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Tubelab Discussion and support of Tubelab products, prototypes and experiments

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Old 19th September 2020, 06:33 AM   #31
frankjoerg is offline frankjoerg  Denmark
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Default solder cathode resistor

hi
the cathode resistor middle of the board may look like it is not soldered in one end.
regards frank
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Old 19th September 2020, 08:45 PM   #32
brl0301 is offline brl0301
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it looks like there are a handfull of solder joints tbat are in need of some attention
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Old 20th September 2020, 10:32 PM   #33
capitanalga is offline capitanalga  Spain
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Hi,
It is soldered, itís just that most of the solder is on the other side. Could any of the freds have been damaged with the mistake of the R2 (the one that blew up)? Is it possible to check if freds are ok? How? I may have to change them for new ones...?

Thanks!

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Originally Posted by frankjoerg View Post
hi
the cathode resistor middle of the board may look like it is not soldered in one end.
regards frank
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Old 21st September 2020, 10:06 PM   #34
Tubelab_com is online now Tubelab_com  United States
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I just returned from what was supposed to be a one week vacation in a time share condo on an island off the coast of Florida. On day two hurricane Sally came to visit. Sally was not yet a full fledged hurricane, but still dropped 18 inches of rain on the island in about 14 hours. Power, internet, and water out and the roads were underwater. Most visitors had left before the storm, but we were there for 11 days. That's why I haven't answered any email or posted anything here. I just got back from a 1000 mile drive, and I'm going to sleep I will be able to read through this thread tomorrow or the next day and offer some help.

I took a quick look and believe that the amp is now working except for a loud hum? If this is correct, pull out the 12AT7 and see if the hum (and the music) stops. That will tell me if there is a problem in the input or output stage.

If this is not the case, please describe exactly what is wrong, hum? With music, or is there no sound other than hum? Does the hum change when the input is removed? Does the hum go away when the input wires are disconnected from the board?
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Old 22nd September 2020, 08:10 AM   #35
capitanalga is offline capitanalga  Spain
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Hi George!

That doesn't sound like quiet and relaxed holidays..

Thanx for the answer and sorry for being so insistent, 3 years working on the amp build and now all of a sudden all the hurries come (probably the emotion of being so close to finish just makes me anxious...)

Anyway, I checked what you told and pulled out the 12AT7 (in my case MULLARD CV4024 / 12AT7) plus no input signal and ... deadly quiet... No hum no, no brain-destroying high frequency whistle/noise.
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Old 24th September 2020, 01:40 AM   #36
Tubelab_com is online now Tubelab_com  United States
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OK, next put back the 12AT7 and disconnect the wiring from the input connector. I'm trying to figure out what's oscillating from far away.
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Old 28th September 2020, 08:22 AM   #37
capitanalga is offline capitanalga  Spain
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So, the situation after putting back the 12AT7 and disconnecting the input wires is the following: First 20 seconds deathly quiet, after the 20 seconds a bit of what could be a soft static noise. The annoying high frequency whistle is gone.
I keep the power on for 2 more minutes but nothing changes.

Thanks!!
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Old 6th October 2020, 08:21 AM   #38
capitanalga is offline capitanalga  Spain
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Hi!

Any update on the hum investigation??

Thanks!

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Originally Posted by Tubelab_com View Post
OK, next put back the 12AT7 and disconnect the wiring from the input connector. I'm trying to figure out what's oscillating from far away.
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Old 6th October 2020, 01:06 PM   #39
Tubelab_com is online now Tubelab_com  United States
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I can not "investigate" hum in an amp I did not build from thousands of miles away. I have built at least a dozen SSE's and none of them hum. I can only look at the picture in post #24 and come to many of the same conclusions that other people have already stated. Using speakers of the wrong impedance will not affect the hum or oscillation problem.

You state that the amp is quiet for the first 20 seconds. I assume that you are talking about the time when the tubes are warming up. This shows that there is no coupling from the power transformer to the OPT's from poor mounting.

When all tubes are in the amp it hums and whistles, but the hum and whistling goes away when the 12AT7 is pulled out. Putting back the 12AT7 but disconnecting the input wiring still stops the hum and whistling, but causes a "soft static noise." This hiss could be normal if it is very faint and only heard if your ear is near the tweeter. Each 12AT7 has some background noise, and some are worse than others. This step shows that the board is probably working fine and you have a problem in your input wiring, a ground loop, or both.

Your picture shows a long pair of input wires coiled up and laying on top of the wiring from the OPT's and what looks like a barrier strip that's connected to the grey wires from the power transformer. This is allowing some signal from the output of the amp to get coupled right back into the input which WILL cause instability and oscillation (whistling and screeching). If those grey wires do come from the power transformer, you are also coupling AC line (mains) signal right into the amplifier's input which can cause hum.

Fix this before going any further. The wiring from the input terminal on the board must be as short as possible and run directly from the input jacks to the input terminals on the board. The wires connecting one set of input jacks to the other should also be as short as possible. All input wiring should be placed as close as possible to the metal chassis and not intermingled with any other wiring.

In your case the input wiring must cross the red and blue wires from the OPT. This should happen at one point only and at right angles. Keep the spacing as far away as possible. Place the input wiring directly on the metal chassis, and keep the red and blue wires up away from the chassis and the input wiring.

I don't know what the purpose of the grey wires and barrier strip is, but keep them down near the power transformer and not next to the input wiring.

As shown your chassis has no ground. Add the red wire you show in your picture. The best place to ground your speaker jacks (green wire) is to the T2-SEC connector's ground pin. It is the one closest to the corner of the board.
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Old 14th October 2020, 12:36 PM   #40
capitanalga is offline capitanalga  Spain
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THAT WAS IT!!!

It was the cables messing around that made that noise. I re-organized everything and all of a sudden it became deadly quiet. Now it's working amazingly good with my vintage Infinity Kappa 6.1.

Thanks for the help and the patience!!
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