Can anyone help me biasing a Jadis Defy 7?

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?

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

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.

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.

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