DIY mini T2 Build Thread

[jamesmking]

22 hours ago, micon21 said:

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.

Simplified explanation:

There are multiple methods for changing how a transistor or valve conducts. You can vary the voltage on valve grid or current on the transistor base. Another way is to vary the current supply to the transistor emitter, collector or valve anode or cathode, another way is to vary the voltage at the valve cathode. 

.. Just because the valve grid or transistor base does not have audio directly applied does not mean that there is no audio passing through the device....

• Q1 is constant current. Since its emitter has a constant load and voltage applied and so has a constant current available and the base also has a fixed resistor and fixed voltage. Rv1 can adjust slightly the overall emitter resistance and therefore available current to each half of Q1 so some current balancing can be done to compensate for mismatches in the halves of V1 and V2 etc.
• The Q1 current is shared between the second 6922 (V2) (the one which does not have its grids connected to the audio input) and Q4/Q5.
• If second 6922 (V2) pulls more current then less current must be available to Q4 and Q5 since the total current available is constant and visa versa is true if the 6922 pulls less current.
• So the current through Q4/Q5 is effected by the current pull from second 6922 (V2) whos current draw is effected by the audio on the grid 1 of the input 6922 (V1) . Since the second 6922 (V2) supplies the anode current to the input 6922 (V1).
• if Q4/Q5 are effected by input 6922, Q11 and Q12 must also be since they get their current directly from Q4/Q5 and the base of Q11 and Q12 is held constant by the resistor string .. R36, R49,50,60
• If Q11/Q12 have current draw effected by the audio input 6922 then the voltage drop across R11 and R12 and current through R11/12 must also be based on the audio input... since the current through R11 and R12 is directly controlled by the current through Q11 and Q12
• The current through R11 and R12 control the current at the base of Q8 and Q9 and therefore control the current through Q8 and Q9. Q8 and Q9 control the voltage at the EL34 cathode and that's how the audio gets into the EL34s and that's the AC audio voltage than can be measured with an multimeter at the cathode of the EL34s.

Edited January 6, 2023 by jamesmking

[boinger]

Anyone have the links to the bom and schematic for the group by? 

 

The first post seems to have the links unavailable.

[Pars]

Links in the first post work for me...

[boinger]

I got it to work. Had to open in new tab for some reason. 

[audiostar]

On 3/11/2019 at 2:36 PM, JoaMat said:

Well, I rather not make recommendations but I would go for +400V/-460V and add one 20K/3W resistor on each board between +220V and +400V pads.

I think I want something like this…

...for power supply.

Now, going to order some transformers 🙂

Is there anything speaking against using the following secondaries with this PSU? 

320Vac @ 0.18A -> 400Vdc
365Vac @ 0.18A -> -460Vdc

For the +220Vdc I'll use a separate GRHV, like the kgsshvpssicfetsinglenewright but without the Bias portion on it. 

 

Edited June 14, 2023 by audiostar

[jamesmking]

4 minutes ago, audiostar said:

Going to order transformers. Is there anything speaking against using the following secondaries with this PSU? 

320Vac @ 0.18A -> 400Vdc
365Vac @ 0.18A -> -460Vdc

 

 

thats close to what I used and should work fine.

I used 190Vac 0.18A for the 220VDC rail

[JoaMat]

 

mini T2 one channel lsk389 replacing 6DJ8/6922. Still on drawing table.

[yabba235]

On 9/21/2023 at 9:20 AM, JoaMat said:

 

mini T2 one channel lsk389 replacing 6DJ8/6922. Still on drawing table.

Did you run it with lsa389? What did you change in the schematic?

Have you considered using the SOIC version instead of the TO-71 transistors?

[Helium]

On 6/14/2023 at 2:30 PM, audiostar said:

320Vac @ 0.18A -> 400Vdc
365Vac @ 0.18A -> -460Vdc

Could someone explain the rule for choosing VAC secondaries of HV transformer?

Rule of 0.707 (400VDC as target + ~10V of voltage drop = 290VAC÷0.707) doesn't apply here? Should we add 30-50VAC  (or how many?) for voltage drop and regulation? Thx.

Edited August 27 by Helium

[jamesmking]

1 hour ago, Helium said:

Could someone explain the rule for choosing VAC secondaries of HV transformer?

Rule of 0.707 (400VDC as target + ~10V of voltage drop = 0.707*290VAC) doesn't apply here? Should we add 30-50VAC  (or how many?) for voltage drop and regulation? Thx.

The calculation are based on some assumptions and tradeoffs. Ideally you need to know some things about the high voltage power supply the secondaries will be powering.

1. you need to know the minimum voltage input to the power supply to get the required regulated output at the required current. (the minimum input will usually increase with higher current draw). typically this will be a fairly small additional voltage above the output voltage for low voltage power supplies e.g. a few extra volts for the golden reference low voltage and typically in the few 10s of volts extra for the golden reference high voltage power supply. Since this is the absolute minimum you probably want a little more input voltage than that to be safe. 

2. the voltage out of the wall also varies. In many countries during peak demand the power companies can reduce the voltage - typically by up to about 5 to 10% so you need about 10% more voltage on the output side than you might think to compensate the electrical grid giving you less than you expect. So we multiply the total secondary output voltage by 1.1 i.e. add 10%

3. Competent transformer design is also assumed. The more current you draw from a transformer the more inefficient it becomes. Which is why you specify the output voltage of the secondaries at the current draw you expect. If the manufacturers are crap and only design the transformer secondary output based on no load being applied you can have issues where the psu loads the secondaries and the output of the secondaries drops so much you don't get enough voltage for the psu to maintain regulation.  Conversely if the manufacturers design the transformer properly, specifying a transformer with a much higher secondary current draw than it will actually experience will result in the secondary voltage being higher than expected.

Any voltage going into the psu above the minimum amount required to regulate will be converted into additional heat. Any voltage below the minimum to regulate will result in loss of regulation and the output voltage dropping with greatly increased levels of ripple and noise.

you also need to make sure the input capacitors in the power supply can handle the peak voltage going into them or their life expectancy will be reduced and in more extreme overload conditions they can fail completely. (the output caps are mostly isolated from the varying input voltage and should only see the DC voltage output the power supply is setup to output assuming there is no failure in the psu which causes the regulation to be bypassed/short out and pass all the available voltage to the output e.g. the pass transistor to be fully switched on or become shorted)

 

[Since minimum dc voltage input for regulation - DC output voltage] is the additional voltage needed to regulate at the required current draw.

secondary VAC =  (minimum dc voltage input for regulation *1.1 )*0.707

Or

secondary VAC =  ((DC output voltage + minimum additional voltage for regulation)*1.1 )*0.707

 

so if we assume 10V more input is needed to regulate than output for 400V output we get:

secondary VAC = ((400+10)*1.1)*0.707 = 319VAC 

Edited August 27 by jamesmking

[JoaMat]

18 hours ago, yabba235 said:

Did you run it with lsa389? What did you change in the schematic?

Have you considered using the SOIC version instead of the TO-71 transistors?

Not much (nothing) has happened since my post 11 months ago.

Here is a schematic, original to the left and lsk389 version to the right.

My idea was to replace 6922 tubes with lsk389 and just make necessary changes and else keep the board unchanged. This is a draft - more work is needed.

Of course you can use SOIC lsk389, but I prefer TO-71 since I think it fits better in the layout.

[yabba235]

OK, thanks for explaining - best regards!