Tube Amps / Music Electronics
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|12/24/2000 8:51 PM|
||Paraphase invertor instead of the long tailed. What|
I was told the paraphase invertor doesn't cancel the 2nd harmonics where the long tailed does. Would it be nice in a Fender amp?
|12/25/2000 1:44 AM|
Well I have to admit, I've never heard that 2nd harmonic thing about the inverter but I have heard PLENTY of paraphase inverter Fender amps and they sound "really-really good" driving a couple 6L6s in cathode bias up to about 30 watts through some WeberVST speakers or a couple old AlNiCos!!
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|12/27/2000 2:26 AM|
What's the paraphase, a split load pi? Could you please name a particular circuit that has one? Any idea how it would work with a fixed bias amp? Maybe a 100-watter?
|12/27/2000 3:07 AM|
Look at the early Fender Pro 5D5, and I'm sure other Fender's of that era. But a paraphase is used in there. Of recent amps, the only one I know of is the Bedrock Royal Reverb. Took me a bit and some help to figure it out and get it right. But there is a schem of it posted over at my Boxer site. As I recall, the values used on the Bedrock are very different than the earlier Fenders.
|12/27/2000 5:58 AM|
Some early Deluxes had paraphase too -- 5C3 and 5D3 wide panels anyway. I got the schematics a while back from the Fender Amp Field Guide site. The only functional differences between a paraphase and a Schmitt (blackface) using the same tube are that paraphase has more gain, but there are also more ways it can get out of balance.
I've recently tried Schmitt, paraphase, and cathodyne splitters using a 12AT7 in the same Deluxe and couldn't tell a difference. Other people have told me there is one, but I'm not hearing it. If anybody can shed some light on what to listen for, I'd love to know.
Also, while its possible different splitters may produce different levels of distortion and asymmetry between their output legs, no splitter can "cancel" harmonics. That can only happen in where the signals come back together in the output transformer primary.
|12/27/2000 1:58 PM|
that's a pretty astute observation, and 99 percent true, but not absolutely always correct.
the differential type splitters (i.e. those that share a tail current between two paraphase gain stages) are not often used in such a capacity (or, i should say, advantage of this attribute is not often taken) but they DO have the ability to reduce even harmonic distortion by virtue of their balanced nature.
this occurs due to the curved nature of the gm--there is a dropping off of gm at low plate currents, and an increase at higher plate currents. thus, if the two stages were NOT sharing a tail current, as one tube is turning on, and the other is turning off, the currents would not change at the same rate.
as soon as you enforce current sharing at the tail, the currents will offset one another, and (assuming a perfect current sink in the tail) one must decrease as fast as the other is increasing. in this way the gm is linearized, and the curvature of the gm characteristic that previously lead to the predominant 2nd (and other even) order distortion from the gain stage is decreased.
actually there's additional cancellation going on due to that shared tail current... if the dynamic resistance of the tail is high, you have a high CMRR in the differential pair. iow, any common-mode signal will tend to be amplified LESS than any differential-mode signal.
this is one of the very few topologies that can actually reduce distortion.
just an extra bit of theory on a wednesday morning!
|12/27/2000 3:14 PM|
||break it down|
Back to the average-Joe-doin'-a-mod level, does anyone else have an opinion as to how the various pi's compare to each other vis a vis their effect on the sound? Like, from the ear's perspective?
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