• Disclaimer: This Vendor's Forum is a paid-for commercial area. Unlike the rest of diyAudio, the Vendor has complete control of what may or may not be posted in this forum. If you wish to discuss technical matters outside the bounds of what is permitted by the Vendor, please use the non-commercial areas of diyAudio to do so.

Tubelab SE: Removing MOSFETs?

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
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.
 

Attachments

  • Zeners  NTE5162A  and  IN5384B.JPG
    Zeners NTE5162A and IN5384B.JPG
    128.4 KB · Views: 329
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
 
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
 

Attachments

  • IMG_20170611_165848.jpg
    IMG_20170611_165848.jpg
    426.6 KB · Views: 328
  • IMG_20170611_222305.jpg
    IMG_20170611_222305.jpg
    359.7 KB · Views: 314
  • IMG_20170611_222208.jpg
    IMG_20170611_222208.jpg
    343.6 KB · Views: 326
  • IMG_20170611_222417.jpg
    IMG_20170611_222417.jpg
    377.1 KB · Views: 313
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
 
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
 

Attachments

  • IMG_20170618_165926.jpg
    IMG_20170618_165926.jpg
    399.4 KB · Views: 155
  • IMG_20170618_165742.jpg
    IMG_20170618_165742.jpg
    446.7 KB · Views: 184
  • IMG_20170618_165532.jpg
    IMG_20170618_165532.jpg
    504.1 KB · Views: 176
Last edited:
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
 

Attachments

  • IMG_20170911_095034.jpg
    IMG_20170911_095034.jpg
    468.2 KB · Views: 148
Last edited:
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
 
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
 

Attachments

  • IMG_20170913_110844.jpg
    IMG_20170913_110844.jpg
    447.9 KB · Views: 141
  • IMG_20170913_110915.jpg
    IMG_20170913_110915.jpg
    463.7 KB · Views: 120
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.
Win W5JAG
 

Attachments

  • IMG_20170914_214506.jpg
    IMG_20170914_214506.jpg
    502.6 KB · Views: 354
  • IMG_20170914_220623.jpg
    IMG_20170914_220623.jpg
    392.7 KB · Views: 345
  • IMG_20170914_221328.jpg
    IMG_20170914_221328.jpg
    270.1 KB · Views: 351
Last edited:
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.

Win W5JAG
 
Last edited:
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.

Win W5JAG
 

Attachments

  • cr sse what if.jpg
    cr sse what if.jpg
    107.9 KB · Views: 166
  • IMG_20170918_091003.jpg
    IMG_20170918_091003.jpg
    382.2 KB · Views: 179
  • IMG_20170918_091045.jpg
    IMG_20170918_091045.jpg
    333.6 KB · Views: 165
  • fm mule sheet 4 alternate IF unused.jpg
    fm mule sheet 4 alternate IF unused.jpg
    275.3 KB · Views: 156
  • fm mule sheet 3 st decoder.jpg
    fm mule sheet 3 st decoder.jpg
    259.2 KB · Views: 144
  • fm mule sheet 2 IF.jpg
    fm mule sheet 2 IF.jpg
    351.3 KB · Views: 325
  • fm mule sheet 1 tuner.jpg
    fm mule sheet 1 tuner.jpg
    402.7 KB · Views: 329
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.

.... 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 ......

Win W5JAG
 

Attachments

  • IMG_20171023_165123.jpg
    IMG_20171023_165123.jpg
    940.2 KB · Views: 161
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.

Win W5JAG
 
I got back into town last night, and stuck my head in the ham shack this morning to see what the temp was ~ 30F.

I saw the 2A3 filaments in the DHT SSE lit up. Oops, I left it on. For how long, can't say for sure. Possibly a month, maybe a bit less.

The tubes are CBS branded, RCA 2A3. I guess I'll check them when I get a chance. I hate burning 500+ hours off their life for zilch, but I guess this establishes the DHT SSE is pretty stable and not at all environmentally sensitive ....

Win W5JAG
 
So, a year later, the DHT modded SSE continues to work flawlessly, still on the same CBS (RCA) 2A3 power tubes, no issues of any kind, no component failures, no changes to the amp, haven't moved it from the shelf I sat it on out here in the radio room ...

I was working on another project tonight, and just put it away; I decided I wanted to listen to music through DHT's instead ...

Win W5JAG
 

Attachments

  • IMG_20190106_223903.jpg
    IMG_20190106_223903.jpg
    765.9 KB · Views: 125
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.