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50 triode amp

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What Benifit does Returning the Cathode Cap to B+

Looks like not much, the AC current thru the cap remains the same whether returned to Common or B+. But return to the B+ brings with it the introduction of PS Ripple into the signal path.
Follow the Switches.:eek:
 

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From your simulations the conclusion can be drawn PS ripple plays a greater role when the cathode gets decoupled to PS instead of ground. With actual numbers (ripple < 20mV) the difference would be small. Though, on micro levels some influence from power supply AC exists.
To isolated the input stage from PS, I replaced the anode load with a CCS set for 5mA. Next I'll have a look at the 50 workpoint and alter the PS setup as the 10 and 50 share the same PS, isolated by just the impedance of their loads. Some more cyphers to follow.

Rough Estimate of Damping Factor, taken from 8Ω tab.
1,00V 1KHz in 8R resistor, output without load= 1488mVrms.
Internal resistance ΔE= 0,488V ΔI= 0,125A 0,488/0,125= 3,90Ω
According to the rough estimate DF= 8/3,9= 2
 
5K in place, bass seems better controlled but less deep. Dissipation is 22W, tomorrow I'll take some measurements.

I wonder what the limit is for the interstage. It's a C-core with enamelled wire, poured in with silicon compound. The power supply is at its top now. I could exchange the tranny for a 1000V CT unit and employ a thermal rectifier...
 
Under ordinary operating conditions all triodes have about the same damping factor. Simply depends on the relation between the plate resistance & the OPT impedance reflected from the load. Then the wire resistance in the OPT, lower is better. This article explains it well.:)
 

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PRR

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Joined 2003
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Do you have 7.5k load on 7.5r winding?
The ripple voltage set is arbitrary, probably much better in practice.:)

That is not what I was asking.

I realize the ripple source is arbitrary.

But a 1000:1 mis-fit of load to tube (through transformer) radically changes the ripple around the loops, masking the effect you are trying to show.
 
Comparison of the two OPTs learned there's not much difference to be had with this particular loudspeaker. Below is a chart for measurements up to 10KHz, the horizontal scale is not linear.

Further refinement could be to investigate HF behaviour as Tango XE-20S is sensible for ringing at high frequencies. A RC combination at the 50 anode might work.

From the ancient Sylvania manual the thought arose 3K5 was a suitable load for 400V plate voltage. 5K seems slightly better wrt damping and distortion at the expense of half a watt output power. Remember, Platevoltage == Vak.

The power supplies is built from parts at hand, you would certainly do better than this designing your own.

It's a lovely little amp that deserves a good loudspeaker as I found out. Perhaps a Lowther or Philips 12 inch broadband unit. Bright emitter 10 tubes have a shorter life span (1500 hours) than the original oxide coated type (5000 hours).
 

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That is not what I was asking.

I realize the ripple source is arbitrary.

But a 1000:1 mis-fit of load to tube (through transformer) radically changes the ripple around the loops, masking the effect you are trying to show.
OK, got it. I missed that completely. I will attribute that to rushing too much..........or just too old. Lots going on here as usual, manual labor, cutting the dead ash down, Etc.
So here is the series of measurements again, this time the load corrected. The bottom line still holds, returning the cathode cap to B+ is not the best plan.:)
 

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PRR

Member
Joined 2003
Paid Member
> returning the cathode cap to B+ is not the best plan.

Yes, for ripple rejection. Cap to B+ injects ripple to cathode where it amplifies about as Mu, cap to ground doesn't.

Where you find this connection most is old W.E. designs with massive B+ filtering, so that ripple is small in any case.
 
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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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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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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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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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.
 

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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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