Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: worth on December 25, 2010, 09:08:57 am
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What is the primary impedence of this output transformer ? It uses 4 X 7591. Would it work in a 2 X 6L6 build ?
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7591 is a 7868 in an octal base config.
7868 wants to see close to the same primary imp as a 6L6 (if I remember correctly)....~~ 2k for 4 bottles, & ~~4k for 2 bottles
So, if you use 4x 6l6's then 2ndary taps stay the same (as labeled on the amp), but if you go with 1/2 the tubes (2x 6L6) then double the 2ndary imps.
IOW, if you use 2x 6L6's, your labeled 4-8-16 ohm taps (using 4x bottles) become 8-16-32 ohms using 2x bottles.
G
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Most old tube data calls for 6k6 for a pair of 7591s, so for a quad that would drop to 3k3. It IS similar to the 5881/7581/6L6 specs, but is a true pentode, and drives similar to an EL84/6BQ5.
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What's your power supply??
If you have lots of power, use four 6L6/EL34 or two 6550 for 100W.
If you don't have heaps of power, you can re-label the taps and run two 6L6 at 50W.... but then why carry 100W worth of iron for 50W output?
If you have the original power transformer (PTs and OTs should stay together-- it's a known-working well-engineered plan), four big bottles or two BIG bottles will work but you need to devise a new bias supply.
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I have measured lots of them and (as I recall), they measure out to around 2700 ohms. If not that, then something in that neighborhood.
Either way, they would work just fine with a pair of 6L6's. Just relabel the taps (IE the 4 ohm tap becomes 8 ohms and so forth).
Dave
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Don't forget to figure in the supply voltage which, together with the primary impedance, determines the maximum power the output tubes will handle. This must be within the tube spec...
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The one I have here measures 2500 ohms for the primary impedance. It is using four 7868's with about 470v B+, and in my modified circuit, with fixed bias, puts out 58 watts RMS and about 102 watts peak. It sounds very nice too. The 7868's are pretty thick and juicy tubes when overdriven. Nicer than 6L6's in my opinion.
Greg