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Old 31st May 2008, 12:38 PM   #1
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Default KT66 alternatives in Quad II amps?

I need to get a pair of Quad II working for a friend. He has 3 working old KT66 but doesn't want to pay silly money for a fourth.

What other output tubes do you suggest? I have 5881 and EL39 for example - may also have some EL34. EL39 have top caps, but that's OK - nice tube!
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Old 31st May 2008, 01:44 PM   #2
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Andy,

The KT66 is a "glorified" 6L6. An obvious replacement is the SED (=C=) 6L6GC. Tweak the amps' voicing by "rolling" small signal tubes.
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Old 31st May 2008, 02:35 PM   #3
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What's the voltage on the anode and screen grid with the Quad II?

Need to observe those! Andy
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Old 31st May 2008, 02:56 PM   #4
JoshK is offline JoshK  Canada
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Doesn't Shuguang make a bespoke KT66?
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Old 31st May 2008, 03:59 PM   #5
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I'll have to look at the current crop of KT66.

Meanwhile, as to substitutes:

Quad circuit says
anode 340v 65mA 22.1W
screen 330v 7mA 2.3W
cathode 26v resitor 180 (shared)

Is 5881 a bit wimpy? Data states 23W anode max and 3W screen
That would put it on max? Max anode and screen voltage is covered at 400v

I actually have some 6550 just remembered. That shuld be OK. Except that would add 1.2 amps to heaters. Probably OK if preamp not used. I'm open to correction on this.

EL39 should be interesting - trying to find details.
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Old 31st May 2008, 06:12 PM   #6
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EL39 is an odd-duck tube - I've seen EL38s labeled as 'EL38/EL39'.

you may want to check out the EL38, though it is a true pentode and requires a plate cap.
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Old 31st May 2008, 07:45 PM   #7
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Rewire the socket for 6BG6GA. The later Sylvania ones have large 30 watt anodes just like 6L6GC/7027A/7581A. Heater current is only 0.9 amp, which your power transformer will appreciate. These come up on that auction place regularly and sell for about 4 USD each.

Wade
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Old 31st May 2008, 08:10 PM   #8
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My pal 7N7 reckons put in a 5881 and it should bias up fine. I'll have a look!

Still curious about the EL39, except looks like screen max is 325v - needs to get to 330v. Not a huge difference. Not sure if it's one of the 18 watters or 25. I have the ST shape ones with black glass
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Old 31st May 2008, 08:52 PM   #9
cerrem is offline cerrem  United States
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Quote:
Originally posted by tubewade
Rewire the socket for 6BG6GA. The later Sylvania ones have large 30 watt anodes just like 6L6GC/7027A/7581A. Heater current is only 0.9 amp, which your power transformer will appreciate. These come up on that auction place regularly and sell for about 4 USD each.

Wade

Wade....
That is the best and most usefull idea I heard in a long time, regarding QUAD II tube substitution....
I like the part about the reduced heater current.....
I will look into this...

Chris
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Old 31st May 2008, 09:03 PM   #10
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Rewire the socket for 6BG6GA>>

Data says 20W dissipation, so it's a case of distinguishing which are which, if later ones were 30W
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