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50 triode amp
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Old 13th September 2019, 06:37 AM   #31
disco is offline disco  Netherlands
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For auditive comparison it's an easy way to evaluate an unknown big electrolyte. Those low value oil filled caps are the least offending to my ears. 30uF/160V is enough for decoupling to ground.

There's a mistake in the last schematic, this is the correct one:
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Old 13th September 2019, 04:42 PM   #32
jhstewart9 is offline jhstewart9  Canada
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If it sounds OK, that's good. But a simulation shews that only 4 microF as the 50 bypass cap leaves the spectrum below 100 Hz dropping. So some of the LF audio is dissipated in the cathode resister. And plate resistance rises due to cathode degeneration so DF goes down. Translates to less control of the loudspeaker.
A 10 MicroF cathode bypass would probably fix all.
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File Type: jpg 50 Amp D1.jpg (153.4 KB, 156 views)
File Type: jpg 50 Amp D2.jpg (73.0 KB, 144 views)
File Type: jpg 50 Amp D3.jpg (75.0 KB, 146 views)
File Type: jpg 50 Amp D4.jpg (160.9 KB, 144 views)
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Old 13th September 2019, 09:25 PM   #33
disco is offline disco  Netherlands
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Well... I can't argue with the result of your simulation but I started with a big electrolyte to ground, soon replaced with 2,2uF to PS. That set the cut off too high. I tried both 4uF and 8uF without audible distiction.

*dong* It just hit me that the last was without label.... (and indeed, it turns out to be 4uF as well). Hahaha, good to have you around mister Stewart. I must admit being disappointed in a so highly appraise bottle but here is the reason for that LF decline I measured (and heared). Oh well, there's another day.

Meanwhile I've been exchanging tubes. The 50 was replaced by the 300Bxls. A complete different bird. The first thing striking was the lack of top end, what the 50 does so beautiful. But low end -wow- even that horrible 8ohm bassreflex comes alive. Good, controlled grip while I finally witnessed some cone movement. Perhaps a 26 can be of service at the front end. I'll check it out in the next week.

The old PS could not cope, voltage drooped to a meager 459V while 475 is the target. When I look at the Kron advised operating point, I wonder why someone would use it as it's destined for distortion.

Doing the math these numbers roll out. I have not accounted for transformer losses, what would that be for a 40W C-core, perhaps 5 to 8%?

I did the math and these numers roll out, not accounting for winding and core losses. What could those run in to for a 40W C-core, perhaps 5 to 8%?
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File Type: jpg specs.JPG (49.1 KB, 36 views)
File Type: png 300Bxls-475V.png (143.2 KB, 39 views)
File Type: jpg 300BXLS-2.JPG (157.8 KB, 27 views)
File Type: png 300Bxls-546V.png (142.9 KB, 30 views)
File Type: jpg calculations.JPG (47.0 KB, 4 views)
File Type: jpg calculations.JPG (46.2 KB, 47 views)
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Old 14th September 2019, 03:52 AM   #34
PRR is offline PRR  United States
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"560Vpp in 3K5 relates to 26,8Vpp in 8ohm == P=32W"

Some error there. 26.8V pp is 9.5Vrms which in 8ohms is 11.2 Watts, no?
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Old 14th September 2019, 07:47 AM   #35
disco is offline disco  Netherlands
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Along the route is a square function. Outcome differs when one would first convert to Vrms. This was my reasoning:

26,8Vpp/8R= 3,35A => 11,2x8= 89,8 => 89,8/2rt2= 32W


Different approach

560Vpp swing with 2x 57mA. 200Vrms X 0,114A= 22,8W
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Last edited by disco; 14th September 2019 at 08:08 AM.
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Old 14th September 2019, 10:03 PM   #36
PRR is offline PRR  United States
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So you are expecting 32 Watts Sine RMS from a triode working at 45 Watts idle. That is wonderfully excellent performance. A 6V6 Champ at 12W Pdiss barely manages 5.6W of nearly-Sine output.
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Old 15th September 2019, 05:35 AM   #37
disco is offline disco  Netherlands
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Efficiency would be 25% at most probably. So, 450V x 0,1A idle equals 45W. 26,8/2rt2= 9,47Vrms. That in 8 ohm= 11,2W. Ballpark

What do you make of those anode curves? How would they arrive at the acclaimed 24W output power? If they calculate swing and current (like I arrived at 22,8W in the previous message) that would be an explication...

OK, new setup with #26 on a current source heater supply by Tentlabs. Without shielding it nicely picked up 100Hz from the nearby PS so this tube is probably not a definitive choice. Despite the lousy wiring and improper placement output hum is down to 6,6mV full gain.

The leftmost scope image is taken at 1KHz, output level 16Vrms in a 6,4Ω resistor. If I'm right that is 16x16/6,4= 40W. The other scope image is 10KHz, showing some ringing. I'm testing a 50pF-100KΩ combination at the OPT right now. Final scope image is of 30Hz with the 8Ω loudspeaker connected. I can't explain for the slope but proably it's common for a SE amplifier.
Attached Images
File Type: jpg 300Bxls schema1.JPG (47.6 KB, 26 views)
File Type: jpg PS 495V.JPG (93.4 KB, 25 views)
File Type: jpg 1KHz - 14V.jpg (104.7 KB, 17 views)
File Type: jpg 10KHz - 16V.jpg (107.0 KB, 16 views)
File Type: jpg 30Hz - 5V.jpg (99.7 KB, 17 views)
File Type: jpg residual B.jpg (143.1 KB, 21 views)
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Old 17th September 2019, 12:23 PM   #38
jhstewart9 is offline jhstewart9  Canada
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Pout is usually measured driven by a sine wave. Your RMS responding VM will indicate high on a square wave.
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Old 17th September 2019, 01:30 PM   #39
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Thanks, again. How about this 10KHz square, 28KHz not too low? Seems to clear up the midrange and cymbals sound more natural.
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File Type: jpg 10KHz - 13V RC.jpg (107.0 KB, 101 views)
File Type: jpg 56pF100K.jpg (130.4 KB, 93 views)
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Old 17th September 2019, 04:55 PM   #40
jhstewart9 is offline jhstewart9  Canada
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Final scope image is of 30Hz with the 8Ω loudspeaker connected. I can't explain for the slope but proably it's common for a SE amplifier.

The poor response to the 30 Hz square wave might be due to the interstage transformer. Put the scope on the 300B grid, that will tell you. Nice Philips Scope, used to be lots of them here in the 60s & 70s, especially at the schools. They were competitive to the lower cost lines offered by TEK & HP.
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