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Biasing Stax SRM-T1?

post #1 of 12
Thread Starter 
Since I spotted a tiny burst in one of the tubes of my SRM-T1, I replaced them. Now I'm wondering however, whether it's necessary to rebias it with the new tubes. Operation of the amp is normal, nothing weird happening, but I feel better when I know it's set up correctly. Can somebody help me with this?
post #2 of 12
Spritzer sent me the clearest of explanations but I lost it after the crash
post #3 of 12
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.


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


This is the procedure I got from a Stax tech. Believe me it was hard to get.

All adjustments are to 0 volts.
Adjustments for pots Tvr 3 & Tvr 4 .
1- Tvr-3 put voltmeter leads between testpoints of Tp1(little silver circles on the board near each tube) and adjust tvr3 pot
to 0 volts.
2-Tvr4 put voltmeter leads between testpoints of tp2 and adjust to 0 volts.
3- Tvr1 put one voltmeter lead to tp1the other to tp2 and adjust tvr1 to 0 volts.
4- Tvr2 One voltmeter lead to tp1 the other to ground and adjust tvr2 to 0 volts.
Repeat procedure for other channel.
doesn't matter which channel you start with.
You may have to repeat this procedure a few times untill tubes stabilize.

1)Yes you are measuring DC voltage, set meter to dc volts.
2) The amp has to be turned on and plugged in and stabilized. let it warm up for a few minutes.


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


Take a voltmeter and put the probes on + and - on an output jack and adjust TVR1. When it is hovering close to 0v you take the - probe and connect it to ground on the amp (i.e. the case) and adjust TVR2 to 0v. See the pic for the pinout


In a stax 006 or 007 there are 2 pots per channel that
need adjusting when you change out the tubes.

In a 006 you try to balance the sections of a tube.
In a 007 you try to balance 2 tubes to be the same.
(2 tubes per channel instead of 1)


One pot is the dc balance between the sections of a tube
and is measured between pins 3 and 4 of the headphone
connector. (2 and 5 for other channel) Set it to zero.

The other pot is the dc offset and is measured between
pin 3 and chassis (pin 2 and chassis for other channel).
Set it to zero.

You will have to repeat this a number of times to get it right.


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


have an 007 here i'm doing a little work on...

1- Tvr-3 put voltmeter leads between testpoints of Tp1(little silver circles on the board near each tube) and adjust tvr3 pot
to 0 volts.

This balances the two sections of one tube.


2-Tvr4 put voltmeter leads between testpoints of tp2 and adjust to 0 volts.

This balances the two sections of the other tube.

3- Tvr1 put one voltmeter lead to tp1 the other to tp2 and adjust tvr1 to 0 volts.

this balances the output of tube 1 with respect to tube 2

4- Tvr2 One voltmeter lead to tp1 the other to ground and adjust tvr2 to 0 volts.

this balances both tubes with respect to ground

Repeat procedure for other channel.

You will have to repeat this many many times...

Best to use a tube tester and check the tube for balance
between the sections of the same tube first. The closer
the two sections are to each other, the less trouble you will
have.
post #4 of 12
That's it! Thanks. Must be stickied
post #5 of 12
Thread Starter 
Thanks a lot for the information!
post #6 of 12
Thread Starter 
So, a week ago I did this and everything went fine. Those pots are super sensitive by the way! Anyway, last night I measured again and the right channel is now at 2V while I set it around 0V (fluctuating between -0.8V and 0.8V). How come? How much offset is acceptable? Furthermore, what exactly am I doing all of this for? Is it better for the sound, for the 'phones, for the tubes maybe?

Oh and another question... The instruction says that I should measure between - and ground (chassis) when adjusting TVR2. Is this still correct in case my amp isn't grounded (I don't have grounded sockets in my room), or should I measure between - and a real ground?

Thanks in advance for answering these burning questions.

Edit: Yet another question: can I just leave the 'phones plugged in and measure at one of the other outputs?
post #7 of 12
Quote:
Originally Posted by Michgelsen View Post
So, a week ago I did this and everything went fine. Those pots are super sensitive by the way! Anyway, last night I measured again and the right channel is now at 2V while I set it around 0V (fluctuating between -0.8V and 0.8V). How come? How much offset is acceptable? Furthermore, what exactly am I doing all of this for? Is it better for the sound, for the 'phones, for the tubes maybe?

Oh and another question... The instruction says that I should measure between - and ground (chassis) when adjusting TVR2. Is this still correct in case my amp isn't grounded (I don't have grounded sockets in my room), or should I measure between - and a real ground?

Thanks in advance for answering these burning questions.

Edit: Yet another question: can I just leave the 'phones plugged in and measure at one of the other outputs?
The pots are too sensitive since they are single turn so I'm going to try and swap them for multi turn units in my SRM-007t. Should be a fun experiment.

The DC offset should be as close to 0v as you can get. As to why you are biasing then you are adjusting the gain of the two triodes in each tube so they match each other and then eliminating as much DC voltage as is possible on the output.

The signal ground works fine so there is no need for earth ground.

It doesn't matter if there are phones plugged in or not but I normally leave them unplugged.
post #8 of 12
Thread Starter 
That's about as clear as it can get.
post #9 of 12
Quote:
Originally Posted by milkpowder View Post
That's it! Thanks. Must be stickied
X2!
post #10 of 12
Anyone got a schematic for one of those things? I'm trying to build an amplifier inspired by Audio Research SP11 but im still curious about how stax is doing it.
post #11 of 12
Thread Starter 
Maybe you should post that question in the DIY subforum. You will probably get a better response over there. Good luck.
post #12 of 12

Great thread! Thanks KG, Spritz, and others for the info.

 

I just refurbed my T1 tonight replacing all the caps and popping in new tubes. After I powered everything back up I noticed a bit of static in the right channel. After an initial rebiasing according to the directions in this thread the right-channel static is now 90% gone. I'll leave the amp on overnight to let the new tubes stabilize a bit before I re-bias the thing again in the morning. Hopefully that will get rid of the static for good.

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