Can anyone help me biasing a Jadis Defy 7?

Why am I getting different voltage readings from each end of the fuse?

I need your help to see if it is serious or not. 
I was adjusting for the second time the voltage, and putting 5.05v in all the four rows, and suddenly in the resistor I was measuring, a little explosion and a little smoke appeared, and the voltmeter show a value equal to zero in all rows, even the amplifier switches on the light and all tubes are light on. 

What do you think the damage is?

Joao 

I replaced all 12 resistors with Jadis supplied 180 ohm resistors that came with the amp.

The Gold Lions still do not bias properly and measure differently from each other by more than double in some cases when the pots are set for minimum.

I put the original JJ kt88 tubes back in and had the amp biased to 4.5 volts in less than 5 minutes.

Should any brand kt88 be biasable in my amp? It seems that the gain in the Gold Lions I received is much higher than the JJ tubes.

Sounds like an extremely promising discovery, Matt! The "5w" undoubtedly refers to the wattage that the resistor is rated to handle, which is consistent with the shape you are describing (a power resistor).

If Brooks can't readily supply them, any major electronics distributor should have suitable equivalents. Digikey.com is one such place.

Good luck!
-- Al

Almarg -

I found something... possible the root cause of my biasing problem.

I found a bad resistor labeled 5w150 ohm j under one of the tubes. I measure the othe 11 and they have ground on one side and 150 ohms on the other. It looks like this one resistor is open.

The resistors look like small white boxes. I have not been able to source them yet. I am going to try Brooks in California to see if he has any. I have a good tech here in Chicago that I trust to install it.

Then, I hope that I can get this sounding like it should. Thanks for hanging in there with me.

Matt

All voltages are much higher than 33mv.

Do the voltages vary as you turn the bias pots? And how much higher than 33mv are the readings? I would imagine that if they were WAY higher than 33mv, and the circuit is presently wired such that the voltage across them represents bias current, and if you are measuring correctly, then the plates of the tubes would be glowing red or orange (they aren't, are they?).

Also, just to make sure nothing simple is being overlooked, are you using the dc (not ac) voltage function on the meter, and are you connecting the meter leads to the two ends of the resistor, and not for instance between one end of the resistor and chassis?

Beyond those things, I'm not sure what to suggest.

Best regards,
-- Al

I keep swapping and tweaking.... the sound is improving, but I still have not reached the magic.

I am going to bed soon, but I thought that it might be the combination of 12au7 and 12ax7 tubes that might be causing some of the difficulties.

Is anyone else using Gold Lions in a Defy7 and is so, what 12ax7 and 12au7 tubes seem to work for you?

Thanks Al...
Speakers are attached.

Tracing the connections of the resistor might require some help. The colors on the resistor are next to impossible to see.... very brown looking. But the voltage readings of other neighboring resistors is much higher than 33mv, so I am feeling that these are the correct resistors that Brooks told me to measure.

Fuses are all good.

All voltages are much higher than 33mv.

The tubes are Gold Lion KT88.

Matt,

Not sure what all that adds up to, but the following questions seem to suggest themselves:

1)Can you discern what the two ends of the resistor are connected to (i.e., which pin(s) of the 6550, and/or chassis ground, or some other circuit point)? And are the resistors marked as 1.5 ohms, or if not do they have color-coded bands encircling them, and if so what are the colors (from which we could determine the resistor value).

2)What range of voltages CAN you get across the resistors?

3)Have you checked the fuses with an ohmmeter or multimeter?

4)I would strongly suggest that you ignore the instruction that it is ok to have the speakers not connected. As you probably know it is certainly not ok to have no speaker load present while a music signal is being processed through a tube amp (other than an otl, possibly). However, even with no music or other signal present, if you were to turn the amp off with no speaker load present, it seems to me that an internally generated turn-off transient could lead to an inductive kickback effect that would damage the output transformers, just as might happen if a music signal were being processed with no load.

Regards,
-- Al

My thoughts are that Da-Hong Seetoo performed a mod to this amp that brought it back to the regular way of biasing between chassis and fuses.

Yes.... it should be irrelevant. I cannot get any of the tubes to read at 33 mv. So at this point, I am thinking that the tech notes that I have from Da-Hong Seetoo may reveal that the modification he performed on 3/7/91 updated the amp to be biased to the chassis and fuse readings.
He wrote his report to include the following:

1 - installed new 6550a tubes.
2 - modified bias power supply.
3 - updated protection circuit.
4 - removed cathode follower.
5 - replaced output cables. (stated he used Furakawa cable)
6 - installed new power caps.
7 - modified pre-drive circuit.
8 - adjusted bias.
9 - burn in tested.
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I also received an envelope containing a bunch of Jadis 180 ohm resistors and a box of quarter amp Littlefuses. Not sure what to do with the resistors.
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I also have an email from Jadis@jadis-electronics.com dated 5/19/03 suggesting setting the bias voltage between 4.5 and 5.5 volts.
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I have a typewritten letter than came from the seller with everything else I have mentioned. I am not sure of the origin, but it has what look to be biasing instructions and reads as follows:

Here are the instructions to bias the Defy-7 II power amp.

1 Check all 12 160mA slo-blo fuses on the board. Very often some of the fuses are broken. When ever one tube goes it also takes out its fuse with it.

2 After you put the new tube in the amp, make sure you turn all 4 trim pots all the way up clockwise before turning on the amp. (Turn to right)

3 Make sure there is no input signal. Speakers could either be connected or disconnected to the amp. (No load is necessary)

4 After letting the amp warm up for 10 minutes, use a DC voltage meter to measure between the ground and the fuse holders. The target reading should be about 4 volts.

5 There are 4 rows of tubes and 4 trim pots respectively. each trim pot controls one row of tubes. (3 tubes) Turn the trim pot slowly counter clockwise to bring up the bias reading. You will find that changing the bias of any given row will effect the other 3 rows. Therefore, you must repeat the biasing process many times until they all read 4 volts.

6 Since it is a tube power amp, the tolerance of the tubes will greatly effect the bias reading. Even when you use super matched tubes, the readings still could be as far away from each other as 20%. (From 3.2 - 4.8 volts) Don't go nuts over it if you can't get them to be within couple of % of each other. Try to swap the tubes amount (sic) 12 of them so within each row all 3 will have a relatively close bias setting after it is done.

7 Recheck the bias again after about an hour of usage. The better the tube, the closer the 2nd reading will be against the first one.

8 As usual, when everything fails, call the guy who knows what he is talking about. (718-xxx-xxxx and ask for you know who!

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I tried that number and it has been disconnected.

Matt -- Not sure I follow that. Per your previous post, shouldn't you be looking for 33mv across the resistor, and shouldn't the voltage at the fuses be irrelevant?

Best regards,
-- Al

Still having a rough time shifting these tube around and trying to find some balance. The voltages are ranging from in the 3 volt range up to the 7 volt range..... it seems that when I finally get 6 tubes close to 5 volts, the remaining tubes I have left are (at this point in time), not going to be able to get to being close to 5 volts.

My feeling is that I may not have a matched full set of 12 and instead have 3 matched quads (plus a couple of spares).

If this is the case, am I chasing my tail?

I remember backs in the mid 1990s taking my Defy 7 to Brooks to retube,setup..It cost me over $800 for 12 matched 6550s and pre tubes,and that was a good guy cash under the table deal..Sold it shortly after that and went solid state. Never looked back,lol

Schipo, loved your post, so funny but so true.

I once had Defy 7 monoblocks, if you can imagine twice the headache. I took it a step further with output tube rolling and once reaching that, found I didn't care the sound of the amps and sold them.

A word about safety, beyond what was said in the article Schipo quoted. And this is particularly applicable if you are using an uninsulated metallic screwdriver to adjust the bias pots.

There is an old adage that when working under the chassis of powered up tube equipment you should have one hand tied behind your back. Obviously you don't literally have to do that, but the point is that if the hand you are doing the work with, or a conductive object that it is holding, were to inadvertently come in contact with high voltage you MUST not simultaneously have your other hand, or any other part of your body, in contact with the chassis, or with any other circuit point or conductive surface.

The reason, of course, being that if one hand is in contact with high voltage, and the other hand is in contact with ground or some significantly different voltage, there would be a large flow of current directly through your body.

Best regards,
-- Al

here's another thread on the topic

http://forum.audiogon.com/cgi-bin/fr.pl?aamps&1206759992&openflup&6&4#6

Did own the Mk4 and to be sure I never attempted any such adjustments. And yes, I did buy mine from Brooks and brought it in for service.--The best service and waaaay; the price. A hard combo to beat in this day of "stone handed" over charging repair guys. It was this amp's lack of user biasing that sent me to the CJ 8's.
Side note;ain't this a great site where such information is shared? U-bet.

Mattzack2-
If you bought matched tubes as I suggested, you should be able to keep them together and avoid a lot of the hassle in Schipo's post. The pot sets the bias voltage, and if the tubes are closely matched the voltage readings should be nearly equal. Ultimately they should be close to achieve maximum power, but will work if it isn't. What you are doing is measuring the idle current flowing through a resistor via the voltage drop across that resistor.

It will be worth the effort, that midrange is fabulous.

Well... I spoke with Brooks Berdan and he shed a whole new light on the particular Defy. It has the s/n 213 and it turns out that Jadis built all defy-7 amps from something like s/n 200 to s/n 399 a little bit different.
On these amps, you do not bias by measuring voltage between chassis and fuse. You measure across a greenish brown 1.5 ohm resister that sits under the power tubes.
You have to aim for 33 mv.
You also have to swap tubes around to try to get the best match while biasing. I am glad to have read the previous posts and hear that the lengthy biasing procedure will be worth the effort.
Thanks folks.

Thanks Wendell. Yes I'm sure it is a magnificent sounding amp, and I mis-phrased my comment, which was not intended to be pejorative in any way.

Regards,
-- Al

Almarg, it is a truly great amp and one of the best I've ever heard. Biasing is a pain but the sound is worth it.

Wendell

OMG! Somehow I don't think that amp will appear on my short list when it comes time to upgrade.

Can't offer any suggestions on the question being asked, but if you want to purchase insulated alignment tools as the article quoted by Schipo suggests, they can be found here (search under "Tools"):

http://www.radiodaze.com/

Regards,
-- Al

I hope this helps you...

Biasing the Power Tubes

As with all power amplifiers that are not self-biasing, hand biasing the twelve power tubes is essential for optimal performance. Jadis recommends that you bias the power tubes anywhere between 4 and 5 volts. Pierre has even experimented with biasing to 6 and higher.

Choosing the right bias for your musical taste is essential to getting the best from the DA-7. In my experience, the higher the bias, the richer the sound. But higher bias also means that tubes run hotter and burn out faster. It can also produce sound overly rich and full, to they extent that the amp begins to sound too sweet, warm, thick, or even syrupy (depending upon your choice of tubes).

It would be very nice if Jadis had made biasing the DA-7’s twelve power tubes as simple as adjusting twelve individual settings on the top of the amp and reading the results via a built-in gauge. No such luck. Instead, Jadis’ diminutive bias adjustment screws are located inside the amp. You must turn off the amp, remove the protective tube cage on top, turn the amp on its side, remove the 9 screws that hold the bottom plate in place and get the plate out of the way before the bias adjustments become visible.

Once you turn the amp over, expose the adjustment screws, and turn the amp back on until it stabilizes, you must work with four adjustment screws, one for each row of three tubes, rather than twelve screws, one for each tube. Jadis forces you to bias an entire row of three tubes at a time. And while the screws are easy to turn, the tiniest movement can make a big difference in bias.

Given that no two power tubes measure exactly alike, and that bias requires readjustment as tubes age (differently), you must bias the three tubes in a given row to an average level, and recalibrate them every three or four months. To facilitate the process, you will find yourself mapping out the twelve tubes on a sheet of paper, writing down initial bias levels, and adjusting from there.

If you encounter a marked disparity in bias levels in a particular row of three tubes, you must then begin to swap tubes from row to row until the bias range in a particular row is as close as your particular assortment of twelve tubes allows. Assuming you don’t burn a hand or drop a tube in the process, getting it right can be fun. It’s a bit like a treasure hunt, and ideal for compulsive-obsessive personalities.

It’s an especial joy when you discover that the bias of an individual tube is to some extent dependent upon the bias of the other two tubes in its row. When you move a tube that measures 5.3 volt to another row, it may suddenly measure 4.6. Hence what initially looks like a good move may not prove efficacious.

Moving power tubes between sockets and rows is not as intellectually challenging as playing chess, but it does require attention and patience. Much experimentation is required to get your twelve tubes positioned optimally. You may even have to purchase new tubes after you discover that eight tubes are easily adjustable while the four others variously measure much higher or much lower than the others. This becomes more of an issue as tubes age.

Operating a voltmeter (I use a Radio Shack) to measure the bias is easy. You put one lead on the chassis, another on the end of one of the twelve fuses (one for each power tube) easily accessible inside the amp.

Warning. Turning the bias screws is fraught with danger. Brush against the wrong bit of metal with a metal screwdriver and you may be shocked blue by 500 Volts. This may help cure your schizophrenia, but at the cost of the hair on your head or, if you don’t have any . . . let’s not go there.

The trick is to use a plastic screwdriver to adjust bias. Touch the wrong thing, and no harm is done.

Not so fast. Finding a plastic screwdriver in this day and age may require another little treasure hunt. It seems Radio Shack no longer stocks plastic screwdrivers. Nor do major automotive tool distributors (at least in N. California).

I spent an entire afternoon searching for a plastic screwdriver. I went to carpet stores, auto parts stores, a locksmith, hardware stores, you name it. I tried everything short of driving 45 minutes each way to a computer chain outlet that couldn’t tell you what they stocked when you called them on the phone.

Finally, a local locksmith suggested I check out “the old guy in the TV repair shop across the street who keeps irregular hours.” Inside a virtual museum of television history, open for 60 years and stocked with more antiquated TV consoles than I wish to recount, stood a man well into his 80s who boasts that he still works 12 hours a day. Behind him, on his tool display, hung six long Zenith TV adjustment screwdrivers in their original packaging. One had a zip code for the Zenith address; one other was manufactured before zip codes came on the scene. I bought one of each size available. One fit the Jadis. I am still alive to tell the tale.

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