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Old 18th March 2017, 06:03 AM   #91
w5jag is offline w5jag  United States
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Played around tonight with 47 and both transformers, the Edcor and dual primary.

Lost a 10M45 on the 5 pin side, but not in the usual way - instead of failing to a dead short, it began oscillating. The LED bias revealed this straight away - the LED was too bright on that side. The current set resistor measured correct. I checked the 1K suppressor in circuit, and it read low, about 700 ohms. I lifted a leg, and it measured correct, so I replaced the 10M45, but not before taking a few measurements on the oscillating side - plate voltage was about double what it should be, and the 10M45 and tube were way high in temperature. Cathode voltage was about 0.4 volts higher than nominal.

I've spent the last few days skimming material on partial triode, distributed load, ultra linear - whatever one wants to call it, notably the 1955 articles by F. Langford-Smith and Chesterman, and that zombie thread on distributed load in the main forum, that has just jumped back to life.

Langford-Smith refers to per cent impedance in his work, while it seems like the current transformer makers refer to per cent of turns. I think the former is more useful. Referencing per cent impedance, the 5K tap on the dual impedance transformer provides a 77% distributed load when the 5K tap is wired to a grid, and 29% when wired backwards. The Edcor spec on the XSE 15 simply says 40%, which if per cent of turns, is 16% distributed load if my arithmetic is correct. L-S's paper is good reading, but the bottom line is we're pretty well stuck with what the manufacturers offer, so it's mostly academic. The Merit A-3130 PP Opt's with the 10% tertiary winding look to be a bit of an odd bird. I think they are likely PA OPT's but L-S suggests 5% is optimum for that application, while they are less than what is suggested for optimum fidelity. They might work very well for PP 6V6, though. I got them for 6146 and sweep tubes, and they may work well enough for that. Time will eventually tell.

I got a reasonably complete set of data for a 5W4 / 12BH7 / 47 combination with both OPT's prior to the 10M45 failure. I settled on 200Hz for the low tone, and 6 kHz for the high tone. I'll probably try to graph the data, but the bottom line is pretty clear: with both OPT's, 47 just does not gain much of anything in a distributed load configuration. With the dual impedance transformer, 71% distributed load provides a marginal improvement above 600 mw over 7K full triode, and marginally worse below 600 mw. This transformer is pretty well done by 500 mw.

The Edcor performs very well with the adjusted low frequency tone up to about 750 mw. However, 16% UL is measurably worse than full triode at all power levels up to 1000 mw, although the delta narrows considerably between 750 mw and 1000 mw. Pentode with Schade feedback is comparable to 16% UL up to about 500 mw, where the two differentiate, with UL performing significantly better. Triode is absolutely the way to go here.

I haven't tried 46 yet. It will do very well as a triode, likely better than 47. It will work with the 71% distributed load, but may be better or worse than full triode in that configuration. Since connecting grid 1 and 2 together makes a zero bias triode, it is not likely to work well with the 16% or 29% distributed load on grid no. 2.

Win W5JAG
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Old 18th March 2017, 09:55 PM   #92
w5jag is offline w5jag  United States
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A few caveats:

the front end is modified to pull less current; this adversely affects 12AT7, I think.

I tried higher voltage and less current on the 46, but didn't see any improvement. There might be merit with lower voltages. I would have to set it up on a bench supply for that.

I did not test for max power. It will obviously make more than one watt with the Edcor. The Poshan is not very efficient, so it might not with that OPT. Eventually I will get to this.

46 obviously makes a terrific triode; why it is not more popular is a mystery.

Win W5JAG
Attached Images
File Type: png Rev 46 Edcor 5K 12AT7 triode.png (20.0 KB, 85 views)
File Type: png Rev 46 Edcor 5K 12BH7 triode.png (19.6 KB, 86 views)
File Type: png Rev 46 Poshan 7K 12AT7 triode.png (18.9 KB, 84 views)
File Type: png Rev 46 Poshan 7K 12BH7 triode.png (20.4 KB, 82 views)
File Type: png Rev 46 Poshan 71% UL.png (20.8 KB, 83 views)
Attached Files
File Type: zip Type 46 03_18_2017.zip (2.7 KB, 3 views)
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Old 18th March 2017, 10:00 PM   #93
w5jag is offline w5jag  United States
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47 was developed as a pentode, but I didn't get very good results that way. More work is needed it seems. The schade graphs are running in pentode - it helped some, not as much I expected.

It makes a good triode, but not as good as 46.

It has very high gain, so I did not use a 12AT7 with it.

Win W5JAG
Attached Images
File Type: png Rev 47 Edcor 5K triode.png (19.4 KB, 19 views)
File Type: png Rev 47K Edcor 5K 16% UL.png (20.4 KB, 18 views)
File Type: png Rev 47 Edcor 5K Schade.png (18.9 KB, 19 views)
File Type: png Rev 47 Poshan 29% UL.png (19.3 KB, 21 views)
File Type: png Rev 47 Poshan 71% UL.png (19.2 KB, 22 views)
File Type: png Rev 47 Poshan 7K triode.png (18.6 KB, 18 views)
File Type: png Rev 47 Poshan 7K schade.png (19.7 KB, 19 views)
Attached Files
File Type: zip Type 47 03_18_2017.zip (2.8 KB, 2 views)

Last edited by w5jag; 18th March 2017 at 10:04 PM.
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Old 19th March 2017, 06:20 PM   #94
w5jag is offline w5jag  United States
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I didn't expect 16% UL to work with 46, so I didn't try it earlier.

I gave it a quick try this morning, and this is what I got.

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File Type: png 46 Edcor 16% UL 12BH7.png (21.0 KB, 21 views)
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Old 20th March 2017, 04:02 AM   #95
w5jag is offline w5jag  United States
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2A3 using the Edcor XSE15-5K.

test conditions:

CBS 2A3 grey plate, FAA RCA 12AT7 blackplate, RCA 5Y3GT

B+ 333
Vk 57
Rk 1200
Ik 48ma
Pd 13

SMPTE IMD 4:1 200Hz:6kHz

SE amp cad predicts 3% 2nd harmonic distortion; 0.1% 3rd at 1700 mw

Win W5JAG
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File Type: png edcor xse 5K 2A3.png (22.2 KB, 24 views)
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Old 20th March 2017, 04:36 AM   #96
w5jag is offline w5jag  United States
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45 using the Edcor XSE15-5K.

test conditions:

unknown 45, branded Delco, FAA RCA 12AT7 blackplate, National Union 5W4GT

B+ 339
Vk 58
Rk 1500
Ik 39 ma
Pd 11

SMPTE IMD 4:1 200Hz:6kHz

SE amp cad predicts 3.1% 2nd harmonic distortion; 0.1% 3rd at 968 mw

Win W5JAG
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File Type: png Edcor XSE 5K 45.png (22.6 KB, 21 views)
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Old 20th March 2017, 03:56 PM   #97
w5jag is offline w5jag  United States
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I don't see a practical way to eliminate the unresolved variable of the front end tube.

I've monkeyed around with the front end to try to make it work more consistently with the lower voltages for these tubes. That has included reducing the CCS current to about 6 ma. The amplification factor of the tubes is also different. 45 and 2A3 are similar, and less than 46, and all three are orders of magnitude less than 47 when it is run as a pentode. 12BH7 is less than 12AT7. So, the way things played out, on *this* DHT modified SSE board in its present state, it's clear that 46 and 47 work best with the lower gain 12BH7, and 45 and 2A3 need the higher gain 12AT7. Further tinkering could produce better results.

It's clear, even at this point of unrefinement, that the modified SSE board makes a very good simple DHT amplifier. Obviously better transformers produce better results as they always do, but a good DHT amplifier could be made for very little money with the modified SSE board and inexpensive transformers. It also seems clear that 46 and 47 make very good triodes on the modified SSE board, maybe better than 45 - but I don't have any way to do spectrum analysis. 45 could be better in that regard. It also seems clear that, as concerns the modified SSE board, that UL at the normal % impedances is unacceptable for 46, and no advantage for 47. Better UL transformers would move the overall distortion in each mode down, but I don't see better transformers changing the fundamental conclusion that 47 does not gain anything by using UL in single ended mode. Pentode is probably worth further exploring, but it will be less simple, maybe a lot less simple. 46 and 47 are spec'ed for a 7K load, and they tested a lot better at 7K than at 5K with the Poshan OPT, so with something like the Edcor at 7 or 8K, they may test better still. I didn't graph them with 5K on the Poshan OPT, but that data is in the attached file.

If I did not already have all the tubes to do this, and if I was the type of crazy person that was willing to roll the dice on cutting out the power tube sockets and risking total and irremediable destruction of the pcb, out of this lot I would probably choose 2A3 or 47 for purely practical reasons. 47 is cheap and abundant on eBay. Audiophiles have not discovered it, or turn their nose up at it. 2A3 is still in production, as are 12AT7 and 12BH7. 2A3 gives about 3 dB more headroom, so, if you need that, 2A3 is the way to go.

Note that I am not, and would never, advocate drilling out socket holes on a pcb and installing new sockets in their place, to make the board do something it was never intended by its designer to do.

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Old 20th March 2017, 08:17 PM   #98
spendorite is offline spendorite  Canada
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Quote:
Originally Posted by w5jag View Post

If I did not already have all the tubes to do this, and if I was the type of crazy person that was willing to roll the dice on cutting out the power tube sockets and risking total and irremediable destruction of the pcb, out of this lot I would probably choose 2A3 or 47 for purely practical reasons. 47 is cheap and abundant on eBay. Audiophiles have not discovered it, or turn their nose up at it. 2A3 is still in production, as are 12AT7 and 12BH7. 2A3 gives about 3 dB more headroom, so, if you need that, 2A3 is the way to go.

Note that I am not, and would never, advocate drilling out socket holes on a pcb and installing new sockets in their place, to make the board do something it was never intended by its designer to do.

Win W5JAG
Agreed. I would never butcher one of my boards to do these kinds of experiments even if I had the tubes and other components to do this which I don't.
Anyhow, W5JAG your posts in this thread make for interesting reading about the the kind of things that can be done with the SSE board.
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Old 21st March 2017, 03:58 AM   #99
w5jag is offline w5jag  United States
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Default 307A / VT-225

Okay, I have 307A/VT-225 running on the DHT modified SSE board right now.

I didn't want an extra transformer on the bench, so I used the existing non ct 6.3 volt transformer that is running the front end, and built a virtual center tap to attach the 307A cathode resistor/bypass capacitor. I also have G3 attached to the virtual center tap.

It biased up right where I calculated and seems to be working as expected. is it permissible to use one filament transformer for evaluation purposes, or do I need to give the 307A it's own transformer?

The only curious result thus far is that 307A, like 47, does not seem to respond well to distributed load. I had it hooked up to the Poshan with the 5k lead to G2 for 71% impedance on G2, and this was marginally worse than full triode.

I'm running it at the max data sheet values - 300 volts; -30 on G1; 43 ma, but with a 7K load as a triode. Sounds clean and crisp. I briefly ran it up to about 25 watts, and it seemed fine there.

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Old 22nd March 2017, 03:34 PM   #100
w5jag is offline w5jag  United States
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I swapped G3 from the cathode to the plate, and it lit up like Times Square in between the suppressor and the plate. Is this normal behavior for 307A, or is this a gassy tube?

I bumped up the voltage to see if it would burn off, and after an hour or so, the blue area diminished, but was still quite visible, and was still confined between the suppressor and the plate. I only have one 307A here at the house, the rest are at the warehouse of junk, so I don't have any others for comparison.

It seems to be working normally as far as I can tell. I do think it sounds better with G3 tied to the plate.

First pic is G3 tied to the cathode. Second pic is G3 tied to the plate.

Win W5JAG
Attached Images
File Type: jpg IMG_20170321_223002.jpg (470.2 KB, 22 views)
File Type: jpg IMG_20170321_222618.jpg (701.0 KB, 18 views)
File Type: jpg IMG_20170321_221934.jpg (613.3 KB, 28 views)
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