kevin gilmore

stax t8000 clone (well sorta)

104 posts in this topic

Posted (edited)

On 2017-6-1 at 3:57 AM, kevin gilmore said:

I will have to look that later, but I took the tube piece from the GG design

edit: this is the same as the T2, so it is what I wanted.

Connecting G3 to anode is theoretically better. But EL34 is a pentode so it has a real suppressor grid so tying G3 to anode should be fine. 6CA7 is a beam tetrode, so the G3 is actually a beam forming plate. By connecting G3 to plate will make it unable to form electron beams. I don't know if this will make the tube kinks or acts strangely as the plate was originally trying to avoid secondary emission. KT77 has G3 internally connected to cathode and pin 6 jumped. So far as I know EL34, 6ca7, KT77 worked fine in GG. I wonder if it will be the same under T2 configuration. 

Edited by joehpj

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On Friday, May 12, 2017 at 11:09 PM, kevin gilmore said:

Well its not like the stax mafia aren't busy

and we certainly are not going to do an output stage with 8ma of current, and therefore no reason for a 4 stage amp, and since tube inputs seem to be the big thing lately, well might as well take the kgsshv-carbon design and replace the input fet with an input triode. bonus, no low voltage supplies.  not a bonus, ksa1156 and ksc5026 run at close to 1 watt each.

I should have a real t8000 end of june, but this should keep some of you busy. I expect joamat to have one built in less than 24 hours.

no, no circuit board yet

about 70 volts on the plate of the tube, same as t2.

optical servo from kgsshv-carbon will work fine (but then you need the low voltage supplies)

 

kgsshvcarbontubeinput.PDF

 

 

 

 

 

you got the T80000 yet, bro? God bless USPS!!!

 

 

 

 

 

 

-

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He will have it soon. 

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Posted (edited)

On 2017-06-16 at 5:03 PM, JoaMat said:

No blowing up - no fun.:(   One HV up, other HV down - so no fun there either.:mikey2:

Some progress. The terrible unbalance was due to an unconnected small tube pin. During the milling process the CNC machine drilled a couple of unauthorized holes which I completely forgot when soldering the board.

170617132413148486625.jpg.ca0518199b7a7ef27618db0d92f72f9d.jpg

When corrected the amplifier works. The offset is done by opto servo and it works as supposed. Balance control is also done by servo and I've no trimmer. I've same balance servo here as in my amplifiers with LSK389 input and there it works without any problem. With the tube input you can probably expect more differences between the sides. First tube I tried gave 260 V unbalance (offset worked, so +130V and -130V) and the second tube gave 30V unbalance. The balance servo (in current version) is not capable to make it balanced if the tube isn't enough matched. The input is supplied by +100V/-15 and all components there are surface mounted. Current source is 01N100D/LT1021.

 

GG to the left and tube input GG to the right.
DSC00339.thumb.JPG.a3f3d02619fa0e34d4dc51a70d2b7694.JPG

Edited by JoaMat
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happy it works. now for a version with tubes as the constant current sources :D

And a cherub has just informed me a t8000 is headed my way

I like retirement. going to a hamfest tomorrow to see what I can find to update some of my very antique test gear

 

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I've always wanted to go to a hamfest...  :wub: 

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I built this yesterday. There's some noise that I need to track down, but otherwise it seems to work.

The offset adjusts to within a few volts using the balance pot; the offset pot doesn't seem to do much. Offset drifts a bit and I'm not sure I'm adjusting it right. Is there some way to disable the servo?

 

 

tsandwich.JPG

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Welcome MattN, great work.

The offset servo is a new version and I'm not familiar with the board you are using. I guess the offset servo is working and reason the trimmer seems not affecting the offset might be because the resistor in series with the trimmer has a bit high value? One way to disable the servo might be to remove the ptza06 next to the 12V Zener diode.

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21 hours ago, kevin gilmore said:

happy it works. now for a version with tubes as the constant current sources :D

And a cherub has just informed me a t8000 is headed my way

I like retirement. going to a hamfest tomorrow to see what I can find to update some of my very antique test gear

 

 

 

 

hamfisting the T80000, yoohoo!!!!

 

 

 

-

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3 hours ago, MattN said:

I built this yesterday. There's some noise that I need to track down, but otherwise it seems to work.

The offset adjusts to within a few volts using the balance pot; the offset pot doesn't seem to do much. Offset drifts a bit and I'm not sure I'm adjusting it right. Is there some way to disable the servo?

Not familiar with the circuit and the board you are using. But if it's like other KG ES amps, you need to disable the offset servo when you adjust the offset trim pot then enable the servo afterwards.

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If that's the board from page 2 of this thread, I don't think it has a servo on board...

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The second version of both boards has the servo from Kerry's schematic. There's no obvious way (to me anyway) to disable it.

With the servo components removed the offset pot works and I can adjust the offset and balance to a volt or two from zero. I guess that means the servo works.

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Next time you can just take out the opto chip and put it back in when you are done adjusting

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Try adjusting to +25v or so, then enable the servo.

Just now, mypasswordis said:

Next time you can just take out the opto chip and put it back in when you are done adjusting

This one's not using the opto chip.

Maybe we can add something to the board to disable it in the future.

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4 hours ago, Kerry said:

This one's not using the opto chip.

Kevin's boards show a resistor marked "open"

this connects Q2 base, the input of the servo to ground

which if grounded, wouldn't it turn Q3 off disabling the servo?

Or what is its purpose?

 

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That resistor was for lowering the gain when using the original opamp and a pair of diodes.

will add a jumper once the board is verified

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On ‎2017‎-‎06‎-‎17 at 4:13 PM, kevin gilmore said:

... now for a version with tubes as the constant current sources :D
....

Can you put that in a high altitude extended holding for a while? Need to save fuel.  IMG_0594.jpg.de4ad0f0c14bc0f22184746a9908cf38.jpg

Almost finished the layout for the other channel. This is the first board layout I’ve done from schematic in the software. By doing it this way I hopefully can avoid mistakes in the process of copy, mirror from the right channel board to the left …

5947a4290b429_UNTITLED-Proteus8Professional-3DVisualizer.thumb.jpg.e30aaf7b3088d817c8d491e5c343260f.jpg

Thanks Kevin, Kerry and others for new ideas that I can exploit.

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On 6/17/2017 at 8:13 AM, kevin gilmore said:

 now for a version with tubes as the constant current sources :P

 

Oh, cool!  Cascoded pentodes would work really well.  Lots of heat, lots of filament supplies. :P

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On ‎2017‎-‎06‎-‎17 at 3:32 PM, JoaMat said:

                                                                                                                                                                           ...Balance control is also done by servo and I've no trimmer. I've same balance servo here as in my amplifiers with LSK389 input and there it works without any problem. With the tube input you can probably expect more differences between the sides. First tube I tried gave 260 V unbalance (offset worked, so +130V and -130V) and the second tube gave 30V unbalance. The balance servo (in current version) is not capable to make it balanced if the tube isn't enough matched. ...

Lowered the resistor value at OP27 output. Now both offset and balance are in the mV area. Very happy with the outcome.

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Posted (edited)

no more off-topic posts please

 

 

T80000000:Dtear down, one and only

Edited by fat joey

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The chassis belonged to a Grounded Grid, now transformed to something similar with tube input. Need to make a top plate else I think it’s finished.

DSC00341.thumb.JPG.fecdd4393c807d97d7d8b810b0b677ef.JPG

 

Ordered 50 pieces of ceramic discs. Got them in 50 plastic bags…..

DSC00342.thumb.JPG.85bd550defadec91439e018dd5bda0dc.JPG

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I can’t hear any noise - same as lsk389 input. Be aware my ears are old and I also suffer from some light tinnitus.

Seems you have 500 ohms at current sink input stage while 300 at other of your amplifiers. Any particular reason?

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