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Old 9th March 2005, 02:52 AM   #1
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Default Peculiar Sovtek EL34 bevaviour

I have been happily using Sovtek EL34s in a stereo p-p amp that I have been developing for the past year or so. I am using fixed bias for the EL34s, with a separate 10 ohm resistor in each cathode so I can check quiescent current to adjust the bias. The EL34s are connected as pentodes.

Yesterday, I measured the cathode currents and found that one pair of EL34s was behaving normally but the other pair had gone completely nutzoid. One tube of the 'bad' pair was drawing almost no cathode current at all (about 1mA), while its mate was drawing 125mA! I had previously biased them for 40mA each and I found that readjusting the bias pots made no difference. I tried swapping over the 'bad' pair of tubes. I also tried moving the 'bad' pair from the right-channel to the left-channel, and vice versa. Whatever I did, the same tube consistenly drew almost no current while its mate guzzled 125mA.

I'm sure that the circuit cannot be to blame for this phenomenon, since moving the tubes about makes no difference to which tubes are misbehaving. I must, therefore, have a pair of bad tubes - but what I can't understand is: why one tube should suddenly stop conducting while the other suddenly starts taking 3x its normal current? Any ideas, please?
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Old 9th March 2005, 03:25 AM   #2
SY is offline SY  United States
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Have you done a swap with known-good tubes? IOW, if you had tubes A and B in the left channel (bad), and tubes C and D in the right channel, did you also pair them up AC and BD to see which one is flaky?
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Old 9th March 2005, 05:32 AM   #3
Enzo is offline Enzo  United States
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In other words break up the pairs, my first thought as well. That 125 ma guy might be responsible for the other one getting no current.
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Old 9th March 2005, 06:50 AM   #4
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I suspect that the tube that is drawing no current , is shorted in the grid , so all the negative bias voltage of that pair disappear.
Then , the other tube of the pair, is working hard without any negative bias.

Inspect that..

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Old 9th March 2005, 09:25 AM   #5
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I'm talking about a short, between the cathode and the grid , in that case , the negative variable bias supply sag.

This will cause , the other tube to be without negative bias and consequently to pass high current.
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Old 9th March 2005, 03:03 PM   #6
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Hi there....I had a fault like this which costed a pair of good 88's in an MI amp....fault suprisingly was a busted g1 contact in the valve holder. Thinking it was a tranny problem and o/p stage in HF take-off, the other half was glowing rather....until I knocked the replacement tube into contact just before the other was going to melt down.... I always check the soldered contacts on the tube pins on octals.

richj
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Old 15th March 2005, 01:53 AM   #7
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Thanks for your replies. I have tried various combinations since I last posted, including the "bad" tubes on their own (single-ended). Nothing seems to help: 2 of the tubes are OK and the other 2 are not.

I've been doing this testing with dummy load resistors in place of the speakers and I've noticed that the channel with the "bad" tubes has a large output signal, which increases in amplitude if I increase the volume control. It's as if whichever channel happens to have the "bad" tubes is oscillating and/or is extremely sensitive to noise at the amp's input. The channel with the "good" tubes seems perfectly silent.

I believe the design of my circuit should mean that each push-pull tube is independent of its mate, in terms of both grid bias and cathode current. So, I think they should operate independantly and I can see no reason why one tube drawing excessive current would cause the other tube to draw virtually no current at all (or vice versa).

I've checked with an ohmmeter for short-circuits between electrodes in both "bad" tubes and found nothing wrong. However, richwalters' suggestion to check the soldered contacts at the tube pins makes good sense and I'll do that ASAP.
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Old 15th March 2005, 04:52 PM   #8
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Hi there.....I'm not sure how you want to proceed but my way is given...beware of the hoax.......With fixed bias UL circuits one side can effect the other.....not only though the bias......How ? Through the UL screen/ anode tap as the output tranny is common to both tubes. If one tube is duff or really unmatched/ gain current hungry it can stir up the leakage induct/cap reactance properties into ultrasonic oscillation activated by a small amount of gain from the good tube...and go into crazy motor-boat conduction pulling down g1....this condition is a suspect hallmark of UL using high gm tubes. A standard DVM will go display crazy with the RF......the only sure way to detect oscillation is with an osc'scope. However;

This is a serious bench test.....My way of checking instability in the output stage or feedback loop is to temporarily remove the feedback loop from the secondary of the output transformer (that goes to an input tube). Beware an amp in this condition only requires a few 10's mV for full output. Dummy load the sec with good wattage resistors....then see how the DC conditions of the output stage behaves with no input signal. Measure currents and if settles down then we shall deal with the next problem. The nfb loop can cause oscillation if any component is busted.........you can try this test with the other channel for comparison. .........that will reveal if you have a real DC or AC problem. If this is too much then try this:

A simpler way is to remove the output stage driver tube/s (if there are any) that also effectively breaks the nfb loop. (so long they have interstage caps) Then you will be running the o/p stage alone.......and should be peace and quiet to measure.

I'm not sure of your diagnostic ability....beware of the DVM with the B+...not all meters like it mixed with RF....
In any case I've mentioned the above ......for others to comment.

richj
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Old 16th March 2005, 02:06 AM   #9
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Hi Rich,

Quote:
I'm not sure of your diagnostic ability....
I'm OK in my field - it's only when I get into towns that I have a problem

Your suggestions about testing make sense, although they can't all be applied in my amp. My only feedback loops are cross-coupled, from the OP tube plates to the 6SL7 LTP phase-splitter plates. In between are 6SN7 drivers. Also, I'm not using UL - my OPTs don't have the necessary taps or tertiary windings, otherwise I probably would. And I don't have a DVM, just a moving coil multimeter with 50uA basic movement.

The fact that both channels behave normally with the "good" tubes (as far as I can tell), and both behave abnormally with the "bad" tubes, leads me to conclude that the EL34 tubes themselves are the problem.

I haven't had time, yet, to try remelting the solder joints at the ends of the pins but I'll certainly give it a go. Who knows, it might fix the problem. Suppose, for instance, that the tube that draws almost no current has a break in the cathode connection. Then the only current that could flow through the cathode resistor would be from the suppressor grid. And suppose that the guzzling tube has a break in the suppressor grid connection (making it a tetrode). Could this cause excessive plate current, do you think?
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Old 16th March 2005, 07:49 AM   #10
Jaap is offline Jaap  Netherlands
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Default peculiar behaviour Sovtek 2A3

I have some Sovtek 2A3 tubes which I bought some years ago from a guy in Russia. At least I suppose it is sovtek. It only has 2A3 in small black letters on it and a number 007, the other 008 and so on.
I tried 3 of them in a dc coupled se amp. They all measured fine at the start. After about 2 hours 2 of the 3 tubes began to play softer and one of them started to distort (I did stopped earlier with the other bad tube for safety reasons). I could not measure during playing. I switched the tubes on my two monoblocks and the problems switched to to the channel with the bad tube.

Just curious
What can this be ?
Should I put them in the trash can in stead of in a amp ?

regards,

Jaap
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