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SRM 007Tii MOD to use 6S4A Tubes

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Hi All,

I would like to mod my SRM007Tii so that it can use 6S4A tubes. However I am almost new in messing around with electronics circuit. Need some guidance from you all.

Pictures below show my 007Tii board with re-routing & termination labels (Incomplete). I took quite sometimes to compare snapshots available online posted by Spritzer & Justin. All of them have slightly different wirings. The one I understand the most is the one Spritzer posted for the first time, so my preliminary outcome is similar to his one although it is still incomplete. Therefore I would like to clear my doubts before I start messing around with the PCB.

 

6S4A%20amp%206CG7.jpg

This is my guidance to get the idea where should I re-wiring to. I also have a fluke 175 to check the continuity between 2 points.

 

The Back part of the PCB

SRM007tii%20Mod%20Back.jpgURL]

Q1: For the Yellow wiring, can I skip the plate 2 and connect directly on plate 1?

Q2 : According to the Basing Diagram Pin 3 & Pin 6 are linked. Can I wiring Pin 6 to Pin 3 or vice versa? (Example drawn on Left Bottom tube)

 

The Top part of the PCB

SRM007tii%20Mod%20Front.jpg

I cant find much info about the Mod on the top of the PCB. Logically some traces should be cut for termination. Please kindly advise if I labeled them correctly.

 

Hopefully my understanding (routing wise) is good enough so I can proceed further. I am really new in this therefore alot of questions popped on me. The safest way for me to have this successfully modded is to fully get to know how to convert 6CG7 routing to adapt 6S4A tubes plus some really careful soldering jobs. In terms of technical I have no idea at all..

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Internally connected pins means you can use either one so no worries.  Otherwise it is just grid to grid, anode to anode but with the cathode I always bypass the balancing circuit next to each tube. 

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5 hours ago, spritzer said:

Internally connected pins means you can use either one so no worries.  Otherwise it is just grid to grid, anode to anode but with the cathode I always bypass the balancing circuit next to each tube. 

"Internally connected pins means you can use either one so no worries."

If my understanding is correct it means Pin 3 & Pin 6 is connected internally in the tube therefore no external wiring is required. I guess the best way to check continuity between Pins is to put on all the 6S4A tubes during modification?

 

"Otherwise it is just grid to grid, anode to anode.."

The 6S4A pins layout consists only 1 pair of internally connected pins which is Pin 3 & Pin 6. Think this is what you were referring.

 

"but with the cathode I always bypass the balancing circuit next to each tube."

I don't understand this. Do you mean I can skip the Cathode 2 and connect to Cathode 1 directly?

 

 

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If you check the 007t schematic (which is out there) you see that the cathode goes through two low value resistors and the pot so you need to bypass those. 

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I just tried to google around but couldnt get the schematics at all. 

Found some other Stax amps but not the 007t.

However I found T1 which is similar to 006t. 006t used 2 tubes though..

006t.jpg

Lets ignore the 6FQ7 tube, think you are suggesting me to skip 2 resistors and 1 Pot for each tube, therefore the 2 resistors and 1 Pot is remained unused in the circuit?

Thanks!

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The T1 is not equal to a 007t!!  What you would do is feed -350V straight to the cathode and do a lot of crazy things. 

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Wow! A full high res schematics for 007t!

I need to read the whole thing again now. That will make things more clear. Thanks Birgir & KG!

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srm007t-Schematics-Cathod.jpg

Ok. Just had a quick look.

One question, I managed to find only 1 resistor & 1 pot to be bypassed which is differ from 2 resistors & 1 pot mentioned by Birgir..?

Now that explains why Birgir removed TVR3 & TVR4 in his latest Modified 007tii wiring snapshot....

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Just check opened the up amp again and compare. Noticed that there is difference between 007t schematics and my 007tii amp. Not sure if I interpret them wrongly..

srm007t-Schematics-2.jpg

Is this what you mean in your previous post? However I cant find any 200 omh resistor around the tube, not even the label...

And i noticed components from Pin 3 & Pin 8 to TVR2 in my 007tii is quite different from the schematics. My amp has transistor, 4~5k resistor, 2x 1.5k resistor but doesnt have the 200 resistor...

 

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What part numbers are on q17 and q18

should be something like this

 

to hard to read the real values, so all the R# and values are clearly wrong.

 

dualtube.PDF

Edited by kevin gilmore

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So I opened the amp again and try to draw the Cathode circuit. This is what I got:

IMG_20160607_210949-Edited.jpg

And it looks quite identical to the dual tube schematics KG posted here except that the resistor values are different..This is my first time interpreting electronics stuff that detailed because of Stax!

And regarding the Q17 Q18 part number, the labeling is C1815 B4 4F if I am not seeing wrongly.

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Back-Edited_1.jpg

Just opened up the amp again and draw out the cathode circuit, it is exactly the same as the 007t. 

Since now I have the proper schematics for Cathodes and Plates to visualize easier, I have a clearer picture where should I rewire. And this also helped me a lot to understand Spritzer's latest rewiring snapshot, now I know why some of the components are removed to ease the rewiring :lol:

Removed components are :2x 100 omh resistors. And a Pot (TVR4/3).

However, I still have some questions though.

1.) For the Cathode part, Spritzer mentioned it is to be connected to TVR2. However I figured out that there is a 1.5k resistor before it. Should I bypass it or remain it before TVR2?

2.) Let say my unit is successfully modded to be able to use 6S4A tubes. Is there any impact on the bias adjustment process?

Thanks!

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Ok, found my answers for my previous questions and modded my amp yesterday.

Now my modded 007tii is running quite fine I guess, no smokes and humming etc etc.

However I find the biasing process is not as smooth and easy because the wild voltage swing, I could get 300-500mV at one point then the next minute when I measure it become >1V. For your information my tubes are not matched quads, only 2 pairs of matched pair per each channel which I bought new online. Is that normal that the swing is so great? Or my bad soldering skill....

And because of my considered half done biasing I noticed that the right channel is slightly louder (more bass, more sparkles). Really slight not sure if that is my psychological play.

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Power up the amp for 30 minutes, adjust the trimpot, power off the amplifier let it cool then power it up again and readjust if necessary.

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Reworked my soldering and tubes are warmed up more than 20 hours. DC voltage still swing greatly everytime I measure. Not sure if this is normal.

And the slight weaker bass (impact) in Left channel is really annoying me..Is there any way to detect channel imbalance on amp other than measuring bias?

And I noticed when I turned the volume to 3 o'clock, without putting the L700 on my head, there is kind of mechanical vibration noise when the bass note hits on Left driver. Right driver is totally fine though.

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You can easily measure amplitude with a function generator and an oscilloscope. 

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That will be beyond my capability already...

I am considering to replace the tubes with matched quad 6S4A. Those I am currently using is matched pairs not sure if this is the problem.

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It's not the problem if the tubes are within their operating spec. 

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Still can't figure out what's the potential problem on the amp. I do notice when the voltage swing, both L and R channel will swing similar magnitude. For example, when I set both at 0.5V, the next minute I measure will be 2V. Although both are not very close to 0 but still pretty the same, or balanced.

Another potential problem can be from my L700. I sent back to retailer 1 month ago due to Left side of the driver making noise on certain bass note turns out to be dented driver. In fact I noticed the issue the second day I bought it, definitely not caused from my side. So they replaced my Left side driver and the noise is gone. Before I modded the amp, the returned L700 sounds fine, but now I noticed the bass on the Left is slightly less impact, the difference is even greater at higher volume. Modded 007tii did improve bass, depth but my Left driver's bass is still weaker than the right. Can be headphone problem since it is flawed once and not entirely replaced with new phones.

 

Edited by microCuts

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From experience it is always good to have at least two headphones and two amps to compare, otherwise it is often impossible to say, if the problem originates from the headphones or the amp or even the source. Maybe you can borrow another headphone from someone?

Another possibility to exclude the source as the problem is to switch the cables from the source to the amp.

I dont know if its possible with the output of the srm007tii but if there is a cable from the stax jack to the board, you could switch channels there. If the problem remains on the same side, its the headphones, if it switches the side, its the amp.

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It's possibly a good idea to figure out the issue. I will see what can I do with the channel swap. Also I will improve the previous circuit cuts with nail polish to ensure no continuity at all. I afraid I cut not deep enough that caused the circuits run wrongly. Thanks insanity.

Edited by microCuts

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