60mA quies on Svet 88's at 530V without fuss.
JJ88's appear to run better but bias isn't quite same as Svet 88's- roughly 10V lower with JJ. THD is also up a notch.

richj
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Old 22nd August 2006, 07:10 PM   #8
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Its been a few years since I've messed with it but it was up near the top end of what a el34 could handle. The 40ma is the same as what the el34 runs at on a stock 4040. I know the 40ma is low and I was going to play around with that assuming the power supply in the 4040 was up to it but it sounds loads better than the el34s did in this unit and no more fried tubes..

Dave
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Old 22nd August 2006, 07:37 PM   #9
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Through the decades of tube amp design I've been far more performance impressed by running output stage in parallel pairs UL p-p and at quite low B+ i.e 270V or so. The tubes run at 80mA quies have a very long life and near "quasi'' class A performance.
parallel pairs Kt88 at 265V B+ UL will give a little over 30W poke but it sounds way louder when compared to my GEC 88-50 Watt. That runs at 500V and it doesn't sound so good in the mid-range.
The emphasis is then on the power supply.
I did buy a set of KT90's and completely dissapointed with'em. the anodes glow red at 60mA whereas JJ88's just sit black.

Some time ago I stirred up quite a fuss regarding the Velleman performance basically of using an ECC83 to drive KT88's. Perhaps this was a typeset error but I was latercinformed it was an 83. What !

There must be something radically wrong with the amp design if EL34's get gobbled up. These tubes in their hayday were the best class audio ouput tube produced.

richj
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Old 22nd August 2006, 10:51 PM   #10
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I am not entirely sure, but the driver tube has to do 2 things current wise...

charge the input cap of the output valve

and supply current to the grid if it starts to conduct, these 2 may be related.

so point 2 is largely irrelevant if you stick to class 1.

As to point 1, it will limit top end and slew rate, and again that may not be consequential, I have seen ecc83s driving 3 pairs of el34s, and even some faily hi cap triodes.

if paralleled, it will double current, the top end of the output tx may be more of a factor. ecc83s do give a gutsy sound that I quite like. guitar amps use them almost exclusively.

I am going to try driving a 300b with an 83, not done, but I will judge on the sound, it will raise eyebrows

just listen and see if you like it, a lot of the time, the theory doen'st sound good in practise.
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