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Tubelab Discussion and support of Tubelab products, prototypes and experiments

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Old 21st December 2017, 03:40 AM   #21
bibin3210 is offline bibin3210  India
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Hi George,

Great job. I am very much interested and will be following this thread closely.

Thanks,
Bibin
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Old 22nd December 2017, 12:37 PM   #22
colnago55 is offline colnago55  United States
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I love the idea of having several schemes for using the board made available. KT88's, 300B's. Mono-blocks or a Stereo amp.

Sounds great.
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Old 22nd December 2017, 04:01 PM   #23
Tubelab_com is offline Tubelab_com  United States
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Quote:
Perhaps 5000 ohms would be a good compromise.......I just happen to have two excellent sets of 5K OPT's. That's for another test day.........
Yesterday was the "another day."

I have a pair of Lundahl LL1663/PP OPT's that are 5K but are rated for "40 watts at 30Hz." I think I can get more than that from these tubes, so......

I have two different pair of Plitron OPT's that are 1.25K and are rated for "400 watts at 20 Hz." I have previously used one set of them at 5K ohms by connecting the 8 ohm load to the 2 ohm output tap. Maybe I won't get 400 watt capability this way, but I have seen 100+ watts. One pair of the Plitrons are seen in post #5 connected to a pair of transmitting tubes and wired for 2500 ohms (8 ohm load on 4 ohm tap). 250 watts flowed forth from this combination, but I didn't run a frequency response test. I decided to use their bigger brothers for this test! Everything else is as ran in the last test with the GE 6550A's. I just swapped the OPT's.

The first two pictures show the huge OPT. These things cost about $900 per pair when Plitron took them out of their catalog. I paid far less for a used pair. We found out last time that expensive tubes produced better MEASUREMENTS than the budget stuff, now does a $400 OPT do better than a $16 OPT........it better:

The results for a 5000 ohm OPT:

power THD B+ current output stage efficiency

0.1 W 0.086% 90 mA 0.2 %
0.5 W 0.074% 93 mA 1.2 %
1.0 W 0.083% 95mA 2.3 %
2.0 W 0.126% 98 mA 4.5 %
5.0 W 0.273% 111 mA 10.0 %
10 W 0.520 % 130 mA 17.1 %
20 W 0.855 % 160 mA 27.7 %
50 W 1.73 % 220 mA 50.5 %

Maximum tube dissipation is reached somewhere between 20 and 20 watts output, and is about 26 watts. It drops to 24.5 watts at 50 watts output. Maximum tube current is reached at maximum power (50 watts) and is 110 mA. Both of these are conservative for KT88 and 6550.

Conclusions.....Unless you need 70 watts, the 5K load is the optimum spot for triode wired KT88's on 450 volts. The distortion numbers can not be directly compared since the better OPT's obviously made better numbers. The power and efficiency numbers are about right.

So if I had frequency response numbers on the 3300 and 6600 cases, I would have been measuring the OPT's and not the amplifier circuitry. This may still be the case with the Plitrons, but I like the numbers. the distortion numbers put that "first watt" the squeaky clean zone below 1/10th of a percent.

Many amps measure the frequency response at something like 1 watt, but those numbers were beyond the range of my test equipment, so I decided to measure the frequency response at FULL POWER, 50 watts.

The low frequency -3db point was somewhere below 10 Hz, the 8903 is not accurate below 20 Hz. OPT saturation effects became visible on the scope around 8 Hz at 50 watts. The distortion at 20 Hz and 50 watts is 1.83% (picture #3). The frequency response at 20 Hz is 0.16 dB down at 20 Hz compared to 1KHz. (picture #4)

The high frequency -3db point is 78,568 Hz. The distortion at 20 KHz and 50 watts is 2.464% (picture #5). The frequency response at 20 KHz is up 0.07 dB compared to 1KHz. (picture #6)

I had planed to move on and test some different tubes, but these numbers have convinced me to build up the second channel, and maybe a power supply to see what this thing SOUNDS like! I plan to order enough parts to build a few more UD boards.....I still have my dead 300Beast amp that I built about 12 years ago. It's the only HiFi amp I saved from the past, still awaiting a total rebuild.

Progress and posting will be slow and sporadic until after the first of the year. With 4 grandkids, the Christmas season insanity is already out of hand, and Sherri has the next week off, so that we can catch up on life, and all the tasks that have been ignored for the last few weeks.
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Old 22nd December 2017, 09:38 PM   #24
mkane77g is offline mkane77g  United States
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Merry Christmas to you and your family George. Enjoy
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Old 22nd December 2017, 10:19 PM   #25
Tubelab_com is offline Tubelab_com  United States
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Thanks.......

Sherri went to the movies with friends, so the soldering iron is smoking........
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Old 23rd December 2017, 12:31 PM   #26
tomlang is offline tomlang  United States
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Wow! With careful measurements to boot! George, do you take all those distortion readings manually by nulling, etc each and every time?
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Old 23rd December 2017, 02:16 PM   #27
Tubelab_com is offline Tubelab_com  United States
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If you look at the pictures in post #23 and many of my other posts for the last 10 years you can see a HP8903A Audio Analyzer (on top of the power supply to the left of the scope). It replaced the old HP33X series in the 1980's (I still have an HP331A). It does the same thing, except for having a better notch filter for a lower distortion floor, and it's somewhat automated. The automation is primitive by today's standards, but tuning and nulling the notch filter is automated.

Those distortion readings were made by simply pushing the distortion button on the front panel. It is only a basic THD + noise measurement, but good for comparisons on things like tubes and OPT's. It will do an automated frequency response sweep and send the output to an analog HP plotter, but you can't recover the data any other way. More automated results require an external computer and some programming.

I worked in the test equipment calibration lab at Motorola when these things came out, and I spent quite a bit of time playing with them. The measurements that make are accurate, and better than the old HP33X series due to the deeper notch filter. The HP8903B series is a bit better, and a lot more reliable. I have repaired this one 4 times in 20 years.

The instrument I have actually came from Motorola when the first large Florida plant closed down and all the stuff was scrapped, auctioned, or donated to the local college.

I use a PC with a 24/192 sound card for FFT measurements (individual harmonics), but I haven't made it work right since I moved 3 years ago. I lost the license dongle for WinMLS, so I will try some free software soon.

The second channel PC board is now stuffed with the same parts that is in the board seen throughout this thread. All this stuff will not fit on the plywood base I have, and I'm not about to head out to Lowes today, so I'm repacking it all to fit the available space. The OPT's will have to move to another piece of wood. I have a pair of 100 watt Edcors that need to be tested too!
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Old 23rd December 2017, 02:28 PM   #28
zman01 is online now zman01  Bangladesh
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George,

Great to see you working on new stuff and sharing the findings again... hope you manage to get more time going forward to continue the R&D (followed by good sales!)

Wish you Merry Christmas and a Happy New Year!
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Old 24th December 2017, 01:53 PM   #29
Tubelab_com is offline Tubelab_com  United States
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Quote:
more time going forward to continue the R&D (followed by good sales!)
Sales of my PC boards, DIY audio, and small project DIY stuff in general have always been cyclical. People tend to do more indoor building during their local winter than in their summer months. Sales are very slow during the summer months, then tend to pick up around October, peak in January, then taper off toward April or May.

That did not happen this year. I had 4 sales in October, 3 in November, and the third for December just came in. Despite an unusual uptick in sales that started late last year, 2017 has been the worst year for board sales since 2007. Tubelab Inc. has never made a lot of money, and lost money in several years which was not a problem when I had a full time engineering job with a fat paycheck.

Now that I'm retired and trying to support two families, Tubelab must pay for itself, which it has barely done in the last 3 years. In order for Tubelab to continue I need to lower costs and generate new revenue.

I plan to try an offshore PC board vendor for a test batch or two to see if I can get a better board for less than I'm paying now.

The cost of shipping continues to go up every year, but I have not raised my rates. That may have to change.

I had planned to offer parts kits for the SSE and restock the parts kits for the SPP, but I'm buying parts from the same places everybody else is (Digikey, Mouser....) so the only way that works is to buy large quantities Unfortunately the payoff for this takes years, and a large cash outlay is needed up front. Not likely to happen soon.

I have several vacuum tube PC boards part way through the design process. These should turn into new vacuum tube products.

There are a few non vacuum tube audio related projects that are a possibility, and maybe even RF (ham radio) projects.

In the short term, I can sell these boards. For now the only construction info is what's right here. The boards are $25 USD plus the usual $8 shipping in the US, and $15 international for up to 4 boards.

I have finished populating the second board. It works great stand alone. Assembly of a stereo test amp is underway, but likely stalled for a few days.....4 grandkids, and 2 parents are scheduled to invade this house tomorrow morning. All objects that could be considered hazardous to 3 and 5 year old kids who think that they are Ninja' in training must be hidden!
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Old 24th December 2017, 10:19 PM   #30
Sprags is offline Sprags  United States
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My career became messed up last year ...and the job I'm at now pays no where near what I made before ...but I had an interview that I hope will be in line with the kind of stuff I do for a salary close to what I was make...

I hope things change for the better for you...
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