Tube Change/Biasing a Stax 006t II

[ColoradoTom]

I'm trying to confirm the following procedure to change tubes and set dc offset and balance in a Stax 006t II.

Inside the amplifier are two sets of potentiometers labeled Tvr1 (dc balance) and TVR2 (dc offset). I assume that since there are two pairs of pots that each pair relates to the left/right channel.

The procedure should be:

Turn off and unplug the SRM-006t II

Open up the SRM-006t II

Remove the two 6FQ7 (6CG7) tubes

Replace the tubes with the new tubes

Plug-in and turn on the SRM-006t II letting it warm up for 30 minutes

Adjust DC balance to zero

Adjust DC offset to zero.

Since changes in DC balance will change DC offset and vice versa the adjustments will probably be repeated and checked several times.

Close the SRM-006t II

Slap on music and enjoy

Now.... when making the measurement for dc offset I was going to cut an old interconnect, plug it into each input (left then right), and clip the voltmeter to the +/- of the interconnect and make the adjustment.

How do I make the connection to adjust dc balance??


Thanks for the help

 

[WilCox]

Here's an earlier post by Elephas that I saved as a Word file. Unfortunately, I couldn't locate the link just now.

--------------

The following are copy & pasted text (original sources: spritzer and kevin gilmore).

--------------

You use a normal multimeter set at DC voltage, the lowest voltage level (20volts or less). The amp is powered on when you do this with the new tubes inside. Never pull out tubes when amps are turned on. Only do so when the amp is disconnected from the wall. Leave the amp on for two hours so the new tubes can stabilize. You measure in the Stax plug on the front.



First you measure between + and - on one channel and adjust TVR1 and then put the multimeter between + and the ground post at the back and adjust TVR2. Both are adjusted for zero. They pots are clearly marked on the PCB for each channel.

Be very careful inside the amp because there are some hundreds of volts floating around. I use a special screwdriver that is only bare metal at the very tip to adjust but plastic screwdrivers are also good.


--------------


There are 2 pots per channel.

One of them is the differential pot, and you put a voltmeter between The two output pins and adjust for zero. I believe this pot is labeled TVR1
For left channel voltmeter goes between yellow and green for right channel it is white and red

The other is the dc offset pot, you put a voltmeter on either output pin and ground and adjust for zero.
I believe this pot is labeled TVR2. For left channel voltmeter goes between yellow and ground, for right channel it is white and ground.

You have to do this a number of times, and new tubes drift like crazy.

 

[ColoradoTom]

Wilcox....

Thanks, if I'm reading this correctly the first part of the text relates to using the output jacks to make the adjustment and would be measuring across the hot/shield of the output jack and the hot OR shield of the output jack and the groundpost. The second part of the text looks like the procedure to use (possibly) the headphone jack.

Tom

 

[ColoradoTom]

Could someone confirm the following...especially the pin designations.


The procedure should be as follows:

Turn off and unplug the SRM-006t II
Open up the SRM-006t II
Remove the two 6FQ7 (6CG7) tubes
Replace the tubes with the new tubes
Plug-in and turn on the SRM-006t II letting it warm up for 30 minutes
Adjust DC balance to zero
Adjust DC offset to zero.
***Since changes in DC balance will change DC offset and vice versa the adjustments will probably be repeated and checked several times.***
Turn off and unplug the SRM-006t II
Close the SRM-006t II
Connect to source and listen to music.........

If I face the front of the amplifier the pins of the connector should be as follows – AGAIN this is looking at the front of the amplifier towards the back and looking INTO the “female” connector

____3____

4_______2

__5____1__

(underlines being used to keep spacing)

To measure the DC balance I put my voltmeter probe into 2 (right +) and 5 (right -) and adjust potentiometer TVR1 for the right channel to read zero. I then move the probes to 3 (left +) and 4 (left -) and adjust potentiometer TVR1 for the left channel to read zero.


To measure DC offset I connect one probe to 2 (right +) and the ground post at the back of the amp and adjust TVR2 for the right channel to read zero. I then move the probe to 3 (left +) and leave the other probe on the ground post and adjust TVR2 for the left channel to read zero.

I want to make sure this is correct as I don’t want to put a probe into pin 1 – the bias voltage thinking it was pin 5. This probably wouldn't be good for me or the amplifier.


Thanks for your help.......................

 

[WilCox]

Everything you're looking for should be in the Stax Thread over in the High End Forum:

http://www.head-fi.org/forums/f113/s...ad-new-223263/

Use the search function or post your question there and you will find a lot of helpful Stax experts.

 

[ColoradoTom]

Thanks Wilcox......

I plan on changing out the tubes and confirming the prodedure and then doing a detailed writeup with detailed photo's when I'm done. The info has been scattered in bits and pieces in several places with some procedures seemingly contradicting others. Hopefully it will be useful for people in the future.

Tom

 

[Frihed89]

Quote:

Originally Posted by WilCox /img/forum/go_quote.gif Everything you're looking for should be in the Stax Thread over in the High End Forum: http://www.head-fi.org/forums/f113/s...ad-new-223263/ Use the search function or post your question there and you will find a lot of helpful Stax experts.

There's 500 pages on that thread!

Tom, have you gotten in touch with Kevin Gilmore?

 

[ColoradoTom]

Frihed89,

Yup.... in all the groups that I visit that has to be one of the longest threads I've ever seen. The only others close to that length are when someone starts talking about the Beatles on Steve Hoffman's site!!!

I'm real careful about contacting people directly, some people don't mind and others get a little cranky. I did get a PM suggesting getting in contact with Spritzer and so I fired off a quick note. At this time I think I've got all I need. What I'll do is go through the procedure myself and take a bunch of photo's and then put out a write-up that people can view/re-view.

Thanks for the suggestion.....

 

[krmathis]

Quote:

Originally Posted by Frihed89 /img/forum/go_quote.gif There's 500 pages on that thread!

Yeah, its great isn't it?
Pretty much all you ever want to know about Stax collected in one place... Love it!

 

[spritzer]

I could just as well answer the PM here. That's all correct but be patient as Stax still sticks to those cheap single turn pots and adjusting them can be a pain.

 

[ColoradoTom]

But the problem is that you get sucked into this time warp if you start reading it... I started at page 1 ........ and all of a sudden I realized that it was four hours later and I was still in the first 50 pages!!! You start jumping into other linked-in threads and the next thing you know hours have vanished and your wife is screaming at you because she thinks you're looking at "naughty pictures" on your laptop!!

Tom

 

[ColoradoTom]

Quote:

Originally Posted by spritzer /img/forum/go_quote.gif I could just as well answer the PM here. That's all correct but be patient as Stax still sticks to those cheap single turn pots and adjusting them can be a pain.

Thanks Spritzer.... I'm kind of anal about things. Probably comes from my woodworking days using equipment that can cut off limbs if you make the smallest error!! I like to know EXACTLY what I'm doing before I do it! Since there seemed to be a possibility of confusion I wanted to hear from some people that had actually gone through the procedure in the past and TRUELY knew what they were talking about. High voltages and casual mistakes don't mix well!

Thanks again for the confirmation.... I will post a detailed "users guide" with photos as soon as I'm finished

 

[spritzer]

Quote:

Originally Posted by ColoradoTom /img/forum/go_quote.gif High voltages and casual mistakes don't mix well!

Sure it does, 350v doesn't hurt that much...

 

[ColoradoTom]

Quote:

Originally Posted by spritzer /img/forum/go_quote.gif Sure it does, 350v doesn't hurt that much...

I didn't have a camera at the time..... but this is a pretty good likeness of my expression during my first electrical mistake!!

 

[ColoradoTom]

Guys......

I've just spent about 30 minutes checking and adjusting the original tubes that came with the 006t. My first measurement was the dc balance in the left channel - I nearly died when I measured 6.0 volts (not mV). I was able to adjust this down to 90 mV but couldn't do any better than that as the readings were jumping between -20mV to +110 mV. I then measured the left channel dc offset which started at 1.2 volts (not mV). I was able to get this to 40 mV. When everyone says that the potentiometers are sensitive they are NOT kidding. Using the face of a clock as a guide adjusting the potentiometer from "1" to "2" can change the values in the multiple volts range - blinking your eyes while you make an adjustment can change the value several hundred mV!! The values for the right channel were similar... initial dc balance was 4.0 volts and dc offset was around 1.8 volts.

Question to you experts... what's up with these original values being in the multiple volt range? Does anybody have some guidelines as to what are acceptable values for these measurements. Finally, does it seem right that the original factory settings were SO far off or do these STAX amplifiers have a VERY wide acceptable range in dc balance and dc offset... If these measurements were from a power amp I'd be thinking that there was a problem, not to mention that with that much dc offset my speaker cones wouldn't be very happy campers!

Thanks.....

Tom