DIY mini T2 Build Thread

[micon21]

you won't believe it, many missiles from russia were shot down over my city in the morning, and I was sitting there trying to figure out why the board wasn't working properly. it was more interesting to me....

[blessingx]

Stay safe (and sane) 

[jamesmking]

3 hours ago, micon21 said:

Thank you for helping.

I have to use the translator, it doesn't give me a full understanding of what you wrote, alas.  
sorry. It is not at all clear what you mean >I assume you scope is set for 10M input and NOT 50ohm... also I assume you scope probes are set to x10 so you don't overload the front end of your scope.

some (usually expensive)  scopes have the option of setting their input impedance to 50ohms or 1Mohm. stax headphones are high impedance and draw very little current from the amplifier and so the scope should be set to high impedance. Setting the scope to 50ohm input impedance would draw far too much current and risk damaging the scope. If your scope does not have switchable input impedance  then it probably has high impedance inputs by default.

Stax amplifiers are can output 100V or more of audio. Most scopes with 1:1 probes will overload and can get damaged with that level of input. X10 probes reduce the voltage by 10 times providing more protection for the scope as well as raising the input impedance from 1Mohm to 10Mohm (usually).

3 hours ago, micon21 said:

There is no regulator on the input of the amplifier. I connect the signal directly from the player. I look on the first leg of the first 6922 +67v, on the third leg 0.6v
on EL34 8pin -334v// signal 1v, on 1pin  signal 3.4 v

on the second lamp, 1 and 6 pin +183v
on pins 2 and 7 +57v

 

Apparently, the first tube does not amplify the signal  

[JoaMat]

6 hours ago, micon21 said:

I look on the first leg of the first 6922 +67v, on the third leg 0.6v
on EL34 8pin -334v// signal 1v, on 1pin  signal 3.4 v

on the second lamp, 1 and 6 pin +183v
on pins 2 and 7 +57v

If +220V out from PSU you should have +200V on pin1 and 6 and +69V on pin 2 and 7 upper small tube. Lower small tube should have +71V on pin 1 and 6. Your -334V on EL34 Pin 8 seems to be too high, I would expect something like -400V or lower.

This, of course, is in a perfect world. What supply voltages do you read on the amplifier board?

[jamesmking]

29 minutes ago, micon21 said:

Before connecting to the board, I set the necessary voltages on the stabilizers. When I turned on the board, all the high voltages dropped, +368, -408, 208. Since they dropped evenly, I believe that this is not the reason for the very small output swing.  

the psu clearly has poor voltage regulation. The voltages have not all decreased by the same percentage but I don't think this is why you have almost no voltage amplification.

if you connect the test signal to the + input and ground the - input do you get a similar output on both out + and out -?

if you connect the test signal to the - input and ground the + input do you get a similar output on both out + and out -?

how are the + and - 15V dc rails under load? (the -15v also acts as the input 6922 anode current sink) 

and what frequency is the oscillation when both + and - input are grounded? 

 

Edited December 29, 2022 by jamesmking

[micon21]

12 hours ago, blessingx said:

Stay safe (and sane) 

life is short, but music is eternal....

[micon21]

filed 220, 400, 460, but alas, the result is not happy.

!!! fed the input BALANCED  signal 0.35v, photo 1. the output received a swing of 82v  and  84v, but in terrible condition, photo 2

 

P.S. I get an undistorted signal with a swing of 20v at the output if I input 0.170v 

It's not enough for headphones.

Edited December 30, 2022 by micon21

[micon21]

on the first leg of the first lamp, photo 1. on the third leg of photo 2

[micon21]

Tester measurements are as follows: 

lamp one, 1 pin - 74v.
3pin - 2v
The second top lamp: 6pin-200v.  7pin- 61v
el34, 8 pin - minus 365v

[jamesmking]

13 hours ago, micon21 said:

OK, and I'll take care of the power supplies, make them adjustable, what to do 400, 460 
I had microcircuits to make +12 and -12. 
I assumed that if I feed 12 instead of 15, it won't make much difference. or am I wrong?

have you built the amp board using the components and values recommended in the schematic? or have you also used different values or components?

[micon21]

1 hour ago, jamesmking said:

have you built the amp board using the components and values recommended in the schematic? or have you also used different values or components?

I understand that it is very important that all resistors match the ratings.   I strictly adhered to the specified values.
I used one 197 and one 356, as JoaMat recommended

Edited December 30, 2022 by micon21

[jamesmking]

12 minutes ago, micon21 said:

I understand that it is very important that all resistors match the ratings.   I strictly adhered to the specified values.
I used one 197 and one 356, as JoaMat recommended

how about the transistors, op amps and voltage references are they also the same as the schematic?

[micon21]

9 hours ago, jamesmking said:

how about the transistors, op amps and voltage references are they also the same as the schematic?

Thank you for trying to help me.

I got a good undistorted sinusoid, with a large sweep of more than 110v at the output, with an input signal of 1.2v.

after adjusting the resistors R43, R44, R7, R8.
In the end I replaced 100k with 270k and 100 ohms with 2k.

now also on the third leg of the first lamp, I see a nice not crooked sine wave.

This change did not change the voltages on all legs of both 6922 lamps. 

Edited December 31, 2022 by micon21

[jamesmking]

14 hours ago, micon21 said:

Thank you for trying to help me.

I got a good undistorted sinusoid, with a large sweep of more than 110v at the output, with an input signal of 1.2v.

after adjusting the resistors R43, R44, R7, R8.
In the end I replaced 100k with 270k and 100 ohms with 2k.

now also on the third leg of the first lamp, I see a nice not crooked sine wave.

This change did not change the voltages on all legs of both 6922 lamps. 

i'm glad you are making progress. As originally designed the mini t2 should be able to output about 500Vrms with around 0.02% distortion with about a 0.5Vrms input. At 100Vrms output distortion should be about 0.004%.I think this is what you should be aiming for.

I would also strongly advise that you build the golden reference HV power supply for the amp. Its an extremely high performance power supply and is the core of most of the headphones amps designs here. Its also all through hole and easy to build. It will also provide the 580VDC bias needed for the stax headphones.

Edited December 31, 2022 by jamesmking

[JoaMat]

R43,44 are the feedback resistors and R7,8 are the lower tube cathode resistors. LTspice simulation indicates that they all determine the gain of the amplifier. 100K and 100R, as in the original schematics, gives a gain of roughly 1000. With 270K and 2K resistors LTspice indicates a gain of 140. The resistor changes you have done will not change DC voltages at the small tube pins.

Maybe your trouble is somewhere else than at the feedback and cathode resistors.

But don’t give. It seems you are more than competent to build a mini T2 and, even more important, to fix it if it’s broken. I guess you will have a fully working mini T2 sooner than you now think.

[micon21]

1 hour ago, JoaMat said:

R43,44 are the feedback resistors and R7,8 are the lower tube cathode resistors. LTspice simulation indicates that they all determine the gain of the amplifier. 100K and 100R, as in the original schematics, gives a gain of roughly 1000. With 270K and 2K resistors LTspice indicates a gain of 140. The resistor changes you have done will not change DC voltages at the small tube pins.

Maybe your trouble is somewhere else than at the feedback and cathode resistors.

But don’t give. It seems you are more than competent to build a mini T2 and, even more important, to fix it if it’s broken. I guess you will have a fully working mini T2 sooner than you now think.

I checked everything yesterday, all resistors, transistors, no fault found, I put a different bulb, it didn't help, and was very upset. 

i planned to throw the board away and be done with it. but i accidentally discovered that increasing 100k leads to an increase in signal. The result is much more interesting than it was. the sinusoid is beautiful, not crooked. in the evening I will plug in my headphones and listen to how it plays.... 

Edited December 31, 2022 by micon21

[micon21]

?

Maybe I'm wrong, but I understood it this way: the signal goes only to the first lamp 2pin. output from 3pin.

through the second lamp 6922 signal does not pass, it has a different purpose, and you can use cheaper brands.

EL34 also does not receive a signal, the signal goes through R43, 44

Edited December 31, 2022 by micon21

[jamesmking]

1 hour ago, micon21 said:

?

Maybe I'm wrong, but I understood it this way: the signal goes only to the first lamp 2pin. output from 3pin.

through the second lamp 6922 signal does not pass, it has a different purpose, and you can use cheaper brands.

EL34 also does not receive a signal, the signal goes through R43, 44

you need to consider the current flow....

audio goes into the grid of the input 6922 and the audio effects how much current the valve allows to flow from its anode to its cathode. The input valves anode current supply comes from the second 6922 cathode... therefore the current flow through the input 6922 effects the current flow through the second 6922...

I guarantee to you (in a properly working min T2)  the audio goes through the el34s... think about it.. how do we get 500Vrms audio output from a single 6922? A single 6922 can't do that kind of voltage swing.

 

 

Edited December 31, 2022 by jamesmking

[JoaMat]

I don’t think mini T2 works the way you think, micon21.

Here is an excellent description how to build the original Blue Hawaii.

There is also a nice explanation how it works. Mini T2 works mainly as the Blue Hawaii does. Read the article (I read it several times – have slow brain) and I think you will get a better understanding how the dots are connected.

Good luck and keep on with your good work.

Edited December 31, 2022 by JoaMat

[micon21]

20 hours ago, jamesmking said:

you need to consider the current flow....

audio goes into the grid of the input 6922 and the audio effects how much current the valve allows to flow from its anode to its cathode. The input valves anode current supply comes from the second 6922 cathode... therefore the current flow through the input 6922 effects the current flow through the second 6922...

I guarantee to you (in a properly working min T2)  the audio goes through the el34s... think about it.. how do we get 500Vrms audio output from a single 6922? A single 6922 can't do that kind of voltage swing.

 

 

I understand and see that the first tube works only as a preamplifier, especially I see a small gain on the 3pin output. 

Edited January 1, 2023 by micon21

[jamesmking]

 

On 12/30/2022 at 4:23 AM, micon21 said:

Tester measurements are as follows: 

lamp one, 1 pin - 74v.
3pin - 2v
The second top lamp: 6pin-200v.  7pin- 61v
el34, 8 pin - minus 365v

To help people with troubleshooting here are the valve DC operating points measured with respect to ground on a known good stock mini T2. Inputs are shorted to ground. No load on output. All components stock values and DC PSU voltage rails within 1% of schematic when under load. Obviously the two el34s should have identical DC operating points and so should each half of a 6922, the other channel should also match. Exact measured values are going to vary a bit depending upon the valves, exact PSU voltages component, tolerances etc etc.

NOTE the EL34 heaters sit at approximately -400VDC. The 6922 heaters are approximately at 0VDC

 

I get a signal gain of approximately 940x with a single ended 1Khz sine wave 0.1Vrms input and an output of 94Vrms between the + and - output pins into a 10Mohm (multimeter) load. 

Audio enters the EL34s on the cathode and for 0.1Vrms 1khz input I get approximately 5.17Vrms of audio on the cathode, the EL34 amplifies this by about 9 times and gives 46.7Vrms on the anode with respect to ground. The other EL34 operates similarly but opposite phase.

Edited January 11, 2023 by jamesmking

[jamesmking]

On 12/31/2022 at 11:52 AM, micon21 said:

?

Maybe I'm wrong, but I understood it this way: the signal goes only to the first lamp 2pin. output from 3pin.

through the second lamp 6922 signal does not pass, it has a different purpose, and you can use cheaper brands.

EL34 also does not receive a signal, the signal goes through R43, 44

the audio signal goes into the cathode of the EL34 and is amplified by each EL34 by about 9 times. (5.17Vrms into the cathode and 46.7Vrms out of the anode)

[micon21]

Alas, I got a good result only after some changes in the board. 

I found that R32, R33 are detrimental to the signal, I do not understand why they are needed. 
I increased R43, R44 to 300k, R7, R8 to 470 oms got undistorted sine waves with a large range - 140v. I plugged the headphones, for normal listening it is enough for me on the output and 70v.

It is also not clear what improves the section Servo, on one board I did not set the elements for Servo. 
I looked both on and off, but did not see the essence.

Translated with www.DeepL.com/Translator (free version)

Edited January 6, 2023 by micon21

[micon21]

16 hours ago, jamesmking said:

the audio signal goes into the cathode of the EL34 and is amplified by each EL34 by about 9 times. (5.17Vrms into the cathode and 46.7Vrms out of the anode)

Sorry,  but how does the signal get to pin8. I see that the cathode feeds transistor Q8, no signal comes to Base Q8 through R11.

Edited January 5, 2023 by micon21

[JoaMat]

I think you can say the last stage of mini T2 is Q8/V3 and Q9/V4. The signals come via R11 and R12 to base Q8 and base Q9.