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Old 16th March 2006, 12:07 AM   #21
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Thanks Chris,

Since I have dealt with this elsewhere (although a long time ago), I will summarise.

It is difficult to note the plate gauge (unless you are wealthy enough to smash one for the purpose of investigating!), but I look at (1) the G1 cooling fins (on top) and (2) the general mounting below. For the classic GE 6L6GC (7581A etc.) the mounting pins are relatively short and thick. I found them thinner for the Ruskies, and sometimes the general construction seemed just a little, er, well, less rugged.

We have an importer of tubes here in Johannesburg RSA, not far from me. (He is an insurance appraiser and does this for a hobby.)He had Sovtek WXT, EH, Svetlana etc., and I found the Svetlana best looking, but relatively expensive. They had rugged grooved anodes, the classic oval shape. Then came the rest with box shaped anodes (no grooves or ridges), etc. Of those the EHs seemed to look somewhat more rugged, but I am now getting to subjective impressions. This guy sells 95% of his power valves to music bands of one kind or another. Imports are not matched, but he will match for a nominal fee.

Mentioning cost would not mean a lot; the EHs go for the equivalent of about US $18, which might appear cheap - but keep in mind that our whole economy here is a little different to yours.

Regards
Johan
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Old 16th March 2006, 01:19 AM   #22
anatech is offline anatech  Canada
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Hi Johan,
Prices are all relative. BTW, 7581A is a 35W plate.

-Chris
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Old 16th March 2006, 09:49 PM   #23
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Yep - how I wish I could afford a dozen or so! I recently refurbished a pair of Quad IIs fitted with those.

Suppose I should now get back to thread. How are things, Stueyp?
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Old 18th March 2006, 11:23 AM   #24
stueyp is offline stueyp  United Kingdom
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Just a quick check, what wattage and type of those 100ohm resistors are these suposed to be? Would a higher watage resisitor be of any advantage in this case?
Many thanks,
Stuart.
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Old 18th March 2006, 07:37 PM   #25
stueyp is offline stueyp  United Kingdom
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Also, if I remove the relay and take a picture, would anyone be able to tell me how to hard wire into triode, of would you need a schematic?
Thanks,
Stuart.
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Old 18th March 2006, 08:28 PM   #26
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Stuart,

I have not checked all the posts back: I presume you talk of 100 ohm screen-to-anode resistors - those could be 1/2 W or less; they only dissipate about 50 mW or so.

Btw, I asked some time before: You do not have screen taps on the output transformer? (Would be advantagous if you could use UL.)

Hard-wiring to triode would simply mean mounting the 100 ohm resistors between screen and anode posts on the socket, but a circuit might be useful just to check whether other things like capacitors lurk around the output transformer. There is also the question of altering feedback component values if you have.

Regards.
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Old 20th March 2006, 02:59 PM   #27
stueyp is offline stueyp  United Kingdom
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Hi guys,
Heres a coupe of pics of what I have done...

http://www.fossilfarm.co.uk/stuebay/board1.jpg
http://www.fossilfarm.co.uk/stuebay/board2.jpg

The scorch mark went right the way through the board (literally, its almost a hole), hence the black patch still there. Ive checked the caps and resistors etc , all appear ok.

I have a new relay coming, but the existing one works fine, and the whole amp SEEMS to be fine.
Please let me know if I have got my wires crossed with some of the advice given, and whether my solution is more dangerous than an unattended chip pan.

The resistor is 1w, but I have smaller ones if I should make a change. I chose a 3 amp quick blow fuse, looking at the 5v on the relay.

Another thing, i finally got the darned manual translated from chinese and it turns out the bias was set too high from the factory. The manual suggests drawing 40-45mA, before the explosion, they were running at over 55mA (without me tinkering with the settings). Could this have contributed to the fireworks?

Im remaining vigilant. In a couple of days I'll check the transformers. But theres only so much PCB green I can take in one day...

Many thanks,
Stu.
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Old 20th March 2006, 03:00 PM   #28
stueyp is offline stueyp  United Kingdom
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Johan,
Im not sure what you mean by screen taps?
Sorry,
Stu.
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Old 20th March 2006, 03:45 PM   #29
anatech is offline anatech  Canada
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Hi Stu,
Remove all the charred board with an exact-o (hobby knife) knife, it's conductive or can be in higher humidity. You can fill it in with epoxy if you wish.

The higher bias could have certainly helped the tube to fail. Just think, the plates weren't glowing red yet but I'll bet the screen grid was. The control grid may have got hot enough to get the tube into thermal runaway too.

R18 looks like it may have gotten warm there. The 3A fuse is useless where it is, the resistor will burn out first. Speaking of resistors, I see you have a carbon film as a replacement. You should be using a metal oxide type as the original was. Since you have to dig out all the burned stuff, replace the resistor with the proper type. Sorry.

-Chris
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Old 20th March 2006, 03:52 PM   #30
stueyp is offline stueyp  United Kingdom
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Whoops!
Ok, i'll let you know how I get on.
Thanks.
Stu.
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