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Tubelab Discussion and support of Tubelab products, prototypes and experiments

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Old 9th June 2017, 02:43 PM   #121
spendorite is offline spendorite  Canada
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Thanks for that info W5JAG.
What is the value of the capacitor C1 in the regulator block ?

It looks like I can substitute the zener NTE5162A with 1N5384B, their specs looks identical,
see attachment which I clipped from their respective datasheets.

I haven't had a chance yet to redo the Pete Millet regulator, maybe in the next week.
I'll try this one after that.
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File Type: jpg Zeners NTE5162A and IN5384B.JPG (128.4 KB, 201 views)
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Old 9th June 2017, 03:30 PM   #122
w5jag is offline w5jag  United States
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10 uF.

NTE, which used to be ECG, I think, is just a parts rebrander, so any part with the same specs would be fine.

I am looking at rebuilding it to allow for more voltage at the high end, more screen current, and a smaller physical footprint, so I'm not done with it yet.

Pete's regulator is way better, but I didn't have the parts in stock. I build around what I have on hand. It seems to me his regulator can pretty easily be made to have two or three selectable fixed voltage outputs just by manipulating the series zener string. Two or three fixed voltages is about all you need, I think.

Win W5JAG
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Old 12th June 2017, 08:58 PM   #123
w5jag is offline w5jag  United States
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Here is the screen/bias board installed, sort of, in my SSE. The ground bus on the board is attached to the unused terminal of the SSE terminal block as the PT CT, to avoid ground loops.

A neater installation could be made if the board was just hardwired in, but for the moment I am not finished with it.

The bias transformer is tucked underneath the chassis, directly beneath the choke, using one of the choke mounting holes and hardware for it's attachment. It has has three ac outputs of about 18 volts, 9 volts, and 4.5 volts. Full wave rectified, the 18 volt tap gives up to about - 26 volts negative bias, the 9 volt tap gives up to about -14 volts negative bias.

The 4.5 volt tap gives about + 6.5 volts dc, full wave rectified.

I am using the 18 volt tap for bias. I pulled the 220K grid resistors, and placed a 100K device standing on end, and attached the bias line from the board to the free end. The cathode bypass caps are left in place on the board, but the jumper connecting the negative end to ground has been removed, effectively taking them out of circuit. A 10 ohm resistor is installed in place of the normal cathode resistor.

I do not have the screen supply wired in at present. Using a low voltage screen supply entails pentode operation, and feedback is required. Local plate to plate ( Schade ) is the simple way to go here, but I ran out of time to fool with it. The four empty spots on the terminal block are for the Schade resistors, for easy experimentation. They are also near a ground point, if some capacitance to ground is required.

The low voltage DC supply is to power the possible addition of some electronic "analog" voltmeters, Martel Electroncis QM-130M, pictured. They have a native 0-1 volt DC capability, hence the 10 ohm resistor shunt installed where the cathode resistor would be placed. They provide an analog indication of about 9 discrete segments. As pictured, it is indicating the static current in a 6V6 biased to about 35 ma.

The interesting thing about them, that I did not expect, is their ability to show the dynamic variation in the cathode current. A DMM measuring the cathode voltage will show a bit of jiggle, not much. The QM-130M will show the current slamming the "peg" at both ends of the scale.

Win W5JAG
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File Type: jpg IMG_20170611_165848.jpg (426.6 KB, 195 views)
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File Type: jpg IMG_20170611_222208.jpg (343.6 KB, 190 views)
File Type: jpg IMG_20170611_222417.jpg (377.1 KB, 185 views)
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Old 17th June 2017, 05:24 PM   #124
w5jag is offline w5jag  United States
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I've used 46 for a few hours - enough to determine that, as a triode, it will not die immediately at this elevated voltage.

I made a few quick harmonic distortion tests; as before, distortion is reduced by increasing the standing current in the tube. On the DHT modified SSE board, 12BH7 is a much better performer with 46 than 12AT7, and comparable to the IMD results. For example, at one bias setting and running a bit more than one watt output to a dummy load, 46/12AT7 showed more than 6% THD, while 46/12BH7 was less than 1%.

Win W5JAG
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Old 19th June 2017, 03:49 PM   #125
w5jag is offline w5jag  United States
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Here is the full implementation of the screen / bias board wired into the low voltage SSE, pictured running GE 6W6GT tubes.

I took power for the screen board from the top of the resistor (R4?) that is used to elevate the heaters at DC. I tried using one of the terminal blocks, but my screen wiring is sized smaller than the plate wiring, and getting a good connection was unreliable. Screen wiring out of the board is orange; B+ into the board is red.

Pentode operation requires additional negative feedback, imo. Local plate to plate (Schade) negative feedback is easy and effective.

Feedback wiring out of the power tubes is blue. I took the feedback directly from pin 3 of the tube socket, and it runs to the terminal block holding the feedback resistors. Feedback into the driver tube plates is grey. I put the feedback in at the pad for the leg of the coupling cap connected to the plate side of the driver triode. The connection between the two is via the schade feedback resistor at the terminal block.

The Schade is quite effective. Just casually playing around with it after wiring up the board and doing the initial tests, bringing the Schade feedback online took THD with a 6W6 at one watt from over about 7% or 8% down to under 1%, as well as smoothing out and normalizing the frequency response.

Having the resistors on the terminal block will allow for easy tinkering with the value, as well as allow adding some capacitance, if necessary. it can be disabled simply by removing the resistor.

My board is built topside, so all of my new wiring is topside. It shouldn't make any difference in operation. There are no issues with wiring everything in one bundle in the middle of the board as pictured, but, if you get careless and cross connect the feedback, you will get a funky mode of operation where it appears both the power tubes and the triode oscillate in a different mode of operation from each other, simultaneously. So, don't do that.

6W6 was all I had at home at the moment. I'm running the screens at about 125 volts, the tubes biased to about 32 mils. Anode voltage is 260 with a 6087. Just casually testing the various bits, I did notice some relationship between screen voltage and distortion, but it didn't look significant to me.

As pictured, this little beast will run pretty much any tube, in any mode of operation, within the voltage and current limitations of the power supplies, just by swapping connections with a screwdriver. A soldering iron would be required only for going back to cathode bias operation, but there is little to no reason to do that.

The versatility and expandablity of the SSE is astonishing.

Win W5JAG
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File Type: jpg IMG_20170618_165532.jpg (504.1 KB, 81 views)
File Type: jpg IMG_20170618_165742.jpg (446.7 KB, 79 views)
File Type: jpg IMG_20170618_165926.jpg (399.4 KB, 65 views)

Last edited by w5jag; 19th June 2017 at 04:13 PM.
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Old 5th July 2017, 03:58 PM   #126
w5jag is offline w5jag  United States
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And with the "meters" installed, showing a pair of 6K6GT biased to 25 ma, straight up pentode - 250 VDC on the plate and screens.

Win W5JAG
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File Type: jpg IMG_20170702_162828.jpg (393.9 KB, 69 views)
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Old 11th September 2017, 05:15 PM   #127
w5jag is offline w5jag  United States
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This thread has gone quiet - not necessarily from lack of interest, just lack of attention.

The DHT SSE is working well enough that I think I am going to mount some rubber bumpers on the bottom of the breadboard, and put some convenient input jacks on it, and put it in service in the ham shack. There are still some things I want to try with it, so I can't say it is done, yet, but it is done enough to go in service, and just be moved back to the bench to diddle with as time / opportunity allows.

Once I pick a team, that is - it is still sporting one four pin socket and one five pin socket. I'm not crazy about 2A3, but I will probably take my own advice and go with 2A3, since the small Yamaha's that dwell in the ham shack are not all that sensitive, and I can get a bit of extra headroom with them.

Mule 3A and B is going to be the long suffering, set on fire at least once, TSE board last seen running 801A's. It will probably be a two part breadboard - 3B for the power supply board, and 3A for the carcass of the TSE board. In lieu of punching out the tube sockets, a little fixture will be on the 3A breadboard allowing the mounting of different tube sockets to go with the on board four pin sockets. The separate power supply board will be purposed to incorporate multiple tube types - hopefully to include RF power tubes such as 572B and 811A, but for sure stuff like 307A, SV811 and SV572. Maybe others.

At least that's the plan in the back of my head.

In the meantime, I am also looking at some add on boards for my low power SSE. The screen voltage board is up and running - I'm kind of indifferent about that - it works OK, but I think when I get time I'm going to rebuild it to use Pete's MOSFET regulator.

I've also got the bug to put an FM tuner in the low voltage SSE, and add some input audio switching for convenience. The tuner is mostly done - I'm still doing minor diddling with it, mostly regarding frequency control. Haven't started on the audio switching, yet, but I've thought about it. I have enough room under the chassis of the low voltage SSE, but front panel space, particularly with the meters installed, looks problematic.

It might be easier just to build a new model, incorporating all the stuff from the start. The SPP looks like the preferred board for the driver / power amp part of the build.

Not a single part was fried or set on fire ( so far ) building the tuner - that seemed a little weird. It is pretty minimalistic, but works surprisingly well, and sounds quite good, on a good FM station.

Win W5JAG
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Last edited by w5jag; 11th September 2017 at 05:17 PM.
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Old 12th September 2017, 09:41 PM   #128
jdrouin is offline jdrouin  United States
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That was some really great work, Win. I would like to chime in more but don't feel I have anything to add to the technical discussion. Please know that I read every post and follow it with interest.
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Old 12th September 2017, 10:43 PM   #129
w5jag is offline w5jag  United States
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Thanks, Jeff, it's been a lot of fun, really, in addition to the learning experience. The cathode biased version of the DHT SSE is really a nice little amp.

Someone could use external to the board tube sockets, similar to the way you are doing your 300B experiments, just soldering the leads to the SSE PCB, and, at the cost of some chassis space, have a nice, simple, high quality, low cost DHT without any destructive mods to the SSE board.

I took my own advice, and last night removed the five pin socket, and put a four pin in it's place. Since we have learned that 46 and 47 make excellent triodes in the DHT SSE, but don't seem to gain much in UL or Pentode ( 47 ) mode, they can still be used with a five to four pin socket adapter, as triodes. Which I already happen to have ....

I'm tentatively not done threadjacking and trainwrecking your thread, yet, though. I'm expecting delivery of another line transformer, the Bogen T- 725, to try out on the push pull mule 1, that should offer up a wider range of load impedances. I should have that Friday and will probably get it mounted up on the mule pretty quick, but it may be a while before I can test it.

And Mule 3A and 3B will be semi oddball to oddball DHT tubes in the TSE, and mainstream indirectly heated tubes. Who knows, maybe something useful or interesting will come out of that, too.

I'm debating whether or not to put up anything on the little fm tuner ^^^^ pictured above, that I'm building. The window of opportunity to easily build a simple analog fm tuner is really closing rapidly. A long time ago, in an analog world, far, far, away, National Semiconductor, published an application note, AN-147, http://www.tij.co.jp/jp/lit/an/snoa640/snoa640.pdf, for a simple stereo receiver, that I wanted to build, but just always had something else to do. I came back to the idea about six weeks ago, just to find out that suitable parts are almost unobtainum now, and will be unobtainum in a very short time frame. It would be kind of buried here, but it will ultimately be installed in, or connected to, a Tubelab power amp ....

Win W5JAG
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Old 13th September 2017, 05:44 PM   #130
w5jag is offline w5jag  United States
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The Bogen T-725 came in a few days early.

I have a case of the Atlas Sound HT-87 here in the storeroom at the office ( I have parts everywhere - it's maddening ) so I lined them up for a size comparison. The Bogen is puny compared to the Atlas, which makes a quite good p-p opt, with the right tube. The Bogen is the usual line transformer size.

What interested me about the Bogen is the wide range of loads it offers. If my arithmetic is correct, I can get 39K, 19K, and 10K, out of it, while the normal line transformer is usually about 5K, max. I'm thinking the 19K load might work for p-p 6G6 and 6AK6, maybe 6K6. Hopefully next week I can get it installed, and take some measurements.

Might be a way to use these as interstage transformers, as well. Possibly it will have enough inductance to use as a plate / grid choke.

Win W5JAG
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