|
Tubes Asylum Questions about tubes and gear that glows. FAQ |
|
|
|
Tubes Asylum Questions about tubes and gear that glows. FAQ |
|
In Reply to: RE: I suggest caution when upgrading some amps posted by Allen Wright on March 30, 2010 at 03:26:34:
When Douglas Chu imported Jadis into Australia during late 80's and early 90's I was the Jadis service agent.
The Jadis are the most quirky of all tube amplifiers I have seen over a period of thirty years plus. Allen describes accurately the very limited bandwidth they have and recall working on the Jadis 500 monsters that kept blowing up. These were so big and heavy I had to lay down a carpet on the floor and work on them on all fours. First time I felt like a motor mechanic. For an amplifier that was rated 500 Watts I recall getting less than 5 Watts at 20KHz. Also they were running high tension that often went over 820V. We managed, and I admit it took several goes, to tame the beast and make it reliable. WE ended up buying a bunch of Manley KT90's as they seemed to tolerate the odd conditions better.
The other odd thing that it used was combined partial cathode and partial (so-called) fixed bias - also found in many Jadis amps but thankfully not all (look for Standby/Operate switch as a give-away). It's hard to explain but the fixed bias is fed from a high value resistor into a capacitor that gets charged. It's takes a long time (a long time-constant) during which the output tubes are exposed as the cathode bias resistor will be the wrong value. If the two are not synced, only the incomplete cathode bias cannot prevent the output tubes being bias too hard. The result is as one DA-60 owner experienced (Audiogon):
"I bought my Jadis DA60 from new about five years ago. Over the last three years the amp has blown two output transformers. The ..."
I have only had one single unsuccessful attempt at fixing ONE Jadis amp (and I was willing to persist). Forgive me for my non-infallibility.
BTW, since starting to use 10R 1/4W fusing resistor in cathodes of power tubes, I have not lost a single transformer, NOT ONE!
So if the fuse resistor blows, that is a real inconveninence (and rare), but that may just have saved you a big wad of bills.
Cheers, Joe
PS: As for mods, I have often DISCOURAGED mods to certain tube amps. Other times I have been approached as a last resort to help a particular amp that had consistent realiability problems. Rarely did I not succeed - so I shouldn't be blamed for some reallly botched designs. Like the VTL amp that turned out to be Tetrode and not Ultra-Linear as the badge on the front panel so proudly announced. With 600V B+ and the Screens of the 6550's directly to it... it blew up regularly. Want to hear other stories? Like a brand name British Tube amp that ran 85mA Bias and nearly 50 Watts on the Anodes. When I was finished with it, only then did it become reliable - and it was NOT modded, just fixed.
If you were mad, would you know it?
This post is made possible by the generous support of people like you and our sponsors:
Follow Ups
- RE: I suggest caution when upgrading some amps - I sort of agree... - Joe Rasmussen 03/30/1023:31:58 03/30/10 (0)
Top of Page ]
[ Contact Us ]
[ Support/Wish List ]
[ Copyright © 2025, Audio Asylum, all rights reserved ]
