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Old 16th Jan 2008, 4:27 pm   #1
mastermanx2001
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Default Quad 2 Valve power amp questions

I have just brought my Quad 2 valve power amps back to life the first time in about 25 years. (see earlier thread today). They do sound fantastic at the moment through some vintage B&W DM4 loudspeakers. The amps have original line up of valves, which includes GEC KT66s. I understand that it is normal for the mains transformers to run quite hot. Mine run at 39 deg. C. However, I may wish to try and reduce this temperature, so as to make the amplifier more reliable by putting less strain on the mains transformer. This may be by using an output valve that draws less heater current. Out on the web there seems to be contradictions about heater currents for the various suitable valves. I have thought the 6L6 draws less than GEC KT66. Can anyone expand on this, perhaps recommending the best valves.

Also, some protection for the HT may be sensible. Can anyone recommend where to put fuses and what values to use.

Regards, Paul.

I have posted this here because more sensible and technical people seem to be on this site than say a Quad specialist site.
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Old 16th Jan 2008, 5:24 pm   #2
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Default Re: Quad 2 Valve power amp questions

Quote:
Originally Posted by mastermanx2001 View Post
I have posted this here because more sensible and technical people seem to be on this site...
How true! Whether it's vintage radios, fridge thermostats or computers, there's always someone here who will give you a friendly, sensible and honest answer
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Old 16th Jan 2008, 6:06 pm   #3
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Default Re: Quad 2 Valve power amp questions

The 6L6 does take less heater current but it's a lower power valve too. I suspect to use it, ideally you would need to rebias the output stage and probably consider lowering the screen voltage. In any case the Quad output stage is somewhat unique, so any other effects might be too!

You can find data sheets for both valves (and many others) here: http://www.tubedata.org/

39C doesn't sound particularly hot for electronics- bear in mind that many Quad II amps would have been hidden away in cabinets when new- operating it out in the open probably reduces the temperature usefully. Possibly a quiet fan would do more to reduce temperatures than drastic action like changing valves.

Chris
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Old 16th Jan 2008, 6:38 pm   #4
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Default Re: Quad 2 Valve power amp questions

Hi Paul, fusing is usually in the centre tap lead of the HT winding as well as on the mains input side; this protects against most faults, say 1A for the mains and 150/250mA for the HT.
39C is not that hot. Typically the hot spot temp is allowed to 120C max.
If you really want to reduce it you could add an external heater transformer and disconnect the heater wires from the normal one.

Ed
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Old 16th Jan 2008, 8:22 pm   #5
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Default Re: Quad 2 Valve power amp questions

Soory about this, but I just remembered that about 25 years ago someone gave me four new General electric 5851A valves. The boxes are marked 7781A/KT66. I have just found them (see photo). Would these do the trick of lowering the heater current? and perhaps they would not lower the output as the 6L6 would. If this is the case these would seem to be the ideal valves to use. Can someone tell me?. Regards, Paul
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Old 16th Jan 2008, 8:31 pm   #6
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Default Re: Quad 2 Valve power amp questions

The 7581A is a bolt in replacement for the KT66. Quad used to supply these (about 10 years ago) as replacements for the original KT66's. They have as near as dammit the same electrical specs - including heater current.
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Old 16th Jan 2008, 8:31 pm   #7
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Default Re: Quad 2 Valve power amp questions

Of course I made a mistake above, the valves are of course 7581A/KT66 as in the photo. Sorry again, Paul
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Old 16th Jan 2008, 9:50 pm   #8
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Default Re: Quad 2 Valve power amp questions

I'm in danger of answering my own question, but it looking like the GE 7518A/KT66 valve is the one to replace the GEC KT66. Audiomm mentions that the heater current is the same for both, but there appears to be comment on the internet that the 7581A/KT66 has a heater current of 0.9A and the GEC KT66 a heater current of 1.3 A If my calculations are correct,this would be a 31% decrease of heater power for the output valves. This must be good for the mains transformer life. Perhaps someone would confirm this. Regards, Paul
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Old 16th Jan 2008, 10:10 pm   #9
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Default Re: Quad 2 Valve power amp questions

If everything else in the amp is working OK then your transformer should be happy too. The trouble is it's one of the weakest links, so if some kind of fault develops then it can be the first thing to go. And replacing it can be eye-wateringly expensive. And the paint colour never seems to match.

So the trick is to keep an eye on all the other stuff. In fact the components most vulnerable to heat can be the capacitors. If the 0.1uF coupling caps go leaky or (less likely) if the resistance of the R or C in the KT66 cathode circuit should go low then the KT66s can draw enough current to see the transformer off. I've replaced all these components in my Quad IIs using modern, reliable alternatives. Not very authentic I'm afraid but I like to think Peter Walker would've used polyprops instead of paper-in-oils and polyesters instead of electrolytics if only he'd had them available. A check of the KT66 cathode and grid voltages will tell you if the old caps are going west. Watch out also for second hand KT66s which have higher-than-usual emission. The old ones are rare now and people will sell "hot" ones claiming that their high current means they've got loads of life left. Maybe they have, but if you put them in your Quad II then maybe it won't have ...

Another problem, apparently, is that the HT choke can migrate inside its can until it shorts to the pins holding the cathode components. Unless you've got the sort of kit that was used to take the X-ray on Keith Snook's page http://www.dc-daylight.ltd.uk/Valve-...D-II-Mods.html you don't get any warning of this (maybe tar oozing from the choke ?).

Sorry if all this sounds a bit gloomy. As Nick Ross says, don't have nightmares ! Once they're up and running these amps are pretty reliable. The best thing for them is to keep enjoying them

Cheers,

GJ
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Old 16th Jan 2008, 10:26 pm   #10
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Default Re: Quad 2 Valve power amp questions

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Originally Posted by mastermanx2001 View Post
there appears to be comment on the internet that the 7581A/KT66 has a heater current of 0.9A and the GEC KT66 a heater current of 1.3 A
I think you're right. Tung-Sol and GE say the 7581 is one of the 6L6 famly with a 0.9A heater see http://www.pentodepress.com/tubes/7581.pdf or http://www.pentodepress.com/tubes/7581A.pdf It also has a lower anode current at the standard test point (250V on the anode and screen, -15V on the control grid). You get 85mA from a GEC KT66 at this point but only about 70mA from the 6L6 family. So although it will work OK it won't be quite the same as "the real thing". Actually the reduced heater current only saves you about 5W of transformer load out of a total of more than 80W (10W in the GZ32 heater, 20W plus between the KT66s and EF86s and 50W plus in the HT supply). However if replacing the valves also cuts the HT current you will save a bit more. If you get a chance to make some DC current measurements with the 7581s vs the KT66s in your Quad IIs I'd be interested to see the results. Just measuring the voltage across the cathode resistor would be a good start.

Cheers,

GJ
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Old 17th Jan 2008, 11:34 am   #11
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Default Re: Quad 2 Valve power amp questions

Hi GJ and everyone, I can't get over the enthusiasm on this site, I would like to take in all the info in a while, say few days, and write a sumary so that it may help others. This afternoon, I may fit the GE 7581A and measure across the cathode resistor as you say. I know that my resistors are generally a bit high in this amp, about 10% from nominal, but I will include the measured value of the cathode resistor. regards, Paul, believe it or not, I used to work for GEC Marconi.
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Old 17th Jan 2008, 1:53 pm   #12
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Default Re: Quad 2 Valve power amp questions

Hi GJ and anyone interested, I have done the tests suggested by GJ. On my completely standard Quad II (GEC KT66 and GZ32, used valves but probably good) The HT is 337 V before the smoothing choke and 332V after the choke. The voltage across the 180 Ohm cathode resistor (actually measures 193 Ohms) is 25 Volts.

I put in the new pair of GE 7581A/KT66 valves (and did the same measurements: HT before smoothing Choke is 343 V and after the choke is 338 V and the voltage across the cathode resistor (measured valve 193 Ohms) is also 25 V.

I hope that this is if interest. Would you please explain the significance of these results, regards, Paul.


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Originally Posted by GrimJosef View Post
I think you're right. Tung-Sol and GE say the 7581 is one of the 6L6 famly with a 0.9A heater see http://www.pentodepress.com/tubes/7581.pdf or http://www.pentodepress.com/tubes/7581A.pdf It also has a lower anode current at the standard test point (250V on the anode and screen, -15V on the control grid). You get 85mA from a GEC KT66 at this point but only about 70mA from the 6L6 family. So although it will work OK it won't be quite the same as "the real thing". Actually the reduced heater current only saves you about 5W of transformer load out of a total of more than 80W (10W in the GZ32 heater, 20W plus between the KT66s and EF86s and 50W plus in the HT supply). However if replacing the valves also cuts the HT current you will save a bit more. If you get a chance to make some DC current measurements with the 7581s vs the KT66s in your Quad IIs I'd be interested to see the results. Just measuring the voltage across the cathode resistor would be a good start.

Cheers,

GJ
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Old 17th Jan 2008, 2:29 pm   #13
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Default Re: Quad 2 Valve power amp questions

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Originally Posted by mastermanx2001 View Post
Hi GJ and anyone interested, I have done the tests suggested by GJ. On my completely standard Quad II (GEC KT66 and GZ32, used valves but probably good) The HT is 337 V before the smoothing choke and 332V after the choke. The voltage across the 180 Ohm cathode resistor (actually measures 193 Ohms) is 25 Volts.

I put in the new pair of GE 7581A/KT66 valves (and did the same measurements: HT before smoothing Choke is 343 V and after the choke is 338 V and the voltage across the cathode resistor (measured valve 193 Ohms) is also 25 V.

I hope that this is if interest. Would you please explain the significance of these results, regards, Paul.
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Old 17th Jan 2008, 4:05 pm   #14
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Default Re: Quad 2 Valve power amp questions

Hi Ed, Thanks for the info, The Quad II already has a a 2A mains fuse, perhaps this could be reduced to nearer your recommendation of 1A. My pair of IIs still have the original Belling Lee fuses in (could be from the late 50s!) Looking at the circuit diag, fot the II, I see the HT centre tap to earth, but cannot see it on the transformer tag strip (see photo). I presume that it must be connected internally, so may not be easy to gert at. Any suggestions? Regards, Paul.

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Originally Posted by Ed_Dinning View Post
Hi Paul, fusing is usually in the centre tap lead of the HT winding as well as on the mains input side; this protects against most faults, say 1A for the mains and 150/250mA for the HT.
39C is not that hot. Typically the hot spot temp is allowed to 120C max.
If you really want to reduce it you could add an external heater transformer and disconnect the heater wires from the normal one.

Ed
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Old 17th Jan 2008, 4:11 pm   #15
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Default Re: Quad 2 Valve power amp questions

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Originally Posted by mastermanx2001 View Post
Hi Ed, Thanks for the info, The Quad II already has a a 2A mains fuse, perhaps this could be reduced to nearer your recommendation of 1A. My pair of IIs still have the original Belling Lee fuses in (could be from the late 50s!) Looking at the circuit diag, fot the II, I see the HT centre tap to earth, but cannot see it on the transformer tag strip (see photo). I presume that it must be connected internally, so may not be easy to get at. Any suggestions? Regards, Paul.
Having problems posting photo!

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Old 17th Jan 2008, 6:47 pm   #16
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Default Re: Quad 2 Valve power amp questions

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I see the HT centre tap to earth, but cannot see it on the transformer tag strip (see photo). I presume that it must be connected internally
Right again Paul, the E terminal on the mains side of the transformer tagboard is where the centre tap for both the HT and the 6.3V heaters is brought out. I'm trying to think if it would still be safe to put the additional fuse in this line. In principle the heaters should only induce a very small current in it since they are balanced. But what happens if one half of the GZ32 goes short ? Does this blow the fuse ? If it does then are the heaters now connected, through the short, to whatever charge is still left in the smoothing capacitor ? Maybe this scenario is way too far-fetched to worry about and maybe various other things save you even if it does happen

The idea of fusing the HT is a good one though. So good that Quad actually did it in versions of the Quad II they supplied to Rediffusion. I've never seen one in the flesh but there's a photo on this page http://www.geocities.com/ResearchTri...diiwipmar.html Apparently it was a 500mA fuse in the HT, presumably between the rectifier cathode and the smoothing cap or between the cap and the output transformer/choke.

Some people are so concerned about meltdown that they add a solid-state circuit to monitor the cathode voltage and, if it rises, switch the power off via a mains relay. Details are on this very long page http://www.turneraudio.com.au/quad2powerampmods.html This seems a bit OTT to me, but can't hurt I guess.

The significance of the cathode resistor voltage is a) that all the KT66 current (anode plus screen for both valves) passes through it and b) that it tells you, more or less, what the grid bias is. In your case the total current is 25V divided by 193 ohms i.e. 130mA. According to the circuit it should be 144mA total so the KT66s are running a little light. Assuming the grids are close to 0V (it's important that they are) the valves' bias voltage will be about 27V because there will be 2V or so dropped across the cathode windings of the output transformer which you have to add to the 25V. So it sounds like your KT66s have 305V screen-to-cathode and 310V anode-to-cathode and they're drawing 65mA total current each. The graph at the top of p7 of the KT66 data sheet http://www.dc-daylight.ltd.uk/Valve-...a/KT66-GEC.pdf lets you check these numbers against what a new valve should do. If you run your finger along the bottom to about 310V and then run it up to cross the different grid voltage curves you'll see that you cross the 30V bias curve at about 38mA and the 20V curve just off the top of the graph, let's say 104mA. So with 27V bias you'd expect the current to be 38mA +(104-38)mA x 3V/10V which comes to 58mA. So your KT66s measure up rather well against the GEC data sheet and it seems the Quad numbers are optimistic. The reason your total current is a bit low is the low HT voltage (can you try another rectifier ?). It's not anything wrong with the KT66s themselves.

I have to say in my experience the Quad circuit currents are about right but the HT voltages are usually higher than your 340V/330V by at least 20V. This makes the KT66s currents and bias voltages consistent with the GEC data sheet and it's just the Quad circuit HT voltages which are awry. Has anyone got any more info about this ?

If you want you could repeat these sums for the 7581s and compare with their data sheet. I'd do it but my wife's just got home with the week's shopping and if I leave her to unload the car on her own in the rain the consequences will be too awful to contemplate.

One last thing. There may not be any significance in the different HT voltages you measure with the KT66s and the 7581s. In my house mains voltage fluctuations routinely cause bigger changes than this.

Cheers,

GJ

Last edited by GrimJosef; 17th Jan 2008 at 6:55 pm.
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Old 17th Jan 2008, 6:52 pm   #17
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Default Re: Quad 2 Valve power amp questions

Hi Paul, on some potted transformers like the Quad the connection will go to the screen and posssibly and earthed heater line as well before exiting the can. This appears to happen in your case. It is probably not a good idea to fuse this point as if the fuse goes due to a fault, then a highish voltage may be imposed across valve heaters to cathodes.
It could be worth reducing the size of the mains fuse to 1A or even less as long as you use a time delay/ anti surge type; otherwise you may get fuse blowing at switch on due to surges. Try measuring the input current when running at full power, this will be an indication of the current the fuse will se in use.
The HT centre tap fuse is used because there are some faults that can cause the transformer to burn out, but not draw sufficient current to blow the input fuse.

Ed
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Old 17th Jan 2008, 7:40 pm   #18
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Default Re: Quad 2 Valve power amp questions

Hi GF and Ed and everone interested. Right! With the used GEC KT66s I have put a new (old stock) Pinnacle GZ34 in and rechecked the voltages. HT before choke = 358V, after choke = 352 V and the voltage across the 193 Ohm cathode resistor is now 27.5V. What does this mean. I'm not too good with the graphs etc. Oh yes, the mains voltage here is pretty stable.

Also, still not sure what is best as regards the HT fusing. I saw the Redifusion amps, but cannot see where in the circuit the fuse is. Regards, Paul.
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Old 17th Jan 2008, 11:12 pm   #19
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Default Re: Quad 2 Valve power amp questions

Hi Paul of course I am tempting fate here so I am tyhking this post with my finhgs crossed! But I have very happily used my Quad 2s almost continuously since I bought them second hand in 1969. I have of course tried a few newer amps over the years, but in the end I have always returned ‘home’ to Mr Walkers grey doorstops. I do though have a pair of new (1970) ELS57s to go with them, and I find this antique combination (driven from a startlingly recent Audio Research LS7) pretty much all that I want for a bit of acoustic Jazz or nice Classical. Actually the Hi-Fi system sometime sounds rather better that some concerts I've been to, because of poor acoustics or Bl..dy modern PA systems.

Like GrimJosef though I have updated the beasts with some small mods and newer components. A long time ago I replaced the cathode bypass caps with something new and 100UF, the adjacent cathode resistor was replaced with a big ceramic thing. The HT smoothing got a couple of 47UF across the (I thought wrongly) paper-in-oil boxes, and the rest of the caps replaced mostly with expensive non-polarised yellow things we used at work. Metal film resistors and some silver plated Teflon wire was fitted in odd places (I did all this so long ago so that memory fails). Oh and the dreadful Jones socket was replaced with a nice and shiny gold-plated phono socket, and plated same binding posts for the speakers (the shame of it!). Original fuses though…

While many would grumble at the vandalism perpetrated on my own amplifiers (and I would not have a problem with that) I would though have a problem with changing the KT66s to something else. Change the KT66s? Change the KT66s? The Quad 2 IS KT66s! You know, that mains transformer always ran hot enough to poach and egg on it, always did, it was a feature! I suppose like Turville some have removed their GZ32s and converted to semiconductor diodes (well it was supposed to make them produce 30 Watts) That would give the mains transformer something to think about I imagine (and the KT66s), but with a reasonably standard and just lightly restored pair of amps you should have almost unlimited life (er, amplifier wise). Did I mention that most of my KT66s are original?

Seriously, welcome to what is a probably rather diminishing club. I suppose Quad 2s are rather like vintage cars, not everyone likes them but there again I don’t know what else there is to run a ELS57 on. And as far as the ELS57 goes I am still waiting for a significantly better speaker to be invented.

Weren’t those Rediffusion amps 100 Volt line units?

Cheers
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Old 18th Jan 2008, 12:24 am   #20
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Default Re: Quad 2 Valve power amp questions

Hi Cformat, I pretty well agree with all you say, I may have given the wrong impression, I have owned these Quad II amps for more than 30 years, but not used them for more than 20 years. I have also tried many other amps over the years (including Quad 303, 405-2, transistor Radfords, Sugden etc etc.), another favourite is Radford STA25 which does drive the my Quad ESL 57s nicely and I have been using this quite a lot over the years. I am not interested in trying to get much more power out of the II, it seems fine as it is. However I am interested in trying the GE 7581A/KT66 firstly to preserve the GEC KT66s, secondly to reduce the loading on the mains transformer. I understand that Quad at one time supplied GE 7581A/KT66 (this is marked on the valve), so they must be OK. These days my Hi Fi front end is the PC with a good sound card. I feed the outputof this direct to the Power amp, not even a volume control potentiometer in-between. It seems to work very well.

I found the same with concerts, some years ago I went to a jaz concert that was recorded for broadcast later. The sound where I was sitting was reinforced with some not very good equipment. some time later, I heard the FM broadcast of the concert over my ESL57s and it was far better than the so called live experience. Best regards, Paul. PS if you like the ESL57, you may like the B&W DM4s, I find them a bit more practical and still very good.

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