HIWATT 400 - any queries?

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iskrem
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Joined: Thu Apr 17, 2014 9:29 pm
Location: Norway
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Re: HIWATT 400 - any queries?

Post by iskrem »

Hi there,

Just curious, did you measure actual RMS power output? I've built 2 of these with transformers from TAD, and 300 watts is the power I got from them. Even if pushed into class AB2.. Not that it prevents them from shaking walls though..

thanks,
Lars
Iskrem Electro - www.iskrem.org
gingertube
Posts: 530
Joined: Mon Nov 14, 2011 2:29 am
Location: Adelaide, South Oz

Re: HIWATT 400 - any queries?

Post by gingertube »

No I did not measure output power but 300 Watt is what I would expect to see. This is based on the datasheet which says 100W per pair of KT88.
Cheers
Ian
gingertube
Posts: 530
Joined: Mon Nov 14, 2011 2:29 am
Location: Adelaide, South Oz

Re: HIWATT 400 - any queries?

Post by gingertube »

Well finally sent it home to its owner.
Cathodes wired straight to 0V so no way to check idle current of each tube and there are only 2 bias adjustments one for each of the three tubes on each side of the push pull.
So I found the output tranny primary centre tap B+ feed and measured the DC resistance of each half primary, that is to each set of three output tubes. By measuring voltage drop from centre tap to each side I could at least set equal idle currents in each push pull side.
Thought about bias levels, for KT88 or 6550 you should never bias hotter than 35W,even though they are rated at 42W(if you take special precautions in the design). While I had matched each set of three output tubes equal current sharing between them is probably IFFY, so I calculated voltage drops for each half OT primary based on 32W per tube and set that.
CAUTION: On using this method of measuring voltage drop across each half primary, one of your multimeter leads is at +700V and the other lead is at +697V, clip the positive lead to the B+ centre tap before powering up so you have just one lead to move about.
Ran all the small tubes on the valve tester and discovered one triode of the 12AT7 output tube driver was stone dead , ditto for one side of the 12AX7 phase spitter dual triode. SUS. Replacing those tubes was POSSIBLY all it needed, the old output tubes might have been OK.
Anyway the owner has taken it off to give it an acceptance test hammering.
Cheers,
Ian
gingertube
Posts: 530
Joined: Mon Nov 14, 2011 2:29 am
Location: Adelaide, South Oz

Re: HIWATT 400 - any queries?

Post by gingertube »

Report from owner. The hammering test was a big success. Now using it as his 1st choice bass amp.
Cheers,
Ian
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