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DIY mini T2 Build Thread

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Thanks folks.

Aerial picture of old and new “mini T2”. The new just got a top plate, looks rather nice when newly polished. Small holes in front of small tubes are for trimmers. I drilled two 6.5mm holes in the old to plate (between big tubes) for the balance servo jumpers.

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Made this jumper with a piece of cable and heat shrink.
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the size means we can get creative with the casing again, I'm a fan

Sent from my Pixel 2 XL using Tapatalk

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On ‎3‎/‎23‎/‎2019 at 6:06 PM, JoaMat said:

The only thing is that left channels of all the three amplifiers have a bloody annoying hum during the first minutes after start up. I’ve no idea what causes it.

I took these two fellows..
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…on a ride to big brother who has his own original Kevin DIY T2 PSU (reduced voltages - everything else as original). With this PSU both mini T2 are silent - no annoying hum.

Here they are all together.
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Beautiful. I have to do this amp right now. 

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Quote

The only thing is that left channels of all the three amplifiers have a bloody annoying hum during the first minutes after start up. I’ve no idea what causes it.

Quote

…on a ride to big brother who has his own original Kevin DIY T2 PSU (reduced voltages - everything else as original). With this PSU both mini T2 are silent - no annoying hum.

These kind of problems are fascinating.  IME there are two ways to approach troubleshooting them: cause-and-effect (work the list from most likely theoretical cause on down) and logical elimination (what's changed?  Voltage, PS itself (grounding etc), umbilical(s), etc.)  Out of interest was it electrical or mechanical* hum?  Assume the former since it was channel specific?  

* of course we all know the reason transformers hum - because they don't know the words..  ?

Edited by struts

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On ‎3‎/‎23‎/‎2019 at 6:06 PM, JoaMat said:

Heat sinks, all connectors (STAX, Neutrik, Amphenol) are from one of my full size DIY T2 (RIP – I’ll build a new one with all original parts)

Half a year ago I promised to build a new original DIY T2. Now it’s done. After some difficulties with left channel and PSU she is playing as an original DIY T2 does!!!

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Thinking of retire and feeding ducks. No more modifications to an amplifier that can’t be improved.

 

…never trust an addict

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Joamat, at the sound level, have you found any significant difference between the mini T2 and the DIY T2?

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8 hours ago, jose said:

Joamat, at the sound level, have you found any significant difference between the mini T2 and the DIY T2?

The original DIY T2 I just finished sounds outstanding .

Now, I don’t want to compare the mini T2 with The Famuos DIY T2 - which I believe is the ultimate electrostatic amplifier a DIYer can build.

But I’m very happy with the outcome of the mini T2.

1 hour ago, mumford said:

Are you heating the tubes with AC or DC?

All heaters are AC.

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Its force is its footprint with Kerry's mini power supplies.

For sure no way to shrink the transformers, but still.

Will definitely build one one day.

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My two mini T2 have been working for half  a year now. I’m still pleased with them. But when I checked the outputs I got offset voltages for all four channels in the range 2 – 20 volts. They should be close to zero. After some investigations I found that all OP27 measured zero volt between + and – input pins in diode test mode both ways. After replacing  opamps the offset and balance are perfect again.

I’ve no idea why four opamps have deteriorated. But I do feel that it will happen again so I appreciate any ideas and thoughts that could make the offset servo more reliable.

Below is schematic of the offset servo.
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Reduce resistance of R38 to 10K? Look for another opamp? More?

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Maybe I say nonsense but what about an over voltage in any of the power lines?

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No, that’s no nonsense. I’ve been thinking of that possibility as well. 

But… There is the balance servo also OP27 on the same power lines which works alright. Second, the offset servo works and keeps the balance quite stable but not at zero as supposed. When I built my three layer Carbon a few years ago I experienced the same problem with offset OP27. So until today I’ve had five OP27 deteriorations/failures.

Anyhow, ordered LF356 and OPA197 with same footprint as OP27 today. Expected to arrive on Friday 1 p.m..

According to OPA197 datasheet the OPA197 has an unique input protection. As I understand OP27 has diode input protection that might be less robust than OPA197’s input protection… but this is probably well beyond my understanding.

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The input seems pretty well protected. Maybe something coming in from the output. 

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On ‎10‎/‎30‎/‎2019 at 1:35 PM, JoaMat said:

Anyhow, ordered LF356 and OPA197 with same footprint as OP27 today. Expected to arrive on Friday 1 p.m..

FedEx came an hour earlier than expected.

One amplifier has now “new” op amps OPA197. The other has new OP27, one channel with 10K instead of 25K at -input pin.

So now I just have to wait and see. I expect the OP27 with 25K resistor to fail sooner or later. For the rest - wish me luck! :D

  • Haha 1

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On 11/1/2019 at 1:37 PM, JoaMat said:

So now I just have to wait and see. I expect the OP27 with 25K resistor to fail sooner or later.

So – now it has failed (couple of days ago), both outputs at +HV. First I put a 650R resistor cross Q10 collector/emitter which brought offset voltage down to +60V increasing to about +90V when warm. Changed the failed OP27 to OPA197 today and now it’s working again.

Edited by JoaMat

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Frankly I expected OP27 to fail sooner or later. I’ve had several OP27 failures, only the offset servo – no balance servo failure so far. I hope that the OPA197 is more robust.  I’ll use OPA197 from now in new builds… as long as they work properly…

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I don't know whether this is the case for the OP27, but some op-amps can get internally unhappy in some applications.

A good example is the LM4562, a dual op-amp which has truly superb specs - but if you use it in follower mode it works perfectly as a radio receiver. It is impossible to measure noise performance because it is swamped in radio. Nothing you can do stops this - battery power, screened enclosure - nothing. Wire it as a gain block, or a filter, or just about anything else and all is sweetness and light. Follower mode - RF. Possibly there is some internal RF instability in this mode and it acts as a mixer.

Not saying that the OP27 suffers woes when operating as a servo, but there seems to be some evidence that all is not well in the camp.

Edited by Craig Sawyers

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JoaMat,

Can I ask what are the specifications for the transformer you are using? also do I need seperate heater windings for the el34s and 6922s.?

thank you for your help

James

 

Edited by jamesmking

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Well, I use some old DIY T2 supplies for all my amplifiers.

For LV and filament transformer I have the following windings
15V + 15V / 0.5A (center tap)
6.3V / 4A
6.3V / 4A
6.3V / 1.5A
6.3V / 1.5A
You don’t want to use same winding for small and output tubes. Separate windings for channels. So total four filament windings.

Demands for HV is 60mA for output stage and 20mA for input.

With +220V supply you I suggest
195V/0,06A
320V/0.18A
365V/0.18A   \\135VA

Without +220V supply, feeding input positive via 20k power resistor, I suggest
320V/0.24A
365V/0.18A   \\143VA        

Calculations are based on 50V headroom and current factor of three (some builders may want another factor).

Reservation for errors!

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