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Question about RLa-a for *LW6 in triode connection?
Question about RLa-a for *LW6 in triode connection?
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Old 13th June 2018, 09:49 PM   #1
kodabmx is offline kodabmx  Canada
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Default Question about RLa-a for *LW6 in triode connection?

I'm thinking of building a 36LW6 PP triode amp but I'm having a hard time figuring out a good RLa-a. I'm thinking about 300V B+ and fixed bias at about 50ma. I use 12AV5GA at this level into 3k RLa-a and get a nice 20W, 30W with still less than 10% THD. Since the 36LW6 has more power though I was thinking of boosting the B+ but worried about the max screen rating. I assume the screen stopper will save the screen from excessive current though. Maybe a B+ of 380V?
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Old 13th June 2018, 10:49 PM   #2
smoking-amp is offline smoking-amp  United States
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George (Tubelab) has mentioned that the long term stability (reliability) of the _LW6 tubes is not so good in triode mode, subject to runaway. So keeping the screen V down for triode would seem a good idea. Power wise the _LW6 tubes should be able to handle down to 2200 Ohms P-P if you want the power.

However, figuring the Triode Rp of the tubes as Mu/gm = 10^6 x 3.7/12000 = 308 Ohms, you would want at least 3X (up to 5X) that loading for each tube for reasonable triode distortion, then 4X that for P-P would put you at min. of 3700 Ohm P-P (class A op). Some class AB range in there could lower that. You will probably want to sacrifice some power for reliability, so maybe not boosting the B+.

A variable power supply with current limiting for testing could be a big help.
These PS500XT things are cheap:
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File Type: jpg Hoefer_PS500XT.jpg (175.2 KB, 253 views)

Last edited by smoking-amp; 13th June 2018 at 10:51 PM.
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Old 13th June 2018, 10:54 PM   #3
kodabmx is offline kodabmx  Canada
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Cheers. Yes, I've heard of these before. My "Adjustable" power supply is in the form of taps. 110 160 200 220 280 380 400 440 560 760
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Old 13th June 2018, 11:27 PM   #4
smoking-amp is offline smoking-amp  United States
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OOPs, that 3700 Ohm P-P I calc'd above would be for class aB Op already. More class A could lower that Zp-p.

Current limiting power is useful in any case. Even the timer on those Hoefer things can be useful for when the listener falls asleep on the couch auditioning the sound.
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Old 14th June 2018, 12:09 AM   #5
kodabmx is offline kodabmx  Canada
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What about class AB1 but class A limit of about 1W? The 6AV5GA when triode connected is basically a 2A3, Could the 6LW6 in triode sub for a 300B I wonder...

Last edited by kodabmx; 14th June 2018 at 12:11 AM.
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Old 14th June 2018, 02:33 AM   #6
smoking-amp is offline smoking-amp  United States
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Quote:
What about class AB1 but class A limit of about 1W?
I think that would basically fit the usual class aB description.

Quote:
Could the 6LW6 in triode sub for a 300B I wonder...
a) 36LW6 in triode, 50 V/div horiz., 50 mA/div vert., Mu approx. 3.7 at 125 mA

b) 300B Mu 3.9 at 60 mA

c) $4 tube, for reference, best used as an emulant tube in inverse triode

d) $1 tube, 10 Watt, frame grid, Mu 30 (curve tracer selected)

e) same tube in Crazy/Twin drive (perfect pentode now!)

The _LW6 looks better, but probably just a poor curve tracer got used for the 300B datasheet.
Attached Images
File Type: jpg rsz_36lw6_t.jpg (54.4 KB, 226 views)
File Type: jpg 300B.JPG (739.7 KB, 213 views)
File Type: jpg Perfect_triode.JPG (163.5 KB, 232 views)
File Type: jpg TFH_in_triode.jpg (97.9 KB, 214 views)
File Type: jpg Crazy_TFH.JPG (172.3 KB, 9 views)

Last edited by smoking-amp; 14th June 2018 at 03:02 AM.
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Old 14th June 2018, 06:25 AM   #7
jazbo8 is offline jazbo8
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Question about RLa-a for *LW6 in triode connection?
Quote:
Originally Posted by smoking-amp View Post
a) 36LW6 in triode, 50 V/div horiz., 50 mA/div vert., Mu approx. 3.7 at 125 mA.
What's the grid step used? BTW, does anyone have all the pages of the 6LW6 datasheet?
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Old 14th June 2018, 03:43 PM   #8
smoking-amp is offline smoking-amp  United States
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I think those would be -9V grid1 steps for the 36LW6 curves. That fits the Mu 3.7 spec, and 90V was the max step range available back when 10 steps (11 curves) were available on my Tek 576 tracer (my 1st modification).

Later modification (#2) to the Tek 576 allows 14 steps (15 curves) and 168V step range (so 14 steps of 12V)

------------------

The 300B curves look like they might have been combined curves from 2 step ranges. The last two curves (HV curves on RT) look inconsistent with the left 6 curves.
I used to use the same kluge when I needed more than a 90V step range, just wire in a fixed DC V supply in series with the grid1 lead wire and trace again. Been there, done that. The inconsistency on the 300B graph is probably due to capacitance to ground of the floating supply. (possibly some R to ground too)

-------------------

The 6LW6 datasheet is available online as the M2057 (different bottle and base for RF) The actual (and a nice clean copy) of the 6LW6 datasheet is available, along with all the other hard to find tube datasheets (like 6LR6), on DVD here:

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Last edited by smoking-amp; 14th June 2018 at 03:50 PM.
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Old 14th June 2018, 04:02 PM   #9
jazbo8 is offline jazbo8
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Question about RLa-a for *LW6 in triode connection?
Thanks! Lots of good information.

Quote:
Originally Posted by kodabmx View Post
I'm thinking of building a 36LW6 PP triode amp but I'm having a hard time figuring out a good RLa-a. I'm thinking about 300V B+ and fixed bias at about 50ma. I use 12AV5GA at this level into 3k RLa-a and get a nice 20W, 30W with still less than 10% THD. Since the 36LW6 has more power though I was thinking of boosting the B+ but worried about the max screen rating. I assume the screen stopper will save the screen from excessive current though. Maybe a B+ of 380V?
What's the key spec you are shooting for? e.g., max power or best fidelity?
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Old 14th June 2018, 07:56 PM   #10
kodabmx is offline kodabmx  Canada
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Probably a combo of the two. I'm thinking class A triode connection, but without the tube specs it's hard for me to calculate... I'm also not opposed to pentode connection with stabilized screen.

I also would prefer to use a B+ of about 300V but higher current which is why I'm thinking of the *LW6.
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