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Old 27th April 2009, 09:25 AM   #1
Tekko is offline Tekko
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Hey! I found your 833A amp project very interesting and i happen to have a 833A, actually it was when i saw GU81M amp on youtube.

After i saw it i searched 833A and came across your project, then after that i built my own, but my little dinky 25W tubeamp doesent have enuf power to drive this 833A to much more than 30-40W before i get distortion.

Did you tell the transformer winder to layer the plate and output windings or is it just wound like a normal transformer ? that it one winding on top of the other.

In my experiment i just took a microwave oven transformer, cut the welds out and removed the primary, then filled up the space left w some 1.5mm2 cable for secondary, then i put a few sheets of paper in the gap to make an airgap. the thing is good for bass but the upper ranges just dont sound very good which is due to the crappy transformer im using as well as no feedback loop to compensate.
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Old 27th April 2009, 12:51 PM   #2
Tubelab_com is offline Tubelab_com  United States
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Did you tell the transformer winder to layer the plate and output windings or is it just wound like a normal transformer ?
The monster transformer was made by Transcendar who is well known on these forums for making good sounding SE OPT's for tube amps. The operator of Transcendar is semi retired from a lifetime of transformer design and manufacturing. At the time he told me that my request was "too big" but he would try it for time and materials cost. I heard the "too big" statement from other transformer winders as well.

It seems that big, SE, and OPT are all conflicting requirements. Make it big enough so that it doesn't saturate at low frequencies and the winding capacitance combined with leakage inductance will create a notch in the HF region. Magnetic and resistive losses get pretty bad too.

Most well experienced transformer winders state that 50 watts is the limit for an SE transformer without resorting to exotic materials. Hammond lists a 75 watt model in their catalog, but I never found anyone who actually used one in a high powered amp. My experiences with their 1628SE and 1628SEA has convinced me that it wouldn't work too good.
Tubelab, it's 5 year mission. To explore strange new tubes, to seek out new circuits and topologies, to boldly go where no tube has gone before......
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Old 27th April 2009, 05:42 PM   #3
isaacc7 is offline isaacc7  United States
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I have no idea what I'm talking about here, but would a parafeed arrangement be more tractable?
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Old 27th April 2009, 05:49 PM   #4
Tekko is offline Tekko
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But now most tubeamps above 30-50W are usually used for bass, either for the instrument or to drive the woofers in a biamp setup.

Anyways i tried a special circuit today to drive the 833A but id say it cannot make enuf current to drive the grid, on full volume i only get a few watts out.

This is the circuit:

A finnish friend of mine insist that this cir will work great and my test does not convine him, he just claim im doing something wrong.
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Old 27th April 2009, 08:01 PM   #5
Alastair E is offline Alastair E  Wales
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Your existing scheme will not give the required drive....Build the driver-stage with a MOSFET as Tubelab did (powerdrive scheme) and you may get somewhere....

Be aware, that the 833A needs a pretty heavy drive and supply voltage!

MOT modified will not work well as O/P. It has no air-gap in the CORE and the windings are not interleaved. MOT are OK 'ish' for parafeed chokes....
Das Beste Oder Nichts
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Old 27th April 2009, 08:21 PM   #6
Tekko is offline Tekko
Join Date: Jan 2005
It does have an airgap, if u read my first post ull see that i wrote that i had cut the welds and made an airgap using a few sheets of paper.

Interleaved windings is not necessary unless your doing a hifi amp, this amp if i choose actually build it will be used for bass only.

Ive been thinking about the powerdrive circuit. that 12AU7 cir was just a test to show that finnish dude that it doesent have enuf drive but he still doesent believe me.
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