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disco 4th September 2019 01:19 PM

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Optimizing for lowest distortion I'm looking at the curves for this #10. Right now the powersupply has 400V and can be raised to circa 470V. The anode is at 373V, Ia is 20mA, anode load is 10K. Would it be wise (linearity, aging) to reach for the right most limit at Vg=-60V?

Current situation: ΔV= 531-215= 316 volt, ΔI= 28-12= 16mA (ca. 5VA)
optimal (?): ΔV= 579-215= 364 volt, ΔI= 28-9 = 19mA (ca. 6,9VA)

Hearinspace 4th September 2019 06:00 PM

The 50 grid's bias is -65V. How much more than that do you want to drive it?

I used 10s as outputs for a few years, driving a horn, so not much power needed. After some experimenting I settled on plate voltages between 250 and 300 and current up to 32mA. Output transformer primaries tried were 16k, 10K, 5K. All sounded very nice. In other words, you have options. Best to try different operating points and see what works for you.

My 46 (that's 2 adjusted for inflation since 1914 here. It's 66 if you want to go back to 1760!)

6L6 4th September 2019 06:55 PM


ST envelope 50's are probably the most beautiful tube ever built.


disco 4th September 2019 07:18 PM

Gain is approximately 65, I have not measured exactly. In the end I'd like to drive a 300B XLS (biassed at 95V) with this front end. The interstage holds at 450Vdc and full signal swing, it's a sturdy build :D


Originally Posted by 6L6 (

ST envelope 50's are probably the most beautiful tube ever built.


urrr... the ST is a #10, the balloon is a #50. Both nice tubes :)

What's the better habit to allow for tube aging? Staying away of the bunching, limiting the Va excursion, or both?

Hearinspace 4th September 2019 08:17 PM

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To my knowledge, (value documented above ! : )

As long as you aren't damaging the tube by going outside the various declared limits, the issue with aging is emission. The more you use, the faster the tube ages.

So operating point at moderate voltage and current will give longest life.

Swinging into bunching = distortion.

Also, read the attached paper on thoriated filament life extension.

disco 4th September 2019 09:16 PM

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It was more the effect of tube aging on the circuit at hand I was after but this is an excellent point to consider as well. Indeed, running them less hot prolonges tube life. So does a constant filament voltage. Filament voltage for those bright emitting thoriated tungsten tubes can be reduced it seems. For the dull oxide type it's not recommended... or is this only with the big emitter tubes?

Hearinspace 5th September 2019 01:56 AM

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Another Doc for you. Pretty straightforward reading. You probably already know what's below but . . . . .

Recommended drop for thoriated tungsten filaments is 5%. On the 10 , 6.3V is 16% and looking at the stars in Fig. 8 of the CPI doc I'd say that's too low. Notice also that the management program isn't to be started until after 200 hours of use at nominal voltage.

Doing the math given on the CPI doc will tell if the thoriated tungsten filament in question is in the right range for the program.

As far as focusing on tube aging, my personal notion is that if the plate voltage is held constant, on the plate curves the operating point will fall vertically downward in a straight line over the plate voltage shown on the graph and so will the load line. In other words, I believe the load line at the reduced plate current will necessarily be in parallel with the load line at the original operating point. If a CCS is used then operating point and the load line will shift to the right as the tube ages.

And if neither voltage nor current are regulated then operating point will move as determined by resistive drops in the circuit.

'nuther doc. in German. Strangely, Google translate doesn't do a very good job on this one.

disco 5th September 2019 06:32 AM

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Extract from the RCA paper:

For turning on filament power [of pure Tungsten filament tubes], a filament starter should be used so as to increase the voltage gradually and to limit the high initial rush of current through the filament. ... Similarly, as an added precaution, the filament power should be turned off gradually to prevent cooling strains in the filament. ... It should be noted that a reduction of 5 per cent in the filament voltage [while maintaining regulation] will approximately double their life.

If this recommendation holds for all Tungsten filament tubes (and not only for big transmitting tubes), Rod has a new task to fulfill: slow turn off.

As recommended by Upton I 'baked' a VT25 for half an hour at 10,0 volt. This restored emission, which was ca 15% back on this tube.

The 200nF coupling capacitor was sufficient but in this test setup changing parts is very easy. The added capacitance does have effect on the weight of bass notes in modern popular music.
I had calculated F-3dB at 4Hz with 470uF cathode bypass on the 27 but 1000uF still made an improvement. Perhaps the reason lays in the age of the old cap, dating from the seventies. Wow, this amp sings. Bass is to die for while it's all silk and honey :D

Next I'll examine decoupling the #50 cathode to AC ground, see what it brings. If I'm right it's custom to set the dominant LF-pole at the front of the amplifier. Why is this?

PRR 5th September 2019 08:55 PM


Originally Posted by disco (
Extract from the RCA paper:

[I]For turning on filament power [of pure Tungsten filament tubes], a filament starter should be used..."..

Pure Tungsten filament has to be run MUCH hotter than our Oxide-coated filaments and cathodes. Different situation.

disco 6th September 2019 07:39 AM

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Transmitting tube hot cathode characteristics:

After exchanging my easy playing 4 ohm speaker for a nasty 8 ohm bass reflex (with lots of caps and coils going on) reproduction suffered from boom. The lowest register was somewhat out of control. A simple exchange of the 27 and 50 cathode bypass (to HT) made an improvement. Not belonging to the incrowd familiar with Chebyshev's theorem I took the empirical route. Output impedance must have dropped just enough to make a difference. Exchanging this lil tubegiant for the old Marantz PM7200 made no difference in bass reproduction.

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