Jadis Defy 7 Schematic

[carl 33]

HI .DEFY 7 HAS BLOWN RESISTORS. I ALSO HAVE SOME SMOKED AREAS . ANYONE HAVE SCHEMATIC ? PLEASE CONTACT ME CARLWADE3@GMAIL.COM:

[llwhtt]

Carl 33,

Search this forum for Jadis Defy 7 and you'll find "Why are my amps doing this" post, there is a schematic from Apogee. Don't know if this is the correct version, might be close.

Craig

[gingertube]

Carl33,
Forgive the "rave" BUT one of my buttons was just pressed.

The Jadis Defy7 has a few problems from a "back to basics" sort of fellow like me. Its just NOT a good design.

The Grid 1 resistors (474K) are too big for 6550 in fixed bias. Any grid current will cause a tube to "run away".
That is:
Grid current flows, causes the coupling capacitors to charge up, that reduces the negative bias at the grid, more anode current flows and therefore more grid current flows and the whole thing builds up until you get smoke (but hopefully not flames).

The obvious fix for this is lower value grid 1 resistors BUT since those resistors are the effective load on the driver section we have another problem. The driver section is just too wimpy to drive the load if we reduce those resistors.

From an AC signal drive point of view things MAY not be quite as bad as first glance would suggest. You would need to confirm this for the Defy7 BUT I can confirm that in the Jadis JA80 monoblocks at least the cathode feedback is POSITIVE feedback and it relies on the NEGATIVE feedback of the Ultralinear connection to keep it stable. This makes the output tubes look like a higher impedance AC load.

It pretty much means that you have to use specially selected output tube sets from Jadis. They are selected not just for matched operating points BUT for low grid current as well.

When I did a restoration on a pair of JA80's this got right "up my wick" and I was determined to find output tubes myself. In the JA80 the problem is slightly worse in that each of the push pull sides is cathode biased and matching of tubes was even more critical. On the other hand they run just 2 tubes (KT88) each side not 3 as for the Defy7. I ended up purchasing 4 matched quads of JJ KT88 to be able to find 2 sufficiently matched quads with low grid current to get the amps running reliably-ish. ('ish meaning that I returned them to the owner running well, but with all my fingers and toes crossed that they would stay that way beyond tomorrow or the day after).

My advise to the guy I did this restoration for was to sell the amps fast while they were working because sooner or later they were going to "blow up" again. The shift to fixed bias in the Defy7 is at least a step in the right direction - otherwise all the warts from the JA80's have just been copied into the Defy7.

If you get the idea that I'm not to impressed with these very expensive examples of so called Hi-End Audio Amplifiers, then you are reading me exactly right. I have yet to see a Jadis Amp I would be comfortable running long term in my system.

Others, if you have an opportunity to purchase a 2nd hand pair of Jadis Power Amps at what seems to be a bargain price, my advice is don't.

Cheers,
Ian

[kevinkr]

Originally posted by gingertube
Carl33,
Forgive the "rave" BUT one of my buttons was just pressed.

The Jadis Defy7 has a few problems from a "back to basics" sort of fellow like me. Its just NOT a good design.

<big snip>

Others, if you have an opportunity to purchase a 2nd hand pair of Jadis Power Amps at what seems to be a bargain price, my advice is don't.

Cheers,
Ian

I've got to agree 100% with Ian based on identical experience. I've worked on a number of Defy 7 amplifiers and some JA-80s as well. I believe the last time I agreed to work on one was about 10yrs ago. They are a service tech's nightmare for all of the reasons enumerated above, and in addition the Defy-7 is particularly annoying because of the level of disassembly required to replace myriad fried components when the amplifier experiences the inevitable melt down.

In addition to what I will charitably label as at best an "eccentric" design, many of the passive components are underrated for power dissipation, voltage or both.

I will never work on another one of these amplifiers. I desperately want to say something about a group of trained monkeys and that design, but I'll refrain.

Edit: The sad thing is the iron is great, the chassis nice, and they even sound pretty good all things considered when working properly. Just don't do it!!!