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Old 31st July 2003, 03:26 AM   #1
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Default Wavac MD300B

Anyone tried doing this amp with Inverted Interstage Trans Coupling?
Is there a website where the schematic is published?
TIA
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Old 31st July 2003, 12:36 PM   #2
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Default Re: Wavac MD300B

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Originally posted by hardcore
Anyone tried doing this amp with Inverted Interstage Trans Coupling?
Is there a website where the schematic is published?
TIA
This amp doesn't use IIT; it's not mentioned on the WAVAC page. The inverting interstage is used in class A2 and means that with careful design, the grid current passing through the secondary matches the anode current of the tube in the previous stage, which means it can be made more like a parafeed IT, without the coupling caps. 300B's do not dig A2 operation.
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Old 31st July 2003, 08:31 PM   #3
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Default Re: Re: Wavac MD300B

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Originally posted by Brett

The inverting interstage is used in class A2 and means that with careful design, the grid current passing through the secondary matches the anode current of the tube in the previous stage, which means it can be made more like a parafeed IT, without the coupling caps. 300B's do not dig A2 operation.
the iit is set up with the secondary reversed so the A2 grid current partially offsets the plate current of the driver. i suspect the approach gained favor because if it doesn't offset, it adds... and adding more DC just makes the IT that much more compromised.

for the longest time i wanted to do a perfect offset so the iron could be optimised, but the numbers never worked out, and i came to the realization that the IT needs a gap anyways so a small amount of DC wasn't so bad.

i chased the whole small gap / current cancelling thing for some time, and it never showed me the results i had expected...a well designed gapped trannie always sounded better.... except for one case where the only way to get it done was with the current cancel concept.

A2 drive is still an interest to me... but those inverted slightly less than unity turns ratio IT's are a beee-atch to design.

i'd like to get my hands on one of the tangos to see what they look like.

dave
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Old 1st August 2003, 01:06 AM   #4
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Default Re: Re: Re: Wavac MD300B

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Originally posted by dave slagle
the iit is set up with the secondary reversed so the A2 grid current partially offsets the plate current of the driver.
Thanks, forgot to mention the reversal specifically, though it's implied in the design.

Looking at the 805 schem, it looks like the P and S currents will balance reasonably well, allowing a smaller gap than required for the 6L6 on it's own.

Tx's may be a mongrel to design, but neither of us are Shisheido-san.

Cheers
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Old 1st August 2003, 02:24 AM   #5
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Default Re: Re: Re: Re: Wavac MD300B

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Originally posted by Brett


Thanks, forgot to mention the reversal specifically, though it's implied in the design.

Looking at the 805 schem, it looks like the P and S currents will balance reasonably well, allowing a smaller gap than required for the 6L6 on it's own.

Tx's may be a mongrel to design, but neither of us are Shisheido-san.

Cheers
don't forget the traditional 1:0.7 stepdown requiring 1.4X the A2 grid current to cancel the plate current.... you always seem to end up chasing your tail.

i still say the best approach is to keep a traditional IT design and rely on the IIT to lighten the load on a stressed core/run the driver tube at a bit more current. a small tweek down of the gap to increase inductance is fine, but going beyond that imo sends you in the wrong direction.

dave
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