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Old 17th Jan 2008, 3:53 pm   #12
mastermanx2001
Retired Dormant Member
 
Join Date: Jun 2007
Location: London, UK.
Posts: 366
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.


Quote:
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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