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Old 10th October 2021, 08:46 PM   #111
spiggs is offline spiggs  United States
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Originally Posted by Francois G View Post
...but instead are trimming R104 to change the bias on Q101. When you decrease the Q101 bias the V101 tube conducts more current and therefore lowering the plate voltage due to a bigger drop across the load resistor R108. Did I get this right?
Correct. All I am doing is playing with trimmer R104 and observing it's influence on the FFT spectrum.
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Old 11th October 2021, 03:32 AM   #112
spiggs is offline spiggs  United States
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Here is the REW plot I am getting at 1W
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Old 12th October 2021, 03:29 PM   #113
spiggs is offline spiggs  United States
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The mains noise that is shown in the REW plot is audible. I have been trying to cleanup the grounding but can still hear a buzz through the speakers. Right now I have a ground from each opt speaker outputs, one from left input to pcb, toroidal ps shield, and mains earth all meeting at one point using clip leads. Any advice on what to try next?
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Old 18th October 2021, 04:16 AM   #114
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Originally Posted by Tubelab_com View Post
...OPT secondary... This ground should be returned to the PC board, not through the chassis.

I currently have the OPT secondary grounded to C9 (the two black clip leads). This has proved to be the quietest so far but doesn't seem the best place for when I put this in a chassis. Is there another recommended place to bring these grounds? The closest options seem to be the vias by the output tubes. On my SSE I connected the OPT secondaries ground to a single point with the input ground.
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Old 18th October 2021, 09:00 PM   #115
Francois G is offline Francois G  United States
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Sorry you are having problems with that hum. While we wait for George’s definitive reply I would venture that it wouldn’t make much difference where on the PCB you ground the output transformer, since there should be no potential difference on the “pseudo ground plane” of the PCB. For practical reasons I plan OT/PCB ground at C2 where the copper is wider.

So what could be the problem? Have you practiced star-ground? I would look for ground loops due to multiple grounding points. I found Rod Ellliott’s piece helpful, especially figure 5 in section 9, and separating “dirty ground” from “clean or signal ground” with the ground loop breaker.

Earthing (Grounding) Your Hi-Fi - Tricks and Techniques

My building has been progressing slowly with soldering most the PCB parts, but I will be away on a three week vacation starting this weekend, so I hope to fire imy UNSET up somewhere in early December.
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Old 20th October 2021, 06:28 AM   #116
zintolo is offline zintolo  Italy
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I give my experience based on instrument amps: I ground the secondary to the main supply cap (close to the rectifier) and this is the quietest point.
Concerning this specific amp, I still don't know because I've not yet completed it.

I would like to use one amp with 500V B+, 6E5P on preamp, KT88 on poweramp with 1k5 Toroidy OT used as 3k. Screens connected to 40% UL. 6E5P loaded with resistor or CCS. The second amp with GU50 in pentode mode.
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Old 21st October 2021, 01:51 PM   #117
Tubelab_com is offline Tubelab_com  United States
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Quote:
Originally Posted by Francois G View Post
I would venture that it wouldn’t make much difference where on the PCB you ground the output transformer, since there should be no potential difference on the “pseudo ground plane” of the PCB. For practical reasons I plan OT/PCB ground at C2 where the copper is wider.
For hum purposes connecting something that draws ZERO AC or DC current should not make much difference where it connects to the board. For the purpose of killing the potential for high frequency oscillation, it does often matter since lots of gain and even small inductances and stray capacitances can cause oscillation.

If you look carefully at the PC board there are three separate “pseudo ground planes” and they meet at the star ground point, the negative terminal of C2. The output tubes, the driver tubes, and the power supply all have separate grounds, meeting at C2. All off board ground connections except for the inputs should go to the T1-RED-YEL terminal as this is the star ground point for the whole amp. The ground vias near the output tubes are probably OK for the OPT grounds if the output jacks are isolated from the chassis.

There should be exactly one connection from the chassis itself (and the transformer cases) to the PC board. Experts are divided on where this should occur. Some say it should be at C2, and I would tend to agree with this.

Others say it should be where the input jacks pass through the chassis. I have done this on some of my amps with good results, but only because the little RCA jacks that I use are hard to isolate. With nearby sweep tubes hitting large current peaks, and capable of high power oscillation at tens of MHz, I would tend to avoid trying this here.

I have not yet tried to stuff an UNSET board into a chassis, and my FFT box is not operational on the bench yet. A major bench clean up is needed to make space.

The residual hum, noise, and crud on my speaker jacks is in the 2 mV range with a similarly connected board. My black clip leads are connected to the ground end of R1, which is the power supply ground, out of convenience. C9 is actually the far end of the driver ground. This should be about the worst place to connect the OPT ground from a stability standpoint.

Next time I fire my board up, I'll play with the OPT ground point and see if it makes any difference.

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Originally Posted by spiggs View Post
The mains noise that is shown in the REW plot is audible. I have been trying to cleanup the grounding but can still hear a buzz through the speakers..... Any advice on what to try next?
Your spectral plot shows considerable 60 Hz with lots of harmonics. This would indicate that the hum could be originating in the power supply.

Some more info could be obtained by performing a few simple tests.

Fire up the board so that you can get the same plot.

Disconnect the input wiring from the board. Obviously there will be no 1 KHz tone, but do the 60 Hz and it's harmonics remain present, and at the same level.

If so, power down, remove the driver tubes and repeat the test. If the hum remains with only the output section alive, then there must be some ripple on the supply, or other means of hum coupling.

For the last few weeks I have been using the bench supply so that I can play "dial a volt" with several different tubes. I need to hook the transformer back up and make some tests.
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Old 22nd October 2021, 11:43 PM   #118
Francois G is offline Francois G  United States
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I would like to report my progress after orders arrived. Would like some feedback if any, and I present it for the information of present and future builders.

Photos are of my build as of this afternoon. Most resistors and capacitors on the PCB are soldered now. Next I have to do the semis/heatsinks and wiring. I am planning to use 6EJ7 and 6DQ5 tubes, with about 425 Vdc at the B+ supply, employing Toroidy output transformers (TTG-KT88SE ) with Za-a of 3k ohms.
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File Type: jpg D082D42E-6F94-4DFA-9DA0-88673F244729.jpg (778.9 KB, 29 views)
File Type: jpg 6018FDAF-2759-4D95-990F-0EB27D85161E.jpg (968.7 KB, 28 views)

Last edited by Francois G; 22nd October 2021 at 11:47 PM. Reason: Adding photos
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Old Yesterday, 01:19 AM   #119
Francois G is offline Francois G  United States
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George, my feedback for future UNSET boards, based on my initial experience with the Beta boards:

1. For this (and future) UNSET boards, I expect very few tube rectifiers will be used (versus SS rectifiers) due to the high currents required by the TV sweep tubes, especially at higher power than your “common 300B SE amp”, the rai·son d'ę·tre for the UNSET. Why have an on-board tube rectifier? I’m using 1200 V SIC (STPSC2H12D STMicroelectronics | Mouser) rectifiers and I plan to report on how that goes. But the question is, why have the rectifier socket on board if it is not used?

2. This PCB has the power tube sockets built in (I think the original reason for this with Tubelabs was ease of supporting various levels of builders), but it still requires the builder to select, and mix and match all the inputs to the power tubes. Why have the power tubes on board if it would allow much more flexibility when off-board, and about the same “confusion factor”?

3. Same for input tubes.

After completing the Beta boards I will have more feedback. Especially on the “notes for building the UNSET beta board”.
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