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Tubelab Discussion and support of Tubelab products, prototypes and experiments

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Old 15th September 2017, 03:40 PM   #131
w5jag is offline w5jag  United States
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Join Date: Feb 2007
Location: Arkansas
A few weeks ago I bought one of the little Mega 328 component testers off eBay. They are a pretty handy device - not only will they identify many types of components, they will give many of the parameters, such as ESR.

It is not quite as useful for inductance as it is for other components, as it only measures down to 2 microhenries or so, and a lot of RF coils are less than that. Still, for L above that threshold, it beats hooking up a known capacitance and trying to find resonance, like a full house beats two of a kind.

It does quite well at reading the large inductances on power supply type chokes, and primary inductance on conventional audio transformers.

I tried to measure the inductance on the little Bogen T-725, and came up with some oddball results, that I don't have a very high degree of confidence in, unfortunately.

Measuring the 39K tap, I got around 16 Henries. Oddly, measuring the 10K tap, I got around 50 Henries. Lower impedance taps gave more nonsensical results. I tried another line transformer I had laying around, and got the same kind of bewildering results, so, these are strictly FWIW. Which may be not much.

I tried to figure out the frequency it was using to measure the L, and hooked up my scope to an unused tap on the line transformer, and it looks like it pulses the component with a sawtooth type of AC wave, at stepped intervals and frequencies, probably 4, 8, 12, and 16 Hz. I did not look at the DC component, if any.
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File Type: jpg IMG_20170914_221328.jpg (270.1 KB, 108 views)
File Type: jpg IMG_20170914_220623.jpg (392.7 KB, 109 views)
File Type: jpg IMG_20170914_214506.jpg (502.6 KB, 109 views)

Last edited by w5jag; 15th September 2017 at 03:48 PM.
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Old 18th September 2017, 05:45 AM   #132
w5jag is offline w5jag  United States
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Join Date: Feb 2007
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I mounted the Bogen T-725 on Mule 1, and fiddled with it a bit.

8 ohms is the only output impedance this line transformer provides. Similar to the other line transformers, the "center tap" is two taps up from the end tap in use. It has both 25 and 70 volt lines.

I had the amp set up for 6K6, cathode bias, running about 6 watts in each tube of the push pull pair. At 10k load, I got about 3.125 watts out of it. I also tried 19K load, and got the same power output, but saw higher THD at 19K.

Even though it was set up for 6K6, I also tried 6G6. They biased up almost identically to 6K6; a little bit more than 5 watts per tube. 6G6 seemed to respond better to the 19k load. Power output dropped, but distortion also dropped relative to 10K, and at one watt output, wasn't all that bad, about 1.6%, IIRC. I didn't take any notes of exact numbers.

When I had it set up on the 19K load with 6K6, I looked briefly at frequency response. Relative to one watt output at 1 Khz, the -3 dB point was about 55 Hz. I didn't check the high frequency response. With the standard 60Hz:6KHz 4:1 set up, and 55 Hz -3dB point, SMPTE IMD was poor - about 5% at 0.125 watt.

I'll fool with it some more, and get some better measurements at some point - it's a bit of a tedious process with all the combinations of tubes, taps, and operating points - it will need to be set up on a bench supply and rigged for fixed bias; right now other projects are a higher priority. For now, about all I can say for sure is that it didn't immediately self destruct at mid voltages ( ~ 275 VDC ) and low currents ( 20 - 22 ma).

I did not listen to it. It might sound okay or bad.

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Last edited by w5jag; 18th September 2017 at 05:53 AM.
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Old 18th September 2017, 04:38 PM   #133
w5jag is offline w5jag  United States
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Default Analog FM Tuner board drawings

For anyone interested, here are the schematics for the analog FM tuner board. They are a bit helter skelter as I just kept making changes to the drawings as I changed things, in lieu of drawing a new one. Also attached is an IF amp I did not use, but worked well.

It's pretty well done. I may still fiddle with some of the IF selectivity, but that is just part substitution, maybe adding an additional filter. The frequency display is a programmable frequency counter module from eBay > $12 USD to my door.

It should be obvious that I am not an engineer, but if there are any questions, I'll try to answer them.

Construction used both sides of the board. My soldering sucks compared to what I did twenty years ago. It caused me some real problems with the stereo decoder. I'm not adapting well to the new solders / parts.

The tuner works well enough, and is pretty hot. I live on a pretty good hill in the river valley; standing on my roof I am line of sight to the foothills of the Boston Mountains to the north, and to the horizon east and west. A ridge four or five miles to the south limits line of sight in that direction. One of my target stations was a 47 KW station 53 miles south, down in the Ouachita Mountains. The tuner limits, stereo decodes, and fully quiets on this station with a couple of clip leads for an antenna.

Connected to a test dipole cut for 98 MHz and mounted about 17 feet AGL; there is a 100 KW station 11 miles north of me, line of sight, and a 250 watt translator spaced 600 KHz below the 100 KW station, also line of sight. The tuner has no difficulty receiving the 250 watt station.

I haven't started on the audio input switching yet - I lack a few needed parts. I think I can get it all in the existing SSE, except for the frequency display. There just isn't enough front panel space.

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File Type: jpg fm mule sheet 1 tuner.jpg (402.7 KB, 92 views)
File Type: jpg fm mule sheet 2 IF.jpg (351.3 KB, 88 views)
File Type: jpg fm mule sheet 3 st decoder.jpg (259.2 KB, 34 views)
File Type: jpg fm mule sheet 4 alternate IF unused.jpg (275.3 KB, 28 views)
File Type: jpg IMG_20170918_091045.jpg (333.6 KB, 41 views)
File Type: jpg IMG_20170918_091003.jpg (382.2 KB, 41 views)
File Type: jpg cr sse what if.jpg (107.9 KB, 38 views)
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Old 24th October 2017, 04:52 PM   #134
w5jag is offline w5jag  United States
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Quote:
my 845SE which is sitting on the shelf waiting for something between a power supply upgrade, and a total rebuild. There are multiple instances of exposed electricity up to 1050 volts and grandkids come here.....
I know, when / if you come up with a solution to this dilemma, I'm all ears. If anyone noticed, experiments with the DHT SSE pretty much ended back in May, although there was / is a lot that needed to be done. My third harmonic turned five in late summer, and he likes coming out to the ham shack ( detached building ), a few times when I've not been around. Consequently, the DHT SSE was finished out as a working breadboard amp, and stuck up on a high shelf, where there are at least a few barriers to him getting fingers into it.

The power supply in the DHT SSE is probably good for 500 volts in a CLC configuration, and I think 46's probably are too, but I've throttled it back to an operating voltage of about 300 volts, and if I can remember to grab a 5W4 from the warehouse, I'll drop it to about 280 - 275.

I wired the electric in my shack with a separate sub panel, and I keep the 230 off at all times ( I don't use it anyway ), and I can kill the relevant 120 leg that powers the tube gear and most of the work bench as a fail safe, but when he sees me flip that breaker one time, that fail safe is gone.

So, there has been a sudden lurch back to solid state, audio accessory type stuff, the fm stereo tuner, a preamp / audio switch, finishing out the low voltage / child resistant SSE, the old SSB rig, etc., and probably a lowish voltage, low power, SPP, in a full and complete enclosure.

Quote:
.... Those pads actually only isolate the B+ voltage to the mosfets. The limiting factor for B+ on the board is heat and breakdown voltage in the mosfets. This jumper, and the -Vin pad next to the Fred diodes aloe operating the mosfets from an external power supply. I used +/- 150 volts in my 845 SE, and ran the TSE board from an external 500 volt supply.

To run 811A's or other big voltage tubes, build the TSE with enough power supply to generate the drive for the big tubes, then just tie the red wires from your OPT's to a power supply with enough voltage for the big tubes. That supply never needs to touch the TSE board at all!
That probably saved some fireworks ..... As noted above, I haven't gone forward on Mule 3, yet, because of the situation involving exposed high voltage in progress just laying around on the workbench. Also, much of my work now is really late at night/early morning, and not the best time to be working with transmitting tube voltages .....

I was at the warehouse yesterday and, spotted this enclosure that needs something built into it ......

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Old 17th November 2017, 03:45 PM   #135
w5jag is offline w5jag  United States
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At this point, I have quite a few hours on the DHT SSE with 2A3 at normal voltages ( i.e. less than 300 VDC ), and it appears to be a completely issue free amplifier - entirely stable, including the front end modified to draw slightly less current through the 10M45's.

I'm not an audiophile, but it really sounds pretty good; even though the OPT's are sub optimal, they seem to be a reasonable match to the Yamaha's in my ham shack that do not have a deep bass response. The highish IMD seen in the measurements with these OPT's, doesn't seem to be that big of a deal in the real world. I've been putting some hours in my ham shack on another trainwreck project, and typically have the DHT SSE running in the background. In some circumstances, I can hear the IMD come up during the combination of high power output with complex inputs, but for 99 + % of listening, it just doesn't seem to be an issue.

I'm even starting to like 2A3, or put a different way, dislike them less.

Has Google Play Music upped their streaming quality? it sounds like 320K or better.

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