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PostPosted: Tue Mar 01, 2022 4:56 pm 
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Hi all

I received my triwatt kit at the beginning of February this year, and after working on it each day for the last few weeks, I finally have a completed, working amplifier!

Have to say, I was reasonably terrified at the first power up with tubes out. I was entirely sure something would explode, or at least spontaneously combust, due to some error I had made. But nothing went wrong, and after completing all the tests in the manual, I finally plugged in my SG and made some sounds.

The amp sounds amazing! Absolutely the cleanest, noise free sound I have ever had out of any amp. Even the solid state amps I've been playing for the past few years had some hum or noise when I wasn't playing. I could literally forget this amp is on and walk away, it's that quiet. I even went without the tube shields, since the mounting base of the shields were too small for the Sovteks and Tungsol tubes to fit in.

That being said, I have to ask about the out of spec voltages I measured across the phase invertor - is it acceptable tolerance stackup from other components, or should I be de-soldering and looking for a bad cap or resistor?

Pin#____Nominal Voltage____Voltage measured
1_______274____________________257.6
2_______71_____________________66.7
3_______74_____________________69.7

6_______236____________________269
7_______64.4___________________50.2
8_______74_____________________70.5

As you can see, the voltages across the first half are within 10%, all on the low side. But the second phase is questionable; pin 6 (the plate, if I have learned anything at all) is high by 14%! And pin 7 (the grid?) is low by 22%.

From reading Stephens' basic troubleshooting post, high plate voltages could be caused by a bad bypass capacitor or plate resistor. When measuring all the components before I started soldering, I noted a high resistance of 89k for R30: this is called out as an 82k 5% resistor, but 89 is well outside of the 5% tolerance range. Could the high resistance of R30 cause lower voltage to pin 1, and result in a higher voltage at pin 7?

Keep in mind this is my first amp build, and my knowledge is very limited. I will appreciate any responses I get here, so please don't be afraid to sound patronizing.

Note: I attached some pictures, because I just can't help it!


Attachments:
File comment: Wiring
3.jpg
3.jpg [ 324.38 KiB | Viewed 4170 times ]
File comment: Almost done...
1.jpg
1.jpg [ 264.52 KiB | Viewed 4170 times ]
File comment: Complete with cab!
2.jpg
2.jpg [ 206.5 KiB | Viewed 4170 times ]
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PostPosted: Thu Mar 03, 2022 3:22 pm 
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Those voltages aren't too far off, and if the amp sounds good then its probably ok.
The plate resistors for V4 are carbon comp, and typically drift more (usually upwards) than carbon film with age. An 89k reading for an 82k-banded CC resistor isn't that unusual.

Can you provide a close up of V4? Did you double-check the resistor value mounted on the socket between pin 2 and 7?

Also a close-up of the PI section of the board...where you have the 91k and 82K resistors. Are they swapped?

What is your B+ voltage (measure power tube pin 3)? Is it pretty close to the layout (~440)?


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PostPosted: Thu Mar 03, 2022 3:23 pm 
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Also...nice looking build!


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PostPosted: Thu Mar 03, 2022 6:59 pm 
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Attached is a cropped picture I had from a little earlier in the build state, without all of the shielded coax installed. I definitely have the 82K and 91K resistors in the right spots, but it jumps out at me now that the tolerance bands are not the same, with the 82K having a silver band designating a 10% tolerance instead of the 5% noted in the parts list. I hadn't taken note of that detail when I was testing components, and now that 89K reading makes more sense.

I can understand in todays world with part shortages running rampant that Stephen may have had to substitute the wider tolerance in.

As far as double checking the 1 meg across V4, I checked everything before starting the build, and then one again before soldering the components in. So I am very confident in the value of each resistor.

I would have to pull the head back apart to check B+, as I checked it at initial power up and it was near 450v, but that was before biasing, and I did not check again after I set the bias. Once I make the feet for the head, I have to remove the bottom panel and drill the holes to install them, so I will double check B+ on both power tubes at that time.

Not sure if it's worth noting, but the tubes I bought were tested, and the seller was nice enough to write the values on the boxes. I did not research what the values mean yet, but the Tung Sol 12AT7W had the following markings on the box; 45.9/42.3 and 2300/1960.

And thanks for the comment about my build, I spent a lot of time on the woodworking, and learned a fair bit there too!


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PostPosted: Fri Mar 04, 2022 11:24 am 
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Thanks for the extra pic...everything looks right based on the layout/schematic.

I think that you should be ok. I've seen some build guides where if you were +/- 30% you are still ok (just checked...the Triwatt builder's guide says 50%...page 44). My guess is that with the 82K plate resistor actually reading at 89k, that could explain some of the differences. The only really "weird" thing is the higher reading on pin 6, but even then you are only "off" by 14%. What was the reading of the 91K resistor?

Again, if it is sounding good to your ears, you are probably "close enough". To answer your question, I wouldn't worry about it.


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PostPosted: Sat Mar 05, 2022 8:06 pm 
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Glad to hear you think it is within reason. I went back and looked at the manual and you are right, 50% tolerance on gain stages cathodes, grids and plates. Also, the comment about 1/2-1/3 B+ voltage on the plates pretty much covers my deviation as well.

As far as the 91K, I must have measured within 5% of that. I only wrote down the values of parts that were questionable. As it were, the only other items I didn't get within tolerance readings on were the 16 and 33uf electrolytic caps. Not sure why but my multiscope kept reading OVER when testing these. I dont have an oscilloscope or function generator that I could otherwise test with, so I rolled the dice with these two.

That being said, I was unable to test any of the other caps for polarity, like Stephen said in the manual. I was convinced that I was soldering them all in backwards, and would be rewarded with a wall of white noise the first time I fired it up!

Just another reason I am so pleased/surprised with the amps quietness


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PostPosted: Sat Mar 12, 2022 12:24 pm 
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w201028 wrote:
That being said, I was unable to test any of the other caps for polarity, like Stephen said in the manual. I was convinced that I was soldering them all in backwards, and would be rewarded with a wall of white noise the first time I fired it up!

Just another reason I am so pleased/surprised with the amps quietness


Testing the film caps using a guitar cable is hit or miss....sometimes I can tell a major difference when the outer shield is swapped to the tip or sleeve, and other times there is no change in hum at all.

I think the Triwatt's layout is so incredibly optimized it almost takes effort to make it sound bad!


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