Hello,
I've got a pair of quad II's that I've been refurbishing and I have some worries about the results :
Both amps have all new resistors, bias bypass cap and C1. One amp has a pair of new matched Marshall KT66 and the other has a new old stock pair of Tungsol KT66 that came with it and probably aren't matched.
One of the issued is a flash at power ON at the inside base of one of the EF86 input tubes (not on the socket).
The other is that one of the KT66 has some very light red plating after a few minutes warm up and that goes together with 6V on its grid (11V without the tube so probably faulty coupling cap ??). The other tubes have around 1V so also a little high. I will probably replace all four coupling caps but I'm not sure to be able to get metal boddied caps so I'll add a little 33pF to ground from the grids. Any suggestions on what caps to use or will any good MKP at 250v or more do the job just fine ?
Also I've read almost anything and its oposite on blue glow, all four KT66's have both a dark blue glow on the glass, a little more purpleish on the Tungsol pair and a light bright blue glow inside the plate cage. I'm pretty confident the glow on the glass is normal but I've read the inside shouldn't glow and is probably a sign of a gassy tube which could also cause the red plating.
If anyone can clear this up for me that would be great.
Thanks
I've got a pair of quad II's that I've been refurbishing and I have some worries about the results :
Both amps have all new resistors, bias bypass cap and C1. One amp has a pair of new matched Marshall KT66 and the other has a new old stock pair of Tungsol KT66 that came with it and probably aren't matched.
One of the issued is a flash at power ON at the inside base of one of the EF86 input tubes (not on the socket).
The other is that one of the KT66 has some very light red plating after a few minutes warm up and that goes together with 6V on its grid (11V without the tube so probably faulty coupling cap ??). The other tubes have around 1V so also a little high. I will probably replace all four coupling caps but I'm not sure to be able to get metal boddied caps so I'll add a little 33pF to ground from the grids. Any suggestions on what caps to use or will any good MKP at 250v or more do the job just fine ?
Also I've read almost anything and its oposite on blue glow, all four KT66's have both a dark blue glow on the glass, a little more purpleish on the Tungsol pair and a light bright blue glow inside the plate cage. I'm pretty confident the glow on the glass is normal but I've read the inside shouldn't glow and is probably a sign of a gassy tube which could also cause the red plating.
If anyone can clear this up for me that would be great.
Thanks
Definitely replace all the (C2/C3) coupling caps before testing again.
Russian K40-9y and K42Y are popular choices. Scrape any paint or coating off first to ground their cases.
If you have not seen it there is information here: QUAD II Valve Power Amplifier Information
Some KT66s do have a blue glow, not a sign of a problem.
Russian K40-9y and K42Y are popular choices. Scrape any paint or coating off first to ground their cases.
If you have not seen it there is information here: QUAD II Valve Power Amplifier Information
Some KT66s do have a blue glow, not a sign of a problem.
Thank you for the link, this is where I got my info so far, it's also in this article that I read the blue glow behind the plates is a sign of gassy tubes (not the regular outer blue glow that gives these tubes such a good look). That's what got me worried.
I did a search for these (k40y-9) but didn't find anything at first, I now found someone selling these on ebay so I will probably order a set.
I did a search for these (k40y-9) but didn't find anything at first, I now found someone selling these on ebay so I will probably order a set.
Hmm flash stays on the same EF86 when moved so that will have to go as well. It did sound very good for the little time I left it running before noticing the red plating.
Some say the blue inner glow can appear after running the tube at too high plate voltages/red plating and thus liberating gass. I hope it didn't ruin the tube after 5 minutes, the red glow was barely noticable, I cross my fingers it was only just above normal use.
Some say the blue inner glow can appear after running the tube at too high plate voltages/red plating and thus liberating gass. I hope it didn't ruin the tube after 5 minutes, the red glow was barely noticable, I cross my fingers it was only just above normal use.
Sounds like the "refurbish" was more like "rerubbish". I'd say open the the case and check the wiring and parts vs. the Quad II schematics. Fix all wirking issues and replace wrong parts. I'd tend to stay away from exotic parts. Good film/foil (MKP) caps from reliable manufacturers are the way to go.
That's a bit harsh mbrennwa, all the parts I replaced are good quality correct replacements suggested by keith snook who seems to know his way around these amps and wiring has been checked several times. Also except from the grid voltages, all the rest checks out compared to the schematics. The only thing I apparently did wrong was to leave the coupling caps after testing them and had been told they were unlikely to go bad (guess not).
Don't worry about the EF86 flash. It's normal. And as for KT66, marshall don't make tubes, they rebrand. My recommendation is to get a matched quad KT66 from eurotubes.com ( or any other good dealer that offers warranty and matching worth the name) Read all info from keith snook, don't make any other jobs on the quads !
Indeed, tubes are made by Shuguang but matched and rebadged by Marshall for use in their gear, I would expect them to be serious about that no ?
I've thoroughly read through all the info on keith snook's website and replaced everything he suggested to get them up and running without blowing the budget. I will first give it a quick test after replacing the coupling caps and see how the tubes I have on hand react and if necessary will order a new set.
I've thoroughly read through all the info on keith snook's website and replaced everything he suggested to get them up and running without blowing the budget. I will first give it a quick test after replacing the coupling caps and see how the tubes I have on hand react and if necessary will order a new set.
Of what colour is that EF86 flash? I bet it resembles incandescent light bulbs. If so, it is quite normal and due to some uncoated heater filament area.
What happens if you short circuit each KT66's grid leak resistor? Do the bias currents and the blueish glow stay the same?
I wouldn't bother with antique Russian capacitors. They're just a hype. Put in modern industrial quality foil capacitors instead if the original ones prove to be faulty (see above).
Best regards!
What happens if you short circuit each KT66's grid leak resistor? Do the bias currents and the blueish glow stay the same?
I wouldn't bother with antique Russian capacitors. They're just a hype. Put in modern industrial quality foil capacitors instead if the original ones prove to be faulty (see above).
Best regards!
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Absolute 100% rubbish. These caps are by now of proven reliability, tolerance, and subjective audio quality, and have been used by thousands of DIYers and restorers over the years. Probably far better made than many of today’s ‘audiophile’ caps. Keith Snook also knows his stuff with Quad IIs, and wouldn't use nor recommend these caps if they were ‘a hype’.
Yes the flash looks like a simple incandescent light bulb, no worries than, good to know.
I ordered the russian caps because I really want to keep that extra capacitance to ground without having to add an extra component in there. When I see people advising to go with old electrolytics I have my doubts but with film caps, when it comes from people who really seem to know their stuff I give it a try. Also I really didn't pay that much.
I'll first install the new caps and see what the results are and if that still doesn't change things I'll try shorting the grid leak resistors.
I ordered the russian caps because I really want to keep that extra capacitance to ground without having to add an extra component in there. When I see people advising to go with old electrolytics I have my doubts but with film caps, when it comes from people who really seem to know their stuff I give it a try. Also I really didn't pay that much.
I'll first install the new caps and see what the results are and if that still doesn't change things I'll try shorting the grid leak resistors.
The extra capacitance to ground is claimed to have been designed into the circuit, but I've never seen any actual evidence for that, or that it was even there when the original couplimg capacitors were new. The amplifier works perfectly well without it. The KT66s already have Cag of about 30pF, and that is the only pole required for stability.
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