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Old 5th March 2010, 06:48 PM   #11
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Join Date: Aug 2004
Location: Mansfield, Connecticut
Great! Thanks for that information.
I too want to play around with this type of design for a little more power. What are you using for an OT? I'm currently running my dual monoblock build with Hammond 1628SEA's. I think these will handle two KT88's in parallel as they are rated for 30 watts.

I think adding two more KT88's will put me over the top on my PT though. I'm again using a Hammond 273BX, and it's sized correctly for the tube compliment I'm running right now. Time for a different PT I guess.

I'm still really enjoying the LED bias on the power tubes, best change I've made so far!

"Imagination is more important than knowledge"
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Old 5th March 2010, 10:53 PM   #12
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Just a comment; to parallell tubes means that you have to halve the OPT primary impedance to get the desired doubling of power. If you have 8 ohm speakers and want to stay with the 1628 you should try to hook them up to the 4 ohm connection. That will give you a reflected 2.5k primary, should be about right for parallell KT88's

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Old 6th March 2010, 02:04 AM   #13
AKSA is offline AKSA  Australia
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You could use a higher gain tube, running 10mA or so, and a 2:1 step down interstage, which will give you four times the drive.

One which comes to mind and is a good tube is the 6GK5. Rp is around 5400R, so four to one would give you a Zout around 1K4, plenty to drive a stiff grid bias resistor, too.

Downside is overload margin at the grid, not much more than 3Vpp, but you will have very good overall gain.

Another approach - with no interstage - might be to use Pete Millet's 12HG7 driver with 400V B+. This has a Zout around 4K, and very low distortion with excellent overload margin. BUT, this tube is a pentode, and lots of folks bring out the wooden cross when the word comes up in conversation.....

See attached.
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Aspen Amplifiers P/L (Australia)

Last edited by AKSA; 6th March 2010 at 02:15 AM.
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