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Tubelab Discussion and support of Tubelab products, prototypes and experiments

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Old 3rd June 2016, 05:18 PM   #11
w5jag is offline w5jag  United States
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You know, I had totally, I mean totally, forgotten about 307A.

I bought a half dozen Ken Rad's at Fair Radio the last time I went to Dayton.

Win W5JAG
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Old 5th June 2016, 03:06 AM   #12
tomchr is offline tomchr  Canada
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Quote:
Originally Posted by tubelab.com View Post
The mosfet or a good cathode follower (high Gm pentode)
Yep. High gm is needed for a good cathode follower.

Quote:
Originally Posted by tubelab.com View Post
Some CD players can not drive a 300B to clipping in some TSE's depending on OPT and speakers.
... a problem which gets further compounded by the folks using their phones/tablets/laptops to drive amps. Many of the modern sources only provide about 900 mV RMS of output swing. Need a preamp with some stones...

Tom
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Old 20th June 2016, 05:27 PM   #13
w5jag is offline w5jag  United States
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I have the power supply and a power amplifier final stage running.

This is a test mule, so no effort at beauty is being made - the nasty looking hardware and input / output terminals are from a dumpster dive Eico HF-32, I stripped down a few weeks ago. The PT is a used hamfest scrounge; the other parts were NOS hamfest scrounge, with the exception of the OPT, which is an Ebay deal. I spent a couple of days slowly reforming the 60 year old Sprague filter cap. The spray paint was supposed to be orange, but twenty years in a hot garage apparently made it more brownish.

The power supply is conventional: 6X5 rectifier, choke input. Not much to say here.

The PA is a 46, for now; a 47 will come later. AC on the filaments, cathode bias; the cathode resistor and bypass are on the filament transformer CT. I had a spot of trouble here - my target was a cathode voltage of DC 16 volts, and on power up, I got 0.12 volts. After about ten minutes, it dawned on me that I had left out the grid resistor. I installed a 220K resistor since I will be using the Tubelab front end in some form.

I repowered it up, and, of course, it still didn't work. This took a bit more thought. The 46 is a dual grid tube. I assumed the two grids were more or less identical, and I wired them to each other, simulating a triode. For whatever reason, this didn't work. I backed up, and wired the grid nearest the plate, what I call the second grid, to the plate, and repowered the device.

This time, it worked - cathode voltage settled at 16, exactly as I had planned. Anode voltage was 194 volts, pretty close to what PSU predicted for a choke input. That's the first time I've had that happen, but this is the first time I've used measured data on all the parts in my power supply.

The bias resistor is 470 ohms, bypassed with a 100 uF cap ( because I had a bunch of them, and figured that was a reasonable starting point ). That's about 30 mils, and about 6 watts.

Dizzy with success, I jumpered a signal generator to the first grid input cap, fed it a 1 kHz tone, and got a clean, hum free, 1 kHz tone from a test speaker. For fun, I fed the output of the AA-1 into it - it could drive it to about 90 milliwatts, and showed two tone IMD at a bit less than 6%, as shown in the second pic.

The remaining hole in the "chassis" can take either a 7 or 9 pin socket, so I have some flexibility here. Once I've tweaked the final and have it working the way I want, I"ll graft an SSE front end unto it, as a start.

The odd second grid of 46 needs to be explored. The OPT will need to be changed to a UL type at some point. The existing one has taps at 5K and 7K ( I'm using the 7K tap ) so of course I'll try to use the 5K tap as a feedback element before I spring for a new OPT.

Win W5JAG
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File Type: jpg WP_20160619_22_46_18_Pro.jpg (615.9 KB, 358 views)
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Old 27th June 2016, 06:10 PM   #14
w5jag is offline w5jag  United States
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The mule has a functional front end now - essentially the SSE front end sans ( for now ) the CCS, and the cathode bypass cap is 1000 uF, because I didn't have a 1500 uF handy. In lieu of the CCS, a 10K resistor is the anode load. My plan is to put the CCS on a small perfboard, so it can be installed and removed as one component.

For now, I used a 7 pin front end socket, so the front end is either 6AB4 ( 12AT7) or 6C4 ( 12AU7 ). 6C4 and 6AB4 have similar enough pin out that it is possible to wire the socket to accept either tube.

Loaded B+ comes in at about 260 volts with a CLC power supply.

I've done some preliminary testing with both tubes, and 46 and 47 wired as triodes. My notebook is back at home, but this is what I recall:

Both tubes do about 850 mw output as triodes. 46 is running at ten watts, so is at about its limit, while 47 is at about eight watts and can ( and will be ) further exploited.

Both tubes can be easily driven to clipping with the 6AB4, taking about 0.5 to 0.7 volts RMS. It takes about 1.4 to 1.6 volts RMS to drive to clipping with the 6C4. The 47 drives easier than the 46. That could be problematic in some applications. Adding the CCS may change this, I don't know.

Two tone IMD for all combinations is pretty similar, about 8 to 9% at 100 mw output, and 4-5% at 50 mw output. Adding the CCS may change this, I don't know

This OPT is not good as a hi fi OPT, it's high frequency response is fine beyond 20kHz, but it is down 5 dB at 100Hz. This is probably why the amplifier is unexpectedly and completely free of residual AC hum. A better OPT is in order. It works fine as-is as a medium fi utility amplifier where response below 200Hz is not required. It has already put in a fair amount of time in that application.

Using the 5k tap as a quasi UL feed appears to have merit with the 46. Attaching the 5K tap to the second grid gave an immediate increase in power output to 1.45 watts at clipping, and an across the board two tone IMD reduction of 2-3% at all power levels. I don't know how it affects input sensitivity, if at all. I haven't tried 47 that way, yet.

Member hareynolds also did some work with 46 in UL and that thread is in this subforum.

Pics are attached, still wired as triodes; there's not much to it really - very few components required to make a working amplifier. No leads were cut so the parts can be recycled for further use.

I'm already looking ahead to possibly a push pull amp using 45, 46, or 47 - I have enough tubes to do either. For some reason, almost all my 46 and 47 are globes, which is kinda cool. Not sure how that happened.

Win W5JAG
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Old 27th June 2016, 06:47 PM   #15
jdrouin is offline jdrouin  United States
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Great stuff, Win! Sorry I haven't had a chance to respond yet: we're in the middle of moving so I'm not as tuned in as usual. But this is fascinating. I know you said it's OK as a mid-fi utility amp, but can you describe the sound characteristics?

It looks really cool, btw. Love the orange.
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Old 27th June 2016, 08:53 PM   #16
w5jag is offline w5jag  United States
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Quote:
Originally Posted by jdrouin View Post
.... I know you said it's OK as a mid-fi utility amp, but can you describe the sound characteristics? ....
Jeff, I'm releuctant to, yet, for several reasons: first, the OPT is just to get it working - it's not intended to be hi fi, yet. Second, I actually haven't listened to it through anything but my test speaker, which may not even be mid fi.

I used it all weekend listening to field day and talk radio, and it worked well for that. I've also got some baseline numbers to build from.

I found that I have some more of this same transformer with the 5K and 7K secondaries, but about 50% larger, and they might have a better low frequency extension. I hope they do, because I'm starting to think that a quasi, or true, UL 46 could be a project with merit, and that would save me having to buy yet another set of SE OPT's.

I thought it was interesting that two tone IMD actually went down when the tube was moved from triode, to quasi UL. I didn't expect it to work at all with that OPT.

I'm also really intrigued with the idea of a very simple, very low cost, directly heated push pull amp, and I kind of think that this is one of the directions your thread will veer off into - a mosfet less simple quasi UL 46 SE, and a mosfet less simple PP quasi UL 46.

Get your 46's before the price skyrockets ....

Win W5JAG
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Old 2nd July 2016, 05:53 PM   #17
w5jag is offline w5jag  United States
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Someone more knowledgeable help me out here, please:

If my arithmetic is correct, with the 8 ohm secondary, the 7K tap has a turns ratio of 29.5, and the 5K tap has a turns ratio of 25.

So, by using the 5K tap as a UL feedback point, is this a 16% tap? Or something else?

Win W5JAG
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Old 3rd July 2016, 10:01 AM   #18
gcom is offline gcom  Ireland
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W5jag
Is an 8ohm secondary and a 5k primary not a turn ratio of 625?
George
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Old 3rd July 2016, 01:32 PM   #19
w5jag is offline w5jag  United States
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I thought you take the square root of that number?

I'm at our weekend house for the holiday, and I actually have a hardback of Radiotron Designers Handbook 4 here, so, near four year old willing, I may try to read up on feedback and transformers.

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Old 4th July 2016, 12:41 PM   #20
w5jag is offline w5jag  United States
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In the little time I had to look, I didn't see much in RDH. There is a whole chapter on feedback, which I didn't even have time to skim.

In the amplifier chapter, it describes partial triode operation at 570:

"... while triode operation is obtained when the screen is connected to the plate end of the primary. Any desired intermediate condition can be obtained by connecting the screen to a suitable tap on the primary. In such intermediate condition the valve operates as a pentode having negative feedback applied to the screen with a section of the load impedance common to both electrodes, and minimum high level distortion with push pull operation is usually obtained when the tapping point is about 20% of the total primary turns." (my emphasis added)

This just leaves me with more questions. The 5K tap as a feedback point would be at about the right point - if you start at the opposite end of the primary from the usual way it is done. Does this make any difference?

edit: have I made two dissimilar triodes in parallel by doing it this way?

It would seem not to in this instance, but the second grid in 46 isn't a screen; also, distortion went down from pure triode operation, instead of increasing, so maybe it is something different altogether. I don't have any regular UL OPT's free at the moment, for comparison. You can look straight down at the grids in a globe 46 - I don't recall the second grid showing signs of distress in this mode.

I guess I've lost sight of the mule's purpose - clearly, the SSE front end can drive these alternative flea power directly heated tubes to clipping, without the intervening mosfet follower.

Win W5JAG

Last edited by w5jag; 4th July 2016 at 12:56 PM.
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