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DIY mini T2 Build Thread

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Recovered the GRHV,  I take a look  to  the amplifier. Interestingly, all damage is in the area of Servo/OPA27.

It seems that most of the transistors are fine although I suspect that the IXTP01N100D are damaged (continuity between two of their legs). 

Tomorrow I will begin to dismantle the most damaged channel. 

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Great, quick recovery. What parts in GRHV needed to be replaced?

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HC´s number one rule: have more components in your warehouse than Mouser.

Follow the trail of burned components

Certified dead: CL resistor, r12, c2m1000170d, 2x bc557b, 2n3904. njm7915fa

Changed as a precaution: Q7 10m90s. Q8 & Q9 ksc5026m and 24v diode.  

The rest of the components passed the test.

Total: A few euros and a couple of hours.

 

 

Edited by jose
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Thanks, good to know what components that are at risk. I’m using a KGSShv style PSU and I’ve a small inventory of FQPF8N80C, 30V Zener and 15R power resistors and so far that has been enough. Also omitted current limiter since it hasn’t protected me. 

One other question, @jose. Do you have four separate 6.3 VAC for the tube heaters?

Edited by JoaMat

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The resistance I have in the current limiter is about 3R. It is interesting what you have commented about removing the CL resistor. Perhaps without this resistor the damage would have been greater ¿?¿?¿?¿?¿?¿?

Currently my plan is to build a BH or BHmini PSU just for testing.

As for the heating filaments, my T2 mini is configured with the E88CC filaments in DC (with a separate transformer and a PSU for this work) and EL34 with the "standard" way;  four tubes with AC with its own transformer. Should I use separate transformers?

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I haven’t removed “current limiter” resistor – here I use 15R/3W – just removed the 2n3904 transistor.

I’m only used to 4 separate heater windings so I can’t tell about running all big tubes as you do. Mini T2 has big tubes heaters tied to roughly -400 V via R53-56 voltage divider, so you actually connect left and right channel’s dividing points with your single filament transformer. My brain is to slow to figure out what, if any, implications that might have. Perhaps someone out there can enlighten us.

But never use same heater windings for big and small tubes. Small tubes don’t like heaters to be tied to -400 V and besides their heaters are tied to ground via 47R resistors.

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I believe all the 6922s might be run off one winding since the heaters are ground referenced and the current draw is much less than the el34s but I have not tried it. As joamat says the el34s cant share a heater winding with the 6922s since the el34 heaters are -400V with respect to ground. I have not tried running both channels el34s from a single winding because a. thats a lot of current draw 6A ignoring switch on inrush and magnetic losses b like joamat I don't know if it would be safe to do so.  c separate windings provides a bit more isolation if/when things go wrong. You might want to message Kevin Gilmore for his advice....

P.S. jose have you signed up to the please add screw terminals to the mini t2 amp boards request yet 👿

 

Edited by jamesmking

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2 hours ago, JoaMat said:

I haven’t removed “current limiter” resistor – here I use 15R/3W – just removed the 2n3904 transistor.

I agree with this. The transistor does not provide protection against shorts, so I remove it as well and use the 15R resistor.  When I feel it necessary, I use an external current regulator. 

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I think I have not expressed myself well:

For the EL34 I use a single trafo with two windings. One winding per channel (2 EL34 per winding)
For the E88CC I use another trafo with its own psu for DC heater. 

Both types of tubes DO NOT share transformer.

Edited by jose

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I'm running + and - 15v, 6922 and el34 from the same transformer all heaters are AC. As long as the insulation on the windings can handle the 400V between el34 and the other windings its fine.

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2 hours ago, jamesmking said:

I'm running + and - 15v, 6922 and el34 from the same transformer all heaters are AC. As long as the insulation on the windings can handle the 400V between el34 and the other windings its fine.

That's similar to the DIY T2 and works just fine.

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On 6/1/2020 at 4:34 PM, jose said:

My idea is to remove all parts of that servo and run the amp without it or using the servo on top. I understand that it is possible.

To find out I removed Q10 (also removed vol618) and replaced it with 1K trimmer (just put it in Q10 position) to control the offset. With balance trimmer (jumper removed) and offset trimmer I’ve no problem to manage the amplifier, but this is just after a day. Maybe I tell more later.

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40 minutes ago, JoaMat said:

To find out I removed Q10 (also removed vol618) and replaced it with 1K trimmer (just put it in Q10 position) to control the offset. With balance trimmer (jumper removed) and offset trimmer I’ve no problem to manage the amplifier, but this is just after a day. Maybe I tell more later.

Thanks for advice Joachim!

I´ll work in that direction.

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Think I like trimmers. Instead of servos – miniature stepper motor driven trimmers controlled by Arduino?

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3 hours ago, JoaMat said:

Think I like trimmers. Instead of servos – miniature stepper motor driven trimmers controlled by Arduino?

Sounds like a battle between the Mechanical Engineer and the Electrical Engineer inside of all of us.  

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A detail that I forgot to comment, the slow 1A fuses that I use at the output each HV line of my PSU were blown.

The fuse on the HV + rail protected its side of the GRHV but the one on the side - it wasn't so lucky.

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On 6/5/2020 at 11:47 AM, JoaMat said:

To find out I removed Q10 (also removed vol618) and replaced it with 1K trimmer (just put it in Q10 position) to control the offset. With balance trimmer (jumper removed) and offset trimmer I’ve no problem to manage the amplifier, but this is just after a day. Maybe I tell more later.

A week without offset servo and most of the time jumpers for balance servo have been removed. Today amplifier started out with offsets up to -40V. After half of an hour offsets between 0V and -10V and unbalance 5V and 10V. I think this is acceptable.

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