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Old 27th December 2010, 06:45 PM   #11
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Here are some curve sets for 29GK6 which is essentially the same as 6BQ5.
Screen V at 300 V, 250, 200, 150, 100, 50, and 50 again. Vertical plate current scaling and g1 step V's reducing progressively with lower screen V's.

The last 50 V plot is shown at the same vertical current scale as the 100 V plot. Just to show that the plate resistance Rp may actually be increasing slightly (flatter lines), or the same, with lower V screens, despite the more ramped appearance from the lower vertical scaling on the other graphs.
Attached Images
File Type: jpg 29GK6_300.jpg (26.7 KB, 226 views)
File Type: jpg 29GK6_250.jpg (25.7 KB, 224 views)
File Type: jpg 29GK6_200.jpg (25.9 KB, 223 views)
File Type: jpg 29GK6_150.jpg (27.1 KB, 221 views)
File Type: jpg 29GK6_100.jpg (27.7 KB, 220 views)
File Type: jpg 29GK6_50.jpg (28.7 KB, 4 views)
File Type: jpg 29GK6_50_b.jpg (26.6 KB, 9 views)
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Last edited by smoking-amp; 27th December 2010 at 06:50 PM.
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Old 27th December 2010, 08:20 PM   #12
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Thanks Guys.
Perhaps there was some sort of "rule" like this in the very early days as suggested by Yves, I can understand that, and of course it would have absolutely no bearing on modern tubes.

Perhaps it also has something to do with the tube tv's and the use of sweep tubes in them. In any case I will stick to the "books".

Smoking Amp, those pics were very thoughtful of you to add and interesting!

I am going to take a look at the 29GK6 at least.

Andrew
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Old 28th December 2010, 02:07 AM   #13
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I don't know where the half voltage screen statement come from either. Yes some sweep tubes may run with the screen at half the supply, but that may be just coincidence. I have been experimenting with lots of sweep tubes in Petes big red board. The tubes intended for vertical sweep (essentially audio tubes) tend to want the screen near the plate voltage. The tubes intended for horizontal sweep (switched CCS's in normal operation) tend to work best with the screen at or near 150 volts regardless of the plate voltage. Tests were run on little 12 watt tubes all the way up to 40 watt big boys, from 300 volts to 650 volts on the plate. All of them worked the best at 150 volts on G2.

For those who haven't delved deep into tube type TV, there are two sweep circuits.

The vertical sweep circuit moves the CRT's electron beam up and down on the CRT face using electromagnets mounted on the CRT neck (the deflection yoke). The vertical oscillator generates a ramp that must be amplified in a linear manner by the vertical output tube. The vertical output tube functions exactly the same as an audio amp except that it works at only 60 Hz (50 Hz). Early TV's used audio tubes like the 6V6, 6K6, 6W6 and even the 6SN7. Later speciallized tubes incorporating the oscillator and output tube in one envelope were designed. Some use a triode output section, others use a pentode. All of them make good audio amps.

The horizontal sweep circuit moves the beam from side to side, by driving the yoke, and it generates the high voltage (10 to 30 KV) needed for operating the CRT by pulsing the flyback transformer. A clever engineer somewhere made use of the CCS characteristics of a pentode, and the fact that an inductor will charge at a linear rate when fed a constant current, to do both functions in a single circuit. The horizontal oscillator generates a square wave that switches the output tube between cutoff and G1 = 0V. The voltage on G2 sets the current through the output tube. This is why you will find screen curves with G1 = 0V in the data sheets. The tube is switched on and the flyback transformer begins to charge at a linear rate. A winding on the flyback feeds the deflection signal to the yoke. As the current reaches max the beam is moved to the right side of the TV screen. The output tube switches off (like the breaker points in an old car) and the current through the flyback ceases creating a huge voltage spike. The flyback has a HV winding like the ignition coil in your car that feeds a rectifier tube generating the HV. The damper tube squelches (damps) the ringing inherent in a flyback transformer capturing the ringing energy reusing it to create a boosted B+ voltage for other circuits. The current for the horizontal sweep circuit passes through the damper tube, voltages can exceed 500 volts on the cathode, and the plate sees 1KV+ pulses. Yes, these tubes make excellent rectifiers in our amps too.

Horizontal sweep tubes are not designed or intended for linear operation. Some but not all will work in audio applications. The low G2 rating precludes UL operation although a few will work in triode.

Back to the EL84. There are so many tubes out there being called EL84 that it is impossible to characterize them all based on the performance of a few, but I have been experimenting on a Simple P-P board with two power supplies. I find that for every plate voltage there is a screen voltage that produces the lowest THD, and a different voltage that minimizes the higher order harmonics. These voltages vary quite a bit with different versions of EL84. I decided that the JJ EL84 is the best of the reasonably priced current production tubes. At plate voltages around 300 volts the screen wants to be near 300 to get any power output. At plate voltages in the 400 volt range the screen wants to be around 325 volts. 25 watts are possible at THD numbers in the 0.5% range!
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Old 28th December 2010, 02:40 AM   #14
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Using a screen supply voltage of half the B+ was very common practice amongst guitar amplifier builders here in Australia during the 1970's. The output valve of choice was the 6CA7, and the Philips data sheets gave operating conditions for this configuration.

A voltage doubler power supply was used to produce a B+ of 750-800 volts and the screen supply was taken from the mid-point of the doubler. This configuration worked quite reliably with the Mullard and Philips 6CA7's of that era, but as the quality of valves steadily declined over the years, suitable replacements became problematic.
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Old 28th December 2010, 03:58 AM   #15
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So George, that means that a distortion analyzer is critical to getting the optimum performance out of pentode outputs. In your experiments was the minimum high order distortion point also the point that minimized HD3 in general? It is somewhat discomforting that different versions of the same tube show such variation.
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Old 28th December 2010, 05:40 AM   #16
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I'd agree with darry-h and Wieslaw Lipowsk that powering was a major influence - there are a variety of power rectifier configurations that naturally generate a 50% HT rail. KT88's enjoyed a 50% screen as well for higher powered PP amps. Wieslaw's rectifier example was also used by Philips in their PA commercial amps - eg. with EL36/PL36/6CM5.

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Old 28th December 2010, 05:44 AM   #17
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Here's an interesting production vintage amp with a really high plate V. and half the screen. Looks like a regular mono amp and not PA? Has a separate PS trans winding for screen V. What's the reason on this one and will the Output tubes last longer?

Randy
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Last edited by rmyauck; 28th December 2010 at 05:50 AM.
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Old 28th December 2010, 05:54 AM   #18
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rmyauck, that's for PA use - check out the speaker output winding configurations.
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Old 28th December 2010, 06:14 AM   #19
rmyauck is offline rmyauck  Canada
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Right and in Pics it looks like one too! Could circuit be modded for audiophile quality sound or is the use for PA the result of low screen V?
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Old 28th December 2010, 07:04 AM   #20
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Imho the amp can be modded somewhat to reduce distortion related affects - it's then up to your golden ears. Half voltage screen does not per se relegate an amp as far as distortion.

The first thing would be to measure the turns ratio's of all the secondary segments, and see what opportunity exists for reconfiguring winding segments - with the aim of using as much of the secondary copper as possible for a speaker matched output.

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