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Old 14th March 2006, 02:05 PM   #11
amperex is offline amperex  United States
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Buy some NOS tubes. Siemens made an EL34 that could handle very high voltages. But, you best check the operating bias.
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Old 14th March 2006, 03:10 PM   #12
Nordic is offline Nordic  South Africa
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Just press F4 on that link, it most certainly works, the site is just a little busy some times. you have to click on DOwnload here once the page opens.
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Old 14th March 2006, 04:16 PM   #13
kevinkr is offline kevinkr  United States
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Hi Stueyp,
Excellent pictures.. I recommend very carefully cleaning the board with rubbing alcohol (90% if you can get it) your local apothecary should have it.. Then once you have done that look at the traces connecting to the burned components, they hopefully won't be damaged. The carbon residue on the board is probably somewhat conductive and needs to removed, usually alcohol and a stiff acid brush will do the job nicely. Blot the area with paper towel and repeat until it is as clean as you can get it. Hopefully you won't have to remove the board to clean the other side.

Check all of the resistors connected to that socket (replaced or not) and the ones connected to the other valve and make sure that they measure to within a couple of % of the same location on the other channel.

Be sure to check the bias pots! (4 pin foot print with pot on other side of board.) Also it looks like there was (hehe) a capacitor on the screen grid of that tube that exploded? Can you check the voltage on the remaining caps, this should be marked somewhere on the capacitor. Are those relays used for switching from triode to UL mode, or do you know?

It might not be a bad idea to install fuses in series with the cathodes of the EL34.

The Siemens EL34 are wonderful, but IMHO they will cost more than what you paid for this amplifier. I am not incidentally a big fan of most Russian made EL34, and would recommend a quartet of new JJ instead. Their QC seems more consistent, although people do rave about the sound of the EH EL34. Make sure any you buy in the future are matched and burned in for at least 24 hrs - if you can't get them that way contact me as I can...

Need to think about how to power this amp up safely once you have made repairs. I recommend a variac to do this, or alternately just a ballast lamp in series with the power cord to the amplifier could be used to limit current in the event of a fault. (3 x 100W bulb in parallel, all of which should be connected in series with the amplifier power cord.)
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Old 14th March 2006, 08:18 PM   #14
anatech is offline anatech  Canada
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Well, just to throw a different view in.

I am using 6CA7EH's in my own amp, better than the EL34's I tried. I have used 6CA7EH's in Mesa Baron amps that eat EL34's (pick your brand), if you know that amp you also know what I mean. It should be stuffed with 6L6GC's or better.

So I am very happy with Electroharmonix as some of you know. You may have had a defective tube, can happen. In general I find the fat bottle 6CA7/EL34 more robust. These are beam tetrodes as opposed to a true EL34 which is a pentode.

Be aware too that a defective output transformer can ruin the tube, so can an open in the load (not in your case as you heard it oscillating). Also check the original solder joints to the plate and screen leads, and grid for defects. It would be nice to know the why of this.

-Chris
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Old 14th March 2006, 08:30 PM   #15
anatech is offline anatech  Canada
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Hi Stu,
Those relays switch G2 between the transformer and a resistor to plate (triode to pentode mode). One wonders .... many places where you could have a problem here.

Can you clean up the damaged area and wash it with alcohol, then take another picture?

Also, try to get a schematic so you can service the amp in the future.

-Chris
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Old 15th March 2006, 07:42 AM   #16
stueyp is offline stueyp  United Kingdom
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Hi guys,
I will check the solder points, the ouput from the transformer, and clean up the bombsite. I'll take another picture of the relay. Installing fuses sounds like a good idea, but what pins should I put them between, and what sort of fuse? The reason I ask is I have a surplus of 3.15A glass fast-blow types from an Audio Analogue Puccini. (Lf.F3.15AL250V)

By the way, the amp as you can see in the pictures has two sets of speaker posts, one for 4 Ohm and one for 8 Ohm. If I have a pair of speakers running at a nominal 6 Ohms, which post do I put them in??

Thanks again,
Stu.
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Old 15th March 2006, 03:33 PM   #17
EC8010 is offline EC8010  United Kingdom
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Lordy, lordy, what a bonfire! The last time I had a bonfire like that was inside a Tektronix curve tracer. I would echo all of kevinkr's advice and add that fibreglass brushes are good for scrubbing burnt printed circuit board away. You really do need to get rid of all the damaged parts and to wash the affected area with isopropyl alcohol (sometimes available from chemists but always available from Farnell or RS).

After that, a circuit diagram is essential. It is not enough to replace the damaged parts - you need to know what caused the problem. I would suspect a failure of negative grid bias on the destroyed valve.
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Old 15th March 2006, 08:45 PM   #18
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Quote:
Originally posted by amperex
Siemens made an EL34 that could handle very high voltages. But, you best check the operating bias.
Ahh..ah and also the price ? Nup.
I can't say much for siemens nos EL34's, the ones I got (fawn coloured box with blue end) tend to have stars glowing inside on the grids. This implies non cleansiness in assembly.

Re burnt out amp..I wasn't able to download all pics.../details.....but it sounds if only a solid state power supply could do that kind of damage, if a GZ or 5Uxx was doing the rectification, they would immediately glow red hot on a short circuit and the drama would stop there. From the details I gleaned it mentions a ring transformer/or toroid...capable of high current at low impedance.
You mention triode cum pentode c/o switch. A defective switch may not show up on a meter, because internal arcing could be via switch body to chassis and would certainly re-track/arc internally the next time B+ appears and the problem is ready to destroy the replacement tube.. Switch replacement is the only option or hard wire the problem out.
The other point is if the anode was shorted to chassis via int arcing from a duff switch, the screen (g2) may have it's supply derived from another low impedance source and the current immediately destroy the tube. One needs the circuit diagram.

When checking the output transformer resistance with some digital multimeters, the ohms range may not settle due to the slow rise time v.s inductance....simply shortout a secondary winding. If the tranny is duff this problem is automatically solved.

Again, with Sstate psu's, this emphasises the correct fuses.

richj
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Old 15th March 2006, 09:27 PM   #19
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Stueyp,

Oh man, my heart burns for you! As a man of many years and several such experiences in my youth, I know how you must feel. (Assure you it is worse if one discovers afterwards that all the devastation was due to one's own stupidity....)

Just to add a bit about the output transformer. I did not download the circuit but presume that the connections are known. One way to test is to remove connections (removing power tubes would in most cases be sufficient), and feed a small voltage into the secondary and measure on the primaries. If you do not have a signal generator the mains will do - I mean suitably reduced by a transformer, of course!!

The ratio of secondary/primary would be very roughly of the order of 20 - 30:1, so a few volt should do. Avoid shocking yourself! You should notice if there is a short somewhere in windings, by not measuring equal voltages where you should, e.g. the anodes, G2 taps, etc. This will be more revealing than a d.c. ohms test - the latter will, of course, show any open circuit. One problem might remain undetected, i.e. a winding short only on high (signal) voltages.

Regards, and success!

But I also note with concern the comments on the different tube makes. I have just purchased 10 x 6L6EH (supposed to be 6L6GC), and carefully read the different experiences. They will work under 460V G2; 550V Va (fixed bias). I hope I will not need to open a similar post some time from now! It might be a little off- thread, but can someone kindly comment on any experience with JJ tubes, please?

Thanks.
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Old 15th March 2006, 10:50 PM   #20
anatech is offline anatech  Canada
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Hi Johan,
I am told the plates are a little heavier than spec for a 6L6GC. They do look in between a 6L6GC and 7581A.

Where did you get them and did you buy any matched? As for me, the hardest service I have seen them in are Fender Twins. They seem to be happy and no complaints. Even the guitar guys like the tone.

I have used quite a few, so let me know how they work out for you. I've heard the JJ's are very good too. Since I am happy so far I have not bothered to try the JJ's.

-Chris
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