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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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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!!! Thinking of retire a

A few points regarding the boards Michael have had made on my gerbers. They are basically the same as my kitchen made boards I made  five months ago. The small tube footprint has no hole for the

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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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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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  • 1 month later...

While working with latest mini T2 (version .23) boards today I discovered that op amps on left channel have wrong (old) values. The op amp on none tube side, close to VOL168A, should be OPA197. Op amp on the other side should be LF356. Silkscreen right channel seems to be correct.

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Warning!

0.1u/1kV film cap pads on right channel have 80-30 pads meaning hole diameter is 0.030 in or 0.76 mm. This is to tight for most film caps that have lead diameter of 0.8mm. There are some Panasonic, Cornell Dubilier, Nichicon and Vishay caps with 0.6 mm leads and 15 mms lead spacing.  Do not drill the holes for the film caps. The through hole plating is needed as it ties ground planes together and also needed for -460V to be fed to none tube side of the board.

Left board have 90-40 pads – 40th or 1.0 mm. 

This mistake is done by me and the blame is entirely on me. I’m sorry.

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Now completed mini T2 with the new boards version .23. 

Squeezed in film caps with 0.8 mm leads on right channel with 0.76 mm hole diameter, elevated and soldered both sides (see below).
IMG_0324.thumb.jpg.265e198452d3bae9b70e06b09cd70ea2.jpg

Both channels worked right away. Only issues I’ve found with those new boards are mentioned in posts above.

 

Edited by JoaMat
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  • 4 weeks later...

Here are some distortion figures and harmonic analysis up to 50Khz.

Test setup groove tubes el34 matched, electro harmonix gold pin 6922 matched, 1Khz sine from a 192K 24bit rme soundcard spdif out to a v90 dac to the mini t2 inputs, measured on a keithley 2015thd. distortion graph is the average of 20 measurements. All other readings are not averaged:  Distortion is less than 0.02% up to 500V rms output with second harmonic being the highest and a little bit of third above the noise floor. At just under 500V the 4th harmonic is above the noise floor, above 500V rms clipping starts and at 600V rms distorion is ~2.5%  with the second harmonic at -36db relative to the 1khz signal level. On the right hand top side of each graph under the measurement section is the measurements for the 2nd harmonic in db and mVolts average, min and max over 20 measurements. Noise is the ac voltage not in a harminic bin (i.e. a bin which is an integer multiple of 1Khz). Analysis is with 749 bins going from 20hz to 50khz. A full fft with all bins is included for the 100Vrms output only at the bottom of the post.

I wrote the software myself, its very much a rapid prototype kludgy mess but I hope to make it available to community members with keithley 2015 when the code is not so offensive. At the moment there is no guarantee that it will not open up a dimensional hole and make all your original NOS Japanese transistors disappear.

 

1105861145_t11k100vdistortion.thumb.jpg.8ca41b4689ed7c3533a433ad7213c296.jpg1388388288_t11k200vdistortion.thumb.jpg.ae5ecad3838d9049361d617a278f1dbe.jpg1391844372_t11k300vdistortion.thumb.jpg.e6e026d9f12bd6a276d3d3ca55142a9d.jpg2115333200_t11k400vdistortion.thumb.jpg.484e57581686c36511a5c056c8acd162.jpg844482534_t11k500vdistortion.thumb.jpg.f8853d14d4dbaa84b63ac2f25ff2703f.jpg1836862417_t11k511vdistortion.thumb.jpg.d4c2dba9dc3d0bc70ac31952203a15cc.jpg534397009_t11k540vdistortion.thumb.jpg.c6da3f776c8ffdfc2bc0fda5c90a5d3e.jpg

FFT analysis 1Khz 100Vrms output. 20hz to 50khz

 

999300007_fft1khz100v749bins.thumb.jpg.606eedcde8f52820a47cad5c9f8af526.jpg

 

 

 

Edited by jamesmking
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