Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: kagliostro on May 21, 2015, 09:40:34 am
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At the begin I was thinking
Oh, one other triode pentode tube that may be usable for audio (pentode has 9W plate dissipation)
but then I see the .gif file and I've think to a typo
so I give a look also to the datasheet and B+ and G2 voltages are the same of the .gif file ????
(http://www.mif.pg.gda.pl/homepages/frank/short/001/e/ECL85.gif)
http://www.mif.pg.gda.pl/homepages/frank/sheets/030/e/ECL85.pdf (http://www.mif.pg.gda.pl/homepages/frank/sheets/030/e/ECL85.pdf)
http://tdsl.duncanamps.com/link.php?target=2F258316 (http://tdsl.duncanamps.com/link.php?target=2F258316)
Don't you think that a 65v B+ vs 210v G2 is really unusual ??
K
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Hello Franco,
Yes, that must be a print error ?
I think it really should be :
Anode 285v @ 65mA
What do you think ?
Regards, Max.
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.....And now I look at other Data sheets for the 6GV8 and I see that maybe it is correct ???
Now I'm confused as well ! 285mA ........?
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I read
the pentode is optimised for pulse output operation into the magnetic load of the scanning coils
but a so high G2 voltage respect B+ voltage confuses me :w2:
Franco
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It's just one point on the Eg1=-1V curve for reference purpose, not a recommended operating condition, certainly not for audio applications...
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I have seen this 6GV8 used as audio amplifier output just like the 6GW8 / ECL86 and it works very well.
I have never noticed that data specification before, I learn something new everyday !
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Here it is in a schematic.
An Australian 'Kriesler_11-99 ' Radio.
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Many Thanks for the schematic Kde
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Jazbo8 I agree that the data were not for audio purpose
the odd is a so high G2 with respect to Plate
Franco
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It's just one point on the Eg1=-1V curve for reference purpose, not a recommended operating condition ...
+1.
Look at the Philips data sheet Kagliostro linked in the 1st post. Scroll down to the plate curves on the 6th page, then look at the -1v gridline. That line has a point at about 285mA and 60v plate-to-cathode.
I think the confusing chart is showing a single data point of max power output (-1v is commonly held as a safe grid voltage prior to the onset of grid current and serious distortion) which is at the knee of the curve and tell you peak plate current, plate saturation voltage, and allows you to calculate the plate load impedance needed for your proposed supply voltage.
It also hints at that supply voltage (more than the 210v on G2) and shows the maximum screen current at that moment of peak plate current.
The sheet says max voltage is 250v, so the total peak voltage could be as much as 250v-60v = 190v. The 285mA peak current is given by the chart and graph, when the grid is driven by a peak positive input signal to momentarily reach -1v. Taken together, this implies a power output in class AB of (190v*0.285A)/2 = ~54w/2 = ~27w, when there is a load of 4 * (190v/0.285A) = 4 * ~670Ω = ~2700Ω plate-to-plate.
The load seems low and the power output seems hopeful for such a small glass bulb. I'd guess the realistic output is less than 27w for a pair but more than 1/2 that, and that a higher load impedance would be needed for audio use. The very high peak current possible seems more realistic in a pulsed-operation setting, as suggested by the data sheet's note this is a frame oscillator or pulse-amplifier tube.
For example, a 5kΩ load for class AB yields (for the same 250v plate and 60v plate saturation) 190v/1.25kΩ = 0.152A and (190v * 0.152A)/2 = ~14.5w. May or may not be all this tube could deliver, but more refinement would take more than back-of-napkin figuring.
The dynamic characteristic on page 5 of the Philips sheet implies the tube's Gm. For the G2=210v curve, the points of 100mA and 200mA of plate current coincide with grid voltages of -15v and -8v for a Gm of (200mA-100mA)/(15v-8v) = 100mA/7v = ~14.3mA/v. The 152mA of peak current for our 5kΩ plate-to-plate load case then requires 152mA/14.3mA = 10.6v of peak input. Leaving at least 1v extra to stay away from grid current, this suggests a bias voltage of at least -12v.
Adventuresome tinkerers might try biasing cooler and pushing the load lower, but that might be best figured out experimentally.
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Many Thanks HotBluePlates
very interesting explanation
now I must study it trying to put deep on my brain all that info
Grazie
Franco
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> Look at the Philips data sheet .... Scroll down to the plate curves on the 6th page
First look at the top of the first page.
"Frame Output Tube"
In american we say "Horizontal Sweep Tube".
What is that? A TV tube. What does it do? It pulls a *huge* pulse of current, then rests, then does it again. The high current sweeps the CRT electron beam left to right to scan-out one line of picture. Then it turns off, to jerk the beam right to left for the next scan.
Note also the current on the GIF short-data. 285mA!!
In TV sweep/frame service, the tube is powered from about 210V, switched from 200+mA at 65V drop to "zero" mA at 1,500V kick-up from a coil. The GIF only shows one end of that switching.
Yes, it can be used as linear audio. You probably want to keep G2 voltage quite low; you have no need for 285mA. 100V ought to be ample for any linear purpose.
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Many Thanks PRR
This help to understand better how this tube works
Grazie
Franco