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Quad II Amplifier KT66 Valves/Tubes and Carbon Composition Resistors
Quad II Amplifier KT66 Valves/Tubes and Carbon Composition Resistors
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Old 20th April 2016, 03:20 PM   #1
Alnasir is offline Alnasir
diyAudio Member
Join Date: Oct 2014
Default Quad II Amplifier KT66 Valves/Tubes and Carbon Composition Resistors

Greetings from Portugal,
A couple of years ago I bought a pair of the Quad II amplifiers and restored them with recent manufacture electrolytic capacitors and NOS (New Old Stock) Russian PIO (Paper in Oil) capacitors. I will share this heartwarming experience in a different post with photos and more in depth information. Hopefully members can benefit from this and in case they do not have any training/experience with vintage electronics but as long as they are handy with a soldering iron and take extra care in reading and following instructions with photos, a "normal refurbishment" is actually quite easy on an old but working amplifier. Failing that, your local repair shop is perfectly capable of putting in these parts!
When I took possession of the pair of Quads, they were in a truly sorry state with an assortment of old valves that did not appear identical and one of the mains input transformer would overheat within seconds producing a nasty smell! I sold all the old valves on eBay together with all the old capacitors that I had replaced. This allowed me to buy a third untested amplifier with the intention of replacing the bad transformer but there must be a universal law (Murphy´s?) against this because the 3rd mains transformer did not seem to be an “original”, because the terminals of the windings were handwritten and it worked fine apart from the first 30 seconds after switch on during which time it made all sorts of crackling and popping noises!
It took me weeks to find the parts for refurbishment because I wanted to maintain the vintage or retro look. Also, 3 different suppliers were required to fulfill my parts list thereby making the final costs (including shipping!) quite expensive. So, I ordered surplus parts and sold them as kits. At present, all my kits are sold out and the Russian PIO capacitor dealers have practically run out of their stock thereby causing a doubling in price compared to 2 years ago. I have sourced other suppliers and am awaiting delivery of the capacitors but with a higher voltage and bigger size for testing, which brings me to the topic at hand! Sorry, but I like to tell my story properly!
All this time, I have been using my Quads with the Russian 6S3P valves after having changed the original 3W bias resistor R12 180 Ohms to 220 Ohms at 5W to accommodate these new valves which are quite inexpensive with the added advantage of being smaller and much narrower than the original KT66s. They also draw a smaller current hence a twofold advantage for my transformers because being narrower, they do not heat up the transformers as much as the fat coke bottle KT66. I am very pleased with their performance but on the downside the amplifiers have lost their original appealing looks that the coke bottle KT66 impart to them!
So, I am now in the market for 4 new recent manufacture non audiophool priced KT66 tubes and would like some pointers and advice, please!
Also, I intend to replace the original Carbon Composition (CC) resistors with current production 1/2Watt 350VDC Carbon Composition resistors to maintain their vintage looks…..any comments will be appreciated!
Apologies for the long post but it allows me to introduce myself as well!
Thanks in advance and regards to all.
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Old 20th April 2016, 05:48 PM   #2
Eli Duttman is offline Eli Duttman  United States
diyAudio Member
Join Date: Apr 2004
Location: Monroe Township, NJ
Purchase the "reissue" Gold Lion KT66s, from a reliable dealer.

In the unlikely event that OEM carbon comp. parts have not drifted out of tolerance or gone noisy, leave them be. Which matters more to you, performance or appearance? Carbon comp. parts are superb in grid stopper positions and problematic elsewhere. Modern carbon film parts will retain the "warm" voice, while eliminating noise and drift.
Eli D.
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Old 21st April 2016, 12:41 AM   #3
Alnasir is offline Alnasir
diyAudio Member
Join Date: Oct 2014
I have been busy locating datasheets from different manufacturers and have made a list of the heater currents of the different brands so far. It seems that the heater current is one of the most important parameters that will differentiate a 6L6 (usually around 0.9A) valve from a KT66 (usually around 1.3A) and the other parameter being anode power dissipation (30W for the 6L6 and 25W for the KT66). I hope with these parameters one can “scientifically” identify the Wolf in Sheep´s Clothing: because after all it is much more profitable to sell the cheaper 6L6 bottled as the more expensive KT66 and no one is any the wiser.
GEC KT66 heater current 1.3A as per 1977 datasheet
Genalex Gold Lion KT66 specified at 1.27A New Sensor Corp old datasheet
JJ KT66 at 1.3A from a recent datasheet
Tung Sol KT66 a nominal 0.9A so probably is a 6L6 and I read somewhere that this valve is used in the new QUAD Classic Amplifiers!
Please help compile this list for all to see and use!
Thanks and regards,
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