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Old 18th November 2015, 12:22 PM
bikerhifinut bikerhifinut is offline
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Join Date: Jan 2006
Location: Penrith, Cumbria
Posts: 828
Default Re: valve testing el84 only

Quote:
Originally Posted by Phil Y View Post
Andy, as I have already said, 3 of the 4 times I have had valve failure was before I put the amp in pentode mode so I see no reason to suspect this. Also, the reason I did it this way was basically because all the designs I looked at online have the screen feed not taken from the same point as the main feed to the o/p Tx. Normal practice seems to be to take it from a more smoothed, slightly lower voltage point, further down the R-C filter chain. I was just trying to copy what seems to be standard practice. I did consider "splitting" the 2.2k dropper in the HT to create a feed but with a PCB design I decided this would be a bit of a messy solution.

http://www.r-type.org/articles/art-003e.htm Scroll down for circuit.

And this one: http://www.drtube.com/schematics/vox/ac156202.gif

I am not a designer, just a repair technician, so I am unable to argue the finer points with you. My plan for now is to leave the amp in use for a couple of weeks and then lift the bonnet again and re-check all voltages, re-test the valves, and see if anything has changed.

Regards, Phil.
Sorry Phil It's me going round in Circles. Not you.
It just looks to me like you have had a bad run of luck with EL84's.
For what it's worth, I think there's a lot of "snake oil" Valves out in retail land.
Certainly for current production there are very few manufacturers still making them.
Contrary to all the dire warnings that i had at the time (2003 have I been messing with valves that long?) when i look back, the best batch I had were the original replacements I got when refurbing my stereo20. A set of Chinese "Golden Dragon" valves from Tube-shop which is the retail arm of PM components afaik. I could have got them a bit cheaper i think but on reflection, I had no duffers, the EL84 were matched as a quad, and the ECC83 are still doing their thing and measuring well. So i have the feeling that Chinese valves are either better than they were, or the premium paid for a reseller to weed out the dross is worth it?
Currently I have a quad of Sylvania US 6BQ5 in. I think they have a slightly higher Va and Pa than bog standard EL84. Whatever, the amp sounds as good as ever with them in.
Just wish I had snapped up the quad of NOS Philips EL 84 I spotted for not a lot more than a set of Sovteks. They were on examination, Philips branded but Blackburn made jobs. (Philips as we know owned mullards since before WW11). Philips valves were no different anyway, as you would expect as it was their design, as far as I know.
A snippet of info i picked up recently from someone who has been in the Valve business for a very long time was that Telefunken had a lot of their stock made for them by RFT in the DDR. This was a trick pulled by much of West German business, they would get assemblies and components made for their expensive branded gear in the east. Which is fine, the East Germans were excellent engineers and their stuff was as good as the next door neighbours, just not as well dressed up or finished when presented to their internal and export market as a finished product.
Food for thought maybe, or just me on one...............

Fingers crossed for the amps Phil.

A.
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