The Headcase Stax thread

[spritzer]

Loo roll Spritzer. Those SFI drivers love some loo roll damping.

Way ahead of you... I'm now moving upscale with some Tork's fiber paper to see if I can get some more bass. I've also angled the earpads to good effect as the POS Koss frame if too weak for a firm seal. The epoxy is still not ready so they just sit here playing in the mean time.

No kidding. A tube that is badly matched with the other one in its channel can make the amp impossible to bias correctly. Reasonably matched pairs are absolutely necessary. Nothing quite as frustrating as watching the voltage get closer and closer to 0 on the meter, and hitting the limit of the trimpot before you get there.

It's a nightmare all right. I spent one evening trying to beat 3 SRM-T1's into submission... ohh the horror!!!

Thanks for the info.

I bought 8 tubes, and had the seller give me the tube tester readings of each of them. I think the numbers are transconductance values, which isn't ideal, but I guess it's better than nothing. Hopefully, I'll be able to group them into 4 pairs, but I am a bit worried about the sections not being well matched on some of the tubes.

That can certainly be an issue and will make it much harder to bias properly.

[Smeggy]

I need help from our technical Staxies.

I'm going to replace the ECR-500 cable with a Stax Gamma cable and need to know which of the Gamma leads is + and - so I don't blow up the Sony electrets

I'm going to run the Sony ECR from a Stax transformer.

Pics...

ECR-500

Stax Gamma

I'm off to make new cups for them shortly and hope to be all done this weekend, so any help would be appreciated. I think I know but want to make sure

[spritzer]

The Stax ribbon cable is the following for the 3 wires entering each housing:

Wire with stripe or dotted line is the +, the middle wire is the bias and the last wire is the -. If you look carefully at the plastic housing of the Gamma drivers you can see the markings for + and -.

[cclragnarok]

Uh oh.... I think I have much bigger problems now than the tubes not biasing properly. :'(

I opened up the 007t today to swap the tubes out. Before I actually did anything, I saw that one side of the fuse holder was bent, and wasn't actually clamping down the fuse. There was still contact though, so the amp was working before.

I figured that it would be better if the fuse was actually clamped down on both ends, so I used a piece of floss, and restored the fuse holder to the shape that it was supposed to be. Obviously I didn't notice it at this point, but this also caused that side of the fuse holder to touch one of the pins on the power transformer.

I plugged the power cord back in and switched on the 007t. Right away, I saw a few small flashes, and a bunch of smoke (which kind of smelled like solder) started coming out of the power supply section, around where the big capacitors are. I switched the 007t off and unplugged the power cord, but I guess the damage has been done.

The fuse also blew. I'm not sure when it did, but it probably wasn't fast enough.

Here are some pictures I just took after all this happened:

What do you guys think (how bad is the damage, what parts need to be replaced, etc)?

[kevin gilmore]

fuse now shorting against one of the transformer pins.

needs to be fixed. Transformer needs to be loosened up and

moved a bit, and some kind of insulation between the fuse holder

and the transformer.

[cclragnarok]

fuse now shorting against one of the transformer pins.

needs to be fixed. Transformer needs to be loosened up and

moved a bit, and some kind of insulation between the fuse holder

and the transformer.

Thanks a lot for the quick reply.

I understand that the shorting problem definitely needs to be fixed before using the amp again. Considering that smoke was coming out of the unit, do you think some other parts might need to be replaced?

[kevin gilmore]

well if you fried the transformer, its over, or expensive, or both.

fix the fuse issue and turn it on, you will know soon.

[cclragnarok]

well if you fried the transformer, its over, or expensive, or both.

fix the fuse issue and turn it on, you will know soon.

Thanks for the suggestion. I will go buy a replacement fuse and try it out.

[Smeggy]

The Stax ribbon cable is the following for the 3 wires entering each housing:

Wire with stripe or dotted line is the +, the middle wire is the bias and the last wire is the -. If you look carefully at the plastic housing of the Gamma drivers you can see the markings for + and -.

Thanks Birgir,

as I suspected, thanks for the confirmation. Modding underway.

I may start a wee photo diary of my progress in the DIY forum.

[cclragnarok]

well if you fried the transformer, its over, or expensive, or both.

fix the fuse issue and turn it on, you will know soon.

Moving the transformer looked kind of complicated, so I bent the pin away using a plier. I just plugged in a new fuse, and everything seems to work (and sound) perfectly fine.

I wasn't looking directly at the amp when the shorting happened. Now that I thought about it a little, the little flash probably just came from the fuse. I wonder where the smoke came from though. Maybe the insulation on the wires.

Anyway, thanks a lot for the help.

[spritzer]

Thanks Birgir,

as I suspected, thanks for the confirmation. Modding underway.

I may start a wee photo diary of my progress in the DIY forum.

Moving the transformer looked kind of complicated, so I bent the pin away using a plier. I just plugged in a new fuse, and everything seems to work (and sound) perfectly fine.

I wasn't looking directly at the amp when the shorting happened. Now that I thought about it a little, the little flash probably just came from the fuse. I wonder where the smoke came from though. Maybe the insulation on the wires.

Anyway, thanks a lot for the help.

The flash was probably from the fuse but the smoke is harder to pin down. Since you basically short circuited the transformer it could come from a number of components. If it works it works so no worries and listen to some tunes...

[kevin gilmore]

You did put some insulation between the too... RIGHT??

its really going to damage something if it shorts again with

the capacitors charged up. Then you really will damage the

thing. Small sheet of fiberglass works great.

Very few of the companies selling tubes actually match

section to section on an indivual tube. They just match

tube to tube. On the sr-007 it is very important for the

2 sections of each tube to be closely matches. The additional

adjustment for each section has a very small range.

[cclragnarok]

The flash was probably from the fuse but the smoke is harder to pin down. Since you basically short circuited the transformer it could come from a number of components. If it works it works so no worries and listen to some tunes...

I'll do that when I finish burning in and biasing the tubes.

By the way, how much burn in do they need before the bias settles down?

You did put some insulation between the too... RIGHT??

its really going to damage something if it shorts again with

the capacitors charged up. Then you really will damage the

thing. Small sheet of fiberglass works great.

There is maybe 1.5mm of space between the pin and the fuse holder now. I haven't put in any insulation yet, but I will soon. I don't have any fiberglass though. Would electrical tape work?

Very few of the companies selling tubes actually match

section to section on an indivual tube. They just match

tube to tube. On the sr-007 it is very important for the

2 sections of each tube to be closely matches. The additional

adjustment for each section has a very small range.

I tried using the tubes with better-matching (according to the transconductance measurements given by the seller) sections first. I actually didn't have to adjust the section matching trimpots at all. All of them ended up within 0.1V before any adjustments.

For the right channel, I also haven't adjusted the tvr1 (matching tubes to each other). The value drifts a bit, but it never goes above 1v.

For the left channel, the tvr1 adjustment was actually quite off even before I swapped out the old EI tubes. I am trying to adjust tvr1, but it's really hard to get it right since the value swings wildly every time I move the trimpot even a little bit. It was even worse last night. I'd be making tiny adjustments to tvr1, and the multimeter reading would swing back and forth from ~100V to ~-100V.

[cclragnarok]

While I am able to get the tvr1 adjustment down to less than 5V, I am wondering whether the left channel is a little messed up somehow. I think the previous owner of the amp also reported some difficulty with the biasing of the left channel, LED D3 not lighting up, etc.

I noticed that some parts (Q6 and R13) on the left channel look like they have been replaced with parts that look different from the original. R13 also looks like it has been charred/fried.

Here are some pics:

Here are some voltage readings (taken from the leg that is easier to access):

Left channel R13: -95V

Left channel Z1: 13V

Left channel Z2: -198V

Left channel R14: -290V

Right channel R13: -232V

Right channel Z1: -130V

Right channel Z2: -130V

Right channel R14: -230V

[luvdunhill]

lol, even Stax is running out of the dual parts

[The Monkey]

Heard plenty of Stax at CanJam. I feel ready to take the plunge soon. I have to say that I still really like the 404. It just sounds effortless and musical to me.

[Smeggy]

Heard plenty of Stax at CanJam. I feel ready to take the plunge soon. I have to say that I still really like the 404. It just sounds effortless and musical to me.

Cool, I'm glad you're liking them. I'm one of those who prefers the old SR-lambda to the 404 but it's just a personal preference. I like the rehoused Gamma Pro best of all. Just spectacular to my ears... but I have strange ears.

[cclragnarok]

lol, even Stax is running out of the dual parts

I guess so. \

The problem is the R13 resistor though. I think something happened that caused the discoloring on the circuit board, and R13, Q6, and Z1 had to be replaced. The outer shell of the new R13 resistor is now cracking and peeling off. It probably still works (considering the fact that the amp sounds fine), but it sure isn't pretty.

[spritzer]

I guess so. \

The problem is the R13 resistor though. I think something happened that caused the discoloring on the circuit board, and R13, Q6, and Z1 had to be replaced. The outer shell of the new R13 resistor is now cracking and peeling off. It probably still works (considering the fact that the amp sounds fine), but it sure isn't pretty.

It could very well have been replaced at some point and with the wrong part. I think it's time to go resistor shopping.

[spritzer]

I finished the variable bias supply destined for the BH earlier today and I was given the task to try out the Koss ESP950 at 620v bias. Holy crap is the first impression when listening to Megadeth out of a SRM-T1. The bass is much more controlled and so very deep that it gives the 4070 a run for it's money. The sound seemed a little bit forward as first but after some time I don't notice it any more. Now the bias supply should be able to reach 880v so should I see just how much bass they can produce.

[deepak]

I finished the variable bias supply destined for the BH earlier today and I was given the task to try out the Koss ESP950 at 620v bias. Holy crap is the first impression when listening to Megadeth out of a SRM-T1. The bass is much more controlled and so very deep that it gives the 4070 a run for it's money. The sound seemed a little bit forward as first but after some time I don't notice it any more. Now the bias supply should be able to reach 880v so should I see just how much bass they can produce.

This is good news indeed...

How did you implement the variable bias supply?

[n_maher]

Very interesting results, I noticed on Justin's BHSE that the 950's definitely had less bass than the O2's. Not necessarily in a bad way, just different, but it wouldn't hurt my feelings if running the bias supply higher was all that was needed to round things out a bit more.

[luvdunhill]

How did you implement the variable bias supply?

trade secret

[grawk]

I'm going to need to talk to someone about a nice but inexpensive 620v stax/koss amp

[luvdunhill]

Spritzer:

Would it be possible to stuff the goods into an energizer box of some sorts? My layout is rather long, but I'm betting it would be possible to stack the two components and save some space.