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Old 24th December 2012, 11:40 PM   #11
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Location: Little Rock
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Originally Posted by Armstrong View Post
I see the EL84 is the same as a 6BQ5? I have some of those here.
Yes, and if you have four good ones, you've already begun your design. These tubes need a power supply of about 300 to 350 (tops) volts B+ and some 6.3 VAC heater power.

There are zillions of variations and driver circuits, and with these excellent output transformers you'll have a really good sounding amplifier. The Dynaco ST-35 circuit works well and is easy for a first project, but you'll really want to improve the bias arrangement as soon as possible. Lots of excellent possibilities; one fun variation is the "Red Light District", which is also an excellent and original design.

All good fortune,
Chris
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Old 25th December 2012, 01:53 AM   #12
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Eli might step in here for this one.
D@mned right!

That's pretty good looking O/P "iron". As long as you don't go for full bass extension, UL mode "finals" will be fine. The trafos will go right into an "El Cheapo". Any 6V6 family tube can be used without changing parts values in the uploaded schematic. Jim McShane has all the small parts, along with tubes, for the project. Ask the man for a pair of DPDT switches suitable for triode/UL mode switching too. The 1 KOhm screen grid resistors get connected to the wipers and the "ends" get connected to the associated anode/UL trafo taps. NEVER switch mode while the unit is powered up! Power O/P is approx. 6 WPC in triode mode and 12 WPC in UL mode. Mid 90s sensitive speakers are appropriate.

The Heath O/P trafos are good enough to allow the use of 0.068 μF. caps. in the I/P high pass filter, instead of the value shown. There is a risk of core saturation, if you go for full bass extension, and saturation is highly undesirable. Allowance must made for a GNFB induced LF error correction signal. With 0.068 μF. in the high pass position, the LF limit is 23.4 Hz. and that's well below the lowest note a double bass can play.

A full set of power "iron" can ordered from Allied. 2X stock # 70218526 take care of primary B+. These days, a pair of N-68Xs cost less than a N-77U. Stock # 70218344 takes care of "12" V. heater power and B+ boost. Stock # 70218145 takes of the B+ filter choke. Stock # 70009000 is the little 6K27VF B- and 12AT7 heater power trafo.
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Last edited by Eli Duttman; 25th December 2012 at 01:55 AM. Reason: grammar
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