|
Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
|
Thread Tools |
24th Sep 2020, 7:54 am | #21 |
Dekatron
Join Date: Nov 2010
Location: Oxford, UK.
Posts: 4,942
|
Re: KT66 gm?
Apart from lots of stupidity, there are several 0.1uF metal shell capacitors in 600V rating here https://www.hificollective.co.uk/ from about a fiver each. Not sure of the dimensions needed though.
Craig |
24th Sep 2020, 8:48 am | #22 | |
Dekatron
Join Date: Sep 2007
Location: Oxfordshire, UK.
Posts: 4,310
|
Re: KT66 gm?
Quote:
If further support is needed for this, I see that when Quad re-released the amp as the Quad II Classic they fitted polyprop capacitors in the C2 and C3 positions, each with a 22pF disc ceramic from the output wire to ground. Cheers, GJ
__________________
http://www.ampregen.com |
|
24th Sep 2020, 10:20 am | #23 |
Nonode
Join Date: Mar 2008
Location: Aberdeen, UK.
Posts: 2,839
|
Re: KT66 gm?
Thanks GJ, for posting Keith Snook's diagram. He clearly expects the KT66's to draw their full wollop of 65mA. My knowledge of Quads is just about nil, but I can see & understand that the amp specialists here have concerns about imbalance.
How old is peterp1's Amp I wonder ? Still no info yet on the "electronics man's" Valve Tester or proper Ia & mA/V results at the Book's value for Vg ? Regards, David |
24th Sep 2020, 10:58 am | #24 |
Moderator
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,803
|
Re: KT66 gm?
Oh, well, no prize for me, but I'm glad it's been done.
Kudos to Keith Snook for spotting a bit of jiggery pokery and demystifying solutions. relying on unspecified strays of components never looks wonderful when you're found out. Things like this are a lot foxier in the RF world than in the audio world. Capacitance direct to ground at these points will tend to promote RF oscillation in the KT66, but that doesn't seem to be a common problem with Quad IIs though I've seen mention of people fitting G1 stoppers and other mentions of care needed with screen decoupler choice including adding smaller high frequency types. David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done |
24th Sep 2020, 11:39 am | #25 |
Dekatron
Join Date: Sep 2007
Location: Oxfordshire, UK.
Posts: 4,310
|
Re: KT66 gm?
There isn't an explicit KT66 screen decoupler in the Quad II. The 'shifting round' of the common element of the output transformer windings from the screen circuit to the cathode means that the screens are held to AC ground just by the 16uF HT smoothing cap. Although the wiring goes round the houses a bit you're right, there really doesn't seem to be a problem with output stage oscillation in this amp.
I'm not sure how critical this stray capacitance is. I've seen plenty of Quad IIs with C2 and C3 replaced by polyprops and they seem to work fine. In that sense I suspect Peter Walker wasn't really 'relying' on it. I suppose it's interesting though that he chose a Hunts Mouldseal for C1 but used the, presumably more expensive, metal-canned type L45 for C2 and C3. I never met him but I understand he could be, shall we say, 'robust' if challenged technically. I suspect he'd have argued a) that his 1950's/60's customers weren't likely to start swapping components in and out, and if they did and doing so bit them then they only had themselves to blame, and b) given the probably more serious impact of all sorts of strays in the output transformer he wasn't going to feel too guilty if the capacitor he'd explicitly selected had a stray that turned out to help a bit . Cheers, GJ
__________________
http://www.ampregen.com |
24th Sep 2020, 4:17 pm | #26 |
Nonode
Join Date: Mar 2008
Location: Aberdeen, UK.
Posts: 2,839
|
Re: KT66 gm?
Just a thought, not having heard from peterp1 - - who was his local "electronics man" with a valve tester? I recall, two or three months or so back, there was a flurry of Forum thread posts from someone down in Cornwall who had knackered a CT160 & a VCM MK3. Cant remember if it was Launceston, or elsewhere. (Testing KT66's perhaps !).
Regards, David |
24th Sep 2020, 11:36 pm | #27 | |
Octode
Join Date: Mar 2006
Location: Watford, Hertfordshire, UK.
Posts: 1,270
|
Re: KT66 gm?
Quote:
e.g. AVO use an Anode Current of 65mA for the KT66 and 57mA for the KT88. For the KT66 they chose -15V to give that 65mA and expect a gm of 6.3. The meter on an AVO tester will have a Red Fail zone for a gm of 4 or less. The Green Pass zone is for a gm of 5.6 to 10.4. So another tester type may choose a different Anode Current point on the graphs and the gm may be subsequently different. I use my valve tester to do a plot with various grid voltages and I consider it to be a most excellent way of matching valves in addition to verifying their health Les-G4CNH
__________________
Whether the Top Cap is Grid or Anode - touching it will give you a buzz either way! |
|
25th Sep 2020, 12:25 am | #28 | |
Moderator
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,803
|
Re: KT66 gm?
Quote:
They could have imagined us having flying cars, conversational robots, holidays on Mars, but tube rollers and directional cables? Not a chance! I think they'd have something very direct to say. David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done |
|
25th Sep 2020, 1:17 am | #29 |
Tetrode
Join Date: May 2019
Location: Melbourne, Victoria, Australia.
Posts: 66
|
Re: KT66 gm?
I always thought AVO's 65mA expected anode current was a typo and if you go by the datasheet and other parametric tester valve books, 85mA should be right around the ballpark for the expected plate current reading based off the 250/250v plate/screen voltage and -15v for grid bias.
|
25th Sep 2020, 6:44 am | #30 | |
Nonode
Join Date: Jan 2009
Location: Papamoa Beach, Bay of Plenty, New Zealand
Posts: 2,943
|
Re: KT66 gm?
Quote:
Cheers, |
|
25th Sep 2020, 9:28 am | #31 |
Octode
Join Date: Dec 2017
Location: Morden, Surrey, UK.
Posts: 1,552
|
Re: KT66 gm?
And of course the famous (possibly apocryphal) story of the Quad engineer being asked at a demo what was the special orange cable being used to connect the speakers - "Lawnmower cable" was the answer.
|
25th Sep 2020, 6:23 pm | #32 |
Nonode
Join Date: Mar 2008
Location: Aberdeen, UK.
Posts: 2,839
|
Re: KT66 gm?
Lacking a response or further info from "peterp1", those of us who are valve testing &/or audio amp enthusiasts are floundering in the surf. We are tossing, hither & thither, lifebelts - without knowing exactly where the troubled swimmer is.
I.e. is this a new acquisition by Peter, or perhaps a previous owner has done some mods ? Or perhaps his local "electronics man" has been delving inside ? Here speaks an ex RAF S & R guy, who had collegues down at St Mawgan. We were used to helping guys & damsels in distress. Regards, David |
25th Sep 2020, 7:18 pm | #33 |
Moderator
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,803
|
Re: KT66 gm?
I guess there's not a lot more can be said without some more information to go on. Would be nice to get some feedback or an outcome, though.
David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done |
26th Sep 2020, 7:41 pm | #34 |
Diode
Join Date: Sep 2020
Location: Launceston, Cornwall, UK.
Posts: 3
|
Re: KT66 gm?
Thank you gentlemen for your kind responses.
I do not know the make or model of his valve tester. He has marked some of the valves thus: 5.9/43 4.3/37 6.3/53 5.5/41 5.7/41 They were from Quad 2 amplifiers. Is there any harm in using them? They sound ok in use. Regards, Peter, Launceston, Cornwall. |
26th Sep 2020, 7:47 pm | #35 |
Moderator
Join Date: Jan 2003
Location: Ipswich, Suffolk, IP4, UK.
Posts: 21,193
|
Re: KT66 gm?
That differs from what you said in post #1
__________________
Graham. Forum Moderator Reach for your meter before you reach for your soldering iron. |
26th Sep 2020, 8:03 pm | #36 |
Dekatron
Join Date: Jun 2005
Location: Warnham, West Sussex. 10 miles south of DORKING.
Posts: 9,145
|
Re: KT66 gm?
I'm confused..J.
|
26th Sep 2020, 8:09 pm | #37 |
Moderator
Join Date: Jan 2003
Location: Ipswich, Suffolk, IP4, UK.
Posts: 21,193
|
Re: KT66 gm?
I'm assuming the first figure is mA/V and the second anode current in mA.
__________________
Graham. Forum Moderator Reach for your meter before you reach for your soldering iron. |
26th Sep 2020, 8:21 pm | #38 |
Dekatron
Join Date: Nov 2011
Location: Surrey, UK.
Posts: 4,385
|
Re: KT66 gm?
Perhaps the intended meaning was something along the lines, "Some of the KT66s I've had tested show low gm compared to the listed specification, only around 3/4 of the figure for a "good" valve". That would apply to e.g the one labelled "4.3/37" if the 4.3 means the gm of that particular valve with 6.3 being a good KT66 figure. (OK, not exactly 3/4 the value, but maybe there are a few low ones in the region of 3/4 "good"). I'm further guessing that the second figure is anode current in mA at a particular bias setting.
"Harm" is unlikely to ensue, but pairing up valves with similar characteristics is good practice- in the case of the Quad II, giving similar standing currents precedence over gm figures, so that a valve with a somewhat higher current at a given bias doesn't tend to hog current over the other- a characteristic of the common cathode resistor set-up. The 4.3/37 specimen would seem to be in its twilight years compared to the others. |
26th Sep 2020, 8:46 pm | #39 |
Nonode
Join Date: Mar 2008
Location: Aberdeen, UK.
Posts: 2,839
|
Re: KT66 gm?
Me thinks that Peter aught to research Push-Pull Amp circuitry &/or get someone to coach or mentor him.
Non of the valves are anywhere up to scratch, except for the one with 6.3mA/V & an Ia of 53mA. For O/P configuration it needs to be paired with another KT66 of very close Ia & Gm. The two 41mA ones might possibly do for drivers in another type of amp. Peter, are you just using your amp, or amps to blast out Status Quo, or for your own guitar playing ? Regards, David |
27th Sep 2020, 3:33 pm | #40 | |
Dekatron
Join Date: Sep 2007
Location: Oxfordshire, UK.
Posts: 4,310
|
Re: KT66 gm?
Quote:
Amp damage resulting from a gassy valve usually makes itself very clear, but unfortunately not until it's too late to do much about it. If you're running successfully with the valves now, and have been for extended periods, then they may be OK. Far and away the best test for gas is to monitor the valve's DC grid voltage with respect to ground. It should be less than 0.5V and stay that way when the amp gets properly hot. Cheers, GJ
__________________
http://www.ampregen.com |
|